Channelrhodopsin
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Channelrhodopsins are ion channels that are directly opened by light. They therefore may be very useful molecules, enabling the use of light to control intracellular acidity, calcium influx, and electrical excitability.
Two channelrhodopsins are currently known: Channelrhodopsin-1 and Channelrhodopsin-2 are both light gated proton channels, but Channelrhodopsin-2 exhibits in addition some conductance for cations. Both proteins serve as sensory photoreceptors in the green alga Chlamydomonas controling behavioural responses like photophobic and phototaxic responses at high light intensities.
Structurally, channelrhodopsins are retinylidene proteins. They are thought to be seven-transmembrane proteins like rhodopsin, and contain the light-isomerizable vitamin A derivative all-trans-retinal. However, whereas most opsins are G-protein coupled receptors that open other ion channels indirectly via messengers, channelrhodopsins form a channel pore itself. This makes cellular depolarization extremely fast, robust, and useful for bioengineering and neuroscience applications, including photostimulation. Peak absorbance of the Channelrhodopsin-2 retinal complex is about 460 nm. Channelrhodopsin-2 and the yellow light-activated chloride pump halorhodopsin together enable multiple-color optical activation and silencing of neural activity.
