Exotic properties of a voltage-gated proton channel from the snail<i>Helisoma trivolvis</i>

Abstract

Voltage-gated proton channels, H_V1, were first reported in <i>Helix aspersa</i> snail neurons. These H⁺ channels open very rapidly, two to three orders of magnitude faster than mammalian H_V1. Here we identify an H_V1 gene in the snail <i>Helisoma trivolvis</i> and verify protein level expression by Western blotting of <i>H. trivolvis</i> brain lysate. Expressed in mammalian cells, HtH_V1 currents in most respects resemble those described in other snails, including rapid activation, 476 times faster than hH_V1 (human) at pH_o 7, between 50 and 90 mV. In contrast to most H_V1, activation of HtH_V1 is exponential, suggesting first-order kinetics. However, the large gating charge of ∼5.5 <i>e</i>₀ suggests that HtH_V1 functions as a dimer, evidently with highly cooperative gating. HtH_V1 opening is exquisitely sensitive to pH_o, whereas closing is nearly independent of pH_o Zn²⁺ and Cd²⁺ inhibit HtH_V1 currents in the micromolar range, slowing activation, shifting the proton conductance-voltage (<i>g</i>_H-<i>V</i>) relationship to more positive potentials, and lowering the maximum conductance. This is consistent with HtH_V1 possessing three of the four amino acids that coordinate Zn²⁺ in mammalian H_V1. All known H_V1 exhibit ΔpH-dependent gating that results in a 40-mV shift of the <i>g</i>_H-<i>V</i> relationship for a unit change in either pH_o or pH_i This property is crucial for all the functions of H_V1 in many species and numerous human cells. The HtH_V1 channel exhibits normal or supernormal pH_o dependence, but weak pH_i dependence. Under favorable conditions, this might result in the HtH_V1 channel conducting inward currents and perhaps mediating a proton action potential. The anomalous ΔpH-dependent gating of HtH_V1 channels suggests a structural basis for this important property, which is further explored in this issue (Cherny et al. 2018. <i>J. Gen. Physiol.</i> https://doi.org/10.1085/jgp.201711968).