(Cellular systems for the transduction of external stimuli into intracellular signals are essential components of the cellular plasma membrane. An emerging paradigm is that such “transmembrane signaling systems” often involve three distinct components. First, there are the specific receptors exposed at the external surface of the cell membrane that recognize and interactwith ligands such as hormones or drugs or respond to sensory stimuli such as light. Second, exposed at the cy-toplasmic surface are effector enzymes such as adenylate cyclase, which generates the second messenger cAMP (Lefkowitz et al., 1983). Effector enzymes also include phospholipase C (PLC)(whichhydrolyzes phosphatidylinositol 4, 5-bisphosphate to inositol 1, 4, 5-trisphosphate and 1, 2-di-acylglycerol)(Taylor & Merritt, 1986) as well as cGMP phosphodiesterase (Klein, 1984). Other effectors may act as ion channels (Pfaffinger et al., 1985; Breitwiesser & Szabo, 1985). Third, interposed physically and functionally between many receptor and effector molecules are transducing or coupling proteins, which bind and hydrolyze GTP. These are the guanine nucleotide regulatory proteins, or “G proteins”(Gilman, 1984). Recent evidence indicates that signaling systems of this type demonstrate a degree of structural, functional, and regulatory homologythat had not previously been appreciated.