OCCURRENCE OF COLLAGEN AND PROTEOGLYCAN FORMS OF TYPE-IX COLLAGEN IN CHICK-EMBRYO CARTILAGE - PRODUCTION AND CHARACTERIZATION OF A COLLAGEN FORM-SPECIFIC ANTIBODY

Type IX collagen from chick embryonic cartilage is a proteoglycan bearing a single chondroitin sulfate chain covalently linked to the alpha-2(IX) polypeptide chain. We have isolated type IX collagen metabolically labeled with [H-3]proline using an antibody to type IX collagen and have found that the molecule is synthesized in two forms, a collagen form (COLIX) and a proteoglycan form (PGIX). In cultured chondrocytes, the two forms of type IX collagen showed a different ability to be deposited in the matrix. We have suggested the possibility that both forms may arise from an alternative substitution of a chondroitin sulfate chain to the NC3 domain of the alpha-2(IX) chain. Based on the reported amino acid sequence at the NC3 domain of alpha-2(IX), we have synthesized undecapeptides containing the sequence around the glycosaminoglycan attachment site of the alpha-2(IX) chain. Antibody against the peptide, which was raised in rabbit, only recognized COLIX and made it possible to distinguish COLIX from PGIX. Evidence shows that this could be due to a difference in antigenicity of the NC3 domain of the alpha-2(IX) chain between COLIX and PGIX caused by the substitution of a chondroitin sulfate chain to the serine residue in this domain. Therefore, this antibody may be useful as a probe for studies on the functions of glycosaminoglycan substitution in type IX collagen.