Stimulation of the Na+/K+ pump by external [K+] is regulated by voltage-dependent gating

Wild-type and mutants with alpha-subunits truncated at the N terminus of Na+/K+ pumps of Torpedo electroplax were expressed in Xenopus oocytes by injection of cRNAs encoding for one of the alpha-subunits and for the beta-subunit. Currents generated by the pump were investigated under voltage clamp in Na(+)-free solution, a condition where stimulation by external [K+] is the only voltage-dependent and rate-determining step in the pump cycle (Rakowski, R. F., Vasilets, L. A., LaTona, J., and Schwarz, W. (1991) J. Membr. Biol. 121, 177-187). Voltage dependence of the apparent Km value for pump stimulation and of maximum transport activity was investigated. Truncation of the intracellular N-terminal end of the alpha-subunit at the trypsin-accessible site (alpha delta K37, leaving Lys37) leads to nearly complete inhibition of pump current at physiological potentials, whereas ouabain binding capacity is retained indicating an essential involvement of the N-terminal end in the process of ion translocation. Truncation at the N-terminal end leaving Lys28 (alpha delta K28) or Thr29 (alpha delta T29) leads to removal of 6 or 7 lysine residues, respectively, and has no effect on maximum transport activity. On the other hand, the mutated pumps with alpha delta K28 or alpha delta T29 exhibit more pronounced voltage dependences for stimulation of pump current by external [K+] compared with the wild-type Torpedo pump. In particular, a pronounced increase in voltage dependence of the apparent affinity of pump stimulation is obtained by the removal of the Lys28. The results support the view that the lysine-rich region in the N-terminal end affects the cation binding to the pump molecule and that Lys28 is important.