Rhodopsin

Rhodopsin (opsin 2, rod pigment) (retinitis pigmentosa 4, autosomal dominant)
Sensory rhodopsin II (rainbow colored) embedded in a lipid bilayer (heads red and tails blue) with Transducin below it. Gtα is colored red, Gtβ blue, and Gtγ yellow. There is a bound GDP molecule in the Gtα-subunit and a bound retinal (black) in the rhodopsin. The N-terminus terminus of rhodopsin is red and the C-terminus blue. Presumed anchoring of transducin to the membrane has been drawn in black.
Available structures PDB Ortholog search: Template:Homologene2PDBe PDBe, Template:Homologene2uniprot RCSB List of PDB id codes 1eds​, 1edx​, 1f88​, 1gzm​, 1hzx​, 1jfp​, 1l9h​, 1ln6​, 1u19​, 2g87​, 2hpy​, 2i35​, 2i36​, 2i37​
Identifiers
Symbols	RHO ; MGC138309; MGC138311; OPN2; RP4
External IDs	Template:OMIM5 Template:MGI HomoloGene: 68068
Gene ontology Molecular function Template:GNF GO Template:GNF GO Cellular component Template:GNF GO Template:GNF GO Template:GNF GO Biological process Template:GNF GO Template:GNF GO Template:GNF GO Template:GNF GO Template:GNF GO Template:GNF GO
RNA expression pattern
File:PBB GE RHO 206454 s at tn.png
File:PBB GE RHO 206455 s at tn.png
More reference expression data
Orthologs
Template:GNF Ortholog box
Species	Human	Mouse
Entrez	n/a	n/a
Ensembl	n/a	n/a
UniProt	n/a	n/a
RefSeq (mRNA)	n/a	n/a
RefSeq (protein)	n/a	n/a
Location (UCSC)	n/a	n/a
PubMed search	n/a	n/a

Rhodopsin, also known as visual purple, is expressed in metazoan photoreceptor cells. It is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the class of G-protein coupled receptors. Rhodopsin is extremely sensitive to light, and enables night-vision. Exposed to white light, the pigment immediately bleaches, and it takes about 30 minutes to regenerate fully in humans.

Structure

Rhodopsin consists of two building blocks, an opsin protein called scotopsin and a reversibly covalently bound cofactor, retinal (retinaldehyde). The structure of rhodopsin consists of a bundle of seven transmembrane helices that surround the photoreactive chromophore, 11-cis retinal. Retinal, the chromophore portion of rhodopsin, is made in the retina from Vitamin A. Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the opsin that activates the associated G protein and triggers a second messenger cascade.

Rhodopsin of the rods most strongly absorbs green-blue light and therefore appears reddish-purple, which is why it is also called "visual purple". It is responsible for the monochromatic vision in the dark.

File:Bovine rhodopsin.png

Bovine rhodopsin

Several closely related opsins, the photopsins, exist that differ only in a few amino acids and in the wavelengths of light that they absorb most strongly. These pigments are found in the different types of the cone cells of the retina and are the basis of color vision. Humans have three different other opsins beside rhodopsin, with absorption maxima for yellowish-green (photopsin I), green (photopsin II), and bluish-violet (photopsin III) light.

The photoisomerization of rhodopsin has been studied in detail via x-ray crystallography on rhodopsin crystals. 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. Several models (a.o. 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 .

Rhodopsin and retinal disease

Mutation of the rhodopsin gene is a major contributor to various retinopathies such as retinitis pigmentosa. The disease causing protein generally 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 . Other mutations on rhodopsin lead to congential stationary night blindness, mainly due to constitutive activation, when the mutations occur around the chromophore binding pocket of rhodopsin (Mendes et al., 2005). Several other pathological states relating to rhodopsin have been discovered including poor post-Golgi trafficing, dysregulative activation, rod outer segment instability and arrestin binding .

Microbial rhodopsins

Some prokaryotes express proton pumps called bacteriorhodopsin, proteorhodopsin, xanthorhodopsin to carry out phototrophy. Like rhodopsin, these contain retinal and have seven transmembrane alpha helices; however they are not coupled to a G protein. Bacterial halorhodopsin is a light-activated chloride pump. Finally, an alga is known to have an opsin that contains its own monolithic light-gated ion channel, channelrhodopsin. While bacteriorhodopsin, halorhodopsin, and channelrhodopsin all have significant sequence homology to one another, they have no detectable sequence identity to G-protein coupled receptor (GPCR) family where rhodopsins belong. Nevertheless, bacterial rhodopsins and GPCR are possibly evolutionary related, based on similarity of their three-dimensional structures. Therefore, they have been assigned to the same superfamily in Structural Classification of Proteins

Further reading

• Humphries P, Kenna P, Farrar GJ (1992). "On the molecular genetics of retinitis pigmentosa". Science. 256 (5058): 804–8. PMID 1589761.CS1 maint: Multiple names: authors list (link)
• Edwards SC (1995). "Involvement of cGMP and calcium in the photoresponse in vertebrate photoreceptor cells". The Journal of the Florida Medical Association. 82 (7): 485–8. PMID 7673885.
• al-Maghtheh M, Gregory C, Inglehearn C; et al. (1993). "Rhodopsin mutations in autosomal dominant retinitis pigmentosa". Hum. Mutat. 2 (4): 249–55. doi:10.1002/humu.1380020403. PMID 8401533.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Garriga P, Manyosa J (2002). "The eye photoreceptor protein rhodopsin. Structural implications for retinal disease". FEBS Lett. 528 (1–3): 17–22. PMID 12297272.
• Mendes HF, van der Spuy J, Chapple JP, Cheetham ME (2005). "Mechanisms of cell death in rhodopsin retinitis pigmentosa: implications for therapy". Trends in molecular medicine. 11 (4): 177–85. doi:10.1016/j.molmed.2005.02.007. PMID 15823756.CS1 maint: Multiple names: authors list (link)
• Inglehearn CF, Keen TJ, Bashir R; et al. (1993). "A completed screen for mutations of the rhodopsin gene in a panel of patients with autosomal dominant retinitis pigmentosa". Hum. Mol. Genet. 1 (1): 41–5. PMID 1301135.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Farrar GJ, Findlay JB, Kumar-Singh R; et al. (1993). "Autosomal dominant retinitis pigmentosa: a novel mutation in the rhodopsin gene in the original 3q linked family". Hum. Mol. Genet. 1 (9): 769–71. PMID 1302614.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Robinson PR, Cohen GB, Zhukovsky EA, Oprian DD (1992). "Constitutively active mutants of rhodopsin". Neuron. 9 (4): 719–25. PMID 1356370.CS1 maint: Multiple names: authors list (link)
• Fujiki K, Hotta Y, Hayakawa M; et al. (1992). "Point mutations of rhodopsin gene found in Japanese families with autosomal dominant retinitis pigmentosa (ADRP)". Jpn. J. Hum. Genet. 37 (2): 125–32. PMID 1391967.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Olsson JE, Gordon JW, Pawlyk BS; et al. (1992). "Transgenic mice with a rhodopsin mutation (Pro23His): a mouse model of autosomal dominant retinitis pigmentosa". Neuron. 9 (5): 815–30. PMID 1418997.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Andréasson S, Ehinger B, Abrahamson M, Fex G (1993). "A six-generation family with autosomal dominant retinitis pigmentosa and a rhodopsin gene mutation (arginine-135-leucine)". Ophthalmic paediatrics and genetics. 13 (3): 145–53. PMID 1484692.CS1 maint: Multiple names: authors list (link)
• Inglehearn CF, Lester DH, Bashir R; et al. (1992). "Recombination between rhodopsin and locus D3S47 (C17) in rhodopsin retinitis pigmentosa families". Am. J. Hum. Genet. 50 (3): 590–7. PMID 1539595.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Fishman GA, Stone EM, Gilbert LD, Sheffield VC (1992). "Ocular findings associated with a rhodopsin gene codon 106 mutation. Glycine-to-arginine change in autosomal dominant retinitis pigmentosa". Arch. Ophthalmol. 110 (5): 646–53. PMID 1580841.CS1 maint: Multiple names: authors list (link)
• Keen TJ, Inglehearn CF, Lester DH; et al. (1992). "Autosomal dominant retinitis pigmentosa: four new mutations in rhodopsin, one of them in the retinal attachment site". Genomics. 11 (1): 199–205. PMID 1765377.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Dryja TP, Hahn LB, Cowley GS; et al. (1991). "Mutation spectrum of the rhodopsin gene among patients with autosomal dominant retinitis pigmentosa". Proc. Natl. Acad. Sci. U.S.A. 88 (20): 9370–4. PMID 1833777.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Gal A, Artlich A, Ludwig M; et al. (1992). "Pro-347-Arg mutation of the rhodopsin gene in autosomal dominant retinitis pigmentosa". Genomics. 11 (2): 468–70. PMID 1840561.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Sung CH, Davenport CM, Hennessey JC; et al. (1991). "Rhodopsin mutations in autosomal dominant retinitis pigmentosa". Proc. Natl. Acad. Sci. U.S.A. 88 (15): 6481–5. PMID 1862076.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)
• Jacobson SG, Kemp CM, Sung CH, Nathans J (1991). "Retinal function and rhodopsin levels in autosomal dominant retinitis pigmentosa with rhodopsin mutations". Am. J. Ophthalmol. 112 (3): 256–71. PMID 1882937.CS1 maint: Multiple names: authors list (link)
• Sheffield VC, Fishman GA, Beck JS; et al. (1991). "Identification of novel rhodopsin mutations associated with retinitis pigmentosa by GC-clamped denaturing gradient gel electrophoresis". Am. J. Hum. Genet. 49 (4): 699–706. PMID 1897520.CS1 maint: Explicit use of et al. (link) CS1 maint: Multiple names: authors list (link)

External links

• The Rhodopsin Protein
• Photoisomerization of rhodopsin, animation.
• Rhodopsin and the eye, summary with pictures.
• UMich Orientation of Proteins in Membranes families/superfamily-6 - Calculated spatial positions of rhodopsin-like proteins in membrane
• Rhodopsin at the US National Library of Medicine Medical Subject Headings (MeSH)