A membrane-bound human placental protein kinase activated by endogenous polypeptides

A protein kinase (PPdPK) was purified from plasma membranes of human placenta. Phosphorylation of casein, but not of phosvitin or lactalbumin, by [gamma-32 P]ATP in the presence of PPdPK was stimulated about 10-fold by naturally occurring polypeptides prepared from a variety of sources similar to the procedure of Roberts et al. (Proc. Natl. Acad. Sci. U.S.A. 77, 3494-3498, 1980). The amino acid phosphorylated on casein was serine. According to gel exclusion chromatography the mol.wt. of PPdPK was 95 000. In autoradiograms, following polyacrylamide-gel electrophoresis, the autophosphorylation of PPdPK was greatly enhanced by the polypeptide activators.