![]()
|
|
||||||||
J. Biol. Chem., Vol. 279, Issue 41, 42924-42928, October 8, 2004
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||


From the
Department of Cell and Molecular Biology, Uppsala University, SE-751 24 Uppsala, Sweden, the ¶Department of Cell and Molecular Biology, Section for Molecular Pathogenesis, University of Lund, SE-221 00 Lund, Sweden, and the ||Department of Structural Chemistry, Biovitrum, SE-112 76 Stockholm, Sweden
Received for publication, June 22, 2004 , and in revised form, July 20, 2004.
| ABSTRACT |
|---|
|
|
|---|
| INTRODUCTION |
|---|
|
|
|---|
The anaerobic bacterium Finegoldia magna (formerly Peptostreptococcus magnus) is present in the indigenous flora of the skin, the oral cavity, and the gastrointestinal and urogenital tracts. However, these bacteria are also important human pathogens connected with conditions such as soft tissue abscesses and deep wound infections (6). Some isolates of F. magna bind HSA to their surface, and the molecule responsible for this is called protein PAB (7). Protein PAB contains a domain showing high sequence homology (60%) to the albumin binding domains (ABDs) of protein G. This ABD, the GA module, was found to have been transferred from protein G into the gene-encoding protein PAB through a recombination event including a conjugational plasmid. Although the biological function(s) of the GA module is not known in detail, the acquisition of the GA module seems to add selective advantages to the bacterium in terms of growth and also increases its virulence (8, 9).
HSA is the most abundant protein in plasma, where it acts as a transporter of an exceptionally broad spectrum of compounds that are predominantly fatty acids but also amino acids, bile acids, and steroids (10). It is also capable of binding and transporting a wide range of therapeutic substances. Its binding abilities have been probed in a number of studies, and crystal structures are available for HSA in complex with fatty acids (11, 12), hemin (13, 14), and local anesthetics (15). In this paper we present the first crystal structure for a protein-protein complex of HSA, the HSA-GA complex. This complex is formed when the GA module from protein PAB binds to HSA. The data provide insights into factors influencing the affinity and specificity of the interactions between albumins from different animal species and bacterial albumin-binding proteins, with interesting evolutionary implications. The structure might also prove useful in the study of HSA in the context of structure-aided drug design.
| MATERIALS AND METHODS |
|---|
|
|
|---|
Crystallization of the HSA-GA ComplexCrystallization was achieved by vapor diffusion at 18 °C using the hanging drop method with a crystallization solution consisting of 2.2 M (NH4) 2SO4 and 0.1 M citrate, pH 6. The crystals belong to space group C2221 with unit cell dimensions of a = 96.3 Å, b = 134.8 Å, c = 122.5 Å, and
=
=
= 90° and one complex per asymmetric unit. Crystals were frozen in liquid nitrogen using mineral oil as a cryoprotectant.
Crystallographic Data Collection and Structure Determination Data statistics are summarized in Table I. The data were indexed using Denzo and XdisplayF and scaled with Scalepack (17). A molecular replacement search in MOLREP (18) using chain A of the apo form of HSA (Protein Data Bank entry 1AO6
[PDB]
) as a search model gave the solution for the albumin part of the complex with an R-factor of 48.9 and a correlation coefficient of 56.2. After cycles of rigid body refinement in REFMAC5 (19), maps were generated using xdlMAPMAN (20). Manual inspection of normalized positive difference density maps (|Fo| - |Fc|) at a
level of 3 in the program O (21) indicated the location of the GA three-helix bundle. The minimized average NMR structure of the GA module (Protein Data Bank entry 1PRB
[PDB]
) was manually built into the positive difference density. The resulting complex was subsequently subjected to several cycles of restrained refinement in REFMAC5 using the CCP4i interface (22) and manual rebuilding in O. Toward the end of model building, TLS refinement in REFMAC (version 5.2) was also carried out. Attempts were made to refine the model without geometric restraints. However, at this limited resolution such attempts not only result in fitting to noise (higher free R-factor), but also to improbable torsion angles for the main chain and side chains. Therefore, the restraints were kept throughout the refinement. The model is available through the Protein Data Bank, accession code 1TF0
[PDB]
(RCSB022596). An additional map calculation was performed using ARP/wARP (23). Model validation was carried out using PROCHECK (24). Molecular graphics illustrations were generated with PyMol (25).
|
| RESULTS |
|---|
|
|
|---|
|
60° against HSA helix 3 in domain IIA. Although this arrangement is almost identical to the orientation reported for the all
-helical affibody-ZSPA complex (26), the respective binding sites on the Z domain and the GA module do not overlap. The three-helix bundle Z domain of protein A and the GA module share a striking similarity in their overall structure. Even so, it is helix 1 of the Z domain that packs against the affibody helices, not the second and third as in the HSA-GA case. This illustrates the versatility inherent in an all-helical binding surface.
|
The Structure of the GA ModuleNo significant conformational changes in GA occur upon HSA binding, as judged by comparison of the present crystal structure and the previously reported solution structure (27). The three helices of the GA module form a tight three-helix bundle with a distinct hydrophobic core. Both the N and C terminus of the GA module are well ordered in the crystal. The N-terminal residue Thr-1 is hydrogen bonded to the side chain of a symmetry-related Lys-536, whereas the penultimate residue at the C-terminal end, His-52, is stabilized by a hydrogen bond to the main chain oxygen of Ala-21 in the GA module.
The main chain of GA helix 1 was corrected, and side chain orientations throughout the entire chain also had to be amended. The rebuilding resulted in that almost 90% of the GA residues in the crystal structure came within core regions in the Ramachandran plot, compared with 64% in the NMR solution structure.
The HSA-GA InterfaceThe interface is a predominantly flat surface of
700 Å2 involving one-fourth of the total GA surface area (Fig. 2B). The binding surface consists of a hydrophobic core flanked by two hydrogen bond networks.
The hydrophobic core of the interface is lined with residues Phe-228, Ala-229, Ala-322, Val-325, Phe-326, and Met-329 from HSA and residues Phe-27, Ala-31, Leu-44, and Ile-48 from GA. In this context it is worth mentioning the phenyl side chain of Phe-27, which is buried in a cleft formed by the HSA residues Phe-309, Phe-326, and Met-329 (Fig. 2C). In addition, this interaction extends the aromatic cluster of HSA domain II, which consists of Phe-309, Phe-326, Phe-330, Phe-374, Phe-377, Tyr-334, and Tyr-353, to also include Phe-27 and Tyr-28 of GA.
Adjacent to the hydrophobic core of the interface is a hydrogen bond network between helix 7 in the HSA domain IIB and the loop preceding helix 2 in GA (Fig. 3A). HSA residue Glu-321 forms two hydrogen bonds, one each with the main chain nitrogens of Thr-24 and Ser-25 in GA. Furthermore, the side chain hydroxyl group of Ser-25 in GA forms another hydrogen bond with the side chain oxygen of Asn-318 from HSA. The main chain oxygen of Asn-318 is, in turn, involved in a hydrogen bond with the side chain hydroxyl group of Tyr-28 in the GA module.
|
A Fatty Acid in the Binding InterfaceAn additional hydrogen bond at the HSA-GA interface occurs between GA residue Glu-47 in helix 3 and HSA residue Lys-212. Adjacent to this region, in the tunnel-shaped cavity previously described as fatty acid binding site 6, we observed density compatible with a medium-length fatty acid. Two molecules of the saturated ten-carbon fatty acid decanoate were built into the model in a linear tail-to-tail orientation. In the final model, the carboxylate head of one of the fatty acids is stabilized by a hydrogen bond with Ser-232 (Fig. 4A). The fatty acid is thereby brought within 4.5 Å from the side chain of Glu-47 in GA. Local side chain rotations occur in HSA upon binding of the fatty acid molecules and the GA module simultaneously. The side chain of Lys-212, which in defatted HSA forms a 3.6-Å salt bridge with the carboxyl group of Glu-208, rotates slightly, pushed by the fatty acid so that its terminal amine group forms a tighter, 2.6-Å ion pair bond to the glutamic acid side chain (Fig. 4B). One molecule of decanoate was also built in fatty acid site 7. To account for density in Sudlow site I, one molecule of citrate was built into this site.
|
| DISCUSSION |
|---|
|
|
|---|
A Fatty Acid at the Binding InterfaceThe presence of fatty acid in the HSA-GA binding interface might influence complex formation, although its precise role in that case is not evident because it is not within hydrogen bonding distance from any of the GA module side chains. However, binding of GA might be influenced indirectly, because the binding of the fatty acid causes the side chain of Lys-212 to be pushed aside slightly into a position that allows it to form a hydrogen bond to GA. Whether completely defatted HSA would be able to bind GA is not known. Therefore, it is uncertain whether the presence of a fatty acid is required for or only tolerated in the binding of GA.
Structural Basis for Species SpecificityThe GA module in this study is homologous to an ABD of protein G of group G streptococci as a result of it being transferred by exon shuffling from protein G to protein PAB. The binding site for protein G on HSA has been suggested to be located on the second and third domain of HSA and involves a single site consisting of a segment spanning residues 330 to 548 (28). This study demonstrates that HSA residues 212, 309, 318, 321, 326, and 329 are important for GA binding, indicating that there is at least partial overlap between the two sites.
Despite the high sequence homology between the two domains, the wide species affinity displayed by ABD is not retained in the GA module. Whereas affinities for human and rhesus monkey albumin are similar in GA and ABD, the affinity of GA for mouse albumin is more than a hundred times lower than that of ABD (29).
Considering that the interaction in the hydrophobic core of the interface between Phe-27 in the GA module and Met-329 in HSA is crucial, we suggest a structural basis for the difference in species selectivity between ABD and the GA module. A superposition of the
carbons in the structures of ABD (Protein Data Bank entry 1GJS
[PDB]
) and GA shows that the phenylalanine in GA position 27 corresponds to Tyr-39 in the ABD (not shown). Furthermore, a sequence alignment of albumins of different species shows that the methionine in HSA position 329 corresponds to polar or charged side chains such as serine and lysine in albumins of other species (Fig. 5). As other interactions at the surface seem to be conserved across species, the bootstrapped hydrophobic Met-Phe interaction emerges as a crucial interaction at the HSA-GA interface, and we stipulate that it plays a major part in the species specificity observed for the GA module compared with that of ABD. The polar nature of the hydroxyl group in Tyr-39 of the ABD domain enables it to interact with the range of polar side chains displayed by the albumins from different species, whereas its GA counterpart, Phe-27, is restricted to interact with a hydrophobic side chain, i.e. methionine in the case of human and rhesus monkey albumin.
|
Biological ImplicationsIn 1979, Kronvall et al. (30) first described the binding of HSA to bacterial surface structures and found that groups A, C, and G streptococci specifically absorbed HSA from plasma. Subsequently, some strains of F. magna were also found to bind HSA (31). In the case of groups C and G streptococci, protein G is responsible for albumin binding, whereas the corresponding protein of F. magna is called PAB. Analysis of the gene encoding protein PAB revealed that the HSA binding domain had been transferred from the protein G gene by the action of a conjugational plasmid from a third bacterial species, Enterococcus faecalis (7). This finding represents the first described case of contemporary module shuffling, and the fact that PAB-expressing strains of F. magna are tetracycline resistant2 suggests that antibiotics provide the selective pressure behind the evolution of this novel HSA-binding protein. As mentioned above, F. magna is part of the normal human bacterial flora, but strains that express protein PAB are mostly isolated from patients with localized suppurative infections, suggesting that the binding of HSA to the bacterial surface increases the pathogenic potential of F. magna. The study by de Château et al. (8) did indeed show that HSA enhances the growth rate of streptococcal and F. magna strains expressing HSA-binding surface proteins, and the structural data of the present work indicate that the binding of HSA to the GA module could provide growing bacteria with fatty acids and, possibly, other nutrients transported by HSA.
Whereas group C and G streptococci infect most mammalian species, F. magna has been isolated only from humans. This finding is reflected also in the albumin binding properties of proteins G and PAB, where protein G has a much broader specificity than PAB, which binds preferentially primate albumins. This observation represents an unusually clear and beautiful example of microbial adaptation to its host(s) at the molecular level. As described in the previous paragraph, the sequence differences between the ABDs of proteins G and PAB and between different albumins, in relation to the HSA-GA interface, help to explain the structural basis for this adaptation.
Implications for the Rational Design of Albumin LigandsIn this study we have obtained reasonably well diffracting crystals of HSA that endure cryoconditions during data collection. This might prove useful, especially in the context of minimizing the HSA affinity of drug molecules by structure-based design. We speculate that the readily available, cryo-enduring HSA-GA complex could be an entry point to enabling high throughput structure determination in the study of HSA-drug interactions. Further studies will be performed to investigate the consistency of HSA binding ligands in the presence of the GA module.
Based on the molecular interactions in the HSA-GA interface, it might be possible to design and screen for compounds that will interfere with the binding of HSA to bacterial surfaces in vivo. Several observations have shown that the binding of HSA adds selective advantages to the bacteria and increases their virulence, suggesting that such compounds could be used to treat infections caused by HSA binding bacterial pathogens.
| FOOTNOTES |
|---|
* This work was supported by the Swedish Research Council Project Grants 7480 and 14379 and the Foundations of Crafoord and Österlund. 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. ![]()
To whom correspondence should be addressed. Tel.: 46-18-4714451; Fax: 46-18-511755; E-mail: sara{at}xray.bmc.uu.se.
1 The abbreviations used are: HSA, human serum albumin; GA, protein G-like albumin binding module; ABD, albumin binding domain. ![]()
2 I.-M. Frick and L. Björck, unpublished results. ![]()
| ACKNOWLEDGMENTS |
|---|
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
A. Jonsson, J. Dogan, N. Herne, L. Abrahmsen, and P.-A. Nygren Engineering of a femtomolar affinity binding protein to human serum albumin Protein Eng. Des. Sel., August 1, 2008; 21(8): 515 - 527. [Abstract] [Full Text] [PDF] |
||||
![]() |
T. Goto, A. Yamashita, H. Hirakawa, M. Matsutani, K. Todo, K. Ohshima, H. Toh, K. Miyamoto, S. Kuhara, M. Hattori, et al. Complete Genome Sequence of Finegoldia magna, an Anaerobic Opportunistic Pathogen DNA Res, February 7, 2008; (2008) dsm030v1. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. Stork, D. Muller, and R. E. Kontermann A novel tri-functional antibody fusion protein with improved pharmacokinetic properties generated by fusing a bispecific single-chain diabody with an albumin-binding domain from streptococcal protein G Protein Eng. Des. Sel., November 3, 2007; (2007) gzm061v1. [Abstract] [Full Text] [PDF] |
||||
![]() |
P. A. Alexander, Y. He, Y. Chen, J. Orban, and P. N. Bryan The design and characterization of two proteins with 88% sequence identity but different structure and function PNAS, July 17, 2007; 104(29): 11963 - 11968. [Abstract] [Full Text] [PDF] |
||||
![]() |
Y. He, Y. Chen, D. A. Rozak, P. N. Bryan, and J. Orban An artificially evolved albumin binding module facilitates chemical shift epitope mapping of GA domain interactions with phylogenetically diverse albumins Protein Sci., July 1, 2007; 16(7): 1490 - 1494. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| All ASBMB Journals | Molecular and Cellular Proteomics |
| Journal of Lipid Research | ASBMB Today |