Type XI collagen is mainly found as a minor constituent in type II-containing fibrils and presents a alpha 1(XI)alpha 2(XI)alpha 3(XI) stoichiometry. This molecule was shown to be partially processed in its intact tissue form, Moreover, alternative splicing has been demonstrated in the variable region of the N-terminal domain of alpha 1(XI) and alpha 2(XI) chains. In this work, the processing of a major intact form of alpha 1(XI) from matrix laid down by chick chondrocytes in culture was identified using N-terminal sequencing and antibodies to synthetic peptides corresponding to the N-terminal propeptide cDNA-derived sequence, The results show that the fully processed form of alpha 1(XI) beans at Gln(254) Of the N-terminal propeptide, seven residues before the end of the proline/arginine-rich protein region encoded by exon I (Zhidkova, N. I., Justice, S. K., and Mayne, R. (1995) J. Biol, Chem, 270, 9486-9493). This sequence is immediately followed by a sequence encoded by exon III, The processing takes place at an Ala-Gln sequence that corresponds to a consensus sequence for procollagen N-proteinase. The antibody raised against a sequence located within the region corresponding to exon IV (anti-P8) fails to recognize this fully processed form of the alpha 1(XI) chain, It recognizes, however, two minor bands of high molecular mass, These results suggest that a major cartilage form of alpha 1(XI) is the product of alternative splicing in which sequences encoded by both exons II and IV are shipped, The presence of a highly acidic subdomain en coded by exon III at the N terminus of the major form of the alpha 1(XI) chain, as predicted by these data, provides potential sites for interaction of collagen XI with other molecules.
