Mutational Analysis of Ionizable Residues Proximal to the Cytoplasmic Interface of Membrane Spanning Domain 3 of the Multidrug Resistance Protein, MRP1 (ABCC1) GLUTAMATE 1204 IS IMPORTANT FOR BOTH THE EXPRESSION AND CATALYTIC ACTIVITY OF THE TRANSPORTER

Abstract The multidrug resistance protein MRP1 is an ATP-dependent transporter of organic anions and chemotherapeutic agents. A significant number of ionizable amino acids are found in or proximal to the 17 transmembrane (TM) helices of MRP1, and we have investigated 6 of these at the cytoplasmic interface of TM13-17 for their role in MRP1 expression and transport activity. Opposite charge substitutions of TM13 Arg1046 and TM15 Arg1131 did not alter MRP1 expression nor did they substantially affect activity. In contrast, opposite charge substitutions of TM16 Arg1202 and Glu1204 reduced protein expression by >80%; however, MRP1 expression was not affected when Arg1202 and Glu1204 were replaced with neutral or same-charge residues. In addition, organic anion transport levels of the R1202L, R1202G, and R1202K mutants were comparable with wild-type MRP1. In contrast, organic anion transport by E1204L was substantially reduced, whereas transport by E1204D was comparable with wild-type MRP1, with the notable exception of GSH. Opposite charge substitutions of TM16 Arg1197 and TM17 Arg1249 did not affect MRP1 expression but substantially reduced transport. Mutants containing like-charge substitutions of Arg1197 or Arg1249 were also transport-inactive and no longer bound leukotriene C4. In contrast, substrate binding by the transport-compromised E1204L mutant remained intact. Furthermore, vanadate-induced trapping of azido-ADP by E1204L was dramatically increased, indicating that this mutation may cause a partial uncoupling of the catalytic and transport activities of MRP1. Thus, Glu1204 serves a dual role in membrane expression of MRP1 and a step in its catalytic cycle subsequent to initial substrate binding.