title: Drug-stimulated ATPase activity of human P-glycoprotein is blocked by disulfide cross-linking between the nucleotide-binding sites.
abstract: P-glycoprotein (P-gp) is an ATP-dependent drug pump that contains two nucleotide-binding domains (NBDs). Disulfide cross-linking analysis was done to determine if the two NBDs are close to each other. Residues within or close to the Walker A (GNSGCGKS in NDB1 and GSSGCGKS in NBD2) sequences for nucleotide binding were replaced with cysteine, and the mutant P-gps were subjected to oxidative cross-linking. Cross-linking was detected in two mutants, G427C(NBD1)/Cys-1074(NBD2) and L439C(NBD1)/Cys-1074(NBD2), because the cross-linked proteins migrated slower in SDS gels. Mutants G427C(NBD1)/Cys-1074(NBD2) and L439C(NBD1)/Cys-1074(NBD2) retained 10% and 82%, respectively, of the drug-stimulated ATPase activity relative to that of Cys-less P-gp. The cross-linking properties of the more active mutant L439C(NBD1)/Cys-1074(NBD2) were then studied. Cross-linking was reversed by addition of dithiothreitol and could be prevented by pretreatment of the mutant with N-ethylmaleimide. Cross-linking was also inhibited by MgATP, but not by the verapamil. Oxidative cross-linking of mutant L439C(NBD1)/Cys-1074(NBD2) resulted in almost complete inhibition of drug-stimulated ATPase activity. More than 60% of the drug-stimulated ATPase activity, however, was recovered after treatment with dithiothreitol. The results indicate that the two predicted nucleotide-binding sites are close to each other and that cross-linking inhibits ATP hydrolysis.
authors.affiliation: Medical Research Council Group in Membrane Biology, Departments of Medicine and Biochemistry, 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 / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001665:Binding Sites / Q000187:drug effects; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects; D003545:Cysteine / Q000378:metabolism; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism*; D004229:Dithiothreitol / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D006801:Humans; D008954:Models, Biological; D016296:Mutagenesis; D009711:Nucleotides / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism; D017434:Protein Structure, Tertiary; D013439:Sulfhydryl Reagents / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D004220:Disulfides; D009711:Nucleotides; D013439:Sulfhydryl Reagents; D014700:Verapamil; D000251:Adenosine Triphosphatases; D003545:Cysteine; D005033:Ethylmaleimide; D010100:Oxygen; D004229:Dithiothreitol