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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

Classification

There are two types of opsins, type 1 and type 2. Though similar in structure and function, evidence suggests that they evolved independently of one another in bacteria and animals.[1]

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Prokaryotic opsins (type 1)

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.[1]

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. [2]

Classical type 2 opsin 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 thermally stable opsins 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:[3] 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".

Other animals may have different number of photoreceptor proteins, whose optical absorption spectra are shifted compared to those in human photoreceptors. This leads to a different perception of light and visual images in humans and other 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 type 2 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 opsins[4].

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 al.[10]. Lys296 is conserved in all known opsins and serves as the site for the Schiff base linkage with the chromophore. Cys138 and Cys110 form a highly conserved disulfide bridge. Glu113 serves as the counterion, stabilizing the protonation of the Schiff linkage between Lys296 and the chromophore. The Glu134-Arg135-Tyr136 is another highly conserved motif, involved in the propagation 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 Yokoyama[11] and Deeb[12]. The impact of spectral tuning sites on λmax differs between different opsin groups and between opsin groups of different species.

External links

References

  1. ^ a b Fernald, Russell D. (September 2006), "Casting a Genetic Light on the Evolution of Eyes", Science 313 (5795): 1914–1918, doi:10.1126/science.1127889, PMID 17008522 
  2. ^ Römpler H, Stäubert C, Thor D, Schulz A, Hofreiter M, Schöneberg T (February 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 
  3. ^ Terakita A (2005). "The Opsins". Genome Biology 213 (6(3)): 213. doi:10.1186/gb-2005-6-3-213. 
  4. ^ Bellingham J, Foster RG (2002). "Opsins and mammalian photoentrainment". Cell and Tissue Research 309 (1): 57–71. doi:10.1007/s00441-002-0573-4. 
  5. ^ 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. 
  6. ^ 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. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. 
  11. ^ 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. 
  12. ^ 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. 
Name Gene Notes
Melanopsin OPN4 best studied novel opsin involved in circadian rhythms and pupillary reflex
Pineal Opsin (Pinopsin)[5] wide range of expression in the brain, most notably in the pineal region
Vertebrate Ancient (VA) opsin[6] 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 [7]
Extraretinal (or extra-ocular) Rhodopsin-Like Opsins (Exo-Rh)[8] 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[9] 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
Neuropsin OPN5

Simple English

Opsins are the universal photoreceptor molecules of all visual systems in the animal kingdom.[1][2]

They change from a resting state to a signalling state by absorbing light. This activates the G protein, resulting in a signalling cascade which produces physiological responses.

This process of capturing a photon and transforming it into a physiological response is known as phototransduction.

Five 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.

References

  1. Plachetzki, D.; Fong, C.; Oakley T. 2010. "The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway". Proceedings of the Royal Society / Biological sciences 277 (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMID 20219739.
  2. Shichida Y.; Matsuyama T. 2009. "Evolution of opsins and phototransduction". [1] Philosophical transactions of the Royal Society of London. Series B, Biological sciences. 364 (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMID 19720651

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