PROTEIN-KINASE C-MEDIATED PHOSPHORYLATION DOES NOT REGULATE DRUG TRANSPORT BY THE HUMAN MULTIDRUG-RESISTANCE P-GLYCOPROTEIN

P-glycoprotein (P-gp) is an active transporter that can confer multidr ug resistance by pumping cytotoxic drugs out of cells and tumors, P-gp is phosphorylated at several sites in the ''linker'' region, which se parates the two halves of the molecule, To examine the role of phospho rylation in drug transport, we mutated P-gp such that it could no long er be phosphorylated by protein kinase C (PRC), When expressed in yeas t, the ability of the mutant proteins to confer drug resistance, or to mediate [H-3]vinblastine accumulation in secretory vesicles, was indi stinguishable from that of wild type P-gp. A matched pair of mammalian cell lines were generated expressing wild type P-gp and a non-phospho rylatable mutant protein. Mutation of the phosphorylation sites did no t alter P-gp expression or its subcellular localization The transport properties of the mutant and wild type proteins were indistinguishable . Thus, phosphorylation of the linker of P-gp by PRC does not affect t he rate of drug transport, In fight of these data, the use of agents t hat alter PRC activity to reverse multidrug resistance in the clinic s hould be considered with caution.