- Retinylidene protein
Retinylidene proteins are a family of proteins that use
retinal aschromophore forlight reception. This group of proteins is also commonly referred to asrhodopsin s. They are the molecular basis for a variety of light-sensing systems fromphototaxis inflagellate s toeyesight inanimal s.Structure
All rhodopsins consist of two building blocks, a
protein moiety and a reversiblycovalent ly bound non-protein cofactor,retinal (retinaldehyde). The protein structure of rhodopsin consists of a bundle of seven transmembrane helices that form an internal pocket binding the photoreactive chromophore. They form a superfamily with other membrane-bound receptors containing seven transmembrane domains, for exampleodor andchemokine receptors.cite journal |author=Sakmar T |title=Structure of rhodopsin and the superfamily of seven-helical receptors: the same and not the same |journal=Curr Opin Cell Biol |volume=14 |issue=2 |pages=189–95 |year=2002 |pmid=11891118 |doi=10.1016/S0955-0674(02)00306-X]Mechanism of light reception
Instead of being activated by binding chemical
ligand s like their relatives, rhodopsins contain retinal which changes conformation in reaction to light viaphotoisomerization and thus are activated by light. The retinal molecule can take on several different cis-trans isomeric forms, such as all-"trans", 11-"cis" and 13-"cis".Photoisomerization (light -dependentisomerization ) of retinal from "cis" to "trans" or vice versa induces a conformational change in the receptor protein. This change acts as a molecular switch to activate asignal transduction mechanism within the cell. Depending on the type of rhodopsin, it either opens anion channel (for example in bacteria) or activates an associatedG protein and triggers asecond messenger cascade (for example in animal eyes).Types of rhodopsins
Retinylidene proteins or rhodopsins are present in many species from bacteria to algae and animals. They can be divided into two distinct groups based on their sequence as well as the retinal isomer they contain at the ground state and their signal transduction mechanisms.
Ion channels and pumps
Rhodopsins found in prokaryotes and algae commonly contain an all-"trans" retinal isomer at the ground state that isomerizes to 13-"cis" upon light activation, also known as microbial-type chromophore. Examples are bacterial sensory rhodopsins,
channelrhodopsin ,bacteriorhodopsin ,halorhodopsin , andproteorhodopsin . They act as light-gated ion channels and can be further distinguished by the type of ion they channel. Bacteriorhodopsin functions as aproton pump , whereas halorhodopsin act as achloride channel . Their functions range from bacterialphotosynthesis (bacteriorhodopsin) to drivingphototaxis (channelrhodopsins inflagellate s). Signal transduction in phototaxis involvesdepolarization of the cell membrane.cite journal |author=Nagel G, Szellas T, Kateriya S, Adeishvili N, Hegemann P, Bamberg E |title=Channelrhodopsins: directly light-gated cation channels |journal=Biochem Soc Trans |volume=33 |issue=Pt 4 |pages=863–6 |year=2005 |pmid=16042615 |doi=10.1042/BST0330863]G protein-coupled receptors
The retinylidene proteins of the animal kingdom are also referred to as
opsin s.Vertebrate s contain five subfamilies of (rhod)opsins andarthropod s three subfamilies [ [http://www.gpcr.org/7tm/seq/001_004/001_004.html G Protein-Coupled Receptor Data Base] ] . Opsins belong to the class ofG protein-coupled receptor s and bind an 11-"cis" isomer of retinal at the ground state that photoisomerizes to an all-"trans" retinal upon light activation. They are commonly found in the light-sensing organs, for example in thephotoreceptor cell s of vertebrateretina where they facilitate eyesight. Animal opsins can also be found in theskin of amphibians, thepineal gland s of lizards and birds, thehypothalamus of toads, and the humanbrain . They can be categorized into several distinct classes including:*visual opsins (classical
rhodopsin and relatives),
*melanopsin s
*peropsin s
*neuropsin s
*encephalopsin sVisual perception
The "visual purple"
rhodopsin (opsin-2) of therod cell s in the vertebrate retina absorbs green-blue light. Thephotopsin s of thecone cell s of the retina differ in a fewamino acid s resulting in a shift of their light absorption spectra. The three human photopsins absorb yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light and are the basis ofcolor vision , whereas the more light-sensitive "visual purple" is responsible for the monochromatic vision in the dark. Light signal transduction involves an enzyme cascade of G-proteins (transducin ),cGMP phosphodiesterase , closure of acation channel and ultimately hyperpolarization of the visualphotoreceptor cell .cite journal |author=Terakita A |title=The opsins |journal=Genome Biol |volume=6 |issue=3 |pages=213 |year=2005 |pmid=15774036 |doi=10.1186/gb-2005-6-3-213]The visual rhodopsins of
arthropod s andmollusc s differ from the vertebrate proteins in their signal transduction cascade involving G-proteins,phospholipase C , and ultimatelydepolarization of the visual photoreceptor cell.Others
Other
opsin s found in humans include encephalopsin (or panopsin, opsin-3), melanopsin (opsin-4), neuropsin (opsin-5) and peropsin.Melanopsin is involved in the light entrainment of thecircadian clock in vertebrates. Encephalopsins and neuropsins are highly expressed in nerve cells and brain tissue, but so far their function is unknown. Peropsin binds all-"trans" retinal (microbial-type chromophore) and might function as aphotoisomerase to return retinal to the 11-"cis" isomer form needed in visual perception.See also
*
Opsin
*Rhodopsin
*Visual cycle
*Visual phototransduction References
External links
* [http://opm.phar.umich.edu/families.php?superfamily=6 Calculated positions of retinylidene proteins in the lipid bilayer]
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