Endogenous mono-ADP-ribosylation of the free Gβγ prevents stimulation of phosphoinositide 3-kinase-γ and phospholipase C-β2 and is activated by G-protein-coupled receptors

Abstract

We have recently demonstrated that the β subunit of the heterotrimeric G-proteins is endogenously mono-ADP-ribosylated in intact cells. The modified βγ heterodimer loses its ability to inhibit calmodulin-stimulated type 1 adenylate cyclase and, remarkably, is de-ADP-ribosylated by a cytosolic hydrolase that completes an ADP-/de-ADP-ribosylation cycle of potential physiological relevance. In the present study, we show that this ADP-ribosylation might indeed be a general mechanism for termination of βγ signalling, since the ADP-ribosylated βγ subunit is also unable to activate both phosphoinositide 3-kinase-γ and phospholipase C-β2. Moreover, we show that β subunit ADP-ribosylation is induced by G-protein-coupled receptor activation, since hormone stimulation of Chinese-hamster ovary plasma membranes leads to increases in β subunit labelling. This occurs when βγ is in its active heterodimeric conformation, since full inhibition of this modification can be achieved by binding of GDP-αi3 to the βγ heterodimer. Taken together, these findings delineate a pathway that arises from the activation of a G-protein-coupled receptor and leads to the inhibition of βγ activity through its reversible mono-ADP-ribosylation.