Advertisement

The Chlamydia pneumoniae Adhesin Pmp21 Forms Oligomers with Adhesive Properties*

Open AccessPublished:August 22, 2016DOI:https://doi.org/10.1074/jbc.M116.728915

      Abstract

      Chlamydiae sp. are obligate intracellular pathogens that cause a variety of diseases in humans. The adhesion of Chlamydiae to the eukaryotic host cell is a pivotal step in pathogenesis. The adhesin family of polymorphic membrane proteins (Pmp) in Chlamydia pneumoniae consists of 21 members. Pmp21 binds to the epidermal growth factor receptor (EGFR). Pmps contain large numbers of FXXN (where X is any amino acid) and GGA(I/L/V) motifs. At least two of these motifs are crucial for adhesion by certain Pmp21 fragments. Here we describe how the two FXXN motifs in Pmp21-D (D-Wt), a domain of Pmp21, influence its self-interaction, folding, and adhesive capacities. Refolded D-Wt molecules form oligomers with high sedimentation values (8–85 S). These oligomers take the form of elongated protofibrils, which exhibit Thioflavin T fluorescence, like the amyloid protein fragment β42. A mutant version of Pmp21-D (D-Mt), with FXXN motifs replaced by SXXV, shows a markedly reduced capacity to form oligomers. Secondary-structure assays revealed that monomers of both variants exist predominantly as random coils, whereas the oligomers form predominantly β-sheets. Adhesion studies revealed that oligomers of D-Wt (D-Wt-O) mediate significantly enhanced binding to human epithelial cells relative to D-Mt-O and monomeric protein species. Moreover, D-Wt-O binds EGFR more efficiently than D-Wt monomers. Importantly, pretreatment of human cells with D-Wt-O reduces infectivity upon subsequent challenge with C. pneumoniae more effectively than all other protein species. Hence, the FXXN motif in D-Wt induces the formation of β-sheet-rich oligomeric protofibrils, which are important for adhesion to, and subsequent infection of human cells.

      Introduction

      Chlamydiae are Gram-negative bacteria with compact genomes, and some species represent significant threats to human health. Chlamydia trachomatis is the most prevalent sexually transmitted bacterial pathogen worldwide (
      • Schachter J.
      Infection and Disease Epidemiology.
      ). Furthermore, it causes trachoma, a form of ocular conjunctivitis characterized by massive inflammation that leads to scarring of the inner epithelial lining of the eyelid and eventually to blindness (
      • Wright H.R.
      • Turner A.
      • Taylor H.R.
      Trachoma.
      ). Chlamydia pneumoniae is an important respiratory pathogen, causing pneumonia, pharyngitis, sinusitis, and bronchitis. Moreover, it is associated with several chronic diseases including atherosclerosis, central nervous system disorders, and Alzheimers disease (
      • Blasi F.
      • Tarsia P.
      • Aliberti S.
      Chlamydophila pneumoniae.
      ,
      • Harris S.A.
      • Harris E.A.
      Herpes simplex virus type 1 and other pathogens are key causative factors in sporadic Alzheimer's Disease.
      • Kuo C.C.
      • Jackson L.A.
      • Campbell L.A.
      • Grayston J.T.
      Chlamydia pneumoniae (Twar).
      ). Despite the clinical relevance of Chlamydia, no vaccine is available for use in humans (
      • Brunham R.C.
      • Rey-Ladino J.
      Immunology of Chlamydia infection: implications for a Chlamydia trachomatis vaccine.
      ,
      • Puolakkainen M.
      Innate immunity and vaccines in chlamydial infection with special emphasis on Chlamydia pneumoniae.
      ). Chlamydia species have a unique biphasic developmental cycle, alternating between the infectious elementary body (EB)
      The abbreviations used are: EB, elementary body; Pmp, polymorphic membrane protein; EGFR, epidermal growth factor receptor; ThT, Thioflavin T; Aβ42, amyloid β42; SEC, size-exclusion chromatography; MALS, multiangle light scattering; AUC, analytical ultracentrifugation; RFU, relative fluorescent unit; TEM, transmission electron microscopy.
      and the intracellular, metabolically active, reticulate body that replicates in eukaryotic cells (
      • Moulder J.W.
      Interaction of Chlamydiae and host-cells in vitro.
      ,
      • Hatch T.P.
      Development Biology.
      ).
      Polymorphic membrane proteins (Pmps) found in different species of the Chlamydiaceae are adhesion-mediating proteins (
      • Crane D.D.
      • Carlson J.H.
      • Fischer E.R.
      • Bavoil P.
      • Hsia R.C.
      • Tan C.
      • Kuo C.C.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is a species-common pan-neutralizing antigen.
      • Kalman S.
      • Mitchell W.
      • Marathe R.
      • Lammel C.
      • Fan J.
      • Hyman R.W.
      • Olinger L.
      • Grimwood J.
      • Davis R.W.
      • Stephens R.S.
      Comparative genomes of Chlamydia pneumoniae and C-trachomatis.
      ,
      • Longbottom D.
      • Russell M.
      • Dunbar S.M.
      • Jones G.E.
      • Herring A.J.
      Molecular cloning and characterization of the genes coding for the highly immunogenic cluster of 90-kilodalton envelope proteins from the Chlamydia psittaci subtype that causes abortion in sheep.
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ). Bioinformatic analysis has shown that the Pmp protein family is composed of 9 members (PmpA to PmpI) in C. trachomatis and 21 (Pmp1 to Pmp21) in C. pneumoniae (
      • Kalman S.
      • Mitchell W.
      • Marathe R.
      • Lammel C.
      • Fan J.
      • Hyman R.W.
      • Olinger L.
      • Grimwood J.
      • Davis R.W.
      • Stephens R.S.
      Comparative genomes of Chlamydia pneumoniae and C-trachomatis.
      ,
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ). The pmp gene family has been subdivided, on phylogenetic grounds, into six subtypes (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ). The subtypes from both species have retained a significant degree of sequence similarity across species. Thus, the level of identity between the PmpD subtype members PmpD and Pmp21 of C. trachomatis serovar E and C. pneumoniae CWL029, respectively, is 33%, indicating some level of functional similarity across chlamydial species (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ).
      The various Pmp families show a unique overrepresentation of repeats of the motifs GGA(I/L/V) and FXXN (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ,
      • Rockey D.D.
      • Lenart J.
      • Stephens R.S.
      Genome sequencing and our understanding of Chlamydiae.
      ). For example, the FXXN motif is found on average 11.3 times in the Pmps of C. pneumoniae, whereas its average incidence in the rest of the proteome is 0.84 (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ).
      Furthermore, Pmp proteins are united by their predicted autotransporter characteristics. Thus all Pmps share an N-terminal Sec-dependent leader sequence followed by a passenger domain and a C-terminal β-barrel (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ,
      • Henderson I.R.
      • Lam A.C.
      Polymorphic proteins of Chlamydia spp.: autotransporters beyond the proteobacteria.
      ). Structure predictions have suggested that a large region of the passenger domain of Pmp6 folds into a parallel β-strand in a helical pattern with three faces that form a β-helix (
      • Bradley P.
      • Cowen L.
      • Menke M.
      • King J.
      • Berger B.
      BETAWRAP: successful prediction of parallel β-helices from primary sequence reveals an association with many microbial pathogens.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ). β-Helical structures are a characteristic of autotransporters and might be required for their efficient translocation across the outer membrane and for folding (
      • Junker M.
      • Schuster C.C.
      • McDonnell A.V.
      • Sorg K.A.
      • Finn M.C.
      • Berger B.
      • Clark P.L.
      Pertactin β-helix folding mechanism suggests common themes for the secretion and folding of autotransporter proteins.
      ). Moreover, it has been speculated that Pmp β-helices could associate with each other to generate oligomers (
      • Hegemann J.H.
      • Moelleken K.
      Chlamydial adhesin and adhesins.
      ). Like other autotransporter proteins, many Pmps undergo complex proteolytic processing (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Kiselev A.O.
      • Skinner M.C.
      • Lampe M.F.
      Analysis of pmpD expression and PmpD post-translational processing during the life cycle of Chlamydia trachomatis serovars A, D, and L2.
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Saka H.A.
      • Thompson J.W.
      • Chen Y.S.
      • Kumar Y.
      • Dubois L.G.
      • Moseley M.A.
      • Valdivia R.H.
      Quantitative proteomics reveals metabolic and pathogenic properties of Chlamydia trachomatis developmental forms.
      • Swanson K.A.
      • Taylor L.D.
      • Frank S.D.
      • Sturdevant G.L.
      • Fischer E.R.
      • Carlson J.H.
      • Whitmire W.M.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
      ). Several of the C. pneumoniae and all C. trachomatis Pmps have been shown to be located on the chlamydial surface (
      • Crane D.D.
      • Carlson J.H.
      • Fischer E.R.
      • Bavoil P.
      • Hsia R.C.
      • Tan C.
      • Kuo C.C.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is a species-common pan-neutralizing antigen.
      ,
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Kiselev A.O.
      • Skinner M.C.
      • Lampe M.F.
      Analysis of pmpD expression and PmpD post-translational processing during the life cycle of Chlamydia trachomatis serovars A, D, and L2.
      ,
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Swanson K.A.
      • Taylor L.D.
      • Frank S.D.
      • Sturdevant G.L.
      • Fischer E.R.
      • Carlson J.H.
      • Whitmire W.M.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
      ,
      • Montigiani S.
      • Falugi F.
      • Scarselli M.
      • Finco O.
      • Petracca R.
      • Galli G.
      • Mariani M.
      • Manetti R.
      • Agnusdei M.
      • Cevenini R.
      • Donati M.
      • Nogarotto R.
      • Norais N.
      • Garaguso I.
      • Nuti S.
      • Saletti G.
      • Rosa D.
      • Ratti G.
      • Grandi G.
      Genomic approach for analysis of surface proteins in Chlamydia pneumoniae.
      • Tan C.
      • Hsia R.C.
      • Shou H.
      • Carrasco J.A.
      • Rank R.G.
      • Bavoil P.M.
      Variable expression of surface-exposed polymorphic membrane proteins in in vitro grown Chlamydia trachomatis.
      ).
      All C. trachomatis Pmps as well as Pmp6, Pmp20, and Pmp21 from C. pneumoniae have been found to serve as adhesins, mediating the attachment of chlamydial EBs to human epithelial cells. In addition, blocking experiments using recombinant Pmp proteins have provided direct evidence for the critical role of the Pmp proteins in chlamydial pathogenesis (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Becker E.
      • Hegemann J.H.
      All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function.
      ) More recently, the epidermal growth factor receptor (EGFR) was identified as the host receptor for the C. pneumoniae adhesin Pmp21, and binding to EGFR was shown to induce the uptake of the chlamydial EB, thus qualifying Pmp21 as an invasin (
      • Mölleken K.
      • Becker E.
      • Hegemann J.H.
      The Chlamydia pneumoniae invasin protein Pmp21 recruits the EGF receptor for host cell entry.
      ).
      Pmp21 occurs on the surface of infectious EBs in various processed forms, referred to as N-M-C-Pmp21, N-M-Pmp21, N-Pmp21 (31–670 amino acids (aa)), M-Pmp21 (671–1145 aa), and C-Pmp21 (1146–1609 aa) (Fig. 1C) (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Grimwood J.
      • Olinger L.
      • Stephens R.S.
      Expression of Chlamydia pneumoniae polymorphic membrane protein family genes.
      ). Interestingly, Pmp21 possesses multiple adhesion domains (
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ), and truncation experiments have demonstrated (Fig. 1, D and E) that adhesion critically depends on the presence of the repetitive GGA(I/L/V) and FXXN motifs. Thus, targeted mutagenesis has revealed that at least one GGA(I/L/V) and one FXXN motif (present in Pmp21-A) (Fig. 1E) or two FXXN motifs (present in Pmp21-D) (Fig. 1D) are required and sufficient for significant adhesion to HEp-2 cells (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ).
      Figure thumbnail gr1
      FIGURE 1.A, schematic representation of Pmp21. The full-length protein is depicted with the N-terminal signal sequence (SS) and the C-terminal β-barrel domain (β-barrel). Each of the tetrapeptide motifs GGA(I/L/V) (in yellow) and FXXN (in red) in the central passenger domain is marked. Two known proteolytic cleavage sites in Pmp21 are indicated by arrows (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). B, structure of M-Pmp21 as predicted by I-Tasser (C-score: −1.58) (
      • Roy A.
      • Kucukural A.
      • Zhang Y.
      I-TASSER: a unified platform for automated protein structure and function prediction.
      ,
      • Yang J.
      • Yan R.
      • Roy A.
      • Xu D.
      • Poisson J.
      • Zhang Y.
      The I-TASSER Suite: protein structure and function prediction.
      • Zhang Y.
      I-TASSER server for protein 3D structure prediction.
      ). The β-sheets are displayed in yellow, and random coils are in gray. C, predominant forms of Pmp21 in vivo, as detected by proteome analysis (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Grimwood J.
      • Olinger L.
      • Stephens R.S.
      Expression of Chlamydia pneumoniae polymorphic membrane protein family genes.
      ). D and E, the recombinant full-length Pmp21 passenger domain (PD-Pmp21), the processed forms N-Pmp21 and M-Pmp21, and the truncated subdomains Pmp21-A to Pmp21-D (D-Wt) mediate adhesion. A mutant form of Pmp21-D (D-Mt) in which each FXXN motif is replaced by SXXV was found to be incapable of mediating adhesion (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ).
      Strikingly, immunoaffinity enrichment of PmpD, the C. trachomatis homologue of Pmp21, from infectious EBs resulted in the isolation of high molecular weight structures, which included full-length PmpD and two proteolytically processed forms. The functional significance of these structures remains unknown (
      • Swanson K.A.
      • Taylor L.D.
      • Frank S.D.
      • Sturdevant G.L.
      • Fischer E.R.
      • Carlson J.H.
      • Whitmire W.M.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
      ).
      High molecular weight structures with adhesive characteristics have been identified on the surface of a number of pathogenic bacteria. In Enterobacteriaceae, including Escherichia coli, highly aggregative surface fibers called curli have been found (
      • Bokranz W.
      • Wang X.
      • Tschäpe H.
      • Römling U.
      Expression of cellulose and curli fimbriae by Escherichia coli isolated from the gastrointestinal tract.
      ,
      • Chapman M.R.
      • Robinson L.S.
      • Pinkner J.S.
      • Roth R.
      • Heuser J.
      • Hammar M.
      • Normark S.
      • Hultgren S.J.
      Role of Escherichia coli curli operons in directing amyloid fiber formation.
      ). First observed in 1989 in fibronectin binding E. coli isolates from bovine fecal samples (
      • Olsén A.
      • Wick M.J.
      • Mörgelin M.
      • Björck L.
      Curli, fibrous surface proteins of Escherichia coli, interact with major histocompatibility complex class I molecules.
      ), curli fibers have been shown to mediate interactions between individual bacteria, bacteria and host tissues, and bacteria and inert surfaces like Teflon and stainless steel, which are usually refractory to bacterial colonization (
      • Bokranz W.
      • Wang X.
      • Tschäpe H.
      • Römling U.
      Expression of cellulose and curli fimbriae by Escherichia coli isolated from the gastrointestinal tract.
      ,
      • Gophna U.
      • Barlev M.
      • Seijffers R.
      • Oelschlager T.A.
      • Hacker J.
      • Ron E.Z.
      Curli fibers mediate internalization of Escherichia coli by eukaryotic cells.
      • Pawar D.M.
      • Rossman M.L.
      • Chen J.
      Role of curli fimbriae in mediating the cells of enterohaemorrhagic Escherichia coli to attach to abiotic surfaces.
      ,
      • Ryu J.H.
      • Kim H.
      • Beuchat L.R.
      Attachment and biofilm formation by Escherichia coli O157:H7 on stainless steel as influenced by exopolysaccharide production, nutrient availability, and temperature.
      ,
      • Uhlich G.A.
      • Cooke P.H.
      • Solomon E.B.
      Analyses of the red-dry-rough phenotype of an Escherichia coli O157:H7 strain and its role in biofilm formation and resistance to antibacterial agents.
      • Zogaj X.
      • Bokranz W.
      • Nimtz M.
      • Römling U.
      Production of cellulose and curli fimbriae by members of the family Enterobacteriaceae isolated from the human gastrointestinal tract.
      ). Subsequently, curli fibers were shown to be made up of an amyloid-like protein that binds the amyloid-specific dye Thioflavin T (ThT) (
      • Barnhart M.M.
      • Chapman M.R.
      Curli biogenesis and function.
      ,
      • Wang X.
      • Smith D.R.
      • Jones J.W.
      • Chapman M.R.
      In vitro polymerization of a functional Escherichia coli amyloid protein.
      ). Amyloids are insoluble protein aggregates derived from the conversion of protofibrils into amyloid fibrils, which are formed by proteins characterized by a typical β-sheet structure. The commercially available human amyloid β42 (Aβ42) is an amyloid-like peptide that can also form amyloid fibrils in vitro.
      In this study we focus on Pmp21-D (D-Wt), a C-terminal fragment derived from the naturally occurring M-Pmp21. It is the smallest Pmp21 fragment identified thus far that exhibits adhesion and infection blocking capacity (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ) (Fig. 1E). Our data demonstrate that the monomer (D-Wt-M) forms oligomers that adopt an amyloid-like structure. The Pmp21-D oligomers (collectively referred to as D-Wt-O) are comparable in size and shape to protofibrils of Aβ42. Comparison of the oligomerization capacity of D-Wt with that of a previously analyzed mutant form (D-Mt) with poor adhesion properties revealed that the FXXN motif in D-Wt contributes significantly to the formation of oligomers. Interestingly, both the adhesion to its host cell receptor EGFR and the neutralization capacity of D-Wt require its oligomerization.

      Results

      Recombinant Pmp21-D Can Exist in Monomeric and Diverse Homo-oligomeric Forms

      When we performed an automated structural characterization based on the C. pneumoniae Pmp21 protein sequence, we found that the processing product M-Pmp21 is predicted to form a long, right-handed β-helix domain, which could provide a rigid platform on which multiple adhesive sites can be presented and could self-associate to generate oligomers (Fig. 1B). To test this hypothesis experimentally, we expressed Pmp21-D (D-Wt; Fig. 1E), an adhesive C-terminal fragment of M-Pmp21 (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ), in E. coli. We then purified the recombinant protein under denaturing conditions via its N-terminal His tag and allowed it to refold (at a concentration of 3.1 mg ml−1) before elution from the affinity column (see “Experimental Procedures”). The eluate was first analyzed by SDS-PAGE, which revealed that the major species migrated at ∼30 kDa as expected for the monomeric D-Wt protein (M, Fig. 2A). However, a small amount of the SDS-resistant dimer was also detected (D, ∼60 kDa; Fig. 2A). Western blotting using an anti-His antibody confirmed the identity of the ∼30 kDa and ∼60 kDa bands as recombinant D-Wt (Fig. 2D).
      Figure thumbnail gr2
      FIGURE 2.A, analysis of an aliquot of the recombinant D-Wt by SDS-PAGE before SEC revealed monomers (*, ∼30 kDa) and SDS-resistant dimers (**, ∼60 kDa) (independent replicates n = 3). B, preparative SEC (HiLoad 16/600 Superdex 200 pg) of D-Wt refolded at a concentration of 3.1 mg ml−1 and dissolved in PBS. The flow rate used for separation of diverse oligomeric forms (D-Wt-O1–3) from the monomer (D-Wt-M) was 0.5 ml min−1 (independent replicates n = 3) blue dextran as well as the molecular weights of the standard proteins are indicated above the chromatogram. a.u., absorbance units. C, the SEC eluate was collected in 1.2-ml fractions. Every second fraction was analyzed by SDS-PAGE (independent replicates n = 3). D and E, analysis of A and C by Western blotting with anti-His-tag antibodies (independent replicates n = 3).
      We then analyzed the homogeneity of the purified and refolded protein by size-exclusion chromatography (SEC). A broad peak emerging near the void volume (46–70 ml) was observed together with one distinct later peak eluting at around 79 ml (with an apparent size of 57 kDa) (Fig. 2B). Analysis of every second elution fraction by SDS-PAGE (Fig. 2C) and Western blot (Fig. 2E) verified the identity of the main protein species detected by Coomassie staining in Fig. 2C as D-Wt.
      We additionally repeated the experiment by eluting denatured D-Wt from the nickel column and refolded it by dialysis for 36 h in PBS before SEC. In this SEC we observed the same elution profile as for the previously described “on column refolding” procedure (data not shown).
      To determine the molecular mass of the protein species in the second peak, analytical size-exclusion chromatography coupled to multiangle light scattering detection (SEC-MALS) was performed. The data revealed that this peak (Fig. 3) corresponds to the monomeric species D-Wt-M (23 kDa). Thus, under non-denaturing conditions, only a minor fraction of recombinant D-Wt exists in the monomeric form, which in turn suggested that the broad first peak was made up of high molecular weight oligomers (D-Wt-O). Representative fractions of D-Wt-O were chosen from this peak and its descending limb (D-Wt-O1–3 with apparent sizes of 839, 554, and 366 kDa, respectively) (Fig. 2B) for further experiments.
      Figure thumbnail gr3
      FIGURE 3.Analytical SEC (Superdex 200 10/300 GL; flow rate 0.3 ml min−1 in PBS) (solid line) of the D-Wt-M peak isolated by preparative SEC (B). The eluate was monitored by multi-angle light scattering (broken line) and a monomer of 23 kDa was identified. a.u., absorbance units.

      Pmp21-D Oligomers Form Rod-like Structures

      The size distribution of the D-Wt species in these fractions was analyzed in solution by analytical ultracentrifugation (AUC). Because the AUC assay has a running time of several hours, we first determined the stability of D-Wt-O1 by analytical SEC on the time scale required for AUC sample preparation and measurement. Even after an incubation period for 20 h in PBS at 4 °C, D-Wt-O1 was still stable and had not dissociated into lower molecular weight species (Fig. 4A).
      Figure thumbnail gr4
      FIGURE 4.A, analytical SEC (Superdex 200 HR 10/30; flow rate 0.3 ml min−1) of the D-Wt-O1 fraction isolated by preparative SEC (B) was used to monitor the dissociation of D-Wt-O1 into D-Wt-M over time (independent replicates n = 2). a.u., absorbance units. B and E, size distribution of D-Wt monitored by AUC. B, subset of the raw data for velocity sedimentation of D-Wt-M in a Beckman An-60 Ti rotor at 20 °C, 230 nm, and 40,000 rpm, recorded over a period of 280 scans (at 1 scan/1.5 min). C, distribution of sedimentation coefficients for D-Wt-M analyzed using a continuous c(s) distribution model (independent replicates n = 2). D, subset of the raw sedimentation data obtained for the D-Wt SEC input sample (D-Wt refolded at a concentration of 3.1 mg ml−1). B, same conditions as A. E, distribution of sedimentation coefficients for D-Wt, analyzed as in C. Input was analyzed using a continuous c(s) distribution model. Residuals after data fitting are shown together with the calculated S values (independent replicates n = 2).
      The peak fraction of monomeric D-Wt-M (Fig. 3) was analyzed by measuring its sedimentation velocity with AUC at 40,000 rpm and 20 °C. Subsequent data analysis by SEDFIT, assuming a continuous c(s) distribution model, indicated particles of 1.6S with a small side peak at 2.6S and an overall frictional ratio f/f0 of 1.9 (Fig. 4, B and C). Similarly, D-Wt-O1 was analyzed at a sedimentation velocity of 20,000 rpm. The results suggested that D-Wt-O1 accounted for ∼87.4% of D-Wt, with a size distribution ranging from 8S to 85S, with a mean size of 23.8S and a frictional ratio of f/f0 = 2.8 (Fig. 4, D and E). This frictional ratio suggests the presence of long rod-like particles in D-Wt-O, whereas the data for D-Wt-M are typical for a monomeric protein of this size.

      Oligomers of Pmp21-D Are Protofibril-like in Shape

      To obtain a better understanding of the structure of the D-Wt-O species observed in the SEC experiments, we imaged them by electron microscopy (EM). Representative SEC fractions of D-Wt-O (D-Wt-O1-3) as well as D-Wt-M (Fig. 2B) were centrifuged at 10,000 × g for 10 min and applied to carbon-coated copper grids. After a brief washing with H2O, the grids were stained with 1% uranyl acetate. D-Wt-O1 appeared in a form we refer to as protofibrils due to their morphological similarity to amyloid-like proteins (
      • Walsh D.M.
      • Lomakin A.
      • Benedek G.B.
      • Condron M.M.
      • Teplow D.B.
      Amyloid β-protein fibrillogenesis: detection of a protofibrillar intermediate.
      • Bitan G.
      • Kirkitadze M.D.
      • Lomakin A.
      • Vollers S.S.
      • Benedek G.B.
      • Teplow D.B.
      Amyloid β-protein (Aβ) assembly: Aβ40 and Aβ42 oligomerize through distinct pathways.
      ,
      • Nybo M.
      • Svehag S.E.
      • Holm Nielsen E.
      An ultrastructural study of amyloid intermediates in Aβ(1–42) fibrillogenesis.
      • Serpell L.C.
      Alzheimer's amyloid fibrils: structure and assembly.
      ) (Fig. 5, A and B). The D-Wt-O1 protofibrils had a mean width of 8.8 ± 1.3 nm and an average length of 61.9 nm (Fig. 5, C and D). The proteins in the D-Wt-O2 fractions were also fibrillar in shape (Fig. 5, A and B) and had a similar mean width (10.7 nm) to the D-Wt-O1 oligomers, but were shorter, with a mean length of 27.7 nm (Fig. 5, C and D). Fraction D-Wt-O3 contained more spherically structured particles (Fig. 5, A and B) with mean lengths and widths of 11.9 nm (Fig. 5E). No oligomeric structures were detectable by EM in the D-Wt-M fraction (Fig. 5, A and B). This was in agreement with the previous data showing that this protein species has a size of only 23 kDa as determined by SEC-MALS (Fig. 3). Moreover, this result indicates that D-Wt-M has no tendency to spontaneously form D-Wt-O structures during the course of sample preparation (∼2 h).
      Figure thumbnail gr5
      FIGURE 5.TEM (80 kV) of samples (1 μm) of D-Wt (D-Wt-O1–3 and D-Wt-M) isolated by SEC (B). Samples were applied to carbon-coated copper grids, negatively stained with 1% uranyl acetate, and imaged at 50,000 × magnification (A) and 85.000 × magnification (B, scale bar = 100 nm). The images shown are representative of the whole grids (independent replicates n = 2). The widths (C) and lengths (D) of 300 D-Wt particles from each sample were determined with ImageJ. The data are displayed in box plots. Maximum and minimum values are indicated by the whiskers; 1st and 3rd quantiles represent 50% of the total data. The means are represented by the red dots within the boxes. E, relative (ThT fluorescence assay. The indicated protein samples were incubated (at a final concentration of 10 μm) with 10 μm ThT for 48 h. BSA and preincubated Aβ42 were used as negative and positive controls, respectively (independent replicates n = 3). At the 0-h time point all protein samples except for Aβ42 exhibited low RFUs. Data shown are the means ± S.D. Statistical significance was assessed with Student's t test (*, p = 0.05; **, p = 0.01; ***, p = 0.001).

      Oligomers of Pmp21-D Exhibit Amyloid-specific ThT Fluorescence

      The benzothiazole dye ThT is a sensitive probe for amyloid fibril detection. The ThT test is based on its unique ability to form highly fluorescent complexes with amyloid and amyloid-like proteins (
      • Kuznetsova I.M.
      • Sulatskaya A.I.
      • Uversky V.N.
      • Turoverov K.K.
      A new trend in the experimental methodology of amyloid fibril structural investigation with the use of thioflavin T.
      ,
      • Sulatskaya A.I.
      • Kuznetsova I.M.
      • Turoverov K.K.
      interaction of thioflavin T with amyloid fibrils: fluorescence quantum yield of bound dye.
      ). To ask whether the Pmp protofibrils detected by EM (Fig. 5, A and B) had amyloid-like characteristics, all SEC fractions studied by EM (except D-Wt-O3, in which protein concentrations were too low) were tested for ThT fluorescence. BSA, which served as the negative control, the positive control amyloid β42 (Aβ42), and the Pmp21 species D-Wt-M, D-Wt-O2, and D-Wt-O1 were incubated at 37 °C for 48 h in the presence of 10 μm ThT, and the change in ThT fluorescence emission at 480 nm was monitored. PBS and BSA both showed only a very low emission during the entire assay with maximally 325 relative fluorescent units (RFU) (Fig. 5E). In contrast, the positive control Aβ42 started with an RFU of 831 and ended with 2898 ± 688 RFU at 48 h. Most strikingly, emission levels with D-Wt-O1 and D-Wt-O2 protofibrils were even higher, starting with 4570 RFU and 3407 RFU, respectively, at time point 0 h and increasing with time to 7766 RFU and 6441 RFU, respectively, at the end of the assay. Notably, D-Wt-M exhibited with 234 RFU a very low fluorescence emission at time point 0 h, which increased over time to an RFU of 2505 ± 86 (48 h), indicating slow oligomer formation. Taken together with our EM data, these results confirm that D-Wt has a strong tendency to form ThT-sensitive, amyloid-like structures.

      The FXXN Motif Strongly Promotes the Formation of D-Wt Oligomers

      In previous studies we found that mutating the two FXXN motifs present in D-Wt to SXXV (D-Mt) results in complete loss of adhesive capacity (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). Thus we speculated that these motifs might also be relevant for oligomer formation. Therefore, recombinant D-Wt and D-Mt were refolded at four different concentrations (0.7, 1.7, 2.4, and 3.1 mg ml−1), centrifuged, and analyzed by SEC. The area under the oligomer peaks was then expressed as a percentage of the total area under the curve. To get an impression of the ratios easily, the monomer peaks were set to 1 (Fig. 6A). At a concentration of 0.7 mg ml−1 22% of total D-Wt protein eluted as oligomeric D-Wt-O. The figure for oligomeric mutant D-Mt-O was only 4%. Increasing the total concentration of either monomer in the assay also increased the percentage of oligomers formed (Fig. 6B). However, D-Wt always gave rise to a significantly higher proportion of oligomers than the same concentration of the mutated D-Mt protein. This difference was very pronounced at a concentration of 1.7 mg ml−1, at which the mutant form D-Mt produced ∼10-fold fewer oligomers (5%) compared with D-Wt (53%). Thus, these data demonstrate that both D-Wt and D-Mt can form oligomers and that the capacity to form oligomers is significantly enhanced by the two FXXN motifs.
      Figure thumbnail gr6
      FIGURE 6.The FXXN motif is essential for oligomer formation by D-Wt-M. A, D-Wt and D-Mt were refolded at 3.1 mg ml−1 and analyzed by SEC for monomer and oligomer formation. To get an impression of the ratios easily, the monomer peaks were set to 1 (independent replicates n = 2). a.u., absorbance units. B, percentage of oligomers found in the total input after refolding of D-Wt and D-Mt at the indicated concentrations (independent replicates n = 2). C, CD spectroscopy (at 20 °C) of D-Wt-O1 (solid line), D-Wt-M (dashed line), and D-Mt-M (dashed-dotted line). Samples were dissolved at 10 μm in 10 mm sodium phosphate buffer, 75 mm sodium fluoride, pH 7.5 (independent replicates n = 2). Because very little D-Mt-O1 was formed (dotted line), these measurements were performed in the running buffer PBS directly after SEC. D and E, TEM (80 kV) of samples (1 μm) of D-Mt (D-Mt-O1–3 and D-Mt-M) isolated by SEC (SEC data not shown). The analyzed fractions O1–3 and M correspond to the elution volumes taken for the corresponding D-Wt species (see B). Samples were applied to carbon-coated copper grids, negatively stained with 1% uranyl acetate, and imaged at 50,000× magnification (D) and 85,000× magnification (E, scale bar = 100 nm). The images shown are representative for the whole grids (independent replicates n = 2).
      The fact that D-Mt has a strongly reduced propensity to form oligomers points to structural differences between the monomers and possibly their respective oligomeric forms too. Therefore, the secondary structure of wild-type and mutant monomer and oligomer forms was determined by circular dichroism (CD) spectroscopy (Fig. 6C). Data analysis using the program CONTINLL revealed that D-Wt-O1 consists of ∼11.5% β-sheets and ∼3.4% α-helices, whereas D-Wt-M displayed a random-coil structure similar to that of a disordered protein lacking any secondary structure. D-Mt-O1 showed strong similarities to D-Wt-O1 but with some tendency to adopt the random coil pattern of D-Wt-M. Finally, D-Mt-M produced a random-coil spectrum comparable with that of D-Wt-M. Hence, oligomers of D-Wt-O1 and D-Mt-O1 differ somewhat in their secondary structures, whereas both monomers, D-Wt-M and D-Mt-M, show predominantly random-coil structures.
      We next analyzed representative SEC fractions corresponding to D-Mt-O1–3 and D-Mt-M by TEM (Fig. 6, D and E). Interestingly, D-Mt-O1 showed rod-shaped protofibrilar structures, very similar to those found for D-Wt-O1. The smaller D-Mt oligomers (O2–3) appeared globular. As for D-Wt-M, no oligomeric structures were detectable by EM in the D-Mt-M fraction.

      FXXN-induced Oligomerization of Pmp21-D Is Crucial for Its Ability to Adhere to Human Cells

      In previous studies, Pmp6, -20, and -21 from C. pneumoniae and all nine Pmps from C. trachomatis serovar E, were characterized as bacterial adhesins that are important for the infection of human epithelial cells (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Becker E.
      • Hegemann J.H.
      All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function.
      ). The motifs GGA(I/L/V) and FXXN are characteristic for Pmps and essential for adhesion (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). Our results so far have demonstrated that these motifs are additionally responsible for protein oligomerization (Fig. 6, A and B). To analyze which of the different protein species represents the adhesion-competent conformation and whether adhesion is mediated by the FXXN motifs alone, we performed bead-based adhesion assays with all D-Wt and D-Mt protein species (Fig. 7A). Protein-coated green fluorescent latex beads were incubated with human epithelial HEp-2 cells. The coating efficiency estimated by immunoblotting revealed only slight differences in the amount of each protein coupled to the beads (data not shown). The negative control, BSA-coated beads, only bound to 15.9 ± 3% HEp-2 cells. Importantly, D-Wt-O1 showed a significant adhesion capacity with 50 ± 4.3% HEp-2 cells carrying bound beads. In contrast, the monomeric protein species D-Wt-M and D-Mt-M as well as the oligomeric mutant form D-Mt-O1 mediated comparatively little binding of beads to HEp-2 cells (20.6 ± 3.2%, 20.5 ± 2.1%, and 20.3 ± 5%, respectively). Hence, the bead assay suggests that only the oligomeric wild-type protein species D-Wt-O1 exhibits strong adhesion capacity. This implies that both the formation of oligomers as well as presence of FXXN motifs is crucial for binding to HEp-2 cells.
      Figure thumbnail gr7
      FIGURE 7.Adhesion of D-Wt to human cells depends on the presence of FXXN motifs and oligomerization. Binding of fluorescent latex beads coated with BSA or the indicated recombinant protein species isolated by preparative SEC (100 μg ml−1) to HEp-2 cells was analyzed as described under “Experimental Procedures.” Fluorescent latex beads were coated with BSA or the recombinant proteins after preparative SEC (100 μg ml−1), and 1 × 106 beads were incubated with confluent HEp-2 cells at 37 °C for 1 h. The number of HEp-2 cells with bound beads was determined by flow cytometry. Results were derived from three independent experiments (independent replicates n = 3). Data shown are the means ± S.D. Statistical significance was assessed with Student's t test (**, p = 0.01; ***, p = 0.001).

      The Oligomeric Form of Pmp21-D Is Important for the Binding to the EGFR

      In previous studies the EGF receptor was identified as the receptor for the M-Pmp21 protein domain, which occurs naturally (
      • Mölleken K.
      • Becker E.
      • Hegemann J.H.
      The Chlamydia pneumoniae invasin protein Pmp21 recruits the EGF receptor for host cell entry.
      ). To analyze whether the monomeric and oligomeric D-Wt species have identical or different affinities for EGFR, we performed pulldown assays with D-Wt-O1 and D-Wt-M. As positive control we used recombinant M-Pmp21 (
      • Mölleken K.
      • Becker E.
      • Hegemann J.H.
      The Chlamydia pneumoniae invasin protein Pmp21 recruits the EGF receptor for host cell entry.
      ), whereas other controls were recombinant GST, the C. pneumoniae adhesin OmcB-BD (
      • Moelleken K.
      • Hegemann J.H.
      The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding.
      ), and the C. trachomatis adhesin and invasin Ctad1 (
      • Stallmann S.
      • Hegemann J.H.
      The Chlamydia trachomatis Ctad1 invasin exploits the human integrin β 1 receptor for host cell entry.
      ) (Fig. 8A). After biotinylation the recombinant proteins were incubated with human epithelial HEp-2 cells, cross-linked, and affinity-purified using a streptavidin resin. Interaction partners were eluted after cross-link removal. Western blots of whole cell lysates cross-linked to the recombinant proteins revealed the presence of identical amounts of EGFR (Fig. 8B). After pulldown the EGFR signals were found to be of similar strength for the positive control M-Pmp21 and the oligomeric D-Wt-O1 (1.03 and 1.0, respectively) (Fig. 8C). In contrast, the pulldown using the monomeric D-Wt-M brought down only ∼50% of the amount brought down by the oligomeric form (0.5 relative intensity). Recombinant GST as well as OmcB-BD failed to bring down detectable amounts of EGFR. A very weak signal was observed for Ctad1. These results provide evidence that both D-Wt-M and D-Wt-O1 can interact with EGFR; however, the efficiency of the oligomeric species is twice as high as that of monomeric species. This again strongly suggests the relevance of oligomer formation for the infection process.
      Figure thumbnail gr8
      FIGURE 8.A, Coomassie-stained SDS-PAGE of the recombinant proteins GST, Ctad1, M-Pmp21, D-Wt-O1, C-Wt-M, and OmcB-BD used in the pulldown experiment. Shown is a Western blot of the whole cell lysate after cross-linking (B) and of the eluate after pulldown (C) using an anti-EGFR antibody. B, all HEp2 cell lysates harbored similar amounts of EGFR. C, after pulldown and elution from the streptavidin resin different amounts of EGFR were detected, quantified by Image J, and normalized against the EGFR signal obtained for D-Wt-O1, which was set to 1. Relative EGFR amounts are indicated at the bottom of each lane (independent replicates n = 2).

      The Oligomeric Form of Pmp21-D Is Important for Infection

      Next we asked whether the adhesion-competent oligomeric D-Wt-O protein species is also relevant for a chlamydial infection. To this end, soluble recombinant monomeric or oligomeric D-Wt and D-Mt protein species were preincubated with HEp-2 cells before infection. ∼48 h post infection the efficiency of infection was measured by counting the numbers of inclusions formed (Fig. 9, A and B). Pretreatment with the negative control BSA did not significantly reduce the infection upon subsequent exposure to C. pneumoniae EBs, whereas the positive control heparin reduced infectivity by 99%. Interestingly, D-Wt-O1 had the strongest effect, blocking the subsequent infection by almost 50% compared with the PBS control (53.5 ± 2.3% inclusions). D-Wt-M (82.9 ± 5.7% inclusions) and D-Mt-O1 (83 ± 1.7% inclusions) had a much weaker effect, reducing infectivity by only ∼17%, which is not significantly different from the value for the PBS control. The least effective of the protein species tested was D-Mt-M (92.7 ± 2.9%), which inhibited infection to about the same extent as the negative control BSA (96.7 ± 8.8%) (Fig. 9, A and B). These data also show that the wild-type and mutant oligomeric forms of Pmp21-D inhibit infection more effectively than the corresponding monomeric forms.
      Figure thumbnail gr9
      FIGURE 9.The ability of Pmp21-D to inhibit infection of HEp-2 cells by C. pneumoniae depends on the presence of FXXN motifs and protein oligomerization. HEp-2 cells were incubated with PBS, BSA, heparin, or the indicated recombinant proteins (100 μg ml−1) before infection with C. pneumoniae EBs (multiplicity of infection 20). Cells were fixed 48 h post infection, and the number of inclusions formed was determined by microscopy. The number of inclusions per 1 × 103 human cells was determined and expressed as percentage relative to the number of inclusions found in PBS-treated samples. A, results are derived from three independent experiments (independent replicates n = 3). Data shown are the means ± S.D. Statistical significance was assessed with Student's t test (*, p = 0.05; **, p = 0.01; ***, p = 0.001; n.s. = not significant). B, chlamydial infectivity as visualized by immunofluorescence microscopy. Methanol-fixed HEp-2 cells were stained with DAPI (blue). Chlamydial inclusions were detected with an antibody directed against chlamydial LPS (green).

      Discussion

      Successful infection of host cells by C. pneumoniae depends on a variety of virulence factors. These include specialized surface structures, which mediate uptake by host cells (
      • Kline K.A.
      • Fälker S.
      • Dahlberg S.
      • Normark S.
      • Henriques-Normark B.
      Bacterial adhesins in host-microbe interactions.
      ). Chlamydiae enter cells via multiple routes, using mechanisms that are poorly understood (
      • Campbell L.A.
      • Kuo C.-C.
      Interactions of Chlamydia with the host cells that mediate attachment and uptake.
      ). In previous studies we identified the Pmp proteins of C. pneumoniae and C. trachomatis as adhesins that are essential for the successful infection of human cells (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ,
      • Becker E.
      • Hegemann J.H.
      All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function.
      ). Pmps are known to share characteristic features with Type V autotransporters, including proteolytic processing (
      • Wehrl W.
      • Brinkmann V.
      • Jungblut P.R.
      • Meyer T.F.
      • Szczepek A.J.
      From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
      ,
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ,
      • Henderson I.R.
      • Lam A.C.
      Polymorphic proteins of Chlamydia spp.: autotransporters beyond the proteobacteria.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ,
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). Recently, Pmp21 was shown to bind to the host's EGF receptor and to induce its own uptake; hence, Pmp21 acts both as an adhesin and as an invasin (
      • Mölleken K.
      • Becker E.
      • Hegemann J.H.
      The Chlamydia pneumoniae invasin protein Pmp21 recruits the EGF receptor for host cell entry.
      ). Interestingly, the characteristic FXXN and GGA(I/L/V) motifs, which are known to occur in multiple copies exclusively in chlamydial Pmp proteins (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ), have been shown to be crucial for Pmp21-mediated adhesion (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). Structure prediction programs have indicated that the passenger domains of Pmps are dominated by parallel β-strands disposed in a helical pattern with three faces that form a β-helix (
      • Bradley P.
      • Cowen L.
      • Menke M.
      • King J.
      • Berger B.
      BETAWRAP: successful prediction of parallel β-helices from primary sequence reveals an association with many microbial pathogens.
      ,
      • Vandahl B.B.
      • Pedersen A.S.
      • Gevaert K.
      • Holm A.
      • Vandekerckhove J.
      • Christiansen G.
      • Birkelund S.
      The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
      ). It has, therefore, been speculated that these β-helices could associate with each other to generate Pmp oligomers (
      • Hegemann J.H.
      • Moelleken K.
      Chlamydial adhesin and adhesins.
      ).
      Our initial characterization of refolded recombinant Pmp21-D, an adhesion-competent subdomain of Pmp21, by SEC revealed that homo-oligomeric forms (D-Wt-O) were dominant, whereas the monomer (D-Wt-M) made up only a relatively small fraction of the whole (Fig. 2, B and C). Interestingly, we found D-Wt-O to be very stable, as no disaggregation was observed after prolonged incubation in PBS buffer (Fig. 4A). In contrast, with time, the monomeric D-Wt-M (1.6S) gave rise to a new stable species at 2.6S, probably a dimer, which may nucleate the formation of higher order oligomers (Fig. 4C). The in vitro formation of Pmp21-D oligomers is in agreement with earlier findings which indicated that the C. trachomatis homologue of Pmp21, PmpD, is part of a protein complex on the EB cell surface (
      • Swanson K.A.
      • Taylor L.D.
      • Frank S.D.
      • Sturdevant G.L.
      • Fischer E.R.
      • Carlson J.H.
      • Whitmire W.M.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
      ). Moreover, it was reported that Pmps of Chlamydia psittaci also occur in supramolecular complexes (
      • Tanzer R.J.
      • Longbottom D.
      • Hatch T.P.
      Identification of polymorphic outer membrane proteins of Chlamydia psittaci 6BC.
      ).
      Analysis of the sizes of the oligomeric D-Wt-O yielded remarkable large S values of up to 85S with an average of 23.8S, which may reflect the formation of amorphous aggregates by refolding intermediates. However, determination of the frictional ratio f/f0 of 2.75 for the D-Wt sample argues that large elongated structures are formed (Fig. 4E). Typical examples for highly elongated proteins are human fibrinogen (Mr 330) with an S value of 7.6S and an f/f0 = 2.3 and myosin (Mr 570) with an S value of 6.4S and an f/f0 = 3.6 (
      • Tanford C.
      ). However, in contrast to those proteins, Pmp21-D is very small (23 kDa), supporting the idea that it might form long homo-oligomeric protein species.
      Indeed, EM analysis revealed that the D-Wt-O oligomers form protofibril-like structures (Fig. 5, A and B). Interestingly, the three differently sized D-Wt-O species isolated by SEC correspond to protofibrils with almost identical widths (∼10 nm) but different lengths. One might speculate that the largest oligomer W-Wt-O1 (∼60 nm) could be the most mature form produced in vitro, whereas the others could represent intermediates. Interestingly, the D-Wt protofibrils exhibited amyloid-like characteristics, as they bind the dye thioflavin T and strongly enhance its fluorescence, as does the prototypical β-sheet-rich Aβ42 (Fig. 5E). Interestingly and in agreement with the theory, the longest Pmp oligomers (D-Wt-O1) yielded a significantly higher ThT fluorescence emission than the shorter D-Wt-O2. In contrast monomeric D-Wt at time point 0 only showed background ThT fluorescence, indicating the absence of β-sheet-rich oligomer structures. However, over time D-Wt-M also yielded significant ThT fluorescence, likely due to spontaneous protein oligomerization (as observed also in Fig. 4C). Indeed, the folding of Pmp21-D is remarkably stable, as dimers can be detected by SDS-PAGE (Fig. 2A). Such high stability is characteristic for a number of amyloid-like proteins (
      • Chapman M.R.
      • Robinson L.S.
      • Pinkner J.S.
      • Roth R.
      • Heuser J.
      • Hammar M.
      • Normark S.
      • Hultgren S.J.
      Role of Escherichia coli curli operons in directing amyloid fiber formation.
      ,
      • Epstein E.A.
      • Reizian M.A.
      • Chapman M.R.
      Spatial clustering of the curlin secretion lipoprotein requires curli fiber assembly.
      ,
      • Podlisny M.B.
      • Ostaszewski B.L.
      • Squazzo S.L.
      • Koo E.H.
      • Rydell R.E.
      • Teplow D.B.
      • Selkoe D.J.
      Aggregation of secreted amyloid β-protein into sodium dodecyl sulfate-stable oligomers in cell-culture.
      • Walsh D.M.
      • Klyubin I.
      • Fadeeva J.V.
      • Rowan M.J.
      • Selkoe D.J.
      Amyloid-β oligomers: their production, toxicity and therapeutic inhibition.
      ). However, dimer formation has also been observed for amyloid-like proteins upon sample preparation for SDS-PAGE (
      • Pujol-Pina R.
      • Vilaprinyo-Pascual S.
      • Mazzucato R.
      • Arcella A.
      • Vilaseca M.
      • Orozco M.
      • Carulla N.
      SDS-PAGE analysis of Aβ oligomers is disserving research into Alzheimer's disease: appealing for ESI-IM-MS.
      ). Moreover, in vitro ThT-induced amyloid aggregation has been described, and this might also contribute to the formation of oligomeric D-Wt (
      • D'Amico M.
      • Di Carlo M.G.
      • Groenning M.
      • Militello V.
      • Vetri V.
      • Leone M.
      Thioflavin T promotes Aβ(1–40) amyloid fibrils formation.
      ).
      Previous work has demonstrated that the FXXN motifs in D-Wt (Pmp21-D) are essential for adhesion and for the ability of soluble Pmp21 fragments to block infection (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ). In the present study we have now shown that the two FXXN motifs in D-Wt are also crucial for its ability to form protofibrils, as the capacity for oligomerization is strongly reduced when these motifs are mutated (D-Mt) (Fig. 6B). It is well known that specific protein sequences are involved in the induction of amyloid-like structures (
      • Maurer-Stroh S.
      • Debulpaep M.
      • Kuemmerer N.
      • Lopez de la Paz M.
      • Martins I.C.
      • Reumers J.
      • Morris K.L.
      • Copland A.
      • Serpell L.
      • Serrano L.
      • Schymkowitz J.W.
      • Rousseau F.
      Exploring the sequence determinants of amyloid structure using position-specific scoring matrices.
      ). Thus the FXXN motif is very probably an essential part of the amyloid-promoting sequence within Pmp21-D. However, it is worth mentioning that the D-Mt-O1 oligomers formed again exhibited a rod-shaped structure (Fig. 6C), similar to those seen for the corresponding Wt oligomers. Thus the structures of Wt and Mt oligomers are identical, yet the tendency to be formed is highly increased for D-Wt.
      Interestingly, our CD analysis showed that D-Wt-O1 harbors some β-sheet structure, whereas D-Mt-O1 shows a certain shift toward random coils, suggesting that the FXXN motifs play a role in protein folding. The β-sheets may instead adopt β-helical structures. Amyloids and β-helices in general are suggested to share similar motifs (
      • Tsai H.H.
      • Gunasekaran K.
      • Nussinov R.
      Sequence and structure analysis of parallel β helices: implication for constructing amyloid structural models.
      ). Indeed, β-helical amyloids are already known for other organisms in various contexts (
      • Govaerts C.
      • Wille H.
      • Prusiner S.B.
      • Cohen F.E.
      Evidence for assembly of prions with left-handed β 3-helices into trimers.
      • Smirnovas V.
      • Baron G.S.
      • Offerdahl D.K.
      • Raymond G.J.
      • Caughey B.
      • Surewicz W.K.
      Structural organization of brain-derived mammalian prions examined by hydrogen-deuterium exchange.
      ,
      • Tycko R.
      • Wickner R.B.
      Molecular structures of amyloid and prion fibrils: consensus versus controversy.
      • Van Melckebeke H.
      • Wasmer C.
      • Lange A.
      • Ab E.
      • Loquet A.
      • Böckmann A.
      • Meier B.H.
      Atomic-Resolution Three-dimensional structure of HET-s(218–289) amyloid fibrils by solid-state NMR spectroscopy.
      ).
      The results presented here document a functional role for Pmp21-D oligomers during the C. pneumoniae infection. The oligomer Pmp21-D (D-Wt-O1) shows significant adhesion to human epithelial cells, in contrast to monomer and mutant species (Fig. 7). Moreover, D-Wt-O1 interacts significantly more strongly with the Pmp21 receptor EGFR than the monomeric D-Wt (Fig. 8). Finally, recombinant D-Wt-O1 blocks chlamydial infection more efficiently than any other species tested (Fig. 9). These data strongly suggest a functional role of oligomeric Pmp species in the chlamydial infection. Unexpectedly, the pulldown with Ctad1 brought down small amounts of EGFR. We recently reported that Ctad1 binds to integrin β1 (
      • Stallmann S.
      • Hegemann J.H.
      The Chlamydia trachomatis Ctad1 invasin exploits the human integrin β 1 receptor for host cell entry.
      ). As it is known that the integrin and EGFR receptors exhibit intensive cross-talk (
      • Huveneers S.
      • Danen E.H.
      Adhesion signaling: crosstalk between integrins, Src and Rho.
      ), it may be speculated that Ctad1-mediated integrin activation results in the association of both receptors in downstream signaling complexes.
      In conclusion, Pmp21 may belong to the group of functional, oligomeric structures found on microbial cell surfaces, the prototypes of which are CsgA and CsgB, which are secreted by their own apparatus to form the filamentous cell surface structures called curli produced by many Enterobacteriaceae (
      • Barnhart M.M.
      • Chapman M.R.
      Curli biogenesis and function.
      ). Curli shares all of the biophysical properties of amyloids, including the propensity to form ordered β-sheet-rich fibers with a capacity to bind the dye ThT (
      • Chapman M.R.
      • Robinson L.S.
      • Pinkner J.S.
      • Roth R.
      • Heuser J.
      • Hammar M.
      • Normark S.
      • Hultgren S.J.
      Role of Escherichia coli curli operons in directing amyloid fiber formation.
      ), and produces extracellular proteinaceous fibers that contribute to biofilm formation, host colonization, immune activation, and cell invasion (
      • Fernández L.A.
      • Berenguer J.
      Secretion and assembly of regular surface structures in Gram-negative bacteria.
      ,
      • Jonson A.B.
      • Normark S.
      • Rhen M.
      Fimbriae, pili, flagella, and bacterial virulence.
      • Epstein E.A.
      • Chapman M.R.
      Polymerizing the fibre between bacteria and host cells: the biogenesis of functional amyloid fibres.
      ).
      So far, Pmp21 oligomers have not been described for extracellular infectious EBs or dividing reticulate bodies within the inclusion. Either these structures do not survive the harsh fixation protocols used for sample preparation for immunofluorescence and electron microscopy, or the size of the oligomeric structures is well controlled by the Chlamydia, and thus their small in vivo size does not enable their detection. Alternatively, Pmp protofibrils might not form in vivo, because the proteins are physically constrained by their anchorage through their β-barrel in the outer membrane, as has been shown for the Candida albicans adhesins, which are also anchored in the cell wall. Nevertheless, these fungal adhesins form cell surface amyloid patches of arrayed adhesin molecules (“adhesin nanodomains”) 100-1000 nm in size, thus binding ligands with high avidity (
      • Lipke P.N.
      • Garcia M.C.
      • Alsteens D.
      • Ramsook C.B.
      • Klotz S.A.
      • Dufrêne Y.F.
      Strengthening relationships: amyloids create adhesion nanodomains in yeasts.
      ,
      • Otoo H.N.
      • Lee K.G.
      • Qiu W.
      • Lipke P.N.
      Candida albicans Als adhesins have conserved amyloid-forming sequences.
      ). It is tempting to speculate that the flower-like structures observed by EM in affinity-enriched preparations of endogenous PmpD-containing protein complexes might possibly represent the in vivo version of the Pmp21 protofibrils detected in vitro in this study (
      • Swanson K.A.
      • Taylor L.D.
      • Frank S.D.
      • Sturdevant G.L.
      • Fischer E.R.
      • Carlson J.H.
      • Whitmire W.M.
      • Caldwell H.D.
      Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
      ). The data presented here are compatible with the idea that oligomeric Pmp complexes might enhance the chlamydial cell's capacity for adhesion to human epithelial cells and be important for the initial step in infection.
      Finally, there is a striking correlation between chlamydial infection and amyloid formation in the brains of mice (
      • Boelen E.
      • Stassen F.R.
      • van der Ven A.J.
      • Lemmens M.A.
      • Steinbusch H.P.
      • Bruggeman C.A.
      • Schmitz C.
      • Steinbusch H.W.
      Detection of amyloid beta aggregates in the brain of BALB/c mice after Chlamydia pneumoniae infection.
      ,
      • Little C.S.
      • Joyce T.A.
      • Hammond C.J.
      • Matta H.
      • Cahn D.
      • Appelt D.M.
      • Balin B.J.
      Detection of bacterial antigens and Alzheimer's disease-like pathology in the central nervous system of BALB/c mice following intranasal infection with a laboratory isolate of Chlamydia pneumoniae.
      • Little C.S.
      • Hammond C.J.
      • MacIntyre A.
      • Balin B.J.
      • Appelt D.M.
      Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice.
      ). However, there is currently no evidence for a direct or indirect connection between amyloid formation in the mouse brain and Pmp proteins.
      Future work needs to focus on whether or not the different Pmp family members (21 in C. pneumoniae and 9 in C. trachomatis; Refs.
      • Kalman S.
      • Mitchell W.
      • Marathe R.
      • Lammel C.
      • Fan J.
      • Hyman R.W.
      • Olinger L.
      • Grimwood J.
      • Davis R.W.
      • Stephens R.S.
      Comparative genomes of Chlamydia pneumoniae and C-trachomatis.
      and
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ) can interact with each other to form heteromeric protofibrils. If so, this would provide for greater antigenic complexity and enable Chlamydiae to adapt to a larger range of cellular niches (
      • Grimwood J.
      • Stephens R.S.
      Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
      ,
      • Hegemann J.H.
      • Moelleken K.
      Chlamydial adhesin and adhesins.
      ).

      Experimental Procedures

      Bacterial Strains and Culture Conditions

      E. coli strain BL21 (DE3) (Agilent Technologies) was used for protein expression and plasmid amplification. C. pneumoniae strain GiD was propagated in HEp-2 cells (ATCC CCL-23) as described (
      • Jantos C.A.
      • Heck S.
      • Roggendorf R.
      • Sen-Gupta M.
      • Hegemann J.H.
      Antigenic and molecular analyses of different Chlamydia pneumoniae strains.
      ). Chlamydial EBs were purified by using a 30% gastrographin gradient (Schering).

      DNA Manipulations and Plasmid Construction

      Plasmids were generated in Saccharomyces cerevisiae as described in Mölleken et al. (
      • Mölleken K.
      • Schmidt E.
      • Hegemann J.H.
      Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
      ), and their structures were verified by sequence analysis (GATC).

      Protein Expression and Affinity Purification of His6-tagged Proteins

      Growth for protein expression in BL21 (DE3) was performed in LB media + 0.8% glucose at 37 °C. Cells were induced with 1 mm isopropyl-β-d-thiogalactopyranoside (IPTG) at an A600 of 0.6–1 and grown for a further 4 h. The cells were then harvested by centrifugation in a JLA10.500 rotor (Beckman) at 5000 rpm for 10 min at 4 °C.
      The cell pellets were lysed with 10 ml of lysis buffer (20 mm Tris-HCl, pH 8, 6 m guanidine HCl, 0.5 m NaCl, 1 mm β-mercaptoethanol, 5 mm imidazole) per gram of cell pellet overnight. Cell debris was removed by centrifugation in a Type 45Ti rotor (Beckmann) at 42,000 rpm for 1 h at 4 °C.
      Purification under denaturing conditions and refolding of the His6-tagged D-Wt and D-Mt was performed with HiTrap Chelating Nickel-IDA columns (5 ml column volume) at 4 °C using an ÄKTA Prime plus (GE Healthcare). The system was equilibrated with lysis buffer. Subsequently the cleared whole cell lysate was applied to the column. After washing with 50 ml of lysis buffer, the column was washed with 10 ml of washing buffer (20 mm Tris-HCl, pH 8, 8 m urea, 0.5 m NaCl, 1 mm β-mercaptoethanol, 5 mm imidazole). Thereafter washing buffer was gradually replaced with 30 ml with refolding buffer (20 mm Tris-HCl, pH 8, 0.5 m NaCl, 1 mm β-mercaptoethanol, 5 mm imidazole). Proteins were eluted at 4 °C by applying a 40-ml imidazole gradient (0.005–0.5 m in the same buffer), and the eluate was collected in 1-ml fractions. The whole process was monitored by absorbance at 280 nm. All subsequent steps were also performed at 4 °C. The protein-containing fractions were centrifuged at 10,000 × g for 10 min. The two peak fractions were pooled, and protein concentrations were determined with the Bradford assay. Within 2 h after elution from the HiTrap column, the samples were analyzed by preparative SEC. Different concentrations of refolded protein were generated by starting with different amounts of whole cell lysate.

      Size-exclusion Chromatography

      SEC was performed on an ÄKTA Prime plus (GE Healthcare) equipped with a HiLoad 16/600 Superdex 200 pg column (GE Healthcare) at 4 °C. SEC was performed at a flow rate of 0.5 ml/min in phosphate-buffered saline (PBS) (10 mm Na2HPO4, 1.8 mm KH2PO4, 137 mm NaCl, 2.7 mm KCl, pH 7.4), and 1.2-ml fractions were collected. The void volume of the column was determined by using blue dextran (BD Biosciences), and the separation range of the column was verified by standard proteins for gel filtration (Sigma). Within 6 h after elution from the SEC column, the sample were used for further analysis. In the meantime the samples were kept on 4 °C.
      Analytical size-exclusion chromatography was performed on an ÄKTA Prime plus (GE Healthcare) equipped with a Superdex 200 HR 10/30 column (GE Healthcare) at 4 °C. Analytical SEC was performed at a flow rate of 0.3 ml/min in PBS, and 0.3-ml fractions were collected. In both cases the elution profiles were analyzed with the PrimeView software package (GE Healthcare).

      Combined Analytical SEC and MALS

      Analytical SEC-MALS was performed on a HPLC system from Agilent Technologies using a Superdex 200 10/300 GL column (GE Healthcare), equilibrated, and run at a flow rate of 0.3 ml/min in PBS at 4 °C. For MALS analysis, the eluate was monitored with a miniDAWN TREOS triple-angle light-scattering detector in combination with an OPTILab T-rEX differential refractive index detector (both from Wyatt Technology). Typically, 100 μl of purified D-Wt-M (0.5 mg/ml) was loaded onto the Superdex 200 10/300 GL column, and the elution data were analyzed with the ASTRA software package (Wyatt Technology).

      AUC

      AUC was performed in an Optima XL-A analytical ultracentrifuge (Beckman Coulter) with absorbance optics using an An-50 Ti for D-Wt and an An-60 Ti rotor for D-Wt-M. Sedimentation velocity centrifugation was done at 20,000 rpm and 20 °C for D-Wt and at 40,000 rpm and 20 °C for D-Wt-M. The signal intensity for D-Wt was recorded at 280 nm and for D-Wt-M at 230 nm over the 7-h duration of the analysis. Data were fitted to the continuous distribution (c(s)) Lamm equation model with a partial specific volume of 0.7286 cm3/g (based on the D-Wt sequence) using the software package Sedfit. The density and viscosity of the buffer were 1.0053 and 0.01019, respectively, according to SEDNTERP (Sedimentation Interpretation Program, Version 20120828, University of New Hampshire) (
      • Laue T.M.
      • Shah B.D.
      • Ridgeway T.M.
      • Pelletier S.L.
      ). A resolution of 200 was chosen for the S value. The relative amounts of the different species in the D-Wt-M sample were derived from the c(s) distribution exclusive of the area below 0.6 S, which contains a baseline deconvolution artifact. The S values determined were corrected for PBS at 20 °C.

      Transmission Electron Microscopy (TEM)

      D-Wt samples were diluted to 1 μm, and 10-μl aliquots were incubated for 5 min at room temperature on freshly glow-discharged S162 Formvar/carbon-coated copper grids (Plano). The grids were then washed 3 times with 10 μl of H2O and subsequently incubated for 1 min with 10 μl of 1% aqueous uranyl acetate for negative staining, then air-dried for 5 min at room temperature. The samples were examined with an E902 electron microscope (Zeiss) operating at 80 kV.

      ThT Fluorescence Assay

      For fluorescence measurements, ThT (Sigma) was added at a final concentration of 10 μm to protein samples containing 10 μm D-Wt or BSA in PBS or 48 h of preincubated Aβ42 (Bachem) in a final volume of 100 μl of 10 mm sodium phosphate, 50 mm NaCl, pH 7.4. ThT was allowed to bind at 37 °C in a (stationary) round-bottomed 96-well black plate (Nunc) for 48 h. Fluorescence was excited at 442 and measured at an emission wavelength of 484 nm in an Infinite 200pro plate reader (Tecan). The slit width was 10 nm.

      CD Spectroscopy

      Far-UV CD spectra were measured on a JASCO J-815 spectropolarimeter equipped with a 1-mm Suprasil quartz cuvette (Hellma) at 20 °C. Protein species (D-Wt-O1, D-Wt-M, D-Mt-M) were analyzed at a concentration of 10 μm in 20 mm NaPi, 75 mm NaF, pH 7.4, or D-Mt-O1 in PBS. The D-Wt-O1 spectrum was analyzed on the Dichroweb server using the program CONTINLL (
      • Lobley A.
      • Whitmore L.
      • Wallace B.A.
      DICHROWEB: an interactive website for the analysis of protein secondary structure from circular dichroism spectra.
      • Provencher S.W.
      • Glöckner J.
      Estimation of globular protein secondary structure from circular dichroism.
      ,
      • van Stokkum I.H.
      • Spoelder H.J.
      • Bloemendal M.
      • van Grondelle R.
      • Groen F.C.
      Estimation of protein secondary structure and error analysis from circular dichroism spectra.
      ,
      • Whitmore L.
      • Wallace B.A.
      DICHROWEB, an online server for protein secondary structure analyses from circular dichroism spectroscopic data.
      • Whitmore L.
      • Wallace B.A.
      Protein secondary structure analyses from circular dichroism spectroscopy: methods and reference databases.
      ).

      Adhesion Assays with Protein-coated Latex Beads

      Adhesion assays with protein-coated latex beads were performed as described (
      • Becker E.
      • Hegemann J.H.
      All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function.
      ). Coating efficiency with 100 μg/ml His6-tagged proteins was estimated by immunoblotting before use. Confluent monolayers of HEp-2, HeLa, or HUVE cells were grown in 24-well plates and incubated with a 10-fold excess of protein-coated latex beads (Polyscience) (diameter 1 μm, green fluorescent) for 1 h at 37 °C. Cells were washed twice with PBS, detached with cell dissociation solution, fixed with 3% formaldehyde for 20 min at room temperature, and analyzed by flow cytometry using a FACSAria instrument (BD Biosciences).

      EGFR Pulldown Assay

      The EGFR pulldown assay was performed with 200 μg/ml D-Wt, M-Pmp21, GST, Ctad1, or the binding domain of OmcB (OmcB-BD) in a volume of 2 ml. All proteins were biotinylated according to the manufacturer's instructions (Thermo Fisher Scientific). Afterward the biotinylated proteins were incubated with HEp2 cells to allow adhesion. After 1 h of incubation at 37 °C the unbound protein was washed away, and the bound proteins were cross-linked with DTSSP (Thermo Fisher Scientific). The HEp2 cells were lysed, and the soluble lysate was incubated for 12 h at 4 °C with streptavidin resin to allow binding of biotinylated proteins. The streptavidin resin was washed with PBS (3 × 0.5 ml), and afterward the recombinant proteins and their interaction partners were eluted with 100 mm DTT. The elution fractions were analyzed by Western blot with anti-EGFR antibodies (Thermo Fisher Scientific) and quantified by ImageJ.

      Infection Inhibition Assay

      These assays were performed as previously described (
      • Moelleken K.
      • Hegemann J.H.
      The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding.
      ). Briefly, HEp-2 cells were grown on glass coverslips (12-mm diameter) for 48 h and incubated (as confluent monolayers) with 250 μl of medium containing recombinant protein (100 μg/ml in PBS) for 2 h at 37 °C. Purified chlamydial EBs (multiplicity of infection 20) were added to the protein suspension and incubated for 2 h at 37 °C without centrifugation to avoid any influence of the centrifugation procedure on the adhesion/infection process (
      • Moelleken K.
      • Hegemann J.H.
      The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding.
      ). The cells were subsequently washed 3 times, covered with chlamydial growth medium, and incubated for 48 h at 37 °C for C. pneumoniae infection. Subsequently the cells were fixed with 96% methanol for 2 min. Each monolayer was then washed 3 times with PBS. For detection of chlamydial inclusions, a monoclonal fluorescein isothiocyanate (FITC)-conjugated antibody directed against chlamydial LPS (Bio-Rad) was used. Cells were viewed using a C2 confocal microscope (Nikon). Inclusions were counted, and the results were expressed as percentages of the number found in PBS-treated control samples.

      Immunoblot Analysis

      SDS-PAGE and immunoblot analysis were performed as described (
      • Laemmli U.K.
      Cleavage of structural proteins during the assembly of the head of bacteriophage T4.
      ). The PageRulerTM (Thermo Fisher Scientific) set of prestained markers was employed as a molecular weight standard. The His6-tagged recombinant proteins purified from E. coli were detected with a anti-His6 antibody (Qiagen) and visualized with alkaline phosphatase (AP)-conjugated anti-mouse antibody (Promega).

      Author Contributions

      J. H. H. conceived and coordinated the study. J. H. H. and S. E. T. L. designed, performed, and analyzed the experiments. S. H. J. S. and L. S. gave general biochemical advice and support. C. D. and L. N.-S. gave advice and support concerning AUC and ThT assay. J. H. H. and S. E. T. L. wrote the paper. All authors reviewed the results and approved the final version of the manuscript.

      Acknowledgments

      We thank Katja Moelleken, Jan Galle, and Sebastian Haensch for help with human cell culture and chlamydial infection assays. We also thank Klaus Meyer for help with FACS and Marion Nissen for help with the EM. We thank Elisabeth Becker for the initial structural prediction analyses, which suggested the possibility of Pmp oligomerization.

      References

        • Schachter J.
        Infection and Disease Epidemiology.
        in: Stephens R.S. Chlamydia: Intracellular Biology, Pathogenesis and Immunity. ASM Press, Washington, D.C1999: 139-169
        • Wright H.R.
        • Turner A.
        • Taylor H.R.
        Trachoma.
        Lancet. 2008; 371: 1945-1954
        • Blasi F.
        • Tarsia P.
        • Aliberti S.
        Chlamydophila pneumoniae.
        Clin. Microbiol. Infect. 2009; 15: 29-35
        • Harris S.A.
        • Harris E.A.
        Herpes simplex virus type 1 and other pathogens are key causative factors in sporadic Alzheimer's Disease.
        J. Alzheimers Dis. 2015; 48: 319-353
        • Kuo C.C.
        • Jackson L.A.
        • Campbell L.A.
        • Grayston J.T.
        Chlamydia pneumoniae (Twar).
        Clin. Microbiol. Rev. 1995; 8: 451-461
        • Brunham R.C.
        • Rey-Ladino J.
        Immunology of Chlamydia infection: implications for a Chlamydia trachomatis vaccine.
        Nat. Rev. Immunol. 2005; 5: 149-161
        • Puolakkainen M.
        Innate immunity and vaccines in chlamydial infection with special emphasis on Chlamydia pneumoniae.
        FEMS Immunol. Med. Microbiol. 2009; 55: 167-177
        • Moulder J.W.
        Interaction of Chlamydiae and host-cells in vitro.
        Microbiol. Rev. 1991; 55: 143-190
        • Hatch T.P.
        Development Biology.
        in: Stephens R.S. Chlamydia: Intracellular Biology, Pathogenesis and Immunology. ASM Press, Washington, D.C.1999: 29-67
        • Crane D.D.
        • Carlson J.H.
        • Fischer E.R.
        • Bavoil P.
        • Hsia R.C.
        • Tan C.
        • Kuo C.C.
        • Caldwell H.D.
        Chlamydia trachomatis polymorphic membrane protein D is a species-common pan-neutralizing antigen.
        Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 1894-1899
        • Kalman S.
        • Mitchell W.
        • Marathe R.
        • Lammel C.
        • Fan J.
        • Hyman R.W.
        • Olinger L.
        • Grimwood J.
        • Davis R.W.
        • Stephens R.S.
        Comparative genomes of Chlamydia pneumoniae and C-trachomatis.
        Nat. Genet. 1999; 21: 385-389
        • Longbottom D.
        • Russell M.
        • Dunbar S.M.
        • Jones G.E.
        • Herring A.J.
        Molecular cloning and characterization of the genes coding for the highly immunogenic cluster of 90-kilodalton envelope proteins from the Chlamydia psittaci subtype that causes abortion in sheep.
        Infect. Immun. 1998; 66: 1317-1324
        • Wehrl W.
        • Brinkmann V.
        • Jungblut P.R.
        • Meyer T.F.
        • Szczepek A.J.
        From the inside out-processing of the chlamydial autotransporter PmpD and its role in bacterial adhesion and activation of human host cells.
        Mol. Microbiol. 2004; 51: 319-334
        • Grimwood J.
        • Stephens R.S.
        Computational analysis of the polymorphic membrane protein superfamily of Chlamydia trachomatis and Chlamydia pneumoniae.
        Microb. Comp. Genomics. 1999; 4: 187-201
        • Rockey D.D.
        • Lenart J.
        • Stephens R.S.
        Genome sequencing and our understanding of Chlamydiae.
        Infect. Immun. 2000; 68: 5473-5479
        • Henderson I.R.
        • Lam A.C.
        Polymorphic proteins of Chlamydia spp.: autotransporters beyond the proteobacteria.
        Trends Microbiol. 2001; 9: 573-578
        • Bradley P.
        • Cowen L.
        • Menke M.
        • King J.
        • Berger B.
        BETAWRAP: successful prediction of parallel β-helices from primary sequence reveals an association with many microbial pathogens.
        Proc. Natl. Acad. Sci. U.S.A. 2001; 98: 14819-14824
        • Vandahl B.B.
        • Pedersen A.S.
        • Gevaert K.
        • Holm A.
        • Vandekerckhove J.
        • Christiansen G.
        • Birkelund S.
        The expression, processing and localization of polymorphic membrane proteins in Chlamydia pneumoniae strain CWL029.
        BMC Microbiol. 2002; 2: 36
        • Junker M.
        • Schuster C.C.
        • McDonnell A.V.
        • Sorg K.A.
        • Finn M.C.
        • Berger B.
        • Clark P.L.
        Pertactin β-helix folding mechanism suggests common themes for the secretion and folding of autotransporter proteins.
        Proc. Natl. Acad. Sci. U.S.A. 2006; 103: 4918-4923
        • Hegemann J.H.
        • Moelleken K.
        Chlamydial adhesin and adhesins.
        in: Tan M. Bavoil P. Intracellular Pathogens I: Chlamydiales. American Society for Microbiology, Washington, D. C2012: 97-125
        • Kiselev A.O.
        • Skinner M.C.
        • Lampe M.F.
        Analysis of pmpD expression and PmpD post-translational processing during the life cycle of Chlamydia trachomatis serovars A, D, and L2.
        PloS ONE. 2009; 4: e5191
        • Mölleken K.
        • Schmidt E.
        • Hegemann J.H.
        Members of the Pmp protein family of Chlamydia pneumoniae mediate adhesion to human cells via short repetitive peptide motifs.
        Mol. Microbiol. 2010; 78: 1004-1017
        • Saka H.A.
        • Thompson J.W.
        • Chen Y.S.
        • Kumar Y.
        • Dubois L.G.
        • Moseley M.A.
        • Valdivia R.H.
        Quantitative proteomics reveals metabolic and pathogenic properties of Chlamydia trachomatis developmental forms.
        Mol. Microbiol. 2011; 82: 1185-1203
        • Swanson K.A.
        • Taylor L.D.
        • Frank S.D.
        • Sturdevant G.L.
        • Fischer E.R.
        • Carlson J.H.
        • Whitmire W.M.
        • Caldwell H.D.
        Chlamydia trachomatis polymorphic membrane protein D is an oligomeric autotransporter with a higher-order structure.
        Infect. Immun. 2009; 77: 508-516
        • Montigiani S.
        • Falugi F.
        • Scarselli M.
        • Finco O.
        • Petracca R.
        • Galli G.
        • Mariani M.
        • Manetti R.
        • Agnusdei M.
        • Cevenini R.
        • Donati M.
        • Nogarotto R.
        • Norais N.
        • Garaguso I.
        • Nuti S.
        • Saletti G.
        • Rosa D.
        • Ratti G.
        • Grandi G.
        Genomic approach for analysis of surface proteins in Chlamydia pneumoniae.
        Infect. Immun. 2002; 70: 368-379
        • Tan C.
        • Hsia R.C.
        • Shou H.
        • Carrasco J.A.
        • Rank R.G.
        • Bavoil P.M.
        Variable expression of surface-exposed polymorphic membrane proteins in in vitro grown Chlamydia trachomatis.
        Cell. Microbiol. 2010; 12: 174-187
        • Becker E.
        • Hegemann J.H.
        All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function.
        Microbiologyopen. 2014; 3: 544-556
        • Mölleken K.
        • Becker E.
        • Hegemann J.H.
        The Chlamydia pneumoniae invasin protein Pmp21 recruits the EGF receptor for host cell entry.
        PLoS Pathog. 2013; 9: e1003325
        • Grimwood J.
        • Olinger L.
        • Stephens R.S.
        Expression of Chlamydia pneumoniae polymorphic membrane protein family genes.
        Infect. Immun. 2001; 69: 2383-2389
        • Bokranz W.
        • Wang X.
        • Tschäpe H.
        • Römling U.
        Expression of cellulose and curli fimbriae by Escherichia coli isolated from the gastrointestinal tract.
        J. Med. Microbiol. 2005; 54: 1171-1182
        • Chapman M.R.
        • Robinson L.S.
        • Pinkner J.S.
        • Roth R.
        • Heuser J.
        • Hammar M.
        • Normark S.
        • Hultgren S.J.
        Role of Escherichia coli curli operons in directing amyloid fiber formation.
        Science. 2002; 295: 851-855
        • Olsén A.
        • Wick M.J.
        • Mörgelin M.
        • Björck L.
        Curli, fibrous surface proteins of Escherichia coli, interact with major histocompatibility complex class I molecules.
        Infect. Immun. 1998; 66: 944-949
        • Gophna U.
        • Barlev M.
        • Seijffers R.
        • Oelschlager T.A.
        • Hacker J.
        • Ron E.Z.
        Curli fibers mediate internalization of Escherichia coli by eukaryotic cells.
        Infect. Immun. 2001; 69: 2659-2665
        • Pawar D.M.
        • Rossman M.L.
        • Chen J.
        Role of curli fimbriae in mediating the cells of enterohaemorrhagic Escherichia coli to attach to abiotic surfaces.
        J. Appl. Microbiol. 2005; 99: 418-425
        • Ryu J.H.
        • Kim H.
        • Beuchat L.R.
        Attachment and biofilm formation by Escherichia coli O157:H7 on stainless steel as influenced by exopolysaccharide production, nutrient availability, and temperature.
        J. Food Prot. 2004; 67: 2123-2131
        • Uhlich G.A.
        • Cooke P.H.
        • Solomon E.B.
        Analyses of the red-dry-rough phenotype of an Escherichia coli O157:H7 strain and its role in biofilm formation and resistance to antibacterial agents.
        Appl. Environ. Microbiol. 2006; 72: 2564-2572
        • Zogaj X.
        • Bokranz W.
        • Nimtz M.
        • Römling U.
        Production of cellulose and curli fimbriae by members of the family Enterobacteriaceae isolated from the human gastrointestinal tract.
        Infect. Immun. 2003; 71: 4151-4158
        • Barnhart M.M.
        • Chapman M.R.
        Curli biogenesis and function.
        Annu. Rev. Microbiol. 2006; 60: 131-147
        • Wang X.
        • Smith D.R.
        • Jones J.W.
        • Chapman M.R.
        In vitro polymerization of a functional Escherichia coli amyloid protein.
        J. Biol. Chem. 2007; 282: 3713-3719
        • Walsh D.M.
        • Lomakin A.
        • Benedek G.B.
        • Condron M.M.
        • Teplow D.B.
        Amyloid β-protein fibrillogenesis: detection of a protofibrillar intermediate.
        J. Biol. Chem. 1997; 272: 22364-22372
        • Bitan G.
        • Kirkitadze M.D.
        • Lomakin A.
        • Vollers S.S.
        • Benedek G.B.
        • Teplow D.B.
        Amyloid β-protein (Aβ) assembly: Aβ40 and Aβ42 oligomerize through distinct pathways.
        Proc. Natl. Acad. Sci. U.S.A. 2003; 100: 330-335
        • Nybo M.
        • Svehag S.E.
        • Holm Nielsen E.
        An ultrastructural study of amyloid intermediates in Aβ(1–42) fibrillogenesis.
        Scand. J. Immunol. 1999; 49: 219-223
        • Serpell L.C.
        Alzheimer's amyloid fibrils: structure and assembly.
        Biochim. Biophys. Acta. 2000; 1502: 16-30
        • Kuznetsova I.M.
        • Sulatskaya A.I.
        • Uversky V.N.
        • Turoverov K.K.
        A new trend in the experimental methodology of amyloid fibril structural investigation with the use of thioflavin T.
        FEBS J. 2012; 279: 416
        • Sulatskaya A.I.
        • Kuznetsova I.M.
        • Turoverov K.K.
        interaction of thioflavin T with amyloid fibrils: fluorescence quantum yield of bound dye.
        J. Phys. Chem. B. 2012; 116: 2538-2544
        • Moelleken K.
        • Hegemann J.H.
        The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding.
        Mol. Microbiol. 2008; 67: 403-419
        • Stallmann S.
        • Hegemann J.H.
        The Chlamydia trachomatis Ctad1 invasin exploits the human integrin β 1 receptor for host cell entry.
        Cell. Microbiol. 2016; 18: 761-775
        • Kline K.A.
        • Fälker S.
        • Dahlberg S.
        • Normark S.
        • Henriques-Normark B.
        Bacterial adhesins in host-microbe interactions.
        Cell Host Microbe. 2009; 5: 580-592
        • Campbell L.A.
        • Kuo C.-C.
        Interactions of Chlamydia with the host cells that mediate attachment and uptake.
        in: Bavoil P.M. Wyrick P.B. Chlamydia Genomics and Pathogenesis. Norfolk, Horizon Bioscience. Horizon Bioscience, Wymondham, U.K.2006: 505-522
        • Tanzer R.J.
        • Longbottom D.
        • Hatch T.P.
        Identification of polymorphic outer membrane proteins of Chlamydia psittaci 6BC.
        Infect. Immun. 2001; 69: 2428-2434
        • Tanford C.
        Physical Chemistry of Macromolecules. John Wiley and Sons, Inc., New York1961: 381-394
        • Epstein E.A.
        • Reizian M.A.
        • Chapman M.R.
        Spatial clustering of the curlin secretion lipoprotein requires curli fiber assembly.
        J. Bacteriol. 2009; 191: 608-615
        • Podlisny M.B.
        • Ostaszewski B.L.
        • Squazzo S.L.
        • Koo E.H.
        • Rydell R.E.
        • Teplow D.B.
        • Selkoe D.J.
        Aggregation of secreted amyloid β-protein into sodium dodecyl sulfate-stable oligomers in cell-culture.
        J. Biol. Chem. 1995; 270: 9564-9570
        • Walsh D.M.
        • Klyubin I.
        • Fadeeva J.V.
        • Rowan M.J.
        • Selkoe D.J.
        Amyloid-β oligomers: their production, toxicity and therapeutic inhibition.
        Biochem. Soc. Trans. 2002; 30: 552-557
        • Pujol-Pina R.
        • Vilaprinyo-Pascual S.
        • Mazzucato R.
        • Arcella A.
        • Vilaseca M.
        • Orozco M.
        • Carulla N.
        SDS-PAGE analysis of Aβ oligomers is disserving research into Alzheimer's disease: appealing for ESI-IM-MS.
        Scientific Reports. 2015; 5: 14809
        • D'Amico M.
        • Di Carlo M.G.
        • Groenning M.
        • Militello V.
        • Vetri V.
        • Leone M.
        Thioflavin T promotes Aβ(1–40) amyloid fibrils formation.
        J. Phys. Chem. Lett. 2012; 3: 1596-1601
        • Maurer-Stroh S.
        • Debulpaep M.
        • Kuemmerer N.
        • Lopez de la Paz M.
        • Martins I.C.
        • Reumers J.
        • Morris K.L.
        • Copland A.
        • Serpell L.
        • Serrano L.
        • Schymkowitz J.W.
        • Rousseau F.
        Exploring the sequence determinants of amyloid structure using position-specific scoring matrices.
        Nat. Methods. 2010; 7: 237-242
        • Tsai H.H.
        • Gunasekaran K.
        • Nussinov R.
        Sequence and structure analysis of parallel β helices: implication for constructing amyloid structural models.
        Structure. 2006; 14: 1059-1072
        • Govaerts C.
        • Wille H.
        • Prusiner S.B.
        • Cohen F.E.
        Evidence for assembly of prions with left-handed β 3-helices into trimers.
        Proc. Natl. Acad. Sci. U.S.A. 2004; 101: 8342-8347
        • Smirnovas V.
        • Baron G.S.
        • Offerdahl D.K.
        • Raymond G.J.
        • Caughey B.
        • Surewicz W.K.
        Structural organization of brain-derived mammalian prions examined by hydrogen-deuterium exchange.
        Nat. Struct. Mol. Biol. 2011; 18: 504-506
        • Tycko R.
        • Wickner R.B.
        Molecular structures of amyloid and prion fibrils: consensus versus controversy.
        Acc. Chem. Res. 2013; 46: 1487-1496
        • Van Melckebeke H.
        • Wasmer C.
        • Lange A.
        • Ab E.
        • Loquet A.
        • Böckmann A.
        • Meier B.H.
        Atomic-Resolution Three-dimensional structure of HET-s(218–289) amyloid fibrils by solid-state NMR spectroscopy.
        J. Am. Chem. Soc. 2010; 132: 13765-13775
        • Huveneers S.
        • Danen E.H.
        Adhesion signaling: crosstalk between integrins, Src and Rho.
        J. Cell Sci. 2009; 122: 1059-1069
        • Fernández L.A.
        • Berenguer J.
        Secretion and assembly of regular surface structures in Gram-negative bacteria.
        FEMS Microbiol. Rev. 2000; 24: 21-44
        • Jonson A.B.
        • Normark S.
        • Rhen M.
        Fimbriae, pili, flagella, and bacterial virulence.
        Contrib. Microbiol. 2005; 12: 67-89
        • Epstein E.A.
        • Chapman M.R.
        Polymerizing the fibre between bacteria and host cells: the biogenesis of functional amyloid fibres.
        Cell. Microbiol. 2008; 10: 1413-1420
        • Lipke P.N.
        • Garcia M.C.
        • Alsteens D.
        • Ramsook C.B.
        • Klotz S.A.
        • Dufrêne Y.F.
        Strengthening relationships: amyloids create adhesion nanodomains in yeasts.
        Trends Microbiol. 2012; 20: 59-65
        • Otoo H.N.
        • Lee K.G.
        • Qiu W.
        • Lipke P.N.
        Candida albicans Als adhesins have conserved amyloid-forming sequences.
        Eukaryot. Cell. 2008; 7: 776-782
        • Boelen E.
        • Stassen F.R.
        • van der Ven A.J.
        • Lemmens M.A.
        • Steinbusch H.P.
        • Bruggeman C.A.
        • Schmitz C.
        • Steinbusch H.W.
        Detection of amyloid beta aggregates in the brain of BALB/c mice after Chlamydia pneumoniae infection.
        Acta. Neuropathol. 2007; 114: 255-261
        • Little C.S.
        • Joyce T.A.
        • Hammond C.J.
        • Matta H.
        • Cahn D.
        • Appelt D.M.
        • Balin B.J.
        Detection of bacterial antigens and Alzheimer's disease-like pathology in the central nervous system of BALB/c mice following intranasal infection with a laboratory isolate of Chlamydia pneumoniae.
        Front Aging Neurosci. 2014; 6: 304
        • Little C.S.
        • Hammond C.J.
        • MacIntyre A.
        • Balin B.J.
        • Appelt D.M.
        Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice.
        Neurobiol. Aging. 2004; 25: 419-429
        • Jantos C.A.
        • Heck S.
        • Roggendorf R.
        • Sen-Gupta M.
        • Hegemann J.H.
        Antigenic and molecular analyses of different Chlamydia pneumoniae strains.
        J. Clin. Microbiol. 1997; 35: 620-623
        • Laue T.M.
        • Shah B.D.
        • Ridgeway T.M.
        • Pelletier S.L.
        Harding S.E. Rowe A.J. Horton J.C. Analytical Ultracentrifugation in Biochemistry and Polymer Science. Royal Society of Chemistry, Cambridge1992: 90-125
        • Lobley A.
        • Whitmore L.
        • Wallace B.A.
        DICHROWEB: an interactive website for the analysis of protein secondary structure from circular dichroism spectra.
        Bioinformatics. 2002; 18: 211-212
        • Provencher S.W.
        • Glöckner J.
        Estimation of globular protein secondary structure from circular dichroism.
        Biochemistry. 1981; 20: 33-37
        • van Stokkum I.H.
        • Spoelder H.J.
        • Bloemendal M.
        • van Grondelle R.
        • Groen F.C.
        Estimation of protein secondary structure and error analysis from circular dichroism spectra.
        Anal. Biochem. 1990; 191: 110-118
        • Whitmore L.
        • Wallace B.A.
        DICHROWEB, an online server for protein secondary structure analyses from circular dichroism spectroscopic data.
        Nucleic Acids Res. 2004; 32: W668-W673
        • Whitmore L.
        • Wallace B.A.
        Protein secondary structure analyses from circular dichroism spectroscopy: methods and reference databases.
        Biopolymers. 2008; 89: 392-400
        • Laemmli U.K.
        Cleavage of structural proteins during the assembly of the head of bacteriophage T4.
        Nature. 1970; 227: 680-685
        • Roy A.
        • Kucukural A.
        • Zhang Y.
        I-TASSER: a unified platform for automated protein structure and function prediction.
        Nat. Protoc. 2010; 5: 725-738
        • Yang J.
        • Yan R.
        • Roy A.
        • Xu D.
        • Poisson J.
        • Zhang Y.
        The I-TASSER Suite: protein structure and function prediction.
        Nat. Methods. 2015; 12: 7-8
        • Zhang Y.
        I-TASSER server for protein 3D structure prediction.
        Bmc Bioinformatics. 2008; 9: 40