Mutations in Either Nucleotide-Binding Site of P-glycoprotein (Mdr3) Prevent Vanadate Trapping of Nucleotide at Both Sites

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Abstract
Vanadate trapping of nucleotide and site-directed mutagenesis were used to investigate the role of the two nucleotide-binding (NB) sites in the regulation of ATP hydrolysis by P-glycoprotein (mouse Mdr3). Mdr3, tagged with a hexahistidine tail, was overexpressed in the yeast Pichia pastoris and purified to about 90% homogeneity by Ni-affinity chromatography. This protocol yielded purified, reconstituted Mdr3 which exhibited high verapamil stimulation of ATPase activity with a Vmax of 4.2 micromol min-1 mg-1 and a KM of 0.7 mM, suggesting that Mdr3 purified from P. pastoris is highly functional. Point mutations were introduced into the core consensus sequence of the Walker A or B motifs in each of the two NB sites. The mutants K429R, K1072R (Walker A) and D551N, D1196N (Walker B) were functionally impaired and unable to confer cellular resistance to the fungicide FK506 in the yeast Saccharomyces cerevisiae. Single and double mutants (K429R/K1072R, D551N/D1196N) were expressed in P. pastoris, and the effect of these mutations on the ATPase activity of Mdr3 was characterized. Purified reconstituted Mdr3 mutants showed no detectable ATPase activity compared to proteoliposomes purified from negative controls (<5% of wild-type Mdr3). Vanadate readily induced trapping of 8-azido-nucleotide in the wild-type enzyme after a short 10 s incubation, and specific photolabeling of Mdr3 after UV irradiation. No such vanadate-induced trapping/photolabeling was observed in any of the mutants, even after a 60 min trapping period at 37 degrees C. Since vanadate trapping with 8-azido-ATP requires hydrolysis of the nucleotide, the data suggest that 8-azido-ATP hydrolysis is dramatically impaired in all of the mutant proteins (<0.3% activity). These results show that mutations in either NB site prevent single turnover and vanadate trapping of nucleotide in the nonmutant site. These results further suggest that the two NB sites cannot function independently as catalytic sites in the intact molecule. In addition, the N- or C-terminal NB sites appear functionally indistinguishable, and cooperative interactions absolutely required for ATP hydrolysis may originate from both sites.