A Newly Identified Site of Cross-linking between Collagens Ix and Ii: Insights on Molecular Assembly in Cartilage

Introduction The collagenous framework of articular cartilage matures from a covalently cross-linked heteropolymer of types II, IX and XI collagens. Type IX collagen is a quantitatively minor component of the adult tissue but it has an important role in the assembly and function of the matrix. In cartilage, type IX collagen molecules are bound to the surface of type II collagen fibrils and to each other via lysine-derived cross-links (1-3). Mutations in type IX collagen genes can cause multiple epiphyseal dysplasia in which the cartilage collagen fibrils appear abnormal and show evidence of a deficiency in intermolecular crosslinking of type IX collagen (4). The type IX collagen molecule is a heterotrimer of α1(IX), α2(IX), and α3(IX) chains. Each chain contains three relatively short triple helical domains (COL1-3) and four noncollagenous domains (NC1-4). The three chains are linked intramolecularly by disulfide bonds in the NC1 and NC3 domains (5). Analysis of the pepsin-solubilized type II collagen from bovine and human cartilages on SDS-PAGE by Western blotting, we consistently observed a minor fraction of type II collagen in a disulfide bonded complex. By further characterization of the protein, we revealed a previously unrecognized and prominent intermolecular cross-linking site between collagen types II and IX.