Aluminum fluoride associates with the small guanine nucleotide binding proteins

Dissecting the mechanism of atlastin-mediated homotypic membrane fusion at the single-molecule level

Homotypic membrane fusion of the endoplasmic reticulum (ER) is mediated by dynamin-like GTPase atlastin (ATL). This fundamental process relies on GTP-dependent domain rearrangements in the N-terminal region of ATL (ATLcyto), including the GTPase domain and three-helix bundle (3HB). However, its conformational dynamics during the GTPase cycle remain elusive. Here, we combine single-molecule FRET imaging and molecular dynamics simulations to address this conundrum. Different from the prevailing model, ATLcyto can form a loose crossover dimer upon GTP binding, which is tightened by GTP hydrolysis for membrane fusion. Furthermore, the α-helical motif between the 3HB and transmembrane domain, which is embedded in the surface of the lipid bilayer and self-associates in the crossover dimer, is required for ATL function. To recycle the proteins, Pi release, which disassembles the dimer, activates frequent relative movements between the GTPase domain and 3HB, and subsequent GDP dissociation alters the conformational preference of the ATLcyto monomer for entering the next reaction cycle. Finally, we found that two disease-causing mutations affect human ATL1 activity by destabilizing GTP binding-induced loose crossover dimer formation and the membrane-embedded helix, respectively. These results provide insights into ATL-mediated homotypic membrane fusion and the pathological mechanisms of related disease.

The RHO Family GTPases: Mechanisms of Regulation and Signaling

Much progress has been made toward deciphering RHO GTPase functions, and many studies have convincingly demonstrated that altered signal transduction through RHO GTPases is a recurring theme in the progression of human malignancies. It seems that 20 canonical RHO GTPases are likely regulated by three GDIs, 85 GEFs, and 66 GAPs, and eventually interact with >70 downstream effectors. A recurring theme is the challenge in understanding the molecular determinants of the specificity of these four classes of interacting proteins that, irrespective of their functions, bind to common sites on the surface of RHO GTPases. Identified and structurally verified hotspots as functional determinants specific to RHO GTPase regulation by GDIs, GEFs, and GAPs as well as signaling through effectors are presented, and challenges and future perspectives are discussed.

Discernment between candidate mechanisms for KRAS G13D colorectal cancer sensitivity to EGFR inhibitors

Phase three clinical trial evidence suggests that colorectal cancers with the KRAS G13D mutation may benefit from EGFR inhibitors, like cetuximab, in contrast to the other most common KRAS mutations. A mechanism to explain why this mutation behaves differently from other KRAS mutations had long been lacking. Two recent studies have reproduced KRAS G13D specific sensitivity to cetuximab in cellular models, and both have implicated the tumor suppressor NF1 as a critical variable in determining sensitivity and resistance. One study proposes a mechanism that focuses on the inhibition of active, GTP-bound wild-type RAS, which is proposed to occur to a greater extent in KRAS G13D tumors due to the inability of KRAS G13D to bind NF1 well. The other study suggests NF1 can convert GTP-bound KRAS G13D to inactive, GDP-bound KRAS G13D. Here, we report an inability to reproduce cellular and biophysical studies that suggested NF1 has strong GTPase activity on KRAS G13D. We also report additional data that further suggests only WT RAS-GTP levels are reduced with EGFR inhibition and that KRAS G13D is impaired in binding to NF1. These new experiments further support a mechanism in which cetuximab inhibits wild-type (HRAS and NRAS) signals in KRAS G13D colorectal cancers. Video Abstract.

Biochemical and structural characterization of murine GBP7, a guanylate binding protein with an elongated C-terminal tail

Guanylate-binding proteins (GBPs) constitute a family of interferon-inducible guanosine triphosphatases (GTPases) that are key players in host defense against intracellular pathogens ranging from protozoa to bacteria and viruses. So far, human GBP1 and GBP5 as well as murine GBP2 (mGBP2) have been biochemically characterized in detail. Here, with murine GBP7 (mGBP7), a GBP family member with an unconventional and elongated C-terminus is analyzed. The present study demonstrates that mGBP7 exhibits a concentration-dependent GTPase activity and an apparent GTP turnover number of 20 min-1. In addition, fluorescence spectroscopy analyses reveal that mGBP7 binds GTP with high affinity (KD = 0.22 µM) and GTPase activity assays indicate that mGBP7 hydrolyzes GTP to GDP and GMP. The mGBP7 GTPase activity is inhibited by incubation with γ-phosphate analogs and a K51A mutation interfering with GTP binding. SEC-MALS analyses give evidence that mGBP7 forms transient dimers and that this oligomerization pattern is not influenced by the presence of nucleotides. Moreover, a structural model for mGBP7 is provided by homology modeling, which shows that the GTPase possesses an elongated C-terminal (CT) tail compared with the CaaX motif-containing mGBP2 and human GBP1. Molecular dynamics simulations indicate that this tail has transmembrane characteristics and, interestingly, confocal microscopy analyses reveal that the CT tail is required for recruitment of mGBP7 to the parasitophorous vacuole of Toxoplasma gondii.

Ras-Specific GTPase-Activating Proteins-Structures, Mechanisms, and Interactions

Ras-specific GTPase-activating proteins (RasGAPs) down-regulate the biological activity of Ras proteins by accelerating their intrinsic rate of GTP hydrolysis, basically by a transition state stabilizing mechanism. Oncogenic Ras is commonly not sensitive to RasGAPs caused by interference of mutants with the electronic or steric requirements of the transition state, resulting in up-regulation of activated Ras in respective cells. RasGAPs are modular proteins containing a helical catalytic RasGAP module surrounded by smaller domains that are frequently involved in the subcellular localization or contributing to regulatory features of their host proteins. In this review, we summarize current knowledge about RasGAP structure, mechanism, regulation, and dual-substrate specificity and discuss in some detail neurofibromin, one of the most important negative Ras regulators in cellular growth control and neuronal function.

Intermediate step of cohesin's ATPase cycle allows cohesin to entrap DNA

Cohesin is a four-subunit ATPase in the family of structural maintenance of chromosomes (SMC). Cohesin promotes sister chromatid cohesion, chromosome condensation, DNA repair, and transcription regulation. Cohesin performs these functions as a DNA tether and potentially a DNA-based motor. At least one of its DNA binding activities involves entrapment of DNA within a lumen formed by its subunits. This activity can be reconstituted in vitro by incubating cohesin with DNA, ATP, and cohesin loader. Previously we showed that a mutant form of cohesin (DE-cohesin) possesses the ability to bind and tether DNA in vivo. Using in vitro reconstitution assays, we show that DE-cohesin can form stable complexes with DNA without ATP hydrolysis. We show that wild-type cohesin with ADP aluminum fluoride (cohesinADP/AlFx) can also form stable cohesin-DNA complexes. These results suggest that an intermediate nucleotide state of cohesin, likely cohesinADP-Pi, is capable of initially dissociating one interface between cohesin subunits to allow DNA entry into a cohesin lumen and subsequently interacting with the bound DNA to stabilize DNA entrapment. We also show that cohesinADP/AlFx binding to DNA is enhanced by cohesin loader, suggesting a function for loader other than stimulating the ATPase. Finally, we show that loader remains stably bound to cohesinADP/AlFx after DNA entrapment, potentially revealing a function for loader in tethering the second DNA substrate. These results provide important clues on how SMC complexes like cohesin can function as both DNA tethers and motors.

Deciphering the Molecular and Functional Basis of RHOGAP Family Proteins: A SYSTEMATIC APPROACH TOWARD SELECTIVE INACTIVATION OF RHO FAMILY PROTEINS

RHO GTPase-activating proteins (RHOGAPs) are one of the major classes of regulators of the RHO-related protein family that are crucial in many cellular processes, motility, contractility, growth, differentiation, and development. Using database searches, we extracted 66 distinct human RHOGAPs, from which 57 have a common catalytic domain capable of terminating RHO protein signaling by stimulating the slow intrinsic GTP hydrolysis (GTPase) reaction. The specificity of the majority of the members of RHOGAP family is largely uncharacterized. Here, we comprehensively investigated the sequence-structure-function relationship between RHOGAPs and RHO proteins by combining our in vitro data with in silico data. The activity of 14 representatives of the RHOGAP family toward 12 RHO family proteins was determined in real time. We identified and structurally verified hot spots in the interface between RHOGAPs and RHO proteins as critical determinants for binding and catalysis. We have found that the RHOGAP domain itself is nonselective and in some cases rather inefficient under cell-free conditions. Thus, we propose that other domains of RHOGAPs confer substrate specificity and fine-tune their catalytic efficiency in cells.

Fluoride Induces a Volume Reduction in CA1 Hippocampal Slices Via MAP Kinase Pathway Through Volume Regulated Anion Channels

Regulation of cell volume is an important aspect of cellular homeostasis during neural activity. This volume regulation is thought to be mediated by activation of specific transporters, aquaporin, and volume regulated anion channels (VRAC). In cultured astrocytes, it was reported that swelling-induced mitogen-activated protein (MAP) kinase activation is required to open VRAC, which are thought to be important in regulatory volume decrease and in the response of CNS to trauma and excitotoxicity. It has been also described that sodium fluoride (NaF), a recognized G-protein activator and protein phosphatase inhibitor, leads to a significant MAP kinase activation in endothelial cells. However, NaF's effect in volume regulation in the brain is not known yet. Here, we investigated the mechanism of NaF-induced volume change in rat and mouse hippocampal slices using intrinsic optical signal (IOS) recording, in which we measured relative changes in intracellular and extracellular volume as changes in light transmittance through brain slices. We found that NaF (1~5 mM) application induced a reduction in light transmittance (decreased volume) in CA1 hippocampus, which was completely reversed by MAP kinase inhibitor U0126 (10 µM). We also observed that NaF-induced volume reduction was blocked by anion channel blockers, suggesting that NaF-induced volume reduction could be mediated by VRAC. Overall, our results propose a novel molecular mechanism of NaF-induced volume reduction via MAP kinase signaling pathway by activation of VRAC.

Site-specific monoubiquitination activates Ras by impeding GTPase-activating protein function

Cell growth and differentiation are controlled by growth factor receptors coupled to the GTPase Ras. Oncogenic mutations disrupt GTPase activity leading to persistent Ras signaling and cancer progression. Recent evidence indicates that monoubiquitination of Ras leads to Ras activation. Mutation of the primary site of monoubiquitination impairs the ability of activated K–Ras to promote tumor growth. To determine the mechanism of human Ras activation we chemically ubiquitinated the protein and analyzed its function by NMR, computational modeling, and biochemical activity measurements. We established that monoubiquitination has little effect on Ras GTP binding, GTP hydrolysis, or exchange factor activation, but severely abrogates the response to GTPase activating proteins in a site–specific manner. These findings reveal a new mechanism by which Ras can trigger persistent signaling in the absence of receptor activation or an oncogenic mutation.

ER-resident Gi2 protein controls sar1 translocation onto the ER during budding of transport vesicles

In our previous study, fluoride ([AlF(4) ](-) ) disturbed ER-to-Golgi transport through the activation of ER-resident heterotrimeric G protein (ER-G protein). Therefore, ER-G protein may be implicated in ER-to-Golgi transport at the early stage prior to coat protein assembly. Sar1 translocation onto the endoplasmic reticulum (ER) membrane is suppressed by non-selective protein kinase inhibitor H89, suggesting the participation of H89-sensitive kinase in this process. To investigate the involvement of ER-G protein in ER-to-Golgi transport, the effect of G(i) protein activator (mastoparan 7) was examined on Sar1 translocation onto the ER in a cell-free system consisting of microsome membrane and cytosol. Sar1 translocation onto the microsome membrane was induced by addition of GTPγS in the cell-free system. Translocation of Sar1 by GTPγS was suppressed significantly by both H89 and mastoparan 7. Mastoparan 7 suppressed the translocation of Sar1 onto the microsome membrane with dosage dependency, but mastoparan 17, the inactive analog of mastoparan 7, had no effect on Sar1 translocation. The suppressive effect of mastoparan 7 was recovered by treatment with pertussis toxin (IAP). Moreover, G(i2) protein was detected on the microsome membrane by western blotting for heterotrimeric G(i) proteins. These results indicate that ER-G(i2) protein modulated Sar1 translocation onto the ER, suggesting that ER-resident G(i2) protein is an important negative regulator of vesicular transport at the early stage of vesicle formation before coat protein assembly on the ER.

Fluoride complexes of oncogenic Ras mutants to study the Ras-RasGap interaction

Down-regulation of Ras signalling is mediated by specific GTPase-activating proteins (GAPs), which stimulate the very slow GTPase reaction of Ras by 10(5)-fold. The basic features of the GAP activity involve the stabilisation of both switch regions of Ras in the transition state, and the insertion of an arginine finger. In the case of oncogenic Ras mutations, the features of the active site are disturbed. To understand these features in more detail, we have investigated the effects of oncogenic mutations of Ras and compared the GAP-stimulated GTPase reaction with the ability to form GAP-mediated aluminium or beryllium fluoride complexes. In general we find a correlation between the size of the amino acid at position 12, the GTPase activity and ability to form aluminium fluoride complexes. While Gly12 is very sensitive to even the smallest possible structural change, Gly13 is much less sensitive to steric hindrance, but is sensitive to charge. Oncogenic mutants of Ras defective in the GTPase activity can however form ground-state GppNHp complexes with GAP, which can be mimicked by beryllium fluoride binding. We show that beryllium fluoride complexes are less sensitive to structural changes and report on a state close to but different from the ground state of the GAP-stimulated GTPase reaction.

Dissecting the thermodynamics of GAP-RhoA interactions

We describe a detailed study of the RhoA-binding epitope of the GAP domain of Graf, including the determination of the thermodynamic and kinetic parameters of the interaction of wild-type domain, and of its 15 single-site mutants, with cognate GTPases. We show that residues important for the structural integrity of the Arg-finger loop are critical for binding Rho and for the catalytic activity of GAP, but GTPase selectivity appears to be modulated by a much more subtle interplay of electrostatic and hydrophobic interactions involving residues on the periphery of the main interface. The eight residues targeted in this study are involved in three distinct patches on the surface, two of which appear to interact with highly conserved regions of the GTPase, while the third plays a role in GTPase selectivity.

On the significance of Toc-GTPase homodimers

Precursor protein translocation across the outer chloroplast membrane depends on the action of the Toc complex, containing GTPases as recognizing receptor components. The G domains of the GTPases are known to dimerize. In the dimeric conformation an arginine contacts the phosphate moieties of bound nucleotide in trans. Kinetic studies suggested that the arginine in itself does not act as an arginine finger of a reciprocal GTPase-activating protein (GAP). Here we investigate the specific function of the residue in two GTPase homologues. Arginine to alanine replacement variants have significantly reduced affinities for dimerization compared with wild-type GTPases. The amino acid exchange does not impact on the overall fold and nucleotide binding, as seen in the monomeric x-ray crystallographic structure of the Arabidopsis Toc33 arginine-alanine replacement variant at 2.0A. We probed the catalytic center with the transition state analogue GDP/AlF(x) using NMR and analytical ultracentrifugation. AlF(x) binding depends on the arginine, suggesting the residue can play a role in catalysis despite the non-GAP nature of the homodimer. Two non-exclusive functional models are discussed: 1) the coGAP hypothesis, in which an additional factor activates the GTPase in homodimeric form; and 2) the switch hypothesis, in which a protein, presumably the large Toc159 GTPase, exchanges with one of the homodimeric subunits, leading to activation.

The function of two closely related Rho proteins is determined by an atypical switch I region

We show here that the encoded proteins of the two duplicated RHO1 genes from the filamentous fungus Ashbya gossypii, AgRHO1a and AgRHO1b have functionally diverged by unusual mutation of the conserved switch I region. Interaction studies and in vitro assays suggest that a different regulation by the two GTPase activating proteins (GAPs) AgLrg1 and AgSac7 contributes to the functional differences. GAP-specificity and protein function is determined to a large part by a single position in the switch I region of the two Rho1 proteins. In AgRho1b, this residue is a tyrosine that is conserved among the Rho-protein family, whereas AgRho1a carries an atypical histidine at the same position. Mutation of this histidine to a tyrosine changes GAP-specificity, protein function and localization of AgRho1a. Furthermore, it enables the mutated allele to complement the lethality of an AgRHO1b deletion. In summary, our findings show that a simple mutation in the switch I region of a GTP-binding protein can change its affinity towards its GAPs, which finally leads to a decoupling of very similar protein function without impairing effector interaction.

Insight into catalysis of a unique GTPase reaction by a combined biochemical and FTIR approach

Rap1 and Rap2 are the only small guanine nucleotide-binding proteins of the Ras superfamily that do not use glutamine for GTP hydrolysis. Moreover, Rap1GAP, which stimulates the GTPase reaction of Rap1 10(5)-fold, does not have the classical "arginine finger" like RasGAP but presumably, introduces an asparagine residue into the active site. Here, we address the requirements of this unique reaction in detail by combining various biochemical methods, such as fluorescence spectroscopy, stopped-flow and time-resolved Fourier transform infrared spectroscopy (FTIR). The fluorescence spectroscopic assay monitors primarily protein-protein interaction steps, while FTIR resolves simultaneously the elementary steps of functional groups labor-free, but it is less sensitive and needs higher concentrations. Combining both methods allows us to distinguish weather mechanistic defects caused by mutation are due to affinity or due to functionality. We show that several mutations of Asn290 block catalysis. Some of the mutants, however, still form a complex with Rap1*GDP in the presence of BeF(x) but not AlF(x), supporting the notion that fluoride complexes are indicators of the ground versus transition state. Mutational analysis also shows that Thr61 is not required for catalysis. While replacement of Thr61 of Rap1 by Leu eliminates GTPase activation by Rap1GAP, the T61A and T61Q mutants have only a minor effect on catalysis, but change the relative rates of cleavage and (P(i)(-)) release. While Rap1GAP(N290A) is completely inactive on wild-type Rap1, it can act on Rap1(T61Q), arguing that Asn290 in trans has a role in catalysis similar to that of the intrinsic Gln in Ras and Rho. Finally, since FTIR works at high, and thus mostly saturating, concentrations, it can clearly separate effects on affinity from purely catalytic modifications, showing that Arg388, conserved between RapGAPs and mutated in the homologous RheBGAP Tuberin, affects binding affinity severely but has no effect on the cleavage reaction itself.

Structure of a GDP:AlF4 complex of the SRP GTPases Ffh and FtsY, and identification of a peripheral nucleotide interaction site

The signal recognition particle (SRP) GTPases Ffh and FtsY play a central role in co-translational targeting of proteins, assembling in a GTP-dependent manner to generate the SRP targeting complex at the membrane. A suite of residues in FtsY have been identified that are essential for the hydrolysis of GTP that accompanies disengagement. We have argued previously on structural grounds that this region mediates interactions that serve to activate the complex for disengagement and term it the activation region. We report here the structure of a complex of the SRP GTPases formed in the presence of GDP:AlF4. This complex accommodates the putative transition-state analog without undergoing significant change from the structure of the ground-state complex formed in the presence of the GTP analog GMPPCP. However, small shifts that do occur within the shared catalytic chamber may be functionally important. Remarkably, an external nucleotide interaction site was identified at the activation region, revealed by an unexpected contaminating GMP molecule bound adjacent to the catalytic chamber. This site exhibits conserved sequence and structural features that suggest a direct interaction with RNA plays a role in regulating the activity of the SRP targeting complex.

Monitoring the real-time kinetics of the hydrolysis reaction of guanine nucleotide-binding proteins

The conversion of guanosine triphosphate (GTP) to guanosine diphosphate (GDP) and inorganic phosphate (Pi) by guanine nucleotide-binding proteins (GNBPs) is a fundamental enzyme reaction in living cells that acts as an important timer in a variety of biological processes. This reaction is intrinsically slow but can be stimulated by GTPase-activating proteins (GAPs) by several orders of magnitude. In the present study, we synthesized and characterized a new fluorescent nucleotide, 2'(3')-O-(N-ethylcarbamoyl-(5''-carboxytetramethylrhodamine) amide)-GTP, or tamraGTP, which is sensitive towards conformational changes of certain GNBPs induced by GTP hydrolysis. Unlike other fluorescent nucleotides, tamra-GTP allows real-time monitoring of the kinetics of the intrinsic and GAP-catalyzed GTP hydrolysis reactions of small GNBPs from the Rho family.

G alpha12 interaction with alphaSNAP induces VE-cadherin localization at endothelial junctions and regulates barrier function

The involvement of heterotrimeric G proteins in the regulation of adherens junction function is unclear. We identified alphaSNAP as an interactive partner of G alpha12 using yeast two-hybrid screening. Glutathione S-transferase pull-down assays showed the selective interaction of alphaSNAP with G alpha12 in COS-7 as well as in human umbilical vein endothelial cells. Using domain swapping experiments, we demonstrated that the N-terminal region of G alpha12 (1-37 amino acids) was necessary and sufficient for its interaction with alphaSNAP. G alpha13 with its N-terminal extension replaced by that of G alpha12 acquired the ability to bind to alphaSNAP, whereas G alpha12 with its N terminus replaced by that of G alpha13 lost this ability. Using four point mutants of alphaSNAP, which alter its ability to bind to the SNARE complex, we determined that the convex rather than the concave surface of alphaSNAP was involved in its interaction with G alpha12. Co-transfection of human umbilical vein endothelial cells with G alpha12 and alphaSNAP stabilized VE-cadherin at the plasma membrane, whereas down-regulation of alphaSNAP with siRNA resulted in the loss of VE-cadherin from the cell surface and, when used in conjunction with G alpha12 overexpression, decreased endothelial barrier function. Our results demonstrate a direct link between the alpha subunit of G12 and alphaSNAP, an essential component of the membrane fusion machinery, and implicate a role for this interaction in regulating the membrane localization of VE-cadherin and endothelial barrier function.

Golgi targeting of human guanylate-binding protein-1 requires nucleotide binding, isoprenylation, and an IFN-gamma-inducible cofactor

Human guanylate-binding protein-1 (hGBP-1) is a large GTPase, similar in structure to the dynamins. Like many smaller GTPases of the Ras/Rab family, it is farnesylated, suggesting it may dock into membranes and perhaps play a role in intracellular trafficking. To date, however, hGBP-1 has never been associated with a specific intracellular compartment. Here we present evidence that hGBP-1 can associate with the Golgi apparatus. Redistribution from the cytosol to the Golgi was observed by immunofluorescence and subcellular fractionation after aluminum fluoride treatment, suggesting that it occurs when hGBP-1 is in its GTP-bound state. Relocalization was blocked by a farnesyl transferase inhibitor. The C589S mutant of hGBP-1, which cannot be farnesylated, and the previously uncharacterized R48P mutant, which cannot bind GTP, both failed to localize to the Golgi. These two mutants had a dominant-negative effect, preventing endogenous wild-type hGBP-1 from efficiently redistributing after aluminum fluoride treatment. Furthermore, hGBP-1 requires another IFN-gamma-induced factor to be targeted to the Golgi, because constitutively expressed hGBP-1 remained cytosolic in cells treated with aluminum fluoride unless the cells were preincubated with IFN-gamma. Finally, two nonhydrolyzing mutants of hGBP-1, corresponding to active mutants of Ras family proteins, failed to constitutively associate with the Golgi; we propose three possible explanations for this surprising result.

Auxin-dependent cell division and cell elongation. 1-Naphthaleneacetic acid and 2,4-dichlorophenoxyacetic acid activate different pathways

During exponential phase, the tobacco (Nicotiana tabacum) cell line cv Virginia Bright Italia-0 divides axially to produce linear cell files of distinct polarity. This axial division is controlled by exogenous auxin. We used exponential tobacco cv Virginia Bright Italia-0 cells to dissect early auxin signaling, with cell division and cell elongation as physiological markers. Experiments with 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) demonstrated that these 2 auxin species affect cell division and cell elongation differentially; NAA stimulates cell elongation at concentrations that are much lower than those required to stimulate cell division. In contrast, 2,4-D promotes cell division but not cell elongation. Pertussis toxin, a blocker of heterotrimeric G-proteins, inhibits the stimulation of cell division by 2,4-D but does not affect cell elongation. Aluminum tetrafluoride, an activator of the G-proteins, can induce cell division at NAA concentrations that are not permissive for division and even in the absence of any exogenous auxin. The data are discussed in a model where the two different auxins activate two different pathways for the control of cell division and cell elongation.

Assay and Functional Analysis of Dynamin‐Like Mx Proteins

Mx proteins are interferon-induced large guanosine triphosphatases (GTPases) that share structural and functional properties with dynamin and dynamin-like proteins, such as self-assembly and association with intracellular membranes. A unique property of some Mx proteins is their antiviral activity against a range of RNA viruses, including influenza viruses and members of the bunyavirus family. These viruses are inhibited at an early stage in their life cycle, soon after host cell entry and before genome amplification. The association of the human MxA GTPase with membranes of the endoplasmic reticulum seems to support its antiviral function by providing an interaction platform that facilitates viral target recognition, MxA oligomerization, and missorting of the resulting multiprotein complex into large intracellular aggregates.

The arginine finger of bacteriophage T7 gene 4 helicase: role in energy coupling

The DNA helicase encoded by gene 4 of bacteriophage T7 couples DNA unwinding to the hydrolysis of dTTP. The loss of coupling in the presence of orthovanadate (Vi) suggests that the gamma-phosphate of dTTP plays an important role in this mechanism. The crystal structure of the hexameric helicase shows Arg-522, located at the subunit interface, positioned to interact with the gamma-phosphate of bound nucleoside 5' triphosphate. In this respect, it is analogous to arginine fingers found in other nucleotide-hydrolyzing enzymes. When Arg-522 is replaced with alanine (gp4-R522A) or lysine (gp4-R522K), the rate of dTTP hydrolysis is significantly decreased. dTTPase activity of the altered proteins is not inhibited by Vi, suggesting the loss of an interaction between Vi and gene 4 protein. gp4-R522A cannot unwind DNA, whereas gp4-R522K does so, proportionate to its dTTPase activity. However, gp4-R522K cannot stimulate T7 polymerase activity on double-stranded DNA. These findings support the involvement of the Arg-522 residue in the energy coupling mechanism.

Fluoride as an essential element in the prevention of disease

The concentration of fluoride in drinking water has been shown to be inversely correlated with the incidence of dental caries and cancer. It is proposed that dental caries, cancer and possibly other diseases are the result of a nutritional deficiency in fluoride.

Fluoride causes reversible dispersal of Golgi cisternae and matrix in neuroendocrine cells

A role for heterotrimeric G proteins in the regulation of Golgi function and formation of secretory granules is generally accepted. We set out to study the effect of activation of heterotrimeric G proteins by aluminum fluoride on secretory granule formation in AtT-20 corticotropic tumor cells and in melanotrophs from the rat pituitary. In AtT-20 cells, treatment with aluminum fluoride or fluoride alone for 60 min induced complete dispersal of Golgi, ER-Golgi intermediate compartment and Golgi matrix markers, while betaCOP immunoreactiviy retained a juxtanuclear position and TGN38 was unaffected. Electron microscopy showed compression of Golgi cisternae followed by conversion of the Golgi stacks into clusters of tubular and vesicular elements. In the melanotroph of the rat pituitary a similar compression of Golgi cisternae was observed, followed by a progressive loss of cisternae from the stacks. As shown in other cells, brefeldin A induced redistribution of the Golgi matrix protein GM130 to punctate structures in the cytoplasm in AtT-20 cells, while mannosidase II immunoreactivity was completely dispersed. Fluoride induced a complete dispersal of mannosidase II and GM130 immunoreactivity. The effect of fluoride was fully reversible with reestablishment of normal mannosidase II and GM130 immunoreactivity within 2 h. After 1 h of recovery, showing varying stages of reassembly, the patterns of mannosidase II and GM130 immunoreactivity were identical in individual cells, indicating that Golgi matrix and cisternae reassemble with similar kinetics during recovery from fluoride treatment. Instead of a specific aluminum fluoride effect on secretory granule formation in the trans-Golgi network, we thus observe a unique form of Golgi dispersal induced by fluoride alone, possibly via its action as a phosphatase inhibitor.

Biochemical characterization of the Ran-RanBP1-RanGAP system: are RanBP proteins and the acidic tail of RanGAP required for the Ran-RanGAP GTPase reaction?

RanBP type proteins have been reported to increase the catalytic efficiency of the RanGAP-mediated GTPase reaction on Ran. Since the structure of the Ran-RanBP1-RanGAP complex showed RanBP1 to be located away from the active site, we reinvestigated the reaction using fluorescence spectroscopy under pre-steady-state conditions. We can show that RanBP1 indeed does not influence the rate-limiting step of the reaction, which is the cleavage of GTP and/or the release of product P(i). It does, however, influence the dynamics of the Ran-RanGAP interaction, its most dramatic effect being the 20-fold stimulation of the already very fast association reaction such that it is under diffusion control (4.5 x 10(8) M(-1) s(-1)). Having established a valuable kinetic system for the interaction analysis, we also found, in contrast to previous findings, that the highly conserved acidic C-terminal end of RanGAP is not required for the switch-off reaction. Rather, genetic experiments in Saccharomyces cerevisiae demonstrate a profound effect of the acidic tail on microtubule organization during mitosis. We propose that the acidic tail of RanGAP is required for a process during mitosis.

Generation of an erythrocyte vesicle transport system by Plasmodium falciparum malaria parasites

The asexual maturation of Plasmodium falciparum is accompanied by the transport of parasite-encoded proteins to the erythrocyte plasma membrane. Activation of G proteins by treatment with aluminum fluoride produced an accumulation within the erythrocyte cytosol of vesicles coated with Plasmodium homologues of COPII and N-ethylmaleimide-sensitive factor, proteins involved in intracellular transport between the Golgi apparatus and the endoplasmic reticulum. These vesicles contain malarial proteins that appear on the erythrocyte plasma membrane, as well as actin and myosin. It is proposed that the parasite adapted a process well established for intracellular transport to mediate the extracellular movement of its proteins through the erythrocyte cytosol to the surface membrane.

Structural fingerprints of the Ras-GTPase activating proteins neurofibromin and p120GAP

Ras specific GTPase activating proteins (GAPs), neurofibromin and p120GAP, bind GTP bound Ras and efficiently complement its active site. Here we present comparative data from mutations and fluorescence-based assays of the catalytic domains of both RasGAPs and interpret them using the crystal structures. Three prominent regions in RasGAPs, the arginine-finger loop, the phenylalanine-leucine-arginine (FLR) region and alpha7/variable loop contain structural fingerprints governing the GAP function. The finger loop is crucial for the stabilization of the transition state of the GTPase reaction. This function is controlled by residues proximal to the catalytic arginine, which are strikingly different between the two RasGAPs. These residues specifically determine the orientation and therefore the positioning of the arginine finger in the Ras active site. The invariant FLR region, a hallmark for RasGAPs, indirectly contributes to GTPase stimulation by forming a scaffold, which stabilizes Ras switch regions. We show that a long hydrophobic side-chain in the FLR region is crucial for this function. The alpha7/variable loop uses several conserved residues including two lysine residues, which are involved in numerous interactions with the switch I region of Ras. This region determines the specificity of the Ras-RasGAP interaction.

The biochemistry and physiology of metallic fluoride: action, mechanism, and implications

Fluoride is a well-known G protein activator. Activation of heterotrimeric GTP-binding proteins by fluoride requires trace amounts of Al3+ or Be2+ ions. AlFx mimics a gamma-phosphate at its transition state in a Galpha protein and is therefore able to inhibit its GTPase activity. AlFx also forms complexes with small GTP-binding proteins in the presence of their GTPase-activating proteins (GAP). As phosphate analogs, AlFx or BeFx affect the activity of a variety of phosphoryl transfer enzymes. Most of these enzymes are fundamentally important in cell signal transduction or energy metabolism. Al3+ and F- tend to form stable complexes in aqueous solution. The exact structure and concentration of AlFx depend on the pH and the amount of F- and Al3+ in the solution. Humans are exposed to both F and Al. It is possible that Al-F complexes may be formed in vivo, or formed in vitro prior to their intake by humans. Al-F complexes may play physiological or pathological roles in bone biology, fluorosis, neurotoxicity, and oral diseases such as dental caries and periodontal disease. The aim of this review is to discuss the basic chemical, biochemical, and toxicological properties of metallic fluoride, to explore its potential physiological and clinical implications.

AF-6 controls integrin-mediated cell adhesion by regulating Rap1 activation through the specific recruitment of Rap1GTP and SPA-1

In the present study, we showed that SPA-1, a Rap1 GTPase-activating protein (GAP), was bound to a cytoskeleton-anchoring protein AF-6. SPA-1 and AF-6 were co-immunoprecipitated in the 293T cells transfected with both cDNAs as well as in normal thymocytes. In vitro binding studies using truncated fragments and their mutants suggested that SPA-1 was bound to the PDZ domain of AF-6 via probable internal PDZ ligand motif within the GAP-related domain. The motif was conserved among Rap1 GAPs, and it was shown that rapGAP I was bound to AF-6 comparably with SPA-1. RapV12 was also bound to AF-6 via the N-terminal domain, and SPA-1 and RapV12 were co-immunoprecipitated only in the presence of AF-6, indicating that they could be brought into close proximity via AF-6 in cells. Immunostaining analysis revealed that SPA-1 and RapV12 were co-localized with AF-6 at the cell attachment sites. In HeLa cells expressing SPA-1 in a tetracycline-regulatory manner, expression of AF-6 inhibited endogenous Rap1GTP and beta(1) integrin-mediated cell adhesion to fibronectin in SPA-1-induced conditions, whereas it affected neither of them in SPA-1-repressed conditions. These results suggested that AF-6 could control integrin-mediated cell adhesion by regulating Rap1 activation through the recruitment of both SPA-1 and Rap1GTP via distinct domains.

Fluorescently labelled guanine nucleotide binding proteins to analyse elementary steps of GAP-catalysed reactions

Downregulation of small guanine nucleotide-binding proteins (GNBPs) requires the interaction with their corresponding GTPase-activating proteins (GAPs), which increase the slow intrinsic GTPase reaction by several orders of magnitude. On the basis of the structure of H-Ras in complex with the catalytic domain of p120-GAP, we have developed a set of site-specifically labelled Ras-variants, one of which turned out to be particularly sensitive for studying the interaction with Ras-specific GAPs. This specific fluorescent reporter group and the use of manganese to increase the rate of the chemical reaction step allowed us to identify differences in the rate-limiting step of either the GAP-334 or NF1-333 catalyzed reaction. The assay was also applied to study the interaction of the Ras-related protein Rap1B with Rap1GAP, for which no detailed kinetic analysis was available. Single-turnover experiments of this reaction show that the low affinity of the complex (50 microM) is due to a slow association rate as well as a fast dissociation rate. RapGAP promotes AlFx binding to Rap1B, even though it does not contain a catalytic arginine. The rate-limiting step of the RapGAP catalysed reaction is release of inorganic phosphate, which is about five times slower than the chemical cleavage step. Our data reveal marked differences in GAP/target interactions even between closely related systems and suggest that the fluorescent reporter group method might be generally applicable to many other GNBPs and their cognate GAPs.

Ypt32 recruits the Sec4p guanine nucleotide exchange factor, Sec2p, to secretory vesicles; evidence for a Rab cascade in yeast

SEC2 is an essential gene required for polarized growth of the yeast Saccharomyces cerevisiae. It encodes a protein of 759 amino acids that functions as a guanine nucleotide exchange factor for the small GTPase Sec4p, a regulator of Golgi to plasma membrane transport. Activation of Sec4p by Sec2p is needed for polarized transport of vesicles to exocytic sites. Temperature-sensitive (ts) mutations in sec2 and sec4 result in a tight block in secretion and the accumulation of secretory vesicles randomly distributed in the cell. The proper localization of Sec2p to secretory vesicles is essential for its function and is largely independent of Sec4p. Although the ts mutation sec2-78 does not affect nucleotide exchange activity, the protein is mislocalized. Here we present evidence that Ypt31/32p, members of Rab family of GTPases, regulate Sec2p function. First, YPT31/YPT32 suppress the sec2-78 mutation. Second, overexpression of Ypt31/32p restores localization of Sec2-78p. Third, Ypt32p and Sec2p interact biochemically, but Sec2p has no exchange activity on Ypt32p. We propose that Ypt32p and Sec4p act as part of a signaling cascade in which Ypt32p recruits Sec2p to secretory vesicles; once on the vesicle, Sec2p activates Sec4p, enabling the polarized transport of vesicles to the plasma membrane.

Self-assembly of human MxA GTPase into highly ordered dynamin-like oligomers

Human MxA protein is a member of the interferon-induced Mx protein family and an important component of the innate host defense against RNA viruses. The Mx family belongs to a superfamily of large GTPases that also includes the dynamins and the interferon-regulated guanylate-binding proteins. A common feature of these large GTPases is their ability to form high molecular weight oligomers. Here we determined the capacity of MxA to self-assemble into homo-oligomers in vitro. We show that recombinant MxA protein assembles into long filamentous structures with a diameter of about 20 nm at physiological salt concentration as demonstrated by sedimentation assays and electron microscopy. In the presence of guanosine nucleotides the filaments rearranged into rings and more compact helical arrays. Our data indicate that binding and hydrolysis of GTP induce conformational changes in MxA that may be essential for viral target recognition and antiviral activity.

Bfa1 can regulate Tem1 function independently of Bub2 in the mitotic exit network of Saccharomyces cerevisiae

In budding yeast, exit from mitosis is achieved by inactivation of Cdc28/Clb2 activity. Although it is not clear at present how mitotic exit is triggered, a growing body of evidence suggests that the Tem1 GTPase plays a critical role in mediating this pathway and that Bfa1 and Bub2 constitute a two-component GTPase-activating protein to negatively regulate Tem1. Here, we have demonstrated that introduction of bfa1 Delta suppresses the growth defects associated with the cdc5-1 mutation significantly better than that of bub2 Delta, suggesting that Bfa1 may have a previously uncharacterized role in this pathway. Overexpression of BFA1 efficiently arrested the cell cycle at postanaphase even in the absence of BUB2, whereas overexpression of BUB2 weakly induced mitotic arrest only in the presence of BFA1. Coimmunoprecipitation and in vitro binding studies indicate that Bfa1 binds strongly to Tem1 independently of Bub2. Provision of GDP+AlF(4)(-), which mimics the GTPase transition state, enhanced the Bub2-Tem1 interaction both in vitro and in vivo. Interestingly, introduction of bfa1 Delta, but not bub2 Delta, greatly increased the interaction between Tem1 and Cdc15, a step that is thought to be critical for activating the mitotic exit network. Our data suggest that, in addition to its role as a putative, two-component GTPase-activating protein with Bub2, Bfa1 also can play a role in the regulation of mitotic exit by directly inhibiting the interaction between Tem1 and Cdc15 even in the absence of Bub2.

Regulators of GTP-binding proteins cause morphological changes in the vacuole system of the filamentous fungus, Pisolithus tinctorius

Tubule formation is a widespread feature of the endomembrane system of eukaryotic cells, serving as an alternative to the better-known transport process of vesicular shuttling. In filamentous fungi, tubule formation by vacuoles is particularly pronounced, but little is known of its regulation. Using the hyphae of the basidiomycete Pisolithus tinctorius as our test system, we have investigated the effects of four drugs whose modulation, in animal cells, of the tubule/vesicle equilibrium is believed to be due to the altered activity of a GTP-binding protein (GTP gamma S, GDP beta S, aluminium fluoride, and Brefeldin A). In Pisolithus tinctorius, GTP gamma S, a non-hydrolysable form of GTP, strongly promoted vacuolar tubule formation in the tip cell and next four cells. The effects of GTP gamma S could be antagonised by pre-treatment of hyphae with GDP beta S, a non-phosphorylatable form of GDP. These results support the idea that a GTP-binding protein plays a regulatory role in vacuolar tubule formation. This could be a dynamin-like GTP-ase, since GTP gamma S-stimulated tubule formation has only been reported previously in cases where a dynamin is involved. Treatment with aluminium fluoride stimulated vacuolar tubule formation at a distance from the tip cell, but NaF controls indicated that this was not a GTP-binding-protein specific effect. Brefeldin A antagonised GTP gamma S, and inhibited tubule formation in the tip cell. Given that Brefeldin A also affects the ER and Golgi bodies of Pisolithus tinctorius, as shown previously, it is not clear yet whether the effects of Brefeldin A on the vacuole system are direct or indirect.

Crystal structure of the Yersinia pestis GTPase activator YopE

Yersinia pestis, the causative agent of bubonic plague, evades the immune response of the infected organism by using a type III (contact-dependent) secretion system to deliver effector proteins into the cytosol of mammalian cells, where they interfere with signaling pathways that regulate inflammation and cytoskeleton dynamics. The cytotoxic effector YopE functions as a potent GTPase-activating protein (GAP) for Rho family GTP-binding proteins, including RhoA, Rac1, and Cdc42. Down-regulation of these molecular switches results in the loss of cell motility and inhibition of phagocytosis, enabling Y. pestis to thrive on the surface of macrophages. We have determined the crystal structure of the GAP domain of YopE (YopE(GAP); residues 90-219) at 2.2-A resolution. Apart from the fact that it is composed almost entirely of alpha-helices, YopE(GAP) shows no obvious structural similarity with eukaryotic RhoGAP domains. Moreover, unlike the catalytically equivalent arginine fingers of the eukaryotic GAPs, which are invariably contained within flexible loops, the critical arginine in YopE(GAP) (Arg144) is part of an alpha-helix. The structure of YopE(GAP) is strikingly similar to the GAP domains from Pseudomonas aeruginosa (ExoS(GAP)) and Salmonella enterica (SptP(GAP)), despite the fact that the three amino acid sequences are not highly conserved. A comparison of the YopE(GAP) structure with those of the Rac1-ExoS(GAP) and Rac1-SptP complexes indicates that few, if any, significant conformational changes occur in YopE(GAP) when it interacts with its G protein targets. The structure of YopE(GAP) may provide an avenue for the development of novel therapeutic agents to combat plague.

Molecular dynamics simulations of Gly-12-->Val mutant of p21(ras): dynamic inhibition mechanism

The mutant p21(ras) protein is a G protein produced by the point-mutated H-ras gene, and this mutant protein has been shown to cause carcinogenesis due to a reduction in its GTPase activity. However, the mechanism underlying this strange phenomenon has still not been elucidated. In our previous study, we have clarified the mechanism of the GTP-->GDP hydrolysis reaction in the wild-type p21(ras) at the atomic level and concluded that GTPase-activating protein plays a significant role in the supply of H2O molecules for the hydrolysis. The structure of the active site in the mutant is the same as that in the wild type. However, by performing molecular dynamic calculations, we found that the structure of the active site of the enzyme substrate complex in the oncogenic mutant p21(ras) continuously changes, and these continuous changes in the active site would make it difficult for the GTP-->GDP hydrolysis reaction to occur in the mutant. These findings can explain the fact that the GTPase activity in the mutant was only 15% of that in the wild type and the fact that GTPase-activating protein has no reaction-activating effect in the mutant. This is a dynamic inhibition mechanism of a vital reaction that can be explained by considering the molecular dynamics.

Role of prenylation in the interaction of Rho-family small GTPases with GTPase activating proteins

The role of prenylation in the interaction of Rho-family small GTPases with their GTPase activating proteins (GAPs) was investigated. Prenylated and nonprenylated small GTPases were expressed in Sf9 insect cells and Escherichia coli, respectively. Nucleotide binding to and hydrolysis by prenylated and nonprenylated proteins were identical, but three major differences were observed in their reactions with GAPs. (1) Membrane-associated GAPs accelerate GTP hydrolysis only on prenylated Rac1 and RhoA, but they are inactive on the nonprenylated form of these proteins. The difference is independent of the presence of detergents. In contrast to Rac1 and RhoA, nonprenylated Cdc42 is able to interact with membrane-localized GAPs. (2) Full-length p50RhoGAP and p190RhoGAP react less intensely with nonprenylated Rac1 than with the prenylated protein, whereas no difference was observed in the reaction of isolated GAP domains of either p50RhoGAP or Bcr with the different types of Rac1. (3) Fluoride exerts a significant inhibitory effect only on the interaction of prenylated Rac1 with the isolated GAP domains of p50RhoGAP or Bcr. The effect of fluoride is not influenced by addition or chelation of Al(3+). This is the first detailed study demonstrating that prenylation of the small GTPase is an important factor in determining its reaction with GAPs. It is suggested that both intramolecular interactions and membrane targeting of GAP proteins represent potential mechanisms regulating Rac signaling.

Characterization of p190RhoGEF, a RhoA-specific guanine nucleotide exchange factor that interacts with microtubules

Rho family GTPases control numerous cellular processes including cytoskeletal reorganization and transcriptional activation. Rho GTPases are activated by guanine nucleotide exchange factors (GEFs) which stimulate the exchange of bound GDP for GTP. We recently isolated a putative GEF, termed p190RhoGEF that binds to RhoA and, when overexpressed in neuronal cells, induces cell rounding and inhibits neurite outgrowth. Here we show that the isolated tandem Dbl homology/pleckstrin homology domain of p190RhoGEF activates RhoA in vitro, but not Rac1 or Cdc42, as determined by GDP release and protein binding assays. In contrast, full-length p190RhoGEF fails to activate RhoA in vitro. When overexpressed in intact cells, however, p190RhoGEF does activate RhoA with subsequent F-actin reorganization and serum response factor-mediated transcription. Immunofluorescence studies show that endogenous p190RhoGEF localizes to distinct RhoA-containing regions at the plasma membrane, to the cytosol and along microtubules. In vitro and in vivo binding experiments show that p190RhoGEF directly interacts with microtubules via its C-terminal region adjacent to the catalytic Dbl homology/pleckstrin homology domain. Our results indicate that p190RhoGEF is a specific activator of RhoA that requires as yet unknown binding partners to unmask its GDP/GTP exchange activity in vivo, and they suggest that p190RhoGEF may provide a link between microtubule dynamics and RhoA signaling.

Reconstitution of the Golgi reassembly process in semi-intact MDCK cells

The Golgi apparatus, which consists of stacks of cisternae during interphase, is fragmented or dispersed throughout the cytoplasm at the onset of mitosis. A sea sponge metabolite, ilimaquinone (IQ), causes Golgi membranes to vesiculate. And after its removal, the vesiculated membranes reassemble into stacks of cisternae in the perinuclear region. To study the mechanism of Golgi membrane dynamics during mitosis, we have reconstituted the reassembly process of IQ-induced vesiculated Golgi membranes in streptolysin O-permeabilized Mardin-Darby canine kidney (MDCK) cells. Monitoring the dynamics of Golgi membranes labeled with a green fluorescence protein (GFP)-tagged protein, we dissected the process into two elementary components: the reassembly of vesiculated Golgi membranes into punctate structures; and the subsequent reformation of these structures into stacks of cisternae near the nucleus. Using morphometric analysis, we studied the kinetics and biochemical requirements for the process, and revealed that an NEM-sensitive factor, cytoplasmic dynein, and GTP binding protein were involved in the Golgi reassembly.

Byr4 localizes to spindle-pole bodies in a cell cycle-regulated manner to control Cdc7 localization and septation in fission yeast

Cytokinesis and septation in the fission yeast Schizosaccharomyces pombe are studied as a model for mammalian cell division. In fission yeast, septation is positively regulated by Spg1, a Ras family GTPase that localizes to spindle-pole bodies (SPBs) throughout the cell cycle. As cells enter mitosis, Spg1 accumulates in an active, GTP-bound form and binds the Cdc7 protein kinase to cause Cdc7 translocation to SPBs. Cdc7 disappears from one SPB in mid-anaphase and from the second SPB in late mitosis. Byr4 plus Cdc16 negatively regulate septation by forming a two-component GTPase-activating protein for Spg1. These results led us to hypothesize that Byr4 localization to SPBs regulated the nucleotide state of Spg1, due to its ability to form Spg1GAP activity with Cdc16 and thus the binding of Cdc7 to Spg1 at SPBs. To test this hypothesis, Byr4 localization was determined using indirect immunofluorescence. This analysis revealed that Byr4 was localized to SPBs that did not contain Cdc7. In byr4(-) mutants, Cdc7 localized to interphase SPBs and only symmetrically localized to mitotic SPBs. In contrast, Byr4 overexpression prevented Spg1 and Cdc7 localization to SPBs. These results suggest that Byr4 localization to SPBs maintains Spg1 in an inactive form, presumably by stimulating Spg1 GTPase activity with Cdc16, and that loss of Byr4 from mitotic SPBs increases the active fraction of Spg1 and thereby increases Spg1-Cdc7 binding. Byr4 localization to SPBs was decreased in spg1, cdc16, sid4, and cdc11 mutants as well as in several mutants that affect medial F-actin structures, suggesting that multiple pathways regulate Byr4 localization to SPBs.

Functional interaction between Galpha(z) and Rap1GAP suggests a novel form of cellular cross-talk

G(z) is a member of the G(i) family of trimeric G proteins whose primary role in cell physiology is still unknown. In an ongoing effort to elucidate the cellular functions of G(z), the yeast two-hybrid system was employed to identify proteins that specifically interact with a mutationally activated form of Galpha(z). One of the molecules uncovered in this screen was Rap1GAP, a previously identified protein that specifically stimulates GTP hydrolytic activity of the monomeric G protein Rap1 and thus is believed to function as a down-regulator of Rap1 signaling. Like G(z), the precise role of Rap1 in cell physiology is poorly understood. Biochemical analysis using purified recombinant proteins revealed that the physical interaction between Galpha(z) and Rap1GAP blocks the ability of RGSs (regulators of G protein signaling) to stimulate GTP hydrolysis of the alpha subunit, and also attenuates the ability of activated Galpha(z) to inhibit adenylyl cyclase. Structure-function analyses indicate that the first 74 amino-terminal residues of Rap1GAP, a region distinct from the catalytic core domain responsible for the GAP activity toward Rap1, is required for this interaction. Co-precipitation assays revealed that Galpha(z), Rap1GAP, and Rap1 can form a stable complex. These data suggest that Rap1GAP acts as a signal integrator to somehow coordinate and/or integrate G(z) signaling and Rap1 signaling in cells.

Identification and characterization of hPEM-2, a guanine nucleotide exchange factor specific for Cdc42

Guanine nucleotide exchange factors of the Dbl family regulate the actin cytoskeleton through activation of Rho-like GTPases. At present the Dbl family consists of more than thirty members; many have not been phenotypically or biochemically characterized. Guanine nucleotide exchange factors universally feature a Dbl homology domain followed by a pleckstrin homology domain. Employing data base screening we identified a recently cloned cDNA, KIAA0424, showing substantial sequence homology with Rac activators such as Tiam1, Sos, Vav, and PIX within the catalytic domain. This cDNA appears to be the human homologue of the Ascidian protein Posterior End Mark-2 (PEM-2). We refer to this exchanger as hPEM-2. hPEM-2 encodes a protein of 70 kDa and features an N-terminal src homology 3 domain, followed by tandem Dbl homology and pleckstrin homology domains. The gene is highly expressed in brain and is localized on the human X-chromosome. Employing biochemical activity assays for Rho-like GTPases we found that hPEM-2 specifically activates Cdc42 and not Rac or RhoA. Ectopic expression of hPEM-2 in NIH3T3 fibroblasts revealed a Cdc42 phenotype featuring filopodia formation, followed by cortical actin polymerization and cell rounding. hPEM-2 represents an exchange factor, which may have a role in the regulation of a number of cellular processes through Cdc42.

Trimeric G proteins modulate the dynamic interaction of PKAII with the Golgi complex

The Golgi complex represents a major subcellular location of protein kinase A (PKA) concentration in mammalian cells where it has been previously shown to be involved in vesicle-mediated protein transport processes. We have studied the factors that influence the interaction of PKA typeII subunits with the Golgi complex. In addition to the cytosol, both the catalytic (Calpha) and regulatory (RIIalpha) subunits of PKAII were detected at both sides of the Golgi stack, particularly in elements of the cis- and trans-Golgi networks. PKAII subunits, in contrast, were practically absent from the middle Golgi cisternae. Cell treatment with either brefeldin A, AlF(4-) or at low temperature induced PKAII dissociation from the Golgi complex and redistribution to the cytosol. This suggested the existence of a cycle of association/dissociation of PKAII holoenzyme to the Golgi. The interaction of purified RIIalpha with Golgi membranes was studied in vitro and found not to be affected by brefeldin A while it was sensitive to modulators of heterotrimeric G proteins such as AlF(4-), GTPgammaS, beta(gamma) subunits and mastoparan. RII(alphaa) binding was stimulated by recombinant, myristoylated Galpha(i3) subunit and inhibited by cAMP. Pretreatment of Golgi membranes with bacterial toxins known to catalyze ADP-ribosylation of selected Galpha subunits also modified RIIalpha binding. Taken together the data support a regulatory role for Golgi-associated Galpha proteins in PKAII recruitment from the cytosol.

Inhibition of RhoGAP activity is sufficient for the induction of Rho-mediated actin reorganization

It is generally believed that the induction of actin cytoskeleton rearrangements by extracellular stimuli results from the activation of guanine nucleotide exchange factors for the Rho GTPases. Here, we present evidence that the inactivation of RhoGAP (GTPase activating protein) activity is an equally effective means of promoting Rho-mediated cellular processes. We observed that exposure of cultured fibroblasts to sodium fluoride (NaF) results in a rapid and potent stimulation of actin stress fiber formation. This effect is mediated by the Rho GTPase and is associated with the inactivation of cellular RhoGAP activity. Specifically, NaF promotes formation of a high-affinity complex between Rho and the two cellular p190 RhoGAPs in vivo, apparently sequestering limiting amounts of RhoGAP activity, thereby resulting in Rho activation. p190 RhoGAP activity was found to account for approximately 60% of the total RhoGAP activity detected in whole cell extracts, indicating that relatively small changes in cellular RhoGAP activity can have potent effects on Rho activation. We also found that sub-effective concentrations of NaF combined with sub-effective concentrations of the Rho pathway activator, lysophosphatidic acid, which stimulates guanine nucleotide exchange activity on the Rho GTPase, results in the rapid induction of actin stress fibers. Together, these results suggest that the Rho GTPase is regulated by a fine balance of nucleotide exchange and RhoGAP activities, and that inactivation of RhoGAP activity may be a physiologically important regulatory mechanism for activating the Rho GTPase.

The crystal structure of rna1p: a new fold for a GTPase-activating protein

rna1p is the Schizosaccharomyces pombe ortholog of the mammalian GTPase-activating protein (GAP) of Ran. Both proteins are essential for nuclear transport. Here, we report the crystal structure of rna1p at 2.66 A resolution. It contains 11 leucine-rich repeats that adopt the nonglobular shape of a crescent, bearing no resemblance to RhoGAP or RasGAP. The invariant residues of RanGAP form a contiguous surface, strongly indicating the Ran-binding interface. Alanine mutations identify Arg-74 as a critical residue for GTP hydrolysis. In contrast to RasGAP and RhoGAP, Arg-74 could be substituted by lysine and contributed significantly to the binding of Ran. Therefore, we suggest a GAP mechanism for rna1p, which constitutes a variation of the arginine finger mechanism found for Ras GAP and RhoGAP.

The conserved arginine in rho-GTPase-activating protein is essential for efficient catalysis but not for complex formation with Rho.GDP and aluminum fluoride

The Rho family of small GTP-binding proteins are downregulated by an intrinsic GTPase, which is enhanced by GTPase-activating proteins (GAPs). RhoGAPs contain a single conserved arginine residue that has been proposed to be involved in catalysis. Here, the role of this arginine has been elucidated by mutagenesis followed by determination of catalytic and equilibrium binding constants using single-turnover kinetics, isothermal titration calorimetry, and scintillation proximity assays. The turnover numbers for wild-type, R282A, and R282K RhoGAPs were 5.4, 0.023, and 0.010 s-1, respectively. Thus, the function of this arginine could not be replaced by lysine or alanine. Nevertheless, the R282A mutation had a minimal effect on the binding affinity of RhoGAP for either Rho. GTP or Rho.GMPPNP, which confirms the importance of the arginine residue for catalysis as opposed to formation of the protein-protein complex. The R282A mutant RhoGAP still increased the hydrolysis rate of Rho.GTP by 160-fold, whereas the wild-type enzyme increased it by 38000-fold. We conclude that this arginine contributes half of the total reduction of activation energy of catalysis. In the presence of aluminum fluoride, the R282A mutant RhoGAP binds almost as well as the wild type to Rho.GDP, demonstrating that the conserved arginine is not required for this interaction. The affinity of wild-type RhoGAP for the triphosphate form of Rho is similar to that for Rho.GDP with aluminum fluoride. These last two observations show that this complex is not associated with the free energy changes expected for the transition state, although the Rho.GDP.AlF4-.RhoGAP complex might well be a close structural approximation.

The IQGAP-related protein DGAP1 interacts with Rac and is involved in the modulation of the F-actin cytoskeleton and control of cell motility

DGAP1 of Dictyostelium discoideum is a cell cortex associated 95 kDa protein that shows homology to both RasGTPase-activating proteins (RasGAPs) and RasGAP-related proteins. When tested for RasGAP activity, recombinant DGAP1 protein did not promote the GTPase activity of human H-Ras or of Dictyostelium RasG in vitro. Instead, DGAP1 bound to Dictyostelium Rac1A and human Rac1, but not to human Cdc42. DGAP1 preferentially interacted with the activated GTP-bound forms of Rac1 and Rac1A, but did not affect the GTPase activities. Since Rho-type GTPases are implicated in the formation of specific F-actin structures and in the control of cell morphology, the microfilament system of mutants that either lack or overexpress DGAP1 has been analysed. DGAP1-null mutants showed elevated levels of F-actin that was organised in large leading edges, membrane ruffles or numerous large filopods. Expression of actin fused to green fluorescent protein (GFP) was used to monitor the actin dynamics in these cells, and revealed that the F-actin cytoskeleton of DGAP1-null cells was rapidly re-arranged to form ruffles and filopods. Conversely, in DGAP1-overexpressing cells, the formation of cellular projections containing F-actin was largely suppressed. Measurement of cell migration demonstrated that DGAP1 expression is inversely correlated with the speed of cell motility.