rhodopsin

Rhodopsin is a light-sensitive protein found in the rod cells of the retina and is essential for vision in low-light conditions. It is a G protein-coupled receptor composed of the opsin protein bound to a chromophore, 11-cis retinal, covalently linked to a lysine residue (Lys296 in bovine rhodopsin).

Upon photon absorption, 11-cis retinal isomerizes to all-trans retinal, triggering a conformational change that activates rhodopsin to metarhodopsin II. This active form activates the G protein transducin, which stimulates a cGMP phosphodiesterase. The resulting drop in cGMP levels closes cGMP-gated ion channels, hyperpolarizing the rod cell and reducing glutamate release to downstream neurons.

Regeneration of rhodopsin occurs through the visual or retinoid cycle. All-trans retinal is converted to all-trans retinol and transported to the retinal pigment epithelium, where it is ultimately rebuilt into 11-cis retinal and rejoined to opsin, restoring rhodopsin for another phototransduction cycle.

Rhodopsin is highly expressed in rod outer segments and has a spectral sensitivity peak near 498 nanometers. The human RHO gene encodes the protein, and mutations in RHO can cause inherited retinal diseases, most notably autosomal dominant retinitis pigmentosa and congenital stationary night blindness.

As a model GPCR, rhodopsin has been central to studies of receptor structure and signaling, the visual cycle, and related pathologies. It has also found applications in optogenetics and retinal research.