Trapped pore waters in the open proton channel H_V1

Abstract

The voltage-gated proton channel, HV1, is crucial for innate immune responses. According to alternative hypotheses, protons either hop on top of an uninterrupted water wire or bypass titratable amino acids, interrupting the water wire halfway across the membrane. To distinguish between both hypotheses, we estimate the water mobility for the putative case of an uninterrupted wire. The predicted single-channel water permeability 3x10-12cm3s-1 reflects the permeability-governing number of hydrogen bonds between water molecules in single-file configuration and pore residues. However, the measured unitary water permeability does not confirm the prediction, i.e., it is negligible. Osmotic deflation of reconstituted lipid vesicles reveals trapped water inside the HV1 wild-type channel and D174A mutant open at 0 mV. The conductance of 1400 H+s-1 per wild-type channel agrees with the calculated diffusion limit for a ~0.2nm capture radius for protons. Removal of a charged amino acid (D174) at the pore mouth decreases H+ conductance, conceivably by reducing the capture radius. At least one intervening amino acid contributes to H+ conductance while blocking water flow.

Interior pH Sensing Residue of Human Voltage-Gated Proton Channel Hv1 is Histidine 168

“…Under the intermediate pH (3<pH<5), most pore-lining acids are titrated. Here we suggest that ionizable pore-lining acids themselves or combined with water molecules trapped in the pore provide a Grotthuss-like hopping chain for proton transfer 12,18,25 .…”

“…In addition, the net charges of intracellular and extracellular sides are +7 and -2 (e) respectively (Tab. S1), creating an inner electric field which might be large enough to have the hHv1 activated and therefore explain the spontaneous activation of hHv1 in the absence of voltage and pH gradient 25 .…”