Heterotrimeric G proteins function to relay information from cell surface receptors to intracellular effectors (1). 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., adenylyl cyclase), which act to generate one or more intracellular messengers, are less numerous. In mammals, G protein å, ∫ and © polypeptides are encoded by at least 16, 4 and 7 genes, respectively (2-5). Most interest in G proteins has been focused on their a subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four distinct classes of Gå subunits have been identified; these include Gs, Gi, Gq and Ga 12/13 (3,4). The Gi class comprises all the known a subunits that are susceptible to pertussis toxin modifications, including Ga i-1, Ga i-2, Ga i-3, Ga o, Ga t1, Ga t2, Ga z and Ga gust (4). Of these, the three Ga i subtypes function to open atrial potassium channels (6). Ga 16 is a member of the Gq subfamily and is expressed specifically in hematopoietic cells (7).