Molecular determinants of the inhibition of human Kv1.5 potassium currents by external protons and Zn2+

Using human Kv1.5 channels expressed in HEK293 cells we assessed the ability of H; to mimic the previously reported action of Zn2+ to inhibit macroscopic hKv1.5 currents, and using site-directed. As with mutagenesis, we addressed the mechanistic basis for the inhibitory effects of H-o(+) and Zn2+ Zn2+, H-o(+), caused a concentration-dependent, K-o(+)-sensitive and reversible reduction of the maximum conductance (g(max)). With zero, 5 and 140 mm K-o(+) the pK(H) for this decrease of g(max) was 6.8, 6.2 and 6.0, respectively. The concentration dependence of the block relief caused by increasing [K+](o) was well fitted by a non-competitive interaction between H-o(+) and K-o(+) for which the K-D for the K+ binding site was 0.5-1.0 mM. Additionally, gating current analysis in the non-conducting mutant hKv1.5 W472F showed that changing from pH 7.4 to pH 5.4 did not affect Q(max) and that charge immobilization, presumed to be due to C-type inactivation, was preserved at pH 5.4. Inhibition of hKv1.5 currents by H-o(+) or Zn2+ was substantially reduced by a mutation either in the channel turret (H463Q) or near the pore mouth (R487V). In light of the requirement for R487, the homologue of Shaker T449, as well as the block-relieving action of K-o(+), we propose that H+ or Zn2+ binding to histidine residues in the pore turret stabilizes a channel conformation that is most likely an inactivated state.