Vision involves the conversion of light into electrochemical signals that are processed by the retina and subsequently sent to, and interpreted by, the brain. The process of converting light to an electrochemical signal begins when the membrane-bound protein, rhodopsin, absorbs light within the retina. Photoexcitation of rhodopsin causes the cytoplasmic surface of the protein to become catalytically active. In the active state, rhodopsin activates transducin, a GTP binding protein. Once activated, transducin promotes the hydrolysis of cGMP by phosphodiesterase (PDE). The decrease of intracellular cGMP concentrations causes the ion channels within the outer segment of the rod or cone to close, thus causing membrane hyperpolarization and, eventually, signal transmission. Rhodopsin’s activity is believed to be shut off by its phosphorylation followed by binding of the soluble protein arrestin. Arrestins are cytosolic proteins that are involved in G protein-coupled receptor (GPCR) desensitization. Arrestin binding to activated GPCRs is phosphorylation dependent and, once bound, uncouple the GPCR from the associated heterotrimeric G proteins. There are currently 4 known mammalian isoforms, beta-Arrestin 1 (Arrestin 2), beta-Arrestin 2 (Arrestin 3), visual Arrestin (Arrestin 1), and cone arrestin. The beta- isoforms are ubiquitously expressed and are known to interact with acetylcholine and adrenergic receptors. Visual and cone Arrestins are found to interact directly with transducin.
Arrestin is one of the major proteins of the ros (retinal rod outer segments); it binds to photoactivated-phosphorylated rhodopsin, thereby apparently preventing the transducin-mediated activation of phosphodiesterase.
Retina and pineal gland.
Defects in SAG are a cause of congenital stationary night blindness Oguchi type 1 (CSNBO1) [MIM:258100]; also known as Oguchi disease. Congenital stationary night blindness is a non-progressive retinal disorder characterized by impaired night vision. CSNBO is an autosomal recessive form associated with fundus discoloration and abnormally slow dark adaptation.