The Interaction between Protein Kinase C and Lipid Cofactors Studied by Simultaneous Observation of Lipid and Protein Fluorescence (∗)

The interaction of protein kinase C (PKC) with lipids was probed by a dual approach. Pyrene-labeled lipid analogues of diacylglycerol, phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and phosphatidylcholine (PC) were used both as acceptors of tryptophan excitation energy of PKC and as membrane probes for intra- and intermolecular lipid chain collisions by measuring the ratio of excimer-to-monomer fluorescence intensity (EM). Both in micelles of polyoxyethylene 9-lauryl ether and in dioleoyl-PC vesicles, interaction of PKC with monopyrenyl PS (pyr-PS) in the absence of calcium resulted in a relatively slow decrease of the EM value. This effect on the lipid dynamics was accompanied by quenching of the tryptophan fluorescence of PKC. Addition of calcium resulted in a rapid further decrease of the EM ratio of pyr-PS and in additional quenching of the tryptophan fluorescence. When 4 mol % of pyr-PS was replaced by 0.5 mol % of dipyrenyl-labeled diacylglycerol a decrease of the intramolecular excimer formation rate and tryptophan fluorescence could only be detected in the presence of calcium and PS. Strong binding was also observed with dipyrenyl-labeled PIP (dipyr-PIP), but not with the other dipyrenyl-labeled lipids: PI, PS, or PC. In addition, the EM ratios of dipyr-PIP were not affected by phorbol 12-myristate 13-acetate, indicating that phorbol 12-myristate 13-acetate and dipyr-PIP can bind simultaneously to PKC.