Title: Exotic Properties of a Voltage Gated Proton Channel in the Snail Helisoma trivolvis
Abstract: Voltage gated proton currents, HV1, were first reported in snail neurons (Helix aspersa and Lymnaea stagnalis). These H+ channels open very rapidly, 2-3 orders of magnitude faster than mammalian HV1. We identified an HV1 gene in the snail Helisoma trivolvis and expressed it in mammalian cells. The resulting HtHV1 currents in most respects resembled those described in other snails, including rapid activation. In contrast with most HV1, activation of HtHV1 was exponential, suggesting first-order kinetics. Also consistent with first order kinetics, τact and τtail overlapped at intermediate voltages and the voltage at which the gating time constants were slowest occurred at the midpoint of the gH-V relationship. However, the large gating charge of ∼5.5 e0 and the existence of extensive predicted coiled-coil regions in the C terminus both suggest that HtHV1 functions as a dimer, but evidently with more highly cooperative gating than exists in other species. In stark contrast with mammalian HV1, HtHV1 opening is extremely sensitive to pHo whereas closing is nearly independent of pHo. All known HV1 exhibit ΔpH dependent gating that in results in a 40 mV shift of the gH-V relationship for a 1 unit change in either pHo or pHi. This property, called ΔpH dependent gating, is crucial to all of the functions of HV1 in many species and in numerous human cells. The HtHV1 channel exhibits normal or supernormal pHo dependence, but anomalously weak pHi dependence. The average slope of the pHi dependence is 15 mV/unit between pHi 5 and 9, with pHo 7. Evidently, HtHV1 has distinct internal and external pH sensors. The anomalous ΔpH dependent gating of HtHV1 channels provides clues to the structural basis for this important property. Supported by NIH grant GM-102336 and NSF grant MCB-1242985.