A universal allosteric mechanism for G protein activation

G proteins play a central role in signal transduction and pharmacology. Signaling is initiated by cell-surface receptors, which promote guanosine triphosphate (GTP) binding and dissociation of Gα from the Gβγ subunits. Structural studies have revealed the molecular basis of subunit association with receptors, RGS proteins, and downstream effectors. In contrast, the mechanism of subunit dissociation is poorly understood. We use cell signaling assays, molecular dynamics (MD) simulations, and biochemistry and structural analyses 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 of the final committed step of G protein activation. [Display omitted] •Receptors promote GTP-GDP exchange and dissociation of G protein α and βγ subunits•An allosteric Gly-Arg-Glu (G-R-E) network links the γ phosphate of GTP to release of Gβγ•Gly-Arg-Glu mutations prevent subunit dissociation regardless of agonist or GTP binding•Gly-Arg-Glu mutations are responsible for human endocrine and neurological disorders G protein signaling involves binding of agonist to receptor and unbinding of GDP from the G protein. Using integrated molecular and computational approaches, Knight et al. investigate the second, committed step of G protein activation, involving an allosteric “Gly-Arg-Glu” network that links GTP binding to subunit dissociation and pathway activation.