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{{Short description|Molecular photoreceptors}}
{{About|molecular photoreceptors|other types of photoreceptors|Photoreceptor (disambiguation)}}
{{Refimprove|date=March 2007}}
 
'''Photoreceptor proteins''' are light-sensitive [[protein]]s involved in the sensing and response to light in a variety of organisms. Some examples are [[rhodopsin]] in the [[photoreceptor cell]]s of the vertebrate [[retina]], [[phytochrome]] in plants, and [[bacteriorhodopsin]] and bacteriophytochromes in some [[bacterium|bacteria]]. They mediate light responses as varied as [[visual perception]], [[phototropism]] and [[phototaxis]], as well as responses to light-dark cycles such as [[circadian rhythm]] and other [[photoperiodism]]s including control of flowering times in plants and mating seasons in animals.
 
== Structure ==
Photoreceptor proteins typically consist of a [[protein]] moietyattached andto a non-protein [[chromophore]] (sometimes referred as [[photopigment]], thateven so photopigment may also refer to the photoreceptor as a whole). The chromophore reacts to light via [[photoisomerization]] or [[photoreduction]], thus initiating a change of the receptor protein which triggers a [[signal transduction]] cascade. PigmentsChromophores found in photoreceptors include [[retinal]] ([[retinylidene protein]]s, for example [[rhodopsin]] in animals),<ref>{{Cite web|title=Rhodopsin {{!}} biochemistry|url=https://www.britannica.com/science/rhodopsin|access-date=2021-01-21|website=Encyclopedia Britannica|language=en}}</ref> [[Flavin group|flavin]] ([[flavoprotein]]s, for example [[cryptochrome]] in plants and animals)<ref>{{Cite journal|last1=Lin|first1=Chentao|last2=Todo|first2=Takeshi|date=2005-04-29|title=The cryptochromes|journal=Genome Biology|volume=6|issue=5|pages=220|doi=10.1186/gb-2005-6-5-220|pmid=15892880|pmc=1175950|issn=1474-760X|doi-access=free}}</ref> and [[Bilin (biochemistry)|bilin]] ([[biliprotein]]s, for example [[phytochrome]] in plants),.<ref>{{Cite discoveredjournal|last1=Rockwell|first1=Nathan byC.|last2=Su|first2=Yi-Shin|last3=Lagarias|first3=J. McDonald'sClark|date=2006|title=Phytochrome workerstructure Drand signaling mechanisms|journal=Annual Review of Plant Biology|volume=57|pages=837–858|doi=10.1146/annurev.arplant.56.032604.144208|issn=1543-5008|pmc=2664748|pmid=16669784}}</ref> SirThe Sunilplant Penumarthyprotein [[UVR8]] is exceptional amongst photoreceptors in that it contains no external chromophore. Instead, PhDUVR8 absorbs light through [[tryptophan]] residues within its protein [[coding sequence]].<ref>{{Cite journal|last1=Li|first1=Xiankun|last2=Ren|first2=Haisheng|last3=Kundu|first3=Mainak|last4=Liu|first4=Zheyun|last5=Zhong|first5=Frank W.|last6=Wang|first6=Lijuan|last7=Gao|first7=Jiali|last8=Zhong|first8=Dongping|date=2020-08-28|title=A leap in quantum efficiency through light harvesting in photoreceptor UVR8|url= |journal=Nature Communications|language=en|volume=11|issue=1|pages=4316|doi=10.1038/s41467-020-17838-6|pmid=32859932|pmc=7455749|bibcode=2020NatCo..11.4316L|issn=2041-1723|doi-access=free}}</ref>
 
== Photoreceptors in animals ==
''(Also{{See see: [[also|Photoreceptor cell]])''}}
*[[Melanopsin]]: in vertebrate retina, mediates pupillary reflex, involved in regulation of circadian rhythms
*[[Photopsin]]: in vertebrate retina, reception of various colors of light in the [[cone cell]]s of vertebrate retina
*[[Rhodopsin]]: in vertebrate retina, green-blue light reception in the [[rod cell]]s of vertebrate retina
*[[Protein Kinase C]]: mediates photoreceptor deactivation, and retinal degeneration <ref>{{cite journal |last1=Smith, D.|first1=Dean P., |last2=Ranganathan |first2=Rama |last3=Hardy, R.|first3=Robert W., &|last4=Marx al,|first4=Julia e.|last5=Tsuchida (1991)|first5=Tammy |last6=Zuker |first6=Charles S. |title=Photoreceptor deactivationDeactivation and retinalRetinal degenerationDegeneration mediatedMediated by a photoreceptorPhotoreceptor-specificSpecific proteinProtein kinaseKinase C. |journal=Science, |date=1991 |volume=254( |issue=5037), 1478-1478|pages=1478–1484 |doi=10.1126/science.1962207 Retrieved|pmid=1962207 from|id={{ProQuest|213560980}} http://search|jstor=2879432 |bibcode=1991Sci.proquest.com/docview/213560980?accountid=14771.254.1478S }}</ref>
*[[OPN5]]: sensitive to [[UV-light]]<ref>{{cite journal |last1=Kojima |first1=Daisuke |last2=Mori |first2=Suguru |last3=Torii |first3=Masaki |last4=Wada |first4=Akimori |last5=Morishita |first5=Rika |last6=Fukada |first6=Yoshitaka |title=UV-Sensitive Photoreceptor Protein OPN5 in Humans and Mice |journal=PLOS ONE |date=17 October 2011 |volume=6 |issue=10 |pages=e26388 |doi=10.1371/journal.pone.0026388 |pmid=22043319 |pmc=3197025 |bibcode=2011PLoSO...626388K |doi-access=free }}</ref>
 
== Photoreceptors in plants ==
*[[UVR8]]: in plants, UV-B light reception
*[[Cryptochrome]]: inblue plants,and blueUV-A light reception
*[[Phototropin]]: inblue plants,and mediatesUV-A light perception (to mediate phototropism and chloroplast movement)
*[[Zeitlupe]]: blue light entrainment of the [[circadian clock]]
*[[Phytochrome]]: in plants, red and far-red light reception
In plant seeds, the photoreceptor phytochrome is responsible for the process termed photomorphogenesis. This occurs when a seed initially situated in an environment of complete darkness is exposed to light. A brief exposure to electromagnetic radiation, particularly that whose wavelength is within the red and far-red lights, results in the activation of the photorecepter phytochrome within the seed. This in turn sends a signal through the signal transduction pathway into the nucleus, and triggers hundreds of genes responsible for growth and development.
 
<ref>Winslow R. Briggs, Margaret A. Olney, Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome, 2003 <http://www.plantphysiol.org/content/125/1/85.full></ref>
All the photoreceptors listed above allow plants to sense light with wavelengths range from 280&nbsp;[[nanometre|nm]] (UV-B) to 750&nbsp;nm (far-red light). Plants use light of different wavelengths as environmental cues to both alter their position and to trigger important developmental transitions.<ref>{{cite journal |last1=Galvão |first1=Vinicius Costa |last2=Fankhauser |first2=Christian |s2cid=12390801 |title=Sensing the light environment in plants: photoreceptors and early signaling steps |journal=Current Opinion in Neurobiology |date=October 2015 |volume=34 |pages=46–53 |doi=10.1016/j.conb.2015.01.013 |pmid=25638281 |url=https://zenodo.org/record/161783 }}</ref> The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering.<ref>{{Cite journal|last1=Christie|first1=John M.|last2=Briggs|first2=Winslow R.|date=2001-04-13|title=Blue Light Sensing in Higher Plants *|url=https://www.jbc.org/article/S0021-9258(19)46006-7/abstract|journal=Journal of Biological Chemistry|language=English|volume=276|issue=15|pages=11457–11460|doi=10.1074/jbc.R100004200|issn=0021-9258|pmid=11279226|doi-access=free}}</ref> One of the most important processes regulated by photoreceptors is known as [[photomorphogenesis]]. When a seed germinates underground in the absence of light, its stem rapidly elongates upwards. When it breaks through the surface of the soil, photoreceptors perceive light. The activated photoreceptors cause a change in developmental program; the plant starts producing chlorophyll and switches to photosynthetic growth.<ref>{{cite journal |last1=Briggs |first1=Winslow R. |last2=Olney |first2=Margaret A. |title=Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome |journal=Plant Physiology |date=1 January 2001 |volume=125 |issue=1 |pages=85–88 |doi=10.1104/pp.125.1.85 |pmid=11154303 |pmc=1539332 |doi-access=free }}</ref>
 
== Photoreceptors in phototactic flagellates ==
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== Photoreceptors in archaea and bacteria ==
{{see also|Archaea|Bacteria}}
* [[Bacteriophytochrome]]
* sensory [[bacteriorhodopsin]]
* [[Halorhodopsin]]
* [[Proteorhodopsin]]
* [[Cyanobacteriochrome]]
 
== Photoreception and signal transduction ==
* [[Phototransduction]]
* [[Visual cycle]]
* [[Visual phototransduction]]
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* [[Photoperiodism]]
 
==Opn5 See also ==
 
The mammalian neuropsin, OPN5, is a photoreceptor sensitive to ultraviolent light, encoded by the Opn5 gene. This gene was discovered in mouse and human genomes and its mRNA expression was also discovered in neural tissues. One study discovered that upon reconstitution with 11-cis-retinal, mouse Opn5 showed an absorption maximum of 380nm. This caused the conversion of OPN5 into a blue-absorbing photoproduct (470 nm), which was stable in the dark. Orange illumination then caused its re-conversion back into the UV- absorbing state. Human OPN5 was discovered to be the first unknown human opsin with sensitivity in the UV region, with similar properties to mice OPN5. The OPN5 activates a UV-sensitive, heterotrimeric G protein Gi-mediated pathway in mammalian tissues. <ref>Kojima, D. Mori S., Torii M., Wada, A., Morishita R., & Fukada, Y. (2011) UV-Sensitive Photoreceptor Protein OPN5 in Humans and Mice. PLoS ONE, 6(10): e26388. DOI: 10.1371/journal.pone.0026388 </ref>
* [[Biliprotein|Biliproteins]]
 
== References ==
{{Reflist}}
 
{{Protein topics}}
 
{{DEFAULTSORT:Photoreceptor Protein}}
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[[Category:Integral membrane proteins]]
[[Category:Molecular biology]]
 
{{membrane-protein-stub}}
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