In vitro regulation of cartilage matrix assembly by a Mr 54,000 collagen-binding protein.

In cartilage, type II collagen is present as thin, short, randomly oriented fibrils. In vitro, however, type II collagen forms fibrils of large diameter, indicating that additional factors may be involved in the regulation of collagen fibril formation. We have examined extracts of a cartilage-producing tumor for the presence of collagen-binding proteins. In addition to fibronectin and link protein, a Mr 54,000 protein was found to bind to collagen fibrils as well as to native and denatured type II collagen. Immunological studies using antibody against the protein indicate that it is a cartilage matrix protein, not present in bone or in several other tissues. In vitro studies show that the Mr 54,000 protein in combination with cartilage proteoglycan decreases the rate of type II fibril formation and causes the fibrils to be of small diameter (24 +/- 8 nm). These studies indicate that complexes between collagen and proteoglycans mediated by this protein may regulate the assembly of cartilage matrix.