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Originally published In Press as doi:10.1074/jbc.M406417200 on September 28, 2004

J. Biol. Chem., Vol. 279, Issue 50, 51760-51768, December 10, 2004
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Identification and Biochemical Characterization of Two Novel Collagen Binding MSCRAMMs of Bacillus anthracis*

Yi Xu{ddagger}§, Xiaowen Liang{ddagger}, Yahua Chen{ddagger}, Theresa M. Koehler¶, and Magnus Höök{ddagger}

From the {ddagger}The Center for Extracellular Matrix Biology, Texas A&M University System Health Science Center, Albert B. Alkek Institute of Biosciences and Technology, Houston, Texas 77030 and Department of Microbiology and Molecular Genetics, University of Texas Health Science Center, Houston, Texas 77030

Received for publication, June 9, 2004 , and in revised form, September 20, 2004.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Cell wall-anchored proteins play critical roles in the pathogenesis of infections caused by Gram-positive bacteria. Through the analysis of the genome of Bacillus anthracis Ames strain, we identified two novel putative cell wall-anchored proteins, BA0871 and BA5258, which have sequence homology to CNA, a cell wall-anchored collagen adhesin of Staphylococcus aureus. The two proteins have similar domain organization to that of CNA, with typical signal peptide sequences, a non-repetitive A region followed by repeats, and a characteristic cell wall-anchoring region. They are expressed on the surface of B. anthracis. The A regions of the two proteins were predicted to adopt similar structural folds as CNA. Circular dichroism analysis of the recombinant A regions of the two proteins (rBA0871A and rBA5258A) indicate that their secondary structure compositions are similar to those of the A regions of CNA and other cell wall-anchored adhesins. We demonstrate through solid phase binding assays and surface plasmon resonance analyses that rBA0871A and rBA5258A specifically bound type I collagen in a dose-dependent and saturable manner. Their dissociation constants (KD) for collagen are 1.6–3.2 µM for rBA0871A and 0.6–0.9 µM for rBA5258A, respectively. We further demonstrate that BA0871 and BA5258 can mediate cell attachment to collagen when expressed on the surface of a heterologous host bacterium. To our knowledge these are the first two adhesins of B. anthracis described, which may have important implications for our understanding of the pathogenic mechanisms explored by this organism.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
The molecular pathogenesis of bacterial infections consists of multistage processes involving many different factors. Bacillus anthracis, a spore-forming Gram-positive organism that causes anthrax, is not an exception. Anthrax is initiated by the entry of spores into the host through inhalation, ingestion, or via cuts in the skin. The spores are engulfed by macrophages, germinate, become vegetative bacilli that are capable of producing toxins, and disseminate in the host. They eventually reach the blood circulation where they multiply to a density of 107-108 colony-forming units/ml, causing massive septicemia and toxemia. To successfully establish an infection, B. anthracis must survive attacks from the host defense system and successfully colonize different tissues. The known principal virulence factors of B. anthracis are the two exotoxins, lethal toxin and edema toxin, and the poly-{gamma}-D-glutamic acid capsule. The toxins are thought to be largely responsible for the morbidity and mortality associated with anthrax, whereas the capsule has been thought to have antiphagocytic activity and be necessary for in vivo survival (1–3). However, the processes by which germinated bacilli colonize different tissues and cross various barriers in the host to reach the bloodstream while avoiding being killed in the process remain unknown. Furthermore, the three forms of anthrax (cutaneous, gastrointestinal, and inhalational) are likely to involve different sets of virulence factors. To our knowledge adhesins that potentially could participate in the pathogenic process have not previously been identified in B. anthracis.

Collagens are major components in the connective tissue and the most abundant proteins in mammals. There are more than 20 types of collagens, among which type I collagen is a major component of the skin. It is not surprising that many bacteria have evolved to produce collagen adhesins to interact with this group of proteins, e.g. CNA of Staphylococcus aureus (4, 5), YadA of Yersinia enterocolitica (6), FimH of meningitis-associated Escherichia coli O18acK1H7 (7), ACE of Enterococcus faecalis (8), Acm of Enterococcus faecium (9), CNE of Streptococcus equi (10), and RspA/RspB of Erysipelothrix rhusiopathia (11). As have been demonstrated for CNA (4, 5, 12–17) and YadA (18, 19) in various animal models, these interactions can be critical in the establishment and progression of bacterial infections. We recently demonstrated that mice infected with S. aureus strains expressing CNA initially had similar numbers of S. aureus in the joints as mice infected with an isogenic S. aureus strain that expressed a mutated inactive form of CNA; however, as the infection progressed, the former group of mice showed significantly more S. aureus in the joints than the latter group as early as 24 h post-inoculation (17). Thus, it seems that the adhesins allow the bacteria to "hold on" to tissue structures containing their corresponding ligand, and as a result, these adhering bacteria appear to resist clearance by the host defense system. In addition, recombinant fragments of CNA and the recently reported RspA protected mice against challenge by wild type S. aureus (20) and E. rhusiopathia (11), respectively, raising the possibility that these proteins can be used as vaccine targets and underlining their importance in bacterial pathogenesis. Sequence analyses have also identified CNA-like proteins in other bacteria such as Bacillus spp. and Clostridium spp.; however, no functional studies of these proteins have been reported.

Among the collagen adhesins of Gram-positive organisms, CNA of S. aureus is the best characterized. CNA is a cell wall-anchored protein (CWAP)1 that belongs to the MSCRAMM (microbial surface component recognizing adhesive matrix molecules) family of adhesins. It has a domain organization typical of MSCRAMMs from Gram-positive bacteria; a signal peptide sequence at the N terminus, a nonrepetitive A region followed by 1–4 B repeats depending on the strains, and a cell wall-anchoring region including an LPXTG-motif, a transmembrane segment, and a short cytoplasmic tail rich in positively charged residues. The LPXTG motif is recognized by sortase A, a transpeptidase that cleaves the bond between T and G and covalently links the T residue to the peptidoglycan in the cell wall. The A region is responsible for the collagen binding ability of CNA, whereas the B repeats are thought to help display the binding domain on the surface of staphylococci (4). Structural analysis as well as comparison with other MSCRAMMs suggested that the A region of CNA consists of three subdomains rich in {beta}-sheets and fold into immunoglobulin-like (Ig-like) domains. The middle subdomain in the CNA A region provides a trench-like hydrophobic surface in one of the {beta}-sheets that can accommodate a triple helical collagen structure as indicated by molecular modeling experiments (21). Mutations of some of the residues in the postulated collagen binding trench on CNA abolished or greatly reduced the collagen binding ability of the MSCRAMM. However, these residues are not necessarily conserved in the collagen binding A region of ACE (8) or the recently described RspA and RspB (11), suggesting differences in the detailed binding mechanisms of these molecules. The Ig-like folded subdomains have also been found in the binding A regions of other MSCRAMMs, such as the fibrinogen-binding protein ClfA of S. aureus (22) and SdrG of Staphylococcus epidermidis (23). Interactions between the subdomains are believed to be an integral part of the binding mechanisms of these molecules (23).

Through analysis of the genome of the Ames strain (www.tigr.org) we have identified two putative CWAPs of B. anthracis that have some sequence similarity to CNA and are expressed on the surface of B. anthracis. This raised the possibility that these two proteins belong to the family of CNA-like collagen binding MSCRAMMs. In this study we analyzed the sequences of the two putative collagen adhesins of B. anthracis and show that they contain features of typical MSCRAMMs. We confirm that the two proteins are expressed on the surface of B. anthracis. We further demonstrate that recombinant fragments of the two proteins are capable of binding collagen and mediate attachment to collagen when expressed on the surface of a heterologous host bacterium. To our knowledge this is the first report on adhesins of B. anthracis and their interactions with host matrix proteins. This information may have important implications with respect to our understanding of the infection mechanisms of B. anthracis.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Bacterial Strains, Plasmids, and Culture Conditions—E. coli strains were grown at 37 °C overnight in Lennox L broth (LB) (Sigma) or on LB agar supplemented with ampicillin (50 µg/ml) when appropriate. Staphylococcus carnosus strains were grown at 37 °C in tryptic soy broth (Difco) or on tryptic soy agar supplemented with chloramphenicol (10 µg/ml) when necessary. B. anthracis Sterne strain 7702 was grown in LB or brain heart infusion broth (Difco) at 37 °C with shaking at 200 rpm.

Identification of LPXTG Motif-containing Cell Wall-anchored Proteins—The genome of B. anthracis Ames strain was analyzed using a combination of a bioinformatics method as described previously (23, 24) and searching annotated genome sequence (www.tigr.org) with the terms the "LPXTG" and/or "cell wall anchor." Nine such proteins were identified.

Cloning, Expression, and Purification of Recombinant A Regions of BA0871 and BA5258 —Genomic DNA of B. anthracis Sterne strain 7702 was prepared using the G NOME® kit (BIO 101, Carlsbad, CA) according to manufacturer's instructions. DNA fragments encoding the A region (amino acid residues 42–765) of BA0871 and the A region (amino acid residues 32–366) of BA5258 were PCR-amplified from the genomic DNA preparation. Primer pair BA0871L and BA0871RPQE was used for BA0871, and primer pair BA5258L and BA5258RPQE was used for BA5258 (Table I). The PCR products were cloned into pQE30 as described previously (16).


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  		TABLE I
Oligonucleotides used in this study

Underlined letters indicate sites for restriction enzymes.

 
Large scale expression and purification of recombinant proteins were performed as described previously (25) using Ni2+ affinity chromatography and ion exchange chromatography. The purified proteins were analyzed using discontinuous SDS-PAGE with 5% stacking and 10% separating gels. The protein bands were visualized by staining the gels with Coomassie Brilliant Blue R250 (Sigma) in 50% methanol and 10% glacial acetic acid solution and destaining with a solution containing 10% methanol and 10% glacial acetic acid. The sizes of the purified proteins were further confirmed by mass spectrometry at Tufts Protein Chemistry Facility, Tufts University.

Circular Dichroism (CD) Spectroscopy—Recombinant A regions of BA0871 and BA5258 were dialyzed against 1% phosphate-buffered saline (PBS), pH 7.4. Their CD spectra were measured with a Jasco J720 spectropolarimeter at room temperature in a 0.05-cm cuvette as described previously (25). Data were integrated for 1 s at 0.2-nm intervals with a bandwidth of 1 nm and 20 accumulations. Secondary structure compositions were estimated using five deconvolution programs, CD Estima (26), Contin (27), Neural Network (28), Selcon (29), and Varslc1 (30). The results were averaged as described (31).

Enzyme-linked Immunosorbent assays (ELISAs)—Proteins were labeled with digoxigenin (Roche Applied Science) according to the manufacturer's instructions and dialyzed against PBS, pH 7.4. ELISAs were performed based on the method described previously with slight modifications (25). Briefly, the wells of 96-well microtiter plates were coated with 1 µg/well bovine type I collagen or bovine serum albumin (BSA) and then blocked with PBS containing 1% (w/v) BSA and 0.1% Tween 20. Increasing concentrations of digoxigenin-labeled recombinant proteins were added to corresponding wells and incubated for 1–2 h at room temperature. Bound proteins were detected with anti-digoxigenin-AP Fab fragment (Roche Applied Science) (1:5000 dilution). Assays were performed in triplicate, and results were reproducible. Data were presented as the mean value ± S.E. of A405 nm from a representative experiment. Apparent dissociation constants were determined using a one-site-binding nonlinear regression model (GraphPad PrismTM 4) as described previously (25).

Surface Plasmon Resonance (SPR) Analysis of Collagen Binding of Recombinant Proteins—SPR analysis was performed at ambient temperature in a BIAcore 3000 system (BIAcore AB, Uppsala, Sweden) based on the method described previously (25). Briefly, bovine type I collagen was immobilized onto the cells of a Biacore CM5 sensor chip. For the blank control one of the cells on the chip was activated and deactivated in the same manner as for the immobilization of collagen except that buffer instead of collagen was used. Increasing concentrations of recombinant proteins in HEPES-buffered saline (10 mM HEPES, 150 mM NaCl, pH 7.4) were injected into the cells in the sensor chip at a flow rate of 30 µl/min for 5 min. The surfaces were regenerated with 15 mM Tris, 1 M NaCl, pH 9. Responses from the blank control were low and were subtracted from responses from the collagen-coated surface. Analysis of the association and dissociation rates was performed using the BIAevaluation 3.0 software (BIAcore). Scatchard plot and nonlinear regression analysis (GraphPad PrismTM 4 software) was carried out using data from the steady state portion of the sensorgrams as described previously (25). Values for the binding ratio, {nu}bound, and the concentration of free proteins, [P]free, were calculated based on the correlation between the SPR response and change in the mass of total bound proteins (25). In nonlinear regression analysis, both one-site-binding and two-site-binding models were used to fit the data. For both recombinant proteins the two-site-binding model gave a slightly better fit as indicated by higher R2 values than the one-site-binding model. However, closer examinations of the results from the two-site-binding model indicated that the 95% confidence intervals of the two KD values were very wide and overlap with each other in each case. In comparison, the 95% confidence intervals for the KD values from the one-site-binding model were narrow. Thus, the simpler one-site-binding model was chosen over the more complicated two-site-binding model to describe the binding data (32). Similar results were obtained from analyses using Scatchard plot for both proteins. Fitting data from low and high concentration ranges separately did not produce sufficiently different KD values (2–3-fold difference), and their 95% confidence intervals were wide and overlap significantly with each other. Therefore, results obtained using the one-site-binding model were reported.

Construction of a S. carnosus Surface-display Expression Vector— The E. coli staphylococci shuttle vector pLI50 (33) was modified as follows. A fragment containing the promoter and signal peptide region of the cna gene of S. aureus was obtained by digesting plasmid pYX102 (17) with EcoRI and BglII. The fragment was cloned into pLI50 digested with EcoRI and BamHI to form pYX103. To introduce the cell wall-anchoring motif into pYX103, the sequence encoding the last B repeat to 51 nucleotides 3' of the end of cna gene was PCR-amplified using primer pairs CNAB5' and PSTCNA3' (Table I). The product was cloned into pYX103, resulting in plasmid pYX105. The inserted regions were confirmed by DNA sequence analysis.

Generation of S. carnosus Heterologous Strains—Primer pair BA0871L and BA0871RYX105 and pair BA5258L and BA5258RYX105 were used to PCR-amplify DNA segments encoding the A regions of BA0871 and BA5258, respectively. The PCR products were cloned into pYX105. The ligation mixture was transformed into E. coli JM101. Transformants were verified by examining the DNA banding patterns using agarose gel electrophoresis of restriction digestions of plasmid preparations and DNA sequence analysis.

Electrocompetent cells of S. carnosus strain TM300 were prepared by washing 200 ml of exponential phase TM300 cells with 200 ml of ice-cold 0.5 M sucrose twice. The cells were then resuspended in 0.8 ml of ice-cold 0.5 M sucrose, separated into aliquots, and stored at –80 °C until ready to use. 50-µl aliquots of thawed TM300 competent cells were mixed with 5 µl of plasmid DNA that were prepared from 5 ml of overnight cultures of E. coli JM101 clones using the QIAprep Spin Miniprep kit. Electroporation was performed in a BTX electroporation system with a 0.1-cm gap electroporation cuvette (Bio-Rad) using the following parameters: 2.5 kV, 25 microfarads, and 72 ohms. Immediately after electroporation 1 ml of tryptic soy broth was added to the cells. The mixture was incubated at 37 °C for 1 h, plated onto tryptic soy broth agar plates containing 10 µg/ml chloramphenicol, and incubated at 37 °C for 16–24 h. Colonies were examined by use of PCR.

Characterization of Heterologous S. carnosus TM300 Strains— Mouse antisera were obtained by injecting female Balb/c mice with recombinant proteins of BA0871 or BA5258 intravenously. The mice were bled 1 and 2 weeks post-injection. Lysostaphin was used to extract cell wall-anchored proteins from heterologous TM300 strains as described (34). To determine the surface expression of BA0871A and BA5258A, the extracts were subjected to western hybridization with mouse anti-rBA0871 or mouse anti-rBA5258 sera as primary antisera as described previously (17).

Attachment of heterologous TM300 strains to bovine type I collagen was assayed as described previously (17). Briefly, various concentrations of log phase cells were incubated with immobilized bovine type I collagen (10 µg/well). The wells were washed with PBS. Attached bacteria were fixed with 25% formaldehyde and stained with 0.5% crystal violet. After washing, 100 µl of 10% acetic acid was added, and the absorbance at 590 nm was measured. Assays were performed in triplicate, and the results were reproducible. Data were presented as the mean value ± S.E. of A590 nm from a representative experiment.

Western Hybridization Analysis of Protein Expression in B. anthracis—Cells from 0.75 ml of an exponential-phase B. anthracis culture (A600 nm = 0.8) grown in LB were washed twice and resuspended in 0.1 ml of PBS, pH 7.4. 35 µl of 4x SDS reducing gel loading solution was then added to the bacterial suspension. The mixture was boiled for 10 min, and then 30 µl of the mixture was loaded onto a 4% stacking and 10% separating SDS-polyacrylamide gel. After electrophoresis, the proteins were transferred to a polyvinylidene difluoride membrane (Bio-Rad). The membrane was blocked with PBS containing 5% (w/v) milk for 1 h at room temperature and then probed with mouse anti-BA0871 sera or mouse anti-BA5258 sera (1:1000 dilution in PBS, 1% (w/v) milk). Goat-anti-mouse IgG (H+L)-alkaline phosphatase conjugant (Bio-Rad) (1:1000 dilution in PBS, 1% milk (w/v)) was used as the secondary antibody for detection.

Immunofluorescent Detection of Surface Proteins—B. anthracis was cultured overnight in brain heart infusion medium. Cells were washed once with PBS and once with PBS containing 1% SDS and then resuspended in PBS to a final density of A600 = 1.0. 10-µl aliquots of cell suspension were applied to poly-L-lysine-coated glass slides. The slides were air-dried, blocked with PBS containing 2% (w/v) BSA at room temperature for 15 min, and then incubated with mouse anti-BA0871 or anti-5258 sera (1:100 dilution) for 1 h. Serum from mice injected with PBS was used as a negative control. Slides were washed twice with PBS and then incubated with Rhodamine Red-conjugated goat anti-mouse IgG (1:100 dilution) (Molecular Probes) at room temperature for 2 h in the dark. The slides were washed twice with PBS. After removal of excess PBS, a coverslip was placed over each slide. The slides were viewed immediately and photographed using an Eclipse E600 microscope (Nikon) equipped with a Photometrics camera (Diagnostic Instruments).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Identification and Sequence Analysis of BA0871 and BA5258 of B. anthracis—To identify putative CWAPs of B. anthracis, the genome of B. anthracis Ames strain was analyzed using a bioinformatics method developed in-house (23, 24). In addition, the annotated genome (www.tigr.org) was searched with terms "LPXTG" or "cell wall anchor." The results of the two methods were combined, and nine CWAPs were identified. Two of them, BA0871 and BA5258, have low level sequence homology to CNA, the S. aureus collagen adhesin.

The BA0871 open reading frame is 2910 nucleotides (nt) long from position 877850 to 880759 on the chromosome of the Ames strain. The gene is flanked by BA0870, a putative hydrolase, 485 nt upstream at the 5' end and by BA0872, a putative N-acetylmuramoyl-L-alanine amidase, 58 nt downstream at the 3' end (Fig. 1). BA0872 is transcribed from the opposite direction to BA0871. Putative transcription terminator sequences were identified 3' of BA0870 and BA0871 open reading frames, respectively. Therefore, the BA0871 gene does not appear to be in an operon. The BA5258 open reading frame is 1884 nt long from position 4765673 to 4763790 on the chromosome of Ames. It is flanked by hypothetical genes at either end. A putative transcription terminator sequence was found between BA5257 (271 nt upstream of BA5258) and BA5258. At the 3' side 31 nt downstream, open reading frame BA5259 is transcribed in the opposite direction to BA5258. Thus, BA5258 is also unlikely to be in an operon.



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  		FIG. 1.
A, gene organization of the regions encoding BA0871 and BA5258. The direction of transcription of each gene is indicated by the direction of an arrow at the end of each block. Lollipops indicate putative transcription terminators. B, domain organization of BA0871 and BA5258 proteins. BA0871 and BA5258 proteins share similar domain organization as CNA; a signal peptide at the N terminus (black-shaded boxes), a non-repetitive A region (white boxes), repeats (hatched boxes), and a cell wall-anchoring region containing LPXTG motif at the C terminus. Regions containing Ig-like folds are indicated by arrows above the proteins. Numbers below the boxes and above the arrows indicated the position of amino acid residues. Dotted lines define the boundary of the homologous region between CNA and BA0871 and the dashed lines the homologous region between CNA and BA5258.

 
The deduced BA0871 protein is 970 amino acids long with a calculated isoelectric point (pI) of 4.56. The predicted BA5258 protein is 628 amino acids long with a calculated pI of 9.10. Examination of the amino acid sequences of the two proteins revealed that they have similar domain organization to that of CNA (Fig. 1). Both contain a signal peptide sequence at the N terminus, a non-repetitive A region followed by repeats. At the C terminus they contain typical cell wall-anchoring sequences; an LPXTG motif, a transmembrane segment, and a short cytoplasmic region with positively charged residues. After post-translational processing by signal peptidase and sortase, the mature proteins of BA0871 and BA5258 have calculated molecular weights of 102713.2 (residues 40–941) and 62804.1 (residues 30–594), respectively. The repeat region in BA0871 (residues 818–908) consists of 13 tandem repeats of a 7-amino acid-residues-long unit, whereas the repeat region (residues 367–551) in BA5258 consists of two tandem repeats of 94 and 91 amino acids, respectively. Secondary structure prediction using PHDsec at the PredictProtein server (cubic.bioc.columbia.edu/predictprotein) indicated that the A regions of both proteins are mainly {beta}-sheets and loops. Fold predication using 3D-PSSM web server (www.sbg.bio.ic.ac.uk/~3dpssm) showed that residues 267–810 in the A region of BA0871 and the entire mature protein of BA5258 are highly likely to adopt Ig-like folds, with probability E values of 7.33e-06 and 0.0176, respectively. The similarities between CNA and the two proteins are not high; however, the homologous regions cover relatively long areas in the proteins, a 357-amino acid-long stretch with 24% identity and 39% similarity between the A regions of CNA and BA0871 and a 183 amino acid-long stretch with 26% identity and 41% similarity between the A regions of CNA and BA5258 (Fig. 1). Thus, BA0871 and BA5258 appear to belong to the family of CNA-like MSCRAMMs.

Expression, Purification, and Secondary Structure Analysis of the A Regions of BA0871 and BA5258 —The predicted A regions of each protein were expressed in E. coli as His-tag fusion proteins (rBA0871A and rBA5258A). Attempts to purify the two recombinant fusion proteins from E. coli lysates by metal chelating chromatography were unsuccessful because both proteins had weak affinity for the nickel column and were present in the flow-through, wash buffer, and early fractions of the eluant. The latter two groups were pooled, and the recombinant proteins were further purified using ion-exchange chromatography, which yielded proteins of reasonably high purity. The calculated molecular weights for rBA0871A and rBA5258A are 80029.3 and 38383.4 Da, respectively. The two proteins migrated at approximately the expected sizes on SDS-PAGE (Fig. 2A). Mass spectrometry analysis of the two purified proteins indicated that their masses were 80030.9 and 38382.2, respectively, in good agreement with the calculated molecular weights. A smaller, weaker band could be observed in the purified rBA5258A sample; however, mass spectrometry did not detect a smaller molecular weight population in the rBA5258A sample. This could be due to a subpopulation of the full-length recombinant protein being more compactly folded or low level partial fragmentation of the protein.



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  		FIG. 2.
A, SDS-PAGE of purified recombinant A regions of BA0871 (rBA0871A) and BA5258 (rBA5258A). 5 µg of purified recombinant proteins were applied to a 5% stacking/10% separating SDS-gel. Bands were visualized by staining the gel with Coomassie Brilliant Blue R250 after electrophoresis. B, far UV circular dichroism spectra of rBA0871A and rBA5258A. Proteins were in 1% PBS, pH 7.4. Spectra were recorded at room temperature in a 0.05-cm cuvette. The spectrum of 1% PBS was subtracted.

 
To determine the secondary structure composition of the A regions of BA0871 and BA5258, the recombinant proteins were analyzed by CD spectroscopy (Fig. 2B). Deconvolution of the spectra indicated that both proteins are predominantly {beta}-sheets. Recombinant BA0871A has 8.3 ± 2.8% {alpha}-helices, 51.0 ± 16.1% {beta}-sheets, and 30.8 ± 18.9% other random structures, and rBA5258A has 8.0 ± 1.0% {alpha}-helices, 63.3 ± 16.9% {beta}-sheets, and 28.7 ± 17.0% other random structures. These compositions are very similar to those observed for the A regions of the MSCRAMMs of staphylococci and entercocci (8, 21–24, 35, 36).

Analysis of the Binding of the A Regions of BA0871 and BA5258 to Type I Collagen—To determine the binding capabilities of the two proteins, solid phase binding assays were performed. Both rBA0871A and rBA5258A bound bovine type I collagen but not BSA in a dose-dependent and saturable manner (Fig. 3). The apparent dissociation constants (KDapp) are 0.19 ± 0.04 µM for rBA0871A and 0.03 ± 0.003 µM for rBA5258A, respectively.



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  		FIG. 3.
Dose-dependent binding of rBA0871A and rBA5258A to immobilized bovine type I collagen as measured by ELISAs. Increasing concentrations of digoxigenin-labeled rBA0871A (panel A) and rBA5258A (panel B) were incubated with immobilized bovine type I collagen or BSA. Bound proteins were detected with anti-digoxigenin-AP Fab fragment (1:5000 dilution).

 
The binding of the recombinant B. anthracis proteins to collagen was further analyzed by SPR using a Biacore 3000 system. Examination of the sensorgrams for the two proteins indicated that they exhibited different kinetics for binding to immobilized type I collagen, with rBA0871A showing markedly slower association and dissociation rates than rBA5258A (Fig. 4, A and B). The association and dissociation rates (ka and kd) for rBA0871A were calculated to be 2.8 ± 1.4 x 103 M–1s–1, and 4.3 ± 1.9 x 10–3 s–1, respectively, resulting in a dissociation constant (KD) of 1.6 ± 0.1 µM. Scatchard analysis based on the responses at the steady state portion of the rBA0871A sensorgrams (25) indicated a KD of around 2.4 µM (data not shown). The KD of rBA0871A for type I collagen was also calculated using a one-site-binding nonlinear regression model, and a KD of 3.2 ± 0.4 µM was obtained (25) (Fig. 4C). Thus, the dissociation constant for the interaction of rBA0871A with type I collagen is ~1.6–3.2 µM.



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  		FIG. 4.
Surface plasmon resonance analysis of the binding of rBA0871A and rBA5258A to bovine type I collagen. Recombinant proteins were flown over a surface coated with collagen. Representative sensorgrams of increasing concentrations of rBA0871A (A) and rBA5258A (B) are shown. One-site-binding nonlinear regression analysis using responses at steady state was shown (C and D for rBA0871A and rBA5258A, respectively). {nu}bound, the binding ratio; [P]free, the concentration of free protein.

 
In contrast, rBA5258A associates and dissociates with type I collagen much faster; the ka and kd for rBA5258A binding to type I collagen are in fact too rapid to be determined accurately and, therefore, are not reported. To calculate the KD of rBA5258A for type I collagen, the responses at the steady state portion of the sensorgrams were first analyzed by Scatchard plot, giving a KD of around 0.6 µM (data not shown). The binding affinity was also calculated to be 0.9 ± 0.2 µM using a one-site-binding nonlinear regression model (Fig. 4D). Thus, the KD determined for the interaction of rBA5258A with type I collagen using SPR is ~0.6–0.9 µM.

BA0871 and BA5258 Mediate Bacterial Adherence to Collagen—To investigate if BA0871 and BA5258 are capable of mediating bacterial attachment to collagen, we developed a surface display system in a non-pathogenic heterologous host, S. carnosus strain TM300. The display vector pYX105 is capable of replicating in S. carnosus and contains DNA sequences for the promoter, signal peptide, the last B repeat, and the cell wall anchoring region of the S. aureus collagen adhesin CNA. Because recombinant A regions of BA0871 and BA5258 showed specific binding to collagen as has been reported for other collagen binding MSCRAMMs (e.g. CNA, ACE, Acm, and CNE), the A regions were cloned into pYX105 between the signal peptide and the B repeat sequence, allowing the surface display of the A regions in S. carnosus. The constructs were then electroporated into TM300, and the resulting strains were designated TM300(BA0871A) and TM300(BA5258A). The gene products are fusion proteins containing the signal peptide of CNA, the A region of BA0871 or BA5258, respectively, the B repeat, and the cell wall-anchoring region of CNA. After post-translational processing by signal peptidase and sortase, the mature products (BA0871f and BA5258f, respectively) should consist of the A region of BA0871 or BA5258 and the CNA B repeat as well as the first four residues of the LPXTG motif. The expected molecular sizes are 106930.1 Da for BA0871f and 65284.2 Da for BA5258f, respectively. The surface expression of the two A regions were verified by western hybridization of lysostaphin cell wall extracts of the two strains. Proteins of the expected sizes were observed and were indicated by arrows (Fig. 5). In TM300(BA0871A), a smaller band that migrated at a similar rate as the recombinant A region of BA0871 was also observed and may be due to a proteolytic cleavage at the junction between the BA0871 A region and the CNA B repeat in the fusion protein.



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  		FIG. 5.
Expression of the A regions of BA0871 and BA5258 on the surface of S. carnosus strain TM300. Exponential phase cells were incubated with lysostaphin in the presence of protease inhibitors as described under "Experimental Procedures." The mixtures were then centrifuged, and the supernatants were subjected to western hybridization. A, probed with mouse anti-BA0871 sera (1:1000 dilution). Lanes from left to right, molecular weight marker, lysostaphin extracts of strains TM300(pYX105) and TM300(BA0871A), and 1 µg of purified rBA0871A protein. B, probed with mouse anti-BA5258 sera (1:1000 dilution). Lanes from left to right, molecular weight marker, lysostaphin extracts of strains TM300(pYX105) and TM300(BA5258A), and 1 µg of purified rBA5258A protein. Goat-anti-mouse IgG-AP conjugant was used as the secondary antibody. Arrows indicate proteins of the expected molecular weights.

 
To test if these recombinant strains are capable of adhering to immobilized collagen, cell attachment assays were carried out (Fig. 6). Exponential phase cells of TM300(BA0871A) and TM300(BA5258A) as well as TM300-containing vector pYX105 only were incubated with immobilized bovine type I collagen or BSA. The results showed that the expression of BA0871A and BA5258A on the surface of S. carnosus increased the ability of the bacteria to adhere to collagen by ~3–4 fold, whereas their abilities to adhere to BSA remained at the low basal level. This suggests that the two proteins can act as collagen adhesins when displayed on the bacterial surface.



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  		FIG. 6.
Attachment of S. carnosus expressing BA0871 and BA5258 to a collagen coated surface. Exponential phase cells were incubated with immobilized bovine type I collagen or BSA. Bound cells were fixed with formaldehyde and stained with crystal violet. Absorbance at 590 nm was measured. Filled squares, TM300(BA0871A) with collagen; filled inverted triangles, TM300(BA5258A) with collagen; filled circles, TM300(pYX105) with collagen; open squares, TM300(BA0871A) with BSA; open inverted triangles, TM300(BA5258A) with BSA; open circles, TM300(pYX105) with BSA.

 
BA0871 and BA5258 Are Expressed on the Surface of B. anthracis—To test for expression of the two proteins by B. anthracis, whole cell lysates from cells grown in LB were analyzed using western hybridization (Fig. 7). The results indicated that BA0871 and BA5258 are expressed in B. anthracis, and their estimated sizes are in agreement with the calculated molecular weights of the two mature proteins. To determine whether the two proteins are displayed on the bacterial surface, immunofluorescence staining of intact B. anthracis cells was carried out (Fig. 8). Bright fluorescence staining were observed on cells incubated with mouse anti-BA0871 or mouse anti-BA5258 sera but not on cells with sera from mice injected with PBS only, indicating that the two proteins are displayed on the surface of B. anthracis.



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  		FIG. 7.
Western blot analysis of the expression of BA0871 and BA5258 in B. anthracis. Exponential phase B. anthracis cells were boiled in SDS-PAGE reducing loading buffer and applied to western hybridization as described under "Experimental Procedures." A, probed with mouse anti-rBA0871 sera (1:1000 dilution). B, probed with mouse anti-rBA5258 sera (1:1000 dilution). Proteins of expected molecular weights are indicated by arrows.

 



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  		FIG. 8.
Immunofluorescent detection of BA0871 and BA5258 on the surface of B. anthracis strain 7702. Phase contrast (panels A, C, and E) and fluorescent visualization (panels B, D, and F) of B. anthracis cells stained with: control mouse sera (A and B), mouse anti-BA0871 sera (C and D), and mouse anti-BA5258 sera (E and F). Goat-anti-mouse IgG-Rhodamine Red conjugant was used as the secondary antibody. All sera and antibodies were used at 1:100 dilutions.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
CWAPs play important roles in the pathogenesis of many Gram-positive organisms and are often virulence factors; however, none of the CWAPs in B. anthracis have been examined. In this study we characterized the first two CWAPs of B. anthracis, BA0871 and BA5258. We show that their amino acid sequences and structural organizations may be similar to CNA, the collagen adhesin of S. aureus, and that they are expressed on the surface of B. anthracis. We further demonstrate that recombinant versions of the predicted A regions of these proteins are capable of binding type I collagen with reasonably high affinity and that they mediate bacterial attachment to collagen when expressed on the surface of a heterologous host.

The development of anthrax involves several discrete stages from the initial entry of spores to the systemic spread of vegetative bacilli. Each stage may require a different set of virulence factors mediating different host-pathogen interactions. Adherence to host cells and tissues has been shown to be important for the establishment of infections caused by both Gram-negative and Gram-positive bacteria. Although B. anthracis has been studied for more than a hundred years, efforts have been mainly focused on elucidating the molecular mechanisms of the toxins and the capsule (1–3, 37–39), which likely come into play in the later stages of the infection (40). Little is known regarding the early events in the establishment of anthrax. In addition, different factors are likely to be involved in the early stages of the three forms of anthrax. Here we report the first two adhesins of B. anthracis that may enable us to investigate some of the early events in the infection process. The cutaneous form of anthrax generally remains localized, and its mortality rate is the lowest among the three forms. The two adhesins may be particularly important in cutaneous anthrax in which adherence to collagen, a major component of the skin, may help the bacilli to resist host clearance and facilitate the colonization of the skin. It is also tempting to speculate that the adherence may act as a "retention" mechanism to deter the bacilli from spreading to the blood circulation.

Previous studies on CNA indicated that the N terminus A region is responsible for collagen binding. Structural analysis as well as comparison with other MSCRAMMs suggested that the A region of CNA consists of three subdomains, each of which are mainly {beta}-sheets that fold into Ig-like folds. Our CD analysis indicates that rBA0871A and rBA5258A are mainly {beta}-sheets. Fold prediction suggests that a large portion of the BA0871A and the entire BA5258A are highly likely to adopt Ig-like folds. The length of these regions suggests that they may contain multiple Ig-like subdomains as in the A regions of CNA and other MSCRAMMs; however, homology modeling of the three-dimensional structures of BA0871A and BA5258A has not been performed since they do not have sufficiently strong sequence identities with CNA or other MSCRAMMs with known structure.

Binding analysis indicated that rBA0871A and rBA5258A specifically bound type I collagen in a dose-dependent manner, with rBA5258A exhibiting higher affinity for collagen than rBA0871A in both SPR analyses and solid phase binding assays. The KDapp values for the interactions between type I collagen and the two recombinant proteins obtained from sold phase binding assays are lower than the KD values obtained from SPR analyses. This could be due to the intrinsic differences between the two methods and has been observed in the binding analyses of other MSCRAMMs.2 Both rBA0871A and rBA5258A appear to fit a relatively simple binding model with one affinity binding class, similar to that observed for the collagen binding A domain of ACE, the E. faecalis collagen adhesin (KD ~48 µM) (8), but unlike the CNA A domain, which exhibited a range of binding affinities for type I collagen (KD values range from ~0.21 to 35 µM) (41). The apparent KD of the interactions between type I collagen and the A region of Acm, an E. faecium collagen binding protein, was reported to be ~3.8 µM as determined by ELISA; however, no SPR analysis was performed (9). To our knowledge, the binding affinities of YadA, FimH, CNE, RspA, and RspB have not been reported. Thus, the A regions of BA0871 and BA5258 appear to bind type I collagen with better affinity than the A regions of ACE and Acm but weaker than the CNA A region. The binding affinities of these proteins are summarized in Table II.


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  		TABLE II
Summary of the binding affinities of collagen binding MSCRAMMs from different bacteria

 
Kinetics analysis show that the two B. anthracis proteins have very different association and dissociation rates. The binding kinetics of rBA0871A resembles that of CNA with relatively slow association and dissociation rates (41), whereas rBA5258A resembles that of ACE with rapid association and dissociation (8). This also suggests that the detailed collagen binding mechanisms of the two B. anthracis proteins are different. It is possible that the A regions of these collagen binding MSCRAMMs adopt similar structural folds that are capable of accommodating a collagen triple helical structure; however, specific residues in the binding surfaces may determine the unique interactions between each MSCRAMM and collagen.

Our recent animal study results suggested that collagen binding affinities and kinetics could affect the severity and duration of infections in vivo (17). We compared the virulence potential of S. aureus isogenic strains that express different fragments of CNA with different affinities for collagen or the binding A region of ACE in a mouse arthritis model. Mice infected with the strain that showed the highest affinity for collagen had the most severe disease. In addition, mice infected with the ACE-expressing strain appeared to recover more rapidly than mice infected with a CNA-expressing strain that showed similar affinity for collagen, suggesting that the more rapid dissociation rate of ACE may lead to a more efficient clearance of the bacteria. Thus, the differences in the binding mechanisms of the two B. anthracis adhesins may have in vivo consequences.

MSCRAMMs are excellent vaccine targets because of their surface locations. Studies on CNA and RspA, the collagen adhesins of S. aureus and E. rhusiopathia, indicated that recombinant fragments of these proteins protected mice against challenge by wild type S. aureus (20) and E. rhusiopathia (11), respectively. This raises the possibility of BA0871 and BA5258 as vaccine candidates for anthrax. Our results from Western blot analysis and immunofluorescent staining of whole B. anthracis cells clearly indicate that the two proteins are expressed on the surface of B. anthracis under laboratory growth conditions. The expression of many B. anthracis genes both on the two virulence plasmids and on chromosome appear to be regulated by temperature and CO2 through the global regulator atxA (42). Analysis of the surface expression of the two proteins under different conditions is currently under way.

Nothing has been reported regarding the biological functions of cell wall-anchored proteins of B. anthracis or the interactions between B. anthracis and host extracellular matrix proteins. Our finding that two putative cell wall-anchored proteins of B. anthracis are capable of binding collagen and mediating bacteria attachment to collagen substrates may contribute to our understanding of the infection mechanisms of different forms of anthrax. Studies to further examine the biological functions of the two proteins are under way.


   FOOTNOTES
 
* This work was supported by National Institutes of Health Grants AI020624-21 and AR44415 (to M. H.) by AIO6155501 (to Y. X.), and by a grant through the Western Regional Center of Excellence for Biodefense and Emerging Infectious Disease Research, National Institutes of Health Grant U54 AI057156 (to T. M. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

§ To whom correspondence should be addressed: The Center for Extracellular Matrix Biology, Texas A&M University System Health Science Center, Institute of Biosciences and Technology, 2121 West Holcombe Blvd., Suite 603, Houston, TX 77030. Tel.: 713-677-7555; Fax: 713-677-7576; E-mail: yxu{at}ibt.tamushsc.edu.

1 The abbreviations used are: CWAP, cell wall-anchored protein; LB, Lennox L broth; MSCRAMM, microbial surface component recognizing adhesive matrix molecules; PBS, phosphate-buffered saline; ELISA, enzyme-linked immunosorbent assay; BSA, bovine serum albumin; SPR, surface plasmon resonance; nts, nucleotides. Back

2 Y. Xu, X. Liang, and M. Höök, unpublished observations. Back


   ACKNOWLEDGMENTS
 
We thank Dr. Eric Brown and Emily Reisanbicchler at the Center for Extracellular Matrix Biology, Texas A&M University System Health Science Center, Institute of Biosciences and Technology, Houston, TX for generating mouse antisera.



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