Structure-function relationships of Na+/K(+)-pumps expressed in Xenopus oocytes

During the last years we have examined structure-function relationships in the Na+/K(+)-ATPase with respect to interactions of the external cations with the pump molecule. We have analysed in voltage-clamp experiments the influence of extracellular Na+ and K+ on the current generated by Na+/K(+)-pumps expressed in Xenopus oocytes. Our results demonstrated that external Na+ and K+ have to pass an access channel in the electrical field of the membrane to reach their binding sites. This external access, therefore, is voltage-dependent and is affected by lysine residues within the cytoplasmic N-terminus, by glutamic acid residues in intramembraneous domains, the ouabain sensitivity and phosphorylation by protein kinases.