Opsins

Opsins Opsins are light-sensitive, 35-55 kDa membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Five classical groups of opsins are involved in vision, mediating the conversion of a photon of light into an electrochemical signal, the first step in the visual transduction cascade (Terakita A, 2005; Nickle B and Robinson PR, 2007). Another opsin found in the mammalian retina, melanopsin, is involved in circadian rhythms and pupillary reflex but not in image-forming (Hankins MW et al, 2008; Kumbalasiri T and Provencio I, 2005). Guanine nucleotide-binding proteins (G proteins) are involved as modulators or transducers in various transmembrane signaling systems. The G protein transducin, encoded by GNAT genes, is one of the transducers of a visual impulse that performs the coupling between rhodopsin and cGMP-phosphodiesterase. Defects in GNAT1 are the cause of congenital stationary night blindness autosomal dominant type 3, also known as congenital stationary night blindness Nougaret type. Congenital stationary night blindness is a non-progressive retinal disorder characterized by impaired night vision (Dryja TP et al, 1996). Defects in GNAT2 are the cause of achromatopsia type 4 (ACHM4). Achromatopsia is an autosomal recessively inherited visual disorder that is present from birth and that features the absence of color discrimination (Kohl S et al, 2002).