Rhodopsin

Rhodopsin, also known as visual purple, 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 G-protein coupled receptor family and are extremely sensitive to light, enabling vision in low-light conditions. Exposed to white light, the pigment immediately photobleaches, and it takes about 30 minutes to regenerate fully in humans.

tructure

Rhodopsin consists of the protein moiety opsin and a reversibly covalently bound cofactor, retinal. Opsin, a bundle of seven transmembrane helices, binds retinal, a photoreactive chromophore, in a central pocket. Retinal is produced in the retina from Vitamin A. Isomerization of 11-"cis"-retinal into all-"trans"-retinal by light induces a conformational change in 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 "monochromatic" vision in the dark.

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 (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.cite journal | author = Nakamichi H, Okada T | title = Crystallographic analysis of primary visual photochemistry | journal = Angew. Chem. Int. Ed. Engl. | volume = 45 | issue = 26 | pages = 4270–3 | year = 2006 | month = June | pmid = 16586416 | doi = 10.1002/anie.200600595 | url = | issn = ] cite journal | author = Schreiber M, Sugihara M, Okada T, Buss V | title = Quantum mechanical studies on the crystallographic model of bathorhodopsin | journal = Angew. Chem. Int. Ed. Engl. | volume = 45 | issue = 26 | pages = 4274–7 | year = 2006 | month = June | pmid = 16729349 | doi = 10.1002/anie.200600585 | url = | issn = ] cite journal | author = Weingart O | title = The twisted C11-C12 bond of the rhodopsin chromophore--a photochemical hot spot | journal = J. Am. Chem. Soc. | volume = 129 | issue = 35 | pages = 10618–9 | year = 2007 | month = September | pmid = 17691730 | doi = 10.1021/ja071793t | url = | issn = ]

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.cite journal | author = Saliba RS, Munro PM, Luthert PJ, Cheetham ME | title = The cellular fate of mutant rhodopsin: quality control, degradation and aggresome formation | journal = J. Cell. Sci. | volume = 115 | issue = Pt 14 | pages = 2907–18 | year = 2002 | month = July | pmid = 12082151 | | url = http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=12082151 | issn = ] 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.cite journal | author = Mendes HF, van der Spuy J, Chapple JP, Cheetham ME | title = Mechanisms of cell death in rhodopsin retinitis pigmentosa: implications for therapy | journal = Trends Mol Med | volume = 11 | issue = 4 | pages = 177–85 | year = 2005 | month = April | pmid = 15823756 | doi = 10.1016/j.molmed.2005.02.007 | url = | issn = ] Several other pathological states relating to rhodopsin have been discovered including poor post-Golgi trafficking, dysregulative activation, rod outer segment instability and arrestin binding.

Microbial rhodopsins

Some prokaryotes express proton pumps called bacteriorhodopsin, proteorhodopsin, xanthorhodopsin to carry out phototrophy.cite journal | author = Bryant DA, Frigaard NU | title = Prokaryotic photosynthesis and phototrophy illuminated | journal = Trends Microbiol. | volume = 14 | issue = 11 | pages = 488–96 | year = 2006 | month = November | pmid = 16997562 | doi = 10.1016/j.tim.2006.09.001 | url = | issn = ] 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. [ http://scop.mrc-lmb.cam.ac.uk/scop/data/scop.b.g.e.b.html.]

References

Further reading

PBB_Further_reading
citations =
*cite journal | author=Humphries P, Kenna P, Farrar GJ |title=On the molecular genetics of retinitis pigmentosa. |journal=Science |volume=256 |issue= 5058 |pages= 804–8 |year= 1992 |pmid= 1589761| doi=10.1126/science.1589761
*cite journal | author=Edwards SC |title=Involvement of cGMP and calcium in the photoresponse in vertebrate photoreceptor cells. |journal=The Journal of the Florida Medical Association |volume=82 |issue= 7 |pages= 485–8 |year= 1995 |pmid= 7673885 |doi=
*cite journal | author=al-Maghtheh M, Gregory C, Inglehearn C, "et al." |title=Rhodopsin mutations in autosomal dominant retinitis pigmentosa. |journal=Hum. Mutat. |volume=2 |issue= 4 |pages= 249–55 |year= 1993 |pmid= 8401533 |doi= 10.1002/humu.1380020403
*cite journal | author=Garriga P, Manyosa J |title=The eye photoreceptor protein rhodopsin. Structural implications for retinal disease. |journal=FEBS Lett. |volume=528 |issue= 1-3 |pages= 17–22 |year= 2002 |pmid= 12297272| doi=10.1016/S0014-5793(02)03241-6
*cite journal | author=Mendes HF, van der Spuy J, Chapple JP, Cheetham ME |title=Mechanisms of cell death in rhodopsin retinitis pigmentosa: implications for therapy. |journal=Trends in molecular medicine |volume=11 |issue= 4 |pages= 177–85 |year= 2005 |pmid= 15823756 |doi= 10.1016/j.molmed.2005.02.007
*cite journal | author=Inglehearn CF, Keen TJ, Bashir R, "et al." |title=A completed screen for mutations of the rhodopsin gene in a panel of patients with autosomal dominant retinitis pigmentosa. |journal=Hum. Mol. Genet. |volume=1 |issue= 1 |pages= 41–5 |year= 1993 |pmid= 1301135 |doi=
*cite journal | author=Farrar GJ, Findlay JB, Kumar-Singh R, "et al." |title=Autosomal dominant retinitis pigmentosa: a novel mutation in the rhodopsin gene in the original 3q linked family. |journal=Hum. Mol. Genet. |volume=1 |issue= 9 |pages= 769–71 |year= 1993 |pmid= 1302614 |doi=
*cite journal | author=Robinson PR, Cohen GB, Zhukovsky EA, Oprian DD |title=Constitutively active mutants of rhodopsin. |journal=Neuron |volume=9 |issue= 4 |pages= 719–25 |year= 1992 |pmid= 1356370| doi=10.1016/0896-6273(92)90034-B
*cite journal | author=Fujiki K, Hotta Y, Hayakawa M, "et al." |title=Point mutations of rhodopsin gene found in Japanese families with autosomal dominant retinitis pigmentosa (ADRP). |journal=Jpn. J. Hum. Genet. |volume=37 |issue= 2 |pages= 125–32 |year= 1992 |pmid= 1391967| doi=10.1007/BF01899733
*cite journal | author=Olsson JE, Gordon JW, Pawlyk BS, "et al." |title=Transgenic mice with a rhodopsin mutation (Pro23His): a mouse model of autosomal dominant retinitis pigmentosa. |journal=Neuron |volume=9 |issue= 5 |pages= 815–30 |year= 1992 |pmid= 1418997| doi=10.1016/0896-6273(92)90236-7
*cite journal | author=Andréasson S, Ehinger B, Abrahamson M, Fex G |title=A six-generation family with autosomal dominant retinitis pigmentosa and a rhodopsin gene mutation (arginine-135-leucine). |journal=Ophthalmic paediatrics and genetics |volume=13 |issue= 3 |pages= 145–53 |year= 1993 |pmid= 1484692 |doi=
*cite journal | author=Inglehearn CF, Lester DH, Bashir R, "et al." |title=Recombination between rhodopsin and locus D3S47 (C17) in rhodopsin retinitis pigmentosa families. |journal=Am. J. Hum. Genet. |volume=50 |issue= 3 |pages= 590–7 |year= 1992 |pmid= 1539595 |doi=
*cite journal | author=Fishman GA, Stone EM, Gilbert LD, Sheffield VC |title=Ocular findings associated with a rhodopsin gene codon 106 mutation. Glycine-to-arginine change in autosomal dominant retinitis pigmentosa. |journal=Arch. Ophthalmol. |volume=110 |issue= 5 |pages= 646–53 |year= 1992 |pmid= 1580841 |doi=
*cite journal | author=Keen TJ, Inglehearn CF, Lester DH, "et al." |title=Autosomal dominant retinitis pigmentosa: four new mutations in rhodopsin, one of them in the retinal attachment site. |journal=Genomics |volume=11 |issue= 1 |pages= 199–205 |year= 1992 |pmid= 1765377 |doi=
*cite journal | author=Dryja TP, Hahn LB, Cowley GS, "et al." |title=Mutation spectrum of the rhodopsin gene among patients with autosomal dominant retinitis pigmentosa. |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=88 |issue= 20 |pages= 9370–4 |year= 1991 |pmid= 1833777| doi=10.1073/pnas.88.20.9370
*cite journal | author=Gal A, Artlich A, Ludwig M, "et al." |title=Pro-347-Arg mutation of the rhodopsin gene in autosomal dominant retinitis pigmentosa. |journal=Genomics |volume=11 |issue= 2 |pages= 468–70 |year= 1992 |pmid= 1840561 |doi=
*cite journal | author=Sung CH, Davenport CM, Hennessey JC, "et al." |title=Rhodopsin mutations in autosomal dominant retinitis pigmentosa. |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=88 |issue= 15 |pages= 6481–5 |year= 1991 |pmid= 1862076 |doi=
*cite journal | author=Jacobson SG, Kemp CM, Sung CH, Nathans J |title=Retinal function and rhodopsin levels in autosomal dominant retinitis pigmentosa with rhodopsin mutations. |journal=Am. J. Ophthalmol. |volume=112 |issue= 3 |pages= 256–71 |year= 1991 |pmid= 1882937 |doi=
*cite journal | author=Sheffield VC, Fishman GA, Beck JS, "et al." |title=Identification of novel rhodopsin mutations associated with retinitis pigmentosa by GC-clamped denaturing gradient gel electrophoresis. |journal=Am. J. Hum. Genet. |volume=49 |issue= 4 |pages= 699–706 |year= 1991 |pmid= 1897520 |doi=

External links

* [http://macromoleculeinsights.com/rhodopsin.php The Rhodopsin Protein]
* [http://www.blackwellpublishing.com/matthews/rhodopsin.html Photoisomerization of rhodopsin] , animation.
* [http://www.chm.bris.ac.uk/webprojects2003/rogers/998/Rhoeye.htm Rhodopsin and the eye] , summary with pictures.
* - Calculated spatial positions of rhodopsin-like proteins in membrane
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