The guanine nucleotide-binding switch in three dimensions

Guanine nucleotide-binding proteins regulate a variety of processes, including sensual perception, protein synthesis, various transport processes, and cell growth and differentiation. They act as molecular switches and timers that cycle between inactive guanosine diphosphate (GDP)-bound and active guanosine triphosphate (GTP)-bound states. Recent structural studies show that the switch apparatus itself is a conserved fundamental module but that its regulators and effectors are quite diverse in their structures and modes of interaction. Here we will try to define some underlying principles.

Drosophila rab GDI mutants disrupt development but have normal Rab membrane extraction

Rab GTPases are essential for vesicular transport. Rab GDP dissociation inhibitor (GDI) binds to GDP-bound rabs, removes rabs from acceptor membranes and delivers rabs to donor membranes. We isolated lethal GDI mutations in Drosophila and analyzed their developmental phenotypes. To learn how these mutations affect GDI structure, the crystal structure of Drosophila GDI was determined by molecular replacement to a resolution of 3.0 A. Two hypomorphic, missense mutations are located in domain II of GDI at highly conserved positions, but not in previously identified sequence conserved regions. The mutant GDIs were tested for ability to extract rabs from membranes and showed wild-type levels of rab membrane extraction. The two missense alleles showed intragenic complementation, indicating that domain II of GDI may have two separable functions. This study indicates that GDI function is essential for development of a complex, multicellular organism and that puparium formation and pole cell formation are especially dependent on GDI function.

Mechanism of NADPH oxidase activation by the Rac/Rho-GDI complex

The low molecular weight GTP binding protein Rac is essential to the activation of the NADPH oxidase complex, involved in pathogen killing during phagocytosis. In resting cells, Rac exists as a heterodimeric complex with Rho GDP dissociation inhibitor (Rho-GDI). Two types of interactions exist between Rac and Rho-GDI: a protein-lipid interaction, implicating the polyisoprene of the GTPase, as well as protein-protein interactions. Using the two-hybrid system, we show that nonprenylated Rac1 interacts very weakly with Rho-GDI, pointing to the predominant role of protein-isoprene interaction in complex formation. In the absence of this strong interaction, we demonstrate that three sites of protein-protein interaction, Arg66(Rac)-Leu67(Rac), His103(Rac), and the C-terminal polybasic region Arg183(Rac)-Lys188(Rac), are involved and cooperate in complex formation. When Rac1 mutants are prenylated by expression in insect cells, they all interact with Rho-GDI. Rho-GDI is able to exert an inhibitory effect on the GDP/GTP exchange reaction except in the complex in which Rac1 has a deletion of the polybasic region (Arg183(Rac)-Lys188(Rac)). This complex is, most likely, held together through protein-lipid interaction only. Although able to function as GTPases, the mutants of Rac1 that failed to interact with Rho-GDI also failed to activate the NADPH oxidase in a cell-free assay after loading with GTP. Mutant Leu119(Rac)Gln could both interact with Rho-GDI and activate the NADPH oxidase. The Rac1/Rho-GDI and Rac1(Leu119Gln)/Rho-GDI complexes, in which the GTPases were bound to GDP, were found to activate the oxidase efficiently. These data suggest that Rho-GDI stabilizes Rac in an active conformation, even in the GDP-bound state, and presents it to its effector, the p67phox component of the NADPH oxidase.

Crystal structure of the Rac1-RhoGDI complex involved in nadph oxidase activation

A heterodimer of prenylated Rac1 and Rho GDP dissociation inhibitor was purified and found to be competent in NADPH oxidase activation. Small angle neutron scattering experiments confirmed a 1:1 stoichiometry. The crystal structure of the Rac1-RhoGDI complex was determined at 2.7 A resolution. In this complex in which Rac1 is bound to GDP, the switch I region of Rac1 is in the GDP conformation whereas the switch II region resembles that of a GTP-bound GTPase. Two types of interaction between RhoGTPases and RhoGDI were investigated. The lipid-protein interaction between the geranylgeranyl moiety of Rac1 and RhoGDI resulted in numerous structural changes in the core of RhoGDI. The interactions between Rac1 and RhoGDI occur through hydrogen bonds which involve a number of residues of Rac1, namely, Tyr64(Rac), Arg66(Rac), His103(Rac), and His104(Rac), conserved within the Rho family and localized in the switch II region or in its close neighborhood. Moreover, in the switch II region of Rac1, hydrophobic interactions involving Leu67(Rac) and Leu70(Rac) contribute to the stability of the Rac1-RhoGDI complex. Inhibition of the GDP-GTP exchange in Rac1 upon binding to RhoGDI partly results from interaction of Thr35(Rac) with Asp45(GDI). In the Rac1-RhoGDI complex, the accessibility of the effector loops of Rac1 probably accounts for the ability of the Rac1-RhoGDI complex to activate the NADPH oxidase.

Crystallization and preliminary crystallographic studies of RhoGDI in complex with the radixin FERM domain

The Rho guanine nucleotide-dissociation inhibitor (RhoGDI) is a general regulator that forms a complex with the GDP-bound form of Rho-family GTPases and suppresses their activation. The FERM domains of ERM (ezrin/radixin/moesin) proteins bind to RhoGDI and dissociate Rho from RhoGDI. The formation of a complex between RhoGDI and the FERM domain is an important step in the regulatory cycle of Rho activation. In this study, crystals of RhoGDI complexed with the FERM domain of radixin were obtained. The crystals of the binary complex belong to the space group P2(1)2(1)2, with unit-cell parameters a = 130.9 (2), b = 151.2 (2), c = 71.2 (1) A, and contain two protein complexes in the crystallographic asymmetric unit. A 2.9 A resolution data set was collected using synchrotron radiation at SPring-8.

Interactions between Rho GTPases and Rho GDP dissociation inhibitor (Rho-GDI)

The Rho-GDP dissociation inhibitor (Rho-GDI) was used as bait in a two-hybrid screen of a human leucocyte cDNA library. Most of the isolated cDNAs encoded GTPases of the Rho subfamily: RhoA, B, C, Rac1, 2, CDC42 and RhoG. The newly discovered RhoH interacted very poorly with Rho-GDI. Another protein partner shared a homology with RhoA that points to Asp67(RhoA)-Arg68(RhoA)-Leu69(RhoA) as critical for interaction with Rho-GDI. A second screen with RhoA as bait led to the isolation of GDI only. In order to investigate the relative role of protein-protein and protein-lipid interactions between Rho GTPases and Rho-GDI, CAAX box mutants of RhoA were produced. They were found to interact with Rho-GDI as efficiently as wild type RhoA, indicating that protein-protein interactions alone lead to strong binding of the two proteins. The C-terminal polybasic region of RhoA was also shown to be a site of protein-protein interaction with Rho-GDI.

Protein crystallization by rational mutagenesis of surface residues: Lys to Ala mutations promote crystallization of RhoGDI

Crystallization is a unique process that occurs at the expense of entropy, including the conformational entropy of surface residues, which become ordered in crystal lattices during formation of crystal contacts. It could therefore be argued that epitopes free of amino acids with high conformational entropy are more thermodynamically favorable for crystal formation. For a protein recalcitrant to crystallization, mutation of such surface amino acids to residues with no conformational entropy might lead to enhancement of crystallization. This paper reports the results of experiments with an important cytosolic regulator of GTPases, human RhoGDI, in which lysine residues were systematically mutated to alanines. Single and multiple mutations were introduced into two different variants of RhoGDI, NDelta23 and NDelta66, in which the first 23 and 66 residues, respectively, were removed by recombinant methods. In total, 13 single and multiple mutants were prepared and assessed for crystallization and all were shown to crystallize using the Hampton Research Crystal Screens I and II, in contrast to wild-type NDelta23 and NDelta66 RhoGDI which did not crystallize. Four crystal structures were solved (the triple mutants NDelta23:K135,138,141A and NDelta66:K135,138,141A, and two single mutants NDelta66:K113A and NDelta66:K141A) and in three cases the crystal contacts of the new lattices were found precisely at the sites of mutations. These results support the notion that it is, in principle, possible to rationally design mutations which systematically enhance proteins' ability to crystallize.

Inhibition of GDP/GTP exchange on G alpha subunits by proteins containing G-protein regulatory motifs

A novel Galpha binding consensus sequence, termed G-protein regulatory (GPR) or GoLoco motif, has been identified in a growing number of proteins, which are thought to modulate G-protein signaling. Alternative roles of GPR proteins as nucleotide exchange factors or as GDP dissociation inhibitors for Galpha have been proposed. We investigated the modulation of the GDP/GTP exchange of Gialpha(1), Goalpha, and Gsalpha by three proteins containing GPR motifs (GPR proteins), LGN-585-642, Pcp2, and RapIGAPII-23-131, to elucidate the mechanisms of GPR protein function. The GPR proteins displayed similar patterns of interaction with Gialpha(1) with the following order of affinities: Gialpha(1)GDP >> Gialpha(1)GDPAlF(4)(-) > or = Gialpha(1)GTPgammaS. No detectable binding of the GPR proteins to Gsalpha was observed. LGN-585-642, Pcp2, and RapIGAPII-23-131 inhibited the rates of spontaneous GTPgammaS binding and blocked GDP release from Gialpha(1) and Goalpha. The inhibitory effects of the GPR proteins on Gialpha(1) were significantly more potent, indicating that Gi might be a preferred target for these modulators. Our results suggest that GPR proteins are potent GDP dissociation inhibitors for Gialpha-like Galpha subunits in vitro, and in this capacity they may inhibit GPCR/Gi protein signaling in vivo.

Structural consequences of site-directed mutagenesis in flexible protein domains: NMR characterization of the L(55,56)S mutant of RhoGDI

The guanine dissociation inhibitor RhoGDI consists of a folded C-terminal domain and a highly flexible N-terminal region, both of which are essential for biological activity, that is, inhibition of GDP dissociation from Rho GTPases, and regulation of their partitioning between membrane and cytosol. It was shown previously that the double mutation L55S/L56S in the flexible region of RhoGDI drastically decreases its affinity for Rac1. In the present work we study the effect of this double mutation on the conformational and dynamic properties of RhoGDI, and describe the weak interaction of the mutant with Rac1 using chemical shift mapping. We show that the helical content of the region 45-56 of RhoGDI is greatly reduced upon mutation, thus increasing the entropic penalty for the immobilization of the helix, and contributing to the loss of binding. In contrast to wild-type RhoGDI, no interaction with Rac1 could be identified for amino-acid residues of the flexible domain of the mutant RhoGDI and only very weak binding was observed for the folded domain of the mutant. The origins of the effect of the L55S/L56S mutation on the binding constant (decreased by at least three orders of magnitude relative to wild-type) are discussed with particular reference to the flexibility of this part of the protein.

GDP dissociation inhibitor domain II required for Rab GTPase recycling

Rab GTPases are localized to distinct subsets of organelles within the cell, where they regulate SNARE-mediated membrane trafficking between organelles. One factor required for Rab localization and function is Rab GDP dissociation inhibitor (GDI), which is proposed to recycle Rab after vesicle fusion by extracting Rab from the membrane and loading Rab onto newly formed transport intermediates. GDI is composed of two domains; Rab binding is mediated by Domain I, and the function of Domain II is not known. In this study, Domain II of yeast GDI, encoded by the essential GDI1/SEC19 gene, was targeted in a genetic screen to obtain mutants that might lend insight into the function of this domain. In one gdi1 mutant, the cytosolic pools of all Rabs tested were depleted, and Rab accumulated on membranes, suggesting that this mutant Gdi1 protein has a general defect in extraction of Rab from membranes. In a second gdi1 mutant, the endosomal/vacuolar Rabs Vps21/Ypt51p and Ypt7p accumulated in the cytosol bound to Gdi1p, but localization of Ypt1p and Sec4p were not significantly affected. Using an in vitro assay which reconstitutes Gdi1p-mediated membrane loading of Rab, this mutant Gdi1p was found to be defective in loading of Vps21p but not Ypt1p. Loading of Vps21p by loading-defective Gdi1p was restored when acceptor membranes prepared from a deletion strain lacking Vps21p were used. These results suggest that membrane-associated Rab may regulate recruitment of GDI-Rab from the cytosol, possibly by regulating a GDI-Rab receptor. We conclude that Domain II of Gdi1p is essential for Rab loading and Rab extraction, and confirm that each of these activities is required for Gdi1p function in vivo.

Structure-activity relationships in flexible protein domains: regulation of rho GTPases by RhoGDI and D4 GDI

The guanine dissociation inhibitors RhoGDI and D4GDI inhibit guanosine 5'-diphosphate dissociation from Rho GTPases, keeping these small GTPases in an inactive state. The GDIs are made up of two domains: a flexible N-terminal domain of about 70 amino acid residues and a folded 134-residue C-terminal domain. Here, we characterize the conformation of the N-terminal regions of both RhoGDI and D4GDI using a series of NMR experiments which include (15)N relaxation and amide solvent accessibility measurements. In each protein, two regions with tendencies to form helices are identified: residues 36 to 58 and 9 to 20 in RhoGDI, and residues 36 to 57 and 20 to 25 in D4GDI. To examine the functional roles of the N-terminal domain of RhoGDI, in vitro and in vivo functional assays have been carried out with N-terminally truncated proteins. These studies show that the first 30 amino acid residues are not required for inhibition of GDP dissociation but appear to be important for GTP hydrolysis, whilst removal of the first 41 residues completely abolish the ability of RhoGDI to inhibit GDP dissociation. The combination of structural and functional studies allows us to explain why RhoGDI and D4GDI are able to interact in similar ways with the guanosine 5'-diphosphate-bound GTPase, but differ in their ability to regulate GTP-bound forms; these functional differences are attributed to the conformational differences of the N-terminal domains of the guanosine 5'-diphosphate dissociation inhibitors. Therefore, the two transient helices, appear to be associated with different biological effects of RhoGDI, providing a clear example of structure-activity relationships in a flexible protein domain.

Molecular structures of proteins involved in vesicle fusion

We present a summary of the structures of 13 proteins involved in the docking and fusion of intracellular transport vesicles to their target membranes.

Cloning and characterization of bovine low molecular weight GTPases (Rac1 and Rac2) and rho GDP-dissociation inhibitor 2 (D4-GDI)

GTPases of the Rho family play important roles in human leukocyte signal transduction pathways; however, little is known about the function of these proteins in bovine cells. In the present studies, we isolated molecular clones of bovine Rac1, Rac2, and the Rac/Rho GTPase regulatory protein D4-GDP dissociation inhibitor (D4-GDI) from a bovine bone marrow cDNA library. These clones contained complete open reading frames, encoding 192, 192, and 200 amino acids, respectively. Comparison of the bovine amino acid sequences with those of other species demonstrated a high degree of identity of these proteins across all species, suggesting that these proteins likely play conserved functional roles in bovine leukocyte signal transduction pathways. Comparative Western blotting of these proteins in human and bovine neutrophil cytosol demonstrated that Rac2 was the predominant Rac species and that D4-GDI was the predominant GDI species in bovine neutrophil cytosol. Despite the high degree of homology between human and bovine Rac2, some of the anti-peptide antibody probes prepared against human Rac2 failed to recognize the bovine homologue. We also showed by subcellular fractionation techniques that Rac2 is localized primarily to the cytosolic compartment of resting bovine neutrophils, but is translocated to the plasma membrane after stimulation with PMA. These findings suggest that Rac2 does play a role in bovine neutrophil activation. In addition, these data will be helpful in developing more specific probes for investigating the role of these proteins in bovine leukocyte signal transduction pathways and for studying various inflammatory diseases in cattle.

A new functional domain of guanine nucleotide dissociation inhibitor (alpha-GDI) involved in Rab recycling

Guanine nucleotide dissociation inhibitor (GDI) is a 55-kDa protein that functions in vesicular membrane transport to recycle Rab GTPases. We have now determined the crystal structure of bovine alpha-GDI at ultra-high resolution (1.04 A). Refinement at this resolution highlighted a region with high mobility of its main-chain residues. This corresponded to a surface loop in the primarily alpha-helical domain II at the base of alpha-GDI containing the previously uncharacterized sequence-conserved region (SCR) 3A. Site-directed mutagenesis showed that this mobile loop plays a crucial role in binding of GDI to membranes and extraction of membrane-bound Rab. This domain, referred to as the mobile effector loop, in combination with Rab-binding residues found in the multi-sheet domain I at the apex of alpha-GDI may provide flexibility for recycling of diverse Rab GTPases. We propose that conserved residues in domains I and II synergize to form the functional face of GDI, and that domain II mediates a critical step in Rab recycling during vesicle fusion.

Vav, a GDP/GTP nucleotide exchange factor, interacts with GDIs, proteins that inhibit GDP/GTP dissociation

Vav functions as a specific GDP/GTP nucleotide exchange factor which is regulated by tyrosine phosphorylation in the hematopoietic system. Loss of the amino-terminus sequences of Vav was sufficient to control its transforming potential and its function in T cells. We report here the identification of the hematopoietic GDP dissociation inhibitor protein, Ly-GDI, as a protein that interacts with the amino-terminus of Vav. Further analysis confirmed that Vav and Ly-GDI interact both in in vitro and in in vivo assays. This association is maximal only when the amino region of Vav is intact and requires an intact carboxy-terminus of Ly-GDI. The interaction between Vav and Ly-GDI is not dependent on the tyrosine phosphorylation status of Vav. In addition, Rho-GDI, the highly homologous protein to Ly-GDI, associates with Vav as well. The contribution of the interaction between Vav and GDIs, proteins that are involved in the GDP/GTP exchange processes, to the biological function of Vav is further discussed.

Structure of the Rho family GTP-binding protein Cdc42 in complex with the multifunctional regulator RhoGDI

The RhoGDI proteins serve as key multifunctional regulators of Rho family GTP-binding proteins. The 2.6 A X-ray crystallographic structure of the Cdc42/RhoGDI complex reveals two important sites of interaction between GDI and Cdc42. First, the amino-terminal regulatory arm of the GDI binds to the switch I and II domains of Cdc42 leading to the inhibition of both GDP dissociation and GTP hydrolysis. Second, the geranylgeranyl moiety of Cdc42 inserts into a hydrophobic pocket within the immunoglobulin-like domain of the GDI molecule leading to membrane release. The structural data demonstrate how GDIs serve as negative regulators of small GTP-binding proteins and how the isoprenoid moiety is utilized in this critical regulatory interaction.

Human RhoA/RhoGDI complex expressed in yeast: GTP exchange is sufficient for translocation of RhoA to liposomes

The human small GTPase, RhoA, expressed in Saccharomyces cerevisiae is post-translationally processed and, when co-expressed with its cytosolic inhibitory protein, RhoGDI, spontaneously forms a heterodimer in vivo. The RhoA/RhoGDI complex, purified to greater than 98% at high yield from the yeast cytosolic fraction, could be stoichiometrically ADP-ribosylated by Clostridium botulinum C3 exoenzyme, contained stoichiometric GDP, and could be nucleotide exchanged fully with [3H]GDP or partially with GTP in the presence of submicromolar Mg2+. The GTP-RhoA/RhoGDI complex hydrolyzed GTP with a rate constant of 4.5 X 10(-5) s(-1), considerably slower than free RhoA. Hydrolysis followed pseudo-first-order kinetics indicating that the RhoA hydrolyzing GTP was RhoGDI associated. The constitutively active G14V-RhoA mutant expressed as a complex with RhoGDI and purified without added nucleotide also bound stoichiometric guanine nucleotide: 95% contained GDP and 5% GTP. Microinjection of the GTP-bound G14V-RhoA/RhoGDI complex (but not the GDP form) into serum-starved Swiss 3T3 cells elicited formation of stress fibers and focal adhesions. In vitro, GTP-bound-RhoA spontaneously translocated from its complex with RhoGDI to liposomes, whereas GDP-RhoA did not. These results show that GTP-triggered translocation of RhoA from RhoGDI to a membrane, where it carries out its signaling function, is an intrinsic property of the RhoA/RhoGDI complex that does not require other protein factors or membrane receptors.

The Rac-RhoGDI complex and the structural basis for the regulation of Rho proteins by RhoGDI

Rho family-specific guanine nucleotide dissociation inhibitors (RhoGDIs) decrease the rate of nucleotide dissociation and release Rho proteins such as RhoA, Rac and Cdc42 from membranes, forming tight complexes that shuttle between cytosol and membrane compartments. We have solved the crystal structure of a complex between the RhoGDI homolog LyGDI and GDP-bound Rac2, which are abundant in leukocytes, representing the cytosolic, resting pool of Rho species to be activated by extracellular signals. The N-terminal domain of LyGDI (LyN), which has been reported to be flexible in isolated RhoGDIs, becomes ordered upon complex formation and contributes more than 60% to the interface area. The structure is consistent with the C-terminus of Rac2 binding to a hydrophobic cavity previously proposed as isoprenyl binding site. An inner segment of LyN forms a helical hairpin that contacts mainly the switch regions of Rac2. The architecture of the complex interface suggests a mechanism for the inhibition of guanine nucleotide dissociation that is based on the stabilization of the magnesium (Mg2+) ion in the nucleotide binding pocket.

The specific binding of small molecule isoprenoids to rhoGDP dissociation inhibitor (rhoGDI)

The activities of small G-proteins are in part regulated by their interactions with GDI proteins. This binding is thought to be dependent on the C-terminal isoprenoid modification (geranylgeranyl or farnesyl) of these proteins. G-proteins are generally isoprenylated/methylated at their C-terminal cysteine residues. A quantitative fluorescence assay is reported here to evaluate the specificity of binding of rhoGDI. A rhodamine-labeled geranylgeranylated/methylated cysteine derivative is used to measure its binding to rhoGDI. Saturable binding in the low micromolar range is found with various geranylgeranylated/farnesylated analogues. Interestingly, the carboxymethylated derivatives bound significantly better than their free acid counterparts, suggesting that the state of methylation of the analogues is important for binding. The binding is also selective with respect to isoprenoid. Analogues containing hydrophobic modifications other than geranylgeranyl or farnesyl do not bind with significant affinities. These data demonstrate a substantial degree of specificity in the binding of isoprenoids to a protein important in signal transduction.

Mapping the binding site for the GTP-binding protein Rac-1 on its inhibitor RhoGDI-1

BACKGROUND

Members of the Rho family of small GTP-binding proteins, such as Rho, Rac and Cdc42, have a role in a wide range of cell responses. These proteins function as molecular switches by virtue of a conformational change between the GTP-bound (active) and GDP-bound (inactive) forms. In addition, most members of the Rho and Rac subfamilies cycle between the cytosol and membrane. The cytosolic guanine nucleotide dissociation inhibitors, RhoGDIs, regulate both the GDP/GTP exchange cycle and the membrane association/dissociation cycle.

RESULTS

We have used NMR spectroscopy and site-directed mutagenesis to identify the regions of human RhoGDI-1 that are involved in binding Rac-1. The results emphasise the importance of the flexible regions of both proteins in the interaction. At least one specific region (residues 46-57) of the flexible N-terminal domain of RhoGDI, which has a tendency to form an amphipathic helix in the free protein, makes a major contribution to the binding energy of the complex. In addition, the primary site of Rac-1 binding on the folded domain of RhoGDI involves the beta4-beta5 and beta6-beta7 loops, with a slight movement of the 3(10) helix accompanying the interaction. This binding site is on the same face of the protein as the binding site for the isoprenyl group of post-translationally modified Rac-1, but is distinct from this site.

CONCLUSIONS

Isoprenylated Rac-1 appears to interact with three distinct sites on RhoGDI. The isoprenyl group attached to the C terminus of Rac-1 binds in a pocket in the folded domain of RhoGDI. This is distinct from the major site on this domain occupied by Rac-1 itself, which involves two loops at the opposite end to the isoprenyl-binding site. It is probable that the flexible C-terminal region of Rac-1 extends from the site at which Rac-1 contacts the folded domain of RhoGDI to allow the isoprenyl group to bind in the pocket at the other end of the RhoGDI molecule. Finally, the flexible N terminus of RhoGDI-1, and particularly residues 48-58, makes a specific interaction with Rac-1 which contributes substantially to the binding affinity.

Molecular characterization of the Caenorhabditis elegans Rho GDP-dissociation inhibitor

GDP-dissociation inhibitors (GDIs) form one of the classes of regulatory proteins that modulate the cycling of the Ras superfamily of GTPases between active GTP-bound and inactive GDP-bound states. We report here the characterization of the Caenorhabditis elegans RhoGDI (CeRhoGDI) as part of our investigations into Rho-GTPase signalling pathways that are involved in nematode development. CeRhoGDI is a 23-kDa protein that is localized predominantly in the cytosol. CeRhoGDI interacts only with the lipid-modified forms of C. elegans Rho-GTPases, CeRhoA, CeRac1 and Cdc42Ce, in vitro and is able to solubilize the membrane-bound forms of these GTPases. CeRhoGDI recognizes the GTPases in both GTP- and GDP-bound forms; hence it inhibits both the guanine-nucleotide dissociation and GTP-hydrolysis activities. The inhibitory activity towards the GTP-bound GTPases is weak compared with that towards GDP-bound GTPases. CeRhoGDI is expressed throughout development and is highly expressed in marginal and vulval epithelial cells, in sperm cells and spicules. Taken together, our results suggest that CeRhoGDI may be involved in specific morphogenetic events mediated by the C. elegans Rho-GTPases.