Structural and functional comparison of type IX collagen-proteoglycan from chicken cartilage and vitreous humor

Type IX collagen-proteoglycan is a major component of hyaline cartilages where it is located on the surface of the collagen fibrils so that a collagenous domain of the molecule (called COL3) and a non-collagenous domain (called NC4) project at periodic distances away from the surface of the fibril. Type IX collagen-proteoglycan is also present on the surface of the collagen fibrils of the adult chicken vitreous but, unlike cartilage, lacks the NC4 domain and possesses a very long chondroitin sulfate chain which provides an extensive coat to the fibril. A monoclonal antibody (called 4D6) is described which will distinguish cartilage from vitreous type IX collagen. To form the epitope for 4D6 two peptides called C2 and C5 derived, respectively, from the alpha 1(IX) and alpha 3(IX) chains are required. Further analysis shows that specificity for 4D6 resides only in the C2 peptide from cartilage and not in C5. These results are entirely consistent with recent evidence that there are two promoters for transcription of the alpha 1(IX) chain which will result in an alpha 1(IX) chain in which the NC4 domain is either present or absent and that expression of these two promoters has tissue specificity (Nishimura, I., Muragaki, Y., and Olsen, B. R. (1989) J. Biol. Chem. 264, 20033-20041). In addition, the function of type IX collagen in cartilage and vitreous may differ with the long chondroitin sulfate chains of vitreous type IX collagen being responsible for the gel-like matrix of this tissue.