Phosphorylation of extracellular bone and dentine matrix proteins.

Phosphoproteins appear to be involved in several ways in the regulation of the orderly deposition and crystal growth of mineral within the performed collagenous matrix of bone and dentine. The phosphorylation of these proteins is not yet understood. Potential protein kinases were extracted from an osteoblast-like cell line, ROS 17/2.8. The ROS 17/2.8 line was shown to produce a full complement of known kinases. However, neither bone phosphoproteins (BPP) nor dentine phosphophoryn (DPP) could be phosphorylated by the messenger dependent kinases. DPP and dephosphorylated BPP (dBPP) were substrates for a unique messenger independent kinase distinct from casein kinase II, and dDPP was a still better substrate. Thus, BPP and DPP are phosphorylated by a unique kinase or set of kinases which are messenger independent and have very specific substrate sequence requirements.