Exchange Of GDP For GTP Research Articles

Rac1 is a novel interactor of Drosophila guanine nucleotide exchange factor GEFmeso.

Guanine nucleotide exchange factors (GEF) are proteins that regulate the activity of GTPases by catalysing the exchange of GDP for GTP. In this work, we studied the interaction of various Rho family GTPases CDC42, Rac1 and RhoA with GEFmeso, a GEF expressed in Drosophila melanogaster (fruit fly). The interaction of GEFmeso with Rho family GTPases was studied by means of guanine nucleotide exchange assays. Our results suggest that GEFmeso interacts with Rac1 in addition to its interaction with CDC42 and Ral, which was reported earlier. We propose that the guanine nucleotide exchange factor GEFmeso is involved in a developmental process that requires the synergistic action of CDC42 and Rac1 during Drosophila development.

A kinetic model of GPCRs: analysis of G protein activity, occupancy, coupling and receptor-state affinity constants

Context: G protein-coupled receptors are vital macromolecules for a wide variety of physiological processes. Upon agonist binding, these receptors accelerate the exchange of GDP for GTP in G proteins coupled to them. The activated G protein interacts with effector proteins to implement downstream biological functions. Objective: We present a kinetic, quaternary complex model, based on a system of coupled linear first-order differential equations, which accounts for the binding attributes of the ligand, receptor, G protein and two types of guanine nucleotide (GDP and GTP) as well as for GTPase activity. Methods: We solved the model numerically to predict the extents of G protein activation, receptor occupancy by ligand and receptor coupling that result from varying the ligand concentration, presence of GDP and/or GTP, the ratio of G protein to receptor and the equilibrium constants governing receptor pre-coupling and constitutive activity. We also simulated responses downstream from G protein activation using a transducer function. Results: Our model shows that agonist-induced G protein activation can occur with either a net decrease or increase in total receptor-G protein coupling. In addition, we demonstrate that affinity constants of the ligand for both the active and inactive states of the receptor can be derived to a close approximation from analysis of simulated responses downstream from receptor activation. Discussion and conclusion: The latter result validates our prior methods for estimating the active state affinity constants of ligands, and our results on receptor coupling have relevance to studies investigating receptor-G protein interactions using fluorescence techniques.

Abstract LB-10: Rational design of small molecules targeting the Ras GEF, SOS1

Abstract Ras GTPases regulate multiple intracellular signaling processes involved in gene expression, cell proliferation, and cell survival. Driver mutations of Ras signaling components including K,N,H-Ras, EGFR, PDGFR, Raf, PI3K, and Akt have been found in many types of human cancers. The activation of Ras is catalyzed by guanine-nucleotide exchange factors (GEFs), which stimulate the exchange of GDP for GTP. SOS1 is a major RasGEF that transduces receptor tyrosine kinase signals to Ras and associated ERK/Akt pathways. SOS1 gain-of-function mutations have been found in Noonan syndrome patients. In addition, SOS1 has been shown to be differentially expressed in prostate and breast cancer cells, and is involved in malignancies dependent upon aberrant Ras signaling. In the current studies, we undertake a structure-based virtual screening approach, coupled with biochemical and cell biological validations, to identify lead chemical inhibitors targeting the Ras catalytic domain of SOS1. From a 118,500 compound subset of the NCI/DTP Open Chemical Repository, 135 showed optimal docking energy to a grid of the SOS1 catalytic site from which 36 were selected for experimental screening by a guanine nucleotide exchange assay, i.e. FL-GDP dissociation of Ras under the catalysis of SOS1. Two compounds were confirmed as true hits (NSC-674954 and NSC-658497), of which NSC-658497 displayed micromolar IC50s in both inhibiting FL-GDP dissociation (IC50 - 22.2 µM) from Ras and TR-GTP loading (IC50 - 40.8 µM) of Ras catalyzed by Sos1. In addition, NSC-658497 displayed a target binding Kd of ∼7.0 µM as assayed by microscale thermophoresis, along with dose-dependently disrupting the SOS1 interaction with Ras as assayed by a GST-Ras pull-down analysis. Mutagenesis studies mapped NSC-658497 action site to the catalytic site in SOS1, involving specifically residues I825, T828, T829, and Y912. SAR analysis revealed that NSC-658497 elicits its inhibitory effect on SOS1 through a benzopyran and nitrophenyl moiety. In fibroblast cells, NSC-658497 showed dose-dependent efficacy in inhibiting EGF-stimulated Ras and ERK/AKT activities. Furthermore, NSC-658497 dose-dependently inhibited NIH/3T3, MEF, and MIA-PaCa-2 pancreatic cancer cell proliferation in conjunction with their respective downstream Ras-dependent ERK/AKT activities. Thus, we have identified a class of lead chemical inhibitors targeting SOS1 that can serve as useful tools for studying SOS1 mediated oncogenesis and as hits for further development of future cancer therapeutics. Citation Format: Chris Evelyn, Xin Duan, Jacek Biesiada, William Seibel, Jaroslaw Meller, Yi Zheng. Rational design of small molecules targeting the Ras GEF, SOS1. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-10. doi:10.1158/1538-7445.AM2014-LB-10

Abstract 4448: Overexpression of ECT2 promotes proliferation and metastasis of UPSC

Abstract Introduction: UPSC is a rare but aggressive malignancy that accounts for no more than 5 to 10% of uterine cancers, but more than 40% of associated uterine cancer deaths. The molecular events responsible for the poor clinical outcomes observed with UPSC are largely unknown. Epithelial cell transforming sequence 2 oncogene (ECT2) is a guanine nucleotide exchange factor (Rho-GEF) that catalyzes the exchange of GDP for GTP by Rho family GTPases. High levels of ECT2 expression have been reported in brain, lung and breast cancers, where they were shown promote metastasis. However, the role of ECT2 in UPSC has not been previously explored. Methods: After obtaining IRB permission, ECT2 mRNA and protein expression were measured in flash frozen specimens of normal proliferative endometrium (n=8) and UPSC (n =8) by quantitative real-time PCR (qPCR) and Western blot. Established cultures of UPSC cell lines (UPSC-ARK1, UPSC-ARK2) were transfected with either an siRNA targeting ECT2 or a non-targeting control (Dharmacon) using Lipofectamine 2000 (Invitrogen). Reduced expression of ECT2 following transfections with ECT2 siRNAs was confirmed by qPCR and Western blot. Standard MTS and Caspase 3/7 assays (Promega) were utilized to measure proliferation and apoptosis. Colony formation and Boyden chamber assays were used to measure metastatic capacity, migration and invasion in vitro. Results: Our data indicate that ECT2 is overexpressed >4-fold in nearly all UPSC specimens tested (n=8, p<0.001). We also found that ECT2 is robustly expressed in both the UPSC-ARK1 and UPSC-ARK2 cell lines. Utilizing these 2 cell lines, we found that knockdown of ECT2 significantly decreased proliferation (UPSC-ARK1: 0.62-fold versus control; n=3, p<0.05; UPSC-ARK2: 0.34-fold versus control, p<0.05) and increased rates of apoptosis (UPSC-ARK1: 1.3-fold; n=3, p<0.05; UPSC-ARK2: 2.2-fold, n=3, p<0.05) in both cell lines tested. Decreased ECT2 expression targeted by transfection with siRNAs led to decreased rates of colony formation in UPSC-ARK1 and UPSC-ARK2 cell lines. Knockdown of ECT2 also significantly reduced rates of migration and invasion observed in vitro with both cell lines. Conclusions: Overexpression of ECT2 plays a critical role in promoting the growth and metastasis of UPSC. Targeting ECT2 expression may provide an effective strategy for improving outcomes for UPSC and other human cancers where hyperactivation of metastasis-promoting Rho GTPases are observed. Citation Format: Claire M. Mach, Thomas J. Magliaro, Matthew L. Anderson. Overexpression of ECT2 promotes proliferation and metastasis of UPSC. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4448. doi:10.1158/1538-7445.AM2014-4448

The emerging role of guanine nucleotide exchange factors in ALS and other neurodegenerative diseases.

Small GTPases participate in a broad range of cellular processes such as proliferation, differentiation, and migration. The exchange of GDP for GTP resulting in the activation of these GTPases is catalyzed by a group of enzymes called guanine nucleotide exchange factors (GEFs), of which two classes: Dbl-related exchange factors and the more recently described dedicator of cytokinesis proteins family exchange factors. Increasingly, deregulation of normal GEF activity or function has been associated with a broad range of disease states, including neurodegeneration and neurodevelopmental disorders. In this review, we examine this evidence with special emphasis on the novel role of Rho guanine nucleotide exchange factor (RGNEF/p190RhoGEF) in the pathogenesis of amyotrophic lateral sclerosis. RGNEF is the first neurodegeneration-linked GEF that regulates not only RhoA GTPase activation but also functions as an RNA binding protein that directly acts with low molecular weight neurofilament mRNA 3′ untranslated region to regulate its stability. This dual role for RGNEF, coupled with the increasing understanding of the key role for GEFs in modulating the GTPase function in cell survival suggests a prominent role for GEFs in mediating a critical balance between cytotoxicity and neuroprotection which, when disturbed, contributes to neuronal loss.

EGF receptor uses SOS1 to drive constitutive activation of NFκB in cancer cells.

Activation of nuclear factor κB (NFκB) is a central event in the responses of normal cells to inflammatory signals, and the abnormal constitutive activation of NFκB is important for the survival of most cancer cells. In nonmalignant human cells, EGF stimulates robust activation of NFκB. The kinase activity of the EGF receptor (EGFR) is required, because the potent and specific inhibitor erlotinib blocks the response. Down-regulating EGFR expression or inhibiting EGFR with erlotinib impairs constitutive NFκB activation in several different types of cancer cells and, conversely, increased activation of NFκB leads to erlotinib resistance in these cells. We conclude that EGF is an important mediator of NFκB activation in cancer cells. To explore the mechanism, we selected an erlotinib-resistant cell line in which the guanine nucleotide exchange factor Son of Sevenless 1 (SOS1), well known to be important for EGF-dependent signaling to MAP kinases, is overexpressed. Increased expression of SOS1 increases NFκB activation in several different types of cancer cells, and ablation of SOS1 inhibits EGF-induced NFκB activation in these cells, indicating that SOS1 is a functional component of the pathway connecting EGFR to NFκB activation. Importantly, the guanine nucleotide exchange activity of SOS1 is not required for NFκB activation.

Bud3 activates Cdc42 to establish a proper growth site in budding yeast.

Cell polarization occurs along a single axis that is generally determined by a spatial cue, yet the underlying mechanism is poorly understood. Using biochemical assays and live-cell imaging, we show that cell polarization to a proper growth site requires activation of Cdc42 by Bud3 in haploid budding yeast. Bud3 catalyzes the release of guanosine diphosphate (GDP) from Cdc42 and elevates intracellular Cdc42-guanosine triphosphate (GTP) levels in cells with inactive Cdc24, which has as of yet been the sole GDP-GTP exchange factor for Cdc42. Cdc42 is activated in two temporal steps in the G1 phase: the first depends on Bud3, whereas subsequent activation depends on Cdc24. Mutational analyses suggest that biphasic activation of Cdc42 in G1 is necessary for assembly of a proper bud site. Biphasic activation of Cdc42 or Rac GTPases may be a general mechanism for spatial cue-directed cell polarization in eukaryotes.

Nox1 activation by βPix and the role of Ser-340 phosphorylation.

Rac is an activating factor for Nox1, an O2(-)-generating NADPH oxidase, expressed in the colon and other tissues. Rac requires a GDP-GTP exchange factor for activation. Nox1 activation by βPix has been demonstrated in cell lines. We examined the effects of βPix and its phosphomimetic mutant on endogenous Nox1 in Caco-2 cells transfected with Noxo1 and Noxa1. βPix expression enhanced O2(-) production in resting cells and cells stimulated with EGF or phorbol ester. βPix(S340E) further enhanced O2(-) production, while βPix(S340A) eliminated the βPix effect. βPix(S340E), but not βPix(S340A), had higher affinity and GEF activity for Rac than wild-type βPix. These results suggest that βPix phosphorylation at Ser-340 upregulates Nox1 through Rac activation, confirming Rac as a trigger for acute Nox1-dependent ROS production.

Amplification of B cell receptor–Erk signaling by Rasgrf-1 overexpression in chronic lymphocytic leukemia

Rasgrf-1 is a guanine exchange factor (GEF) that catalyzes the exchange of GDP for GTP. In a RNA microarray analysis of chronic lymphocytic leukemia (CLL) specimens (n = 5), this gene was found to be overexpressed in CLL as compared to normal peripheral blood mononuclear cell (PBMC) CD19 + B cells (n = 3). CLL specimens (n = 29) expressed Rasgrf-1 RNA at levels 5–300-fold higher as compared to normal B cells. CLL specimens expressed a 75 kDa isoform that was smaller than the expected full-length protein (140 kDa) and the truncated variant had higher GEF activity. Knockdown of Rasgrf-1 in CLL specimens inhibited active GTP-bound Ras and the Ras/Erk/mitogen-activated protein kinase (MAPK) pathway. Rasgrf-1 was phosphorylated and activated by B cell receptor (BCR) signaling that increased its GEF function, and this phosphorylation was blocked by Src and Bruton's tyrosine kinase (BTK) inhibitors. Rasgrf-1 is a novel GEF protein that has a role in BCR signaling and its overexpression further activates the Ras/Erk/MAPK pathway in CLL specimens.

Atypical G<sub>αi</sub> Signal Transduction

G protein signaling is an extremely complex event that is involved in almost every cellular process. As such, G protein-coupled receptors are the most commonly found type of transmembrane receptors used by cells to initiate intracellular signaling events. However, the widely accepted model of cyclical GDP-GTP exchange in response to ligand binding to 7TMRs, followed by dissociation of the G protein subunits and activation of intracellular signaling cascades, has repeatedly been challenged in recent years. Some of the exceptions that have been brought forth include signaling by a non-dissociated, rearranged heterotrimer and the existence of "reverse-mode", active G proteins that interact with active receptors. Here, we focus on G(αi/o), one of the common G(α) classes, and outline a major exception to the classical model, that of G protein coupling to RTKs. We then describe a novel concept in G(αi/o) signaling, namely that the pathways induced by agonist binding circumvent the typical signaling pathways responsive to decreases in the second messenger cAMP, via adenylyl cyclase inhibition.

RhoGEFs in Cell Motility: Novel Links Between Rgnef and Focal Adhesion Kinase

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

Dual Regulation of G Proteins and the G Protein-Activated Potassium Channels (GIRK) by Lithium

Cellular targets of Li+, such as glycogen synthase kinase 3β and G proteins, have been long implicated in bipolar disorder (BPD) etiology. However, recent genetic studies link BPD to other proteins, in particular ion channels. Li+ affects neuronal excitability, but the underlying mechanisms and the relevance to putative BPD targets are unknown. We discovered a novel, dual regulation of G protein-gated K+ channels (GIRK) by Li+, and determined the underlying molecular mechanisms. In hippocampal neurons, therapeutic doses of Li+, 0.5-2 mM, increased GIRK basal current (Ibasal) but attenuated neurotransmitter-evoked GIRK currents (Ievoked) mediated by Gi/o-coupled G protein-coupled receptors (GPCRs). Molecular mechanisms of these regulations were studied with heterologously expressed GIRK1/2. In excised membrane patches, Li+ increased Ibasal but reduced GPCR-induced GIRK currents. Both regulations were membrane-delimited and G protein-dependent, requiring both Gα and Gβγ subunits. Li+ did not impair direct activation of GIRK by Gβγ, suggesting that inhibition of Ievoked results from a Li+ action on Gα, probably through inhibition of GTP-GDP exchange. In direct binding studies, Li+ promoted GPCR-independent dissociation of GαiGDP from Gβγ by a Mg2+-independent mechanism. This previously unknown Li+ action on G proteins explains the second effect of Li+, the enhancement of GIRK's Ibasal. The dual effect of Li+ on GIRK may profoundly regulate inhibitory effects of neurotransmitters acting via GIRKs. Our findings link between Li+, neuronal excitability, and both cellular and genetic targets of BPD: GPCRs, G proteins and ion channels.

The Human Minor Histocompatibility Antigen1 Is a RhoGAP

The human minor Histocompatibility Antigen HMHA-1 is a major target of immune responses after allogeneic stem cell transplantation applied for the treatment of leukemia and solid tumors. The restriction of its expression to hematopoietic cells and many solid tumors raised questions regarding its cellular functions. Sequence analysis of the HMHA-1 encoding HMHA1 protein revealed the presence of a possible C-terminal RhoGTPase Activating Protein (GAP) domain and an N-terminal BAR domain. Rho-family GTPases, including Rac1, Cdc42, and RhoA are key regulators of the actin cytoskeleton and control cell spreading and migration. RhoGTPase activity is under tight control as aberrant signaling can lead to pathology, including inflammation and cancer. Whereas Guanine nucleotide Exchange Factors (GEFs) mediate the exchange of GDP for GTP resulting in RhoGTPase activation, GAPs catalyze the low intrinsic GTPase activity of active RhoGTPases, resulting in inactivation. Here we identify the HMHA1 protein as a novel RhoGAP. We show that HMHA1 constructs, lacking the N-terminal region, negatively regulate the actin cytoskeleton as well as cell spreading. Furthermore, we show that HMHA1 regulates RhoGTPase activity in vitro and in vivo. Finally, we demonstrate that the HMHA1 N-terminal BAR domain is auto-inhibitory as HMHA1 mutants lacking this region, but not full-length HMHA1, showed GAP activity towards RhoGTPases. In conclusion, this study shows that HMHA1 acts as a RhoGAP to regulate GTPase activity, cytoskeletal remodeling and cell spreading, which are crucial functions in normal hematopoietic and cancer cells.

DrosophilaRic-8 interacts with the Gα12/13subunit, Concertina, during activation of the Folded gastrulation pathway

Heterotrimeric G proteins, composed of α, β, and γ subunits, are activated by exchange of GDP for GTP on the Gα subunit. Canonically, Gα is stimulated by the guanine-nucleotide exchange factor (GEF) activity of ligand-bound G protein-coupled receptors. However, Gα subunits may also be activated in a noncanonical manner by members of the Ric-8 family, cytoplasmic proteins that also act as GEFs for Gα subunits. We used a signaling pathway active during Drosophila gastrulation as a model system to study Ric-8/Gα interactions. A component of this pathway, the Drosophila Gα12/13 subunit, Concertina (Cta), is necessary to trigger actomyosin contractility during gastrulation events. Ric-8 mutants exhibit similar gastrulation defects to Cta mutants. Here we use a novel tissue culture system to study a signaling pathway that controls cytoskeletal rearrangements necessary for cellular morphogenesis. We show that Ric-8 regulates this pathway through physical interaction with Cta and preferentially interacts with inactive Cta and directs its localization within the cell. We also use this system to conduct a structure-function analysis of Ric-8 and identify key residues required for both Cta interaction and cellular contractility.

A Physiologically Required G Protein-coupled Receptor (GPCR)-Regulator of G Protein Signaling (RGS) Interaction That Compartmentalizes RGS Activity

G protein-coupled receptors (GPCRs) can interact with regulator of G protein signaling (RGS) proteins. However, the effects of such interactions on signal transduction and their physiological relevance have been largely undetermined. Ligand-bound GPCRs initiate by promoting exchange of GDP for GTP on the Gα subunit of heterotrimeric G proteins. Signaling is terminated by hydrolysis of GTP to GDP through intrinsic GTPase activity of the Gα subunit, a reaction catalyzed by RGS proteins. Using yeast as a tool to study GPCR signaling in isolation, we define an interaction between the cognate GPCR (Mam2) and RGS (Rgs1), mapping the interaction domains. This reaction tethers Rgs1 at the plasma membrane and is essential for physiological signaling response. In vivo quantitative data inform the development of a kinetic model of the GTPase cycle, which extends previous attempts by including GPCR-RGS interactions. In vivo and in silico data confirm that GPCR-RGS interactions can impose an additional layer of regulation through mediating RGS subcellular localization to compartmentalize RGS activity within a cell, thus highlighting their importance as potential targets to modulate GPCR signaling pathways.

Triacylglycerol mobilization is suppressed by brefeldin A in Chlamydomonas reinhardtii

Brefeldin A suppresses vesicle trafficking by inhibiting exchange of GDP for GTP in ADP-ribosylation factor. We report that brefeldin A suppresses mobilization of triacylglycerols in Chlamydomonas reinhardtii, a model organism of green microalgae. Analyses revealed that brefeldin A causes Chlamydomonas to form lipid droplets in which triacylglycerols accumulate in a dose-dependent manner. Pulse labeling experiment using fluorescent fatty acids suggested that brefeldin A inhibits the cells from degrading fatty acids. The experiment also revealed that the cells transiently form novel compartments that accumulate exogenously added fatty acids in the cytoplasm, designated fatty acid-induced microbodies (FAIMs). Brefeldin A up-regulates the formation of FAIMs, whereas nitrogen deprivation that up-regulates triacylglycerol synthesis in Chlamydomonas does not cause the cells to form FAIMs. These results underscore the role of the vesicle trafficking machinery in triacylglycerol metabolism in green microalgae.

Low Concentrations of Hydrogen Peroxide or Nitrite Induced of Paracoccidioides brasiliensis Cell Proliferation in a Ras-Dependent Manner

Paracoccidioides brasiliensis, a causative agent of paracoccidioidomycosis (PCM), should be able to adapt to dramatic environmental changes inside the infected host after inhalation of air-borne conidia and transition to pathogenic yeasts. Proteins with antioxidant functions may protect fungal cells against reactive oxygen (ROS) and nitrogen (RNS) species generated by phagocytic cells, thus acting as potential virulence factors. Ras GTPases are involved in stress responses, cell morphology, and differentiation in a range of organisms. Ras, in its activated form, interacts with effector proteins and can initiate a kinase cascade. In lower eukaryotes, Byr2 kinase represents a Ras target. The present study investigated the role of Ras in P. brasiliensis after in vitro stimulus with ROS or RNS. We have demonstrated that low concentrations of H2O2 (0.1 mM) or NO2 (0.1–0.25 µM) stimulated P. brasiliensis yeast cell proliferation and that was not observed when yeast cells were pre-incubated with farnesyltransferase inhibitor. We constructed an expression plasmid containing the Byr2 Ras-binding domain (RBD) fused with GST (RBD-Byr2-GST) to detect the Ras active form. After stimulation with low concentrations of H2O2 or NO2, the Ras active form was observed in fungal extracts. Besides, NO2 induced a rapid increase in S-nitrosylated Ras levels. This alternative posttranslational modification of Ras, probably in residue Cys123, would lead to an exchange of GDP for GTP and consequent GTPase activation in P. brasiliensis. In conclusion, low concentrations of H2O2 or NO2 stimulated P. brasiliensis proliferation through Ras activation.

Backbone resonance assignments for G protein αi3 subunit in the GDP-bound state

Guanine-nucleotide binding proteins (G proteins) serve as molecular switches in signaling pathways, by coupling the activation of G protein-coupled receptors (GPCRs) at the cell surface to intracellular responses. In the resting state, G protein forms a heterotrimer, consisting of the G protein α subunit with GDP (Gα·GDP) and the G protein βγ subunit (Gβγ). Ligand binding to GPCRs promotes the GDP-GTP exchange on Gα, leading to the dissociation of the GTP-bound form of Gα (Gα·GTP) and Gβγ. Then, Gα·GTP and Gβγ bind to their downstream effector enzymes or ion channels and regulate their activities, leading to a variety of cellular responses. Finally, Gα hydrolyzes the bound GTP to GDP and returns to the resting state by re-associating with Gβγ. The G proteins are classified with four major families based on the amino acid sequences of Gα: i/o, s, q/11, and 12/13. Here, we established the backbone resonance assignments of human Gαi3, a member of the i/o family with a molecular weight of 41 K, in complex with GDP. The chemical shifts were compared with those of Gα(i3) in complex with a GTP-analogue, GTPγS, which we recently reported, indicating that the residues with significant chemical shift differences are mostly consistent with the regions with the structural differences between the GDP- and GTPγS-bound states, as indicated in the crystal structures. The assignments of Gα(i3)·GDP would be useful for the analyses of the dynamics of Gα(i3) and its interactions with various target molecules.

Andrographolide derivatives inhibit guanine nucleotide exchange and abrogate oncogenic Ras function

Aberrant signaling by oncogenic mutant rat sarcoma (Ras) proteins occurs in ∼15% of all human tumors, yet direct inhibition of Ras by small molecules has remained elusive. Recently, several small-molecule ligands have been discovered that directly bind Ras and inhibit its function by interfering with exchange factor binding. However, it is unclear whether, or how, these ligands could lead to drugs that act against constitutively active oncogenic mutant Ras. Using a dynamics-based pocket identification scheme, ensemble docking, and innovative cell-based assays, here we show that andrographolide (AGP)--a bicyclic diterpenoid lactone isolated from Andrographis paniculata--and its benzylidene derivatives bind to transient pockets on Kirsten-Ras (K-Ras) and inhibit GDP-GTP exchange. As expected for inhibitors of exchange factor binding, AGP derivatives reduced GTP loading of wild-type K-Ras in response to acute EGF stimulation with a concomitant reduction in MAPK activation. Remarkably, however, prolonged treatment with AGP derivatives also reduced GTP loading of, and signal transmission by, oncogenic mutant K-RasG12V. In sum, the combined analysis of our computational and cell biology results show that AGP derivatives directly bind Ras, block GDP-GTP exchange, and inhibit both wild-type and oncogenic K-Ras signaling. Importantly, our findings not only show that nucleotide exchange factors are required for oncogenic Ras signaling but also demonstrate that inhibiting nucleotide exchange is a valid approach to abrogating the function of oncogenic mutant Ras.

Abstract SY32-04: The translation initiation factor elF2α regulates cytoprotective autophagy in Myc-dependent lymphoma.

Abstract SY32-04: The translation initiation factor elF2α regulates cytoprotective autophagy in Myc-dependent lymphoma.