Gouaux and colleagues2 report the crystal structure of the chicken ASIC1 protein, which forms a channel across cell membranes. The channel opens in response to hydrogen ions (protons, H+), so allowing sodium ions into cells. The authors propose the following mechanism. a, Protons bind to a cleft next to a thumb-shaped region (green) of the extracellular part of the protein. b, The proton-binding slightly displaces the thumb, which causes a conformational change that opens the channel.