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.
There are two groups of protein termed opsins; these groups are not homologous, but have convergently evolved a similar form and function. Type I opsins are employed by prokaryotes, whereas animals use type II opsins. No opsins have been found outside these groups (for instance in plants, fungi, or placozoans), although as-yet unconfirmed reports of opsins in sponges would suggest that opsins were present in the ancestral metazoan.
Based on the phylogeny of their sequences, type 1 opsins[verification needed] can be grouped into six families; these families are very distinct, with under 20% of their sequences shared with any other subfamily. The families consist of the vertebrate opsins/encephalopsins; Go opsins; Gs opsins; invertebrate Gq opsins; the photoisomerases and neuropsins. These subfamilies can be grouped according to their expression; the first three are found in ciliary-type photoreceptor cells; Gq opsins in rhabdomeric-type photoreceptor cells; and the latter two are found elsewhere but based on their shared intron positions can be bundled together into the photoisomerases.
Prokaryotic (type 1) opsins
Like eukaryotic opsins, prokaryotic opsins have a seven transmembrane domain structure similar to that found in eukaryotic G-protein coupled receptors. Despite this similarity, there is no evidence that they are evolutionarily related, suggesting that they evolved independently of one another.
Several type 1 opsins, such as proteo-, halo- and bacteriorhodopsin, are used by various bacterial groups to harvest energy from light to fix carbon using a non-chlorophyll-based pathway. Additionally, sensory rhodopsins exist in Halobacteria that induce a phototactic response by interacting with transducer membrane-embedded proteins that have no relation to G proteins.
Animal (type 2) opsins
Ciliary opsins are expressed in ciliary photoreceptor cells, and include the vertebrate opsins/encephalopsins, Go and Gs opsin subfamilies. They convert light signals to nerve impulses via cyclic nucleotide gated ion channels, which work by increasing the charge differential across the cell membrane (i.e. hyperpolarization.
Vertebrate opsins can be further subdivided into rod opsins and four types of cone opsin, based on differential spatial expression, spectral sensitivity, and evolutionary history. Rod opsins (rhodopsins, usually denoted Rh), are used in night vision, are thermally stable, and are found in the rod photoreceptor cells. Cone opsins, employed in color vision, are less-stable 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: 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 known 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".
This type of opsin is expressed throughout the mammalian body. It is also expressed in ciliary photoreceptor cells in annelids, and in the brains of some insects.
Go / Gs opsins
These opsins, absent from higher vertebrates and arthropods, are found in the ciliary photoreceptor cells of molluscs and basal chordates (amphioxus); and in cnidarians, respectively.
Arthropods and molluscs use Gq opsins. Arthropods appear to attain colour vision in a similar fashion to the vertebrates, by the use of three (or more) distinct groups of opsin, distinct both in terms of phylogeny and spectral sensitivity. The Gq opsin melanopsin is also expressed in vertebrates, where it is responsible for the maintenance of circadian rhythms.
Unlike ciliary opsins, these are associated with canonical transient receptor potential ion channels; these lead to the electric potential difference across a cell membrane being eradicated (i.e. depolarization).
The identification of the crystal structure of squid rhodopsin is likely to further our understanding of its function in this group.
Arthropods do use different opsins in their different eye types, but at least in Limulus the opsins expressed in lateral and in compound eyes are 99% identical and presumably diverged recently.
This class of opsins are not coupled to a G-protein, and thus serve to traffic retinal around in response to light, rather than directly in signal-induction.
These opsins are found in nervous tissue in mammals, and despite some genetic similarities to photoisomerases, their function has not yet been identified.
|Melanopsin||OPN4||best studied novel opsin involved in circadian rhythms and pupillary reflex|
|Pineal Opsin (Pinopsin)||wide range of expression in the brain, most notably in the pineal region|
|Vertebrate Ancient (VA) opsin||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 |
|Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh)||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) Opsin||Teleost fish opsin with a wide range of expression|
|Peropsin or "Retinal pigment epithelium-derived rhodopsin homolog"||RRH||expressed in the retinal pigment epithelium (RPE) cells|
|Retinal G protein coupled receptor||RGR||expressed in the retinal pigment epithelium (RPE) and Müller cells|
- ↑ 1.0 1.1 1.2 1.3 Plachetzki, D.; Fong, C.; Oakley, T. (2010). "The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway". Proceedings. Biological sciences / the Royal Society. 277 (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMC 2880087 Freely accessible. PMID 20219739.
- ↑ 2.0 2.1 Fernald, R. D. (2006). "Casting a genetic light on the evolution of eyes". Science (New York, N.Y.). 313 (5795): 1914–1918. doi:10.1126/science.1127889. PMID 17008522.
- ↑ "Porifera: Amphimedon queenslandica (sponge) .. 1 opsin". GenomeWiki. University of California Santa Cruz (UCSC) Genome Bioinformatics Group.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 Shichida, Y.; Matsuyama, T. (2009). "Evolution of opsins and phototransduction". Philosophical transactions of the Royal Society of London. Series B, Biological sciences. 364 (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMC 2781858 Freely accessible. PMID 19720651.
- ↑ Römpler, H.; Stäubert, C.; Thor, D.; Schulz, A.; Hofreiter, M.; Schöneberg, T. (2007). "G protein-coupled time travel: evolutionary aspects of GPCR research". Molecular interventions. 7 (1): 17–25. doi:10.1124/mi.7.1.5. PMID 17339603.
- ↑ Terakita, A. (2005). "The opsins". Genome biology. 6 (3): 213. doi:10.1186/gb-2005-6-3-213. PMC 1088937 Freely accessible. PMID 15774036.
- ↑ Murakami, M.; Kouyama, T. (2008). "Crystal structure of squid rhodopsin". Nature. 453 (7193): 363. doi:10.1038/nature06925. PMID 18480818.
- ↑ Smith, WC; Price, DA; Greenberg, RM; Battelle, BA (1993). "Opsins from the lateral eyes and ocelli of the horseshoe crab, Limulus polyphemus". Proceedings of the National Academy of Sciences of the United States of America. 90 (13): 6150–4. doi:10.1073/pnas.90.13.6150. PMC 46885 Freely accessible. PMID 8327495.
- ↑ 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/S0169-328X(99)00242-9. 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/S0169-328X(03)00059-7. 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/S1350-9462(00)00002-1. PMID 10785616.
- ↑ 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. PMID 15811001.