Photoreceptor protein

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),[1] flavin (flavoproteins, for example cryptochrome in plants and animals)[2] and bilin (biliproteins, for example phytochrome in plants).[3] 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.[4]

• 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[5]
• OPN5: sensitive to UV-light[6]

• 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.[7] The most prominent wavelength responsible for plant mechanisms is blue light, which can trigger cell elongation, plant orientation, and flowering.[8] 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.[9]

(Also see: Eyespot apparatus)

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

• Bacteriophytochrome
• sensory bacteriorhodopsin
• Halorhodopsin
• Proteorhodopsin
• Cyanobacteriochrome

• Visual cycle
• Visual phototransduction

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

• Biliproteins