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.

Overexpression of the Heterotrimeric G-Protein α-Subunit Enhances Phytochrome-Mediated Inhibition of Hypocotyl Elongation in Arabidopsis

Abstract: Plant heterotrimeric G-proteins have been implicated in a number of signaling processes. However, most of these studies are based on biochemical or pharmacological approaches. To examine the role of heterotrimeric G-proteins in plant development, we generated transgenic Arabidopsis expressing the Gα subunit of the heterotrimeric G-protein under the control of a glucocorticoid-inducible promoter. With the conditional overexpression of either the wild type or a constitutively active version of Arabidopsis Gα, transgenic seedlings exhibited a hypersensitive response to light. This enhanced light sensitivity was more exaggerated in a relatively lower intensity of light and was observed in white light as well as far-red, red, and blue light conditions. The enhanced responses in far-red and red light required functional phytochrome A and phytochrome B, respectively. Furthermore, the response to far-red light depended on functional FHY1 but not on FIN219 and FHY3. This dependence on FHY1 indicates that the Arabidopsis Gα protein may act only on a discrete branch of the phytochrome A signaling pathway. Thus, our results support the involvement of a heterotrimeric G-protein in the light regulation of Arabidopsis seedling development.

An obligatory requirement for the heterotrimeric G protein Gi3 in the antiautophagic action of insulin in the liver.

Abstract: Heterotrimeric G proteins of the Gi class have been implicated in signaling pathways regulating growth and metabolism under physiological and pathophysiological conditions. Knockout mice carrying inactivating mutations in both of the widely expressed Gαi class genes, Gαi2 and Gαi3, demonstrate shared as well as gene-specific functions. The presence of a single active allele of Gαi3 is sufficient for embryonic development, whereas at least one allele of Gαi2 is required for extrauterine life. Mice lacking both Gαi2 and Gαi3 are massively growth-retarded and die in utero. We have used biochemical and cell biological methods together with in situ liver perfusion experiments to study Gαi isoform-specific functions in Gαi2- and Gαi3-deficient mice. The subcellular localization of Gαi3 in isolated mouse hepatocytes depends on the cellular metabolic status. Gαi3 localizes to autophagosomes upon starvation-induced autophagy and distributes to the plasma membrane upon insulin stimulation. Analysis of autophagic proteolysis in perfused mouse livers showed that mice lacking Gαi3 are deficient in the inhibitory action of insulin. These data indicate that Gαi3 is crucial for the antiautophagic action of insulin and suggest an as-yet-unrecognized function for Gαi3 on autophagosomal membranes.

Hydrophobicity of residue351 of the G protein Gi1 alpha determines the extent of activation by the alpha 2A-adrenoceptor.

Abstract: Cysteine351 is the site for pertussis toxin-catalyzed ADP-ribosylation in the G protein Gi1 alpha. Alteration of this residue, or the equivalent cysteine in other Gi-family G proteins, has been used to examine specific interactions between receptors and these G proteins. However, no systematic analysis has been performed to determine the quantitative effect of such alterations. To address this we mutated cysteine351 of Gi1 alpha to all other possible amino acids. Each of the G protein mutants was transiently coexpressed along with the porcine alpha 2A-adrenoceptor in HEK 293/T cells. Following pertussis toxin treatment of the cells, membranes were prepared and the capacity of the agonist UK14304 to stimulate the binding of [35S]GTP gamma S to the modified G proteins was measured. A spectrum of function was observed. The presence of either a charged amino acid or a proline at this position essentially attenuated agonist regulation. The wild-type G protein did not result in maximal stimulation by agonist. The presence of certain branched chain aliphatic amino acids or bulky aromatic R groups at amino acid351 resulted in substantially greater maximal stimulation by the alpha 2A-adrenoceptor than that achieved with the wild-type sequence. The degree of activation of the forms of Gi1 alpha correlated strongly with the octanol/water partition coefficient of the amino acid at residue351. Variation in EC50 values for agonist-induced stimulation of binding of [35S]GTP gamma S to the mutant G proteins also correlated with the octanol/water partition coefficient. These results define a central role for hydrophobicity of this residue in defining productive receptor-G protein interactions.

Signal transduction by a nondissociable heterotrimeric yeast G protein.

Abstract: Many signal transduction pathways involve heterotrimeric G proteins. The accepted model for activation of heterotrimeric G proteins states that the protein dissociates to the free Gα (GTP)-bound subunit and free Gβγ dimer. On GTP hydrolysis, Gα (GDP) then reassociates with Gβγ [Gilman, A. G. (1987) Annu. Rev. Biochem. 56, 615–649]. We reexamined this hypothesis, by using the mating G protein of the yeast Saccharomyces cerevisiae encoded by the genes GPA1, STE4, and STE18. In the absence of mating pheromone, the Gα (Gpa1) subunit represses the mating pathway. On activation by binding of pheromone to a serpentine receptor, the Gβγ (Ste4, Ste18) dimer transmits the signal to a mitogen-activated protein kinase cascade, leading to gene activation, arrest in the G1 stage of the cell cycle, production of shmoos (mating projections), and cell fusion. We found that a Ste4-Gpa1 fusion protein transmitted the pheromone signal and activated the mating pathway as effectively as when Ste4 (Gβ) and Gpa1 (Gα) were coexpressed as separate proteins. Hence, dissociation of this G protein is not required for its activation. Rather, a conformational change in the heterotrimeric complex is likely to be involved in signal transduction.