Ace is a collagen-binding MSCRAMM from Enterococcus faecalis

A putative collagen-binding MSCRAMM, Ace, of Enterococcus faecalis was identified by searching bacterial genome data bases for proteins containing domains homologous to the ligand-binding region of Cna, the collagen-binding MSCRAMM from Staphylococcus aureus. Ace was predicted to have a molecular mass of 71 kDa and contains features characteristic of cell surface proteins on Gram-positive bacteria, including a LPXTG motif for cross-linking to the cell wall. The N-terminal region of Ace contained a region (residues 174-319) in which 56% of the residues are identical or similar when compared with the minimal ligand-binding region of Cna (Cna 151-318); the remainder of the Ace A domain has 46% similarity with the corresponding region of the Cna A domain. Antibodies raised against recombinant Ace A domain were used to verify the cell surface expression of Ace on E. faecalis. These antibodies also effectively inhibited the adhesion of enterococcal cells to a collagen substrate, suggesting that Ace is a functional collagen-binding MSCRAMM. Structural modeling of the conserved region in Ace (residues 174-319) suggested a structure very similar to that reported for residues 151-318 of the Cna collagen-binding domain in which the ligand-binding site was identified as a trench transversing a beta-sheet face (Symersky, J., Patti, J. M., Carson, M., House-Pompeo, K., Teale, M., Moore, D., Jin, L., DeLucas, L. J., Höök, M., and Narayana, S. V. L. (1997) Nat. Struct. Biol. 10, 833-838). Biochemical analyses of recombinant Ace and Cna A domains supported the modeling data in that the secondary structures were similar as determined by CD spectroscopy and both proteins bound at multiple sites in type I collagen with micromolar affinities, but with different apparent kinetics. We conclude that Ace is a collagen-binding MSCRAMM on enterococci and is structurally and functionally related to the staphylococcal Cna protein.

Adherence of Borrelia burgdorferi. Identification of critical lysine residues in DbpA required for decorin binding

Borrelia burgdorferi, the causative agent of Lyme disease, expresses on its surface two decorin binding adhesins, DbpA and DbpB. Previous studies have demonstrated that vaccination of mice with DbpA provided protection against challenge with heterologous Borrelia strains despite considerable sequence variability among DbpA in these strains. We have now examined the importance of individual amino acid residues in DbpA for decorin binding. We demonstrated that chemical modification of lysine residues resulted in loss of ligand binding activity. Of the 27 lysine residues in native DbpA from strain 297, 6 are present in most and 5 are conserved in all 30 DbpA sequences examined so far. Analysis of recombinant DbpA in which individual lysine residues have been mutated to alanine suggested that three of the conserved residues distributed throughout the DbpA sequence are required for decorin binding. These mutants lost their ability to bind decorin in Western ligand blot assay and bound reduced amounts of decorin in an ELISA. Furthermore, these mutant DbpA proteins did not inhibit the adherence of B. burgdorferi to a decorin substrata, and they did not recognize decorin in an extracellular matrix established by human fibroblast cultures. We conclude that the three lysine residues Lys-82, Lys-163, and Lys-170 are crucial for the binding of DbpA to decorin.

Trench-shaped binding sites promote multiple classes of interactions between collagen and the adherence receptors, alpha(1)beta(1) integrin and Staphylococcus aureus cna MSCRAMM

Most mammalian cells and some pathogenic bacteria are capable of adhering to collagenous substrates in processes mediated by specific cell surface adherence molecules. Crystal structures of collagen-binding regions of the human integrin alpha(2)beta(1) and a Staphylococcus aureus adhesin reveal a "trench" on the surface of both of these proteins. This trench can accommodate a collagen triple-helical structure and presumably represents the ligand-binding site (Emsley, J., King, S. L., Bergelson, J. M., and Liddington, R. C. (1997) J. Biol. Chem. 272, 28512-28517; Symersky, J., Patti, J. M., Carson, M., House-Pompeo, K., Teale, M., Moore, D., Jin, L., Schneider, A., DeLucas, L. J., Höök, M., and Narayana, S. V. L. (1997) Nat. Struct. Biol. 4, 833-838). We report here the crystal structure of the alpha subunit I domain from the alpha(1)beta(1) integrin. This collagen-binding protein also contains a trench on one face in which the collagen triple helix may be docked. Furthermore, we compare the collagen-binding mechanisms of the human alpha(1) integrin I domain and the A domain from the S. aureus collagen adhesin, Cna. Although the S. aureus and human proteins have unrelated amino acid sequences, secondary structure composition, and cation requirements for effective ligand binding, both proteins bind at multiple sites within one collagen molecule, with the sites in collagen varying in their affinity for the adherence molecule. We propose that (i) these evolutionarily dissimilar adherence proteins recognize collagen via similar mechanisms, (ii) the multisite, multiclass protein/ligand interactions observed in these two systems result from a binding-site trench, and (iii) this unusual binding mechanism may be thematic for proteins binding extended, rigid ligands that contain repeating structural motifs.

Role of fibronectin-binding MSCRAMMs in bacterial adherence and entry into mammalian cells

Most bacterial infections are initiated by the adherence of microorganisms to host tissues. This process involves the interaction of specific bacterial surface structures, called adhesins, with host components. In this review, we discuss a group of microbial adhesins known as Microbial Surface Components Recognizing Adhesive Matrix Molecules (MSCRAMMs) which recognize and bind FN. The interaction of bacteria with FN is believed to contribute significantly to the virulence of a number of microorganisms, including staphylococci and streptococci. Several FN-binding MSCRAMMs of staphylococci and streptococci exhibit a similar structural organization and mechanism of ligand recognition. The ligand-binding domain consists of tandem repeats of a approximately 45 amino acid long unit which bind to the 29-kDa N-terminal region of FN. The binding mechanism is unusual in that the repeat units are unstructured and appear to undergo a conformational change upon ligand binding. Apart from supporting bacterial adherence, FN is also involved in bacterial entry into non-phagocytic mammalian cells. A sandwich model has been proposed in which FN forms a molecular bridge between MSCRAMMs on the bacterial surface and integrins on the host cell. However, the precise mechanism of bacterial invasion and the roles of FN and integrins in this process have yet to be fully elucidated.

Decorin is a Zn2+ metalloprotein

Decorin is ubiquitously distributed in the extracellular matrix of mammals and a member of the proteoglycan family characterized by a core protein dominated by leucine-rich repeat motifs. We show here that decorin extracted from bovine tissues under denaturing conditions or produced in recombinant "native" form by cultured mammalian cells has a high affinity for Zn2+ as demonstrated by equilibrium dialyses. The Zn2+-binding sites are localized to the N-terminal domain of the core protein that contains 4 Cys residues in a spacing reminiscent of a zinc finger. A recombinant 41-amino acid long peptide representing the N-terminal domain of decorin has full Zn2+ binding activity and binds two Zn2+ ions with an average KD of 3 x 10(-7) M. Binding of Zn2+ to this peptide results in a change in secondary structure as shown by circular dichroism spectroscopy. Biglycan, a proteoglycan that is structurally closely related to decorin contains a similar high affinity Zn2+-binding segment, whereas the structurally more distantly related proteoglycans, epiphycan and osteoglycin, do not bind Zn2+ with high affinity.

Acupuncture treatment of vasomotor symptoms in men with prostatic carcinoma: a pilot study

PURPOSE

Most men who undergo castration therapy for prostatic carcinoma will have vasomotor symptoms that usually persist for years. Vasomotor symptoms are elicited from the thermoregulatory center, possibly due to a decrease in hypothalamic opioid activity induced by low sex steroid concentrations. Acupuncture treatment in women, which stimulates hypothalamic opioid activity, alleviates vasomotor symptoms. We report on men treated with acupuncture for relief of vasomotor symptoms after castration therapy.

MATERIALS AND METHODS

We asked 7 men with vasomotor symptoms due to castration therapy to receive acupuncture treatment 30 minutes twice weekly for 2 weeks and once a week for 10 weeks. Effects on flushes were recorded in logbooks.

RESULTS

Of the 7 men 6 completed at least 10 weeks of acupuncture therapy and all had a substantial decrease in the number of hot flushes (average 70% after 10 weeks). At 3 months after the last treatment the number of flushes was 50% lower than before therapy. Therapy was discontinued after 10 weeks because of a femoral neck fracture in 1 man and after 3 weeks due to severe back pain in 1.

CONCLUSIONS

Acupuncture may be a therapeutic alternative in men with hot flushes after castration therapy and merits further evaluation.

Inhibition of Staphylococcus aureus adherence to collagen under dynamic conditions

Staphylococcus aureus is the most common etiological agent of bacterial arthritis and acute osteomyelitis and has been shown to bind to type II collagen under static and dynamic conditions. We have previously reported the effect of shear on the adhesion of S. aureus Phillips to collagen and found that this process is shear dependent (Z. Li, M. Höök, J. M. Patti, and J. M. Ross, Ann. Biomed. Eng. 24[Suppl. 1]:S-55). In this study, we used recombinant collagen adhesin fragments as well as polyclonal antibodies generated against adhesin fragments in attempts to inhibit bacterial adhesion. A parallel-plate flow chamber was used in a dynamic adhesion assay, and quantification of adhesion was accomplished by phase contrast video microscopy coupled with digital image processing. We report that both recombinant fragments studied, M19 and M55, and both polyclonal antibodies studied, alpha-M17 and alpha-M55, inhibit adhesion to varying degrees and that these processes are shear dependent. The M55 peptide and alpha-M55 cause much higher levels of inhibition than M19 and alpha-M17, respectively, at all wall shear rates studied. Our results demonstrate the importance of using a dynamic system in the assessment of inhibitory strategies and suggest the possible use of M55 and alpha-M55 in clinical applications to prevent infections caused by S. aureus adhesion to collagen.

Crystallization of ClfA and ClfB fragments: the fibrinogen-binding surface proteins of Staphylococcus aureus

Recombinant constructs encoding the fibrinogen-binding domains of ClfA and ClfB from Staphylococcus aureus have been crystallized. ClfA was crystallized in the orthorhombic space group P212121 with unit-cell parameters a = 39.58, b = 81.39 and c = 112.65 A. A complete data set was recorded to 2.1 A resolution and had a Vm of 2. 3 A3 Da-1 with 46.5% solvent, suggesting one molecule per asymmetric unit. Co-crystals of ClfA with the 17 amino-acid C-terminal peptide of fibrinogen gamma-chain diffracted to 2.1 A resolution and had unit-cell parameters a = 39.11, b = 81.39 and c = 109.51 A in the space group P212121. ClfB was crystallized in the tetragonal space group P41212 or P43212 with unit-cell parameters a = 96.31, b = 96. 31 and c = 84.13 A and diffracted to 2.45 A resolution. The estimated Vm of 2.6 A3 Da-1 with 53% solvent indicated one molecule in the asymmetric unit.

Crystallization and preliminary X-ray analysis of B-domain fragments of a Staphylococcus aureus collagen-binding protein

Recombinant proteins of monomeric and dimeric B-domain repeats of a Staphylococcus aureus FDA 574 collagen-binding adhesin have been crystallized. The single repeat unit (B1) was crystallized in a body-centered orthorhombic lattice with a = 96.9, b = 101.3, c = 120. 8 A in either the I222 or I212121 space group. These crystals diffracted to 2.5 A resolution and the calculated Vm values of 3.2 and 2.2 A3 Da-1 suggest the possibility of a dimer or a trimer in the asymmetric unit. The two-repeat fragment (B1B2) crystallized in the orthorhombic space group P212121 with cell dimensions a = 42.4, b = 79.4, c = 130.4 A and diffracted to 2.3 A resolution. For this species, the calculated Vm value of 2.2 A3 Da-1 indicates the presence of a monomer in the asymmetric unit.

Surface protein adhesins of Staphylococcus aureus

Staphylococcus aureus can colonize the host to initiate infection by adhering to components of the extracellular matrix. Adherence is mediated by surface protein adhesins (MSCRAMMs). Ligand binding by these fibronectin-, fibrinogen- and collagen-binding proteins occurs by distinct mechanisms that are being investigated at the molecular level.

Multiple specificities of the staphylococcal and streptococcal fibronectin-binding microbial surface components recognizing adhesive matrix molecules

Many pathogenic gram-positive bacteria express fibronectin (Fn)-binding microbial surface components recognizing adhesive matrix molecules (MSCRAMMs), most of which have a similar structural organization with a primary ligand-binding domain consisting of 3-6 repeats of 40-50 amino-acid-residue motifs. The MSCRAMMs appear to preferentially bind to the N-terminal region of Fn, which is composed of five type-I modules. Here we report that the Fn-binding MSCRAMM FnbpA of Staphylococcus aureus contains a second ligand-binding domain located outside the repeat units. In addition, several sites in the Fn N-terminus presented as recombinant type-I module pairs bind to the repeat domain of the MSCRAMM. All of the MSCRAMMs analyzed, which include FnbpA of Staphylococcus aureus, Sfb of Streptococcus pyogenes, and FnbA and FnbB of Streptococcus dysgalactiae, were shown to bind to multiple sites in the N-terminal domain of Fn. By dissecting the repeat domain of FnbpA using synthetic peptides and recombinant fragments, we show that discrete, different motifs are responsible for the binding to individual sites in Fn, rather than a common motif being able to bind to several pairs of type-I Fn modules. The C-terminal half of many of the MSCRAMM repeat units contain a common motif, which is shown here to bind to the type-I module pair 4 and 5. In addition, some of the repeat units of FnbpA contain N-terminal motifs which bound to the type-I module pairs 1-2 and 2-3, respectively. These latter binding motifs appear to be partly overlapping and dependent on flanking sequences. Fluorescence polarization experiments using fluorescein-labeled MSCRAMM peptides and recombinant type-I Fn module pairs revealed dissociation constants of 1-13 microM. It was also shown that the fluorescein-labeled peptides differed in their primary binding sites on Fn.

Domain structure of the Staphylococcus aureus collagen adhesin

Sequence analysis of surface proteins from Gram-positive bacteria indicates a composite organization consisting of unique and repeated segments. Thus, these proteins may contain discrete domains that could fold independently. In this paper, we have used a panel of biophysical methods, including gel permeation chromatography, analytical ultracentrifugation, circular dichroism, and fluorescence spectroscopy, to analyze the structural organization of the Staphylococcus aureus collagen adhesin, CNA. Our results indicate that the structure, function, and folding of the ligand-binding domain (A) are not affected by the presence or absence of the other major domain (B). In addition, little or no interaction is observed between the nearly identical repeat units within the B domain. We propose that CNA is indeed a mosaic protein in which the different domains previously indicated by sequence analysis operate independently.

Antibody response to fibronectin-binding adhesin FnbpA in patients with Staphylococcus aureus infections

We have analyzed antibody reactivity to a fibronectin-binding microbial surface component that recognizes adhesive matrix molecules (MSCRAMM) in blood plasma collected from patients with staphylococcal infections. All patients had elevated levels of anti-MSCRAMM antibodies compared to those of young children who, presumably, had not been exposed to staphylococcal infections. The anti-MSCRAMM antibodies preferentially reacted with the ligand-binding repeat domain of the adhesin. However, these antibodies did not inhibit fibronectin binding. Essentially, all patients had antibodies which specifically recognized the fibronectin-MSCRAMM complex but not the isolated components. Epitopes recognized by these anti-ligand-induced binding sites antibodies were found in each repeat unit of the MSCRAMM. These results demonstrate that staphylococci have bound fibronectin some time during infection and that each repeat unit in the MSCRAMM can engage in ligand binding. Furthermore, our previously proposed model, suggesting that an unordered structure in the MSCRAMM undergoes a conformational change upon ligand binding (K. House-Pompeo, Y. Xu, D. Joh, P. Speziale, and M. Höök, J. Biol. Chem. 271:1379-1384, 1996), is presumably operational in patients during infections.

Decorin-binding adhesins from Borrelia burgdorferi

Lyme disease is a tick-transmitted infection caused by the spirochete Borrelia burgdorferi. Ticks deposit B. burgdorferi into the dermis of the host, where they eventually become associated with collagen fibres. We demonstrated previously that B. burgdorferi is unable to bind collagen, but can bind the collagen-associated proteoglycan decorin and expresses decorin-binding proteins (Dbps). We have now cloned and sequenced two genes encoding the proteins, DbpA and DbpB, which have a similar structure, as revealed by circular dichroism (CD) spectroscopy of recombinant proteins. Competition experiments revealed a difference in binding specificity between DbpA and DbpB. Western blot analysis of proteinase K-treated intact B. burgdorferi and transmission electron microscopy studies using antibodies raised against recombinant Dbps demonstrated that these proteins are surface exposed. DbpA effectively inhibits the attachment of B. burgdorferi to a decorin substrate, whereas DbpB had a marginal effect, suggesting a difference in substrate specificity between the two Dbps. Polystyrene beads coated with DbpA adhered to a decorin-containing extracellular matrix produced by cultured skin fibroblasts, whereas beads coated with OspC did not. Taken together, these data suggest that Dbps are adhesins of the MSCRAMM (microbial surface component-recognizing adhesive matrix molecule) family, which mediate B. burgdorferi attachment to the extracellular matrix of the host.

Clumping factor B (ClfB), a new surface-located fibrinogen-binding adhesin of Staphylococcus aureus

The surface-located fibrinogen-binding protein (clumping factor; ClfA) of Staphylococcus aureus has an unusual dipeptide repeat linking the ligand binding domain to the wall-anchored region. Southern blotting experiments revealed several other loci in the S. aureus Newman genome that hybridized to a probe comprising DNA encoding the dipeptide repeat. One of these loci is analysed here. It also encodes a fibrinogen-binding protein, which we have called ClfB. The overall organization of ClfB is very similar to that of ClfA, and the proteins have considerable sequence identity in the signal sequence and wall attachment domains. However, the A regions are only 26% identical. Recombinant biotinylated ClfB protein bound to fibrinogen in Western ligand blots. ClfB reacted with the alpha- and beta-chains of fibrinogen in the ligand blots in contrast to ClfA, which binds exclusively to the gamma-chain. Analysis of proteins released from the cell wall of S. aureus Newman by Western immunoblotting using antibody raised against the recombinant A region of ClfB identified a 124 kDa protein as the clfB gene product. This protein was detectable only on cells that were grown to the early exponential phase. It was absent from cells from late exponential phase or stationary phase cultures. Using a clfB mutant isolated by allelic replacement alone and in combination with a clfA mutation, the ClfB protein was shown to promote (i) clumping of exponential-phase cells in a solution of fibrinogen, (ii) adherence of exponential-phase bacteria to immobilized fibrinogen in vitro, and (iii) bacterial adherence to ex vivo human haemodialysis tubing, suggesting that it could contribute to the pathogenicity of biomaterial-related infections. However, in wild-type exponential-phase S. aureus Newman cultures, ClfB activity was masked by the ClfA protein, and it did not contribute at all to interactions of cells from stationary-phase cultures with fibrinogen. ClfB-dependent bacterial adherence to immobilized fibrinogen was inhibited by millimolar concentrations of Ca2+ and Mn2+, which indicates that, like ClfA, ligand binding by ClfB is regulated by a low-affinity inhibitory cation binding site.

Conditional adherence of Enterococcus faecalis to extracellular matrix proteins

The adherence of 44 clinical isolates of Enterococcus faecalis, a common cause of endocarditis, and 13 Enterococcus faecium to substrates of six extracellular matrix (ECM) proteins was examined using 35S-labeled bacteria. One E. faecalis strain, isolated from a patient with endocarditis, adhered to collagen types I and IV and another E. faecalis strain adhered to laminin and to collagen types I and IV. However, most isolates showed little adherence ( < 5% of added cells adhered) when grown at 37 degrees C regardless of their source (endocarditis, urine or fecal sample). When grown at 46 degrees C (but not when grown in CO2 or nutrient limited media), most isolates of E. faecalis increased their adherence to immobilized laminin, collagen types I and IV but not to fibronectin, fibrinogen or bovine serum albumin, whereas none of the E. faecium increased adherence when grown at 46 degrees C or 50 degrees C. The adherence of E. faecalis was eliminated by digestion with trypsin, suggesting that a protein is somehow important, directly or indirectly, for adherence to occur. Pre-incubation of bacteria with soluble collagen types I and IV inhibited the adherence to these ECM proteins. These results demonstrate that in E. faecalis, adherence to ECM proteins is produced during routine in vitro growth conditions by occasional isolates and can be produced during certain stressful growth conditions by others. Whether this adherence relates to the propensity of E. faecalis to cause endocarditis remains to be determined.

Vaccination with a recombinant fragment of collagen adhesin provides protection against Staphylococcus aureus-mediated septic death

Staphylococcus aureus is a major cause of nosocomial and community-acquired infections. Morbidity and mortality due to infections such as sepsis, osteomyelitis, septic arthritis, and invasive endocarditis remain high despite the use of antibiotics. The emergence of antibiotic resistant super bugs mandates that alternative strategies for the prevention and treatment of S. aureus infections are developed. We investigated the ability of vaccination with a recombinant fragment of the S. aureus collagen adhesin to protect mice against sepsis-induced death. Actively immunized NMRI mice were intravenously inoculated with the S. aureus clinical isolate strain Phillips. 14 d after inoculation, mortality in the collagen adhesin-vaccinated group was only 13%, compared with 87% in the control antigen immunized group (P < 0.001). To determine if the protective effect was antibody mediated, we passively immunized naive mice with collagen adhesin-specific antibodies. Similar to the active immunization strategy, passive transfer of collagen adhesin-specific antibodies protected mice against sepsis-induced death. In vitro experiments indicated that S. aureus opsonized with sera from collagen adhesin immunized mice promoted phagocytic uptake and enhanced intracellular killing compared with bacteria opsonized with sera from control animals. These results indicate that the collagen adhesin is a viable target in the development of immunotherapeutics against S. aureus.

Active and passive immunity against Borrelia burgdorferi decorin binding protein A (DbpA) protects against infection

Borrelia burgdorferi, the spirochete that causes Lyme disease, binds decorin, a collagen-associated extracellular matrix proteoglycan found in the skin (the site of entry for the spirochete) and in many other tissues. Two borrelial adhesins that recognize this proteoglycan, decorin binding proteins A and B (DbpA and DbpB, respectively), have recently been identified. Infection of mice by low-dose B. burgdorferi challenge elicited antibodies against DbpA and DbpB that were sustained at high levels, suggesting that these antigens are expressed in vivo. Scanning immunoelectron microscopy showed that DbpA was surface accessible on intact borreliae. Passive administration of DbpA antiserum protected mice from infection following challenge with heterologous B. burgdorferi sensu stricto isolates, even when serum administration was delayed for up to 4 days after challenge. DbpA is the first antigen target identified that is capable of mediating immune resolution of early, localized B. burgdorferi infections. DbpA immunization also protected mice from B. burgdorferi challenge; DbpB immunization was much less effective. DbpA antiserum inhibited in vitro growth of many B. burgdorferi sensu lato isolates of diverse geographic, phylogenetic, and clinical origins. In combination, these findings support a role for DbpA in the immunoprophylaxis of Lyme disease and suggest that DbpA vaccines have the potential to eliminate early-stage B. burgdorferi infections.

The fibrinogen-binding MSCRAMM (clumping factor) of Staphylococcus aureus has a Ca2+-dependent inhibitory site

The clumping factor (ClfA) is a cell surface-associated protein of Staphylococcus aureus that promotes binding of fibrinogen or fibrin to the bacterial cell. Previous studies have shown that ClfA and the platelet integrin alphaIIbbeta3 recognize the same domain at the extreme C terminus of the fibrinogen gamma-chain. alphaIIbbeta3 interaction with this domain is known to occur in close proximity to a Ca2+-binding EF-hand structure in the alpha-subunit. Analysis of the primary structure of ClfA indicated the presence of a potential Ca2+-binding EF-hand-like motif at residues 310-321 within the fibrinogen-binding domain. Deletion mutagenesis and site-directed mutagenesis of this EF-hand in recombinant truncated ClfA proteins (Clf40, residues 40-559; and Clf41, residues 221-559) resulted in a significant reduction of affinity for native fibrinogen and a fibrinogen gamma-chain peptide. Furthermore, Ca2+ (or Mn2+) could inhibit the binding of the fibrinogen gamma-chain peptide to Clf40-(40-559) and the adhesion of S. aureus cells to immobilized fibrinogen with an IC50 of 2-3 mM. In contrast, Mg2+ (or Na+) at similar concentrations had no effect on the ClfA-fibrinogen interaction. Far-UV CD analysis of Clf40-(40-559) and Clf41-(221-559) in the presence of metal ions indicated Ca2+- and Mn2+-induced differences in secondary structure. These data suggest that Ca2+ binds to an inhibitory site(s) within ClfA and induces a conformational change that is incompatible with binding to the C terminus of the gamma-chain of fibrinogen. Mutagenesis studies indicate that the Ca2+-dependent inhibitory site is located within the EF-hand motif at residues 310-321.

Structure of the collagen-binding domain from a Staphylococcus aureus adhesin

The crystal structure of the recombinant 19,000 M(r) binding domain from the Staphylococcus aureus collagen adhesin has been determined at 2 A resolution. The domain fold is a jelly-roll, composed of two antiparallel beta-sheets and two short alpha-helices. Triple-helical collagen model probes were used in a systematic docking search to identify the collagen-binding site. A groove on beta-sheet I exhibited the best surface complementarity to the collagen probes. This site partially overlaps with the peptide sequence previously shown to be critical for collagen binding. Recombinant proteins containing single amino acid mutations designed to disrupt the surface of the putative binding site exhibited significantly lower affinities for collagen. Here we present a structural perspective for the mode of collagen binding by a bacterial surface protein.

Characterization of epiphycan, a small proteoglycan with a leucine-rich repeat core protein

The epiphysis of developing bones is a cartilaginous structure that is eventually replaced by bone during skeletal maturation. We have separated a dermatan sulfate proteoglycan, epiphycan, from decorin and biglycan by using dissociative extraction of bovine fetal epiphyseal cartilage, followed by sequential ion-exchange, gel permeation, hydrophobic, and Zn2+ chelate chromatographic steps. Epiphycan is a member of the small leucine-rich proteoglycan family, contains seven leucine-rich repeats (LRRs), is related to osteoglycin (osteoinductive factor) (Bentz, H., Nathan, R. M., Rosen, D. M., Armstrong, R. M., Thompson, A. Y., Segarini, P. R., Mathews, M. C., Dasch, J., Piez, K. A., and Seyedin, S. M. (1989) J. Biol. Chem. 264, 20805-20810), and appears to be the bovine equivalent of the chick proteoglycan PG-Lb (Shinomura, T., and Kimata, K. (1992) J. Biol. Chem. 267, 1265-1270). The intact proteoglycan had a median size of approximately 133 kDa. The core protein was 46 kDa by electrophoretic analysis, had a calculated size of 34,271 Da, and had two approximately equimolar N termini (APTLES ... and ETYDAT ... ) separated by 11 amino acids. There were at least three O-linked oligosaccharides in the N-terminal region of the protein, based on blank cycles in Edman degradation and corresponding serine or threonine residues in the translated cDNA sequence. The glycosaminoglycans ranged in size from 23 to 34 kDa were more heterogeneous than those in other dermatan sulfate small leucine-rich proteoglycans and were found in the acidic N-terminal region of the protein core, N-terminal to the LRRs. A four-cysteine cluster was present at the N terminus of the LRRs, and a disulfide-bonded cysteine pair was present at the C terminus of the protein core. The seventh LRR and an N-linked oligosaccharide were between the two C-terminal cysteines. An additional potential N-glycosylation site near the C terminus did not appear to be substituted at a significant level.

Characterization of the interaction between the Staphylococcus aureus clumping factor (ClfA) and fibrinogen

The ability of Staphylococcus aureus to adhere to adsorbed fibrinogen and fibrin is believed to be an important step in the initiation of biomaterial and wound-associated infections. In this study, we show that the binding site in fibrinogen for the recently identified S. aureus fibrinogen-binding protein clumping factor (ClfA) is within the C-terminus of the fibrinogen gamma chain. S. aureus Newman cells expressing ClfA adhered to microtitre wells coated with recombinant fibrinogen purified from BHK cells, but did not adhere to wells coated with a purified recombinant fibrinogen variant where the 4 C-terminal residues of the gamma chain were replaced by 20 unrelated residues. In addition, a synthetic peptide corresponding to the 17 C-terminal amino acids of the fibrinogen gamma chain effectively inhibited adherence of ClfA-expressing cells to fibrinogen. In western ligand blots, a recombinant truncated ClfA protein called Clf33 (residues 221-550) recognized intact recombinant fibrinogen gamma chains, but failed to recognize recombinant fibrinogen gamma chains where the 4 C-terminal amino acids were altered by deletion or substitution. Previous studies have shown that the C-terminal domain of fibrinogen gamma chains contains a binding site for the integrin alphaIIb beta3 (glycoprotein gpIIb/IIIa) receptor on platelets [Kloczewiak, M., Timmons, S., Bednarek, M. A., Sakon, M. & Hawiger, J. (1989) Biochemistry 28, 2915-1919; Farrell, D. H., Thiagarajan, P., Chung, D. W. & Davie, E. W. (1992) Proc. Natl. Acad. Sci. USA 89, 10729-10732; Hettasch, J. M., Bolyard, M. G. & Lord, S. T. (1992) Thromb. Haemostasis 68, 701-706]. We now show that Clf33 inhibits ADP-induced, fibrinogen-dependent platelet aggregation in a concentration-dependent manner and inhibits adhesion of platelets to immobilized fibrinogen under fluid shear stress, indicating that the binding sites for the platelet integrin and the staphylococcal adhesin overlap. The interaction between Clf33 and fibrinogen was further characterized using the BIAcore biosensor. When soluble Clf33 was allowed to bind to immobilized fibrinogen, a Kd of 0.51 +/- 0.19 microM was experimentally determined using equilibrium binding data. It was also shown that the synthetic C-terminal gamma-chain peptide effectively inhibited this interaction.

Is the sensitivity of cells for FGF-1 and FGF-2 regulated by cell surface heparan sulfate proteoglycans?

We have examined the importance of cell surface heparan sulfate proteoglycans (HSPG) in fibroblast growth factor (FGF) signaling. 3T3 cells grown under conventional conditions were much more sensitive to FGF-2 compared to FGF-1. However, cells were equally sensitive to FGF-1 and FGF-2 using conditions which reduced the effect of endogenous HSPG. Addition of heparin, or treatment with chlorate, an inhibitor of proteoglycan sulfation, resulted in enhanced or reduced growth factor response, respectively, and eliminated the differences between FGF-1 and FGF-2. HSPGs isolated from trypsin digests of 3T3 cells had a much higher affinity for FGF-2 compared to that for FGF-1 when analyzed by affinity chromatography. Glycosaminoglycan chains or core protein fragments derived from the HSPG failed to show the same high apparent affinity for FGF-2, suggesting that an intact proteoglycan structure was important for the high FGF-2 affinity. Addition of HSPG ectodomains, isolated from cultured 3T3 cells or produced as recombinant molecules, to chlorate-treated cultures of 3T3 cells inhibited the mitogenic activity of FGF-2 and eliminated the effect of heparin as a potentiator of either growth factor. These results support the idea that the cell-associated HSPG is an integral component of the FGF signaling system and in 3T3 cells contributes to the increased sensitivity of these cells to FGF-2 compared to FGF-1. Since isolated ectodomains of HSPG inhibited rather than stimulated the mitogenic response of the FGFs, the proper anchoring of the HSPG in the cell membrane appears to be important for a stimulatory effect.

Heparin-dependent fibroblast growth factor activities: effects of defined heparin oligosaccharides

Heparin and related molecules have been identified as important participants in fibroblast growth factor (FGF) signaling although the mechanisms of action remain unclear. We have used heparin oligosaccharides to examine steps in the signaling process which could be affected by the polysaccharide. Immobilized FGF-1 and FGF-2 bound all sizes of oligosaccharides tested, ranging from tetrasaccharide to decasaccharide, at physiological salt concentration. Each group of oligosaccharide was eluted from the FGF affinity columns in several peaks, and larger oligosaccharides showed higher apparent affinity for the immobilized growth factors compared to the shorter ones. Heparin hexasaccharides were the smallest fragments providing complete protection of FGF-1 and FGF-2 against trypsin digestion. Tetrasaccharides, however, were able to provide partial protection. The requirement of heparin for ligand-receptor interaction was evaluated in receptor binding assays using Sf9 insect cells engineered to overexpress different recombinant FGF receptor (FGFR) species including FGFR1 beta, FGFR1 alpha or FGFR4 at the cell surface. In these assays hexasaccharides were the smallest fragments capable of stimulating FGF-receptor interaction. Over the range of concentrations examined, neither hexasaccharides nor octasaccharides were able to stimulate receptor binding to the level attained by intact heparin. In fact, these oligosaccharides interfered with the ability of intact heparin in promoting FGF-receptor binding. The presence of both stimulatory and inhibitory activities in hexasaccharide and octasaccharide populations could be attributed to structural heterogeneity within the oligosaccharide preparations. However, similar observations were obtained with "highly-sulfated" structurally homogeneous preparations of hexasaccharide and octasaccharide, although these molecules generally had greater stimulatory and less inhibitory activity than their structurally heterogeneous counterparts. Hexasaccharides were found to be the smallest fragments able to potentiate the FGF-1-induced 3T3 cell proliferation while their effect on FGF-2 signaling was less clear. These observations suggest that heparin can modulate FGF-signaling at several stages with different end results.