title: The packing of the transmembrane segments of human multidrug resistance P-glycoprotein is revealed by disulfide cross-linking analysis.
abstract: Residues from several transmembrane (TM) segments of P-glycoprotein (P-gp) likely form the drug-binding site(s). To determine the organization of the TM segments, pairs of cysteine residues were introduced into the predicted TM segments of a Cys-less P-gp, and the mutant protein was subjected to oxidative cross-linking. In SDS gels, the cross-linked product migrated with a slower mobility than the native protein. The cross-linked products were not detected in the presence of dithiothreitol. Cross-linking was observed in 12 of 125 mutants. The pattern of cross-linking suggested that TM6 is close to TMs 10, 11, and 12, while TM12 is close to TMs 4, 5, and 6. In some mutants the presence of drug substrate colchicine, verapamil, cyclosporin A, or vinblastine either enhanced or inhibited cross-linking. Cross-linking was inhibited in the presence of ATP plus vanadate. These results suggest that the TM segments critical for drug binding must be close to each other and exhibit different conformational changes in response to binding of drug substrate or vanadate trapping of nucleotide. Based on these results, we propose a model for the arrangement of the TM segments.
authors.affiliation: Medical Research Council Group in Membrane Biology, Department of Medicine, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
authors.forename: T W
authors.initials: TW
authors.lastname: Loo
authors.forename: D M
authors.initials: DM
authors.lastname: Clarke
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000737:chemistry*; D000255:Adenosine Triphosphate / Q000378:metabolism; D001665:Binding Sites; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002460:Cell Line; D003078:Colchicine / Q000494:pharmacology; D003432:Cross-Linking Reagents / Q000494:pharmacology; D016572:Cyclosporine / Q000494:pharmacology; D003545:Cysteine / Q000737:chemistry; D003593:Cytoplasm / Q000378:metabolism; D004220:Disulfides; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry*; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary; D013696:Temperature; D014700:Verapamil / Q000494:pharmacology; D014747:Vinblastine / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002121:Calcium Channel Blockers; D003432:Cross-Linking Reagents; D004220:Disulfides; D004791:Enzyme Inhibitors; D008565:Membrane Proteins; D014747:Vinblastine; D016572:Cyclosporine; D000255:Adenosine Triphosphate; D014700:Verapamil; D003545:Cysteine; D003078:Colchicine