POSTTRANSLATIONAL PROCESSING OF CHICKEN BONE PHOSPHOPROTEINS - IDENTIFICATION OF BONE (PHOSPHO)PROTEIN KINASE

We have detected a protein kinase which phosphorylates bone phosphoproteins (BPPs) in the detergent extract of the membranous fractions in the periosteal bone strips of 12-day-embryonic-chick tibia. This enzyme, tentatively named BPP kinase, has a catalytic subunit of M(r) ~ 39 000, utilizes GTP as well as ATP as a phospho-group donor, is inhibited by 2,3-bisphosphoglycerate and heparin, and is therefore similar to casein kinase II. The enzyme can phosphorylate dephosphorylated proteins such as casein, phosvitin and chicken BPPs, but the last-named are preferred substrates. The in vitro-phosphorylation-assay products of this enzyme in the extract were indistinguishable on an SDS/polyacrylamide gel from the major [32P]phosphoprotein metabolically labelled in the embryonic-chick bone tissue. The regulatory mechanisms of the phosporylation process of BPPs by BPP kinase as well as the potential role of this enzyme in mineralization are discussed.