Binding of fibromodulin and decorin to separate sites on fibrillar collagens.

The small proteoglycans, decorin, fibromodulin, biglycan, and lumican, represent a family of structurally related but genetically distinct molecules present in many types of connective tissues. Fibromodulin and decorin interact with collagens I and II (Hedbom, E., and Heinegård, D. (1989) J. Biol. Chem. 264, 6898-6905). These interactions have been characterized further by using native radiolabeled components from fibroblast cultures and nonlabeled proteoglycans purified from guanidine hydrochloride extracts of bovine tendon. Binding of metabolically labeled macromolecules to collagen I was measured in an assay based on precipitation of collagen fibrils formed in vitro. Among a large number of secreted fibroblast products, decorin and fibromodulin represented the vast majority of the collagen binding components. These molecules showed poor binding to denatured collagen, in contrast to fibronectin, which was also present in the medium. Decorin and fibromodulin bind to different sites on collagen I fibrils, since the binding of either radiolabeled component could be competed for only by the corresponding nonlabeled proteoglycan. Similarly, these proteoglycans showed binding to separate sites on collagen II. Binding of isolated fibromodulin and decorin to collagens in solution was measured in a solid-phase inhibition assay. Each of the proteoglycans interacted with triple helical molecules, but not with denatured collagen chain constituents or fragments. For fibromodulin, the data indicated an average of one binding site per collagen I molecule (Kd = 9.9 nM). The data on decorin indicated additional interactions, some apparently mediated by the dermatan sulfate side chain. The results suggest that the small proteoglycans bind to distinct triple helical sites, apparently differing from several other similar structures within each collagen molecule.