Characterization of the human secreted phosphoprotein 24 gene (SPP2) and comparison of the protein sequence in nine species.

Secreted phosphoprotein 24 (spp24) is a member of the cystatin superfamily, which was first identified in cattle as a minor component of cortical bone and subsequently has been identified as a component of the fetuin-mineral complex. We have localized the human SPP2 gene, which encodes spp24 to chromosome 2q37.1, determined its structure and mapped the start of transcription in liver. There is no CAAT or TATA box in the promoter region but potential transcription factor (TF)-binding sites have been identified. The gene comprises eight exons spread over a region of approximately 27 kb with the cystatin-like region of spp24 encoded by four exons, rather than the three-exon structure typical of the genes encoding the archetypal cystatins. A rare single amino acid polymorphism (p.S38F) has been identified within the mature protein and its significance has been assessed by comparing the sequence of human spp24 with that of eight other species.