Mediates the voltage-dependent proton permeability of excitable membranes. Forms a proton-selective channel through which protons may pass in accordance with their electrochemical gradient (By similarity). Homodimer. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Unlike other voltage-gated ion channels it lacks the pore domain (By similarity). The C-terminal coiled coil region mediates homodimerization. It is essential for normal subcellular localization (By similarity). Belongs to the hydrogen channel family.