Secreted Phosphoprotein 2 (SPP2)

Spp24 encodes a 24-kD secreted phosphoprotein that shares sequence homology with members of the cystatin family of thiol protease inhibitors and is expressed in bone, as is cystatin. Thiol proteases are known to be involved in bone turnover, and Spp24 may function as an inhibitor.

The deduced 211-amino acid protein contains a 29-amino acid signal peptide, followed by a cystatin-like domain, a region of phosphorylated serines, and an arginine-rich C-terminal domain. mRNA dot blot analysis detected highest SPP2 expression in adult and fetal liver, with much weaker expression in adult kidney. The array used for the expression study did not contain bone mRNA.