Osteopontin - OPN, produced by osteoblasts is a glycosylated phosphoprotein with a high acidic amino acid content a. It is thought that osteopontin may act as a bridge between osteoblasts and the apatite mineral of the bone Osteopontin-K is a kidney protein, similar to osteopontin and probably also involved in cell adhesion . It binds calcium and interacts with the vitronectin receptor, avbeta3 integrin. It has been shown to bind covalently to fibronectin via transglutaminase-catalyzed cross-linking. In vivo, elevation in OPN levels are seen during sepsis and metastatic cancers.