Cartilage type IX collagen-proteoglycan contains a large amino-terminal globular domain encoded by multiple exons

Type IX collagen in cartilage consists of molecules composed of three genetically distinct polypeptide subunits. One of the subunits, alpha 2(IX), contains a covalently attached glycosaminoglycan side chain whereas a second subunit, alpha 1(IX), contains a large noncollagenous, amino-terminal domain called NC4. In this report, we describe for the first time the complete primary structure of this noncollagenous domain, based on cloning and sequencing of cDNA and genomic DNA as well as amino acid sequencing of tryptic peptides. Analysis of genomic clones has also allowed determination of the exon structure of NC4. Our results demonstrate that the noncollagenous, amino-terminal domain of alpha 1(IX) chains contains 266 amino acid residues (including the signal peptide) with 5 cysteinyl residues forming two disulfide bridges. The domain is basic with an estimated pI of 9.7, thus supporting the idea that it may participate in ionic interactions with polyanionic glycosaminoglycans in cartilage. Both the sequence and exon structure of the NC4 domain is unique among collagens and there is no obvious homology with the noncollagenous domains of other types of collagen, including the propeptides of fibrillar collagens.