The structure of a 38-kDa leucine-rich protein (chondroadherin) isolated from bovine cartilage

A leucine-rich protein, chondroadherin, has been isolated from dissociative extracts of articular cartilage, and its primary structure has been determined by both direct protein sequencing and DNA sequence analysis of polymerase chain reaction products and cDNA clones. This protein 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 Heinegård, D. (1991) J. Biol. Chem. 266, 20428-20433). It has 337 amino acids and exists in several isoforms. The two major isoforms are a form with a calculated molecular weight of 38,353 and 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 near 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 are 9 cysteines; disulfide bonds have been directly identified between Cys282-Cys324 and Cys284-Cys304. The principal feature of the protein is a series of 10 leucine-rich repeats. The most N-terminal of these repeats contains a cysteine (Cys63) which is not disulfide-bonded and which is difficult to derivatize. It is likely that this free cysteine is involved in structure-stabilizing hydrogen bonding. The mRNA is approximately 1.6 kilobases, of which 511 base pairs is a 3'-untranslated region between the stop codon and the polyadenylation signal. Based on anchored polymerase chain reaction analysis of the mRNA, there is some minor heterogeneity in the position of the 5' end of the message.