An effect of voltage on binding of Na+ at the cytoplasmic surface of the Na+-K+ pump

This work utilizes proteoliposomes reconstituted with renal Na+-K+-ATPase to study effects of electrical potential (40-80 mV) on activation of pump-mediated fluxes of Na+ or Rb+ (K+) ions and on inhibitory effects of Rb+ ions or organic cations. The latter include guanidinium derivatives that are competitive Na+-like antagonists (David, P., Mayan, H., Cohen. H., Tal, D. M., and Karlish, S. J. D. (1992) J. Biol. Chem. 267, 1141-1149). Cytoplasmic side positive diffusion potentials significantly decreased the K-0.5 of Na+ at the cytoplasmic surface for activation of ATP-dependent Na+-K+ exchange but did not affect the inhibitory potency of Rb+ (K+) or any Na+-like antagonist. Diffusion potentials did not affect activation of Rb+-Rb+ exchange by Rb+ ions at the cytoplasmic surface and had only a minor effect on Rb+ activation at the extracellular surface. Previously, we proposed that the cation binding domain consists of two negatively charged sites, to which two K+ or two Na+ ions bind, and one neutral site for the third Na+ (Glynn, I. M., and Karlish, S. J. D. (1990) Annu. Rev. Biochem. 59, 171-205). The present experiments suggest that binding of a Na+ ion in the neutral site at the cytoplasmic surface is sensitive to voltage. By contrast, binding of Rb+ ions at the extracellular surface of renal pumps appears to be only weakly or insignificantly affected by voltage. Inferences on the identity of the charge-carrying steps, based on experiments using proteoliposomes, are discussed in relation to recent evidence that dissociation of Na+ or association of K+ ions, at the extracellular surface, represent the major charge-carrying steps.