Heterotrimeric G proteins function to relay information from cell surface receptors to intracellular effectors. Each of a very broad range of receptors specifically detects an extracellular stimulus (a photon, pheromone, odorant, hormone or neurotransmitter) while the effectors (i.e. adenyl cyclase), which act to generate one or more intracellular messengers, are less numerous. In mammals, G protein (alpha), (beta) and (gamma) polypeptides are encoded by at least 16, 4 and 7 genes, respectively. Most interest in G proteins has been focused on their (alpha) subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four distinct classes of G(alpha) subunits have been identified; these include Gs, Gi, Gq and G(alpha) 12/13. The Gi class comprises all the known (alpha) subunits that are susceptible to pertussis toxin modifications, including G(alpha) i-1, G(alpha) i-2, G(alpha) i-3, G(alpha) o, G(alpha) t1, G(alpha) t2, G(alpha) z and G(alpha) gust. In the well characterized visual system, photorhodopsin catalyzes the exchange of guanine nucleotides bound to the visual transducin G(alpha) subunits (G(alpha) t1 in rod cells and G(alpha) t2 in cone cells).