HORMONAL-STIMULATION OF ADENYLYL CYCLASE THROUGH GI-PROTEIN BETA-GAMMA-SUBUNITS

AGONIST-BOUND receptors activate heterotrimeric (alpha-beta-gamma) G proteins by catalysing replacement by GTP of GDP bound to the alpha-subunit, resulting in dissociation of alpha-GTP from the beta-gamma-subunits. In most cases, alpha-GTP carries the signal to effectors, as in hormonal stimulation 1-4 and inhibition 5,6 of adenylyl cyclase by alpha(s) and alpha(i) respectively. By contrast, genetic evidence in yeast 7 and studies in mammalian cells 8-10 suggest that beta-gamma-subunits of G proteins may also regulate effector pathways. Indeed, of the four recombinant mammalian adenylyl cyclases available for study 11-14, two, adenylyl cyclases II and IV, are stimulated by beta-gamma. This effect of beta-gamma requires costimulation by alpha(s)-GTP 14,15. This conditional pattern of effector responsiveness led to the prediction 15 that receptors coupled to many G proteins will mediate elevation of cellular cyclic AMP, provided that G(s) is also active. We now confirm this prediction. Coexpression of mutationally active alpha(s) with adenylyl cyclase II converted agonists that act through 'inhibitory' receptors (coupled to G(i)) into stimulators of cAMP synthesis. Experiments using pertussis toxin and a putative scavenger of beta-gamma, the alpha-subunit of transducin, suggest that beta-gamma-subunits of the G(i) proteins mediated this stimulation. These findings assign a new signalling function to beta-gamma-subunits of G(i) proteins, the conditional stimulation of cAMP synthesis by adenylyl cyclase II.