Histidine 168 is crucial for ΔpH-dependent gating of the human voltage-gated proton channel, hH V 1.

We recently identified a voltage-gated proton channel gene in the snail Helisoma trivolvis , HtH _V 1, and determined its electrophysiological properties. Consistent with early studies of proton currents in snail neurons, HtH _V 1 opens rapidly, but it unexpectedly exhibits uniquely defective sensitivity to intracellular pH (pH _i ). The H ⁺ conductance ( g _H )- V relationship in the voltage-gated proton channel (H _V 1) from other species shifts 40 mV when either pH _i or pH _o (extracellular pH) is changed by 1 unit. This property, called ΔpH-dependent gating, is crucial to the functions of H _V 1 in many species and in numerous human tissues. The HtH _V 1 channel exhibits normal pH _o dependence but anomalously weak pH _i dependence. In this study, we show that a single point mutation in human hH _V 1-changing His ¹⁶⁸ to Gln ¹⁶⁸ , the corresponding residue in HtH _V 1-compromises the pH _i dependence of gating in the human channel so that it recapitulates the HtH _V 1 response. This location was previously identified as a contributor to the rapid gating kinetics of H _V 1 in Strongylocentrotus purpuratus His ¹⁶⁸ mutation in human H _V 1 accelerates activation but accounts for only a fraction of the species difference. H168Q, H168S, or H168T mutants exhibit normal pH _o dependence, but changing pH _i shifts the g _H - V relationship on average by <20 mV/unit. Thus, His ¹⁶⁸ is critical to pH _i sensing in hH _V 1. His ¹⁶⁸ , located at the inner end of the pore on the S3 transmembrane helix, is the first residue identified in H _V 1 that significantly impairs pH sensing when mutated. Because pH _o dependence remains intact, the selective erosion of pH _i dependence supports the idea that there are distinct internal and external pH sensors. Although His ¹⁶⁸ may itself be a pH _i sensor, the converse mutation, Q229H, does not normalize the pH _i sensitivity of the HtH _V 1 channel. We hypothesize that the imidazole group of His ¹⁶⁸ interacts with nearby Phe ¹⁶⁵ or other parts of hH _V 1 to transduce pH _i into shifts of voltage-dependent gating.
(© 2018 Cherny et al.)