GTP-Binding Protein alpha Subunits

About: GTP-Binding Protein alpha Subunits is a research topic. Over the lifetime, 304 publications have been published within this topic receiving 19915 citations.

Evolving insights regarding mechanisms for the inhibition of insulin release by norepinephrine and heterotrimeric G proteins

Abstract: Norepinephrine has for many years been known to have three major effects on the pancreatic β-cell which lead to the inhibition of insulin release. These are activation of K(+) channels to hyperpolarize the cell and prevent the gating of voltage-dependent Ca(2+) channels that increase intracellular Ca(2+) concentration ([Ca(2+)](i)) and trigger release; inhibition of adenylyl cyclases, thus preventing the augmentation of stimulated insulin release by cyclic AMP; and a "distal" effect that occurs downstream of increased [Ca(2+)](i) to inhibit exocytosis. All three are mediated by the pertussis toxin (PTX)-sensitive heterotrimeric Gi and Go proteins. The distal inhibitory effect on exocytosis is now known to be due to the binding of G protein βγ subunits to the synaptosomal-associated protein of 25 kDa (SNAP-25) on the soluble NSF attachment protein receptor (SNARE) complex. Recent studies have uncovered two more actions of norepinephrine on the β-cell: 1) retardation of the refilling of the readily releasable granule pool after it has been discharged, an action that is mediated by Gαi(1) and/or Gαi(2); and 2) inhibition of endocytosis that is mediated by Gz. Of importance also are new findings that Gαo regulates the number of docked granules in the β-cell, and that Gαo(2) maintains a tonic inhibitory influence on secretion. The latter provides another explanation as to why PTX, which blocks the effect of Gαo(2), was initially called "islet activating protein." Finally, there is clear evidence that overexpression of α(2A)-adrenergic receptors in β-cells can cause type 2 diabetes.

Characterization of G alpha 13-dependent plasma membrane recruitment of p115RhoGEF.

Abstract: The Ras homology (Rho) guanine nucleotide exchange factor (GEF), p115RhoGEF, provides a direct link between the G-protein alpha subunit, alpha(13), and the small GTPase Rho. In the present study, we demonstrate that activated mutants of alpha(13) or alpha(12), but not alpha(q), promote the redistribution of p115RhoGEF from the cytoplasm to the plasma membrane (PM). We also show that the PM translocation of p115RhoGEF is promoted by stimulation of thromboxane A(2) receptors. Furthermore, we define domains of p115RhoGEF required for its regulated PM recruitment. The RhoGEF RGS (regulators of G-protein signalling) domain of p115RhoGEF is required for PM recruitment, but it is not sufficient for strong alpha(13)-promoted PM recruitment, even though it strongly interacts with activated alpha(13). We also identify the pleckstrin homology domain as essential for alpha(13)-mediated PM recruitment. An amino acid substitution of lysine to proline at position 677 in the pleckstrin homology domain of p115RhoGEF inhibits Rho-mediated gene transcription, but this mutation does not affect alpha(13)-mediated PM translocation of p115RhoGEF. The results suggest a mechanism whereby multiple signals contribute to regulated PM localization of p115RhoGEF.

Invited review: Activation of G proteins by GTP and the mechanism of Gα-catalyzed GTP hydrolysis.

Abstract: This review addresses the regulatory consequences of the binding of GTP to the alpha subunits (Gα) of heterotrimeric G proteins, the reaction mechanism of GTP hydrolysis catalyzed by Gα and the means by which GTPase activating proteins (GAPs) stimulate the GTPase activity of Gα. The high energy of GTP binding is used to restrain and stabilize the conformation of the Gα switch segments, particularly switch II, to afford stable complementary to the surfaces of Gα effectors, while excluding interaction with Gβγ, the regulatory binding partner of GDP-bound Gα. Upon GTP hydrolysis, the energy of these conformational restraints is dissipated and the two switch segments, particularly switch II, become flexible and are able to adopt a conformation suitable for tight binding to Gβγ. Catalytic site pre-organization presents a significant activation energy barrier to Gα GTPase activity. The glutamine residue near the N-terminus of switch II (Glncat ) must adopt a conformation in which it orients and stabilizes the γ phosphate and the water nucleophile for an in-line attack. The transition state is probably loose with dissociative character; phosphoryl transfer may be concerted. The catalytic arginine in switch I (Argcat ), together with amide hydrogen bonds from the phosphate binding loop, stabilize charge at the β-γ bridge oxygen of the leaving group. GAPs that harbor "regulator of protein signaling" (RGS) domains, or structurally unrelated domains within G protein effectors that function as GAPs, accelerate catalysis by stabilizing the pre-transition state for Gα-catalyzed GTP hydrolysis, primarily by restraining Argcat and Glncat to their catalytic conformations. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 449-462, 2016.