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14-September-2008 18:02:44 - Opsin 3-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red. 3-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The chromophore is shown in red. Opsins are a group of light-sensitive 35-55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade. Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming. Contents 1 Classification 1.1 Classical groups 1.2 Novel opsin groups 2 Structure and Function 3 External links 4 References Classification Classical groups Two families of vertebrate opsins are generally recognized due to different spatial expression and evolutionary histories. Rhodopsins, which are used in night vision, are high-sensitivity, low-acuity opsins found in the rod photoreceptor cells. Cone opsins, employed in color vision, are low-sensitivity, high-acuity opsins located in the cone photoreceptor cells. Cone opsins are further subdivided according to their absorption maxima λmax, the wavelength at which the highest light absorption is observed. Evolutionary relationships, deduced using the amino acid sequence of the opsins, are also frequently used to categorize cone opsins into their respective group. Both methods predict four general cone opsin groups in addition to rhodopsin:1 For example, humans have the following set of photoreceptor proteins responsible for vision: Rhodopsin Rh1, OPN2, RHO - expressed in rod cells, used in night vision Four cone opsins also knows as photopsins - expressed in cone cells, used in color vision Long Wavelength Sensitive LWS, OPN1LW Opsin - λmax in the red region of the electromagnetic spectrum Middle Wavelength Sensitive RH2 or MWS Opsin - λmax in the green region of the electromagnetic spectrum Short Wavelength Sensitive 2 SWS2 Opsin - λmax in the blue region of the electromagnetic spectrum Short Wavelength Sensitive 1 SWS1 Opsin - λmax in the violet/UV region of the electromagnetic spectrum The human genes for these last three are OPN1MW, OPN1MW2, and OPN1SW, with the first two being medium-wave and the third being short-wave. Other animals may have different number of photoreceptor proteins, whose optical adsorption spectra are shifted compare to that in human photoreceptors. This leads to differen perception of light and visual images in humans and animals. Some insects, for example, can see ultraviolet light, whereas animals with only one type of opsins can see the world only in black and white colors. Novel opsin groups Over the last decade, several novel opsin groups have been discovered that are not involved in vision and that do not group with the five classical groups described above. Much of the research is still ongoing, with the function of many novel opsins unknown. The five classical opsins above are expressed solely in the retina, whereas the new novel opsins have a wide range of expression patterns. Phylogenetic studies have been undertaken to categorize these new opsins and determine their evolutionary relationship to the classical opsins2. Melanopsin OPN4 - best studied novel opsin involved in circadian rhythms and pupillary reflex Pineal Opsin Pinopsin3 - wide range of expression in the brain, most notably in the pineal region Vertebrate Ancient VA opsin4 - has three isoforms VA short VAS, VA medium VAM, and VA long VAL. It is expressed in the inner retina, within the horizontal and amacrine cells, as well as the pineal organ and habenular region of the brain Parapinopsin PP Opsin 5 Extraretinal or extra-ocular Rhodopsin-Like Opsins Exo-Rh6 - Rhodopsin-like protein expressed in the pineal region Encephalopsin or Panopsin OPN3 - originally found in human and mice tissue with a very wide range of expression brain, testes, heart, liver, kidney, skeletal muscle, lung, pancreas and retina Teleost Multiple Tissue TMT Opsin7 - Teleost fish opsin with a wide range of expression Peropsin RRH - expressed in the retinal pigment epithelium RPE cells RGR-opsin - expressed in the retinal pigment epithelium RPE and Müller cells Neuropsin OPN5 and KLK8 Structure and Function Opsin proteins covalently bind to a vitamin A-based retinaldehyde chromophore through a Schiff base linkage to a lysine residue in the seventh transmembrane alpha helix. In vertebrates, the chromophore is either 11-cis-retinal A1 or 11-cis-3,4-didehydroretinal A2 and is found in the retinal binding pocket of the opsin. The absorption of a photon of light results in the photoisomerisation of the chromophore from the 11-cis to an all-trans conformation. The photoisomerization induces a conformational change in the opsin protein, causing the activation of the phototransduction cascade. The opsin remains insensitive to light in the trans form. It is regenerated by the replacement of the all-trans retinal by a newly synthesized 11-cis-retinal provided from the retinal epithelial cells. Opsins are functional while bound to either chromophore, with A2-bound opsin λmax being at a longer wavelength than A1-bound opsin. Opsins contain seven transmembrane α-helical domains connected by three extra-cellular and three cytoplasmic loops. Many amino acid residues, termed functionally conserved residues, are highly conserved between all opsin groups, indicative of important functional roles. All residue positions discussed henceforth are relative to the 348 amino acid bovine rhodopsin crystallized by Palczewski et al8. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromaphore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabalizing the Lys296/chromaphore interaction by neutralizing the positive charge that builds up when the Schiff linkage occurs, The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propegation of the transduction signal once a photon has been absorbed. Certain amino acid residues, termed spectral tuning sites, have a strong effect on λmax values. Using site-directed mutagenesis, it is possible to selectively mutate these residues and investigate the resulting changes in light absorption properties of the opsin. It is important to differentiate spectral tuning sites, residues that affect the wavelength at which the opsin absorbs light, from functionally conserved sites, residues important for the proper functioning of the opsin. They are not mutually exclusive, but, for practical reasons, it is easier to investigate spectral tuning sites that do not affect opsin functionality. For a comprehensive review of spectral tuning sites see Yokoyama9 and Deeb10. The impact of spectral tuning sites on λmax differs between different opsin groups and between opsin groups of different species. External links Illustration at Baldwin-Wallace College Overview at NASA Tree of vertebrate opsin at University of Tokyo MeSH Opsin References ^ Terakita A 2005. The Opsins. Genome Biology 213 63: 213. doi:10.1186/gb-2005-6-3-213. ^ Bellingham J, Foster RG 2002. Opsins and mammalian photoentrainment. Cell and Tissue Research 309 1: 57-71. doi:10.1007/s00441-002-0573-4. ^ Okano T, Yoshizawa T, Fukada Y 1994. Pinopsin is a chicken pineal photoreceptive molecule. Nature 372 6501: 94-7. doi:10.1038/372094a0. PMID 7969427. ^ Philp AR, Garcia-Fernandez JM, Soni BG, Lucas RJ, Bellingham J, Foster RG 2000. Vertebrate ancient VA opsin and extraretinal photoreception in the Atlantic salmon Salmo salar. J. Exp. Biol. 203 Pt 12: 1925-36. PMID 10821749. ^ Blackshaw S, Snyder SH 1997. Parapinopsin, a novel catfish opsin localized to the parapineal organ, defines a new gene family. J. Neurosci. 17 21: 8083-92. PMID 9334384. ^ Mano H, Kojima D, Fukada Y 1999. Exo-rhodopsin: a novel rhodopsin expressed in the zebrafish pineal gland. Brain Res. Mol. Brain Res. 73 1-2: 110-8. doi:10.1016/S0169-328X9900242-9. PMID 10581404. ^ Moutsaki P, Whitmore D, Bellingham J, Sakamoto K, David-Gray ZK, Foster RG 2003. Teleost multiple tissue tmt opsin: a candidate photopigment regulating the peripheral clocks of zebrafish?. Brain Res. Mol. Brain Res. 112 1-2: 135-45. doi:10.1016/S0169-328X0300059-7. PMID 12670711. ^ Palczewski K et al 2000. Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor. Science 289 5480: 739-45. doi:10.1126/science.289.5480.739. PMID 10926528. ^ Yokoyama S 2000. Molecular evolution of vertebrate visual pigments. Progress in Retinal and Eye Research 19 4: 385-419. doi:10.1016/S1350-94620000002-1. ^ Deeb SS 2005. The molecular basis of variation in human color vision. Clinical genetics 67 5: 369-77. doi:10.1111/j.1399-0004.2004.00343.x. v d e Eye proteins Opsin retinylidene protein Rhodopsin - Melanopsin - Photopsin Crystallin Alpha crystallin Other Arrestin - Guanylate cyclase activator - Recoverin - Rhodopsin kinase v d e Transmembrane receptor: G protein-coupled receptors Class A: Rhodopsin like Adrenergic α1 A, B, D, α2 A, B, C, β1, β2, β3 Eicosanoid CysLT 1, 2, LTB4 1, 2, FPRL1, OXE, Prostaglandin DP, EP 1, 2, 3, 4, PGF, Prostacyclin, Thromboxane Neuropeptide B/W 1, 2, FF 1, 2, S, Y 1, 2, 4, 5 Orphan GPR 1, 3, 4, 6, 12, 15, 17, 18, 19, 20, 21, 22, 23, 25, 26, 27, 31, 32, 33, 34, 35, 37, 39, 42, 44, 45, 50, 52, 55, 61, 62, 63, 65, 68, 75, 77, 78, 79, 82, 83, 84, 85, 87, 88, 92, 101, 103, 119, 120, 132, 135, 139, 141, 142, 146, 148, 149, 150, 151, 152, 153, 160, 161, 162, 171, 172, 173, 174, 176, 177, 182 Purinergics Adenosine A1, A2a, A2b, A3, P2Y, 1, 2, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14 Serotonin all but 5-HT3 5-HT1 A, B, D, E, F, 5-HT2 A, B, C, 5-HT 4, 5A, 6, 7 Other Acetylcholine M1, M2, M3, M4, M5 - Adrenomedullin - Anaphylatoxin C3a, C5a - Angiotensin 1, 2 - Apelin - Bile acid - Bombesin BRS3, GRPR, NMBR - Bradykinin B1, B2 - Cannabinoid CB1, CB2 - Chemokine - Cholecystokinin A, B - Dopamine D1, D2, D3, D4, D5 - EBI2 - Endothelin A, B - Estrogen - Formyl peptide 1, L1, L2 - Free fatty acid 1, 2, 3, 4 - FSH - Galanin 1, 2, 3 - Gonadotropin-releasing hormone 1, 2 - Ghrelin - Histamine H1, H2, H3, H4 - Kisspeptin - Luteinizing hormone/choriogonadotropin - Lysophospholipid 1, 2, 3, 4, 5, 6, 7, 8 - MAS 1, 1L, D, E, F, G, X1, X2, X3, X4 - Melanocortin 1, 2, 3, 4, 5 - MCHR 1, 2 - Melatonin 1A, 1B- Motilin - neuromedin B, U 1, 2 - Neurotensin 1, 2 - Opioid Delta, Kappa, Mu, Nociceptin, but not Sigma - Olfactory - Opsin 3, 4, 5, 1LW, 1MW, 1SW, RGR, RRH - Orexin 1, 2 - Oxytocin - Oxoglutarate - PAF - Prokineticin 1, 2 - Prolactin-releasing peptide - Protease-activated 1, 2, 3, 4 - Relaxin 1, 2, 3, 4 - Somatostatin 1, 2, 3, 4, 5 - SREB - Succinate - TAAR 1, 2, 3, 5, 6, 8, 9 - Tachykinin 1, 2, 3 - Thyrotropin - Thyrotropin-releasing hormone - Urotensin-II - Vasopressin 1A, 1B, 2 Class B: Secretin like Brain-specific angiogenesis inhibitor 1, 2, 3 - Cadherin 1, 2, 3 - Calcitonin - CD97 - Corticotropin-releasing hormone 1, 2 - EMR 1, 2, 3 - Glucagon GR, GIPR, GLP1R, GLP2R - Growth hormone releasing hormone - PACAPR1- GPR 56, 64, 97, 98, 110, 111, 112, 113, 114, 115, 116, 123, 124, 125, 126, 128, 133, 143, 144, 157 - Latrophilin 1, 2, 3, ELTD1 - Parathyroid hormone 1, 2 - Secretin - Vasoactive intestinal peptide 1, 2 Class C: Metabotropic glutamate / pheromone Calcium-sensing receptor - GABA B 1, 2 - Glutamate receptor Metabotropic glutamate 1, 2, 3, 4, 5, 6, 7, 8 - GPRC6A - GPR 156, 158, 179 - RAIG 1, 2, 3, 4 - Taste receptors TAS1R 1, 2, 3 TAS2R 1, 3, 4, 5, 8, 9, 10, 12, 13, 14, 16, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50 Frizzled / Smoothened Frizzled 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 - Smoothened Retrieved from http://en..org/wiki/Opsin Categories: G protein coupled receptors | Vision Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages Deutsch Español עברית This page was last modified on 22 June 2008, at 19
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