The voltage sensor is responsible for ΔpH dependence in H_v1 channels

Voltage-gated proton channels (Hv1) regulate acid-base balance by modulating open probability based on transmembrane voltage and pH differences. Prior research has shown Hv1 responds to voltage and ΔpH. However, the exact mechanism by which ΔpH influences Hv1 gating remains unclear. Established models suggest voltage and pH work together, but the specific role of the voltage sensor in this process is not well defined. This gap motivated the need to distinguish between voltage sensor activation and permeation pathway opening as sources of ΔpH dependence. No prior work had resolved whether ΔpH affects Hv1 through the voltage sensor or the permeation pathway. Understanding this distinction could clarify Hv1's functional mechanism. The study addresses this uncertainty by isolating the voltage sensor's role in ΔpH-dependent gating. This approach allows for a more precise interpretation of Hv1's behavior under varying pH conditions.

The study aimed to determine whether ΔpH-dependent gating in Hv1 channels occurs during voltage sensor activation or permeation pathway opening. Researchers sought to isolate the voltage sensor's role by using a nonconducting Hv1 mutant. This approach allowed them to examine voltage sensor behavior without interference from ion permeation. The motivation stemmed from the need to clarify Hv1's molecular mechanism of ΔpH dependence. Prior research had not resolved whether ΔpH affects Hv1 through the voltage sensor or the permeation pathway. The study's focus was on the voltage sensor's pH dependence in Hv1 channels. By measuring gating currents in a nonconducting mutant, the researchers could assess voltage sensor activation independently. This method enabled a direct test of the voltage sensor's role in ΔpH-dependent gating.

The researchers used monomeric Ciona-Hv1 channels to study ΔpH-dependent gating. They measured conductance-voltage (G-V) curves at different ΔpH values in wild-type channels. Gating currents were recorded in a nonconducting Hv1 mutant (D160N) to assess voltage sensor activation. This mutant lacked a functional permeation pathway, isolating the voltage sensor's role. The team analyzed shifts in the G-V and Q-V curves to determine ΔpH effects. They fitted data to a model where Hv1 voltage sensor free energy depends on proton chemical and electrical potentials. The model assumed Hv1 voltage sensor free energy is a function of both ΔpH and voltage. This approach allowed the researchers to quantify the coupling between ΔpH and voltage sensor activation.

Higher ΔpH values shifted the G-V curve leftward in monomeric Hv1 channels. Gating currents in the D160N mutant showed a leftward shift in the Q-V curve with increasing ΔpH. These findings suggest ΔpH-dependent gating arises from voltage sensor modulation. The model fitting revealed that approximately 60% of ΔpH energy couples to voltage sensor activation. This coupling indicates protons influence Hv1 voltage sensor free energy. The data support the hypothesis that ΔpH affects Hv1 through voltage sensor accessibility changes. The Q-V curve shifts demonstrate voltage sensor pH dependence in the absence of permeation. The model's parameters align with Hv1's ΔpH-dependent gating mechanism.

The study shows Hv1 ΔpH dependence arises from voltage sensor activation rather than permeation pathway opening. The voltage sensor's free energy is modulated by ΔpH, as shown by Q-V curve shifts. The researchers propose Hv1 protons alter the voltage sensor's free-energy landscape. This alteration is linked to changes in proton accessibility during voltage sensor activation. The model suggests 60% of ΔpH energy couples to voltage sensor activation. These findings clarify Hv1's ΔpH-dependent gating mechanism. The study supports the role of voltage sensor pH dependence in Hv1 function. The authors suggest protons influence Hv1 voltage sensor activation through accessibility changes.