Photoreceptor protein

{{About|molecular photoreceptors|other types of photoreceptors|Photoreceptor (disambiguation)}}

Photoreceptor proteins are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are rhodopsin in the photoreceptor cells of the vertebrate retina, phytochrome in plants, and bacteriorhodopsin and bacteriophytochromes in some 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 photoperiodisms including control of flowering times in plants and mating seasons in animals.

Structure

Photoreceptor proteins typically consist of a protein attached to a non-protein chromophore (sometimes referred as photopigment, even 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. Chromophores found in photoreceptors include retinal (retinylidene proteins, for example rhodopsin in animals),{{Cite web|title=Rhodopsin {{!}} biochemistry|url=https://www.britannica.com/science/rhodopsin|access-date=2021-01-21|website=Encyclopedia Britannica|language=en}} flavin (flavoproteins, for example cryptochrome in plants and animals){{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}} and bilin (biliproteins, for example phytochrome in plants).{{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}} 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.{{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}}

Photoreceptors in animals

Melanopsin: in vertebrate retina, mediates pupillary reflex, involved in regulation of circadian rhythms
Photopsin: reception of various colors of light in the cone cells of vertebrate retina
Rhodopsin: green-blue light reception in the rod cells of vertebrate retina
Protein Kinase C: mediates photoreceptor deactivation, and retinal degeneration{{cite journal |last1=Smith |first1=Dean P. |last2=Ranganathan |first2=Rama |last3=Hardy |first3=Robert W. |last4=Marx |first4=Julia |last5=Tsuchida |first5=Tammy |last6=Zuker |first6=Charles S. |title=Photoreceptor Deactivation and Retinal Degeneration Mediated by a Photoreceptor-Specific Protein Kinase C |journal=Science |date=1991 |volume=254 |issue=5037 |pages=1478–1484 |doi=10.1126/science.1962207 |pmid=1962207 |id={{ProQuest|213560980}} |jstor=2879432 |bibcode=1991Sci...254.1478S }}
OPN5: sensitive to UV-light{{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 }}

Photoreceptors in plants

UVR8: UV-B light reception
Cryptochrome: blue and UV-A light reception
Phototropin: blue and UV-A light perception (to mediate phototropism and chloroplast movement)
Zeitlupe: blue light entrainment of the circadian clock
Phytochrome: red and far-red light reception

All the photoreceptors listed above allow plants to sense light with wavelengths range from 280 nm (UV-B) to 750 nm (far-red light). Plants use light of different wavelengths as environmental cues to both alter their position and to trigger important developmental transitions.{{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 }} The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering.{{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|url-access=subscription}} 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.{{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 }}