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 Galpha subunits have been identified; these include Gs, Gi, Gq and Galpha 12/13. The Gi class comprises all the known alpha subunits that are susceptible to pertussis toxin modifications, including Galpha i-1, Galpha i-2, Galpha i-3, Galpha o, Galpha t1, Galpha t2, Galpha z and Galpha gust. In the well characterized visual system, photorhodopsin catalyzes the exchange of guanine nucleotides bound to the visual transducin Galpha subunits (Galpha t1 in rod cells and Galpha t2 in cone cells).