Detailed comparison of expressed and native voltage-gated proton channel currents

Two years ago, genes coding for voltage-gated proton channels in humans, mice and Ciona intestinalis were discovered. Transfection of cDNA encoding the human HVCN1 (H V 1) or mouse (mVSOP) ortholog of HVCN1 into mammalian cells results in currents that are extremely similar to native proton currents, with a subtle, but functionally important, difference. Expressed proton channels exhibit high H + selectivity, voltage-dependent gating, strong temperature sensitivity, inhibition by Zn 2+ , and gating kinetics similar to native proton currents. Like native channels, expressed proton channels are regulated by pH, with the proton conductance–voltage ( g H – V ) relationship shifting toward more negative voltages when pH o is increased or pH i is decreased. However, in every (unstimulated) cell studied to date, endogenous proton channels open only positive to the Nernst potential for protons, E H . Consequently, only outward H + currents exist in the steady state. In contrast, when the human or mouse proton channel genes are expressed in HEK-293 or COS-7 cells, sustained inward H + currents can be elicited, especially with an inward proton gradient (pH o < pH i ). Inward current is the result of a negative shift in the absolute voltage dependence of gating. The voltage dependence at any given pH o and pH i is shifted by about −30 mV compared with native H + channels. Expressed H V 1 voltage dependence was insensitive to interventions that promote phosphorylation or dephosphorylation of native phagocyte proton channels, suggesting distinct regulation of expressed channels. Finally, we present additional evidence that speaks against a number of possible mechanisms for the anomalous voltage dependence of expressed H + channels.