Abstract
Vertebrate rod and cone photoreceptors detect light via a specialized organelle called the outer segment. This structure is packed with light-sensitive molecules known as visual pigments that consist of a G-protein-coupled, seven-transmembrane protein known as opsin, and a chromophore prosthetic group, either 11-cis retinal ('A(1)') or 11-cis 3,4-didehydroretinal ('A(2)'). The enzyme cyp27c1 converts A(1) into A(2) in the retinal pigment epithelium. Replacing A(1) with A(2) in a visual pigment red-shifts its spectral sensitivity and broadens its bandwidth of absorption at the expense of decreased photosensitivity and increased thermal noise. The use of vitamin A(2)-based visual pigments is strongly associated with the occupation of aquatic habitats in which the ambient light is red-shifted. By modulating the A(1)/A(2) ratio in the retina, an organism can dynamically tune the spectral sensitivity of the visual system to better match the predominant wavelengths of light in its environment. As many as a quarter of all vertebrate species utilize A(2), at least during a part of their life cycle or under certain environmental conditions. A(2) utilization therefore represents an important and widespread mechanism of sensory plasticity. This review provides an up-to-date account of the A(1)/A(2) chromophore exchange system.