From a study to understand the mechanism of covalent interaction between
collagen types II and IX, we present experimental evidence for a previously unrecognized molecular site of cross-linking. The location relative to previously defined cross-linking sites predicts a specific manner of interaction and folding of
collagen IX on the surface of nascent
collagen II fibrils. The initial evidence came from Western blot analysis of
type IX collagen extracted by
pepsin from fetal human cartilage, which showed a molecular species that had properties indicating an adduct between the alpha1(II) chain and the C-terminal domain (COL1) of
type IX collagen. A similar component was isolated from bovine cartilage in sufficient quantity to confirm this identity by N-terminal sequence analysis. Using an antibody that recognized the putative cross-linking sequence at the C terminus of the alpha1(IX) chain, cross-linked
peptides were isolated by immunoaffinity chromatography from proteolytic digests of human
cartilage collagen. They were characterized by immunochemistry, N-terminal sequence analysis, and mass spectrometry. The results establish a link between a
lysine near the C terminus (in the NC1 domain) of alpha1(IX) and the known cross-linking
lysine at residue 930 of the alpha1(II) triple helix. This cross-link is speculated to form early in the process of interaction between
collagen IX molecules and
collagen II
polymers. A model of molecular folding and further cross-linking is predicted that can spatially accommodate the formation of all six known cross-linking interactions to the
collagen IX molecule on a fibril surface. Of particular
biological significance, this model can accommodate potential interfibrillar as well as intrafibrillar links between the
collagen IX molecules themselves, so providing a mechanism whereby
collagen IX could stabilize a
collagen fibril network.