PHOSPHATIDYLCHOLINE TRANSLOCASE - A PHYSIOLOGICAL-ROLE FOR THE MDR2 GENE

P-glycoproteins (P-gps) encoded by the mouse mdr2 and mdr3 genes were expressed in secretory vesicles (SVs) from the yeast mutant sec6-4, an d their capacity to function as a lipid translocase/flippase was teste d. An assay that uses a fluorescent phosphatidylcholine (PC) analog wa s developed to quantitate asymmetric lipid distribution in the outer a nd inner leaflets of the lipid bilayer of these vesicles. Mdr2 express ion in SVs caused a time- and temperature-dependent enhancement of PC translocation to the inner leaflet of the membrane. The Mdr2-mediated effect was specific since expression of Mdr3 in these vesicles was wit hout effect on the membrane distribution of PC. Increased Mdr2-mediate d PC translocation was strictly ATP and Mg2+ dependent, was abrogated by the ATPase inhibitor vanadate and the P-gp modulator verapamil, but was insensitive to the presence of excess of the multidrug resistance drugs colchicine and vinblastine.