Effect of phospholipid unsaturation on protein kinase C activation.

To examine the hypothesis that physical features of the membrane contribute to protein kinase C activation, phosphatidylcholine/phosphatidylserine/diolein (70:25:5) vesicles of defined acyl chain composition were tested for their ability to activate the enzyme. Maximal activation was found to correlate with the mole percent unsaturation in the system. Unsaturation could be provided by either the phosphatidylserine or the phosphatidylcholine component. Vesicles containing 5 mol% diolein but lacking any unsaturation in the phospholipid did not support activity, indicating that acidic head groups alone are not sufficient for activity. The saturated lipid vesicles could be rendered effective but only at very high (25 mol%) concentrations of diolein. The degree of acyl chain unsaturation and the positioning of the double bond had little effect on the activity, suggesting that the effect of the unsaturation is due to some physical property of the lipid rather than to a specific lipid-protein interaction. Addition of cholesterol to both saturated and unsaturated systems indicated that fluidity, as assessed by fluorescence anisotropy, did not correlate with activity. These results suggest that a physical property of the membrane other than fluidity is important for the activation of protein kinase C. A model for protein kinase C activation involving phase separation and/or head group spacing is discussed.