Cooperative Opening of Voltage Gated Proton Channels Involves Inter-Subunit Interactions

Abstract

Voltage-gated proton (Hv) channels have two subunits. Each subunit has a permeation pathway, but opening of the two pathways is highly cooperative. However, Hv channels lack a classic pore domain. Therefore, it is unclear how Hv channels open their permeation pathways. Using Voltage Clamp Fluorometry (VCF), we detect two fluorescence changes for a fluorophore attached close to the voltage sensor S4 in Hv channels, implying two conformational changes of S4. The first fluorescence change is highly voltage dependent and precedes channel opening, consistent with reporting on independent S4 charge movements in the two subunits in an Hv dimer. The second fluorescence change is less voltage dependent and closely correlates with channel opening and closing. Prevention of dimerization or mutations at the inter-subunit interface alters both the second fluorescence change and channel opening. Our results suggest that, following an initial S4 charge movement in the two subunits, channel opening in Hv channels is due to a second conformational change of S4 involving interactions between the two Hv subunits.