Opsin

Opsins are a family of light-sensitive proteins that function as G protein-coupled receptors (GPCRs). They bind a retinal chromophore and reside in photoreceptor cells of the retina, as well as in several non-visual tissues. When they absorb light, opsins undergo conformational changes that initiate intracellular signaling, converting photons into neural signals or physiological responses.

In vertebrates, the best-known opsins are rhodopsin in rods and the cone opsins in cone cells. Photon absorption causes isomerization of 11-cis-retinal to all-trans-retinal, shifting the opsin to an active state (Meta II). This activates the G protein transducin (Gt), which stimulates phosphodiesterase-6, lowering cGMP levels, closing cGMP-gated channels, and hyperpolarizing the photoreceptor to modulate glutamate release. The chromophore is regenerated in the retinal pigment epithelium to re-form 11-cis-retinal.

Opsins vary in spectral sensitivity due to differences around the chromophore, yielding blue, green, and red sensitivities in humans. The genes OPN1SW, OPN1MW, and OPN1LW encode the short-, middle-, and long-wavelength cone opsins, with red-green color vision arising from gene duplications of OPN1MW and OPN1LW on the X chromosome. In addition to visual opsins, non-visual opsins such as melanopsin (OPN4) participate in circadian photoentrainment, and other opsins are expressed in the brain and pineal gland.

Opsins share a seven-transmembrane domain structure and a lysine residue that forms a Schiff base linkage with the chromophore. In other kingdoms, many light-responsive proteins known as opsins are not GPCRs; microbial opsins like bacteriorhodopsin and halorhodopsin function as light-driven ion pumps or channels and are widely used in optogenetics to control cell activity with light.