Strong cooperativity between subunits in voltage-gated proton channels

Voltage-activated proton (H(V)) channels are essential components in the innate immune response. H(V) channels are dimeric proteins with one proton permeation pathway per subunit. It is not known how H(V) channels are activated by voltage and whether there is any cooperativity between subunits during voltage activation. Using cysteine accessibility measurements and voltage clamp fluorometry, we show data that are consistent with that the fourth transmembrane segment S4 functions as the voltage sensor in H(V) channels from Ciona intestinalis. Surprisingly, in a dimeric H(V) channel, S4 in both subunits have to move to activate the two proton permeation pathways. In contrast, if H(V) subunits are prevented from dimerizing, then the movement of a single S4 is sufficient to activate the proton permeation pathway in a subunit. These results suggest a strong cooperativity between subunits in dimeric H(V) channels.