An aqueous H+ permeation pathway in the voltage-gated proton channel Hv1

Abstract

Hv1 voltage-gated proton channels mediate rapid and selective transmembrane H+ flux and are gated by both voltage and pH gradients. Selective H+ transfer in membrane proteins is commonly achieved by Grotthuss proton ‘hopping’ in chains of ionizable amino acid side chains and intraprotein water molecules. To identify whether ionizable residues are required for proton permeation in Hv1, we neutralized candidate residues and measured expressed voltage-gated H+ currents. Unexpectedly, charge neutralization was insufficient to abrogate either the Hv1 conductance or coupling of pH gradient and voltage-dependent activation. Molecular dynamics simulations revealed water molecules in the central crevice of Hv1 model structures but not in homologous voltage-sensor domain (VSD) structures. Our results indicate that Hv1 most likely forms an internal water wire for selective proton transfer and that interactions between water molecules and S4 arginines may underlie coupling between voltage- and pH-gradient sensing.

Flagellar ion channels of sperm: similarities and differences between species

“…While Hv1 is ubiquitously expressed and plays an important role in a number of physiological processes such as the innate immune system, apoptosis, and cancer metastasis, the presence of Hv1 in human spermatozoa was intriguing. Hv1 is not a true ion channel, but rather a strange mix between a transporter and an ion channel without the pore, that provides rapid movement of protons across a lipid bilayer via a voltage-gated mechanism [116][117][118].…”

“…Unlike traditional voltage-gated ion channel, Hv1 does not possess a pore domain. Several models for the proton conductance pathway have been proposed [116,117,126]. The "water wire" model [117] proposes the movement of protons via Grotthus mechanism from the intracellular water vestibules to the extracellular vestibule formed by the transmembrane (TM) portions of the channel.…”

“…Several models for the proton conductance pathway have been proposed [116,117,126]. The "water wire" model [117] proposes the movement of protons via Grotthus mechanism from the intracellular water vestibules to the extracellular vestibule formed by the transmembrane (TM) portions of the channel. Two vestibules are connected by a narrow bottleneck [117] where proton selectivity occurs via a highly conserved and unique aspartate moiety (Asp112 in human Hv1, [127]).…”

“…The "water wire" model [117] proposes the movement of protons via Grotthus mechanism from the intracellular water vestibules to the extracellular vestibule formed by the transmembrane (TM) portions of the channel. Two vestibules are connected by a narrow bottleneck [117] where proton selectivity occurs via a highly conserved and unique aspartate moiety (Asp112 in human Hv1, [127]). Another model [116] suggests that Hv1 closed state favors electrostatic interactions between hydrophobic residues of TM segments and pull them together to form a hydrophobic plug.…”

Functional diversity of potassium channel voltage-sensing domains

“…The presence of water in these vestibules has been postulated on the basis of continuum electrostatic calculations in Shaker channels, 35,36 and the observation that waters occupy these regions in molecular dynamics calculations of Kv1.2 channels. 28,29,37 Water occupancy of the vestibules of VSDs is also implied by the proposed mechanism of proton permeation via water wires in Hv1 channels 38 and the existence of proton and alkali ion currents in certain mutations of the VSD of Shaker and Na channels, specifically of the charged residues in S4 and S2. [39][40][41][42] The large degree of structural conservation suggest that the mechanism of activation of all voltage sensors is also shared among them, and important clues have been obtained from studies of all of these VSDs.…”