Photoreceptor protein

This article is about molecular photoreceptors. For other types of photoreceptors, see Photoreceptor (disambiguation).

Photoreceptors 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 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

(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

Photoreceptors in plants

• Cryptochrome: in plants, blue light reception
• Phototropin: in plants, mediates phototropism
• 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. After a brief exposure to light, particularly the wavelengths within the red and far-red lights of the electromanetic spectrum, 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.

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

Photoreception and signal transduction

• Phototransduction
• Visual cycle
• Visual phototransduction

Responses to photoreception

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