The ability of Staphylococcus aureus to adhere to adsorbed fibrinogen and fibrin is believed to be an important step in the initiation of bioinaterial 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 y 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 y chain were replaced by 20 unrelated residues. In addition, a synthetic peptide corresponding to the 17 C-terminal amino acids of the fibrinogen y 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 y chains, but failed to recognize recombinant fibrinogen y 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 y chains contains a binding site for the integrin . We now show that Clf33 inhibits ADPinduced, 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 t-0.19 pM was experimentally determined using equilibrium binding data. It was also shown that the synthetic C-terminal y-chain peptide effectively inhibited this interaction.

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.

Bacterial adherence to host tissue involves specific microbial surface adhesins of which a subfamily termed microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) specifically recognize extracellular matrix components. We now report on the biophysical characterization of recombinant fibronectin binding MSCRAMMs originating from several different species of Gram-positive bacteria. The far-UV CD spectra (190 -250 nm) of recombinant forms of the ligand binding domain of the MSCRAMMs, in a phosphate-buffered saline solution at neutral pH, were characteristic of a protein containing little or no regular secondary structure. The intrinsic viscosity of this domain was found to be the same in the presence or absence of 6 M guanidine hydrochloride, indicating that the native and denatured conformations are indistinguishable. On addition of fibronectin NH 2 terminus as ligand to the recombinant adhesin there is a large change in the resulting far-UV CD difference spectra. At a 4.9 M excess of the NH 2 terminus the difference spectra shifted to what was predominately a ␤-sheet conformation, as judged by comparison with model far-UV CD spectra. The fibronectin NH 2 -terminal domain undergoes a minute but reproducible blue-shift of its intrinsic tryptophan fluorescence on addition of rFNBD-A, which contains no tryptophan residues. Since this result indicates that there is no large change in the environment of the tryptophan residues of the NH 2 terminus on binding, the large shift in secondary structure observed by CD analysis is attributed to induction of a predominately ␤-sheet secondary structure in the adhesin on binding to fibronectin NH 2 terminus.Many pathogenic bacteria have been shown to specifically recognize and bind to various components of the extracellular matrix in an interaction which appears to represent a host tissue colonization mechanism. This adherence involves a group of bacterial proteins termed MSCRAMMs 1 (microbial surface components recognizing adhesive matrix molecules) (1, 2). A number of Gram-positive bacteria have been shown to express fibronectin (Fn) binding MSCRAMMs, and in some cases these proteins have been isolated and the corresponding genes cloned and characterized. The primary Fn binding sites in these MSCRAMMs have been localized to domains present in most Fn binding adhesins. This domain is composed of a unit of 37-48 amino acids, repeated three or four times (Fig. 1A). The repeat regions have been overexpressed as recombinant fusion proteins in Escherichia coli where the recombinant Fn binding domains (rFNBD) are linked to a stretch of histidine residues which are utilized for affinity purification of the rFNBD proteins. These proteins have been designated as rFNBD-D, rFNBD-A, rFNBD-B, and rFNBD-P, respectively (Fig. 1A). The rFNBDs were found to exhibit similar binding kinetics and dissociation constants; for example, the dissociation constants of the four recombinant proteins binding to porcine Fn was determined by biosensor analysis to be in the low nM range...

Many parasitic bacteria express fibronectin binding proteins that are located on the cell surface. These proteins may act as adhesins and mediate the adherence of the microorganisms to fibronectin-containing host tissues. The ligand binding sites in the fibronectin receptor proteins from Gram-positive bacteria are composed of unique 37-48 amino acid long motifs that are repeated 3-4 times. We have now expressed the ligand binding sites of fibronectin receptors from Staphylococcus aureus, Streptococcus dysgalactiae (two receptors), and Streptococcus pyogenes as recombinant proteins. The purified recombinant proteins have the expected molecular weights as indicated by electrospray mass spectroscopy although they migrate abnormally on SDS-PAGE. Each recombinant protein effectively inhibited the binding of 125I-labeled intact fibronectin or the N-terminal fibronectin domain to Staphylococcus aureus, Streptococcus dysgalactiae, and Streptococcus pyogenes. The relative inhibitory potency of the different recombinant proteins was similar for all target bacteria and is reflected in their relative affinities for fibronectin. Synthetic peptides corresponding to the repeat units of the ligand binding site of the fibronectin receptor proteins were shown to inhibit the binding of the N-terminal fibronectin fragment to Streptococcus pyogenes cells. Together with amino acid sequence comparison, these data demonstrate that the repeat motif of the fibronectin receptor of Streptococcus pyogenes conforms to the consensus sequence previously reported for the Staphylococcus aureus receptor and to one of the Streptococcus dysgalactiae receptors (McGavin et al., 1993).

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We have identified a discrete collagen-binding site within the Staphylococcus aureus collagen adhesin that is located in a region between amino acids Asp209 and Tyr233. Polyclonal antibodies raised against a recombinant form of the collagen adhesin inhibited the binding of collagen type II to S. aureus. When overlapping synthetic peptides mimicking segments of the adhesin fragment were tested for their ability to neutralize the inhibitory activity of the antibody only one peptide, CBD4 was found to be active. CBD4 bound directly to collagen and at high concentrations inhibited the binding of collagen to S. aureus. A synthetic peptide derivative of CBD4 lacking 2 carboxyl-terminal residues (Asn232, Tyr233) had no inhibitory activity. The importance of these residues for collagen binding was confirmed by biospecific interaction analysis. Mutant adhesin proteins N232-->A and Y233-->A exhibited dramatic changes in collagen binding activity. The dominant dissociation rate for the binding of mutant adhesin protein N232-->A to immobilized collagen II decreased almost 10-fold, while the Y233-->A and the double mutant exhibited even more significant decreases in affinity and apparent binding ratio when compared to the wild type protein.

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