Major non-collagenous bone protein that binds tightly to hydroxyapatite. Appears to form an integral part of the mineralized matrix. Probably important to cell-matrix interaction. Acts as a cytokine involved in enhancing production of interferon-gamma and interleukin-12 and reducing production of interleukin-10 and is essential in the pathway that leads to type I immunity. Ligand for integrin alpha-V/beta-3. Extensively phosphorylated on serine residues. N-glycosylated. Belongs to the osteopontin family.