Signaling By Rod And Cone Photoreceptors: Opsin Properties, G-Protein Assembly, And Mechanisms Of Activation

Photoreceptor cells utilize a G-protein-mediated signaling cascade to convert the energy of a photon into an electric impulse in the first step of vision. The relative abundance of photoreceptors in the retina and the localization of the phototransduction cascade proteins in a specialized modified cilium structure, known as the outer segment (OS), have greatly facilitated the biochemical characterization of phototransduction proteins and the reactions involved in this canonical G protein cascade. In addition, the electrophysiologically measurable response produced by stimulating the cascade with light has resulted in quantitative understanding of the reactions involved in the activation and inactivation of rod and cone photoreceptor G-protein-coupled receptors (GPCRs) and their coupled G protein, transducin. Here, we review the structure of rod and cone visual pigments in the context of photoreceptor function as well as the assembly and function of the heterotrimeric transducin.