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 (e.g. 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. The Gs subfamily of Galpha subunits includes two closely related proteins, Galpha s and Galpha olf, which, respectively,
stimulate adenylate cyclase and mediate response to olfactory stimuli.