The past few years have been an exciting period in the field of receptor biology as remarkable similarities among various signal transduction systems have been revealed. Many such transmembrane signaling systems consist of three membrane-bound protein components:(a) a cell surface receptor;(b) an effector, such as an ion channel or the enzyme adenylyl cyclase; and (c) a guanine nucleotide-binding regulatory protein, or G protein, that is coupled to both the receptor and its effector. G protein-coupled receptors mediate the actions of extracellular signals as diverse as light, odorants, peptide hor mones, and neurotransmitters, and they have been identified in organisms as evolutionarily divergent as yeast and man. Nearly all G protein-coupled receptors bear detectable sequence similarity with one another, and it is thought that all share a similar topological motif consisting of seven hydrophobic (and potentially a-helical) segments that span the lipid bilayer (1).

The primary function of cell surface receptors is to discriminate the appro priate ligands from among multiple extracellular stimuli, and then to activate an effector system that produces an intracellular signal [eg the messenger cyclic AMP (cAMP)]. Hence, receptors not only transmit but also amplify and integrate extracellular signals, thereby controlling cellular processes. In this way, cells can communicate and coordinate their development and ac tions. Such intercellular communication has obvious utility in multicellular organisms, but also plays a role in the life cycle of unicellular eukaryotes.