The modulation of P-glycoprotein by protein kinase Cα (PKCα) was examined in a baculovirus expression system. PGP was phosphorylated in membrane vesicle preparations in vitro only when coexpressed with PKCα, and phosphorylation was Ca2+-dependent and inhibited by the PKC inhibitor Ro 31-8220. PGP and PKCα were tightly associated in membrane vesicles and were coimmunoprecipitated with antibodies against either PGP or PKCα. Photoaffinity labeling of membrane vesicles with [3H]azidopine indicated that drug binding to PGP was slightly increased in the presence of PKCα. In contrast, PGP ATPase activity was increased by PKCα as well as by verapamil, but only PKC-stimulated activity in the presence of verapamil was inhibited by Ro 31-8220. Mutation of serine-671 to asparagine in the linker region of PGP abolished PKCα-stimulated ATPase activity, and also inhibited to a lesser degree verapamil-stimulated ATPase activity. These results indicate that PKCa in a positive regulator of PGP ATPase activity and suggest that this mechanism may account for the increased multidrug resistance observed in MDR1-expressing cells when PKCa activity is elevated.
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