Showing papers on "GTP-Binding Protein alpha Subunits published in 2005"

The GAPs, GEFs, and GDIs of heterotrimeric G-protein alpha subunits.

Abstract: The heterotrimeric G-protein alpha subunit has long been considered a bimodal, GTP-hydrolyzing switch controlling the duration of signal transduction by seven-transmembrane domain (7TM) cell-surface receptors. In 1996, we and others identified a superfamily of "regulator of G-protein signaling" (RGS) proteins that accelerate the rate of GTP hydrolysis by Galpha subunits (dubbed GTPase-accelerating protein or "GAP" activity). This discovery resolved the paradox between the rapid physiological timing seen for 7TM receptor signal transduction in vivo and the slow rates of GTP hydrolysis exhibited by purified Galpha subunits in vitro. Here, we review more recent discoveries that have highlighted newly-appreciated roles for RGS proteins beyond mere negative regulators of 7TM signaling. These new roles include the RGS-box-containing, RhoA-specific guanine nucleotide exchange factors (RGS-RhoGEFs) that serve as Galpha effectors to couple 7TM and semaphorin receptor signaling to RhoA activation, the potential for RGS12 to serve as a nexus for signaling from tyrosine kinases and G-proteins of both the Galpha and Ras-superfamilies, the potential for R7-subfamily RGS proteins to couple Galpha subunits to 7TM receptors in the absence of conventional Gbetagamma dimers, and the potential for the conjoint 7TM/RGS-box Arabidopsis protein AtRGS1 to serve as a ligand-operated GAP for the plant Galpha AtGPA1. Moreover, we review the discovery of novel biochemical activities that also impinge on the guanine nucleotide binding and hydrolysis cycle of Galpha subunits: namely, the guanine nucleotide dissociation inhibitor (GDI) activity of the GoLoco motif-containing proteins and the 7TM receptor-independent guanine nucleotide exchange factor (GEF) activity of Ric8/synembryn. Discovery of these novel GAP, GDI, and GEF activities have helped to illuminate a new role for Galpha subunit GDP/GTP cycling required for microtubule force generation and mitotic spindle function in chromosomal segregation.

Signal Transduction by Tga3, a Novel G Protein α Subunit of Trichoderma atroviride

Abstract: Several fungi belonging to the genus Trichoderma can act as mycoparasites and are used commercially as biological control agents against plant-pathogenic fungi, such as Rhizoctonia solani, Botrytis cinerea, Sclerotium rolfsii, Sclerotinia sclerotiorum, and Pythium spp (5, 6, 18) Mycoparasitic strains can penetrate and kill the host fungi Mycoparasitism is accompanied by the secretion of antifungal metabolites, such as peptaibol antibiotics (32, 47), and morphological changes, such as coiling around the host and development of hooks and/or appressorium-like structures (11) Hydrolytic enzymes, such as chitinases, glucanases, and proteases, also are important for biocontrol activity, since they enable Trichoderma to degrade the host's cell wall and to utilize its cellular contents (20) Chitinase gene expression is induced by colloidal chitin, fungal cell walls, or the chitin monomer N-acetylglucosamine (29) In Trichoderma atroviride the N-acetylglucosaminidase (NAGase)-encoding gene, nag1, has a major impact on the induction by chitin of other chitinases (4) In mycoparasitic interactions between T atroviride and R solani, expression of ech42, which encodes endochitinase 42, is triggered by a low-molecular-weight diffusible factor released from the host prior to physical contact between the two fungi (10, 50) Lectins in the host's cell wall can induce coiling of the mycoparasite around the host (1, 24, 41) Elucidation of the signaling pathways underlying mycoparasitism has only recently begun Heterotrimeric G proteins are composed of α, β, and γ subunits Gα subunits play pivotal roles in the recognition process, virulence, and virulence-dependent development in a number of plant-pathogenic fungi (3, 15, 26, 27, 31) Fungal Gα subunits are highly conserved and can be divided into three major subgroups (3) In T atroviride, the subgroup I Gα protein Tga1 affects light-induced conidiation and mycoparasitism-related coiling (41) The corresponding protein in Trichoderma virens is involved in antagonism against S rolfsii but not in antagonism against R solani ΔtgaA loss-of-function mutants sporulate and have coiling behavior similar to that of the wild type (37), suggesting that the subgroup I Gα subunits have different functions in these mycoparasites The T virens subgroup II G protein α subunit, TgaB, has no specific role in development or mycoparasitism (37) Fungal subgroup III Gα proteins also regulate morphological and developmental processes, such as germination, conidiation, and secondary metabolite production (28, 53), and they may increase intracellular cyclic AMP (cAMP) levels by stimulating adenylyl cyclase (28, 33, 39) The objectives of this study were to isolate a subgroup III Gα subunit deletion mutant of T atroviride and to determine its role in development and mycoparasitism This report is the first report of the role of the Tga3-mediated G protein pathway in mycoparasitism-related properties, such as host recognition, chitinase expression, and secretion

Gα12/Gα13 Subunits of Heterotrimeric G Proteins Mediate Parathyroid Hormone Activation of Phospholipase D in UMR-106 Osteoblastic Cells

Abstract: PTH, a major regulator of bone remodeling and a therapeutically effective bone anabolic agent, stimulates several signaling pathways in osteoblastic cells. Our recent studies have revealed that PTH activates phospholipase D (PLD) -mediated phospholipid hydrolysis through a RhoA-dependent mechanism in osteoblastic cells, raising the question of the upstream link to the PTH receptor. In the current study, we investigated the role of heterotrimeric G proteins in mediating PTH-stimulated PLD activity in UMR-106 osteoblastic cells. Transfection with antagonist minigenes coding for small peptide antagonists to G alpha 12 and G alpha13 subunits of heterotrimeric G proteins prevented PTH-stimulated activation of PLD, whereas an antagonist minigene to G alphas failed to produce this effect. Effects of pharmacological inhibitors (protein kinase inhibitor, Clostridium botulinum exoenzyme C3) were consistent with a role of Rho small G proteins, but not of cAMP, in the effect of PTH on PLD. Expression of constitutively active G alpha12 and G alpha13 activated PLD, an effect that was inhibited by dominant-negative RhoA. The results identify G alpha12 and G alpha13 as upstream transducers of PTH effects on PLD, mediated through RhoA in osteoblastic cells.

Gα Subunit Gpa2 Recruits Kelch Repeat Subunits That Inhibit Receptor-G Protein Coupling during cAMP-induced Dimorphic Transitions in Saccharomyces cerevisiae

Abstract: All eukaryotic cells sense extracellular stimuli and activate intracellular signaling cascades via G protein-coupled receptors (GPCR) and associated heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 and associated Gα subunit Gpa2 sense extracellular carbon sources (including glucose) to govern filamentous growth. In contrast to conventional Gα subunits, Gpa2 forms an atypical G protein complex with the kelch repeat Gβ mimic proteins Gpb1 and Gpb2. Gpb1/2 negatively regulate cAMP signaling by inhibiting Gpa2 and an as yet unidentified target. Here we show that Gpa2 requires lipid modifications of its N-terminus for membrane localization but association with the Gpr1 receptor or Gpb1/2 subunits is dispensable for membrane targeting. Instead, Gpa2 promotes membrane localization of its associated Gβ mimic subunit Gpb2. We also show that the Gpa2 N-terminus binds both to Gpb2 and to the C-terminal tail of the Gpr1 receptor and that Gpb1/2 binding interferes with Gpr1 receptor coupling to Gpa2. Our studies invoke novel mechanisms involving GPCR-G protein modules that may be conserved in multicellular eukaryotes.

Design, synthesis, and preliminary pharmacological evaluation of a set of small molecules that directly activate gi proteins.

Abstract: Heterotrimeric G proteins play a pivotal role in the communication of cells with the environment. G proteins are stimulated by cell surface receptors (GPCR) that catalyze the exchange of GDP, bound to Galpha subunit, with GTP and can per se be the target of drugs. Based on the structure of two nonpeptidic modulators of Gi proteins, a series of new molecules characterized by a long hydrophobic chain and at least two nitrogen atoms protonated at physiological pH was designed. The compounds were tested for their ability to stimulate binding of GTPgammaS to recombinant Gi proteins. Gi activation properties were also evaluated by inhibition of adenylyl cyclase activity in intact lymphocytes. Most compounds were able to stimulate GTPgammaS binding and to inhibit cAMP production at micromolar doses. Among the active compounds, 34 showed good efficacy and was the most potent compound studied, particularly on alpha(o) subtype; its regioisomer, 36, was the most efficacious one. Compound 7 showed also an interesting profile as it showed selectivity toward the alpha(o) subtype, in both efficacy and potency. Some of the compounds synthesized and found to be active may be useful leads to develop more potent and selective Gi protein modulators.

[Endocrine tumors associated to protein Gsalpha/Gi2alpha mutations].

Abstract: Many oncogenic mutations promote tumor growth by inducing autonomous activity of proteins that normally transmit proliferative signal initiated by extracellular factors. G proteins are a family of guanine nucleotide binding proteins, which are structurally homologous and widely distributed in eukaryotic cells. They are composed of three different subunits (alpha, beta e gamma). The alpha subunit, which contains the guanine nucleotide-binding site, is unique to each G protein. The G proteins couple an array of seven transmembrane receptors at the cell surface with a variety of intracellular effectors, which produce second messenger molecules. A subset of endocrine tumors, such as GH- or ACTH-secreting pituitary adenomas, functioning thyroid adenomas, adrenocortical and gonadal tumors were associated with somatic activating mutations in the highly conserved codons of the Gs (Arg201 and Gln227) and Gi (Arg179 and Gln205) proteins. These findings indicated that the G proteins play a role as oncogenes, contributing with the human endocrine tumorigenesis.