Rat liver plasma membranes contain a lipid-dependent protein kinase activity.

The treatment of rat liver plasma membranes with EDTA resulted in substantial release of a protein kinase activity. Using histones as external substrates both phosphatidylserine (PS) and diolein activated the enzyme. The activatory effect of PS but not that of diolein depended on Ca2+. In contrast, the enzyme activity was inhibited by both PS and diolein when the substrate was protamine. The inhibition by PS, unlike by diolein, depended on Ca2+. Data suggest that mobilization of plasma membrane-bound Ca2+ by physiological stimuli may be accompanied by release of the lipid-dependent protein kinase into the cytoplasm.