Is The Hydrophobic Gasket A Secondary Selectivity Filter In The Human Voltage Gated Proton Channel Hh(V)1?

A large family of membrane proteins, the voltage gated ion channels, regulate a vast array of physiological functions in essentially all life forms. The mechanism by which these molecules sense membrane potential and respond by creating ionic conduction is incompletely understood. These channels contain a ring of hydrophobic amino acids near the center of the voltage sensing domain in the membrane, the “hydrophobic gasket,” HG, which is anchored by a highly conserved Phe (part of the Charge Transfer Center) in nearly all voltage-gated ion channels and voltage-sensing phosphatases. Various functions of this structure have been suggested in addition to its evident role of separating internal and external aqueous solutions. During gating, the periodic cationic residues in the S4 helix are thought to ratchet past the HG. Here we identify the HG in voltage-gated proton channels and test the hypothesis that it functions as a secondary selectivity filter. Selectivity is ensured primarily by the interaction of Asp112 and one or more Arg in S4. However, molecular dynamics simulations indicate that cation exclusion may not be complete at the Asp-Arg selectivity filter. Potential of mean-force calculations show that the top of the free energy barrier opposing Na+ permeation coincides with the most hydrophobic region of the pore, corresponding with the HG region in other voltage-sensing domains. We replaced amino acids of the HG with less hydrophobic or hydrophilic ones, both individually and in groups, and measured reversal potentials at various pH and in the presence of Na+. All mutants tested were selective for H+ and did not conduct Na+, with PH/PNa > 107 as a lower limit. We conclude that proton selectivity is accomplished by the primary selectivity filter, comprising the interaction of Asp and Arg.