Post-translational processing of chicken bone phosphoproteins. Identification of the bone phosphoproteins of embryonic tibia

In order to understand the mechanism of the post-translational processing of bone phosphoproteins in embryonic bone, periosteal bone strips isolated from 12-day-embryonic-chick tibiae were cultured and the bone proteins labelled with Na2H32PO4. Of the total radiolabelled proteins recovered from the medium and bone extracts in the absence of SDS (‘medium’, ‘EDTA extract’ and ‘EDTA/guanidinium chloride extract’), nearly 80% of the radioactivity was found in the EDTA extract. The three major radiolabelled phosphoproteins in the EDTA extract of apparent Mr 68,000, 63,000 and 58,000 reacted with polyclonal as well as monoclonal antibodies raised against ‘32-kDa’ and ‘150-kDa’ bone phosphoproteins which were derived from 14-week-old chicken. Therefore these phosphorylated embryonic proteins are identified as chicken bone phosphoproteins. Judging from their common N-terminal sequences, differences in the patterns obtained by labelling them with several radioisotopes, and slightly different amino acid compositions, these components seem to have been derived from the same original protein by sequential proteolytic cleavage and other processing such as glycosylation and phosphorylation.