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.Collagen polypeptides are largely composed of repeats of the GPX tripeptide and associate to form triple-helical monomers. These monomers combine into macroscopic fibers. Prokaryotic and eukaryotic cells bind collagen via receptors on their cell surfaces (1-6). We now hypothesize that to accommodate such an unusually shaped ligand, the collagen-binding surface proteins of these cells must adopt an atypical binding-site structure.Bacterial pathogens utilize this interaction as a means of adherence to collagenous host tissues. Some Staphylococcus aureus strains express an adhesin, Cna, 1 of the MSCRAMM class that binds collagen (1, 7-13). Cna from S. aureus FDA 574 is depicted in Fig. 1a: it contains two major domains, A and B, in addition to features characteristic of cell-surface proteins on Gram-positive bacteria (11). The collagen-binding site has been localized within the Cna A domain (12). Binding analyses demonstrate that (i) a synthetic peptide mimicking a short sequence of the A domain can inhibit collagen binding to S. aureus (8); (ii) the A domain/collagen interaction involves more than one affinity class and multiple sites of contact within a single collagen molecule (8, 10); and (iii) the B domain does not alter the collagen binding ability of the A domain (13).The crystal structure of a truncated form of the Cna A domain reveals a binding-site "trench" on one face of the protein. In molecular modeling studies, this trench was found to accommodate a triple-helical peptide that mimics the collagen structure. Symersky et al. (7) noted that this trench complemented well the structure of a collagen triple helix and binding studies of site-specific mutants of the S. aureus Cna truncate revealed that (i) no single residue or area within the trench was responsible for collagen binding, but rather, a number of contacts contributed to the protein/collagen interaction and (ii) this binding-domain truncate bound to multiple sites along a collagen molecule. The affinity of Cna for an individual site within collagen may be the consequence of the number of "good" and "bad" contacts within the binding trench.Binding of eukaryotic cells to collagen serves not only as a mechanism of tissue adherence, but also may induce a complex signaling cascade in the cell. Attachment of eukaryotic cells to the extracellular matrix is primarily medi...

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 Zn 2؉ Decorin, a small chondroitin/dermatan sulfate proteoglycan, is found in the extracellular matrix of a variety of tissues such as skin (1-3), cartilage (4, 5), and bone (6, 7). This proteoglycan is composed of a 40-kDa core protein and one glycosaminoglycan chain attached to a serine residue in the N-terminal part of the protein. The decorin core protein is dominated by a central region composed of 10 leucine-rich repeat units. Each unit contains 21-26 amino acid residues and is proposed to adopt a characteristic ␣-helix/␤-sheet folding pattern (8, 9). The C-and N-terminal regions of the core protein are believed to form globular structures stabilized by disulfide bonds between sets of cysteine residues. Several proteoglycans have core proteins of similar size and structural organization. These related molecules are considered to form a family called small leucine-rich proteoglycans (SLRP) 1 (9, 10). The family of SLRPs include decorin, biglycan, and epiphycan, all of which contain chondroitin/dermatan sulfate chains attached to the N-terminal domain of the core protein and fibromodulin, lumican, keratocan, PRELP, and osteoglycin which often have keratan sulfate linked to asparagine residues in the central region of the core protein.The extracellular matrix glycoprotein chondroadherin has a structural organization similar to the core proteins of the SLRPs but has not been shown to be substituted with glycosaminoglycan chains (11). The biological importance of the different SLRPs is unclear. In vitro binding studies have shown that decorin, biglycan, and fibromodulin can interact with several types of collagen (12-16) and different SLRPs are believed to be important regulators of collagen fibrillogenesis. In support of this hypothesis, a decorin-deficient mouse was found to have fragile skin with an abnormal organization of collagen fibers (17). The phenotype appears to be largely restricted to the skin, perhaps suggesting that other SLRPs have similar functions and may fulfill this role in other collagenous tissues.In fact, a lumican-deficient mouse also exhibited abnormal collagen fibers both in the skin and cornea (18). Decorin may also affect the production of other extracellular matrix components by regulating the activity of transforming growth factor-␤ (19,20). Additionally, decorin can modulate the interactions of matrix molecules such as fibronectin with cells (21-23). These observations suggest that decorin and perhaps other SLRPs regulate at several levels the production and assembly of the extracellular matrix and hence the remodeling of connective tissue.Zinc, a divalent cation, is one of the essential trace elements for eukaryotic org...

We have previously shown that decorin, a member of the small leucine-rich proteoglycan family of extracellular matrix proteoglycans/glycoproteins is a Zn(2+) metalloprotein at physiological Zn(2+) concentrations (Yang, V. W-C., LaBrenz, S. R., Rosenberg, L. C., McQuillan, D., and Höök, M. (1999) J. Biol. Chem. 274, 12454-12460). We now report that the decorin proteoglycan binds fibrinogen in the presence of Zn(2+). The fibrinogen-binding site is located in the N-terminal domain of the decorin core protein and a 45-amino acid peptide representing this domain binds to the fibrinogen D fragment with an apparent K(D) of 1.7 x 10(-6) m, as determined from fluorescence polarization data. Furthermore, we show that Zn(2+) promotes the self-association of decorin. The N-terminal domain of the core protein also mediates this activity. The results of solid-phase binding assays and gel filtration chromatography suggest that the N-terminal domain of decorin, when present at low micromolar concentrations, forms an oligomer in a Zn(2+)-dependent manner. Thus, Zn(2+) appears to play a pivotal role in the interactions and biological function of decorin.

. We now report that the decorin-fibrinogen interaction alters the assembly, structure, and clearance of fibrin fibers. Relative to fibrinogen, substoichiometric amounts of decorin core protein modulated clotting, whereas an excess of an active decorin peptide was necessary for similar activity. These concentration-dependent effects suggest that decorin bound to the D regions sterically modulates fibrin assembly. Scanning electron microscopy images of fibrin clotted in the presence of increasing concentrations of decorin core protein showed progressively decreasing fiber diameter. The sequestration of Zn 2؉ ions from the N-terminal fibrinogenbinding region abrogated decorin incorporation into the fibrin network. Compared with linear thicker fibrin fibers, the curving thin fibers formed with decorin underwent accelerated tissuetype plasminogen activator-dependent fibrinolysis. Collectively, these data demonstrate that decorin can regulate fibrin organization and reveal a novel mechanism by which extracellular matrix components can participate in hemostasis, thrombosis, and wound repair.