Inositol Lipids, G-Proteins and Signal Transduction

Activation of cell surface receptors generates intracellular signals which are the main mediators of cellular responses. The number of different agonists available to interact with their target receptors are enormous compared to the number of intracellular signals. Cyclic AMP is one such intracellular signal, whose synthesis (catalyzed by adenylate cyclase) is well understood. The adenylate cyclase signalling system consists of three kinds of components: a receptor, a guanine nucleotide regulatory protein (G-protein) and the catalytic unit, adenylate cyclase. Receptors either stimulate or inhibit the formation of cAMP by communicating with a pair of homologous G-proteins, Gs which mediates stimulation of adenylate cyclase and Gi which is responsible for its inhibition (Rodbell 1980; Gilman 1984). The G proteins that regulate adenylate cyclase belong to a larger family of homologous G proteins that includes transducin, a molecule that couples light-sensitive rhodopsin to cyclic cGMP phosphodiesterase (Stryer 1983) and Go (Worley et al. 1986), a molecule present in large amounts in brain whose function is not known.