Free Energy Simulations of Ion Translocation through Voltage-Gated Proton Channel Hv1

Abstract

The human voltage-gated proton channel (hHV1) is a transmembrane protein that is responsible for the selective permeation of protons across cell membranes in nasal mucosa, sperm, and white blood cells. hHV1 is a four-helix bundle (S1-S4) with anionic Asp112 on S1 forming a salt bridge with cationic Arg residues on helix S4 in the narrow region of the pore [Kulleperuma et al., J. Gen. Physiol. 141, 445-465 (2013)]. Mutation of Asp112 to Val abrogates channel properties. Unexpectedly, replacing Asp112 by a smaller neutral residue such as Ser turns HV1 into an anion selective channel that conducts Cl-, as does the double mutant D112V-V116S [Musset et al., Nature 480, 273-277 (2012); Morgan et al., J. Gen. Physiol. 142, 625-640 (2013)]. Although HV1 and its mutants exhibit drastic differences in ion permeation, the molecular basis of proton selectivity in WT and anion selectivity in mutants remains unexplained.As the first step towards elucidating the charge selectivity of HV1, we perform molecular dynamics simulations with umbrella sampling to compute the free energy profile for the translocation of Na+ and Cl- ions through the pore of a homology model of HV1 and its mutants. The calculations are repeated in conformational states of the channel differing in the extent of hydration of the pore and in the relative arrangements of pore residues. Results show how ion solvation and electrostatic interactions with charged side chains in the pore lumen modulate the energetics of ion permeation in HV1 and suggest a structural basis for charge selectivity.