RAPID Gating Kinetics of a Voltage Gated Proton Channel in the Snail Helisoma Trivolvis

Voltage gated proton currents, HV1, were first reported in snail neurons (Helix aspersa and Lymnaea stagnalis). The most obvious difference between proton currents in snails and in other species is that snail H+ channels open very rapidly, 2-3 orders of magnitude faster than HV1 in mammals, dinoflagellates, coccolithophores, fish, and insects. Preliminary analysis of RNA-seq data from the snail Helisoma trivolvis suggests that HV1 is differentially expressed in two neurons. We cloned a HV1 gene from H. trivolvis brains, confirmed its protein expression in H. trivolvis brain tissue, and expressed it in mammalian cells. The resulting HtHV1 currents in most respects resembled those described in other snails, including rapid activation. Another species with rapidly activating proton currents is the sea urchin, Strongylocentrotus purpuratus (SpHV1). Sakata et al (2016, BBA 1858:2972-2983) localized a single amino acid at the inner end of the S3 transmembrane helix that appeared to account for this difference in kinetics. Replacing His164 in mHV1 with Ser, the corresponding amino acid in SpHV1, accelerated mHV1 gating by an order of magnitude. We focused on this position, and replaced the corresponding His168 in human HV1 with Gln, the corresponding residue in HtHV1. The mutant did activate 10 times faster, but remained two orders of magnitude slower than HtHV1. We conclude that this position does influence kinetics, but does not account for the rapid gating of snail HV1. Supported by NIH grant GM-102336 and NSF grant MCB-1242985.