A protein kinase (PPdPK) was purified from plasma membranes of human placenta. Phosphorylation of casein, but not of phosvitin or lactalbumin, by [γ-32P]ATP in the presence of PPdPK was stimulated about 10-fold by naturally occurring polypeptides prepared from avariety of sources similar to the procedure of Roberts et al. (Proc. Natl. Acad. Sci. U.S.A. 77, 3494–3498, 1980). The amino acid phos-phorylated 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.

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