GTP-binding proteins (G-proteins)are a family of heterotrimeric proteins that play a critical role in signal transduction by coupling cell surface, 7-transmembrane domain receptors to intracellular signaling pathways including second messenger generation (such as cyclic AMP, calcium and diacylglycerol), protein phosphorylation, ion channel activation, gene induction, cell growth and differentiation. Receptor activation catalyzes the exchange of GTP for GDP bound to the inactive G protein alpha subunit resulting in a conformational change and dissociation of the complex. The G protein alpha and beta-gamma subunits are capable of regulating various cellular effectors. Activation is terminated by a GTPase intrinsic to the G-alpha subunit.