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 ␣1(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 ␣1(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 ␣1(IX) and the known cross-linking lysine at residue 930 of the ␣1(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.Collagen type IX is a member of the fibril-associated collagen with interrupted triple helix (FACIT) 1 family of collagen molecules, which share homologous domains and are all believed to function in the extracellular matrix in association with collagen fibril surfaces (1, 2). Collagen IX is unique among the FACIT molecules in binding covalently to fibril surfaces. The cross-links are formed through the lysyl oxidase mechanism (3-7). Collagen IX is found mostly in cartilages, but it also occurs in the eye (vitreum (8, 9) and avian cornea (8)), ear (tectorial membrane (10)), and intervertebral disc (11, 12), always in co-existence with type II collagen. A special role for collagen IX in the organization of type II collagen fibril networks has apparently evolved, which for articular cartilage seems to be essential for the long term normal functioning of joints. The evidence for this comes from the study of genetic defects. Mutations in all three collagen IX genes have been linked to a chondrodysplasia syndrome (multiple epiphyseal dysplasia), which features early onset osteoarthritis (13).All three chains of the vertebrate collagen IX molecule, ␣1(IX), ␣2(IX), and ␣3(IX)...