Glutamate residue 90 in the predicted transmembrane domain 2 is crucial for cation flux through channelrhodopsin 2

► Channelrhodopsin 2 is a light-activated cation channel widely used in neurobiology. ► We show its transmembrane segment 2 to be crucial for ion conductance. ► Transmembrane segment 2 forms a hydrophilic amphipathic helix. ► Glutamate 90 located within this helix determines cation flux and selectivity.