Rhodopsin (encoded by the human gene OPN2) (Nathans J and Hogness DS, 1984) is expressed in rod photoreceptor cells used in night vision. In humans, three opsins are expressed in cone cells used for colour vision. The opsin 1 gene OPN1MW encodes a protein called green cone photopigment or medium-wave-sensitive opsin (Nathans J et al, 1986). Defects in OPN1MW are the cause of partial colorblindness called deuteranopia (Winderickx J et al, 1992).
The opsin 1 gene OPN1LW encodes a protein called red cone photopigment or long-wavelength sensitive opsin (Nathans J et al, 1986). Defects in this gene are the cause of partial colorblindness (protanopia) (Winderickx J et al, 1992). The opsin 1 gene OPN1SW encodes for blue-sensitive opsins (BOP) (Nathans J et al, 1986). A deficiency in function or numbers (or both) of BOP results in a selective deficiency of blue spectral sensitivity. This is called Tritanopia, an autosomal dominant genetic disorder of human vision (Weitz CJ et al, 1992).
The human gene OPN3 encodes opsin 3 (encephalopsin, panopsin) (Blackshaw S and Snyder SH, 1999). It is strongly expressed in brain and testis with features of a classical photoreceptive opsin. The human gene OPN5 encodes opsin 5, which is expressed in the eye, brain, testes, and spinal cord (Tarttelin EE et al, 2003).
The visual pigment-like receptor peropsin (RRH) is found only in the eye, where it is localized to the retinal pigment epithelium (RPE) (Sun H et al, 1997). In the RPE, it is localized to the microvilli that surround the photoreceptor outer segments. It may play a role in RPE physiology, either by detecting light directly or by monitoring the concentration of retinoids or other photoreceptor-derived compounds.
The putative RPE-retinal G protein coupled receptor (RGR) (Shen D et al, 1994) covalently binds both all-trans- and 11-cis-retinal after reduction by sodium borohydride. The 32-kDa receptor binds all-trans-retinal preferentially, rather than the 11-cis isomer. Defects in RGR are a cause of autosomal recessive retinitis pigmentosa (ARRP). RP leads to degeneration of retinal photoreceptor cells (Morimura H et al, 1999).
Transducin (also called Gt) is a heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones and couple with these opsins to mediate the stimulation of cGMP hydrolysis.