Photopsins

Photopsins are light-sensitive visual pigments found in the cone photoreceptor cells of the retina in vertebrates. They are a subclass of opsins, which are G protein–coupled receptors that, together with a chromophore, form photosensitive pigments essential for daytime vision and color perception. Cones expressing photopsins enable detection of different wavelengths and contribute to high-acuity vision in bright light.

Each photopsin consists of an opsin protein bound covalently to 11-cis-retinal. Upon photon absorption, retinal isomerizes to all-trans, triggering a conformational change in the opsin and activation of the G protein transducin. This initiates a phototransduction cascade that lowers cGMP levels, closes cyclic nucleotide–gated channels, hyperpolarizes the cone cell, and modulates glutamate release.

In humans, three cone photopsins mediate color vision: S-cone opsin (blue), M-cone opsin (green), and L-cone opsin (red). They are encoded by the OPN1SW, OPN1MW, and OPN1LW genes, respectively. The spectral peaks vary among species, and many animals possess additional or differently tuned cone opsins, reflecting adaptations to their light environments. Color vision deficiencies commonly involve alterations in M- or L-opsin genes.

Photopsins have evolved through gene duplication and amino acid substitutions that tune their light absorption. Their activity under bright lighting supports photopic vision and color discrimination, while rod photoreceptors provide sensitivity in dim light. The distribution and expression of photopsins across the retina underlie species-specific visual capabilities.