Channelrhodopsin 1763319 223189323 2008-07-02T23:36:24Z Gabi481923 7412732 '''Channelrhodopsins''' are [[light-gated ion channel]]s. They are 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]] controlling behavioural responses like photophobic and phototaxic responses at high light intensities. Structurally, channelrhodopsins are [[retinylidene protein]]s. They are thought to be [[seven-transmembrane protein]]s like [[rhodopsin]], and contain the light-isomerizable [[vitamin A]] derivative all-trans-[[retinal]]. However, whereas most opsins are [[G-protein]] coupled [[Receptor (biochemistry)|receptor]]s 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. ==Channelrhodopsin-2== Channelrhodopsin-2 is a [[light-gated ion channel]] of the [[channelrhodopsin]] family. As such, Channelrhodopsin-2 (ChR2) contains not only a light-activated transduction mechanism, but the channel pore itself, which can pass cations like [[sodium]], [[calcium]], and a variety of other small cations such as [[potassium]]. === Mechanics === Channelrhodopsin-2 consists of a [[7-transmembrane]] helix protein, as in many other rhodopsins, but ChR2 has a covalently linked retinal. The peak absorbance of the Channelrhodopsin-2 retinal complex is about 488 nm. When the all-trans retinal complex absorbs light, it induces a conformational change, probably to 13-cis-retinal. This conformational change introduces a further conformational change in the transmembrane protein opening the pore, to at least 6A. The 13-cis-retinal naturally relaxes with time back to the all-trans-retinal which closes the pore, stopping the flow of ions.<ref name="PNAS">[http://www.pnas.org/cgi/content/abstract/100/24/13940"Channelrhodopsin-2, a directly light-gated cation-selective membrane channel"] - PNAS | November 25, 2003 | vol. 100 | no. 24 | 13940-13945 </ref> The 7-transmembrane nature of Channelrhodopsin-2 is fairly rare to ion channels which usually consist of similar repeating parts.<ref name="PNAS">PNAS</ref> === Applications === This makes [[depolarization]] of [[excitable cells]] very fast, robust, and useful for bioengineering and neuroscience applications, including [[photostimulation]] of neurons for probing of neural circuits. Channelrhodopsin-2 and the yellow light-activated chloride pump [[halorhodopsin]] together enable multiple-color optical activation and silencing of neural activity. The [[C-terminal]] end of ChR2 extends well into the intracelluar space, whereas the [[N-terminal]] end consists of the 7-transmembrane section. As such, the C-terminus can be replaced by the [[green fluorescent protein]] (GFP). == References == <references/> == External links == * [http://channelrhodopsin.org Channelrhodopsin mediating optical activation of neurons] *[http://www.stanford.edu/group/dlab/optogenetics Optogenetics Resource Center] * [http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=17442243&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum Using channelrhodopsin in transgenic mice to study brain circuitry] * [http://www.neuron.org/content/article/abstract?uid=PIIS0896627306001760 Using channelrhodopsin potentially to treat blindness] [[Category:Ion channels]] [[Category:Integral membrane proteins]]