Guanine nucleotide binding properties of the mammalian RalA protein produced in Escherichia coli

Unravelling molecular dynamics in living cells: Fluorescent protein biosensors for cell biology

Genetically encoded, fluorescent protein (FP)-based Förster resonance energy transfer (FRET) biosensors are microscopy imaging tools tailored for the precise monitoring and detection of molecular dynamics within subcellular microenvironments. They are characterised by their ability to provide an outstanding combination of spatial and temporal resolutions in live-cell microscopy. In this review, we begin by tracing back on the historical development of genetically encoded FP labelling for detection in live cells, which lead us to the development of early biosensors and finally to the engineering of single-chain FRET-based biosensors that have become the state-of-the-art today. Ultimately, this review delves into the fundamental principles of FRET and the design strategies underpinning FRET-based biosensors, discusses their diverse applications and addresses the distinct challenges associated with their implementation. We place particular emphasis on single-chain FRET biosensors for the Rho family of guanosine triphosphate hydrolases (GTPases), pointing to their historical role in driving our understanding of the molecular dynamics of this important class of signalling proteins and revealing the intricate relationships and regulatory mechanisms that comprise Rho GTPase biology in living cells.

Regulation of GTPase function by autophosphorylation

A unifying feature of the RAS superfamily is a conserved GTPase cycle by which these proteins transition between active and inactive states. We demonstrate that autophosphorylation of some GTPases is an intrinsic regulatory mechanism that reduces nucleotide hydrolysis and enhances nucleotide exchange, altering the on/off switch that forms the basis for their signaling functions. Using X-ray crystallography, nuclear magnetic resonance spectroscopy, binding assays, and molecular dynamics on autophosphorylated mutants of H-RAS and K-RAS, we show that phosphoryl transfer from GTP requires dynamic movement of the switch II region and that autophosphorylation promotes nucleotide exchange by opening the active site and extracting the stabilizing Mg2+. Finally, we demonstrate that autophosphorylated K-RAS exhibits altered effector interactions, including a reduced affinity for RAF proteins in mammalian cells. Thus, autophosphorylation leads to altered active site dynamics and effector interaction properties, creating a pool of GTPases that are functionally distinct from their non-phosphorylated counterparts.

Ral GTPases and cancer: linchpin support of the tumorigenic platform

A confluence of recent observations has indicted the Ras-family G-proteins RALA and RALB as key offenders in the subversion of core biological systems driving oncogenic transformation. Here, we will focus on current developments highlighting the pivotal contribution of Ral proteins to the regulatory framework supporting tumorigenesis, and evaluate mechanistic connections between Ral effector activation and generation of this framework.

Identification of Expression Patterns Associated with Hemorrhage and Resuscitation: Integrated Approach to Data Analysis

BACKGROUND

Although transcriptional profiling is a well-established technique, its application to systematic studying of various biological phenomena is still limited because of problems with high-volume data analysis and interpretation. This research project's objective was to create a comprehensive summary of changes in gene expression after hemorrhagic shock (HS), reliant and impartial of multiple variables, such as resuscitation treatments, organ analyzed, and time after impact.

METHODS

Rat model of severe (40% total blood loss) HS was employed. Hemorrhagic shock was treated with 6 different resuscitation strategies: (1) racemic lactated Ringer's (DL-LR); (2) L-lactated Ringer's (L-LR); (3) ketone Ringer's (KR); (4) pyruvate Ringer's (PR); (5) 6% hetastarch (Hex); (6) 7.5% hypertonic saline (HTS). Nonresuscitated and nonhemorrhaged rats served as controls. Ketone and pyruvate Ringer solutions were identical to the lactated Ringer's solution except for equimolar substitution of lactate with beta-hydroxybutyrate and sodium pyruvate, respectively. Total RNA from liver, lung, and spleen was isolated immediately (0 hour) and 24 hour postresuscitation. Each organ, time point and treatment was profiled using individual cDNA array (1,200 genes), to produce 183 separate data files. Methods of analysis included one-way and unbalanced factorial ANOVA, Sokal-Michener average linkage clustering and contextual mapping.

RESULTS

: Unresuscitated HS produced the highest number (56) of upregulated expressions in spleen and lungs. HEX and HTS affected mostly pulmonary genes (22 and 9). Fourteen genes changed in response to combination of all three factors: treatment, organ, and time. Eighteen genes were identified as treatment-specific. Fifteen genes adjusted expression 24 hour post-treatment. The largest number of genes with altered expression (168) responded differently in all three organs. In this study 15 gene clusters were pinpointed. Contextual mapping identified novel and confirmed known pathways contributing to hemorrhage/resuscitation.

CONCLUSIONS

We have reliably identified genes and pathways that are affected by HS and are responsive to resuscitation. Gene expression in various organs is affected differentially by HS, which can be further modulated by the choice of resuscitation strategy.

Crystal structure of the C3bot-RalA complex reveals a novel type of action of a bacterial exoenzyme

C3 exoenzymes from bacterial pathogens ADP-ribosylate and inactivate low-molecular-mass GTPases of the Rho subfamily. Ral, a Ras subfamily GTPase, binds the C3 exoenzymes from Clostridium botulinum and C. limosum with high affinity without being a substrate for ADP ribosylation. In the complex, the ADP-ribosyltransferase activity of C3 is blocked, while binding of NAD and NAD-glycohydrolase activity remain. Here we report the crystal structure of C3 from C. botulinum in a complex with GDP-bound RalA at 1.8 A resolution. C3 binds RalA with a helix-loop-helix motif that is adjacent to the active site. A quaternary complex with NAD suggests a mode for ADP-ribosyltransferase inhibition. Interaction of C3 with RalA occurs at a unique interface formed by the switch-II region, helix alpha3 and the P loop of the GTPase. C3-binding stabilizes the GDP-bound conformation of RalA and blocks nucleotide release. Our data indicate that C. botulinum exoenzyme C3 is a single-domain toxin with bifunctional properties targeting Rho GTPases by ADP ribosylation and Ral by a guanine nucleotide dissociation inhibitor-like effect, which blocks nucleotide exchange.

Activation of RalA is critical for Ras-induced tumorigenesis of human cells

RalGEFs were recently shown to be critical for Ras-mediated transformed and tumorigenic growth of human cells. We now show that the oncogenic activity of these proteins is propagated by activation of one RalGEF substrate, RalA, but blunted by another closely related substrate, RalB, and that the oncogenic signaling requires binding of the RalBP1 and exocyst subunit effector proteins. Knockdown of RalA expression impeded, if not abolished, the ability of human cancer cells to form tumors. RalA was also commonly activated in a panel of cell lines from pancreatic cancers, a disease characterized by activation of Ras. Activation of RalA signaling thus appears to be a critical step in Ras-induced transformation and tumorigenesis of human cells.

Crystal structures of Ral-GppNHp and Ral-GDP reveal two binding sites that are also present in Ras and Rap

RalA is a GTPase with effectors such as Sec5 and Exo84 in the exocyst complex and RalBP1, a GAP for Rho proteins. We report the crystal structures of Ral-GppNHp and Ral-GDP. Disordered switch I and switch II, located away from crystal contacts, are observed in one of the molecules in the asymmetric unit of the Ral-GppNHp structure. In the other molecule in the asymmetric unit, a second Mg(2+) ion is bound to the GppNHp gamma-phosphate in an environment in which switch I is pulled away from the nucleotide and switch II is found in a tight beta turn. Clustering of conserved residues on the surface of Ral-GppNHp identifies two putative sites for protein-protein interaction. One site is adjacent to switch I. The other is modulated by switch II and is obstructed in Ral-GDP. The Ral structures are discussed in the context of the published structures of the Ral/Sec5 complex, Ras, and Rap.

The use of surface plasmon resonance (SPR) and fluorescence resonance energy transfer (FRET) to monitor the interaction of the plant G-proteins Ms-Rac1 and Ms-Rac4 with GTP

Using an RT-PCR approach a cDNA clone, designated Ms-Rac4 and putatively coding for a small GTPase was isolated from Medicago sativa. Ms-Rac4 and the earlier described Ms-Rac1 [Mol. Gen. Genet. 263 (2000) 761] belong to the class of GTP-binding Rho of plants (Rop) proteins. At the amino acid level they display all conserved regions that are common to GTP-binding proteins. Phylogenetically both are located in the group Ia, but within this group they are well-separated. Computed structure models of both proteins revealed a high degree of structural conservation. Particularly the switch I and switch II region are 100% conserved between Ms-Rac1 and Ms-Rac4 and highly conserved as compared to other Rac-like G-proteins. Both GTPases differ in structure within the fourth loop and the fourth helix. GTP-binding properties of the heterologously expressed Ms-Rac1 and Ms-Rac4 was shown by fluorescence resonance energy transfer (FRET) using mantGTP and by surface plasmon resonance (SPR). By this method the specificity of the G-protein/GTP interaction was shown and the inhibitory effect of GTP, EDTA and Mg(2+) on the Ms-Rac1 and Ms-Rac4 binding to immobilized GTP was characterized. Ms-Rac1 and Ms-Rac4 exhibited the same affinity to GTP and are similarly affected by GTP, EDTA and Mg(2+). Thus, the predicted structural differences do not result in different GTP-binding properties of Ms-Rac1 and Ms-Rac4.

Nitric oxide activates Rap1 and Ral in a Ras-independent manner

Rap1 and Ral, the small GTPases belonging to the Ras superfamily, have recently attracted much attention; Ral because of Ral-specific guanine nucleotide exchange factors which are regulated by direct binding to Ras and Rap1 because of its proposed role as an antagonist of Ras signaling. We have previously demonstrated that nitric oxide (NO) activates Ras and proposed the structural basis of interaction between NO and Ras. In the present study we have shown that NO activates Rap1 and Ral in a time- and concentration-dependent manner. Using activation-specific probes for Rap1 and Ral, it was found that the NO-generating compounds SNP and SNAP could activate both Rap1 and Ral in Jurkat and PC12 cell lines. To investigate the involvement of Ras in NO mediated activation of Rap1 and Ral, we used PC12 cell lines expressing either the Ras mutant C118S (Cys118 mutated to Ser) or N17 (GDP-locked and inactive). We had previously shown that NO fails to activate Ras in these mutant cell lines. However, here it was found that Rap1 and Ral were activated by NO in these cell lines. The evidence presented in this study unambiguously demonstrates the existence of Ras-independent pathways for NO mediated activation of Rap1 and Ral.

Analysis of the open and closed conformations of the GTP-binding protein YsxC from Bacillus subtilis

Genetic analysis has suggested that the product of the Bacillus subtilis ysxC gene is essential for survival of the microorganism and hence may represent a target for the development of a novel anti-infective agent. B.subtilis YsxC is a member of the translation factor related class of GTPases and its crystal structure has been determined in an apo form and in complex with GDP and GMPPNP/Mg2+. Analysis of these structures has allowed us to examine the conformational changes that occur during the process of nucleotide binding and GTP hydrolysis. These structural changes particularly affect parts of the switch I and switch II region of YsxC, which become ordered and disordered, respectively in the "closed" or "on" GTP-bound state and disordered and ordered, respectively, in the "open" or "off" GDP-bound conformation. Finally, the binding of the magnesium cation results in subtle shifts of residues in the G3 region, at the start of switch II, which serve to optimize the interaction with a key aspartic acid residue. The structural flexibility observed in YsxC is likely to contribute to the role of the protein, possibly allowing transduction of an essential intracellular signal, which may be mediated via interactions with a conserved patch of surface-exposed, basic residues that lies adjacent to the GTP-binding site.

Signaling specificity by Ras family GTPases is determined by the full spectrum of effectors they regulate

Ras family GTPases (RFGs) regulate signaling pathways that control multiple biological processes. How signaling specificity among the closely related family members is achieved is poorly understood. We have taken a proteomics approach to signaling by RFGs, and we have analyzed interactions of a panel of RFGs with a comprehensive group of known and potential effectors. We have found remarkable differences in the ability of RFGs to regulate the various isoforms of known effector families. We have also identified several proteins as novel effectors of RFGs with differential binding specificities to the various RFGs. We propose that specificity among RFGs is achieved by the differential regulation of combinations of effector families as well as by the selective regulation of different isoforms within an effector family. An understanding of this new level of complexity in the signaling pathways regulated by RFGs is necessary to understand how they carry out their many cellular functions. It will also likely have critical implications in the treatment of human diseases such as cancer.

Constitutive activation of the MEK/ERK pathway mediates all effects of oncogenic H-ras expression in primary erythroid progenitors

Oncogenic mutations in ras genes frequently occur in patients with myeloid disorders, and in these patients erythropoiesis is often affected. Previously, we showed that expression of oncogenic H-ras in purified mouse primary fetal liver erythroid progenitors blocks terminal erythroid differentiation and supports erythropoietin (Epo)-independent proliferation. As a first step in understanding the underlying molecular mechanisms we examined the signaling pathways downstream of Ras in primary erythroid cells. We found that 3 major pathways are abnormally activated by oncogenic H-ras: Raf/ERK (extracellular signal-regulated kinase), phosphatidyl inositol 3 (PI3)-kinase/Akt, and RalGEF/RalA. However, only constitutive activation of the MEK (MAPK [mitogen-activated protein kinase]/ERK kinase)/ERK pathway alone could recapitulate all of the effects of oncogenic H-ras expression in blocking erythroid differentiation and inducing Epo-independent proliferation. Although expression of a constitutively active Akt kinase (ca.Akt) in erythroid progenitors does not significantly affect erythroid differentiation in the presence of Epo, coexpression of ca.Akt together with a constitutively active MEK causes prolonged Epo-independent proliferation of erythroid progenitors in addition to a block in differentiation. Moreover, the effects of oncogenic H-ras expression on primary erythroid cells are blocked by the addition of U0126, a specific inhibitor of MEK1 and MEK2, allowing normal terminal erythroid proliferation and differentiation. Our data suggest that the interruption of constitutive MEK/ERK signaling is a potential therapeutic strategy to correct impaired erythroid differentiation in patients with myeloid disorders.

Distinct requirements for Ras oncogenesis in human versus mouse cells

The spectrum of tumors associated with oncogenic Ras in humans often differs from those in mice either treated with carcinogens or engineered to sporadically express oncogenic Ras, suggesting that the mechanism of Ras transformation may be different in humans. Ras stimulates primarily three main classes of effector proteins, Rafs, PI3-kinase, and RalGEFs, with Raf generally being the most potent at transforming murine cells. Using oncogenic Ras mutants that activate single effectors as well as constitutively active effectors, we find that the RalGEF, and not the Raf or PI3-kinase pathway, is sufficient for Ras transformation in human cells. Thus, oncogenic Ras may transform murine and human cells by distinct mechanisms, and the RalGEF pathway--previously deemed to play a secondary role in Ras transformation--could represent a new target for anti-cancer therapy.

Calmodulin binds RalA and RalB and is required for the thrombin-induced activation of Ral in human platelets

Ral GTPases may be involved in calcium/calmodulin-mediated intracellular signaling pathways. RalA and RalB are activated by calcium, and RalA binds calmodulin in vitro. It was examined whether RalA can bind calmodulin in vivo, whether RalB can bind calmodulin, and whether calmodulin is functionally involved in Ral activation. Yeast two-hybrid analyses demonstrated both Rals interact directly but differentially with calmodulin. Coimmunoprecipitation experiments determined that calmodulin and RalB form complexes in human platelets. In vitro pull-down experiments in platelets and in vitro binding assays showed endogenous Ral and calmodulin interact in a calcium-dependent manner. Truncated Ral constructs determined in vitro and in vivo that RalA has an additional calmodulin binding domain to that previously described, that although RalB binds calmodulin, its C-terminal region is involved in partially inhibiting this interaction, and that in vitro RalA and RalB have an N-terminal calcium-independent and a C-terminal calcium-dependent calmodulin binding domain. Functionally, in vitro Ral-GTP pull-down experiments determined that calmodulin is required for the thrombin-induced activation of Ral in human platelets. We propose that differential binding of calmodulin by RalA and RalB underlies possible functional differences between the two proteins and that calmodulin is involved in the regulation of the activation of Ral-GTPases.

Interaction of the Rho-ADP-ribosylating C3 exoenzyme with RalA

RhoA, -B, and -C are ADP-ribosylated and biologically inactivated by Clostridium botulinum C3 exoenzyme and related C3-like transferases. We report that RalA GTPase, which is not ADP-ribosylated by C3, inhibits ADP-ribosylation of RhoA by C3 from C. botulinum (C3bot), Clostridium limosum (C3lim), and Bacillus cereus (C3cer) but not from Staphylococcus aureus (C3stau) in human platelet membranes and rat brain lysate. Inhibition by RalA occurs with the GDP- and guanosine 5'-3-O-(thio)triphosphate-bound forms of RalA and is overcome by increasing concentrations of C3. A direct interaction of RalA with C3 was verified by precipitation of the transferase with GST-RalA-Sepharose. The affinity constant (K(d)) of the binding of RalA to C3lim was 12 nm as determined by fluorescence titration. RalA increased the NAD glycohydrolase activity of C3bot by about 5-fold. Although RalA had no effect on glucosylation of Rho GTPases by Clostridium difficile toxin B, C3bot and C3lim inhibited glucosylation of RalA by Clostridium sordellii lethal toxin. Furthermore, C3bot decreased activation of phospholipase D by RalA. The data indicate that several C3 exoenzymes directly interact with RalA without ADP-ribosylating the GTPase. The interaction is of high affinity and interferes with essential functions of C3 and RalA.

The role of Mg2+ cofactor in the guanine nucleotide exchange and GTP hydrolysis reactions of Rho family GTP-binding proteins

The biological activities of Rho family GTPases are controlled by their guanine nucleotide binding states in cells. Here we have investigated the role of Mg(2+) cofactor in the guanine nucleotide binding and hydrolysis processes of the Rho family members, Cdc42, Rac1, and RhoA. Differing from Ras and Rab proteins, which require Mg(2+) for GDP and GTP binding, the Rho GTPases bind the nucleotides in the presence or absence of Mg(2+) similarly, with dissociation constants in the submicromolar concentration. The presence of Mg(2+), however, resulted in a marked decrease in the intrinsic dissociation rates of the nucleotides. The catalytic activity of the guanine nucleotide exchange factors (GEFs) appeared to be negatively regulated by free Mg(2+), and GEF binding to Rho GTPase resulted in a 10-fold decrease in affinity for Mg(2+), suggesting that one role of GEF is to displace bound Mg(2+) from the Rho proteins. The GDP dissociation rates of the GTPases could be further stimulated by GEF upon removal of bound Mg(2+), indicating that the GEF-catalyzed nucleotide exchange involves a Mg(2+)-independent as well as a Mg(2+)-dependent mechanism. Although Mg(2+) is not absolutely required for GTP hydrolysis by the Rho GTPases, the divalent ion apparently participates in the GTPase reaction, since the intrinsic GTP hydrolysis rates were enhanced 4-10-fold upon binding to Mg(2+), and k(cat) values of the Rho GTPase-activating protein (RhoGAP)-catalyzed reactions were significantly increased when Mg(2+) was present. Furthermore, the p50RhoGAP specificity for Cdc42 was lost in the absence of Mg(2+) cofactor. These studies directly demonstrate a role of Mg(2+) in regulating the kinetics of nucleotide binding and hydrolysis and in the GEF- and GAP-catalyzed reactions of Rho family GTPases. The results suggest that GEF facilitates nucleotide exchange by destabilizing both bound nucleotide and Mg(2+), whereas RhoGAP utilizes the Mg(2+) cofactor to achieve high catalytic efficiency and specificity.

Effector recognition by the small GTP-binding proteins Ras and Ral

The Ral effector protein RLIP76 (also called RIP/RalBP1) binds to Ral.GTP via a region that shares no sequence homology with the Ras-binding domains of the Ser/Thr kinase c-Raf-1 and the Ral-specific guanine nucleotide exchange factors. Whereas the Ras-binding domains have a similar ubiquitin-like structure, the Ral-binding domain of RLIP was predicted to comprise a coiled-coil region. In order to obtain more information about the specificity and the structural mode of the interaction between Ral and RLIP, we have performed a sequence space and a mutational analysis. The sequence space analysis of a comprehensive nonredundant assembly of Ras-like proteins strongly indicated that positions 36 and 37 in the core of the effector region are tree-determinant positions for all subfamilies of Ras-like proteins and dictate the specificity of the interaction of these GTPases with their effector proteins. Indeed, we could convert the specific interaction with Ras effectors and RLIP by mutating these residues in Ras and Ral. We therefore conclude that positions 36 and 37 are critical for the discrimination between Ras and Ral effectors and that, despite the absence of sequence homology between the Ral-binding and the Ras-binding domains, their mode of interaction is most probably similar.

Ectopic expression of constitutively activated Ral GTPase inhibits cell shape changes during Drosophila eye development

The small GTP-binding protein Ral is activated by RalGDS, one of the effector molecules for Ras. Active Ral binds to a GTPase activating protein for CDC42 and Rac. Although previous studies suggest a role for Ral in the regulation of CDC42 and Rac, which are involved in arranging the cytoskeleton, its in vivo function is largely unknown. To examine the effect of overexpressing Ral on development, transgenic Drosophila were generated that overexpress wild-type or mutated Ral during eye development. While wild-type Ral caused no developmental defects, expression of a constitutively activated protein resulted in a rough eye phenotype. Activated Ral did not affect cell fate determination in the larval eye discs but caused severe disruption of the ommatidial organization later in pupal development. Phalloidin staining showed that activated Ral perturbed the cytoskeletal structure and cell shape changes during pupal development. This phenotype is similar to that caused by RhoA overexpression. In addition, the phenotype was synergistically enhanced by the coexpression of RhoA. These results suggest that Ral functions to control the cytoskeletal structure required for cell shape changes during Drosophila development.

GEF-mediated GDP/GTP exchange by monomeric GTPases: a regulatory role for Mg2+?

GTPases share highly conserved guanine nucleotide-binding domains and fulfill diverse functions through a common molecular switch. An inactive GDP-bound protein is turned on by a guanine nucleotide exchange factor (GEF) that catalyzes exchange of GTP for GDP, but unfortunately little is known about the mechanism of GEF action. A common mechanism for GDP/GTP exchange can be envisioned wherein GEFs activate monomeric GTPases through transient disruption of Mg2+ coordination in the nucleotide-binding pocket while stabilizing a nucleotide-free (and cation-free) conformation. After guanine nucleotide exchange, Mg2+ coordination is restored to complete the conformational switch to the active GTP-bound state. Evidence in the literature highlighting an important regulatory role for Mg2+ in the mechanism of GEF-mediated GDP/GTP exchange by monomeric GTPases is summarized.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Evidence for a Ras/Ral signaling cascade

It is becoming clear that Ras proteins mediate their diverse biological functions by binding to, and participating in, the activation of multiple downstream targets. Recent work has identified nucleotide-exchange factors for Ral-GTPases as the newest members of the set of putative Ras 'effector molecules'. This new work has also detected two potential downstream targets of Ral proteins, a novel CDC42/Rac GTPase-activating protein and a phospholipase D.

Post-translational modifications of Ras and Ral are important for the action of Ral GDP dissociation stimulator

Ral GDP dissociation stimulator (RalGDS) is a GDP/GTP exchange protein of Ral and a new effector protein of Ras. Therefore, there may be a new signaling pathway from Ras to Ral. In this paper, we examined the roles of the post-translational modifications of Ras and Ral on this new signal transduction pathway. The post-translationally modified form of Ras bound to RalGDS more effectively than the unmodified form. The modification of Ras was required to regulate the distribution of RalGDS between the cytosol and membrane fractions in COS cells. The post-translational modification of Ral enhanced the activities of RalGDS to stimulate the dissociation of GDP from and the binding of GTP to Ral. Furthermore, the modified form of Ral bound to Ral-binding protein 1 (RalBP1), a putative effector protein of Ral, more effectively than the unmodified form. Taken together with the observations that Ras and Ral are localized to the membranes, these results suggest that the post-translational modifications of Ras and Ral play a role for transmitting the signal effectively on the membranes in the signal transduction pathway of Ras/RalGDS/Ral/RalBP1.

A role for the Ral guanine nucleotide dissociation stimulator in mediating Ras-induced transformation

Oncogenic Ras transforms cells through the activation of multiple downstream pathways mediated by separate effector molecules, one of which is Raf. Here we report the identification of a second ras-binding protein that can induce cellular transformation in parallel with activation of the Raf/mitogen-activated protein kinase cascade. The Ral guanine nucleotide dissociation stimulator (RalGDS) was isolated from a screen for Ras-binding proteins that specifically interact with a Ras effector-loop mutant, ras(12V,37G), that uncouples Ras from activation of Raf1. RalGDS, like ras(12V, 37G), cooperates synergistically with mutationally activated Raf to induce foci of growth and morphologically transformed NIH 3T3 cells. RalGDS does not significantly enhance MAP kinase activation by activated Raf, suggesting that the cooperativity in focus formation is due to a distinct pathway acting downstream of Ras and parallel to Raf.

Characterization of human platelet GTPase activating protein for the Ral GTP-binding protein

RalA, a ras p21 related 27 kDa GTP-binding protein, was expressed as a fusion protein in Escherichia coli and purified to homogeneity using an immunoaffinity column. The purified protein was capable of binding and hydrolyzing GTP. Addition of platelet cytosolic or detergent solubilized particulate proteins stimulated the intrinsic GTPase activity of ralA by at least six-fold with maximal effect observed at pH 6.5. Addition of platelet proteins denatured by boiling had no effect on ralA GTPase activity. Analysis of GTPase reaction products by thin layer chromatography demonstrated that in samples containing ralA, 78.5 +/- 6.3% of the radioactivity was recovered in the GTP form while samples containing ralA plus platelet cytosol or particulate proteins, only 7.5 +/- 0.2% and 9.0 +/- 1.4% of the radioactivity was in the GTP form respectively. The GTPase activating protein(s) in the cytosolic and particulate fraction was further characterized by measuring GAP activity in proteins eluted from gel slices after sodium dodecyl sulfate polyacrylamide gel electrophoresis. The ralA GTPase activating protein present in the cytosol and particulate fractions was recovered in a single gel slice of identical apparent molecular weight. The molecular mass of the ral specific GTPase activating protein was estimated to be 34 +/- 2 kDa. This protein did not stimulate the intrinsic GTPase activity of ras p21, G25K/CDC42Hs or rab3A GTP-binding proteins. Results demonstrate that in human platelets, the activity/function of ral-related GTP-binding protein(s) is under the regulation of a specific GTPase activating protein of molecular mass of 34 +/- 2 kDa that is distributed equally in the cytosol and particulate fraction.

Influence of Mg2+ on the structure and function of Rab5

Mg2+ inhibits GDP release from Rab5WT but not from Rab5S34N, a mutant lacking Ser34 critical for Mg2+ coordination in the nucleotide binding pocket. Thus, inhibition of GDP release is apparently exerted via coordination of Mg2+ between Rab5 and GDP. Mg2+ also induces conformational changes in Rab5WT, demonstrated by increased tryptophan fluorescence intensity and a red shift in lambda max for the GDP-bound protein. Mg(2+)-induced fluorescence changes are not observed for Rab5S34N. The correlation between Mg2+ effects on nucleotide exchange and the fluorescence properties of Rab5 suggests that a conformation promoted through Mg2+ coordination with Ser34 also contributes to inhibition of GDP release. The role of structural changes in GDP release was investigated using C- and N-terminal truncation mutants. Similar to Rab5WT, Mg2+ inhibits GDP release and alters the fluorescence of Rab5(1-198) but only partially inhibits release from Rab5(23-198) and fails to induce changes in the latter's fluorescence properties. Since Rab5(23-198) maintains Ser34 necessary for Mg2+ coordination, the lack of Mg(2+)-induced fluorescence changes suggests a requirement for the N-terminal domain to promote a conformation blocking GDP release. A model for mechanisms of interaction between Ras-like proteins and their exchange factors is proposed.

Bridging Ral GTPase to Rho pathways. RLIP76, a Ral effector with CDC42/Rac GTPase-activating protein activity

Ra1A and Ra1B are GTPases of unknown function and are activated by proteins, Ra1GDS, that interact with the active form of another GTPase, Ras. To elucidate Ral function, we have searched for proteins interacting with an activated form of Ra1A using the two-hybrid method and a Jurkat cell library. We have identified a partial cDNA encoding a protein, RLIP1, which binds to activated Ra1A and this binding requires an intact effector domain of Ra1A. Biochemical data with purified Ra1A confirm the genetic results. This protein also bears a region of homology with GTPase-activating protein (GAP) domains that are involved in the regulation of GTPases of the Rho family and, indeed, RLIP1 displays a GAP activity acting upon Rac1 and CDC42, but not RhoA. This GAP region is not required for RLIP1 binding to Ra1. The whole cDNA was cloned, and it encodes a 76-kDa polypeptide, RLIP76, which also binds RalA. The Rho pathway is involved in membrane and cytoskeleton modifications after mitogenic stimulation and acts in parallel to and synergistically with the Ras pathway. We propose that these pathways are linked through a cascade composed of Ras --> Ra1GDS --> Ra1 --> RLIP76 --> CDC42/Rac1/Rho, allowing modulation of the Rho pathway by the Ras pathway.

GTP-blot analysis of small GTP-binding proteins. The C-terminus is involved in renaturation of blotted proteins

Recombinant c-Ha-ras, ralA and rap2, but not rap1A or rap1B proteins retained their ability to bind [alpha-32P]GTP after SDS/PAGE and transfer to nitrocellulose. Recombinant c-Has-ras missing the C-terminal 23 amino acid residues failed to bind [alpha-32P]GTP after the blot, and the ability of recombinant ralA missing the C-terminal 28 amino acid residues to bind [alpha-32P]GTP was decreased many-fold. The presence of nonionic detergents of the polyoxyethylene type such as Tween 20, Triton X-100, Nonidet P40 or Lubrol PX in the incubation buffer was necessary to induce renaturation of blotted recombinant c-Ha-ras protein, whereas other types of detergents were ineffective. We propose that detergents of the polyoxyethylene type induce the refolding of some types of blotted small GTP-binding proteins and that the C-terminus is involved in the refolding process. Membranes from NIH3T3 fibroblasts overexpressing c-Ha-ras protein showed much weaker binding of [alpha-32P]GTP as expected from the level of ras immunoreactivity. Treatment of fibroblasts with lovastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase, caused the accumulation of the unfarnesylated form of c-Ha-ras in the cytosol. Examination of [alpha-32P]GTP-binding and immunoreactivity for cytosolic and membrane-bound c-Ha-ras revealed that binding of [alpha-32P]GTP to unprocessed c-Ha-ras was increased about threefold compared to the same amount of processed c-Ha-ras. Our results demonstrate that detection and quantification of small GTP-binding proteins in eukaryotic cells by GTP-blot analysis is hampered by the fact that these proteins differ strongly in their ability to renature after blotting to nitrocellulose.

Association between GTPase activators for Rho and Ras families

The ras-related low-molecular-mass GTPases participate in signal transduction involving a variety of cellular functions, including cell-cycle progression, cellular differentiation, cytoskeletal organization, protein transport and secretion. The cycling of these proteins between GTP-bound and GDP-bound states is partially controlled by GTPase activating proteins (GAPs) which stimulate the intrinsic GTP-hydrolysing activity of specific GTPases. The ras GTPase-activating protein (Ras-GAP) forms a complex with a second protein, p190 (M(r) 190,000), in growth-factor stimulated and tyrosine-kinase transformed cells. At its carboxy-terminal end, p190 contains a region that is conserved in the breakpoint cluster region, n-chimaerin, and Rho-GAP. Each of these three proteins exhibits GAP activity for at least one member of the rho family of small GTPases. We have tested recombinant p190 protein for GAP activity on GTPases of the ras, rho and rab families, and show here that p190 can function as a GAP specifically for members of the rho family. Consequently, the formation of a complex between Ras-GAP and p190 in growth-factor stimulated cells may allow the coupling of signalling pathways that involve ras and rho GTPases.