Signal transduction by guanine nucleotide binding proteins

High affinity binding of guanine nucleotides and the ability to hydrolyze bound GTP to GDP are characteristics of an extended family of intracellular proteins. Subsets of this family include cytosolic initiation and elongation factors involved in protein synthesis, and cytoskeletal proteins such as tubulin (Hughes S.M. (1981) FEBS Lett. 164, 1-8). A distinct subset of guanine nucleotide binding proteins is membrane-associated; members of this subset include the ras gene products (Ellis R.W. et al. (1981) Nature 292, 506-511) and the heterotrimeric G-proteins (also termed N-proteins) (Gilman A.G. (1984) Cell 36, 577-579). Substantial evidence indicates that G-proteins act as signal transducers by coupling receptors (R) to effectors (E). A similar function has been suggested but not proven for the ras gene products. Known G-proteins include G_s and G_i, the G-proteins associated with stimulation and inhibition, respectively, of adenylate cyclase; transducin (TD), the G-protein coupling rhodopsin to cGMP phosphodiesterase in rod photoreceptors (Bitensky M.W. et al. (1981) Curr. Top. Membr. Transp. 15, 237-271; Stryer L. (1986) Annu. Rev. Neurosci. 9, 87-119), and G_o, a G-protein of unknown function that is highly abundant in brain (Sternweis P.C. and Robishaw J.D. (1984) J. Biol. Chem. 259, 13806-13813; Neer E.J. et al. (1984) J. Biol. Chem. 259, 14222-14229). G-proteins also participate in other signal transduction pathways, notably that involving phosphoinositide breakdown. In this review, I highlight recent progress in our understanding of the structure, function, and diversity of G-proteins.