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 Gßγ 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 Gßγ 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.

NMR identification of the binding surfaces involved in the Salmonella and Shigella Type III secretion tip-translocon protein-protein interactions.

The type III secretion system (T3SS) is essential for the pathogenesis of many bacteria including Salmonella and Shigella, which together are responsible for millions of deaths worldwide each year. The structural component of the T3SS consists of the needle apparatus, which is assembled in part by the protein-protein interaction between the tip and the translocon. The atomic detail of the interaction between the tip and the translocon proteins is currently unknown. Here, we used NMR methods to identify that the N-terminal domain of the Salmonella SipB translocon protein interacts with the SipD tip protein at a surface at the distal region of the tip formed by the mixed α/ß domain and a portion of its coiled-coil domain. Likewise, the Shigella IpaB translocon protein and the IpaD tip protein interact with each other using similar surfaces identified for the Salmonella homologs. Furthermore, removal of the extreme N-terminal residues of the translocon protein, previously thought to be important for the interaction, had little change on the binding surface. Finally, mutations at the binding surface of SipD reduced invasion of Salmonella into human intestinal epithelial cells. Together, these results reveal the binding surfaces involved in the tip-translocon protein-protein interaction and advance our understanding of the assembly of the T3SS needle apparatus. Proteins 2016; 84:1097-1107. © 2016 Wiley Periodicals, Inc.

A neurodevelopmental disorder mutation locks G proteins in the transitory pre-activated state.

Many neurotransmitter receptors activate G proteins through exchange of GDP for GTP. The intermediate nucleotide-free state has eluded characterization, due largely to its inherent instability. Here we characterize a G protein variant associated with a rare neurological disorder in humans. GαoK46E has a charge reversal that clashes with the phosphate groups of GDP and GTP. As anticipated, the purified protein binds poorly to guanine nucleotides yet retains wild-type affinity for G protein ßγ subunits. In cells with physiological concentrations of nucleotide, GαoK46E forms a stable complex with receptors and Gßγ, impeding effector activation. Further, we demonstrate that the mutant can be easily purified in complex with dopamine-bound D2 receptors, and use cryo-electron microscopy to determine the structure, including both domains of Gαo, without nucleotide or stabilizing nanobodies. These findings reveal the molecular basis for the first committed step of G protein activation, establish a mechanistic basis for a neurological disorder, provide a simplified strategy to determine receptor-G protein structures, and a method to detect high affinity agonist binding in cells.

Catalytic site mutations confer multiple states of G protein activation.

Heterotrimeric guanine nucleotide-binding proteins (G proteins) that function as molecular switches for cellular growth and metabolism are activated by GTP and inactivated by GTP hydrolysis. In uveal melanoma, a conserved glutamine residue critical for GTP hydrolysis in the G protein α subunit is often mutated in Gαq or Gα11 to either leucine or proline. In contrast, other glutamine mutations or mutations in other Gα subtypes are rare. To uncover the mechanism of the genetic selection and the functional role of this glutamine residue, we analyzed all possible substitutions of this residue in multiple Gα isoforms. Through cell-based measurements of activity, we showed that some mutants were further activated and inactivated by G protein-coupled receptors. Through biochemical, molecular dynamics, and nuclear magnetic resonance-based structural studies, we showed that the Gα mutants were functionally distinct and conformationally diverse, despite their shared inability to hydrolyze GTP. Thus, the catalytic glutamine residue contributes to functions beyond GTP hydrolysis, and these functions include subtype-specific, allosteric modulation of receptor-mediated subunit dissociation. We conclude that G proteins do not function as simple on-off switches. Rather, signaling emerges from an ensemble of active states, a subset of which are favored in disease and may be uniquely responsive to receptor-directed ligands.

Molecular annotation of G protein variants in a neurological disorder.

Heterotrimeric G proteins transduce extracellular chemical messages to generate appropriate intracellular responses. Point mutations in GNAO1, encoding the G protein αo subunit, have been implicated in a pathogenic condition characterized by seizures, movement disorders, intellectual disability, and developmental delay (GNAO1 disorder). However, the effects of these mutations on G protein structure and function are unclear. Here, we report the effects of 55 mutations on Gαo conformation, thermostability, nucleotide binding, and hydrolysis, as well as interaction with Gßγ subunits, receptors, and effectors. Our effort reveals four functionally distinct groups of mutants, including one group that sequesters receptors and another that sequesters Gßγ, both acting in a genetically dominant manner. These findings provide a more comprehensive understanding of disease-relevant mutations and reveal that GNAO1 disorder is likely composed of multiple mechanistically distinct disorders that will likely require multiple therapeutic strategies.