The cartilage collagens: structural and metabolic studies

The molecular sites of crosslinking between types II and IX collagens of bovine articular cartilage have been defined. All 3 chains of the type IX molecule, alpha 1 (IX), alpha 2 (IX) and alpha 3 (IX), contain an hydroxylysine close to the N-terminus of the central COL2 triple helical domain that can crosslink to an hydroxylysine aldehyde in an alpha 1 (II) N-telopeptide. One chain, alpha 3 (IX), also contains a triple helical hydroxylysine farther into COL2 that crosslinks specifically to an alpha 1 (II)C-telopeptide. The metalloprotease, stromelysin, is capable of depolymerizing the type II-type IX complex by bisecting the type IX collagen molecule and cleaving telopeptides from type II collagen. We discuss the significance of these findings in understanding the structure, growth and remodeling of the heterotypic collagen network of articular cartilage.