Anticancer drugs, ionophoric peptides, and steroids as substrates of the yeast multidrug transporter Pdr5p.

Pdr5p is the yeast Saccharomyces cerevisiae ATP-binding cassette transporter conferring resistance to several unrelated drugs. Its high overproduction in Pdr1p transcription factor mutants allows us to study the molecular mechanism of multidrug transport and substrate specificity. We have developed new in vivo and in vitro assays of Pdr5p-mediated drug transport. We show that in spite of little sequence homology, and inverted topology in respect to that of mammalian P-glycoproteins, Pdr5p shares with them common substrates. Pdr5p extrudes rhodamines 6G and 123, from intact yeast cells in an energy-dependent manner. Plasma membrane preparations from a Pdr5p-overproducing strain exhibit ATP hydrolysis-dependent, osmotically sensitive rhodamine 6G fluorescence quenching. The quenching is competitively inhibited by micromolar concentrations of many anticancer drugs, such as vinblastine, vincristine, taxol, and verapamil, and of ionophoric peptides as well as steroids. In contrast, other anticancer drugs, like colchicine and some multidrug resistance modifiers, such as quinidine, exert noncompetitive inhibition. Our experimental system opens new possibilities for the analysis of structure-function relationship of multidrug transporter substrates and inhibitors.