Photoreceptor protein

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 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
 * 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

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 clock
 * Photoperiodism