THE MODULATION OF PROTEIN-KINASE-C ACTIVITY BY MEMBRANE LIPID BILAYERSTRUCTURE

The hypothesis that protein kinase C (PKC) activity is sensitive to ph ospholipid head group interactions was tested using lipid bilayers of defined composition with PKC purified from rat brain. The head group i nteractions were modulated by varying phosphatidylcholine cis-unsatura tion, vesicle curvature, and by the addition of phosphatidylethanolami ne and cholesterol. With unilamellar vesicles (including 20 mol % brai n phosphatidylserine), increased phosphatidylcholine unsaturation pote ntiated basal and phorbol ester stimulated PKC activity. By contrast, in the presence of phosphatidylethanolamine, the activity decreased wi th increasing phosphatidylcholine unsaturation. Weakening phospholipid head group interactions spaces the head group region and increases in terstitial water, and this effect was assessed from its effect on the fluorescence intensity of the phospholipid-labeled fluorophore 1-palmi toyl-2-N(4- 2-oxa-1,3-diazole)aminohexanoylphosphatidylcholine (C-6-NB D-PC). When the PKC activities with vesicles of varying phosphatidylch oline unsaturation, with and without phosphatidylethanolamine, were pl otted as a function of the fluorescence intensity of C6NBD-PC-labeIed vesicles, a biphasic profile was obtained, which had an optimum value of intensity, relating to head group spacing, that corresponded to a m aximal enzyme activity. A similar biphasic curve was also found when P KC activities were plotted as a function of published bilayer intrinsi c curvature x-ray diffraction data, a parameter closely related to hea d group spacing. By contrast, no simple relationship was evident betwe en PKC activity and 1,6-diphenyl-1,3,5-hexatriene anisotropy, taken as a measure of Lipid order or fluidity. Therefore, increasing the level of phosphatidylcholine unsaturation, phosphatidylethanolamine, or cho lesterol either potentiates or attenuates PKC activity, dependent on w hether the initial condition is above or below its optimum.