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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 journal|last1=Rockwell|first1=Nathan C.|last2=Su|first2=Yi-Shin|last3=Lagarias|first3=J. Clark|date=2006|title=Phytochrome structure and 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> The plant protein [[UVR8]] is exceptional amongst photoreceptors in that it contains no external chromophore. Instead, UVR8 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]]: reception of various colors of light in the [[cone cell]]s of vertebrate retina
*[[Rhodopsin]]: 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, D.|first1=Daisuke |last2=Mori S.,|first2=Suguru |last3=Torii M.,|first3=Masaki |last4=Wada, A.,|first4=Akimori |last5=Morishita R., &|first5=Rika |last6=Fukada, Y. (2011)|first6=Yoshitaka |title=UV-Sensitive Photoreceptor Protein OPN5 in Humans and Mice. PLoS|journal=PLOS ONE, |date=17 October 2011 |volume=6( |issue=10): |pages=e26388. DOI: |doi=10.1371/journal.pone.0026388 <http://www|pmid=22043319 |pmc=3197025 |bibcode=2011PLoSO.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0026388>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]]
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== Photoreception and signal transduction ==
* [[Phototransduction]]
* [[Visual cycle]]
* [[Visual phototransduction]]
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* [[Circadian rhythm]] (body clock)
* [[Photoperiodism]]
 
== See also ==
 
* [[Biliprotein|Biliproteins]]
 
== References ==
{{Reflist}}
 
{{Protein topics}}
 
{{DEFAULTSORT:Photoreceptor Protein}}
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