The multidrug resistance protein, MRP1 (ABCC1), is an ATP-binding cassette transporter that confers resistance to chemotherapeutic agents. MRP1 also mediates transport of organic anions such as leukotriene C-4 (LTC4), 17beta-estradiol 17-(beta-D-glucuronide) (E(2)17betaG), estrone 3-sulfate, methotrexate (MTX), and GSH. We replaced three charged amino acids, Lys(332), His(335), and Asp(336), predicted to be in the sixth transmembrane (TM6) helix of MRP1 with neutral and oppositely charged amino acids and determined the effect on substrate specificity and transport activity. All mutants were expressed in transfected human embryonic kidney cells at levels comparable with wild-type MRP1, and confocal microscopy showed that they were correctly routed to the plasma membrane. Vesicular transport studies revealed that the MRP1-Lys(332) mutants had lost the ability to transport LTC4, and GSH transport was reduced; whereas E(2)17betaG, estrone 3-sulfate, and MTX transport were unaffected. E(2)17betaG transport was not inhibited by LTC4 and could not be photolabeled with [H-3]LTC4, indicating that the MRP1-Lys(332) mutants no longer bound this substrate. Substitutions of MRP1-His(335) also selectively diminished LTC4 transport and photolabeling but to a lesser extent. Kinetic analyses showed that V-max (LTC4) of these mutants was decreased but K-m was unchanged. In contrast to the selective loss of LTC4 transport in the Lys(332) and His(335) mutants, the MRP1-Asp(336) mutants no longer transported LTC4, E(2)17betaG, estrone 3-sulfate, or GSH, and transport of MTX was reduced by >50%. Lys(332), His(335), and Asp(336) of TM6 are predicted to be in the outer leaflet of the membrane and are all capable of forming intrahelical and interhelical ion pairs and hydrogen bonds. The importance of Lys(332) and His(335) in determining substrate specificity and of Asp(336) in overall transport activity suggests that such interactions are critical for the binding and transport of LTC4 and other substrates of MRP1.
