Received for publication November 21, 2007.
Revised July 8, 2008.
Accepted for publication July 8, 2008.

Functional Role of Arginine 375 in Transmembrane Helix 6 of Multidrug Resistance Protein 4 (MRP4/ABCC4)

Abstract

Multidrug resistance protein 4 (MRP4) transports a variety of endogenous and xenobiotic organic anions. MRP4 is widely expressed in the body and specifically localized to the renal apical proximal tubule cell membrane, where it mediates the excretion of these compounds into urine. To characterize the MRP4 substrate-binding site, the amino acids Phe³⁶⁸, Phe³⁶⁹, Glu³⁷⁴, Arg³⁷⁵, and Glu³⁷⁸ of transmembrane helix 6, and Arg⁹⁹⁸ of helix 12, localized in the intracellular half of the central pore, were mutated into the corresponding amino acids of MRP1 and MRP2. Membrane vesicles isolated from HEK293 cells over-expressing these mutants showed significantly reduced methotrexate (MTX) and cyclic guanosine monophosphate (cGMP) transport activity compared to vesicles that expressed wild type MRP4. The only exception was substitution of Arg³⁷⁵ with serine, which had no effect on cGMP transport, but significantly decreased the affinity of MTX. Substitution of the same amino acid with a positively charged lysine returned the MTX affinity to that of the wild type. Furthermore, MTX inhibition of MRP4-mediated cGMP transport was non-competitive and the inhibition constant was increased by introduction of the R375S mutation. A homology model of MRP4 showed that Arg³⁷⁵ and Arg⁹⁹⁸ face right into the central aqueous pore of MRP4. We conclude that positively charged amino acids in transmembrane helices 6 and 12 contribute to the MRP4 substrate-binding pocket.

Key words: MDR/p-Glycoprotein, Structure-activity relationships and modeling, Mutagenesis/Chimeric approaches