Proton and cation transport activity of the M2 proton channel from influenza A virus

The M2 protein is a small, single-span transmembrane (TM) protein from the influenza A virus. This virus enters cells via endosomes; as the endosomes mature and become more acidic M2 facilitates proton transport into the viral interior, thereby disrupting matrix protein/RNA interactions required for infectivity. A mystery has been how protons can accumulate in the viral interior without developing a large electrical potential that impedes further inward proton translocation. Progress in addressing this question has been limited by the availability of robust methods of unidirectional insertion of the protein into virus-like vesicles. Using an optimized procedure for reconstitution, we show that M2 has antiporter-like activity, facilitating K+ or Na+ efflux when protons flow down a concentration gradient into the vesicles. Cation efflux is very small except under conditions mimicking those encountered by the endosomally entrapped virus, in which protons are flowing through the channel. This proton/cation exchange function is consistent with the known high proton selectivity of the channel. Thus, M2 acts as a proton uniporter that occasionally allows K+ to flow to maintain electrical neutrality. Remarkably, as the pH inside M2-containing vesicles (pH_in) decreases, the proton channel activity of M2 is inhibited, but its cation transport activity is activated. This reciprocal inhibition of proton flux and activation of cation flux with decreasing pH_in first allows accumulation of protons in the early stages of acidification, then trapping of protons within the virus when low pH_in is achieved.