THE STRUCTURE OF A 38-KDA LEUCINE-RICH PROTEIN (CHONDROADHERIN) ISOLATED FROM BOVINE CARTILAGE

A leucine-rich protein, chondroadherin, has been isolated from dissoci ative extracts of articular cartilage, and its primary structure has b een determined by both direct protein sequencing and DNA sequence anal ysis of polymerase chain reaction products and cDNA clones. This prote in is identical to the 36-kDa protein which was isolated by Larsson et al. (Larsson, T., Sommarin, Y., Paulsson, M., Antonsson, P., Hedbom, E., Wendel, M,, and Heinegard, D. (1991) J. Biol. Chem. 266, 20428-204 33). It has 337 amino acids and exists in several isoforms. The two ma jor isoforms are a form with a calculated molecular weight of 38,353 a nd a pI of 9.76 and a smaller form with a calculated molecular weight of 37,304 and a pI of 9.5. The two isoforms result from a cleavage nea r the C terminus. A further level of heterogeneity is found in that an extra alanine can be found prior to the N-terminal cysteine. There ar e 9 cysteines; disulfide bonds have been directly identified between C ys(282)-Cys(324) and Cys(284)- Cys(304). The principal feature of the protein is a series of 10 leucine-rich repeats. The most N-terminal of these repeats contains a cysteine (Cys(63)) which is not disulfide-bo nded and which is difficult to derivatize. It is likely that this free cysteine is involved in structure-stabilizing hydrogen bonding. The m RNA is approximately 1.6 kilobases, of which 511 base pairs is a 3'-un translated region between the stop codon and the polyadenylation signa l. Based on anchored polymerase chain reaction analysis of the mRNA, t here is some minor heterogeneity in the position of the 5' end of the message.