COLLAGEN-IX - EVIDENCE FOR A STRUCTURAL ASSOCIATION BETWEEN NC4 DOMAINS IN CARTILAGE AND A NOVEL CLEAVAGE SITE IN THE ALPHA-1(IX) CHAIN

Collagen IX, a structural component of the extracellular matrix of con nective tissues, is synthesized as long and short forms which contain or lack, respectively, a 27 kDa non-collagenous (NC) 4 domain at the N -terminus of the alpha 1(IX) chain of the molecule. The long form occu rs in cartilage and developing cornea, but not in vitreous, suggesting a specialized function for the NC4 domain, perhaps by interacting wit h other macromolecules. To test this hypothesis, embryonic chick carti lage was treated with DTSSP, dissociated with bacterial collagenase, a nd the NC4-containing DTSSP-cross-linked protein complexes examined an d purified. Analysis of cartilage extracts using an anti-NC4 antibody, and of purified NC4-containing complexes, identified a predominant NC 4 dimer. A naturally-occurring N-terminal fragment of the alpha 1(IX) chain, whose size is equivalent to the NC4-COL3-NC3 domains of the cha in, was identified. Association of collagen IX molecules via NC4 domai ns and the existence of a cleavage site close to the NC3 domain of the molecule are likely to be of primary importance in the growth and rem odeling processes of cartilage, in health and disease.