THE PRIMARY STRUCTURE OF A BASIC LEUCINE-RICH REPEAT PROTEIN, PRELP, FOUND IN CONNECTIVE TISSUES

We have determined the primary structure of a connective tissue matrix protein from the nucleotide sequence of a clone isolated from a human articular chondrocyte cDNA Library. The major part of the amino acid sequence has also been determined by direct protein sequencing. The tr anslated primary sequence corresponds to 382 amino acid residues, incl uding a 20-residue signal peptide. The molecular mass of the mature pr otein is 41,646 Da. The main part of the protein consists of 10 leucin e-rich repeats ranging in length from 20 to 26 residues, with asparagi ne at position 10 (B-type). The N-terminal part is unusual in that it is basic and rich in arginine and proline. There are four potential N- linked glycosylation sites present. In three of these sites, post-tran slational modifications are likely to be present since Asn was not fou nd by direct protein sequencing. The amino- and carboxyl-terminal part s contain four and two cysteine residues, respectively, probably formi ng disulfide bonds by analogy with the other members of this family. T he protein shows highest identity (36%) to fibromodulin and 33% to bov ine lumican, two other leucine-rich repeat connective tissue proteins. Northern blot analysis showed the presence of an similar to 3.8-kilob ase mRNA in different types of bovine cartilage and cultured osteoblas ts, whereas RNAs isolated from bovine kidney, skin, spleen, thymus, an d trabecular bone and rat calvaria were negative. Human articular chon drocyte and rat chondrosarcoma cell RNAs contained an additional mRNA of similar to 1.6 and 1.8 kilobases, respectively.