Target Synonyms and Classification: Channelrhodopsin 2 (ChR2) belongs to the Ion-translocating Microbial Rhodopsin Family (MRF, TCDB: 3.E.1), which catalyzes light-driven ion translocation across microbial cytoplasmic membranes or serve as light receptors.
Function and Mechanism: ChR2 is a light-gated cation-selective ion channel that transports both monovalent and divalent cations. Upon light adsorption (λmax = 480 nm), the retinal chromophore undergoes a conformational change which is linked to pore opening. After milliseconds, the retinal relaxes, closing the pore and stopping the flow of ions.
Organism and Localization: Channelrhodopsins are found in unicellular green algae. ChR2 from the model organism Chlamydomonas reinhardtii was among the first discovered channelrhodopsins. ChR2 shows dynamic localiation and shuttles between flagella and eyespot of Chlamydomonas in a light-dependent manner: After prolonged light exposure, ChR2 is only found within the eyespot and mediates the photoreceptor current. In the dark, ChR2 additionally triggers the flagellar photocurrent that brings the change in calcium flux to control the movement.
Substrates and Inhibitors: ChR2 is cation-selective and transports both, monovalent and divalent cations (H+, Na+, K+, Ca2+). It is rather nonspecific - especially at the beginning of a light pulse. But it was shown that ChR2 becomes more specific for protons during longer periods of light exposure, reflecting a second opening state.
Related Transporters: All Rhodopsins share the properties of a covalently bound retinal coupling light adsorption with ion translocation. But in opposite to the light gated ion channel ChR2, most Rhodopsins function as light driven ion pumps, e.g. Bacteriorhodopsin. ChR1 from Chlamydomonas reinhardtii differs from ChR2: it is proton-selective and shows smaller photocurrents. Channelrhodopsins are often used in optogenitics: They enable light to control electrical excitability of membranes. Different genetic modifications – point mutations and Chimeras between different Channelrhodopopsin variants - are known that alter the properties of ChR2: most importantly absorption spectra (ReaChR and Chrimson show red spectral shifts), ion selectivity (CatCh prefers Ca2+; ChloC translocates Cl-), translocation speed (ChETA has faster kinetics) and current amplitude (ChIEF demonstrates largest photocurrents).