Different roles for Gi and Go proteins in modulation of adenylyl cyclase type-2 activity.

The effect of Gi/o protein-coupled receptors on adenylyl cyclase type 2 (AC2) has been studied in Sf9 insect cells. Stimulation of cells expressing AC2 with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) led to a twofold stimulation of cAMP synthesis that could be blocked with the protein kinase C inhibitor GF109203X. Activation of a coexpressed alpha2A-adrenoceptor or muscarinic M4 receptor inhibited the stimulation by TPA almost completely in a pertussis toxin-sensitive manner. Activation of Gs proteins switched the response of the alpha2A-adrenoceptor to potentiation of prestimulated AC2 activity. The potentiation, but not the inhibition, could be blocked by a Gbetagamma scavenger. A novel methodological approach, whereby signalling through endogenous G proteins was ablated, was used to assess specific G protein species in the signal pathway. Expression of Go proteins (alphao1 + beta1gamma2) restored both the inhibition and the potentiation, whereas expression of Gi proteins (alphai1 + beta1gamma2) resulted in a potentiation of both the TPA- and the Gs-stimulated AC2 activity. The data presented supports the view of AC2 as a molecular switch and implicates this isoform as a target for Go protein-linked signalling.