Photoreceptor protein

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), flavin (flavoproteins, for example cryptochrome in plants and animals) and bilin (biliproteins, for example phytochrome in plants). 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.

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
• OPN5: sensitive to UV-light

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. The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering. 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.

Photoreceptors in phototactic flagellates

(Also see: Eyespot apparatus)

• Channelrhodopsin: in unicellular algae, mediates phototaxis
• Chlamyopsin and volvoxopsin
• Flavoproteins

Photoreceptors in archaea and bacteria

• Bacteriophytochrome
• sensory bacteriorhodopsin
• Halorhodopsin
• Proteorhodopsin
• Cyanobacteriochrome

Photoreception and signal transduction

• Visual cycle
• Visual phototransduction

Responses to photoreception

• Visual perception
• Phototropism
• Phototaxis
• Circadian rhythm (body clock)
• Photoperiodism

References

References

1. "Rhodopsin {{!}} biochemistry".
2. (2005-04-29). "The cryptochromes". Genome Biology.
3. (2006). "Phytochrome structure and signaling mechanisms". Annual Review of Plant Biology.
4. (2020-08-28). "A leap in quantum efficiency through light harvesting in photoreceptor UVR8". Nature Communications.
5. (1991). "Photoreceptor Deactivation and Retinal Degeneration Mediated by a Photoreceptor-Specific Protein Kinase C". Science.
6. (17 October 2011). "UV-Sensitive Photoreceptor Protein OPN5 in Humans and Mice". PLOS ONE.
7. (October 2015). "Sensing the light environment in plants: photoreceptors and early signaling steps". Current Opinion in Neurobiology.
8. (2001-04-13). "Blue Light Sensing in Higher Plants *". Journal of Biological Chemistry.
9. (1 January 2001). "Photoreceptors in Plant Photomorphogenesis to Date. Five Phytochromes, Two Cryptochromes, One Phototropin, and One Superchrome". Plant Physiology.