Human bone sialoprotein. Deduced protein sequence and chromosomal localization

Abstract

A cDNA encoding the human bone sialoprotein was isolated from a lambda Zap expression library (made from cultured human bone cell poly(A)+ RNA) using radiolabeled rat bone sialoprotein cDNA (Oldberg, A., and Heinegard, D. (1988) J. Biol. Chem. 263, 19430-19432) as a probe. A 5' 1-kilobase EcoRI fragment of the purified 3-kilobase clone was sequenced and found to contain the entire protein-encoding region. The deduced protein sequence revealed a 317-amino acid protein (34,982 Da) containing a 16-amino acid hydrophobic signal sequence and a 33,352-Da protein destined to undergo extensive post-translational modifications before being secreted from the cell. A comparison of the human and rat protein sequences showed extensive (greater than 70%) amino acid identities including the Arg-Gly-Asp (RGD) tripeptide thought to confer the cell attachment activity observed previously for this protein. Also conserved were three regions rich in acidic amino acids and three regions rich in tyrosine. While all three tyrosine-rich regions appear to be composed of a nominal repeat structure, only the two carboxyl-terminal regions that flank the RGD sequence fit all three of the requirements for extensive tyrosine sulfation. Interestingly, human bone sialoprotein, whose final secreted product is approximately 50% carbohydrate, contains no cystines. Northern analysis showed that while bone cells are the major source of bone sialoprotein message production, other tissues may contain trace amounts of this message. Southern hybridization of DNA from human-rodent somatic cell hybrids that have segregated human chromosomes indicated that the gene is located on human chromosome 4. The human bone sialoprotein gene is a single copy gene unlikely to exceed 11.1 kilobases in length. No restriction fragment length polymorphisms were observed with 12 different restriction enzymes in 10 normal individuals.