Gating of the Hv1 Proton Channel

Abstract

Voltage-gated proton channels mediate proton extrusion from the cytoplasm of a wide variety of cells. Hv1, the first and so far only cloned voltage-gated proton channel, is composed of a voltage-sensing domain, consisting of transmembrane segments S1-S4, but lacks the two transmembrane segments that form the pore domain of the “classical” voltage-gated channels. Hv1 channels associate as dimers, each subunit containing its own pore. Earlier work showed that the opening of the two pores occurs in a cooperative manner. As in “classical” voltage-gated channels, the arginine-rich S4 transmembrane helix is the voltage sensor in Hv1. However, the molecular mechanism of opening is still ill-defined. Using Voltage Clamp Fluorometry, we are searching for the part of the protein that undergoes a structural change during the opening transition. Environmentally-sensitive fluorophores attached at the outer end of S4 report on protein motions that are as fast as opening, but are dominated by slower signals that occur at negative voltage where channels do not open. However, sites elsewhere in the protein nicely track the opening and closing transitions. We are using this information to reconstruct the opening transition.