Rhodopsin and phototransduction: a model system for G protein-linked receptors.

Rhodopsin is the photoreceptor protein in rod cells of the vertebrate retina and the first member of the class of G protein-coupled receptors for which the amino acid sequence was determined. Rhodopsin is available in greater quantities than any other receptor of its class and therefore has been studied biochemically and biophysically by methods difficult or impossible to apply to its fellow receptors. Such studies support a model in which rhodopsin consists of seven transmembrane helices that form a binding pocket for its ligand, 11-cis retinal. Insights into the structure and function of rhodopsin serve as a model for understanding the structure and function of other members of the receptor class. Rhodopsin undergoes a change in conformation upon photoexcitation and activates a G protein, transducin, and is phosphorylated by a receptor-specific kinase, rhodopsin kinase. The phosphorylated photoactivated rhodopsin is bound by arrestin, thereby terminating activity of the receptor in the signal transduction process. These auxiliary proteins that function with rhodopsin on rod cells serve as models for understanding how other members of the receptor family may function in conjunction with other G proteins, kinases, and arrestin-like proteins.