Channelrhodopsin: Ideas about Gating and Ion Transport

Channelrhodopsins (ChRs) are directly light-gated ion channels in halo- and glycophytic Chlorophyceae where they serve as sensory photoreceptors for behavioral responses (phototaxis). ChRs are used for the depolarization of selected cells in neural networks of tissues and living animals to gain information about the brain function. The operation of ChRs is determined by two fundamental functions: gating and transport. Within the voltage- pH-, time- and light-sensitive gating function (photocycle), an early and a late conductance with different transport properties were identified. The voltage- and substrate-sensitive transport function of the early conductance can quantitatively be described by enzyme kinetics, where H+ > Na+ > K+ > Ca2+ > Mg2+ are translocated competitively. Experiments with specific mutants, and 3D models based on structures of other microbial proteins, allow preliminary assignments of specific ChR functions to structural details.