Control of Hv1 Voltage-Gated Proton Channel Opening by Changes in the Transmembrane Voltage and pH Gradients

Abstract

Hv1 proton channels contain a voltage sensor (VS) domain that is homologous to that of tetrameric voltage-gated cation channels (VGCs) and voltage-sensitive phosphatases (VSPs), but cooperative channel gating by changes in both membrane potential (Vm) and the transmembrane pH gradient (ΔpH = pHO - pHI) is apparently unique to Hv1. The mechanism underlying Vm-ΔpH coupling in Hv1, which results in a linear −40 mV/ΔpH unit shift in the GH+-V relation, is not well understood. In order to isolate ΔpH-dependent steps in the Hv1 activation pathway, we measured the effect of changing ΔpH on expressed H+ currents using patch clamp electrophysiology. We show here that Hv1 opening exhibits both voltage-dependent and voltage-independent kinetic components, but closing is purely voltage-dependent. The forward time constant for the final charge-translocating step in Hv1 activation gating is steeply ΔpH-dependent at acidic pHI but only weakly sensitive to ΔpH at alkaline pHI. In addition, we find that at high pHI, a voltage-independent transition becomes rate-limiting for channel opening. Analysis of the relative contributions of voltage-dependent and -independent Hv1 gating components demonstrates that both are necessary to explain the linear Vm-ΔpH relation between pHI 4.5 and 7.5. The effect of changes in ΔpH on Hv1 gating contrasts with Ci-VSP, indicating that Hv1 is particularly well tuned to respond to small perturbations in ΔpH over the physiological range (pHI ≈ pHO). We propose a new multi-state model to account for complex gating in Hv1 in which VS activation and channel opening occur in distinct transitions and display different sensitivities to changes in ΔpH. Vm-ΔpH coupling in Hv1 thus represents an integrated response to changes in both voltage-dependent and -independent gating steps.