Hv1 Proton Channel Opening Is Preceded by a Voltage-independent Transition

The voltage sensing domain (VSD) of the voltage-gated proton channel Hv1 mediates a H+-selective conductance that is coordinately controlled by the membrane potential (V) and the transmembrane pH gradient (pH). Allosteric control of Hv1 channel opening by Delta pH (V-Delta pH coupling) is manifested by a characteristic shift of approximately 40 mV per Delta pH unit in the activation. To further understand the mechanism for V-Delta pH coupling in Hv1, H+ current kinetics of activation and deactivation in excised membrane patches were analyzed as a function of the membrane potential and the pH in the intracellular side of the membrane (pH(I)). In this study, it is shown for the first time to our knowledge that the opening of Hv1 is preceded by a voltage-independent transition. A similar process has been proposed to constitute the step involving coupling between the voltage-sensing and pore domains in tetrameric voltage-gated channels. However, for Hv1, the VSD functions as both the voltage sensor and the conduction pathway, suggesting that the voltage independent transition is intrinsic to the voltage-sensing domain. Therefore, this article proposes that the underlying mechanism for the activation of Hv1 involves a process similar to VSD relaxation, a process previously described for voltage-gated channels and voltage-controlled enzymes. Finally, deactivation seemingly occurs as a strictly voltage dependent process, implying that the kinetic event leading to opening of the proton conductance are different than those involved in the closing. Thus, from this work it is proposed that Hv1 activity displays hysteresis.