Histidine is crucial for ΔpH-dependent gating of the human voltage-gated proton channel, hH1.

We recently identified a voltage-gated proton channel gene in the snail , HtH1, and determined its electrophysiological properties. Consistent with early studies of proton currents in snail neurons, HtH1 opens rapidly, but it unexpectedly exhibits uniquely defective sensitivity to intracellular pH (pH). The H conductance ()- relationship in the voltage-gated proton channel (H1) from other species shifts 40 mV when either pH or pH (extracellular pH) is changed by 1 unit. This property, called ΔpH-dependent gating, is crucial to the functions of H1 in many species and in numerous human tissues. The HtH1 channel exhibits normal pH dependence but anomalously weak pH dependence. In this study, we show that a single point mutation in human hH1-changing His to Gln, the corresponding residue in HtH1-compromises the pH dependence of gating in the human channel so that it recapitulates the HtH1 response. This location was previously identified as a contributor to the rapid gating kinetics of H1 in His mutation in human H1 accelerates activation but accounts for only a fraction of the species difference. H168Q, H168S, or H168T mutants exhibit normal pH dependence, but changing pH shifts the - relationship on average by <20 mV/unit. Thus, His is critical to pH sensing in hH1. His, located at the inner end of the pore on the S3 transmembrane helix, is the first residue identified in H1 that significantly impairs pH sensing when mutated. Because pH dependence remains intact, the selective erosion of pH dependence supports the idea that there are distinct internal and external pH sensors. Although His may itself be a pH sensor, the converse mutation, Q229H, does not normalize the pH sensitivity of the HtH1 channel. We hypothesize that the imidazole group of His interacts with nearby Phe or other parts of hH1 to transduce pH into shifts of voltage-dependent gating.