The invention relates to the atomistic functional understanding of the M2 protein from the influenza A virus. This acid-activated selective proton channel has been the subject of numerous conductance, structural, and computational studies. Previously, little was known at the atomic level about the heart of the functional mechanism of this tetrameric protein, a tetrad of HxxxW residues. The structure of the M2 conductance domain in a lipid bilayer is disclosed and displays the defining features of the native protein that have not been attainable from structures solubilized by detergents. A detailed mechanism for acid activation and proton conductance, involving a strong hydrogen bond between two adjacent histidines and specific interactions with the tryptophan gate, is provided and elucidates many observations on the M2 proton conductance.