Mutations in the nucleotide-binding sites of P-glycoprotein that affect substrate specificity modulate substrate-induced adenosine triphosphatase activity.
The amino- and carboxy-terminal nucleotide-binding domains (NBD1 and NBD2) of P-glycoprotein (P-gp) share over 80% sequence identity. Almost all of NBD1 can be exchanged by corresponding NBD2 segments with no significant loss of function, except for a small segment around the Walker B motif. Within this segment, we identified two sets of residues [ERGA --> DKGT (522-525) and T578C] that, when replaced by their NBD2 counterparts, cause dramatic alterations of the substrate specificity of the protein [Beaudet, L., and Gros, P. (1995) J. Biol. Chem. 270, 17159-17170]. We wished to gain insight into the molecular basis of this defect. For this, we overexpressed the wild-type mouse Mdr3 and variants bearing single or double mutations at these positions in the yeast Pichia pastoris. P-gp-specific ATPase activity was measured in yeast plasma membrane preparations after detergent solubilization and reconstitution in Escherichia coli proteoliposomes. P-gp proteoliposomes from P. pastoris showed a strong verapamil- and valinomycin-stimulated ATPase activity, with characteristics (KM, Vmax) similar to those measured in mammalian cells. Mutations did not appear to affect the KM for Mg2+ATP ( approximately 0.4 mM), but maximum velocity (Vmax) of the drug-stimulated ATPase activity was severely affected in a substrate/modulator-specific fashion. Indeed, all mutants showed complete loss of verapamil-induced ATPase, while all retained at least some degree of valinomycin-induced ATPase activity. Photolabeling studies with [125I]iodoarylazidoprazosin, including competition with MDR drugs and modulators, suggested that drug binding was not affected in the mutants. The altered drug resistance profiles of the ERGA --> DKGT(522-525) and T578C mutants in vivo, together with the observed alterations in substrate-induced ATPase activity of these proteins, suggest that the residues involved may form part of a signal pathway between the membrane regions (substrate binding) and the ATP binding sites.
Beaudet
L
L
Department of Biochemistry, McGill University, Montréal, Québec, Canada.
Urbatsch
I L
IL
Gros
P
P
D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000096:biosynthesis / Q000235:genetics* / Q000378:metabolism*; D018528:ATP-Binding Cassette Transporters / Q000096:biosynthesis / Q000235:genetics; D000251:Adenosine Triphosphatases / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001386:Azides / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics; D007457:Iodine Radioisotopes; D051379:Mice; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D019920:Photoaffinity Labels; D010843:Pichia / Q000201:enzymology / Q000235:genetics; D011224:Prazosin / Q000031:analogs & derivatives / Q000378:metabolism; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D013379:Substrate Specificity / Q000235:genetics
D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D018528:ATP-Binding Cassette Transporters; D001386:Azides; D007457:Iodine Radioisotopes; D019920:Photoaffinity Labels; D011994:Recombinant Proteins; D000255:Adenosine Triphosphate; C041552:azidoprazosin; C095122:multidrug resistance protein 3; D000251:Adenosine Triphosphatases; D011224:Prazosin