Photoreceptor protein

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This article is about molecular photoreceptors. For other types of photoreceptors, see 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[edit]

Photoreceptor proteins typically consist of a protein moiety and a non-protein photopigment that reacts to light via photoisomerization or photoreduction, thus initiating a change of the receptor protein which triggers a signal transduction cascade. Pigments 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).

Photoreceptors in animals[edit]

(Also see: 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
  • Rhodopsin: in vertebrate retina, green-blue light reception
  • Protein Kinase C: mediates photoreceptor deactivation, and retinal degeneration [1]

Photoreceptors in plants[edit]

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. [2]

Photoreceptors in phototactic flagellates[edit]

(Also see: Eyespot apparatus)

Photoreceptors in archaea and bacteria[edit]

Photoreception and signal transduction[edit]

Responses to photoreception[edit]

Opn5[edit]

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 such example, HEK293S cell lining stably expressed human OPN5. One study discovered that upon reconstitution with 11-cis-retinal, mouse Opn5 showed an absorption maximum of 380 nm. 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.[3]

References[edit]

  1. ^ Smith, D. P., Hardy, R. W., & al, e. (1991). Photoreceptor deactivation and retinal degeneration mediated by a photoreceptor-specific protein kinase C. Science, 254(5037), 1478-1478. Retrieved from http://search.proquest.com/docview/213560980?accountid=14771
  2. ^ 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>
  3. ^ 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 <http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0026388>