Most organisms use photoreceptors to sense and respond to light. Examples of photoreceptors include bacteriorhodopsins and bacteriophytochromes in some bacteria, phytochromes in plants, and rhodopsins in the photoreceptor cells of the vertebral retina. The light-sensitive property of these receptors is because of the bound chromophores, such as bilin in the phytochromes and retinal in the rhodopsins.
Rhodopsins belong to the family of cell surface proteins called G-protein coupled receptors, which are involved in signal transduction across biological membranes. The rhodopsin family of proteins acquired divergent functions during evolution, such as acting as light receptors, ion pumps, and ion channels. One such subfamily of rhodopsins is channelrhodopsins or ChR that function as light-sensitive ion channels. Naturally occurring ChRs were first discovered in green microalgae, Chlamydomonas reinhardtii. Their role in algae involves coupling light to flagellar motion, thereby allowing the organism to acquire optimum light for photosynthesis and survival.
The structure of ChR protein includes seven membrane-spanning domains and a covalently bound light-sensitive protein called an all-trans-retinal chromophore. Upon illumination with blue light, ChR undergoes a conformational change and opens to allow the diffusion of cations, such as H+, Na+, K+, and Ca+, passively down their concentration gradient. The influx of ions across the membrane produces large photocurrents that activate the cell. Due to their fast activation by light, ChR have given rise to a new research approach in neurobiology called optogenetics.
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