Rhodopsin Explained

Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene^[1] and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction in rods. Rhodopsin mediates dim light vision and thus is extremely sensitive to light.^[2] When rhodopsin is exposed to light, it immediately photobleaches. In humans, it is regenerated fully in about 30 minutes, after which the rods are more sensitive.^[3] Defects in the rhodopsin gene cause eye diseases such as retinitis pigmentosa and congenital stationary night blindness.

Names

Rhodopsin was discovered by Franz Christian Boll in 1876.^{[4] [5] [6]} The name rhodospsin derives from Ancient Greek for "rose", due to its pinkish color, and for "sight".^[7] It was coined in 1878 by the German physiologist Wilhelm Friedrich Kühne (1837–1900).^{[8] [9]}

When George Wald discovered that rhodopsin is a holoprotein, consisting of retinal and an apoprotein, he called it opsin, which today would be described more narrowly as apo-rhodopsin.^[10] Today, the term opsin refers more broadly to the class of G-protein-coupled receptors that bind retinal and as a result become a light sensitive photoreceptor, including all closely related proteins.^[11] When Wald and colleges later isolated iodopsin from chicken retinas, thereby discovering the first known cone opsin, they called apo-iodopsin photopsin (for its relation to photopic vision) and apo-rhodopsin scotopsin (for its use in scotopic vision).^[12]

General

Rhodopsin is a protein found in the outer segment discs of rod cells. It mediates scotopic vision, which is monochromatic vision in dim light.^{[3] [13]} Rhodopsin most strongly absorbs green-blue light (~500 nm)^{[14] [15]} and appears therefore reddish-purple, hence the archaic term "visual purple".

Several closely related opsins differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. Humans have, including rhodopsin, nine opsins,^[16] as well as cryptochrome (light-sensitive, but not an opsin).^[17]

Structure

Rhodopsin, like other opsins, is a G-protein-coupled receptor (GPCR).^{[18] [19]} GPCRs are chemoreceptors that embed in the lipid bilayer of the cell membranes and have seven transmembrane domains forming a binding pocket for a ligand.^{[20] [21]} The ligand for rhodopsin is the vitamin A-based chromophore 11-cis-retinal,^{[22] [23] [24] [25] [26]} which lies horizontally to the cell membrane^[27] and is covalently bound to a lysine residue (lys296)^[28] in the seventh transmembrane domain^[29] through a Schiff-base.^{[30] [31]} However, 11-cis-retinal only blocks the binding pocket and does not activate rhodopsin. It is only activated when 11-cis-retinal absorbs a photon of light and isomerizes to all-trans-retinal,^{[32] [33]} the receptor activating form,^{[34] [35]} causing conformal changes in rhodopsin (bleaching), which activate a phototransduction cascade.^[36] Thus, a chemoreceptor is converted to a light or photo(n)receptor.^[37]

The retinal binding lysine is conserved in almost all opsins, only a few opsins having lost it during evolution. Opsins without the lysine are not light sensitive,^{[38] [39] [40]} including rhodopsin. Rhodopsin is made constitutively (continuously) active by some of those mutations even without light.^{[41] [42] [43]} Also wild-type rhodopsin is constitutively active, if no 11-cis-retinal is bound, but much less.^[44] Therefore 11-cis-retinal is an inverse agonist. Such mutations are one cause of autosomal dominant retinitis pigmentosa.^[43] Artificially, the retinal binding lysine can be shifted to other positions, even into other transmembrane domains, without changing the activity.^[45]

The rhodopsin of cattle has 348 amino acids, the retinal binding lysine being Lys296. It was the first opsin whose amino acid sequence^[46] and 3D-structure were determined. Its structure has been studied in detail by x-ray crystallography on rhodopsin crystals.^[47] Several models (e.g., the bicycle-pedal mechanism, hula-twist mechanism) attempt to explain how the retinal group can change its conformation without clashing with the enveloping rhodopsin protein pocket.^{[48] [49] [50]} Recent data support that rhodopsin is a functional monomer, instead of a dimer, which was the paradigm of G-protein-coupled receptors for many years.^[51]

Within its native membrane, rhodopsin is found at a high density facilitating its ability to capture photons. Due to its dense packing within the membrane, there is a higher chance of rhodopsin capturing proteins. However, the high density also provides a disadvantage when it comes to G protein signaling because the diffusion becomes more difficult in a crowded membrane that is packed with the receptor, rhodopsin.^[52]

Phototransduction

Rhodopsin is an essential G-protein coupled receptor in phototransduction.

Activation

In rhodopsin, the aldehyde group of retinal is covalently linked to the amino group of a lysine residue on the protein in a protonated Schiff base (-NH⁺=CH-). When rhodopsin absorbs light, its retinal cofactor isomerizes from the 11-cis to the all-trans configuration, and the protein subsequently undergoes a series of relaxations to accommodate the altered shape of the isomerized cofactor. The intermediates formed during this process were first investigated in the laboratory of George Wald, who received the Nobel prize for this research in 1967.^[53] The photoisomerization dynamics has been subsequently investigated with time-resolved IR spectroscopy and UV/Vis spectroscopy. A first photoproduct called photorhodopsin forms within 200 femtoseconds after irradiation, followed within picoseconds by a second one called bathorhodopsin with distorted all-trans bonds. This intermediate can be trapped and studied at cryogenic temperatures, and was initially referred to as prelumirhodopsin.^[54] In subsequent intermediates lumirhodopsin and metarhodopsin I, the Schiff's base linkage to all-trans retinal remains protonated, and the protein retains its reddish color. The critical change that initiates the neuronal excitation involves the conversion of metarhodopsin I to metarhodopsin II, which is associated with deprotonation of the Schiff's base and change in color from red to yellow.^[55]

Phototransduction cascade

See main article: Visual phototransduction.

The product of light activation, Metarhodopsin II, initiates the visual phototransduction second messenger pathway by stimulating the G-protein transducin (G_t), resulting in the liberation of its α subunit. This GTP-bound subunit in turn activates a cGMP phosphodiesterase. The cGMP phosphodiesterase hydrolyzes (breaks down) cGMP, lowering its local concentration so it can no longer activate cGMP-dependent cation channels. This leads to the hyperpolarization of photoreceptor cells, changing the rate at which they release transmitters.^[56]

Deactivation

Meta II (metarhodopsin II) is deactivated rapidly after activating transducin by rhodopsin kinase and arrestin.^[57] Rhodopsin pigment must be regenerated for further phototransduction to occur. This means replacing all-trans-retinal with 11-cis-retinal and the decay of Meta II is crucial in this process. During the decay of Meta II, the Schiff base link that normally holds all-trans-retinal and the apoprotein opsin (aporhodopsin) is hydrolyzed and becomes Meta III. In the rod outer segment, Meta III decays into separate all-trans-retinal and opsin.^[57] A second product of Meta II decay is an all-trans-retinal opsin complex in which the all-trans-retinal has been translocated to second binding sites. Whether the Meta II decay runs into Meta III or the all-trans-retinal opsin complex seems to depend on the pH of the reaction. Higher pH tends to drive the decay reaction towards Meta III.^[57]

Diseases of the retina

Mutations in the rhodopsin gene contribute majorly to various diseases of the retina such as retinitis pigmentosa. In general, the defect rhodopsin aggregates with ubiquitin in inclusion bodies, disrupts the intermediate filament network, and impairs the ability of the cell to degrade non-functioning proteins, which leads to photoreceptor apoptosis.^[58] Other mutations on rhodopsin lead to X-linked congenital stationary night blindness, mainly due to constitutive activation, when the mutations occur around the chromophore binding pocket of rhodopsin.^[59] Several other pathological states relating to rhodopsin have been discovered including poor post-Golgi trafficking, dysregulative activation, rod outer segment instability and arrestin binding.^[59]

See also

• Bacteriorhodopsin, used in some halobacteria as a light-driven proton pump.

Further reading

• Humphries P, Kenna P, Farrar GJ . On the molecular genetics of retinitis pigmentosa . Science . 256 . 5058 . 804–808 . May 1992 . 1589761 . 10.1126/science.1589761 . 1992Sci...256..804H .
• Edwards SC . Involvement of cGMP and calcium in the photoresponse in vertebrate photoreceptor cells . The Journal of the Florida Medical Association . 82 . 7 . 485–488 . July 1995 . 7673885 .
• al-Maghtheh M, Gregory C, Inglehearn C, Hardcastle A, Bhattacharya S . Rhodopsin mutations in autosomal dominant retinitis pigmentosa . Human Mutation . 2 . 4 . 249–255 . 1993 . 8401533 . 10.1002/humu.1380020403 . 28459589 . free .
• Garriga P, Manyosa J . The eye photoreceptor protein rhodopsin. Structural implications for retinal disease . FEBS Letters . 528 . 1–3 . 17–22 . September 2002 . 12297272 . 10.1016/S0014-5793(02)03241-6 . 41860711 . free .
  • Inglehearn CF, Keen TJ, Bashir R, Jay M, Fitzke F, Bird AC, Crombie A, Bhattacharya S . A completed screen for mutations of the rhodopsin gene in a panel of patients with autosomal dominant retinitis pigmentosa . Human Molecular Genetics . 1 . 1 . 41–45 . April 1992 . 1301135 . 10.1093/hmg/1.1.41 .
• Farrar GJ, Findlay JB, Kumar-Singh R, Kenna P, Humphries MM, Sharpe E, Humphries P . Autosomal dominant retinitis pigmentosa: a novel mutation in the rhodopsin gene in the original 3q linked family . Human Molecular Genetics . 1 . 9 . 769–771 . December 1992 . 1302614 . 10.1093/hmg/1.9.769 .
• Robinson PR, Cohen GB, Zhukovsky EA, Oprian DD . Constitutively active mutants of rhodopsin . Neuron . 9 . 4 . 719–725 . October 1992 . 1356370 . 10.1016/0896-6273(92)90034-B . 13172583 .
• Fujiki K, Hotta Y, Hayakawa M, Sakuma H, Shiono T, Noro M, Sakuma T, Tamai M, Hikiji K, Kawaguchi R . Point mutations of rhodopsin gene found in Japanese families with autosomal dominant retinitis pigmentosa (ADRP) . The Japanese Journal of Human Genetics . 37 . 2 . 125–132 . June 1992 . 1391967 . 10.1007/BF01899733 . free .
• Olsson JE, Gordon JW, Pawlyk BS, Roof D, Hayes A, Molday RS, Mukai S, Cowley GS, Berson EL, Dryja TP . Transgenic mice with a rhodopsin mutation (Pro23His): a mouse model of autosomal dominant retinitis pigmentosa . Neuron . 9 . 5 . 815–830 . November 1992 . 1418997 . 10.1016/0896-6273(92)90236-7 . 37524461 .
• Andréasson S, Ehinger B, Abrahamson M, Fex G . A six-generation family with autosomal dominant retinitis pigmentosa and a rhodopsin gene mutation (arginine-135-leucine) . Ophthalmic Paediatrics and Genetics . 13 . 3 . 145–153 . September 1992 . 1484692 . 10.3109/13816819209046483 .
• Inglehearn CF, Lester DH, Bashir R, Atif U, Keen TJ, Sertedaki A, Lindsey J, Jay M, Bird AC, Farrar GJ . Recombination between rhodopsin and locus D3S47 (C17) in rhodopsin retinitis pigmentosa families . American Journal of Human Genetics . 50 . 3 . 590–597 . March 1992 . 1539595 . 1684283 .
• Fishman GA, Stone EM, Gilbert LD, Sheffield VC . Ocular findings associated with a rhodopsin gene codon 106 mutation. Glycine-to-arginine change in autosomal dominant retinitis pigmentosa . Archives of Ophthalmology . 110 . 5 . 646–653 . May 1992 . 1580841 . 10.1001/archopht.1992.01080170068026 .
• Keen TJ, Inglehearn CF, Lester DH, Bashir R, Jay M, Bird AC, Jay B, Bhattacharya SS . Autosomal dominant retinitis pigmentosa: four new mutations in rhodopsin, one of them in the retinal attachment site . Genomics . 11 . 1 . 199–205 . September 1991 . 1765377 . 10.1016/0888-7543(91)90119-Y .
• Dryja TP, Hahn LB, Cowley GS, McGee TL, Berson EL . Mutation spectrum of the rhodopsin gene among patients with autosomal dominant retinitis pigmentosa . Proceedings of the National Academy of Sciences of the United States of America . 88 . 20 . 9370–9374 . October 1991 . 1833777 . 52716 . 10.1073/pnas.88.20.9370 . free . 1991PNAS...88.9370D .
• Gal A, Artlich A, Ludwig M, Niemeyer G, Olek K, Schwinger E, Schinzel A . Pro-347-Arg mutation of the rhodopsin gene in autosomal dominant retinitis pigmentosa . Genomics . 11 . 2 . 468–470 . October 1991 . 1840561 . 10.1016/0888-7543(91)90159-C .
• Sung CH, Davenport CM, Hennessey JC, Maumenee IH, Jacobson SG, Heckenlively JR, Nowakowski R, Fishman G, Gouras P, Nathans J . Rhodopsin mutations in autosomal dominant retinitis pigmentosa . Proceedings of the National Academy of Sciences of the United States of America . 88 . 15 . 6481–6485 . August 1991 . 1862076 . 52109 . 10.1073/pnas.88.15.6481 . free . 1991PNAS...88.6481S .
• Jacobson SG, Kemp CM, Sung CH, Nathans J . Retinal function and rhodopsin levels in autosomal dominant retinitis pigmentosa with rhodopsin mutations . American Journal of Ophthalmology . 112 . 3 . 256–271 . September 1991 . 1882937 . 10.1016/s0002-9394(14)76726-1 . free .
• Sheffield VC, Fishman GA, Beck JS, Kimura AE, Stone EM . Identification of novel rhodopsin mutations associated with retinitis pigmentosa by GC-clamped denaturing gradient gel electrophoresis . American Journal of Human Genetics . 49 . 4 . 699–706 . October 1991 . 1897520 . 1683182 .

External links

• • Web site: Webvision Home Page: The organization of the retina and visual system . Kolb H, Fernandez E, Nelson R, Jones BW . 2010-03-01 . University of Utah .
• The Rhodopsin Protein
• Photoisomerization of rhodopsin, animation.
• Rhodopsin and the eye, summary with pictures.

Notes and References

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