Real-time detection of basal and stimulated G protein GTPase activity using fluorescent GTP analogues

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins, composed of Gα, Gβ, and Gγ subunits, are a class of signal transduction complexes with broad roles in human health and agriculturally relevant plant physiological and developmental traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. We sought to develop improved methods for heterologous expression and rapid purification of Gα subunits, initially targeting GPA1, the sole canonical Gα subunit of the model plant species, Arabidopsis thaliana. Compared to conventional methods, our expression methodology and rapid StrepII-tag mediated purification facilitates substantially higher yield, and isolation of protein with increased GTP binding and hydrolysis activities. Human GNAI1 purified using our approach displayed the expected binding and hydrolysis activities, indicating our protocol is applicable to mammalian Gα subunits, potentially including those for which purification of enzymatically active protein has been historically problematic. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate that the inherent instability of GPA1 is a function of the interaction between the Ras and helical domains. Additionally, we found that GPA1-GNAO1 domain swaps partially uncouple the instability from the rapid nucleotide binding kinetics displayed by GPA1. In summary, our work provides insights into methods to optimally study heterotrimeric G proteins, and reveals roles of the helical domain in Gα kinetics and stability.

Clinical and Molecular Profiling in GNAO1 Permits Phenotype-Genotype Correlation

BACKGROUND

Defects in GNAO1, the gene encoding the major neuronal G-protein Gαo, are related to neurodevelopmental disorders, epilepsy, and movement disorders. Nevertheless, there is a poor understanding of how molecular mechanisms explain the different phenotypes.

OBJECTIVES

We aimed to analyze the clinical phenotype and the molecular characterization of GNAO1-related disorders.

METHODS

Patients were recruited in collaboration with the Spanish GNAO1 Association. For patient phenotyping, direct clinical evaluation, analysis of homemade-videos, and an online questionnaire completed by families were analyzed. We studied Gαo cellular expression, the interactions of the partner proteins, and binding to guanosine triphosphate (GTP) and G-protein-coupled receptors (GPCRs).

RESULTS

Eighteen patients with GNAO1 genetic defects had a complex neurodevelopmental disorder, epilepsy, central hypotonia, and movement disorders. Eleven patients showed neurological deterioration, recurrent hyperkinetic crisis with partial recovery, and secondary complications leading to death in three cases. Deep brain stimulation improved hyperkinetic crisis, but had inconsistent benefits in dystonia. The molecular defects caused by pathogenic Gαo were aberrant GTP binding and hydrolysis activities, an inability to interact with cellular binding partners, and reduced coupling to GPCRs. Decreased localization of Gαo in the plasma membrane was correlated with the phenotype of "developmental and epileptic encephalopathy 17." We observed a genotype-phenotype correlation, pathogenic variants in position 203 were related to developmental and epileptic encephalopathy, whereas those in position 209 were related to neurodevelopmental disorder with involuntary movements. Milder phenotypes were associated with other molecular defects such as del.16q12.2q21 and I344del.

CONCLUSION

We highlight the complexity of the motor phenotype, which is characterized by fluctuations throughout the day, and hyperkinetic crisis with a distinct post-hyperkinetic crisis state. We confirm a molecular-based genotype-phenotype correlation for specific variants. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Influence of expression and purification protocols on Gα biochemical activity: kinetics of plant and mammalian G protein cycles

Heterotrimeric G proteins are a class of signal transduction complexes with broad roles in human health and agriculturally important plant traits. In the classic paradigm, guanine nucleotide binding to the Gα subunit regulates the activation status of the complex. Using the Arabidopsis thaliana Gα subunit, GPA1, we developed a rapid StrepII-tag mediated purification method that facilitates isolation of protein with increased enzymatic activities as compared to conventional methods, and is demonstrably also applicable to mammalian Gα subunits. We subsequently utilized domain swaps of GPA1 and human GNAO1 to demonstrate the instability of recombinant GPA1 is a function of the interaction between the Ras and helical domains, and can be partially uncoupled from the rapid nucleotide binding kinetics displayed by GPA1.

GNAO1 Mutations Affecting the N-Terminal α-Helix of Gαo Lead to Parkinsonism

BACKGROUND

Patients carrying pathogenic variants in GNAO1 present a phenotypic spectrum ranging from severe early-onset epileptic encephalopathy and developmental delay to mild adolescent/adult-onset dystonia. Genotype-phenotype correlation and molecular mechanisms underlying the disease remain understudied.

METHODS

We analyzed the clinical course of a child carrying the novel GNAO1 mutation c.38T>C;p.Leu13Pro, and structural, biochemical, and cellular properties of the corresponding mutant Gαo-GNAO1-encoded protein-alongside the related mutation c.68T>C;p.Leu23Pro.

RESULTS

The main clinical feature was parkinsonism with bradykinesia and rigidity, unlike the hyperkinetic movement disorder commonly associated with GNAO1 mutations. The Leu ➔ Pro substitutions have no impact on enzymatic activity or overall folding of Gαo but uniquely destabilize the N-terminal α-helix, blocking formation of the heterotrimeric G-protein and disabling activation by G-protein-coupled receptors.

CONCLUSIONS

Our study defines a parkinsonism phenotype within the spectrum of GNAO1 disorders and suggests a genotype-phenotype correlation by GNAO1 mutations targeting the N-terminal α-helix of Gαo. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Clinical Cases and the Molecular Profiling of a Novel Childhood Encephalopathy-Causing GNAO1 Mutation P170R

De novo mutations in GNAO1, the gene encoding the major neuronal G protein Gαo, cause a spectrum of pediatric encephalopathies with seizures, motor dysfunction, and developmental delay. Of the >80 distinct missense pathogenic variants, many appear to uniformly destabilize the guanine nucleotide handling of the mutant protein, speeding up GTP uptake and deactivating GTP hydrolysis. Zinc supplementation emerges as a promising treatment option for this disease, as Zn2+ ions reactivate the GTP hydrolysis on the mutant Gαo and restore cellular interactions for some of the mutants studied earlier. The molecular etiology of GNAO1 encephalopathies needs further elucidation as a prerequisite for the development of efficient therapeutic approaches. In this work, we combine clinical and medical genetics analysis of a novel GNAO1 mutation with an in-depth molecular dissection of the resultant protein variant. We identify two unrelated patients from Norway and France with a previously unknown mutation in GNAO1, c.509C>G that results in the production of the Pro170Arg mutant Gαo, leading to severe developmental and epileptic encephalopathy. Molecular investigations of Pro170Arg identify this mutant as a unique representative of the pathogenic variants. Its 100-fold-accelerated GTP uptake is not accompanied by a loss in GTP hydrolysis; Zn2+ ions induce a previously unseen effect on the mutant, forcing it to lose the bound GTP. Our work combining clinical and molecular analyses discovers a novel, biochemically distinct pathogenic missense variant of GNAO1 laying the ground for personalized treatment development.

Fluorescence Anisotropy Assay with Guanine Nucleotides Provides Access to Functional Analysis of Gαi1 Proteins

Gα proteins as part of heterotrimeric G proteins are molecular switches essential for G protein-coupled receptor- mediated intracellular signaling. The role of the Gα subunits has been examined for decades with various guanine nucleotides to elucidate the activation mechanism and Gα protein-dependent signal transduction. Several approaches describe fluorescent ligands mimicking the GTP function, yet lack the efficient estimation of the proteins' GTP binding activity and the fraction of active protein. Herein, we report the development of a reliable fluorescence anisotropy-based method to determine the affinity of ligands at the GTP-binding site and to quantify the fraction of active Gαi1 protein. An advanced bacterial expression protocol was applied to produce active human Gαi1 protein, whose GTP binding capability was determined with novel fluorescently labeled guanine nucleotides acting as high-affinity Gαi1 binders compared to the commonly used BODIPY FL GTPγS. This study thus contributes a new method for future investigations of the characterization of Gαi and other Gα protein subunits, exploring their corresponding signal transduction systems and potential for biomedical applications.

Gαi2-induced conductin/axin2 condensates inhibit Wnt/β-catenin signaling and suppress cancer growth

Conductin/axin2 is a scaffold protein negatively regulating the pro-proliferative Wnt/β-catenin signaling pathway. Accumulation of scaffold proteins in condensates frequently increases their activity, but whether condensation contributes to Wnt pathway inhibition by conductin remains unclear. Here, we show that the Gαi2 subunit of trimeric G-proteins induces conductin condensation by targeting a polymerization-inhibiting aggregon in its RGS domain, thereby promoting conductin-mediated β-catenin degradation. Consistently, transient Gαi2 expression inhibited, whereas knockdown activated Wnt signaling via conductin. Colorectal cancers appear to evade Gαi2-induced Wnt pathway suppression by decreased Gαi2 expression and inactivating mutations, associated with shorter patient survival. Notably, the Gαi2-activating drug guanabenz inhibited Wnt signaling via conductin, consequently reducing colorectal cancer growth in vitro and in mouse models. In summary, we demonstrate Wnt pathway inhibition via Gαi2-triggered conductin condensation, suggesting a tumor suppressor function for Gαi2 in colorectal cancer, and pointing to the FDA-approved drug guanabenz for targeted cancer therapy.

Characterization of binding kinetics of A2AR to Gαs protein by surface plasmon resonance

Because of their surface localization, G protein-coupled receptors (GPCRs) are often pharmaceutical targets as they respond to a variety of extracellular stimuli (e.g., light, hormones, small molecules) that may activate or inhibit a downstream signaling response. The adenosine A_2A receptor (A_2AR) is a well-characterized GPCR that is expressed widely throughout the human body, with over 10 crystal structures determined. Truncation of the A_2AR C-terminus is necessary for crystallization as this portion of the receptor is long and unstructured; however, previous work suggests shortening of the A_2AR C-terminus from 412 to 316 amino acids (A_2AΔ316R) ablates downstream signaling, as measured by cAMP production, to below that of constitutive full-length A_2AR levels. As cAMP production is downstream of the first activation event-coupling of G protein to its receptor-investigating that first step in activation is important in understanding how the truncation effects native GPCR function. Here, using purified receptor and Gα_s proteins, we characterize the association of A_2AR and A_2AΔ316R to Gα_s with and without GDP or GTPγs using surface plasmon resonance (SPR). Gα_s affinity for A_2AR was greatest for apo-Gα_s, moderately affected in the presence of GDP and nearly completely ablated by the addition of GTPγs. Truncation of the A_2AR C-terminus (A_2AΔ316R) decreased the affinity of the unliganded receptor for Gα_s by ∼20%, suggesting small changes to binding can greatly impact downstream signaling.

Phosphate-Modified Nucleotides for Monitoring Enzyme Activity

Nucleotides modified at the terminal phosphate position have been proven to be interesting entities to study the activity of a variety of different protein classes. In this chapter, we present various types of modifications that were attached as reporter molecules to the phosphate chain of nucleotides and briefly describe the chemical reactions that are frequently used to synthesize them. Furthermore, we discuss a variety of applications of these molecules. Kinase activity, for instance, was studied by transfer of a phosphate modified with a reporter group to the target proteins. This allows not only studying the activity of kinases, but also identifying their target proteins. Moreover, kinases can also be directly labeled with a reporter at a conserved lysine using acyl-phosphate probes. Another important application for phosphate-modified nucleotides is the study of RNA and DNA polymerases. In this context, single-molecule sequencing is made possible using detection in zero-mode waveguides, nanopores or by a Förster resonance energy transfer (FRET)-based mechanism between the polymerase and a fluorophore-labeled nucleotide. Additionally, fluorogenic nucleotides that utilize an intramolecular interaction between a fluorophore and the nucleobase or an intramolecular FRET effect have been successfully developed to study a variety of different enzymes. Finally, also some novel techniques applying electron paramagnetic resonance (EPR)-based detection of nucleotide cleavage or the detection of the cleavage of fluorophosphates are discussed. Taken together, nucleotides modified at the terminal phosphate position have been applied to study the activity of a large diversity of proteins and are valuable tools to enhance the knowledge of biological systems.

A Putative Non-Canonical Ras-Like GTPase from P. falciparum: Chemical Properties and Characterization of the Protein

During its development the malaria parasite P. falciparum has to adapt to various different environmental contexts. Key cellular mechanisms involving G-protein coupled signal transduction chains are assumed to act at these interfaces. Heterotrimeric G-proteins are absent in Plasmodium. We here describe the first cloning and expression of a putative, non-canonical Ras-like G protein (acronym PfG) from Plasmodium. PfG reveals an open reading frame of 2736 bp encoding a protein of 912 amino acids with a theoretical pI of 8.68 and a molecular weight of 108.57 kDa. Transcript levels and expression are significantly increased in the erythrocytic phase in particular during schizont and gametocyte formation. Most notably, PfG has GTP binding capacity and GTPase activity due to an EngA2 domain present in small Ras-like GTPases in a variety of Bacillus species and Mycobacteria. By contrast, plasmodial PfG is divergent from any human alpha-subunit. PfG was expressed in E. coli as a histidine-tagged fusion protein and was stable only for 3.5 hours. Purification was only possible under native conditions by Nickel-chelate chromatography and subsequent separation by Blue Native PAGE. Binding of a fluorescent GTP analogue BODIPY® FL guanosine 5’O-(thiotriphosphate) was determined by fluorescence emission. Mastoparan stimulated GTP binding in the presence of Mg²⁺. GTPase activity was determined colorimetrically. Activity expressed as absolute fluorescence was 50% higher for the human paralogue than the activity of the parasitic enzyme. The PfG protein is expressed in the erythrocytic stages and binds GTP after immunoprecipitation. Immunofluorescence using specific antiserum suggests that PfG localizes to the parasite cytosol. The current data suggest that the putitative, Ras-like G-protein might be involved in a non-canonical signaling pathway in Plasmodium. Research on the function of PfG with respect to pathogenesis and antimalarial chemotherapy is currently under way.

Kinetics of the early events of GPCR signalling

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Little is known of the kinetics of interactions between GPCRs and their signalling partners.

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NTS1 binds Gα_i1 and Gα_s with affinities of 15 ± 6 nM and 31 ± 18 nM (SE), respectively.

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This SPR assay may be applicable to multiple partners in the signalling cascade.

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We provide the first direct evidence for GPCR-G protein coupling in nanodiscs.

Neurotensin receptor type 1 (NTS1) is a G protein-coupled receptor (GPCR) that affects cellular responses by initiating a cascade of interactions through G proteins. The kinetic details for these interactions are not well-known. Here, NTS1-nanodisc-Gα_s and Gα_i1 interactions were studied. The binding affinities of Gα_i1 and Gα_s to NTS1 were directly measured by surface plasmon resonance (SPR) and determined to be 15 ± 6 nM and 31 ± 18 nM, respectively. This SPR configuration permits the kinetics of early events in signalling pathways to be explored and can be used to initiate descriptions of the GPCR interactome.

Double suppression of the Gα protein activity by RGS proteins

Regulator of G protein signaling (RGS) proteins accelerate GTP hydrolysis on G protein α subunits, restricting their activity downstream from G protein-coupled receptors. Here we identify Drosophila Double hit (Dhit) as a dual RGS regulator of Gαo. In addition to the conventional GTPase-activating action, Dhit possesses the guanine nucleotide dissociation inhibitor (GDI) activity, slowing the rate of GTP uptake by Gαo; both activities are mediated by the same RGS domain. These findings are recapitulated using homologous mammalian Gαo/i proteins and RGS19. Crystal structure and mutagenesis studies provide clues into the molecular mechanism for this unprecedented GDI activity. Physiologically, we confirm this activity in Drosophila asymmetric cell divisions and HEK293T cells. We show that the oncogenic Gαo mutant found in breast cancer escapes this GDI regulation. Our studies identify Dhit and its homologs as double-action regulators, inhibiting Gαo/i proteins both through suppression of their activation and acceleration of their inactivation through the single RGS domain.

Conformational biosensors reveal GPCR signalling from endosomes

A long-held tenet of molecular pharmacology is that canonical signal transduction mediated by G-protein-coupled receptor (GPCR) coupling to heterotrimeric G proteins is confined to the plasma membrane. Evidence supporting this traditional view is based on analytical methods that provide limited or no subcellular resolution. It has been subsequently proposed that signalling by internalized GPCRs is restricted to G-protein-independent mechanisms such as scaffolding by arrestins, or GPCR activation elicits a discrete form of persistent G protein signalling, or that internalized GPCRs can indeed contribute to the acute G-protein-mediated response. Evidence supporting these various latter hypotheses is indirect or subject to alternative interpretation, and it remains unknown if endosome-localized GPCRs are even present in an active form. Here we describe the application of conformation-specific single-domain antibodies (nanobodies) to directly probe activation of the β2-adrenoceptor, a prototypical GPCR, and its cognate G protein, Gs (ref. 12), in living mammalian cells. We show that the adrenergic agonist isoprenaline promotes receptor and G protein activation in the plasma membrane as expected, but also in the early endosome membrane, and that internalized receptors contribute to the overall cellular cyclic AMP response within several minutes after agonist application. These findings provide direct support for the hypothesis that canonical GPCR signalling occurs from endosomes as well as the plasma membrane, and suggest a versatile strategy for probing dynamic conformational change in vivo.

Expression, purification and preliminary NMR characterization of isotopically labeled wild-type human heterotrimeric G protein αi1

Molecular-level investigation of proteins is increasingly important to researchers trying to understand the mechanisms of signal transmission. Heterotrimeric G proteins control the activation of many critical signal transmission cascades and are also implicated in numerous diseases. As part of a longer-term investigation of intramolecular motions in RGS and Gα proteins in their apo and complexed forms, we have successfully developed a protocol for preparing milligram quantities of highly purified, isotopically labeled wild-type human Gα(i1) (hGα(i1)) subunit for NMR studies. High levels of expression in Escherichia coli can be attributed to the use of the SUMO fusion protein system, a bacterial strain that produces rare codons, supplementation of minimal medium with small quantities of isotopically labeled rich medium and a lowered induction temperature. Purification of hGα(i1) utilized affinity and size exclusion chromatography, and protein activity was confirmed using fluorescence-based GTP-binding studies. Preliminary NMR analysis of hGα(i1) has shown that high-quality spectra can be obtained at near-physiological temperatures, whereas lower temperature spectra display numerous weak and broadened peaks, providing preliminary evidence for widespread μs-ms timescale exchange. In an effort to further optimize the NMR spectra we prepared a truncated form of hGα(i1) (hGα(i1)-Δ31) in which the 31-residue unstructured N-terminus was removed. This resulted in further improvements in spectral quality by eliminating high-intensity peaks that obscured resonances from structured segments of the protein. We plan to use hGα(i1)-Δ31 in future investigations of protein dynamics by NMR spectroscopy to gain insight into the role of these motions in RGS/Gα binding selectivity.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

Fluorescence-based assay probing regulator of G protein signaling partner proteins

The regulator of G protein signaling (RGS) proteins are one of the essential modulators for the G protein system. Besides regulating G protein signaling by accelerating the GTPase activity of Gα subunits, RGS proteins are implicated in exerting other functions; they are also known to be involved in several diseases. Moreover, the existence of a single RGS protein in plants and its seven-transmembrane domain found in 2003 triggered efforts to unveil detailed structural and functional information of RGS proteins. We present a method for real-time examination of the protein-protein interactions between RGS and Gα subunits. AtRGS1 from plants and RGS4 from mammals were site-directedly labeled with the fluorescent probe Lucifer yellow on engineered cysteine residues and used to interact with different Gα subunits. The physical interactions can be revealed by monitoring the real-time fluorescence changes (8.6% fluorescence increase in mammals and 27.6% in plants); their correlations to functional exertion were shown with a GTPase accelerating activity assay and further confirmed by measurement of K(d). We validate the effectiveness of this method and suggest its application to the exploration of more RGS signaling partner proteins in physiological and pathological studies.

High-throughput flow cytometry bead-based multiplex assay for identification of Rho GTPase inhibitors

Rho family GTPases and their effector proteins regulate a wide range of cell signaling pathways. In normal physiological conditions, their activity is tightly controlled and it is not surprising that their aberrant activation contributes to tumorigenesis or other diseases. For this reason, the identification of small, cell permeable molecules capable of inhibition of Rho GTPases can be extraordinarily useful, particularly if they are specific and act reversibly.Herein, we describe a flow cytometric assay, which allows us to measure the activity of six small GTPases simultaneously. GST-tagged small GTPases are bound to six glutathione bead sets each set having a different intensity of red fluorescence at a fixed wavelength. The coated bead sets were washed, combined, and dispensed into 384-well plates with test compounds, and fluorescent-GTP binding was used as the read-out.This multiplex bead-based assay was successfully used for to identify both general and selective inhibitors of Rho family GTPases.

Structural flexibility of the G alpha s alpha-helical domain in the beta2-adrenoceptor Gs complex

The active-state complex between an agonist-bound receptor and a guanine nucleotide-free G protein represents the fundamental signaling assembly for the majority of hormone and neurotransmitter signaling. We applied single-particle electron microscopy (EM) analysis to examine the architecture of agonist-occupied β(2)-adrenoceptor (β(2)AR) in complex with the heterotrimeric G protein Gs (Gαsβγ). EM 2D averages and 3D reconstructions of the detergent-solubilized complex reveal an overall architecture that is in very good agreement with the crystal structure of the active-state ternary complex. Strikingly however, the α-helical domain of Gαs appears highly flexible in the absence of nucleotide. In contrast, the presence of the pyrophosphate mimic foscarnet (phosphonoformate), and also the presence of GDP, favor the stabilization of the α-helical domain on the Ras-like domain of Gαs. Molecular modeling of the α-helical domain in the 3D EM maps suggests that in its stabilized form it assumes a conformation reminiscent to the one observed in the crystal structure of Gαs-GTPγS. These data argue that the α-helical domain undergoes a nucleotide-dependent transition from a flexible to a conformationally stabilized state.

A high-throughput screening assay for the identification of flavivirus NS5 capping enzyme GTP-binding inhibitors: implications for antiviral drug development

There are no effective antivirals currently available for the treatment of flavivirus infection in humans. As such, the identification and characterization of novel drug target sites are critical to developing new classes of antiviral drugs. The flavivirus NS5 N-terminal capping enzyme (CE) is vital for the formation of the viral RNA cap structure, which directs viral polyprotein translation and stabilizes the 5' end of the viral genome. The structure of the flavivirus CE has been solved, and a detailed understanding of the CE-guanosine triphosphate (GTP) and CE-RNA cap interactions is available. Because of the essential nature of the interaction for viral replication, disrupting CE-GTP binding is an attractive approach for drug development. The authors have previously developed a robust assay for monitoring CE-GTP binding in real time. They adapted this assay for high-throughput screening and performed a pilot screen of 46 323 commercially available compounds. A number of small-molecule inhibitors capable of displacing a fluorescently labeled GTP in vitro were identified, and a second functional assay was developed to identify false positives. The results presented indicate that the flavivirus CE cap-binding site is a valuable new target site for antiviral drug discovery and should be further exploited for broad-spectrum anti-flaviviral drug development.

High-Throughput Fluorescence Polarization Assay for the Enzymatic Activity of GTPase-Activating Protein of ADP-Ribosylation Factor (ARFGAP)

GTPase-activating proteins of ADP-ribosylation factors (ARFGAPs) play key cellular roles in vesicle production and trafficking, adhesion, migration, and development. Dysfunctional regulation of ARFGAPs has been implicated in various diseases, including cancer, Alzheimer disease, and autism. Unfortunately, there are few mechanistic details describing how ARFGAPs contribute to disease states. In this regard, it would be extremely helpful to have a set of small molecules that selectively and directly modulate specific ARFGAPs as probes to dissect ARFGAP-regulated cell signaling under various conditions. Currently, such probes are lacking, and their identification is hampered by the lack of a suitable high-throughput assay to monitor ARFGAP activity. Here, the authors describe and validate a robust high-throughput assay using fluorescence polarization to monitor the ability of diverse ARFGAPs to enhance the capacity of ARF1 to hydrolyze guanosine triphosphate.

Ric-8B is a GTP-dependent G protein alphas guanine nucleotide exchange factor

ric-8 (resistance to inhibitors of cholinesterase 8) genes have positive roles in variegated G protein signaling pathways, including Gα(q) and Gα(s) regulation of neurotransmission, Gα(i)-dependent mitotic spindle positioning during (asymmetric) cell division, and Gα(olf)-dependent odorant receptor signaling. Mammalian Ric-8 activities are partitioned between two genes, ric-8A and ric-8B. Ric-8A is a guanine nucleotide exchange factor (GEF) for Gα(i)/α(q)/α(12/13) subunits. Ric-8B potentiated G(s) signaling presumably as a Gα(s)-class GEF activator, but no demonstration has shown Ric-8B GEF activity. Here, two Ric-8B isoforms were purified and found to be Gα subunit GDP release factor/GEFs. In HeLa cells, full-length Ric-8B (Ric-8BFL) bound endogenously expressed Gα(s) and lesser amounts of Gα(q) and Gα(13). Ric-8BFL stimulated guanosine 5'-3-O-(thio)triphosphate (GTPγS) binding to these subunits and Gα(olf), whereas the Ric-8BΔ9 isoform stimulated Gα(s short) GTPγS binding only. Michaelis-Menten experiments showed that Ric-8BFL elevated the V(max) of Gα(s) steady state GTP hydrolysis and the apparent K(m) values of GTP binding to Gα(s) from ∼385 nm to an estimated value of ∼42 μM. Directionality of the Ric-8BFL-catalyzed Gα(s) exchange reaction was GTP-dependent. At sub-K(m) GTP, Ric-BFL was inhibitory to exchange despite being a rapid GDP release accelerator. Ric-8BFL binds nucleotide-free Gα(s) tightly, and near-K(m) GTP levels were required to dissociate the Ric-8B·Gα nucleotide-free intermediate to release free Ric-8B and Gα-GTP. Ric-8BFL-catalyzed nucleotide exchange probably proceeds in the forward direction to produce Gα-GTP in cells.

Parallel electrophoretic analysis of segmented samples on chip for high-throughput determination of enzyme activities

Capillary electrophoresis (CE) on microfabricated structures has achieved impressive sample throughput by combining fast separation speed and parallel operations. One obstacle to further increasing throughput has been lack of methods for loading and injecting individual samples at a rate that matches analysis speed. To address this issue, we have developed a microfluidic device in which samples stored as nanoliter volume plugs segmented by a fluorocarbon oil are introduced sequentially to an array of three electrophoresis channels. A microfluidic interface consisting of patterned surface chemistry and geometric restriction was used to extract samples from each segmented flow channel and transfer to the respective electrophoresis channel for separation. Fluorescence detection was achieved by imaging the chip using a fluorescence microscope equipped with a charge-coupled device. Characterization of the system shows that injection volume is controlled by sample plug volume, flow rate during introduction, and voltage applied to the electrophoresis channel. The system was tested for a GTPase assay. Peak area ratios of enzyme product and internal standard had 6% relative standard deviations. Cross-contamination between peaks was 7%. Throughput of 120 samples in 10 min was achieved. Further development of the system may allow application to high-throughput applications such as drug screening.

The trimeric G protein Go inflicts a double impact on axin in the Wnt/frizzled signaling pathway

The Wnt/Frizzled signaling pathway plays crucial roles in animal development and is deregulated in many cases of carcinogenesis. We and others have previously demonstrated that Frizzled proteins initiating the intracellular signaling are typical G protein-coupled receptors and rely on the trimeric G protein Go for Wnt transduction in Drosophila. However, the mode of action of Go and its interplay with other transducers of the pathway such as Dishevelled and Axin remained unclear. Here we show that the alpha-subunit of Go directly acts on Axin, the multidomain protein playing a negative role in the Wnt signaling. G alpha o physically binds Axin and re-localizes it to the plasma membrane. Furthermore, G alpha o suppresses Axin's inhibitory action on the Wnt pathway in Drosophila wing development. The interaction of G alpha o with Axin critically depends on the RGS domain of the latter. Additionally, we show that the betagamma-component of Go can directly bind and recruit Dishevelled from cytoplasm to the plasma membrane, where activated Dishevelled can act on the DIX domain of Axin. Thus, the two components of the trimeric Go protein mediate a double-direct and indirect-impact on different regions of Axin, which likely serves to ensure a robust inhibition of this protein and transduction of the Wnt signal.

Identification of a small GTPase inhibitor using a high-throughput flow cytometry bead-based multiplex assay

Small GTPases are key regulators of cellular activity and represent novel targets for the treatment of human diseases using small-molecule inhibitors. The authors describe a multiplex, flow cytometry bead-based assay for the identification and characterization of inhibitors or activators of small GTPases. Six different glutathione-S-transferase (GST)-tagged small GTPases were bound to glutathione beads, each labeled with a different red fluorescence intensity. Subsequently, beads bearing different GTPase were mixed and dispensed into 384-well plates with test compounds, and fluorescent-guanosine triphosphate (GTP) binding was used as the readout. This novel multiplex assay allowed the authors to screen a library of almost 200,000 compounds and identify more than 1200 positive compounds, which were further verified by dose-response analyses, using 6- to 8-plex assays. After the elimination of false-positive and false-negative compounds, several small-molecule families with opposing effects on GTP binding activity were identified. The authors detail the characterization of MLS000532223, a general inhibitor that prevents GTP binding to several GTPases in a dose-dependent manner and is active in biochemical and cell-based secondary assays. Live-cell imaging and confocal microscopy studies revealed the inhibitor-induced actin reorganization and cell morphology changes, characteristic of Rho GTPases inhibition. Thus, high-throughput screening via flow cytometry provides a strategy for identifying novel compounds that are active against small GTPases.

Europium-labeled GTP as a general nonradioactive substitute for [(35)S]GTPgammaS in high-throughput G protein studies

[(35)S]GTPgammaS, the nonhydrolyzable radioactive GTP analog, has been a powerful tool in G protein studies and has set the standards in this field of research. However, its radioactive nature imposes clear limitations to its use in regular laboratory practice and in high-throughput experimentation. The europium-labeled GTP analog (Eu-GTP) has been used as an alternative in the analysis of G protein activation by G protein-coupled receptors in cellular membrane preparations. Here we expand the usage of Eu-GTP and show that it can be applied in other types of assays where [(35)S]GTPgammaS has been previously utilized. We demonstrate the applicability of the modified Eu-GTP binding technology to analysis of heterotrimeric and monomeric G proteins of natural and recombinant sources, from different organisms, in assays with soluble proteins and membrane-containing assays of a high-throughput format. The deci-nanomolar K(D) of Eu-GTP for the tested G proteins is similar to that of other fluorescent-modified GTP analogs, while the sensitivity achieved in time-resolved fluorescence analysis of Eu-GTP exceeds that of the radioactive measurements. Overall, the results of our modified Eu-GTP binding assay present Eu-GTP as a general nonradioactive alternative for G protein studies, especially attractive in high-throughput experiments.

Analysis of flavivirus NS5 methyltransferase cap binding

The flavivirus 2'-O-nucleoside N-terminal RNA methyltransferase (MTase) enzyme is responsible for methylating the viral RNA cap structure. To increase our understanding of the mechanism of viral RNA cap binding we performed a detailed structural and biochemical characterization of the guanosine cap-binding pocket of the dengue (DEN) and yellow fever (YF) virus MTase enzymes. We solved an improved 2.1 A resolution crystal structure of DEN2 Mtase, new 1.5 A resolution crystal structures of the YF virus MTase domain in apo form, and a new 1.45 A structure in complex with guanosine triphosphate and RNA cap analog. Our structures clarify the previously reported DEN MTase structure, suggest novel protein-cap interactions, and provide a detailed view of guanine specificity. Furthermore, the structures of the DEN and YF proteins are essentially identical, indicating a large degree of structural conservation amongst the flavivirus MTases. Guanosine triphosphate analog competition assays and mutagenesis analysis, performed to analyze the biochemical characteristics of cap binding, determined that the major interaction points are (i) guanine ring via pi-pi stacking with Phe24, N1 hydrogen interaction with the Leu19 backbone carbonyl via a water bridge, and C2 amine interaction with Leu16 and Leu19 backbone carbonyls; (ii) ribose 2' hydroxyl interaction with Lys13 and Asn17; and (iii) alpha-phosphate interactions with Lys28 and Ser215. Based on our mutational and analog studies, the guanine ring and alpha-phosphate interactions provide most of the energy for cap binding, while the combination of the water bridge between the guanine N1 and Leu19 carbonyl and the hydrogen bonds between the C2 amine and Leu16/Leu19 carbonyl groups provide for specific guanine recognition. A detailed model of how the flavivirus MTase protein binds RNA cap structures is presented.

Novel peptide ligands of RGS4 from a focused one-bead, one-compound library

Regulators of G protein signaling accelerate GTP hydrolysis by G alpha subunits and profoundly inhibit signaling by G protein-coupled receptors. The distinct expression patterns and pathophysiologic regulation of regulators of G protein signaling proteins suggest that inhibitors may have therapeutic potential. We previously reported the design, mechanistic evaluation, and structure-activity relationships of a disulfide-containing cyclic peptide inhibitor of RGS4, YJ34 (Ac-Val-Lys-c[Cys-Thr-Gly-Ile-Cys]-Glu-NH(2), S-S) (Roof et al., Chem Biol Drug Des, 67, 2006, 266). Using a focused one-bead, one-compound peptide library that contains features known to be necessary for the activity of YJ34, we now identify peptides that bind to RGS4. Six peptides showed confirmed binding to RGS4 by flow cytometry. Two analogs of peptide 2 (Gly-Thr-c[Cys-Phe-Gly-Thr-Cys]-Trp-NH(2), S-S with a free or acetylated N-terminus) inhibited RGS4-stimulated G alpha(o) GTPase activity at 25-50 microM. They selectively inhibit RGS4 but not RGS7, RGS16, and RGS19. Their inhibition of RGS4 does not depend on cysteine-modification of RGS4, as they do not lose activity when all cysteines are removed from RGS4. Peptide 2 has been modeled to fit in the same binding pocket predicted for YJ34 but in the reverse orientation.

Microfabricated channel array electrophoresis for characterization and screening of enzymes using RGS-G protein interactions as a model system

A microfluidic chip consisting of parallel channels designed for rapid electrophoretic enzyme assays was developed. Radial arrangement of channels and a common waste channel allowed chips with 16 and 36 electrophoresis units to be fabricated on a 7.62 x 7.62 cm(2) glass substrate. Fluorescence detection was achieved using a Xe arc lamp source and commercial charge-coupled device (CCD) camera to image migrating analyte zones in individual channels. Chip performance was evaluated by performing electrophoretic assays for G protein GTPase activity on chip using BODIPY-GTP as enzyme substrate. A 16-channel design proved to be useful in extracting kinetic information by allowing serial electrophoretic assays from 16 different enzyme reaction mixtures at 20 s intervals in parallel. This system was used to rapidly determine enzyme concentrations, optimal enzymatic reaction conditions, and Michaelis-Menten constants. A chip with 36 channels was used for screening for modulators of the G protein-RGS protein interaction by assaying the amount of product formed in enzyme reaction mixtures that contained test compounds. Thirty-six electrophoretic assays were performed in 30 s suggesting the potential throughput up to 4320 assays/h with appropriate sample handling procedures. Both designs showed excellent reproducibility of peak migration time and peak area. Relative standard deviations of normalized peak area of enzymatic product BODIPY-GDP were 5% and 11%, respectively, in the 16- and 36-channel designs.

Refolding of G protein alpha subunits from inclusion bodies expressed in Escherichia coli

Heterotrimeric G proteins relay signals from G protein-coupled receptors (GPCRs) to the interior of the cell. The signaling cascades induced by G protein activation control a wide range of cellular processes. The alpha subunit is believed to determine which G protein couples to each GPCR, and is the primary determinant of the type of signal transmitted. Several members of the G(alpha) family have been expressed in active form in Escherichia coli. However, production levels of these proteins are limited: in most cases only approximately 10% of total G(alpha) protein expressed is active; the rest accumulates in inclusion bodies. Although G(ialpha) has been readily expressed in soluble form (to 10 mg/L), other alpha subunits are minimally soluble, and many are exclusively expressed to inclusion bodies. Previous efforts to solubilize and refold G(alpha) from inclusion bodies have not been successful. Here we did a thorough study of the characteristics of G(alpha) subunits (human G(ialpha(1)), human G(salpha(short)), human G(11alpha) and human G(talpha(cone))), solubilized and purified from inclusion bodies. We find that we can obtain soluble protein both by on-column and rapid-dilution techniques. Comparison to native, soluble G(ialpha) expressed from E. coli showed that although the refolded G(alpha) subunits were soluble and retained partial alpha-helicity characteristic of the native, folded G(alpha) subunit, they did not bind GDP or GTP as effectively as native protein. We conclude that the refolded G(ialpha) protein has a native-like secondary structure, but is predominantly in a molten globular state.

Antibodies that identify only the active conformation of G(i) family G protein alpha subunits

Production of antisera able to recognize individual heterotrimeric G protein alpha subunits resulted in rapid expansion of information on their distribution and function. However, no antibodies that specifically recognize the active state have been available. Four-way primary screening of 763 hybridomas generated from mice immunized with guanosine 5'-O-(3-thio)triphosphate-loaded G alpha(i1) and isolated using an automated robotic colony picker identified three antibodies that interacted with the constitutively active, Q(204)L, mutant but neither the constitutively inactive, G(203)A, mutant nor wild-type G alpha(i1). This profile extended to other closely related G(i) family G proteins but not to the less closely related G alpha(s) and G alpha(q)/G alpha(11) families. Each antibody was, however, also able to identify wild-type, GDP-bound G(i) family G proteins in the presence of fluoroaluminate, which mimics the presence of the terminal phosphate of GTP and hence generates an active/transition state conformation. Stimulation of cells coexpressing a wild-type G alpha(i) subunit and the dopamine D2 receptor with the agonist ligand nor-apomorphine also allowed these conformationally selective antibodies to bind the G protein. Such reagents allow the specific identification of activated G proteins in a native environment and may allow the development of label-free screening assays for G protein-coupled receptor-mediated activation of G(i) family G proteins.

Detection of adenylyl cyclase activity using a fluorescent ATP substrate and capillary electrophoresis

A capillary electrophoresis laser-induced fluorescence (CE-LIF) assay was developed for detection of adenylyl cyclase (AC) activity using BODIPY FL ATP (BATP) as substrate. In the assay, BATP was incubated with AC and the resulting mixture of BATP and enzyme product (BODIPY cyclic AMP, BcAMP) separated in 5 min by CE-LIF. Substrate depletion and product accumulation were simultaneously monitored during the course of the reaction. The rate of product formation depended upon the presence of AC activators forskolin or Galpha(s)-GTPgammaS as evidenced by a more rapid BATP turnover to BcAMP compared to basal levels. The CE-LIF assay detected EC50 values for forskolin and Galpha(s)-GTPgammaS of 27 +/- 6 microM and 317 +/- 56 nM, respectively. These EC50 values compared well to those previously reported using [alpha-32P]ATP as substrate. When AC was concurrently activated with 2.5 microM forskolin and 25 nM Galpha(s)-GTPgammaS, the amount of BcAMP formed was 3.4 times higher than the additive amounts of each activator alone indicating a positively cooperative activation by these compounds in agreement with previous assays using radiolabeled substrate. Inhibition of AC activity was also demonstrated using the AC inhibitor 2'-(or-3')-O-(N-methylanthraniloyl) guanosine 5'-triphosphate with an IC50 of 9 +/- 6 nM. The use of a fluorescent substrate combined with CE separation has enabled development of a rapid and robust method for detection of AC activity that is an attractive alternative to the AC assay using radioactive nucleotide and column chromatography. In addition, the assay has potential for high-throughput screening of drugs that act at AC.

Capillary electrophoresis assay for G protein-coupled receptor-mediated GTPase activity

We describe a capillary electrophoresis (CE) assay to detect G protein-coupled receptor (GPCR)-stimulated G protein GTPase activity in cell membranes expressing alpha2A adrenoreceptor-Galphao1 wild-type (wt) or C351I mutant fusion proteins using a fluorescent, hydrolyzable GTP analogue. As no change in total fluorescence is observed by conversion of substrate to product, CE is used to separate the fluorescent substrate (*GTP) from the fluorescent product (*GDP). Using the assay, the alpha2a adrenoceptor agonist UK14,304 was shown to simulate specific production of *GDP in membranes from HEK293T cells expressing receptor-G protein fusion to 525% of basal levels with an EC50 of 0.48 +/- 0.20 microM. The EC50 increased to 9.4 +/- 5 muM with addition of the antagonist yohimbine. Nucleotide hydrolysis was increased further over agonist-stimulated levels with addition of the in vivo modulator protein RGS (regulator of G protein signaling). It is envisioned that this technique could be used for screening for novel GPCR ligands or other G protein signaling modifiers.

Mechanism of action and structural requirements of constrained peptide inhibitors of RGS proteins

Regulators of G-protein signaling (RGS) accelerate guanine triphosphate hydrolysis by Galpha-subunits and profoundly inhibit signaling by G protein-coupled receptors. The distinct expression patterns and pathophysiologic regulation of RGS proteins suggest that inhibitors may have therapeutic potential. We previously reported the design of a constrained peptide inhibitor of RGS4 (1: Ac-Val-Lys-[Cys-Thr-Gly-Ile-Cys]-Glu-NH2, S-S) based on the structure of the Galphai switch 1 region but its mechanism of action was not established. In the present study, we show that 1 inhibits RGS4 by mimicking and competing for binding with the switch 1 region of Galphai and that peptide 1 shows selectivity for RGS4 and RGS8 versus RGS7. Structure-activity relationships of analogs related to 1 are described that illustrate key features for RGS inhibition. Finally, we demonstrate activity of the methylene dithioether-bridged peptide inhibitor, 2, to modulate muscarinic receptor-regulated potassium currents in atrial myocytes. These data support the proposed mechanism of action of peptide RGS inhibitors, demonstrate their action in native cells, and provide a starting point for the design of RGS inhibitor drugs.

Determination of enzymatic activity by capillary electrophoresis

Enzymes are biological catalysts that play an important role in biochemical reactions necessary for normal growth, maturation and reproduction through whole live world. Their accurate quantitation in biological samples is important in many fields of biochemistry, not only in routine biochemistry and in fundamental research, but also in clinical and pharmacological research and diagnosis. Since the direct measurement of enzymes by masses is impossible, they must be quantified by their catalytic activities. Many different methods have been applied for this purpose so far. Although photometric methods are undoubtedly the most frequently used, separation methods will further gain their position in this field. The article reviews different possibilities for the assay of enzymatic activity by means of capillary electrophoresis (CE). Both the off-line and on-line enzyme assays based on CE are discussed.

How sensitive is a nose?

Odor sensitivity may not be due to odor-receptor (OR) binding but rather may be due to emergent properties of transduction pathways and the anatomical convergence of olfactory neurons. A recent study suggests that odor-OR interactions are brief and infrequently activate heterotrimeric GTP-binding proteins (G proteins); in contrast, visual receptors have long-lasting activation states and activate many G proteins. These differences may reflect strategies that evolved to accommodate very different signals, and the mechanisms described might be applicable for receptors across phyla. However, whereas visual receptors (rhodopsin) appeared before protostome-deuterostome separation, ORs may be independently derived in different phyla. Alternatively, phylum-distinct ORs may share common ancestry but be influenced by diversifying selection. Phylum-distinct ORs may imply phylum-specific OR mechanisms, whereas common ancestry may imply common mechanisms. Nonetheless, most animals detect a similar repertoire of olfactory signals, and OR mechanisms may be convergent on those signals independent of receptor relatedness. Thus, recent insights into the molecular characteristics of odor perception in frogs may well be relevant to such processes as how mosquitoes detect host odors for a malaria-transmitting blood meal.

A direct fluorescence-based assay for RGS domain GTPase accelerating activity

Diverse extracellular signals regulate seven transmembrane-spanning receptors to modulate cellular physiology. These receptors signal primarily through activation of heterotrimeric guanine nucleotide binding proteins (G proteins). A major determinant of heterotrimeric G protein signaling in vivo and in vitro is the intrinsic GTPase activity of the Galpha subunit. RGS (regulator of G protein signaling) domain-containing proteins are GTPase accelerating proteins specific for Galpha subunits. In this article, we describe the use of the ribose-conjugated fluorescent guanine nucleotide analog BODIPYFL-GTP as a spectroscopic probe to measure intrinsic and RGS protein-catalyzed nucleotide hydrolysis by Galphao. BODIPYFL-GTP bound to Galphao exhibits a 200% increase in fluorescence quantum yield. Hydrolysis of BODIPYFL-GTP to BODIPYFL-GDP reduces the quantum yield to 27% above its unbound value. We demonstrate that BODIPYFL-GTP can be used as a rapid real-time probe for measuring RGS domain-catalyzed GTP hydrolysis by Galphao. We demonstrate the effectiveness of this assay in the analysis of loss-of-function point mutants of both Galphao and RGS12. This assay should be useful in screening for and analyzing RGS protein inhibitory compounds.