Complete Inhibition of the Pdr5p Multidrug Efflux Pump ATPase Activity by Its Transport Substrate Clotrimazole Suggests that GTP as Well as ATP May Be Used as an Energy Source^†

The yeast Pdr5p transporter is a 160 kDa protein that effluxes a large variety of xenobiotic compounds. In this study, we characterize its ATPase activity and demonstrate that it has biochemical features reminiscent of those of other ATP-binding cassette multidrug transporters:  a relatively high K_m for ATP (1.9 mM), inhibition by orthovanadate, and the ability to specifically bind an azidoATP analogue at the nucleotide-binding domains. Pdr5p-specific ATPase activity shows complete, concentration-dependent inhibition by clotrimazole, which is also known to be a potent transport substrate. Our results indicate, however, that this inhibition is noncompetitive and caused by the interaction of clotrimazole with the transporter at a site that is distinct from the ATP-binding domains. Curiously, Pdr5p-mediated transport of clotrimazole continues at intracellular concentrations of substrate that should eliminate all ATPase activity. Significantly, however, we observed that the Pdr5p has GTPase and UTPase activities that are relatively resistant to clotrimazole. Furthermore, the K_m(GTPase) roughly matches the intracellular concentrations of the nucleotide reported for yeast. Using purified plasma membrane vesicles, we demonstrate that Pdr5p can use GTP to fuel substrate transport. We propose that Pdr5p increases its multidrug transport substrate specificity by using more than one nucleotide as an energy source.