Hormonal stimulation of adenylyl cyclase through G_i-protein βγ subunits

Abstract

AGONIST-BOUND receptors activate heterotrimeric (αβγ) G proteins by catalysing replacement by GTP of GDP bound to the a subunit, resulting in dissociation of α-GTP from the βγ subunits. In most cases, α-GTP carries the signal to effectors, as in hormonal stimulation^1–4 and inhibition^5,6of adenylyl cyclase by α_s and α_i, respectively. By contrast, genetic evidence in yeast⁷ and studies in mammalian cells^8–10 suggest that βγ 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 βγ. This effect of βγ requires costimulation by α_s-GTP^14,15. This conditional pattern of effector responsiveness led to the prediction¹⁵ 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 α_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 ofβγ , the α subunit of transducin, suggest that βγ subunits of the G_i proteins mediated this stimulation. These findings assign a new signalling function to βγ subunits of G_i proteins, the conditional stimulation of cAMP synthesis by adenylyl cyclase II.