G Proteins and Regulation of Adenylate Cyclase (Nobel Lecture)

How is the hormone‐activated synthesis of the second messenger cyclic AMP regulated? Or, in more general terms, how does the interaction of an extracellular agonist with a receptor lead to intracellular enzyme activity when receptor and enzyme are not distinct macromolecules? The mediators are membrane‐bound, guanine nucleotide binding regulatory proteins (G proteins). When the receptor is activated, G proteins dissociate into their subunits. These in turn activate or inhibit enzymes such as adenylate cyclases, which catalyze the synthesis of cyclic AMP. In these signal transmission processes G proteins act as molecular switches and amplifiers.