Electrostatic Interactions in the Closed and Open States of Voltage-Gated Proton Channels

Abstract

Voltage-gated proton channels (Hv1) play important roles in the respiratory burst, in pH regulation, in spermatozoa, in apoptosis, and in cancer metastasis. Unlike other voltage-gated cation channels, the Hv1 channel lacks a centrally located pore formed by the assembly of subunits. Instead, the proton permeation pathway in the Hv1 channel is within the voltage-sensing domain of each subunit. The gating mechanism of this pathway is still unclear. Mutagenic and fluorescence studies suggest that the fourth transmembrane segment, S4, functions as the voltage sensor and that there is an outward movement of S4 during channel activation. Using thermodynamic mutant cycle analysis, we find that the conserved positively charged residues in S4 are stabilized by counter charges in the other transmembrane segments both in the closed and open states. We construct models of both the closed and open states of Hv1 that are consistent with the mutant cycle analysis. These structural models suggest that electrostatic interactions between transmembrane segments in the closed state pull hydrophobic residues together to form a hydrophobic plug in the center of the voltage-sensing domain. Outward movement of S4 removes the hydrophobic plug, as if this hydrophobic plug works as the gate that prevents protons to permeate the Hv1 channel in the closed state.