Binds tightly to hydroxyapatite. Appears to form an integral part of the mineralized matrix. Probably important to cell-matrix interaction. Promotes Arg-Gly-Asp-dependent cell attachment (By similarity). Phosphorylated on serine and threonine residues. Sulfated on either Tyr-272 or Tyr-273. It is possible that the segments of clustered carboxyl groups mediate the strong binding to hydroxyapatite.