Molecular mechanism of Gαi activation by non-GPCR proteins with a Gα-Binding and Activating motif

Heterotrimeric G proteins are quintessential signalling switches activated by nucleotide exchange on G alpha. Although activation is predominantly carried out by G-protein-coupled receptors (GPCRs), non-receptor guanine-nucleotide exchange factors (GEFs) have emerged as critical signalling molecules and therapeutic targets. Here we characterize the molecular mechanism of G-protein activation by a family of non-receptor GEFs containing a G alpha-binding and -activating (GBA) motif. We combine NMR spectroscopy, computational modelling and biochemistry to map changes in G alpha caused by binding of GBA proteins with residue-level resolution. We find that the GBA motif binds to the SwitchII/alpha 3 cleft of G alpha and induces changes in the G-1/P-loop and G-2 boxes (involved in phosphate binding), but not in the G-4/G-5 boxes (guanine binding). Our findings reveal that G-protein-binding and activation mechanisms are fundamentally different between GBA proteins and GPCRs, and that GEF-mediated perturbation of nucleotide phosphate binding is sufficient for G alpha activation.