title: Functional role of arginine 375 in transmembrane helix 6 of multidrug resistance protein 4 (MRP4/ABCC4).
abstract: Multidrug resistance protein (MRP) 4 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 human embryonic kidney 293 cells overexpressing these mutants showed significantly reduced methotrexate (MTX) and cGMP transport activity compared with 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 noncompetitive, 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.
authors.affiliation: Department of Pharmacology and Toxicology, Radboud University Nijmegen Medical Centre, Nijmegen Centre for Molecular Life Sciences, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands.
authors.forename: Azza A K
authors.initials: AA
authors.lastname: El-Sheikh
authors.forename: Jeroen J M W
authors.initials: JJ
authors.lastname: van den Heuvel
authors.forename: Elmar
authors.initials: E
authors.lastname: Krieger
authors.forename: Frans G M
authors.initials: FG
authors.lastname: Russel
authors.forename: Jan B
authors.initials: JB
authors.lastname: Koenderink
mesh_terms: D000409:Alanine / Q000378:metabolism; D019943:Amino Acid Substitution; D001120:Arginine / Q000378:metabolism*; D016367:Baculoviridae / Q000235:genetics; D001665:Binding Sites; D001692:Biological Transport / Q000187:drug effects; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry; D022082:Cellular Structures / Q000378:metabolism; D018076:DNA, Complementary / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D019084:Fluorescent Antibody Technique, Indirect; D006157:Guanosine Monophosphate / Q000378:metabolism; D006801:Humans; D007668:Kidney / Q000166:cytology; D007700:Kinetics; D008727:Methotrexate / Q000494:pharmacology; D008958:Models, Molecular; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary
chemical_list: D000409:Alanine; D001120:Arginine; D018076:DNA, Complementary; D006157:Guanosine Monophosphate; D008727:Methotrexate; D027425:ATP-Binding Cassette, Sub-Family C Proteins; C073492:ABCC4 protein, human