Rab proteins form in vivo complexes with two isoforms of the GDP-dissociation inhibitor protein (GDI)

GTPases of the Rab family play a key role in the regulation of vesicular transport in eukaryotic cells. Several accessory proteins that regulate their GDP/GTP cycle as well as their subcellular localization have been identified within the past few years. The best known is Rab3A GDP dissociation inhibitor protein (GDI), originally identified as an inhibitor of GDP dissociation from Rab3A, a Rab protein specifically expressed in neuronal and neuroendocrine cells. Recent studies have pointed out a role of Rab3A GDI as a chaperone of several Rab proteins during their cycling between cytosol and membranes and Rab3A GDI has been considered so far as a general regulator of Rab function. However, cDNAs encoding potential isoforms of this protein, called GDI beta and GDI-2, have been recently isolated. In this study, we have characterized cytosolic Rab protein complexes in various cell types and tissues using Mono Q chromatography. We show that in rat brain and in insulin-secreting RINm5F cells, the majority of Rab proteins are complexed with Rab3A GDI. In contrast, in Chinese hamster ovary cells, they are mainly complexed to a protein that we have identified as GDI beta. In rat liver cytosol, Rab proteins form complexes with both isoforms. We also show that the proportion of Rab proteins complexed with either isoform depends on the relative abundance of Rab3A GDI and GDI beta in the cytosol. These findings suggest that GDI isoforms are either redundant or could be involved in the fine control of Rab function.

Crystallization and preliminary crystallographic data for Rab guanine nucleotide dissociation inhibitor (RabGDI) from bovine brain

X-ray quality crystals of Rab guanine nucleotide dissociation inhibitor (RabGDI) from bovine brain expressed in Escherichia coli have been obtained from 1.73 M ammonium sulfate. The crystals are prismatic long rods and belong to the monoclinic space group P21 with approximate cell dimensions a = 91.9 A, b = 43.5 A, c = 63.2 A, beta = 104.5 degrees and one molecule per asymmetric unit. The crystals are stable in the X-ray beam and diffract to at least 2.3 A. Reverse screening, streak seeding and macroseeding methods were used to obtain and improve the crystals.

Role of bound GDP in the stability of the rho A-rho GDI complex purified from neutrophil cytosol

The rho A-rho GDI complex purified from bovine neutrophil cytosol was found to contain GDP as the only bound nucleotide at a ratio of 1 mol of GDP per mol of complex. The rho GDI component of the complex (pI 4.8-5.0, apparent molecular mass 28-29 kDa) and the rho A component (pI scattered between 5.0-6.2, apparent molecular mass 24 kDa) were resolved by 2D gel electrophoresis. Upon dephosphorylation of bound GDP by apyrase, the rho A component of the complex was prone to proteolytic cleavage. The integrity of rho A in the presence of apyrase was preserved by addition of excess GTP. These data suggest that rho A liganded by GDP in the rho A-rho GDI complex is maintained in a conformation that escapes action of proteases.

Rab escort protein-1 is a multifunctional protein that accompanies newly prenylated rab proteins to their target membranes

Rab proteins comprise a family of small GTPases that serve a regulatory role in vesicular membrane traffic. Geranylgeranylation of these proteins on C-terminal cysteine motifs is crucial for their membrane association and function. This post-translational modification is catalysed by rab geranylgeranyl transferase (Rab-GGTase), a multisubunit enzyme consisting of a catalytic heterodimer and an accessory component, named rab escort protein (REP)-1. Previous in vitro studies have suggested that REP-1 presents newly synthesized rab proteins to the catalytic component of the enzyme, and forms a stable complex with the prenylated proteins following the transfer reaction. According to this model, a cellular factor would be required to dissociate the rab protein from REP-1 and to allow it to recycle in the prenylation reaction. RabGDP dissociation inhibitor (RabGDI) was considered an ideal candidate for this role, given its established function in mediating membrane association of prenylated rab proteins. Here we demonstrate that dissociation from REP-1 and binding of rab proteins to the membrane do not require RabGDI or other cytosolic factors. The mechanism of REP-1-mediated membrane association of rab5 appears to be very similar to that mediated by RabGDI. Furthermore, REP-1 and RabGDI share several other functional properties, the ability to inhibit the release of GDP and to remove rab proteins from membranes; however, RabGDI cannot assist in the prenylation reaction. These data suggest that REP-1 is per se sufficient to chaperone newly prenylated rab proteins to their target membranes.

Activation of rat liver phospholipase D by the small GTP-binding protein RhoA

Stimulation of phospholipase D by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) in rat liver plasma membranes indicates the involvement of GTP-binding proteins. We used RhoGDI, an inhibitor of GDP dissociation from small GTP-binding proteins of the Rho family, to determine the involvement of these proteins. Incubation, and subsequent washing, of plasma membranes with RhoGDI dose-dependently diminished GTP gamma S-stimulated phospholipase D activity, as determined by accumulation of phosphatidylethanol in the presence of ethanol. Incubation with RhoGDI also caused a rapid and dose-dependent appearance of RhoA in the wash, which was associated with the inhibition of phospholipase D. RhoGDI also rapidly extracted Cdc42 from membranes, but Rac1 was not extracted. Full reconstitution of GTP gamma S-stimulated phospholipase D in RhoGDI-washed membranes was achieved with recombinant RhoA. There was partial reconstitution with Rac1 and no enhancement with Cdc42 or ADP-ribosylation factor. The response to RhoA was dose-dependent (EC50 = 0.5 microM). ADP-ribosylation of RhoA by Clostridium botulinum C3 exoenzyme did not affect its ability to recover GTP gamma S-stimulated phospholipase D activity in RhoGDI-washed membranes. These findings support a role for GTP-binding proteins of the Rho family in the activation of membrane-associated phospholipase D and implicate RhoA as the major protein involved.

The yeast protein Mrs6p, a homologue of the rabGDI and human choroideraemia proteins, affects cytoplasmic and mitochondrial functions

MRS6 is a newly-identified gene in the yeast Saccharomyces cerevisiae. Its product Mrs6p shows significant homology to the mammalian GDP dissociation inhibitor (GDI) of Rab/Ypt-type small G proteins and to the human choroideraemia protein (CHM), the component A of Rab-specific GGTase II. The interaction of Mrs6p with G proteins is indicated by our observation that the MRS6 gene suppresses the effect of a temperature-sensitive ypt1 mutation. Disruption of the MRS6 gene is lethal to haploid yeast cells. This is consistent with the notion that Mrs6p is interacting with Rab/Ypt-type small G proteins, which are known to have essential functions in vesicular transport. Unexpectedly, the MRS6 gene product also affects mitochondrial functions as revealed by the facts that high-copy numbers of MRS6 (1) suppress the pet- phenotype of mrs2-1 mutant strains and (2) cause a weak pet- phenotype in wild-type strains. We conclude from these results that the MRS6 gene product has a vital function in connection with Rab/Ypt-type proteins in the cytoplasm and, in addition, affects mitochondrial functions.

Evolutionary conservation and genomic organization of XAP-4, an Xq28 located gene coding for a human rab GDP-dissociation inhibitor (GDI)

After the development of efficient methods for the construction of transcription maps of defined genomic regions, the rate-limiting step in the analysis of the coding potentials of these regions is the elucidation of function of the novel genes and the examination of their possible involvement in hereditary diseases localized to the region. This can be greatly facilitated by the detection of sequence homology to a gene of known function. XAP-4 is one of the genes identified in the G6PD region of the human Xq28 by direct cDNA selection. The rapid assembly of this gene and the determination of its function was possible because of its sequence homology with the bovine smg p25A/rab3A GDP dissociation inhibitor (GDI). Sequence comparison with other GDIs in the databases has revealed that XAP-4 belongs to one of at least two distinct classes of mammalian rab GDIs. The rab GDIs, which play an important role in the regulation of cellular transport, are highly evolutionarily conserved, as are several other genes identified in the neighborhood of XAP-4. This genomic region is very gene dense, and all the cDNA clones from the approximately 2.5-kb-long transcript of XAP-4 map to a single 7.5-kb genomic EcoRI fragment. The genomic organization of XAP-4 has been examined to determine the distribution of the exonic sequences within this short segment of genomic DNA. It was found that, similar to several other genes from the region, XAP-4 is split into exons of average size, which are interrupted by very short introns.

Differential intracellular localizations of GDP dissociation inhibitor isoforms. Insulin-dependent redistribution of GDP dissociation inhibitor-2 in 3T3-L1 adipocytes

Insulin action on fat cell/skeletal muscle glucose transporter (GLUT4) redistribution to cell surface membranes appears to involve small GTP-binding proteins. It has been recently recognized that at least two GDP dissociation inhibitor (GDI) isoforms, GDI-1 and GDI-2, can bind and release GDP-bound Rab proteins from membranes (Shisheva, A., Südhof, T.C., and Czech, M. P. (1994) Mol. Cell. Biol. 14, 3459-3468). The present studies show that a strikingly higher level of GDI-2 fractionates with total membranes of COS-1 cells, Chinese hamster ovary cells, and 3T3-L1 adipocytes compared to GDI-1, which is virtually totally cytosolic. In 3T3-L1 adipocytes, most of the membrane-bound GDI-2 was present in a low density, intracellular membrane fraction. Immunodepletion of GLUT4-enriched vesicles from this membrane fraction also depleted significant amounts of GDI-2 proteins. Localization of both GDI-2 and GLUT4 in the same perinuclear regions of these cells was established by immunofluorescence microscopy, whereas GDI-1 displayed a diffuse, cytoplasmic distribution. Insulin acutely decreased both GLUT4 and GDI-2 protein levels in the low density microsomes by about 50%. Concomitantly, GLUT4 but not GDI-2 protein content of plasma membranes increased, suggesting release of GDI-2 into the cytoplasm in response to insulin. Taken together, these data suggest functional differences for the GDI-1 and GDI-2 protein isoforms, as well as a potential role of GDI-2 in the action of insulin on membrane movements.

Guanine nucleotide dissociation inhibitor is essential for Rab1 function in budding from the endoplasmic reticulum and transport through the Golgi stack

The small GTPase Rab1 is required for vesicular traffic from the ER to the cis-Golgi compartment, and for transport between the cis and medial compartments of the Golgi stack. In the present study, we examine the role of guanine nucleotide dissociation inhibitor (GDI) in regulating the function of Rab1 in the transport of vesicular stomatitis virus glycoprotein (VSV-G) in vitro. Incubation in the presence of excess GDI rapidly (t1/2 < 30 s) extracted Rab1 from membranes, inhibiting vesicle budding from the ER and sequential transport between the cis-, medial-, and trans-Golgi cisternae. These results demonstrate a direct role for GDI in the recycling of Rab proteins. Analysis of rat liver cytosol by gel filtration revealed that a major pool of Rab1 fractionates with a molecular mass of approximately 80 kD in the form of a GDI-Rab1 complex. When the GDI-Rab1 complex was depleted from cytosol by use of a Rab1-specific antibody, VSV-G failed to exit the ER. However, supplementation of depleted cytosol with a GDI-Rab1 complex prepared in vitro from recombinant forms of Rab1 and GDI efficiently restored export from the ER, and transport through the Golgi stack. These results provide evidence that a cytosolic GDI-Rab1 complex is required for the formation of non-clathrin-coated vesicles mediating transport through the secretory pathway.

Comparison of kinetic properties between MSS4 and Rab3A GRF GDP/GTP exchange proteins

The kinetic properties of MSS4 are studied in comparison with those of Rab3A GRF. MSS4 stimulates the dissociation of [3H]GDP from the lipid-modified and lipid-unmodified forms of Rab3A to the same extent, although Rab3A GRF is more effective on the lipid-modified form than on the lipid-unmodified form. Both MSS4 and Rab3A GRF are inactive on other Rab/Sec/Ypt family members including at least Rab2, Rab5, and Rab11. Rab GDI inhibits the MSS4 and Rab3A GRF effects on the lipid-modified form of Rab3A, but the doses of Rab GDI necessary for this inhibitory effect on Rab3A GRF are lower than those on MSS4. Moreover, Rab GDI slightly inhibits the Rab3A GRF effect on the lipid-unmodified form of Rab3A, but does not affect the MSS4 effect on the lipid-unmodified form of Rab3A. These results suggest that MSS4 and Rab3A GRF are different GDP/GTP exchange proteins for Rab3A.

Molecular cloning and characterization of yeast rho GDP dissociation inhibitor

We have previously isolated rho GDP dissociation inhibitor (rho DGI) from bovine brain and characterized it. Bovine rho GDI is a protein of a M(r) of 23,421 with 204 amino acids. rho GDI inhibits the GDP/GTP exchange reaction of post-translationally lipid-modified small GTP-binding proteins (G proteins) of the rho family, including the rho, rac, and cdc42 subfamilies, and keeps them in the GDP-bound inactive form. In the present study, we first purified rho GDI from the cytosol fraction of the yeast Saccharomyces cerevisiae and isolated its gene. Yeast rho GDI gene had an open reading frame without introns encoding a protein of a M(r) of 23,138 with 202 amino acids. Yeast rho GDI protein was 36% identical with bovine rho GDI. Yeast rho GDI expressed in Escherichia coli was active not only on yeast rho1 but also on mammalian rho family members which were post-translationally modified. Disruption of rho GDI did not induce apparent phenotypes, whereas overexpression of yeast or bovine rho GDI resulted in the inhibition of cell growth. These results indicate that rho GDI exists and regulates the function of the rho family members in yeast.

Tissue-specific variations in the expression and regulation of the small GTP-binding protein Rho

Rho proteins are involved in the regulation of the assembly of the microfilamental cellular network and are known to be specific substrates for the ADP-ribosyltransferase C3 from Clostridium botulinum. Here, we studied the distribution of Rho and Rho-regulating proteins in extracts from various rabbit tissues. The highest amounts of [32P]ADP-ribosylated proteins were detected in cell extracts from lung and kidney. Compared to these tissues, 50-95% reduced labeling of Rho proteins was observed in extracts from liver, spleen, brain, heart and muscle. The level of the C3-mediated [32P]ADP-ribosylation of Rho did not correlate with the amount of RhoA proteins detected by Western analysis. The relative amounts of [32P]ADP-ribosylated proteins located in cytosolic or membrane fractions, respectively, depended on the type of tissue investigated, indicating a tissue-specific variation in the subcellular distribution of Rho proteins. The same was true for the complexation of Rho with other factors and the expression of diverse Rho species. In respect to Rho-regulating proteins, extracts from lung and brain contained the highest amounts of guanine nucleotide dissociation-inhibitor proteins (Rho-GDI). The association of Rho with Rho-GDI however showed tissue specificity and did not correlate with Rho-GDI amounts. The highest Rho-GAP (GAP = GTPase-activating protein) activities were observed in extracts from lung, kidney and spleen, the lowest ones in extracts from muscle and heart. In total, our data demonstrate tissue-specific differences in the expression of RhoA, [32P]ADP-ribosylated proteins and Rho-regulating factors, indicating a tissue-specific variation in the activity and regulation of Rho proteins.

Rab-GDI presents functional Rab9 to the intracellular transport machinery and contributes selectivity to Rab9 membrane recruitment

Rab proteins occur in the cytosol bound to Rab-GDP dissociation inhibitor (GDI). We demonstrate here that cytosolic complexes of Rab9 bound to GDI represent a functional pool of Rab9 protein that can be utilized for transport from late endosomes to the trans Golgi network in vitro. Immunodepletion of GDI and Rab proteins bound to GDI led to the loss of cytosol activity; readdition of pure Rab9-GDI complexes fully restored cytosol activity. Delipidated serum albumin could solubilize prenylated Rab9 protein, but unlike Rab9-GDI complexes, Rab9-serum albumin complexes led to indiscriminate membrane association of Rab9 protein. Rab9 delivered to membranes by serum albumin was functional, but GDI increased the efficiency of Rab9 utilization, presumably because it suppressed Rab9 protein mistargeting. Finally, GDI inhibited transport of proteins from late endosomes to the trans Golgi network, likely because of its capacity to inhibit the membrane recruitment of cytosolic Rab9. These experiments show that GDI contributes to the selectivity of Rab9 membrane recruitment and presents functional Rab9 to the endosome-trans Golgi network transport machinery.

ADP-ribosylation of Rho proteins by Clostridium botulinum exoenzyme C3 is influenced by phosphorylation of Rho-associated factors

Specific [32P]ADP-ribosylation by Clostridium botulinum exoenzyme C3 was used to study the involvement of phosphorylation in the regulation of the low-molecular-mass GTP-binding protein Rho. Dephosphorylation of CHO cell extracts by alkaline phosphatase treatment resulted in a 80-90% reduction in the C3-catalysed [32P]ADP-ribosylation of Rho proteins in both cytosolic and membrane fractions. Similar results were obtained after dephosphorylation with protein phosphatase type-1 from bovine retina, whereas type-2B and type-2C phosphatases had no effect on the level of subsequent [32P]ADP-ribosylation of Rho by C3. Incubation of CHO cell lysate under phosphorylation conditions increased the subsequent C3-mediated [32P]ADP-ribosylation of Rho proteins. The protein kinase inhibitors H7 and H9 had no effect on [32P]ADP-ribosylation at concentrations which are specific for inhibition of protein kinase A or C. Recombinant glutathione S-transferase-RhoA fusion protein (GST-RhoA) was phosphorylated by protein kinase A; however, the phosphorylation had no stimulatory effect on the ADP-ribosylation of GST-RhoA by C3. An approx. 48 kDa phosphoprotein was identified which bound specifically to recombinant GST-RhoA fusion protein. By gel-permeation chromatography, Rho-containing complexes of approx. 50 kDa and 130-170 kDa were detected. The ADP-ribosylation of Rho in the 130-170 kDa complex was reduced by alkaline phosphatase pretreatment. The data suggest that Rho activity is influenced by phosphorylation of Rho-associated regulatory factors. Phosphorylation/dephosphorylation of these Rho-regulating factors appears to alter the ability of Rho to serve as a substrate for C3-induced [32P]ADP-ribosylation.

Molecular cloning and characterization of two rab GDI species from rat brain: brain-specific and ubiquitous types

Rab GDP dissociation inhibitor (GDI) is a regulatory protein for Rab proteins that controls not only the GDP/GTP exchange reaction of Rab proteins but also their translocation between the cytosol and cell membranes. Recently, rab GDI cDNAs have been isolated from human, bovine, rat, and Drosophila, and these Rab GDI proteins indicated an important role in the regulation of intracellular vesicle traffic. In this study, we have isolated two different rab GDI cDNAs, designated rat rab GDI alpha and beta, from a rat brain cDNA library using bovine rab GDI cDNA as a probe. Rat rab GDI alpha and beta show 99 and 86% amino acid identity with bovine rab GDI, respectively, indicating that rat rab GDI alpha is a rat counterpart of bovine rab GDI. Both rat Rab GDI alpha and beta proteins expressed in Escherichia coli showed a similar degree of activity of regulating the GDP/GTP exchange reaction to that of bovine Rab GDI using Rab3A and Rab11 as substrates. Northern blot analysis revealed that rab GDI alpha mRNA was expressed abundantly in brain but much less in other tissues, whereas rat rab GDI beta mRNA was ubiquitously expressed. Reverse transcription-polymerase chain reaction analysis revealed that astrocytes expressed rat rab GDI beta gene but not rat rab GDI alpha gene. These results indicate that Rab GDI alpha is involved in sorting of highly specialized vesicles in brain such as neurosecretory vesicles, whereas Rab GDI beta plays a general role in vesicular trafficking in diverse types of cells.

Determination of structural requirements for the interaction of Rab6 with RabGDI and Rab geranylgeranyltransferase

The importance of geranylgeranylation to the interaction of Rab proteins with RabGDI was investigated with a set of Rab6 mutants post-translationally modified by all known C-terminal lipid combinations. Rab6 proteins geranylgeranylated on CXC or CC motifs were found to be significantly better substrates for membrane extraction by RabGDI than either Rab6 proteins geranylgeranylated on CAAL motifs or Rab6 proteins that were farnesylated and palmitoylated. The methylation status of the CXC motif did not significantly affect interaction of wild type Rab6 with RabGDI. Rab6 protein sequences required for RabGDI interaction were then identified. Consistent with the significant homology between Rab-GDI and the Rab escort protein, a subunit of Rab geranylgeranyltransferase (RabGGTase), we show that there is an overlap between Rab6 motifs required for RabGDI binding and RabGGTase processing. The effector domain, loop3/beta 3 and the hypervariable region of Rab6 are all required for RabGDI binding, whereas loop3/beta 3 and the hypervariable region but not the effector domain are required for efficient processing of Rab6 by RabGGTase. Interestingly, however, loop3/beta 3 of Rab6 when introduced into H-Ras is sufficient to allow some in vivo processing of a C-terminal CSC motif.

Membrane targeting of the small GTPase Rab9 is accompanied by nucleotide exchange

The Rab GTPases are key regulators of vesicular transport. A fraction of Rab proteins is present in the cytosol, bound with GDP, complexed to a protein termed GDI. Rab9 is localized primarily to late endosomes, where it aids the transport of mannose 6-phosphate receptors to the trans-Golgi network. It has been proposed that Rab proteins are delivered to specific membranes by GDI, and that this process is accompanied by the exchange of bound GDP for GTP. In addition, Rab localization requires carboxy-terminal prenylation and specific structural determinants. Here we describe the reconstitution of the selective targeting of prenylated Rab9 protein onto late endosome membranes and show that this process is accompanied by endosome-triggered nucleotide exchange.

Cloning, characterization, and expression of a novel GDP dissociation inhibitor isoform from skeletal muscle

Cellular mechanisms for controlling membrane trafficking appear to involve small GTP-binding proteins such as the Rab proteins. Rab function is regulated by GDP dissociation inhibitor (GDI), which releases Rab proteins from membranes and inhibits GDP dissociation. Here we report the isolation of a full-length cDNA encoding a novel GDI isoform of 445 amino acids (GDI-2) with a deduced molecular weight of 50,649 from mouse skeletal muscle. Full-length and partial cDNA clones encoding a previously reported GDI protein (GDI-1) were also isolated from cDNA libraries prepared from rat brain and mouse skeletal muscle, respectively. The degree of deduced amino acid sequence identity between mouse GDI-2 and our mouse GDI-1 cDNA clone is 86%. Northern (RNA blot) analysis revealed that in human tissues, both GDI-1 and GDI-2 transcripts were abundant in brain, skeletal muscle, and pancreas but were weakly expressed in heart and liver. GDI-1 mRNA was expressed in kidney, whereas GDI-2 was almost absent, while in lung the relative amounts of these mRNA species were reversed. Specific antibodies against mouse GDI-1 and GDI-2 based on unique peptide sequences in the proteins were raised. Differentiation of 3T3-L1 fibroblasts into highly insulin-responsive adipocytes was accompanied by large increases in both mRNA and protein levels of GDI-1 and GDI-2. GDI-1 and GDI-2 expressed as glutathione S-transferase fusion proteins were both able to solubilize the membrane-bound forms of Rab4 and Rab5 in a GDP/GTP-dependent manner. Taken together, these data demonstrate that the protein products of at least two genes regulate the membrane dynamics of Rab proteins in mice.

GDI1 encodes a GDP dissociation inhibitor that plays an essential role in the yeast secretory pathway

GTP binding proteins of the Sec4/Ypt/rab family regulate distinct vesicular traffic events in eukaryotic cells. We have cloned GDI1, an essential homolog of bovine rab GDI (GDP dissociation inhibitor) from the yeast Saccharomyces cerevisiae. Analogous to the bovine protein, purified Gdi1p slows the dissociation of GDP from Sec4p and releases the GDP-bound form from yeast membranes. Depletion of Gdi1p in vivo leads to loss of the soluble pool of Sec4p and inhibition of protein transport at multiple stages of the secretory pathway. Complementation analysis indicates that GDI1 is allelic to sec19-1. These results establish that Gdi1p plays an essential function in membrane traffic and are consistent with a role for Gdi1p in the recycling of proteins of the Sec4/Ypt/rab family from their target membranes back to their vesicular pools.

Activation of NADPH oxidase involves the dissociation of p21rac from its inhibitory GDP/GTP exchange protein (rhoGDI) followed by its translocation to the plasma membrane

Activation of the NADPH oxidase of phagocytes involves the small GTP-binding protein p21rac. In this paper we report that neutrophil cytosol contains predominantly p21rac2 rather than p21rac1, and that the P21rac2 is almost entirely complexed with rhoGDI (GDP dissociation inhibitor) to form a heterodimer with a molecular mass of 45-50 kDa. Activation of superoxide production by phorbol 12-myristate 13-acetate or formylmethionyl-leucyl-phenylalanine in whole cells, and by SDS in the cell-free assay, led to the dissociation of some of the p21rac2 from rhoGDI and its movement to the plasma membrane together with p47phox and p67phox. The appearance of these proteins at the plasma membrane was related to the dose of the agonist and to the rate of superoxide generation. The nucleotide bound to p21rac2 in this complex following isolation was almost exclusively GDP, with less than 2% GTP, and the complex was active in the cell-free assay. Although the rac/GDI complex could activate the NADPH oxidase in the absence of exogenous GTP, the rate of superoxide production was increased 3-fold by the addition of GTP and was almost completely inhibited by GDP. Our findings confirm that rhoGDI serves as GDP dissociation inhibitor and that the release of p21rac2 from this inhibitor is an important step in activation of the NADPH oxidase.

The role of small GTP-binding proteins in leukocyte function

Recent progress in understanding the regulation of the phagocyte NADPH oxidase by the Rac GTP-binding protein(s) provides the first detailed glimpse into the mechanisms of leukocyte regulation by a small GTP-binding protein. Studies over the past year indicate that the activity of NADPH oxidase can be modulated by regulation of the GTP- versus GDP-bound state of Rac. Additional proteins of the Ras superfamily are likely to be involved in a variety of normal leukocyte functions.

Regulation and function of the Rho subfamily of small GTPases

During the past year, the small GTP-binding proteins Rho and Rac have each been shown to regulate a signal transduction pathway linking growth factor receptors to distinct changes in the actin cytoskeleton. A number of regulatory molecules for Rho-like proteins have been identified and some of these may cross-talk with the Ras signalling pathway.

Removal of Rab GTP-binding proteins from Golgi membranes by GDP dissociation inhibitor inhibits inter-cisternal transport in the Golgi stacks

Rab proteins are a family of Ras-like GTPases involved in intracellular membrane traffic. Rab GDI, a cytosolic protein which inhibits the dissociation of GDP from various Rab proteins, is required to maintain a pool of Rab proteins in the cytosol. We describe the purification of a cytosolic factor from bovine liver that inhibits intra-cisternal transport between the Golgi stacks. We identify this factor as a Rab GDI. Half-maximal inhibition of transport was observed in the presence of the same concentration of GDI that is required for removal of Rab proteins from the Golgi.

Ras-related GTP-binding proteins and leukocyte signal transduction

Many aspects of leukocyte function are regulated by both heterotrimeric and Ras-related GTP-binding proteins, but there is little definite information about their roles in the specialized processes utilized by leukocytes for cell killing. Recent progress in understanding the regulation of the phagocyte NADPH oxidase by the Rac GTP-binding proteins provides a basis for defining the operational characteristics of one such phagocyte system. It is clear from various studies that the activity of the NADPH oxidase can be modulated through the regulation of the GTP-GDP state of Rac. Proteins exist in leukocytes able to modify GTP-binding protein function in this manner, and their activity may be regulated by signals generated on phagocyte stimulation. Proteins of the Ras superfamily are likely to be involved in a variety of normal phagocyte functions through their ability to modulate the assembly of actin filaments, direct vesicle trafficking and fusion, and so forth.

Biologically active lipids are regulators of Rac.GDI complexation

Members of the Rho family of GTP-binding proteins are localized in the cytosol of cells by complexation with a protein known as (Rho)GDI. We show by sucrose gradient equilibrium sedimentation analysis that all of the Rac protein present in human neutrophil cytosol exists as a complex with (Rho)GDI under non-activating conditions. This interaction can be disrupted in the presence of various lipids which have been shown to have biological activity in a variety of systems, including NADPH oxidase activation. Particularly effective were arachidonic acid, phosphatidic acid, and phosphatidylinositols. These lipids were active at concentrations from 0.5-50 microM and were capable of disrupting complexation of (Rho)GDI with both GDP- and GTP-bound forms of Rac, although the latter were more sensitive to lipid. These data suggest that certain lipids generated in chemoattractant-stimulated neutrophils may play a role in modulating the activity of Rac and thus NADPH oxidase activity.

Identification of two human Rho GDP dissociation inhibitor proteins whose overexpression leads to disruption of the actin cytoskeleton

Proteins (IEF's 1120, 8118, 8120) sharing similarity to the bovine Rho GDP dissociation inhibitor (GDI) have been identified in the human two-dimensional-gel database of keratinocyte proteins. Molecular cloning of the corresponding cDNAs showed that IEF 8118 is the human homolog of bovine GDI while IEF 8120 is a distinct although related protein. All available information indicates the IEF 1120 is a derivative of IEF 8120. The cDNAs coding for IEF's 8118 and 8120 were recombined into vaccinia virus and expressed in differentiated human keratinocytes and their effect on the actin cytoskeleton was assessed by immunofluorescence using TRITC-phalloidin. The results showed that overexpression of both GDI proteins leads to rounding up of the cells and loss of stress fibers and focal contact sites. In addition, the cell to cell adhesion belts gradually disappeared, an effect that was particularly pronounced in infected cells overexpressing IEF 8120. Taken together, the results imply that Rho GDI's play a role in modulating the activity of the Rho proteins as their overexpression mimics phenotypic changes associated with the inactivation of these proteins.

Identification of a novel protein with GDP dissociation inhibitor activity for the ras-like proteins CDC42Hs and rac I

We have recently cloned the human cDNA for a gene, denoted D4, that encodes a protein 67% identical to the bovine rhoGDI protein, a GDP dissociation inhibitor (GDI) for the ras-related rho-subtype proteins. We now present data on the cloning and structural analysis of the murine D4 cDNA and confirm its preferential expression in hematopoietic tissues. The predicted murine and human D4 proteins are almost 90% identical, indicating that D4 and rhoGDI are different genes and that they are probably members of a related family of genes. Functional studies with the human D4 protein demonstrate that D4 has GDI activity against the CDC42Hs and rac I proteins, but binds to these proteins with a significantly weaker affinity than does the rho-subtype GDI. These data suggest that D4, which will in subsequent communications be denoted as GDI.D4, might be a GDI for other known or as yet unidentified ras-like GTP-binding proteins. Alternatively, D4 could have other biochemical functions. During murine embryogenesis, D4 transcripts are detected in yolk-sac cells, where the earliest hematopoietic precursors are found. When these precursors undergo proliferation and differentiation in vitro, a dramatic increase in D4 expression is seen. D4 probably has a significant function during the growth and development of hematopoietic precursors.

Consequences of weak interaction of rho GDI with the GTP-bound forms of rho p21 and rac p21

rho GDI is an inhibitory GDP/GTP exchange protein for the rho family. Recently, rho GDI has been reported to interact with the GTP-bound form of G25K and rac1 p21 and to inhibit their basal and GTPase-activating protein (GAP)-stimulated GTPase activity. Here, we examined whether rho GDI interacts with the GTP-bound form of rho p21 and rac p21 and inhibits their basal and rho GAP-stimulated GTPase activity, rho GDI interacted with both the GDP- and GTP-bound forms of rhoA p21 and rac1 p21 as estimated by measuring its ability to form a complex with both forms and to inhibit the membrane-binding activity of both forms. The efficiency of rho GDI for interaction with the GTP-bound form was, however, about 10% that for interaction with the GDP-bound form. Moreover, rho GDI inhibited both the basal and rho GAP-stimulated GTPase activity of rhoA p21 and rac1 p21 in a dose-dependent manner. The doses of rho GDI necessary for this action were, however, about 10-fold higher than those necessary for the action to inhibit their GDP/GTP exchange reaction. These results indicate that rho GDI interacts with the GTP-bound form of its substrate small G proteins, as well as with the GDP-bound form, but much less efficiently than with the GDP-bound form.

Carboxyl methylation of the low molecular weight GTP-binding protein G25K: regulation of carboxyl methylation by rhoGDI

A soluble form of G25K from brain co-purified with a 28 kDa protein, which was identified as guanine nucleotide dissociation inhibitor (GDI) protein, similar or identical to rhoGDI. The GDI protein inhibited the dissociation of GDP from G25K. G25K and the GDI protein form a heterodimer and remain associated with either GDP or GTP gamma S bound. The GDI protein inhibited carboxyl methylation of G25K in the presence of magnesium and GDP. The GDI protein appears to be an important regulator of G25K methylation by blocking methylation of G25K in the inactive GDP-bound conformation.

Regulation of the GTPase cycle of the neuronally expressed Ras-like GTP-binding protein Rab3A

The small GTP-binding protein Rab3A (identical to smg p25A) is expressed in neural/exocrine/endocrine cells, is distributed between the cytosol and secretory vesicle membranes, and may cycle between these locations to regulate exocytosis. It is proposed that the GTP/GDP state of Rab3A controls this distribution. In PC12 cells, cytosolic Rab3A is predominantly GDP-bound, whereas membrane-associated Rab3A is approximately 50% GTP-bound. Two cytosolic factors, GDP dissociation inhibitor (GDI) and guanine nucleotide releasing factor (GRF), act only on GDP.Rab3A, and preferentially with post-translationally modified Rab3A. Rab3A GTPase-activating protein (GAP) does not preferentially act on processed Rab3A, and interacts selectively with GTP.Rab3A. GDI antagonizes GRF but not GAP activity toward Rab3A. These data are consistent with the concept of an ordered Rab3A cycle controlled by factors that regulate the guanine-nucleotide binding state of Rab3A.

Translocation of Rac correlates with NADPH oxidase activation. Evidence for equimolar translocation of oxidase components

Activation of the superoxide-generating NADPH oxidase system of human neutrophils involves the assembly of several neutrophil components, some located on the plasma membrane and others in the cytosol. It has recently been established that one of the required components for NADPH oxidase activity is the GTP-binding protein Rac. To further investigate the role of Rac in the NADPH oxidase system, studies were carried out to determine its subcellular distribution in resting and activated human neutrophils. In resting cells, Rac and an associated guanine nucleotide regulatory factor, GDP dissociation inhibitor (GDI), were located only in the cytosol, along with other known oxidase factors, p47-phox and p67-phox. After activation of neutrophils with phorbol 12-myristate 13-acetate or formyl-methionyl-leucyl-phenylalanine, Rac was translocated from the cytosol to the plasma membrane, and this translocation corresponded temporally with the translocation of p47-phox and p67-phox and with the generation of superoxide. GDI remained localized to the cytosol, suggesting activation of the oxidase involved dissociation of the Rac-GDI complex prior to Rac translocation. Determination of the quantities of cytosolic factors associated with the plasma membrane indicated that Rac, p47-phox, and p67-phox are translocated to the plasma membrane simultaneously in equimolar amounts, but that the membrane-associated cytochrome b was present at 3-4-fold molar excess. These findings suggest that Rac may play a role in assembly of the active NADPH oxidase complex.

Interaction of Sec4 with GDI proteins from bovine brain, Drosophila melanogaster and Saccharomyces cerevisiae. Conservation of GDI membrane dissociation activity

Rab GDP dissociation inhibitor (Rab GDI), will induce the dissociation of GDP-bound rab3A from synaptic membranes and will inhibit GDP dissociation from Sec4, a member of the Rab subgroup of the Ras GTPase superfamily which is required for exocytosis in Saccharomyces cerevisiae. We report that Rab GDI releases GDP-bound Sec4 from yeast membranes. dGDI, a Drosophila homologue can similarly inhibit GDP dissociation from Sec4 and release GDP-bound Sec4 from yeast membranes. An activity partially purified from yeast cytosol dissociates GDP-bound Sec4 from yeast membranes, suggesting that yeast also possess a GDI protein that functions to recycle Sec4 from its target membrane.

Role of the rac1 p21-GDP-dissociation inhibitor for rho heterodimer in the activation of the superoxide-forming NADPH oxidase of macrophages

Activation of the superoxide (O2-)-generating NADPH oxidase of phagocytes requires the interaction of membrane-associated cytochrome b559 with three cytosolic components; p47-phox, p67-phox and sigma 1. We proposed that sigma 1 was a heterodimer composed of proteins of 22 kDa and 24 kDa that were tentatively identified as the small GTP-binding protein (G protein) rac1 p21 and GDP-dissociation inhibitor for rho (rho GDI). We now describe a modified procedure for the rapid purification of sigma 1 and demonstrate that the NADPH-oxidase-activating capacity is associated, throughout the purification sequence, with a protein binding 35S-labelled guanosine 5'-[3-O-thio]triphosphate. SDS/PAGE analysis confirmed the absolute association of sigma 1 activity with the presence of both the 22 kDa and 24 kDa proteins. Immunoblotting with a battery of antibodies against the small G proteins demonstrated that the 22-kDa protein was only recognized by antibodies reacting with rac1 p21; no reaction was found with anti-(rac2 p21), anti-[v-ras(H) p21] and anti anti-(rap1 p21). Free rac1 p21 (not in complex with rho GDI) was not detected at any stage of cytosol fractionation. The proteins comprising the sigma 1 heterodimer could be separated by reverse-phase chromatography and amino acid sequencing was performed on peptides derived by trypsin digestion of each of the isolated proteins. This demonstrated the identity of the 22-kDa protein with rac1 p21 and that of the 24-kDa protein with rho GDI. Purified heterodimeric sigma 1 did not require exogenous GTP for activity under conditions that assured the absence of free nucleotides. Treatment of the sigma 1 heterodimer with 1% sodium cholate, followed by gel filtration or anion-exchange chromatography in the presence of 1% sodium cholate, effectively separated rac1 p21 from rho GDI. Monomeric rac1 p21, obtained by these procedures, was able to stimulate cell-free O2- generation. Artificial heterodimeric sigma 1, capable of NADPH oxidase activation, could be reconstituted in vitro by recombining purified monomeric rac1 p21 and rho GDI and removing the sodium cholate used to dissociate the native sigma 1 dimer. Monomeric rac1 p21 exhibited an almost absolute dependence on exogenous GTP following removal of the endogenous nucleotide in low Mg2+ solution. Under similar conditions, heterodimeric sigma 1 was resistant to nucleotide exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Ly-GDI, a GDP-dissociation inhibitor of the RhoA GTP-binding protein, is expressed preferentially in lymphocytes

The Ras-related small GTP-binding proteins are involved in diverse cellular events, including cell signaling, proliferation, cytoskeletal organization, and secretion. The interconversion of the active, GTP-bound form of the protein to the inactive, GDP-bound form is influenced by two types of regulatory proteins, those that alter the intrinsic GTPase activity of the GTP-binding protein and those that affect the rate of GDP/GTP exchange. By utilizing a subtractive hybridization approach, we have isolated a human gene encoding Ly-GDI, a protein that has striking homology to the product of a previously cloned gene, Rho-GDI, which inhibits GDP/GTP exchange on the Rho family of GTPases. In contrast to Rho-GDI, which is ubiquitously expressed, Ly-GDI is expressed only in hematopoietic tissues and predominantly in B- and T-lymphocyte cell lines. The full-length Ly-GDI cDNA encodes a 27-kDa protein which binds to RhoA and inhibits GDP dissociation from RhoA. Stimulation of T lymphocytes with phorbol ester leads to phosphorylation of Ly-GDI, suggesting an involvement of Ly-GDI in lymphocyte activation pathways. Cell type-specific regulators of the Ras-like GTP-binding proteins may provide one mechanism by which different cell types respond uniquely to signals transduced through the same cell surface receptor or may provide a way by which the GTP-binding proteins can be uniquely engaged by tissue-restricted receptors.

Regulation of the human neutrophil NADPH oxidase by rho-related G-proteins

Superoxide production by phagocytic white blood cells requires the assembly of an NADPH oxidase from membrane and cytosolic proteins. Recombinant cytosolic proteins p47phox and p67phox and neutrophil membranes were used to purify a third cytosolic component that is necessary and sufficient for cell-free reconstitution of NADPH oxidase. The component was isolated as a complex of rho-GDP dissociation inhibitor (rho-GDI) and two members of the rho subfamily of ras-related guanine nucleotide binding proteins, rac2 and CDC42Hs. Oxidase reconstitution with these pure cytosolic proteins was unaffected by GTP gamma S but was inhibited by GDP beta S, suggesting that the active complex contained endogenous bound GTP. Direct binding of rho-GDI to the GTP gamma S-bound forms of these G-proteins was demonstrated by gel filtration following exchange with radiolabeled guanine nucleotide. rho-GDI was shown to be nonessential for cell-free oxidase reconstitution in experiments that compared the activities of pure recombinant forms of these G-proteins. Recombinant rac augmented superoxide production, while recombinant CDC42Hs, which shares 70% amino acid sequence identity with rac, did not. Three highly conserved regions of rac1 and rac2 were noted as markedly divergent in CDC42Hs. It is proposed that one or more of these regions of rac may be involved in the specific interaction of rac with the other NADPH oxidase protein(s).

A novel role for RhoGDI as an inhibitor of GAP proteins

RhoGDI inhibits guanine nucleotide dissociation from post-translationally processed Rho and Rac proteins but its biochemical role in vivo is unknown. We show here that N-terminal effector site mutations in the Rac protein do not compromise its interaction with RhoGDI and that, whilst geranylgeranylation and -AAX proteolysis of the C-terminal CAAX motif of Rac1 and RhoA are required for efficient interaction with RhoGDI, methylesterification of the C-terminal cysteine residue is not required. In vitro, RhoGDI can form stable complexes with Rho and Rac proteins in both the GTP and GDP bound states. Furthermore the Rac-GTP--RhoGDI complex is resistent to the action of recombinant RhoGAP and recombinant BCR. Thus GDI, by complexing with Rac-GTP and preventing GAP stimulated GTP hydrolysis, may allow transit of the activated form of the Rac protein between physically separated activator and effector proteins in the cell.

Regulation of cytoplasmic division of Xenopus embryo by rho p21 and its inhibitory GDP/GTP exchange protein (rho GDI)

Evidence is accumulating that the rho family, a member of the ras p21-related small GTP-binding protein superfamily, regulates cell morphology, cell motility, and smooth muscle contraction through the actomyosin system. The actomyosin system is also known to be essential for cytoplasmic division of cells (cytokinesis). In this study, we examined the action of rho p21, its inhibitory GDP/GTP exchange protein, named rho GDI, its stimulatory GDP/GTP exchange protein, named smg GDS, and botulinum ADP-ribosyltransferase C3, known to selectively ADP-ribosylate rho p21 and to impair its function, in the cytoplasmic division using Xenopus embryos. The sperm-induced cytoplasmic division of Xenopus embryos was not affected by microinjection into the embryos of either smg GDS or the guanosine-5'-(3-O-thio)triphosphate (GTP gamma S)-bound form of rhoA p21, one member of the rho family, but completely inhibited by microinjection of rho GDI or C3. Under these conditions, nuclear division occurred normally but the furrow formation, which was induced by the contractile ring consisting of actomyosin just beneath the plasma membrane, was impaired. Comicroinjection of rho GDI with the GTP gamma S-bound form of rhoA p21 prevented the rho GDI action. Moreover, the sperm-induced cytoplasmic division of Xenopus embryos was inhibited by microinjection into the embryos of the rhoA p21 pre-ADP-ribosylated by C3 which might serve as a dominant negative inhibitor of endogenous rho p21. These results indicate that rho p21 together with its regulatory proteins regulates the cytoplasmic division through the actomyosin system.

Carboxyl methylation of Ras-related proteins during signal transduction in neutrophils

In human neutrophils, as in other cell types, Ras-related guanosine triphosphate-binding proteins are directed toward their regulatory targets in membranes by a series of posttranslational modifications that include methyl esterification of a carboxyl-terminal prenylcysteine residue. In intact cells and in a reconstituted in vitro system, the amount of carboxyl methylation of Ras-related proteins increased in response to the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (FMLP). Activation of Ras-related proteins by guanosine-5'-O-(3-thiotriphosphate) had a similar effect and induced translocation of p22rac2 from cytosol to plasma membrane. Inhibitors of prenylcysteine carboxyl methylation effectively blocked neutrophil responses to FMLP. These findings suggest a direct link between receptor-mediated signal transduction and the carboxyl methylation of Ras-related proteins.

GDP dissociation inhibitor prevents intrinsic and GTPase activating protein-stimulated GTP hydrolysis by the Rac GTP-binding protein

The majority of the GTP-binding proteins of the Ras superfamily hydrolyze GTP to GDP very slowly. A notable exception to this are the Rac proteins, which have intrinsic GTPase rates at least 50-fold those of Ras or Rho. A protein (or proteins) capable of inhibiting this GTPase activity exists in human neutrophil cytosol. Since Rac appears to exist normally in neutrophils as a cytosolic protein complexed to (Rho)GDI, we examined the ability of (Rho)GDI to inhibit GTP hydrolysis by Rac. (Rho)GDI produced a concentration-dependent inhibition of GTP hydrolysis by Rac1 that paralleled its ability to inhibit GDP dissociation from the Rac protein. Maximal inhibition occurred at or near equimolar concentrations of the GDI and the Rac substrate. The ability of two molecules exhibiting GTPase activating protein (GAP) activity toward Rac to stimulate GTP hydrolysis was also inhibited by the presence of (Rho)GDI. The inhibitory effect of the GDI could be overcome by increasing the GAP concentration to levels equal to that of the GDI. (Rho)GDI weakly, but consistently, inhibited GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) dissociation from Rac1, confirming an interaction of (Rho)GDI with the GTP-bound form of the protein. These data describe an additional activity of (Rho)GDI and suggest a mechanism by which Rac might be maintained in an active form in vivo in the presence of regulatory GAPs.

Regulation of morphology by rho p21 and its inhibitory GDP/GTP exchange protein (rho GDI) in Swiss 3T3 cells

rho GDP dissociation inhibitor (GDI) is an inhibitory GDP/GTP exchange protein for a group of small GTP-binding proteins including at least rhoA p21, rhoB p21, rac1 p21, rac2 p21, and G25K. Microinjection of rho GDI into Swiss 3T3 cells made the cells round and refractile. This morphological change was accompanied by the disappearance of stress fibers. The rho GDI action was prevented by comicroinjection of rho GDI with the guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-bound form of rhoA p21, but not with the GTP gamma S-bound form of rhoA p21 lacking the C-terminal three amino acids, which was not post-translationally modified with lipids. The GTP gamma S-bound form of rac1 p21, the same form of G25K, the same form of smg p21B, or Ki-rasval12 p21 was ineffective. Microinjection of the bacterial ADP-ribosyltransferase C3 specific for rho p21 into Swiss 3T3 cells induced the similar changes of morphology and stress fibers. This C3 action was not prevented by comicroinjection of C3 with the GTP gamma S-bound form of rhoA p21, but was prevented by comicroinjection with the same form of a rhoA p21 mutant which was not ADP-ribosylated by C3. These results indicate that the rho GDI-rho p21 system regulates cell morphology presumably through the actomyosin system in Swiss 3T3 cells.

Involvement of rho p21 and its inhibitory GDP/GTP exchange protein (rho GDI) in cell motility

Evidence is accumulating that rho p21, a ras p21-related small GTP-binding protein (G protein), regulates the actomyosin system. The actomyosin system is known to be essential for cell motility. In the present study, we examined the action of rho p21, its inhibitory GDP/GTP exchange protein (named rho GDI), its stimulatory GDP/GTP exchange protein (named smg GDS), and Clostridium botulinum ADP-ribosyltransferase C3, known to selectively ADP-ribosylate rho p21 and to impair its function, in cell motility (chemokinesis) of Swiss 3T3 cells. We quantitated the capacity of cell motility by measuring cell tracks by phagokinesis. Microinjection of the GTP gamma S-bound active form of rhoA p21 or smg GDS into Swiss 3T3 cells did not affect cell motility, but microinjection of rho GDI into the cells did inhibit cell motility. This rho GDI action was prevented by comicroinjection of rho GDI with the GTP gamma S-bound form of rhoA p21 but not with the same form of rhoA p21 lacking the C-terminal three amino acids which was not posttranslationally modified with lipids. The rho GDI action was not prevented by Ki-rasVal-12 p21 or any of the GTP gamma S-bound form of other small GTP-binding proteins including rac1 p21, G25K, and smg p21B. Among these small G proteins, rhoA p21, rac1 p21, and G25K are known to be substrates for rho GDI. The rho GDI action was not prevented by comicroinjection of rho GDI with smg GDS. Microinjection of C3 into Swiss 3T3 cells also inhibited cell motility. These results indicate that the rho GDI-rho p21 system regulates cell motility, presumably through the actomyosin system.

Copurification of rho protein and the rho-GDP dissociation inhibitor from bovine neutrophil cytosol. Effect of phosphoinositides on rho ADP-ribosylation by the C3 exoenzyme of Clostridium botulinum

The substrate of the C3 exoenzyme from botulinum toxin is a protein which is particularly abundant in the cytosol of neutrophils [Stasia, M. J., Jouan, A., Bourmeyster, N., Boquet, P., & Vignais, P. V. (1991) Biochem. Biophys. Res. Commun. 180, 615-622]. Optimal conditions for the ADP-ribosylation of the C3 substrate have been established in order to follow the course of its purification from bovine neutrophil cytosol. In particular, phosphoinositides at micromolar concentrations were found to enhance the ADP-ribosylation capacity of the C3 substrate in crude neutrophil cytosol and partially purified fractions. A [32P]ADP-ribosylatable protein, migrating on SDS-PAGE with a mass of 24 kDa, was copurified with a 29-kDa protein by a series of chromatographic steps on DEAE-Sephacel, Biogel P60, and Mono Q. In the case of the C3 substrate, isoelectric focusing revealed two major labeled bands with pI values of 6.2 and 5.6; the pI of the 29-kDa protein was 4.8-5.0. On the basis of the amino acid sequence of peptides resolved after proteolytic digestion, the 24-kDa protein and the 29-kDa protein were identified respectively as rho and the GDP dissociation inhibitor (GDI), suggesting that rho and GDI copurify from bovine neutrophil cytosol in the form of a complex. The presence of a number of amino acid residues specific of rho A in the enzymatic digest originating from rho indicates that, among the rho proteins, at least rho A belongs to the GDI-rho complex.