Gα _s -specific structural elements attenuate interactions with RGS proteins

Heterotrimeric (αβγ) G proteins are molecular switches that are activated by G protein-coupled receptors (GPCRs) and regulate numerous intracellular signaling cascades. Most active Gα subunits are inactivated by Regulators of G protein Signaling (RGS) proteins, which determine the duration of G protein-mediated signaling by accelerating the catalytic turn-off of the Gα subunit. However, the G protein Gα _s does not interact with known RGS proteins. To understand the molecular basis for this divergent phenomenon, we combined a comparative structural analysis of experimental and modelled structures with functional biochemical assays. This analysis showed that Gα _s contains unique structural elements in both the helical and the GTPase domains. Modeling identified helical domain insertions, missing in experimental structures, that project towards the interface with RGS proteins, and residues in the GTPase domain that might interfere with RGS binding. Mutagenesis of Gα _s and measurements of RGS GAP activity showed that three residues in the Gα _s GTPase domain are both necessary and sufficient to prevent Gα _s inactivation by RGSs. Indeed, substitution of all three Gα _s residues with the corresponding residues from Gα _i1 enabled efficient inactivation by RGS proteins. These results shed new light on the mechanistic bases for G protein specificity towards RGS proteins.