Structural dynamics determine voltage and pH gating in human voltage-gated proton channel
Voltage-gated ion channels are key players of electrical signaling in cells. As a unique subfamily, voltage-gated proton (Hv) channels are standalone voltage sensors without separate ion conductive pores. They are gated by both voltage and transmembrane proton gradient (i.e {Delta}pH), serving as acid extruders in most cells. Amongst their many functions, Hv channels are known to regulate the intracellular pH of human spermatozoa and compensate for the charge and pH imbalances caused by NADPH oxidases in phagocytes. Like the canonical voltage sensors, the Hv channel is a bundle of 4 helices (named S1 through S4), with the S4 segment carrying 3 positively charged Arg residues. Extensive structural and electrophysiological studies on voltage-gated ion channels generally agree on an outwards movement of the S4 segment upon activating voltage, but the real time conformational transitions are still unattainable. With purified human voltage-gated proton (hHv1) channel reconstituted in liposomes, we have examined its conformational dynamics at different voltage and pHs using the single molecule fluorescence resonance energy transfer (smFRET). Here we provided the first glimpse of real time conformational trajectories of the hHv1 voltage sensor and showed that both voltage and pH gradient shift the conformational dynamics of the S4 segment to control channel gating. Our results suggested the biological gating is determined by the conformational distributions of the hHv1 voltage sensor, rather than the conformational transitions between the presumptive resting and activated conformations. We further identified H140 as the key residue sensing extracellular pH and showed that both the intracellular and extracellular pH sensors act on the voltage sensing S4 segment to enrich the resting conformations. Taken together, we proposed a model that explains the mechanisms underlying voltage and pH gating in Hv channels, which may also serve as a general framework to understand the voltage sensing and gating in other voltage-gated ion channels.