G-proteins as Receptor-Effector Couplers

“G-proteins” is the term used to designate a distinct subset of the family of guanine nucleotide binding proteins. Like other members of the family, G-proteins bind guaninc nucleotides with high affinity and specificity, possess intrinsic GTPase activity, and undergo a functionally important change in conformation, depending upon that guanine nucleotide, GDP or GTP, are bound. Numerous G-proteins have been identified through classic protein purification and through complementary DNA (cDNA) cloning. The number of receptors and effectors known to be regulated by G-proteins has also increased substantially. This chapter discusses the features of structure and function shared by all G-proteins and details those features that distinguish individual G-proteins. It is already clear that quantitative and qualitative changes in the G-proteins may profoundly influence transmembrane signalling. G-proteins serve as receptor–effector couple. Receptors for many different types of “first messenger” are coupled to G-proteins. Examples include the photon receptor, rhodopsin, neurotransmitter receptors, such as the β-adrenergic and muscarinic cholinergic receptors, classical hormone receptors, such as those for gonadotropins, glucagon, and parathyroid hormone, receptors for prostaglandins, and receptors for chemotactic factors, such as f-Met-Leu-Phe. Transduction quantitative and qualitative changes in G-proteins may be an important cause of alterations in target cell response to extracellular “first messengers.” Covalent modifications of G-proteins, such as adenosine diphosphate (ADP)-ribosylation catalyzed by bacterial toxins provide a striking example. The possibility that endogenous cellular enzymes covalently modify G-proteins remains unproven, but deserves further investigation.