Overexpression of the multidrug resistance-associated protein (MRP1) causes multidrug resistance in cultured cells. MRP1 transports a large number of glutathione, glucuronide, and sulfate-conjugated organic anions by an ATP-dependent efflux mechanism. Six other MRP proteins exist (MRP2-7), and mutations in some of these genes cause major pathological conditions in humans. A detailed characterization of the structure and mechanism of action of these proteins requires an efficient expression system from which large amounts of active protein can be obtained. We report the expression of a recombinant MRP1 in the methylotrophic yeast Pichia pastoris. The protein is expressed in the membrane fraction of these cells, as a stable and underglycosylated 165 kDa peptide. Expression levels are very high, and 30 times superior to those seen in multidrug-resistant HeLa/MRP1 transfectants. MRP1 expressed in P. pastoris binds 8-azido[alpha-(32)P]ATP in a Mg(2+)-dependent and EDTA-sensitive fashion, which can be competed by a molar excess of ADP and ATP. Under hydrolysis conditions (at 37 degrees C), orthovanadate induces trapping of the 8-azido[alpha-(32)P]nucleotide in MRP1, which can be further modulated by known MRP1 ligands. MRP1 is also labeled by a photoactive analogue of rhodamine 123 (IAARh123) in P. pastoris/MRP1 membranes, and this can be competed by known MRP1 ligands. Finally, MRP1-positive membrane vesicles show ATP-dependent uptake of LTC(4). Thus, MRP1 expressed in P. pastoris is active and shows characteristics of MRP1 expressed in mammalian cells, including drug binding, ligand-modulated formation of the MRP1-MgADP-P(i) intermediate (ATPase activity), and ATP-dependent substrate transport. The successful expression of catalytically active and transport-competent MRP1 in P. pastoris should greatly facilitate the efficient production and isolation of the wild type or inactive mutants of MRP1, or of other MRP proteins for structural and functional characterization.