Mechanism of activation of protein kinase C: roles of diolein and phosphatidylserine.

We studied the roles of lipid concentration, phosphatidylserine (PS), and diolein (DG) contents, as well as Ca2+ concentration, on the partitioning of protein kinase C (PKC) between aqueous and membrane environments as well as the relationship of this partitioning to the activation of the enzyme. Physiological concentrations of 1 mol % DG increased the apparent binding constant of PKC to the 3:1 PC/PS membrane 500 times. This increase was proportional to the mol % DG. Over 50% PKC was bound to that membrane at micromolar concentrations of Ca2+ and physiologically relevant total concentration of lipid only when 1 mol % DG was included. PKC bound either to PS alone or to PS and DG was enzymatically competent; however, the rate of phosphorylation was doubled in the presence of 1 mol % diolein. The dependence of PKC binding on the mol % PS was highly sigmoidal. The Hill coefficient was in the range of 4-6, with the higher values found at the lower lipid concentrations. These results suggest that the observed apparent cooperativity is due, at least in part, to the change in dimensionality when PKC binds to the membrane.