A Universal Allosteric Mechanism for G Protein Activation

SUMMARY G proteins play a central role in signal transduction and pharmacology. Signaling is initiated by cell-surface receptors, which promote GTP binding and the dissociation of Gα from the Gβγ subunits. Structural studies have revealed the molecular basis for subunit association with receptors, RGS proteins and downstream effectors. In contrast, the mechanism of subunit dissociation is poorly understood. We used cell signaling assays, MD simulations, biochemistry and structural analysis to identify a conserved network of amino acids that dictates subunit release. In the presence of the terminal phosphate of GTP, a glycine forms a polar network with an arginine and glutamate, putting torsional strain on the subunit binding interface. This “G-R-E motif” secures GTP and, through an allosteric link, discharges the Gβγ dimer. Replacement of network residues prevents subunit dissociation, regardless of agonist or GTP binding. These findings reveal the molecular basis for the final committed step of G protein activation. HIGHLIGHTS Receptors promote GTP-GDP exchange and dissociation of G protein α and βγ subunits We find an allosteric network linking the γ phosphate of GTP with release of Gβγ The network consists of a conserved Gly-Arg-Glu “activation triad” Triad mutations prevent subunit dissociation, regardless of agonist or GTP binding Triad mutations are responsible for human endocrine and neurological disorders