Osteopontin (OPN, also designated Bone Sialoprotein 1, Urinary Stone Protein, spp-1, eta-1, nephropontin, uropontin) is an extracellular matrix cell adhesion phosphoglycoprotein. It is secreted and binds hydroxyapatite with high affinity. The osteoclast vitronectin receptor is found in the cell membrane and may be involved in the binding to this protein. While OPN is a major product of osteoblasts, it is also synthesized by brain and kidney cells. OPNs isolated from or secreted by various tissues have molecular weights between 44 and 75 kDa, due to post-translational modifications. OPN functions as a substrate for transglutaminase and is involved in cell adhesion, chemoattraction and immunomodulation.
