Cellular transformation and guanine nucleotide exchange activity are catalyzed by a common domain on the dbl oncogene product

A novel PDZ domain containing guanine nucleotide exchange factor links heterotrimeric G proteins to Rho

Small GTP-binding proteins of the Rho family play a critical role in signal transduction. However, there is still very limited information on how they are activated by cell surface receptors. Here, we used a consensus sequence for Dbl domains of Rho guanine nucleotide exchange factors (GEFs) to search DNA data bases, and identified a novel human GEF for Rho-related GTPases harboring structural features indicative of its possible regulatory mechanism(s). This protein contained a tandem DH/PH domain closely related to those of Rho-specific GEFs, a PDZ domain, a proline-rich domain, and an area of homology to Lsc, p115-RhoGEF, and a Drosophila RhoGEF that was termed Lsc-homology (LH) domain. This novel molecule, designated PDZ-RhoGEF, activated biological and biochemical pathways specific for Rho, and activation of these pathways required an intact DH and PH domain. However, the PDZ domain was dispensable for these functions, and mutants lacking the LH domain were more active, suggesting a negative regulatory role for the LH domain. A search for additional molecules exhibiting an LH domain revealed a limited homology with the catalytic region of a newly identified GTPase-activating protein for heterotrimeric G proteins, RGS14. This prompted us to investigate whether PDZ-RhoGEF could interact with representative members of each G protein family. We found that PDZ-RhoGEF was able to form, in vivo, stable complexes with two members of the Galpha12 family, Galpha12 and Galpha13, and that this interaction was mediated by the LH domain. Furthermore, we obtained evidence to suggest that PDZ-RhoGEF mediates the activation of Rho by Galpha12 and Galpha13. Together, these findings suggest the existence of a novel mechanism whereby the large family of cell surface receptors that transmit signals through heterotrimeric G proteins activate Rho-dependent pathways: by stimulating the activity of members of the Galpha12 family which, in turn, activate an exchange factor acting on Rho.

The Dbl-related protein, Lfc, localizes to microtubules and mediates the activation of Rac signaling pathways in cells

The possibility that the Dbl family member Lfc can activate Rac1 in cells is investigated in this study. Previously, we demonstrated that both Lfc and Lsc, like their closest relative Lbc, can act catalytically in stimulating the guanine nucleotide exchange activity of RhoA in vitro. Neither Lfc nor Lsc stimulated the in vitro exchange activity of Cdc42 or Rac1; however, Lfc was capable of forming a tight complex with Rac1 in vitro. We show here that Lfc stimulates c-Jun kinase (JNK) activity in COS-7 cells. This stimulation was blocked by a dominant negative mutant of Rac1 and somewhat less effectively by dominant negative RhoA, but not by dominant negative Cdc42. Overexpression of Lfc in NIH 3T3 cells induced the formation of actin stress fibers and membrane ruffles, consistent with the activation of both RhoA and Rac1 signaling pathways, whereas overexpression of Lsc led exclusively to well developed stress fibers. Using a recently developed assay for measuring the cellular activation of Rac, we did not find that expression of Lfc increased the levels of GTP-bound Rac1. However, an examination of the cellular localization of Lfc showed that it was localized to microtubules, similar to what has been reported for activated Rac1, the mixed lineage kinase (MLK) and JNK. Moreover, we have found that the Pleckstrin homology (PH) domain of Lfc specifically associates with tubulin. Taken together, these findings suggest a model where the PH domain-mediated localization of Lfc to microtubules enables the recruitment of Rac to a site proximal to its signaling targets, resulting in JNK activation and actin cytoskeletal changes.

The product of the cph oncogene is a truncated, nucleotide-binding protein that enhances cellular survival to stress

Cph was isolated from neoplastic Syrian hamster embryo fibroblasts initiated by 3-methylcholanthrene (MCA), and was shown to be a single copy gene in the hamster genome, conserved from yeast to human cells, expressed in fetal cells and most adult tissues, and acting synergistically with H-ras in the transformation of murine NIH3T3 fibroblasts. We have now isolated Syrian hamster full-length cDNAs for the cph oncogene and proto-oncogene. Nucleotide sequence analysis revealed that cph was activated in MCA-treated cells by a point-mutational deletion at codon 214, which caused a shift in the normal open reading frame (ORF) and brought a translation termination codon 33 amino acids downstream. While proto-cph encodes a protein (pcph) of 469 amino acids, cph encodes a truncated protein (cph) of 246 amino acids with a new, hydrophobic C-terminus. Similar mechanisms activated cph in other MCA-treated Syrian hamster cells. The cph and proto-cph proteins have partial sequence homology with two protein families: GDP/GTP exchange factors and nucleotide phosphohydrolases. In vitro translated, gel-purified cph proteins did not catalyze nucleotide exchange for H-ras, but were able to bind nucleotide phosphates, in particular ribonucleotide diphosphates such as UDP and GDP. Steady-state levels of cph mRNA increased 6.7-fold in hamster neoplastic cells, relative to a 2.2-fold increase in normal cells, when they were subjected to a nutritional stress such as serum deprivation. Moreover, cph-transformed NIH3T3 cells showed increased survival to various forms of stress (serum starvation, hyperthermia, ionizing radiation), strongly suggesting that cph participates in cellular mechanisms of response to stress.

Signaling to the actin cytoskeleton

The actin cytoskeleton is a highly dynamic network composed of actin polymers and a large variety of associated proteins. The main functions of the actin cytoskeleton are to mediate cell motility and cell shape changes during the cell cycle and in response to extracellular stimuli, to organize the cytoplasm, and to generate mechanical forces within the cell. The reshaping and functions of the actin cytoskeleton are regulated by signaling pathways. Here we broadly review the actin cytoskeleton and the signaling pathways that regulate it. We place heavy emphasis on the yeast actin cytoskeleton.

Cloning and characterization of GEF-H1, a microtubule-associated guanine nucleotide exchange factor for Rac and Rho GTPases

The Rho-related small GTPases are critical elements involved in regulation of signal transduction cascades from extracellular stimuli to cell nucleus and cytoskeleton. The Dbl-like guanine nucleotide exchange factors (GEF) have been implicated in direct activation of these GTPases. Here we have identified a new member of the Dbl family, GEF-H1, by screening a human HeLa cell cDNA library. GEF-H1 encodes a 100-kDa protein containing the conserved structural array of a Dbl homology domain in tandem with a pleckstrin homology domain and is most closely related to the lfc oncogene, but additionally it contains a unique coiled-coil domain at the carboxyl terminus. Biochemical analysis reveals that GEF-H1 is capable of stimulating guanine nucleotide exchange of Rac and Rho but is inactive toward Cdc42, TC10, or Ras. Moreover, GEF-H1 binds to Rac and Rho proteins in both the GDP- and guanosine 5'-3-O-(thio)triphosphate-bound states without detectable affinity for Cdc42 or Ras. Immunofluorescence reveals that GEF-H1 colocalizes with microtubules through the carboxyl-terminal coiled-coil domain. Overexpression of GEF-H1 in COS-7 cells results in induction of membrane ruffles. These results suggest that GEF-H1 may have a direct role in activation of Rac and/or Rho and in bringing the activated GTPase to specific target sites such as microtubules.

The function of the p190 Rho GTPase-activating protein is controlled by its N-terminal GTP binding domain

p190 is a GTPase-activating protein (GAP) for the Rho family of GTPases. The GAP domain of p190 is at the C terminus of the protein. At its N terminus, p190 contains a GTP binding domain of unknown significance. We have introduced a mutation (Ser36 --> Asn) into this domain of p190 that decreased its ability to bind guanine nucleotide when expressed as a hemagglutinin (HA)-tagged protein in COS cells. In vitro, both the wild type and S36N mutant HA-p190 proteins showed similar GAP activities toward RhoA, but when expressed in NIH 3T3 fibroblasts only wild type p190 appeared able to function as a RhoGAP. Wild type HA-p190 induced a phenotype of rounded cells with long, beaded extensions similar to that seen when Rho function is disrupted by ADP-ribosylation. HA-p190(S36N), although expressed at a similar level to the wild type protein, had no discernible effect on the cells. The beaded extension phenotype induced by wild type HA-p190 required GAP function. A GAP-defective mutant, p190(R1283A), had no effect on cell morphology. Moreover, the beaded extension phenotype could be suppressed by co-expression of a gain-of-function Rho mutant, RhoA(G14V), or Rac mutant, Rac1(G12V). Activation of the Jun kinase (JNK) via muscarinic receptors was inhibited by wild type HA-p190, but JNK activity was enhanced by the S36N mutant. Co-expression of HA-p190 with a fragment containing only the mutated GTP binding domain partially inhibited the beaded extension phenotype, suggesting that it may sequester a factor required for p190 function. Taken together these data demonstrate that within the cell, the Rho/Rac GAP activity of p190 can be regulated by the N-terminal GTP binding domain.

Structure and mutagenesis of the Dbl homology domain

Guanine nucleotide exchange factors in the Dbl family activate Rho GTPases by accelerating dissociation of bound GDP, promoting acquisition of the GTP-bound state. Dbl proteins possess a approximately 200 residue catalytic Dbl-homology (DH) domain, that is arranged in tandem with a C-terminal pleckstrin homology (PH) domain in nearly all cases. Here we report the solution structure of the DH domain of human PAK-interacting exchange protein (betaPIX). The domain is composed of 11 alpha-helices that form a flattened, elongated bundle. The structure explains a large body of mutagenesis data, which, along with sequence comparisons, identify the GTPase interaction site as a surface formed by three conserved helices near the center of one face of the domain. Proximity of the site to the DH C-terminus suggests a means by which PH-ligand interactions may be coupled to DH-GTPase interactions to regulate signaling through the Dbl proteins in vivo.

Identification of darlin, a Dictyostelium protein with Armadillo-like repeats that binds to small GTPases and is important for the proper aggregation of developing cells

We purified from Dictyostelium lysates an 88-kDa protein that bound to a subset of small GTPases, including racE, racC, cdc42Hs, and TC4ran, but did not bind to R-ras or rabB. Cloning of the gene encoding this 88-kDa protein revealed that it contained multiple armadillo-like repeats most closely related to the mammalian GTP exchange factor smgGDS. We named this protein darlin (Dictyostelium armadillo-like protein). Disruption of the gene encoding darlin demonstrated that this protein is not essential for cytokinesis, pinocytosis, phagocytosis, or development. However, the ability of darlin null cells to aggregate in response to starvation is severely affected. When starved under liquid medium, the mutant cells were unable to form aggregation centers and streams, possibly because of a defect in cAMP relay signaling. This defect was not due to an inability of the darlin mutants to activate adenylate cyclase in response to G protein stimulation. These results suggest that the darlin protein is involved in a signaling pathway that may modulate the chemotactic response during early development.

Activation of Rac1 by a Crk SH3-binding protein, DOCK180

DOCK180 is involved in integrin signaling through CrkII-p130(Cas) complexes. We have studied the involvement of DOCK180 in Rac1 signaling cascades. DOCK180 activated JNK in a manner dependent on Rac1, Cdc42Hs, and SEK, and overexpression of DOCK180 increased the amount of GTP-bound Rac1 in 293T cells. Coexpression of CrkII and p130(Cas) enhanced this DOCK180-dependent activation of Rac1. Furthermore, we observed direct binding of DOCK180 to Rac1, but not to RhoA or Cdc42Hs. Dominant-negative Rac1 suppressed DOCK180-induced membrane spreading. These results strongly suggest that DOCK180 is a novel activator of Rac1 and involved in integrin signaling.

Regulation of PAK activation and the T cell cytoskeleton by the linker protein SLP-76

Tyrosine phosphorylation of linker proteins enables the T cell antigen receptor (TCR)-associated protein tyrosine kinases to phosphorylate and regulate effector molecules that generate second messengers. We demonstrate here that the SLP-76 linker protein interacts with both nck, an adaptor protein, and Vav, a guanine nucleotide exchange factor for Rho-family GTPases. The assembly of this tri-molecular complex permits the activated Rho-family GTPases to regulate target effectors that interact through nck. In turn, assembly of this complex mediates the enzymatic activation of the p21-activated protein kinase 1 and facilitates actin polymerization. Hence, phosphorylation of linker proteins not only bridges the TCR-associated PTK, ZAP-70, with downstream effector proteins, but also provides a scaffold to integrate distinct signaling complexes to regulate T cell function.

Distinct cellular effects and interactions of the Rho-family GTPase TC10

BACKGROUND

Rho-family GTPases have central roles in cytoskeletal organization, proliferation, differentiation and apoptosis. Multiple factors possessing overlapping specificities for Rho GTPases have been identified. The Rho GTPases Cdc42 and Rac share many regulators and effectors, yet produce different phenotypes when expressed as gain-of-function mutants in cells. The Rho-family member TC10 has remained almost completely uncharacterized, so it was of interest to determine whether TC10 has unique cellular effects and interacts with the same targets as Cdc42 and Rac.

RESULTS

A gain-of-function TC10 mutant protein expressed in fibroblasts induced cell rounding, loss of stress fibers and formation of peripheral extensions. The extensions were longer than those induced by the analogous Cdc42 mutant protein. Cells expressing TC10 also possessed fewer membrane ruffles and stress fibers than those expressing Cdc42. TC10 mRNA was most highly expressed in heart and skeletal muscle. The GTPase activity of TC10 was lower than that of Cdc42, and TC10 possessed a lower affinity for, but greater responsiveness to, the p50Rho GTPase-activating protein (p50RhoGAP) than did Cdc42. TC10 stimulated Jun N-terminal kinase (JNK) and p21-activated kinase (PAK) activities and interacted with a set of effectors (alpha-, beta- and gammaPAK, MRCKalpha/beta, MLK2, N-WASP and MSE55) that overlaps with those for Cdc42 and Rac. TC10 did not interact with MLK3 or WASP, and interacted only weakly with ACK-1.

CONCLUSIONS

TC10 possesses distinct features, but exhibits a phenotype most closely related to that of Cdc42. It interacts with a similar subset of effectors to Cdc42 but not with MLK3, WASP or ACK-1. It is regulated differentially by p50RhoGAP.

Crystal structure of the Dbl and pleckstrin homology domains from the human Son of sevenless protein

Proteins containing Dbl homology (DH) domains activate Rho-family GTPases by functioning as specific guanine nucleotide exchange factors. All known DH domains have associated C-terminal pleckstrin homology (PH) domains that are implicated in targeting and regulatory functions. The crystal structure of a fragment of the human Son of sevenless protein containing the DH and PH domains has been determined at 2.3 A resolution. The entirely alpha-helical DH domain is unrelated in architecture to other nucleotide exchange factors. The active site of the DH domain, identified on the basis of sequence conservation and structural features, lies near the interface between the DH and PH domains. The structure suggests that ligation of the PH domain will be coupled structurally to the GTPase binding site.

NMR structure and mutagenesis of the N-terminal Dbl homology domain of the nucleotide exchange factor Trio

Guanine nucleotide exchange factors for the Rho family of GTPases contain a Dbl homology (DH) domain responsible for catalysis and a pleckstrin homology (PH) domain whose function is unknown. Here we describe the solution structure of the N-terminal DH domain of Trio that catalyzes nucleotide exchange for Rac1. The all-alpha-helical protein has a very different structure compared to other exchange factors. Based on site-directed mutagenesis, functionally important residues of the DH domain were identified. They are all highly conserved and reside in close proximity on two a helices. In addition, we have discovered a unique capability of the PH domain to enhance nucleotide exchange in DH domain-containing proteins.

Adenosine diphosphate (ADP)-ribosylation of the guanosine triphosphatase (GTPase) rho in resting peripheral blood human T lymphocytes results in pseudopodial extension and the inhibition of T cell activation

Scrape loading Clostridium botulinum C3 exoenzyme into primary peripheral blood human T lymphocytes (PB T cells) efficiently adenosine diphosphate (ADP)-ribosylates and thus inactivates the guanosine triphosphatase (GTPase) Rho. Basal adhesion of PB T cells to the beta1 integrin substrate fibronectin (Fn) was not inhibited by inactivation of Rho, nor was upregulation of adhesion using phorbol myristate acetate (PMA; 10 ng/ml) or Mn++ (1 mM) affected. Whereas untreated PB T cells adherent to Fn remain spherical, C3-treated PB T cells extend F-actin-containing pseudopodia. Inactivation of Rho delayed the kinetics of PMA-dependent PB T cell homotypic aggregation, a process involving integrin alphaLbeta2. Although C3 treatment of PB T cells did not prevent adhesion to the beta1 integrin substrate Fn, it did inhibit beta1 integrin/CD3-mediated costimulation of proliferation. Analysis of intracellular cytokine production at the single cell level demonstrated that ADP-ribosylation of Rho inhibited beta1 integrin/ CD3 and CD28/CD3 costimulation of IL-2 production within 6 h of activation. Strikingly, IL-2 production induced by PMA and ionomycin was unaffected by C3 treatment. Thus, the GTPase Rho is a novel regulator of T lymphocyte cytoarchitecture, and functional Rho is required for very early events regulating costimulation of IL-2 production in PB T cells.

Characterization of the association of the actin-binding protein, IQGAP, and activated Cdc42 with Golgi membranes

IQGAP is a recently identified actin-binding protein, which is a putative target for the Cdc42 and Rac GTP-binding proteins. Cdc42 was localized to the Golgi (Erickson, J. W., Zhang, C., Kahn, R. A., Evans, T., and Cerione, R. A. (1996) J. Biol. Chem. 271, 26850-26854), and here we show by immunofluorescence that IQGAP has a perinuclear localization, that it can be co-immunoprecipitated with Cdc42 from Golgi-enriched fractions, and that purified Golgi membranes are recognized by specific antibodies raised against IQGAP and Cdc42 in negative-stain immunogold electron microscopy experiments. Addition of activated, recombinant Cdc42 or solubilization of endogenous Cdc42 from Golgi membranes by the Rho-GDP dissociation inhibitor protein fails to solubilize IQGAP, suggesting that it associates with these membranes in a Cdc42-independent manner. Detergent solubilization of Golgi membranes leaves IQGAP and actin in an insoluble pellet but releases Cdc42 to the supernatant, whereas treatments that release actin from this detergent-insoluble pellet also release IQGAP. Addition of the COOH-terminal half of the IQGAP protein, which contains the Cdc42-binding domain, removes Cdc42 from Golgi membranes in a dose-dependent manner. These data suggest that IQGAP and Cdc42 are part of a cytoskeletal complex in Golgi membranes that may mediate Cdc42-regulated effects on the actin cytoskeleton in these membranes.

Cytoskeletal reorganization by G protein-coupled receptors is dependent on phosphoinositide 3-kinase gamma, a Rac guanosine exchange factor, and Rac

Reorganization of the actin cytoskeleton is an early cellular response to a variety of extracellular signals. Dissection of pathways leading to actin rearrangement has focused largely on those initiated by growth factor receptors or integrins, although stimulation of G protein-coupled receptors also leads to cytoskeletal changes. In transfected Cos-7SH cells, activation of the chemoattractant formyl peptide receptor induces cortical actin polymerization and a decrease in the number of central actin bundles. In this report, we show that cytoskeletal reorganization can be transduced by G protein betagamma heterodimers (Gbetagamma), phosphoinositide 3-kinase gamma (PI3-Kgamma), a guanosine exchange factor (GEF) for Rac, and Rac. Expression of inactive variants of either PI3-Kgamma, the Rac GEF Vav, or Rac blocked the actin rearrangement. Neither wortmannin nor LY294002, pharmacologic inhibitors of PI3-K, could inhibit the actin rearrangement induced by a constitutively active Rac. The inhibition of cytoskeletal reorganization by the dominant negative Vav variants could be rescued by coexpression of a constitutively active form of Rac. In contrast, a Vav variant with its pleckstrin homology (PH) domain missing constitutively induced JNK activation and led to cytoskeletal reorganization, even without stimulation by PI3-Kgamma. This suggests that the PH domain of Vav controls the guanosine exchange activity of Vav, perhaps by a mechanism regulated by D3 phosphoinositides generated by PI3-K. Taken together, these findings delineate a pathway leading from activation of a G protein-coupled receptor to actin reorganization which sequentially involves Gbetagamma, PI3-Kgamma, a Rac GEF, and Rac.

Transformation activity of Cdc42 requires a region unique to Rho-related proteins

The Rho subfamily GTP-binding protein Cdc42 mediates actin cytoskeletal rearrangements and cell cycle progression and is essential for Ras transformation. Expression of a Cdc42 mutant (Cdc42(F28L)) that undergoes spontaneous activation (guanine nucleotide exchange) results in transformation of NIH3T3 fibroblasts. In this report, we show that deletion of residues 120-139 from Cdc42(F28L), which comprise an insert region unique to Rho subfamily proteins but is missing in other GTP-binding proteins, yields a Cdc42 molecule that still undergoes spontaneous GTP-GDP exchange and stimulates both actin cytoskeletal changes and the activation of the cellular targets p21-activated kinase and the c-Jun kinase (JNK1). However, this Cdc42 mutant is unable to transform cells. These findings indicate that the Rho subfamily insert region is dispensable for many of the known signaling pathways initiated by activated Cdc42 but is essential for its regulation of cell growth.

Transforming potential of Dbl family proteins correlates with transcription from the cyclin D1 promoter but not with activation of Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun

The dbl family of oncogenes encodes a large, structurally related, family of growth-regulatory molecules that possess guanine nucleotide exchange factor activity for specific members of the Rho family of Ras-related GTPases. We have evaluated matched sets of weakly and strongly transforming versions of five Dbl family proteins (Lfc, Lsc, Ect2, Dbl, and Dbs) to determine their ability to stimulate signaling pathways that are activated by Rho family proteins. We found that the transforming potential of this panel did not correlate directly with their ability to activate Jun NH2-terminal kinase, p38/Mpk2, serum response factor, or c-Jun. In contrast, transient stimulation of transcription from the cyclin D1 promoter provided a strong correlation with transforming potential, and we found constitutive up-regulation of cyclin D1 protein in Dbl family protein-transformed cells. In addition, we observed that at least two Dbl family members (Lfc and Ect2) induced changes in the actin cytoskeleton and exhibited nuclear signaling profiles that are consistent with a broader range of in vivo substrate utilization than is predicted from their in vitro exchange specificities. In summary, although Dbl family proteins exhibit signaling profiles that are consistent with their in vivo activation of Rho proteins, stimulation of cyclin D1 transcription is the only activity that correlates with transforming potential, thus suggesting that deregulated cell cycle progression may be important for Dbl family protein transformation.

Guanine nucleotide exchange factors regulate specificity of downstream signaling from Rac and Cdc42

The Rac and Cdc42 GTPases regulate diverse cellular behaviors involving the actin cytoskeleton, gene transcription, and the activity of multiple protein and lipid kinases. All of these pathways can potentially become activated when GTP-Rac or GTP-Cdc42 is formed in response to external cell signals, yet it is evident that each activity must also be able to be controlled individually. The mechanisms by which such specificity of GTPase signaling in response to upstream stimuli is achieved remains unclear. We investigated the action of several well characterized guanine nucleotide exchange factors (GEFRho) to activate Rac- and/or Cdc42-dependent kinase pathways. Coexpression studies in COS-7 cells revealed that the ability of individual guanine nucleotide exchange factors (GEFs) to activate the p21-activated kinase PAK1 could be dissociated from activation of c-Jun amino-terminal kinase, even though activation of both pathways requires the action of the GEFs on Rac and/or Cdc42. In contrast, expression of constitutively active forms of Rac or Cdc42 effectively stimulated both downstream kinases. We conclude that GEFs can be important determinants of downstream signaling specificity for members of the Rho GTPase family.

GTPgammaS-induced actin polymerisation in vitro: ATP- and phosphoinositide-independent signalling via Rho-family proteins and a plasma membrane-associated guanine nucleotide exchange factor

In a cell-free system from neutrophil cytosol GTP(γ)S can induce an increase in the number of free filament barbed ends and massive actin polymerisation and cross-linking. GTP(γ)S stimulation was susceptible to an excess of GDP, but not Bordetella pertussis toxin and could not be mimicked by aluminium fluoride, myristoylated GTPgammaS. Gialpha2 or Gbeta1gamma2 subunits of trimeric G proteins. In contrast, RhoGDI and Clostridium difficile toxin B (inactivating Rho family proteins) completely abrogated the effect of GTPgammaS. When recombinant, constitutively activated and GTPgammaS-loaded Rac1, RhoA, or Cdc42 proteins alone or in combination were probed at concentrations >100 times the endogenous, however, they were ineffective. Purified Cdc42/Rac-interactive binding (CRIB) domain of WASP or C3 transferase did not prevent actin polymerisation by GTPgammaS. The action of GTPgammaS was blocked by mM [Mg2+], unless a heat- and trypsin-sensitive component present in neutrophil plasma membrane was added. Liberation of barbed ends seems therefore to be mediated by a toxin B-sensitive cytosolic Rho-family protein, requiring a membrane-associated guanine nucleotide exchange factor (GEF) for its activation by GTPgammaS under physiologic conditions. The inefficiency of various protein kinase and phosphatase inhibitors (staurosporine, genistein, wortmannin, okadaic acid and vanadate) and removal of ATP by apyrase, suggests that phosphate transfer reactions are not required for the downstream propagation of the GTPgammaS signal. Moreover, exogenously added phosphoinositides failed to induce actin polymerisation and a PtdIns(4,5)P2-binding peptide did not interfere with the response to GTPgammaS. The speed and simplicity of the presented assay applicable to protein purification techniques will facilitate the further elucidation of the molecular partners involved in actin polymerisation.

Multiple signalling pathways lead to the activation of the nuclear factor kappaB by the Rho family of GTPases

Members of the Rho family of small GTPases activate the nuclear factor kappaB (NF-kappaB) (Perona, R., Montaner, S., Saniger, L., Sánchez-Pérez, I., Bravo, R., and Lacal, J. C. (1997), Genes & Dev. 11, 463-475). We have investigated whether different members of the family of exchange factors specific for Rho proteins (Dbl family) could activate the transcription factor NF-kappaB and have explored both their specificity under in vivo conditions and the mechanisms involved. Activated forms of Dbl, Ost, and Vav proteins induce NF-kappaB activation. While the activation induced by the Vav oncogen was efficiently inhibited by a dominant negative mutant of Rac1, the corresponding mutant of Cdc42Hs was able to block selectively NF-kappaB activation mediated by Dbl. Finally, mutants of RhoA and Cdc42Hs, but not that of Rac1, inhibited the activation of NF-kappaB by Ost. Thus, under in vivo conditions, different members of the Dbl family are related to specific Rho GTPases for the regulation of NF-kappaB. Activation of NF-kappaB by Rho or Ras proteins is mutually independent. However, there is a link between the NF-kappaB and the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) cascades since a dominant negative mutant of MEKK1 is able to inhibit NF-kappaB activation induced by Rac1 and Cdc42Hs proteins, but not by RhoA. These results indicate that, in mammalian cells, multiple pathways coexist for the activation of NF-kappaB, some of which are mediated by specific members of the Ras and Rho families of small GTPases.

UNC-73 activates the Rac GTPase and is required for cell and growth cone migrations in C. elegans

unc-73 is required for cell migrations and axon guidance in C. elegans and encodes overlapping isoforms of 283 and 189 kDa that are closely related to the vertebrate Trio and Kalirin proteins, respectively. UNC-73A contains, in order, eight spectrin-like repeats, a Dbl/Pleckstrin homology (DH/PH) element, an SH3-like domain, a second DH/PH element, an immunoglobulin domain, and a fibronectin type III domain. UNC-73B terminates just downstream of the SH3-like domain. The first DH/PH element specifically activates the Rac GTPase in vitro and stimulates actin polymerization when expressed in Rat2 cells. Both functions are eliminated by introducing the S1216F mutation of unc-73(rh40) into this DH domain. Our results suggest that UNC-73 acts cell autonomously in a protein complex to regulate actin dynamics during cell and growth cone migrations.

N terminus of Sos1 Ras exchange factor: critical roles for the Dbl and pleckstrin homology domains

We have studied the functional importance of the N terminus of mouse Sos1 (mSos1), a ubiquitously expressed Ras-specific guanine nucleotide exchange factor whose C-terminal sequences bind Grb-2. Consistent with previous reports, addition of a myristoylation signal to mSos1 (MyrSos1) rendered it transforming for NIH 3T3 cells and deletion of the mSos C terminus (MyrSos1-deltaC) did not interfere with this activity. However, an N-terminally deleted myristoylated mSos1 protein (MyrSos1-deltaN) was transformation defective, although the protein was stable and localized to the membrane. Site-directed mutagenesis was used to examine the role of the Dbl and pleckstrin homology (PH) domains located in the N terminus. When mutations in the PH domain were introduced into two conserved amino acids either singly or together in MyrSos1 or MyrSos1-deltaC, the transforming activity was severely impaired. An analogous reduction in biological activity was seen when a cluster of point mutations was engineered into the Dbl domain. The mitogen-activation protein (MAP) kinase activities induced by the various Dbl and PH mutants of MyrSos1 correlated with their biological activities. When NIH 3T3 cells were transfected with a myristoylated Sos N terminus, their growth response to epidermal growth factor (EGF), platelet-derived growth factor, lysophosphatidic acid or serum was greatly impaired. The dominant inhibitory biological activity of the N terminus correlated with its ability to impair EGF-dependent activation of GTP-Ras and of MAP kinase, as well with the ability of endogenous Sos to form a stable complex with activated EGF receptors. The N terminus with mutations in the Dbl and PH domains was much less inhibitory in these biological and biochemical assays. In contrast to wild-type Sos1, nonmyristoylated versions of Sos1-deltaN and Sos1-deltaC did not form a stable complex with activated EGF receptors. We conclude that the Dbl and PH domains are critical for Sos function and that stable association of Sos with activated EGF receptors requires both the Sos N and C termini.

DRhoGEF2 encodes a member of the Dbl family of oncogenes and controls cell shape changes during gastrulation in Drosophila

We have identified a gene, DRhoGEF2, which encodes a putative guanine nucleotide exchange factor belonging to the Dbl family of oncogenes. DRhoGEF2 function is essential for the coordination of cell shape changes during gastrulation. In the absence of maternal DRhoGEF2 gene activity, mesodermal and endodermal primordia fail to invaginate. The phenotype seen in DRhoGEF2 mutants is more severe than the defects associated with mutations in two previously identified gastrulation genes, folded gastrulation and concertina, suggesting that DRhoGEF2 acts in a signaling pathway independent of these genes. Expression of dominant-negative DRhoA during gastrulation results in phenocopies of the DRhoGEF2 mutant, suggesting that a signaling cascade involving DRhoGEF2 and the small GTPase DRhoA is responsible for the regulation of cell shape changes during early Drosophila morphogenesis.

PAK kinases are directly coupled to the PIX family of nucleotide exchange factors

The PAK family of kinases are regulated through interaction with the small GTPases Cdc42 and Rac1, but little is known of the signaling components immediately upstream or downstream of these proteins. We have purified and cloned a new class of Rho-p21 guanine nucleotide exchange factor binding tightly through its N-terminal SH3 domain to a conserved proline-rich PAK sequence with a Kd of 24 nM. This PAK-interacting exchange factor (PIX), which is widely expressed and enriched in Cdc42- and Rac1-driven focal complexes, is required for PAK recruitment to these sites. PIX can induce membrane ruffling, with an associated activation of Rac1. Our results suggest a role for PIX in Cdc42-to-Rac1 signaling, involving the PIX/PAK complex.

Targeting of Tiam1 to the plasma membrane requires the cooperative function of the N-terminal pleckstrin homology domain and an adjacent protein interaction domain

The Rho-like GTPases Cdc42, Rac, and Rho play key roles in the regulation of the actin cytoskeleton and are implicated in transcriptional activation and cell transformation. We have previously identified the invasion-inducing Tiam1 gene, which encodes an activator of Rac. In fibroblasts, Tiam1 induces Rac-mediated membrane ruffling, which requires the N-terminal pleckstrin homology (PHn) domain. Here we show that this PHn domain is part of a protein interaction domain, which mediates membrane localization of Tiam1. After subcellular fractionation, up to 50% of Tiam1 is recovered in the Triton X-100-insoluble high speed pellet that contains small protein complexes. The regions in Tiam1 that are responsible for these protein interactions comprise the PHn domain, an adjacent putative coiled coil region (CC), and an additional flanking region (Ex). Deletions in each of these regions abolish membrane localization of Tiam1 and membrane ruffling, suggesting that they function cooperatively. Indeed, only polypeptides encompassing the PHn-CC-Ex region, and not the PHn-CC or the Ex region, localize at the membrane. These results indicate that the N-terminal PH domain is part of a larger functional Tiam1 domain that mediates protein complex formation and membrane localization of Tiam1.

Interaction between Cdc42Hs and RhoGDI is mediated through the Rho insert region

Members of the Rho subfamily of GTP-binding proteins contain a region of amino acid sequence (residues 122-134) that is absent from other Ras-like proteins and is termed the Rho insert region. To address the functional role of this domain, we have constructed a Cdc42Hs/Ras chimera in which loop 8 from Ha-Ras was substituted for the region in Cdc42Hs that contains the 13-amino acid insert region. Our data indicate that the insert region of Cdc42Hs is not essential for its interactions with various target/effector molecules or for interactions with the guanine nucleotide exchange factor, Dbl, or the Cdc42 GTPase-activating protein (GAP). However, the regulation of GDP dissociation and GTP hydrolysis on Cdc42Hs by the Rho GDP-dissociation inhibitor (GDI) is extremely sensitive to changes in the insert region, such that a Cdc42Hs/Ha-Ras chimera that lacks this insert is no longer susceptible to a GDI-induced inhibition of GDP dissociation and GTP hydrolysis. The insensitivity to GDI activity is not due to the inability of the GDI molecule to bind to the Cdc42Hs/Ha-Ras chimera, and in fact, the GDI is fully capable of stimulating the release of this chimera from membranes.

A novel Cdc42Hs mutant induces cellular transformation

Cdc42Hs is a small GTPase of the Rho-subfamily, which regulates signaling pathways that influence cell morphology and polarity, cell-cycle progression and transcription. An essential role for Cdc42Hs in cell growth regulation has been suggested by the finding that the Dbl oncoprotein is an upstream activator-a guanine nucleotide exchange factor (GEF)-for Cdc42Hs, and that activated mutants of the closely related GTPases Rac and Rho are transforming. As we were unable to obtain significant over-expression of GTPase-defective Cdc42Hs mutants, we have generated a mutant, Cdc42Hs(F28L), which can undergo spontaneous GTP-GDP exchange while maintaining full GTPase activity, and thus should exhibit functional activities normally imparted by Dbl. In cultured fibroblasts, Cdc42Hs(F28L) activated the c-Jun kinase (JNK1) and stimulated filopodia formation. Cells stably expressing Cdc42Hs(F28L) also exhibited several hallmarks of transformation-reduced contact inhibition, lower dependence on serum for growth, and anchorage-independent growth. Our findings indicate that Cdc42Hs plays a role in cell proliferation, and is a likely physiological mediator of Dbl-induced transformation.

The small GTPases Cdc42Hs, Rac1 and RhoG delineate Raf-independent pathways that cooperate to transform NIH3T3 cells

BACKGROUND

Ras-mediated transformation of mammalian cells has been shown to activate multiple signalling pathways, including those involving mitogen-activated protein kinases and the small GTPase Rho. Members of the Rho family affect cell morphology by controlling the formation of actin-dependent structures: specifically, filopodia are induced by Cdc42Hs, lamellipodia and ruffles by Rac, and stress fibers by RhoA. In addition, Rho GTPases are involved in progression through the G1 phase of the cell cycle, and Rac1 and RhoA have recently been directly implicated in the morphogenic and mitogenic responses to transformation by oncogenic Ras. In order to examine the cross-talk between Ras and Rho proteins, we investigated the effects on focus-forming activity and cell growth of the Rho-family members Cdc42Hs, Rac1 and RhoG by expressing constitutively active or dominant-negative forms in NIH3T3 cells.

RESULTS

Expression of Rac1 or RhoG modulated the saturation density to which the cells grew, probably by affecting the level of contact inhibition. Although all three GTPases were required for cell transformation mediated by Ras but not by constitutively active Raf, the selective activation of each GTPase was not sufficient to induce the formation of foci. The coordinated activation of Cdc42Hs, RhoG and Rac1, however, elicited a high focus-forming activity, independent of the mitogen-activated ERK and JNK protein kinase pathways.

CONCLUSIONS

Ras-mediated transformation induces extensive changes in cell morphology which require the activity of members of the Rho family of GTPases. Our data show that the pattern of coordinated Rho family activation that elicits a focus-forming activity in NIH3T3 cells is distinct from the regulatory cascade that has been proposed for the control of actin-dependent structures in Swiss 3T3 cells.

Kalirin, a cytosolic protein with spectrin-like and GDP/GTP exchange factor-like domains that interacts with peptidylglycine alpha-amidating monooxygenase, an integral membrane peptide-processing enzyme

Although the integral membrane proteins that catalyze steps in the biosynthesis of neuroendocrine peptides are known to contain routing information in their cytosolic domains, the proteins recognizing this routing information are not known. Using the yeast two-hybrid system, we previously identified P-CIP10 as a protein interacting with the cytosolic routing determinants of peptidylglycine alpha-amidating monooxygenase (PAM). P-CIP10 is a 217-kDa cytosolic protein with nine spectrin-like repeats and adjacent Dbl homology and pleckstrin homology domains typical of GDP/GTP exchange factors. In the adult rat, expression of P-CIP10 is most prevalent in the brain. Corticotrope tumor cells stably expressing P-CIP10 and PAM produce longer and more highly branched neuritic processes than nontransfected cells or cells expressing only PAM. The turnover of newly synthesized PAM is accelerated in cells co-expressing P-CIP10. P-CIP10 binds to selected members of the Rho subfamily of small GTP binding proteins (Rac1, but not RhoA or Cdc42). P-CIP10 (kalirin), a member of the Dbl family of proteins, may serve as part of a signal transduction system linking the catalytic domains of PAM in the lumen of the secretory pathway to cytosolic factors regulating the cytoskeleton and signal transduction pathways.

Extracting protein alignment models from the sequence database

Biologists often gain structural and functional insights into a protein sequence by constructing a multiple alignment model of the family. Here a program called Probe fully automates this process of model construction starting from a single sequence. Central to this program is a powerful new method to locate and align only those, often subtly, conserved patterns essential to the family as a whole. When applied to randomly chosen proteins, Probe found on average about four times as many relationships as a pairwise search and yielded many new discoveries. These include: an obscure subfamily of globins in the roundworm Caenorhabditis elegans ; two new superfamilies of metallohydrolases; a lipoyl/biotin swinging arm domain in bacterial membrane fusion proteins; and a DH domain in the yeast Bud3 and Fus2 proteins. By identifying distant relationships and merging families into superfamilies in this way, this analysis further confirms the notion that proteins evolved from relatively few ancient sequences. Moreover, this method automatically generates models of these ancient conserved regions for rapid and sensitive screening of sequences.

Regulated membrane localization of Tiam1, mediated by the NH2-terminal pleckstrin homology domain, is required for Rac-dependent membrane ruffling and C-Jun NH2-terminal kinase activation

Rho-like GTPases, including Cdc42, Rac, and Rho, regulate signaling pathways that control actin cytoskeletal structures and transcriptional activation. The Tiam1 gene encodes an activator of Rac1, and similarly to constitutively activated (V12)Rac1, overexpression of Tiam1 in fibroblasts induces the formation of membrane ruffles. Tiam1 contains a Dbl homology (DH) domain and adjacent pleckstrin homology (PH) domain, hallmarks for activators of Rho-like GTPases. Unique for Tiam1 are an additional PH domain and a Discs-large homology region in the NH2-terminal part of the protein. Here we show that both in fibroblasts and COS cells, membrane localization of Tiam1 is required for the induction of membrane ruffling. A detailed mutational analysis, in combination with confocal laser scanning microscopy and immunoelectron microscopy, demonstrates that the NH2-terminal PH domain of Tiam1, but not the DH-adjacent PH domain, is essential for membrane association. This NH2-terminal PH domain of Tiam1 can be functionally replaced by the myristoylated membrane localization domain of c-Src, indicating that the primary function of this PH domain is to localize the protein at the membrane. After serum starvation, both membrane association of Tiam1 and ruffling can be induced by serum, suggesting that receptor stimulation induces membrane translocation of Tiam1. Similar to V12Rac1, Tiam1 stimulates the activity of the c-Jun NH2-terminal kinase (JNK). This Rac-dependent stimulation of JNK also requires membrane association of Tiam1. We conclude that the regulated membrane localization of Tiam1 through its NH2-terminal PH domain determines the activation of distinct Rac-mediated signaling pathways.

Activation of the exchange factor Ras-GRF by calcium requires an intact Dbl homology domain

Ras-GRF is a guanine nucleotide exchange factor that activates Ras proteins. Its activity on Ras in cells is enhanced upon calcium influx. Activation follows calcium-induced binding of calmodulin to an IQ motif near the N-terminus of Ras-GRF. Ras-GRF also contains a Dbl homology (DH) domain C-terminal to the IQ motif. In many proteins, DH domains act as exchange factors for Rho-GTPase family members. However, we failed to detect exchange activity of this domain on well characterized Rho family members. Instead, we found that mutations analogous to those that block exchange activity of Dbl prevented Ras-GRF activation by calcium/ calmodulin in vivo. All DH domains are followed immediately by a pleckstrin homology (PH) domain. We found that a mutation at a conserved site within the PH domain following the DH domain also prevented Ras-GRF activation by calcium in vivo. These results suggest that in addition to playing a role as activators of Rho proteins, DH domains can also contribute to the coupling of cellular signals to Ras activation.

Tyrosine phosphorylation of the vav proto-oncogene product links FcepsilonRI to the Rac1-JNK pathway

Stimulation of high affinity IgE Fc receptors (FcepsilonRI) in basophils and mast cells activates the tyrosine kinases Lyn and Syk and causes the tyrosine phosphorylation of phospholipase C-gamma, resulting in the Ca2+- and protein kinase C-dependent secretion of inflammatory mediators. Concomitantly, FcepsilonRI stimulation initiates a number of signaling events resulting in the activation of mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK), which, in turn, regulate nuclear responses, including cytokine gene expression. To dissect the signaling pathway(s) linking FcepsilonRI to MAPK and JNK, we reconstructed their respective biochemical routes by expression of a chimeric interleukin-2 receptor alpha subunit (Tac)-FcepsilonRI gamma chain (Tacgamma) in COS-7 cells. Cross-linking of Tacgamma did not affect MAPK in COS-7 cells, but when coexpressed with the tyrosine kinase Syk, Tacgamma stimulation potently induced Syk and Shc tyrosine phosphorylation and MAPK activation. In contrast, Tacgamma did not signal JNK activation, even when coexpressed with Syk. Ectopic expression of a hematopoietic-specific guanine nucleotide exchange factor (GEF), Vav, reconstituted the Tacgamma-induced, Syk- and Rac1-dependent JNK activation; and tyrosine-phosphorylation of Vav by Syk stimulated its GEF activity for Rac1. Thus, these data strongly suggest that Vav plays a critical role linking FcepsilonRI and Syk to the Rac1-JNK pathway. Furthermore, these findings define a novel signal transduction pathway involving a multimeric cell surface receptor acting on a cytosolic tyrosine kinase, which, in turn, phosphorylates a GEF, thereby regulating its activity toward a small GTP-binding protein and promoting the activation of a kinase cascade.

Residues of the Rho family GTPases Rho and Cdc42 that specify sensitivity to Dbl-like guanine nucleotide exchange factors

The Dbl-like guanine nucleotide exchange factor (GEF) Lbc oncoprotein specifically activates the small GTP-binding protein Rho in mammalian fibroblasts to induce transformation and actin stress fiber formation, whereas another Dbl-related molecule, Cdc24, stimulates guanine nucleotide exchange of the Rho family GTPase Cdc42 to elicit effects on both gene induction and actin-based cytoskeleton change in Saccharomyces cerevisiae. To understand the mechanism of these functional interactions, we have taken a biochemical approach to probe the sites on Rho and Cdc42 that are involved in coupling to their respective GEFs, the Lbc and Cdc24 proteins. Point mutations in the switch II region of the small G-proteins, many of which would affect the interaction with GEF in the case of Ras, or a mutation in the switch I region that was identified as a contact site between Rab3A and Rab GEF had little effect on RhoA or Cdc42Hs with regard to the ability to interact with Lbc or Cdc24, suggesting that there exists a unique mechanism of regulation of the Rho family proteins by their GEFs. Analysis of a panel of chimeras made between RhoA and Cdc42Hs, which all maintained the ability to respond to Dbl, their mutual GEF, and to GTPase-activating protein, revealed that at least two distinct sites in each of the GTPases are required for activation by the respective GEFs. Further site-directed mutagenesis studies showed that the conserved residue Tyr32 in the putative effector region of both GTPases (numbered by Cdc42Hs) is critical for binding of the GEFs and that specific recognition for Lbc or Cdc24 is achieved at least in part through residues Lys27 of Rho and Gln116 of Cdc42. Moreover, the loss of GEF responsiveness of a RhoA mutation (D76Q) was found to be caused by the impaired GEF catalysis, not by a change in the GEF binding affinity. Together, these results indicate that multiple sites of the Rho GTPases are involved in the regulation by GEFs, contributing to GEF binding or GEF catalysis, and raise the possibility that activation of each Rho family G-protein by a specific GEF may engage in a distinct mechanism.

Taxonomy and function of C1 protein kinase C homology domains

C1 domains are compact alpha/beta structural units of about 50 amino acids which tightly bind two zinc ions. These domains were first discovered as the loci of phorbol ester and diacylglycerol binding to conventional protein kinase C isozymes, which contain 2 C1 domains (C1A and C1B) in their N-terminal regulatory regions. We present a comprehensive list of 54 C1 domains occurring singly or doubly in 34 different proteins. Many C1 domains and C1 domain-containing proteins bind phorbol esters, but many others do not. By combining analysis of 54 C1 domain sequences with information from previously reported solution and crystal structure determinations and site-directed mutagenesis, profiles are derived and used to classify C1 domains. Twenty-six C1 domains fit the profile for phorbol-ester binding and are termed "typical." Twenty-eight other domains fit the profile for the overall C1 domain fold but do not fit the profile for phorbol ester binding, and are termed "atypical." Proteins containing typical C1 domains are predicted to be regulated by diacylglycerol, whereas those containing only atypical domains are not.

Still life, a protein in synaptic terminals of Drosophila homologous to GDP-GTP exchangers

The morphology of axon terminals changes with differentiation into mature synapses. A molecule that might regulate this process was identified by a screen of Drosophila mutants for abnormal motor activities. The still life (sif) gene encodes a protein homologous to guanine nucleotide exchange factors, which convert Rho-like guanosine triphosphatases (GTPases) from a guanosine diphosphate-bound inactive state to a guanosine triphosphate-bound active state. The SIF proteins are found adjacent to the plasma membrane of synaptic terminals. Expression of a truncated SIF protein resulted in defects in neuronal morphology and induced membrane ruffling with altered actin localization in human KB cells. Thus, SIF proteins may regulate synaptic differentiation through the organization of the actin cytoskeleton by activating Rho-like GTPases.

The faciogenital dysplasia gene product FGD1 functions as a Cdc42Hs-specific guanine-nucleotide exchange factor

The Rho family of small GTP-binding proteins plays important roles in the regulation of actin cytoskeleton organization and cell growth. Activation of these GTPases involves the replacement of bound GDP with GTP, a process catalyzed by the Dbl-like guanine-nucleotide exchange factors, all of which seem to share a putative catalytic motif termed the Dbl homology (DH) domain, followed by a pleckstrin homology (PH) domain. Here we have examined the role of a Dbl-like molecule, the faciogenital dysplasia gene product (FGD1), which when mutated in its Dbl homology domain, cosegregates with the developmental disease Aarskog-Scott syndrome. We report that a polypeptide of FGD1 encompassing the DH and PH domains can bind specifically to the Rho family GTPase Cdc42Hs and stimulates the GDP-GTP exchange of the isoprenylated form of Cdc42Hs. Microinjection of this FGD1 polypeptide into Swiss 3T3 fibroblast cells induces the formation of peripheral actin microspikes, similar to that previously observed when cells were injected with a constitutively active form of Cdc42Hs. This effect of FGD1 on actin organization is readily inhibited by coinjection of a dominant-negative mutant of Cdc42Hs. Examination of NIH 3T3 cells expressing the FGD1 fragment revealed that similar to cells expressing Dbl, two independent elements downstream of Cdc42Hs, the Jun NH2-terminal kinase and the p70 S6 kinase, became activated. Hence, our results indicate that FGD1, through its DH and PH domains, acts as a Cdc42Hs-specific guanine-nucleotide exchange factor and suggest that the Cdc42Hs GTPase may have a role in mammalian development.

Faciogenital dysplasia protein (FGD1) and Vav, two related proteins required for normal embryonic development, are upstream regulators of Rho GTPases

BACKGROUND

Dbl, a guanine nucleotide exchange factor (GEF) for members of the Rho family of small GTPases, is the prototype of a family of 15 related proteins. The majority of proteins that contain a DH (Dbl homology) domain were isolated as oncogenes in transfection assays, but two members of the DH family, FGD1 (the product of the faciogenital dysplasia or Aarskog-Scott syndrome locus) and Vav, have been shown to be essential for normal embryonic development. Mutations to the FGD1 gene result in a human developmental disorder affecting specific skeletal structures, including elements of the face, cervical vertebrae and distal extremities. Homozygous Vav-/- knockout mice embryos are not viable past the blastocyst stage, indicating an essential role of Vav in embryonic implantation.

RESULTS

Here, we show that the microinjection of FGD1 and Vav into Swiss 3T3 fibroblasts induces the polymerization of actin and the assembly of clustered integrin complexes. FGD1 activates Cdc42, whereas Vav activates Rho, Rac and Cdc42. In addition, FGD1 and Vav stimulate the mitogen activated protein kinase cascade that leads to activation of the c-Jun kinase SAPK/JNK1.

CONCLUSIONS

We conclude that FGD1 and Vav are regulators of the Rho GTPase family. Along with their target proteins Cdc42, Rac and Rho, FGD1 and Vav control essential signals required during embryonic development.

Lfc and Lsc oncoproteins represent two new guanine nucleotide exchange factors for the Rho GTP-binding protein

Lfc and Lsc are two recently identified oncoproteins that contain a Dbl homology domain in tandem with a pleckstrin homology domain and thus share sequence similarity with a number of other growth regulatory proteins including Dbl, Tiam-1, and Lbc. We show here that Lfc and Lsc, like their closest relative Lbc, are highly specific guanine nucleotide exchange factors (GEFs) for Rho, causing a >10-fold stimulation of [3H]GDP dissociation from Rho and a marked stimulation of GDP-[35S]GTPgammas (guanosine 5'-O-(3-thiotriphosphate) exchange. All three proteins (Lbc, Lfc, and Lsc) are able to act catalytically in stimulating the guanine nucleotide exchange activity, such that a single molecule of each of these oncoproteins can activate a number of molecules of Rho. Neither Lfc nor Lsc shows any ability to stimulate GDP dissociation from other related GTP-binding proteins such as Rac, Cdc42, or Ras. Thus Lbc, Lfc, and Lsc appear to represent a subgroup of Dbl-related proteins that function as highly specific GEFs toward Rho and can be distinguished from Dbl, Ost, and Dbs which are less specific and show GEF activity toward both Rho and Cdc42. Consistent with these results, Lbc, Lfc, and Lsc each form tight complexes with the guanine nucleotide-depleted form of Rho and bind weakly to the GDP- and GTPgammaS-bound states. None of these oncoproteins are able to form complexes with Cdc42 or Ras. However, Lfc (but not Lbc nor Lsc) can bind to Rac, and this binding occurs equally well when Rac is nucleotide-depleted or is in the GDP- or GTPgammaS-bound state. These findings raise the possibility that in addition to acting directly as a GEF for Rho, Lfc may play other roles that influence the signaling activities of Rac and/or coordinate the activities of the Rac and Rho proteins.

Mammalian Cdc42 is a brefeldin A-sensitive component of the Golgi apparatus

In this study, we have used immunocytochemical and fractionation approaches to provide a description of the localization of the mammalian Cdc42 protein (designated Cdc42Hs) in vivo. A specific anti-peptide antibody was generated against the C-terminal region of Cdc42Hs. Using affinity-purified preparations of this antibody in indirect immunofluorescence experiments, Cdc42Hs was found to be localized to the Golgi apparatus. Similar to the well-characterized non-clathrin coat proteins ADP-ribosylation factor (ARF) and beta-COP, the perinuclear clustering of Cdc42Hs is rapidly dispersed upon exposure of the cells to the drug brefeldin A, suggesting that it too may play a role in the processes of intracellular lipid and protein transport. Employing cell lines possessing inducible forms of ARF, we demonstrate here a tight coupling of the nucleotide-bound state of ARF and the subcellular localization of Cdc42Hs. Specifically, the expression of wild-type ARF had no effect on the brefeldin A sensitivity of Cdc42Hs while, as is the case for ARF and beta-COP, expression of a GTPase-deficient form of ARF (ARF(Q71L)) renders these Golgi-localized proteins resistant to brefeldin A treatment (; ). Moreover, the induced expression of a mutant form of ARF with a low affinity for nucleotide resulted in constitutive redistribution of Cdc42Hs in the absence of brefeldin A treatment. These results suggest that Cdc42Hs may play a role in cell morphogenesis by acting on targets in the Golgi that direct polarized growth at the plasma membrane.

Identification of a novel guanine nucleotide exchange factor for the Rho GTPase

The Rho GTPase promotes proliferation and cytoskeletal rearrangements in mammalian cells. To understand the regulation of Rho, it is important to characterize guanine nucleotide exchange factors (GEFs), which stimulate the dissociation of GDP and subsequent binding of GTP. Using Rho as an affinity ligand, we have isolated a 115-kDa protein (p115-RhoGEF) that binds specifically to the nucleotide-depleted state. A full-length cDNA encoding p115-RhoGEF was isolated, and its protein product, which exhibited sequence homology to Dbl and Lbc, catalyzed the exchange of GDP for GTP specifically on Rho and not on the Rac, Cdc42, or Ras GTPases. p115-RhoGEF is capable of regulating cell proliferation, as determined by its ability to induce the transformation of NIH 3T3 cells. Northern and Western analysis suggests that p115-RhoGEF is ubiquitously expressed. These results indicate that p115-RhoGEF may be a general regulator of Rho and its associated cellular phenotypes.

Phosphatidylinositol 4,5-bisphosphate provides an alternative to guanine nucleotide exchange factors by stimulating the dissociation of GDP from Cdc42Hs

Members of the Rho subfamily of Ras-related GTP-binding proteins play important roles in the organization of the actin cytoskeleton and in the regulation of cell growth. We have shown previously that the dbl oncogene product, which represents a prototype for a family of growth regulatory proteins, activates Rho subfamily GTP-binding proteins by catalyzing the dissociation of GDP from their nucleotide binding site. In the present study, we demonstrate that the acidic phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), provides an alternative mechanism for the activation of Cdc42Hs. Among a variety of lipids tested, only PIP2 was able to stimulate GDP release from Cdc42Hs in a dose-dependent manner, with a half-maximum effect at approximately 50 microM. Unlike the Dbl oncoprotein, which requires the presence of (free) guanine nucleotide in the medium to replace the GDP bound to Cdc42Hs, PIP2 stimulates GDP release from Cdc42Hs in the absence of free guanine nucleotide. PIP2, when incorporated into phosphatidylcholine carrier vesicles, binds tightly to the guanine nucleotide-depleted form of Cdc42Hs and weakly to the GDP-bound form of the GTP-binding protein but does not bind to GTP-bound Cdc42Hs, similar to what was observed for the Dbl oncoprotein. However, mutational analysis of Cdc42Hs indicates that the site that is essential for the functional interaction between PIP2 and Cdc42Hs is distinct from the Dbl-binding site and is located at the positively charged carboxyl-terminal end of the GTP-binding protein. The GDP-releasing activity of PIP2 is highly effective toward Cdc42Hs and Rho (and is similar to the reported effects of PIP2 on Arf (Terui, T., Kahn, R. A., and Randazzo, P. A., (1994) J. Biol. Chem. 269, 28130-28135)), is less effective with Rac, and is not observed with Ras, Rap1a, or Ran. The ability of PIP2 to activate Cdc42Hs (or Rho) and Arf provides a possible point of convergence for the biological pathways regulated by these different GTP-binding proteins and may be related to the synergism observed between Arf and Rho-subtype proteins in the stimulation of phospholipase D activity (Singer, W. D., Brown, H. A., Bokoch, G. M., and Sternweis, P. C. (1995) J. Biol. Chem. 270, 14944-14950).

Identification of a putative effector for Cdc42Hs with high sequence similarity to the RasGAP-related protein IQGAP1 and a Cdc42Hs binding partner with similarity to IQGAP2

Cdc42 is a Ras-related GTP-binding protein that has been implicated in the regulation of the actin cytoskeleton and cell morphology. In this study, we have identified a protein with a molecular mass approximately 180 kDa from rabbit liver cytosol (designated p180), which binds preferentially to the GTP- and guanosine 5'-3-O-(thio)triphosphate-bound forms of Cdc42. Binding of p180 to GTP-bound Cdc42 maintains it in the GTP-bound state. Another cytosolic protein, with an apparent molecular mass of 175 kDa (p175), was also found to interact with Cdc42, but this association showed less dependence on guanine nucleotides. Both p180 and p175 were capable of binding to Rac1 but not to RhoA or Ha-Ras. The limit functional domain of the Cdc42-GAP protein did not compete with p180 or p175 for binding to Cdc42. However, the Cdc42-binding domain from mPAK-3, a member of the PAK (p21 activated kinase) family of serine/threonine kinases, competed with both proteins. The binding of p180 or p175 was inhibited by mutations of the putative effector loop of Cdc42. p180 and p175 also bound less effectively to a Cdc42/Ras chimera in which loop 8 from Ras was substituted for the predicted loop 8 in Cdc42 that includes a 13-amino acid insert present in all Rho family members but absent in Ras. Microsequencing of a p180 peptide revealed 92% identity with the human IQGAP1 protein, while two peptides derived from p175 were 89 and 100% identical to human IQGAP2. These findings identify IQGAP1 and IQGAP2 as a new class of target/effectors that utilize both regions of the switch I domain and an insert region distinct to Rho proteins for binding to Cdc42.

Galpha12 and Galpha13 regulate extracellular signal-regulated kinase and c-Jun kinase pathways by different mechanisms in COS-7 cells

Many growth factors and agonists for G protein-coupled receptors activate mitogen-activated protein (MAP) kinase pathways, including the extracellular signal-regulated kinase (ERK) pathway and the c-Jun kinase (JNK) pathway. Transient transfection of dominant negative and constitutively active pathway components in COS-7 cells shows that two G protein subunits, Galpha12 and Galpha13, inhibit the ERK pathway and stimulate the JNK pathway. Constitutively active (GTPase-deficient) Galpha12 and Galpha13 both inhibit ERK pathway activation by epidermal growth factor. A Galpha13/alphaz chimera, which responds to stimulation by Gi-coupled receptors, mediates inhibition of ERK via such a receptor, the dopamine-2 receptor. In addition, expression of a dominant negative mutant of the GTPase, Cdc42, blocks activation of the JNK pathway by Galpha12 and Galpha13 but does not alter inhibition of ERK activation by the same Galpha proteins; conversely, mutationally activated Cdc42 stimulates the JNK pathway but has no effect on the ERK pathway. Our results show that different mechanisms mediate two effects of Galpha12 and Galpha13: the ERK pathway inhibition is mediated at the level of MAP kinase kinase in a Ras- and Raf-independent fashion, whereas the JNK pathway stimulation is mediated by Cdc42.

The pleckstrin homology domain mediates transformation by oncogenic dbl through specific intracellular targeting

The pleckstrin homology (PH) domain is an approximately 100 amino acid structural motif found in many cellular signaling molecules, including the Dbl oncoprotein and related, putative guanine nucleotide exchange factors (GEFs). Here we have examined the role of the Dbl PH (dPH) domain in the activities of oncogenic Dbl. We report that the dPH domain is not involved in the interaction of Dbl with small GTP-binding proteins and is incapable of transforming NIH 3T3 fibroblasts. On the other hand, co-expression of the dPH domain with oncogenic Dbl inhibits Dbl-induced transformation. A deletion mutant of Dbl that lacks a significant portion of the PH domain retains full GEF activity, but is completely inactive in transformation assays. Replacement of the PH domain by the membrane-targeting sequence of Ras is not sufficient for the recovery of transforming activity. However, subcellular fractionations of Dbl and Dbl mutants revealed that the PH domain is necessary and sufficient for the association of Dbl with the Triton X-100-insoluble cytoskeletal components. Thus, our results suggest that the dPH domain mediates cellular transformation by targeting the Dbl protein to specific cytoskeletal locations to activate Rho-type small GTP-binding proteins.

Expression cloning of lsc, a novel oncogene with structural similarities to the Dbl family of guanine nucleotide exchange factors

In a screen for genes with oncogenic potential expressed by the murine B6SUtA1 myeloid progenitor cell line, we isolated a 2. 5-kilobase pair cDNA whose expression causes strong morphological transformation and deregulated proliferation of NIH 3T3 cells. The transforming cDNA encodes a truncated protein (designated Lsc) with a region of sequence similarity to the product of the lbc oncogene. This region includes the tandem Dbl homology and pleckstrin homology domains that are hallmarks of the Dbl-like proteins, a family of presumptive or demonstrated guanine nucleotide exchange factors that act on Rho family GTPases. Lsc requires intact Dbl homology and pleckstrin homology domains for its oncogenic activity. The transforming activity of Lsc in NIH 3T3 cells is reduced by cotransfection with p190 (a GTPase activating protein for Rho family GTPases) and the Rho family dominant-negative mutants RhoA(19N), CDC42(17N), and Rac1(17N). These results indicate a role for the Rho family of GTPases in mediating the transforming activity of Lsc and are consistent with the exchange specificities that have been attributed to Dbl family members. The lsc gene is expressed in a variety of tissues and is particularly abundant in hemopoietic tissues (thymus, spleen, and bone marrow). Lsc is a member of a growing family of proteins that may function as activators of Rho family GTPases in a developmental or tissue-specific manner.

Rhotekin, a new putative target for Rho bearing homology to a serine/threonine kinase, PKN, and rhophilin in the rho-binding domain

Using a mouse embryo cDNA library, we conducted a two-hybrid screening to identify new partners for the small GTPase Rho. One clone obtained by this procedure contained a novel cDNA of 291 base pairs and interacted strongly with RhoA and RhoC, weakly with RhoB, and not at all with Rac1 and Cdc42Hs. Full-length cDNAs were then isolated from a mouse brain library. While multiple splicing variants were common, we identified three cDNAs with an identical open reading frame encoding a 61-kDa protein that we named rhotekin (from the Japanese "teki," meaning target). The N-terminal part of rhotekin, encoded by the initial cDNA and produced in bacteria as a glutathione S-transferase fusion protein, exhibited in vitro binding to 35S-labeled guanosine 5'-3-O-(thio)triphosphate-bound Rho, but not to Rac1 or Cdc42Hs in ligand overlay assays. In addition, this peptide inhibited both endogenous and GTPase-activating protein-stimulated Rho GTPase activity. The amino acid sequence of this region shares approximately 30% identity with the Rho-binding domains of rhophilin and a serine/threonine kinase, PKN, two other Rho target proteins that we recently identified (Watanabe, G., Saito, Y., Madaule, P., Ishizaki, T., Fujisawa, K., Morii, N., Mukai, H., Ono, Y., Kakizuka, A., and Narumiya, S. (1996) Science 271, 645-648). Thus, not only is rhotekin a novel partner for Rho, but it also belongs to a wide family of proteins that bear a consensus Rho-binding sequence at the N terminus. To our knowledge, this is the first conserved sequence for Rho effectors, and we have termed this region Rho effector motif class 1.

ADP-ribosylation of the G protein Rho inhibits integrin regulation of tumor cell growth

Using a gastric derived tumor line, we investigated the involvement of beta 1 integrin and Rho in cell growth regulation in response to collagen. The addition of C3 exoenzyme from clostridium botulinum to specifically ribosylate and inhibit the function of the rho gene products inhibited cellular proliferation in a dose-dependent fashion. C3 exoenzyme exhibited broad cytostatic activity toward a number of tumor lines and induced G0/G1 accumulation, cyclin A inhibition, and pronounced alterations in cell morphology. Integrin-mediated adhesion to collagen led to the expression of the cyclin A gene whose expression could be blocked using anti-beta 1 integrin monoclonal antibodies. Phospholipid levels were induced upon beta 1 integrin-mediated adhesion to collagen, and the phospholipid induction was inhibited by either antibodies to beta 1 integrin or pretreatment of cells with C3 exoenzyme. Significant reduction in phospholipid levels correlated with proliferation for a panel of tumor lines deprived of adhesion to substrate. These results implicate a novel role for integrins and Rho in the regulation of tumour growth in response to matrix.

The 70 kDa S6 kinase complexes with and is activated by the Rho family G proteins Cdc42 and Rac1

The 70 kDa ribosomol S6 kinase (pp70S6k) plays an important role in the progression of cells through G1 phase of the cell cycle. However, little is known of the signaling molecules that mediate its activation. We demonstrate that Rho family G proteins regulate pp70S6k activity in vivo. Activated alleles of Cdc42 and Rac1, but not RhoA, stimulate pp70S6k activity in multiple cell types. Activation requires an intact effector domain and isoprenylation of Cdc42 and Rac1. Coexpression of Dbl, an exchange factor for Cdc42, also activates pp70S6k. Growth factor-induced activation of pp70S6k is abrogated by dominant negative alleles of Cdc42 and Rac1. In addition, Cdc42 and Rac1 form GTP-dependent complex with the catalytically inactive form of pp70S6k in vitro and in vivo, suggesting a mechanism by which these G proteins activate pp70S6k.