Diversity of visual pigments from the viewpoint of G protein activation--comparison with other G protein-coupled receptors.

The visual pigment present in the photoreceptor cells of the retina is a member of the family of G protein-coupled receptors and contains an 11-cis-retinal as a light-absorbing chromophore. Light induces conformational changes in the protein moiety of the visual pigment through cis-trans isomerization of the chromophore, which leads to the activation of a G protein-mediated signal transduction cascade that eventually generates an electrical response of the photoreceptor cells. So far, various types of visual pigments have been identified from a variety of photoreceptor cells and the structure-function relationship of visual pigments has been widely investigated by means of biophysical, biochemical and molecular biological techniques. Recent identifications of visual pigment-like proteins in the extra-ocular cells emphasize the importance of the visual pigment family as the photoreceptive molecules in not only visual but also non-visual photoreception. This article reviews the functional diversity of visual pigments from the viewpoint of the molecular mechanisms of photoreception and G protein activation. In addition, the similarity and difference of G protein activation mechanism between visual pigment and other G protein-coupled receptors are discussed for furthering our understanding of the common mechanism of G protein activation by G protein-coupled receptors.