Activation of the small GTPase Ral in platelets

Phospholipase D and RalA cooperate with the epidermal growth factor receptor to transform 3Y1 rat fibroblasts

3Y1 rat fibroblasts overexpressing the epidermal growth factor (EGF) receptor (EGFR cells) become transformed when treated with EGF. A common response to oncogenic and mitogenic stimuli is elevated phospholipase D (PLD) activity. RalA, a small GTPase that functions as a downstream effector molecule of Ras, exists in a complex with PLD1. In the EGFR cells, EGF induced a Ras-dependent activation of RalA. The activation of PLD by EGF in these cells was dependent upon both Ras and RalA. In contrast, EGF-induced activation of Erk1, Erk2, and Jun kinase was dependent on Ras but independent of RalA, indicating divergent pathways activated by EGF and mediated by Ras. The transformed phenotype induced by EGF in the EGFR cells was dependent upon both Ras and RalA. Importantly, overexpression of wild-type RalA or an activated RalA mutant increased PLD activity in the absence of EGF and transformed the EGFR cells. Although overexpression of PLD1 is generally toxic to cells, the EGFR cells not only tolerated PLD1 overexpression but also became transformed in the absence of EGF. These data demonstrate that either RalA or PLD1 can cooperate with EGF receptor to transform cells.

PDZ-GEF1, a guanine nucleotide exchange factor specific for Rap1 and Rap2

The small GTPase Rap1 has been implicated in a variety of cellular processes including the control of cell morphology, proliferation, and differentiation. Stimulation of a large variety of cell surface receptors results in the rapid activation of Rap1, i.e. an increase in the GTP-bound form. This activation is mediated by second messengers like calcium, cAMP, and diacylglycerol, but additional pathways may exist as well. Here we describe a ubiquitously expressed guanine nucleotide exchange factor of 200 kDa that activates Rap1 both in vivo and in vitro. This exchange factor has two putative regulatory domains: a domain with an amino acid sequence related to cAMP-binding domains and a PDZ domain. Therefore, we named it PDZ-GEF1. PDZ-GEFs are closely related to Epacs, Rap-specific exchange factors with a genuine cAMP binding site, that are directly regulated by cAMP. The domain related to cAMP-binding domains, like the cAMP binding site in Epac, serves as a negative regulatory domain. However, PDZ-GEF1 does not interact with cAMP or cGMP. Interestingly, PDZ-GEF1 also activates Rap2, a close relative of Rap1. This is the first example of an exchange factor acting on Rap2. We conclude that PDZ-GEF1 is a guanine nucleotide exchange factor, specific for Rap1 and Rap2, that is controlled by a negative regulatory domain.

RA-GEF, a novel Rap1A guanine nucleotide exchange factor containing a Ras/Rap1A-associating domain, is conserved between nematode and humans

A yeast two-hybrid screening for Ras-binding proteins in nematode Caenorhabditis elegans has identified a guanine nucleotide exchange factor (GEF) containing a Ras/Rap1A-associating (RA) domain, termed Ce-RA-GEF. Both Ce-RA-GEF and its human counterpart Hs-RA-GEF possessed a PSD-95/DlgA/ZO-1 (PDZ) domain and a Ras exchanger motif (REM) domain in addition to the RA and GEF domains. They also contained a region homologous to a cyclic nucleotide monophosphate-binding domain, which turned out to be incapable of binding cAMP or cGMP. Although the REM and GEF domains are conserved with other GEFs acting on Ras family small GTP-binding proteins, the RA and PDZ domains are unseen in any of them. Hs-RA-GEF exhibited not only a GTP-dependent binding activity to Rap1A at its RA domain but also an activity to stimulate GDP/GTP exchange of Rap1A both in vitro and in vivo at the segment containing its REM and GEF domains. However, it did not exhibit any binding or GEF activity toward Ras. On the other hand, Ce-RA-GEF associated with and stimulated GDP/GTP exchange of both Ras and Rap1A. These results indicate that Ce-RA-GEF and Hs-RA-GEF define a novel class of Rap1A GEF molecules, which are conserved through evolution.

Phospholipase D stimulation by receptor tyrosine kinases mediated by protein kinase C and a Ras/Ral signaling cascade

Stimulation of phospholipase D (PLD) in HEK-293 cells expressing the M(3) muscarinic receptor by phorbol ester-activated protein kinase C (PKC) apparently involves Ral GTPases. We report here that PKC, but not muscarinic receptor-induced PLD stimulation in these cells, is strongly and specifically reduced by expression of dominant-negative RalA, G26A RalA, as well as dominant-negative Ras, S17N Ras. In contrast, overexpression of the Ras-activated Ral-specific guanine nucleotide exchange factor, Ral-GDS, specifically enhanced PKC-induced PLD stimulation. Moreover, recombinant Ral-GDS potentiated Ral-dependent PKC-induced PLD stimulation in membranes. Epidermal growth factor, platelet-derived growth factor, and insulin, ligands for receptor tyrosine kinases (RTKs) endogenously expressed in HEK-293 cells, apparently use the PKC- and Ras/Ral-dependent pathway for PLD stimulation. First, PLD stimulation by the RTK agonists was prevented by PKC inhibition and PKC down-regulation. Second, expression of dominant-negative RalA and Ras mutants strongly reduced RTK-induced PLD stimulation. Third, overexpression of Ral-GDS largely potentiated PLD stimulation by the RTK agonists. Finally, using the Ral binding domain of the Ral effector RLIP as an activation-specific probe for Ral proteins, it is demonstrated that endogenous RalA is activated by phorbol ester and RTK agonists. Taken together, strong evidence is provided that RTK-induced PLD stimulation in HEK-293 cells is mediated by PKC and a Ras/Ral signaling cascade.

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

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

Differential roles of Akt, Rac, and Ral in R-Ras-mediated cellular transformation, adhesion, and survival

Multiple biological functions have been ascribed to the Ras-related G protein R-Ras. These include the ability to transform NIH 3T3 fibroblasts, the promotion of cell adhesion, and the regulation of apoptotic responses in hematopoietic cells. To investigate the signaling mechanisms responsible for these biological phenotypes, we compared three R-Ras effector loop mutants (S61, G63, and C66) for their relative biological and biochemical properties. While the S61 mutant retained the ability to cause transformation, both the G63 and the C66 mutants were defective in this biological activity. On the other hand, while both the S61 and the C66 mutants failed to promote cell adhesion and survival in 32D cells, the G63 mutant retained the ability to induce these biological activities. Thus, the ability of R-Ras to transform cells could be dissociated from its propensity to promote cell adhesion and survival. Although the transformation-competent S61 mutant bound preferentially to c-Raf, it only weakly stimulated the mitogen-activated protein kinase (MAPK) activity, and a dominant negative mutant of MEK did not significantly perturb R-Ras oncogenicity. Instead, a dominant negative mutant of phosphatidylinositol 3-kinase (PI3-K) drastically inhibited the oncogenic potential of R-Ras. Interestingly, the ability of the G63 mutant to induce cell adhesion and survival was closely associated with the PI3-K-dependent signaling cascades. To further delineate R-Ras downstream signaling events, we observed that while a dominant negative mutant of Akt/protein kinase inhibited the ability of R-Ras to promote cell survival, both dominant negative mutants of Rac and Ral suppressed cell adhesion stimulated by R-Ras. Thus, the biological actions of R-Ras are mediated by multiple effectors, with PI3-K-dependent signaling cascades being critical to its functions.

Differential fMet-Leu-Phe- and platelet-activating factor-induced signaling toward Ral activation in primary human neutrophils

We have measured the activation of the small GTPase Ral in human neutrophils after stimulation with fMet-Leu-Phe (fMLP), platelet activating factor (PAF), and granulocyte macrophage-colony stimulating factor and compared it with the activation of two other small GTPases, Ras and Rap1. We found that fMLP and PAF, but not granulocyte macrophage-colony stimulating factor, induce Ral activation. All three stimuli induce the activation of both Ras and Rap1. Utilizing specific inhibitors we demonstrate that fMLP-induced Ral activation is mediated by pertussis toxin-sensitive G-proteins and partially by Src-like kinases, whereas fMLP-induced Ras activation is independent of Src-like kinases. PAF-induced Ral activation is mediated by pertussis toxin-insensitive proteins, Src-like kinases and phosphatidylinositol 3-kinase. Phosphatidylinositol 3-kinase is not involved in PAF-induced Ras activation. The calcium ionophore ionomycin activates Ral, but calcium depletion partially inhibits fMLP- and PAF-induced Ral activation, whereas Ras activation was not affected. In addition, 12-O-tetradecanoylphorbol-13-acetate-induced activation of Ral is completely abolished by inhibitors of protein kinase C, whereas 12-O-tetradecanoylphorbol-13-acetate-induced Ras activation is largely insensitive. We conclude that in neutrophils Ral activation is mediated by multiple pathways, and that fMLP and PAF induce Ral activation differently.

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

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

The small GTP-binding protein R-Ras can influence integrin activation by antagonizing a Ras/Raf-initiated integrin suppression pathway

The rapid modulation of ligand-binding affinity ("activation") is a central property of the integrin family of cell adhesion receptors. The small GTP-binding protein Ras and its downstream effector kinase Raf-1 suppress integrin activation. In this study we explored the relationship between Ras and the closely related small GTP-binding protein R-Ras in modulating the integrin affinity state. We found that R-Ras does not seem to be a direct activator of integrins in Chinese hamster ovary cells. However, we observed that GTP-bound R-Ras strongly antagonizes the Ras/Raf-initiated integrin suppression pathway. Furthermore, this reversal of the Ras/Raf suppressor pathway does not seem to be via a competition between Ras and R-Ras for common downstream effectors or via an inhibition of Ras/Raf-induced MAP kinase activation. Thus, R-Ras and Ras may act in concert to regulate integrin affinity via the activation of distinct downstream effectors.

Ca2+/calmodulin stimulates GTP binding to the ras-related protein ral-A

Ral-A is a Ras-related GTP-binding protein that has been suggested to be the downstream target of Ras proteins and is involved in the tyrosine kinase-mediated, Ras-dependent activation of phospholipase D. We reported recently that Ral-A purified from human erythrocyte membrane binds to calmodulin in a Ca2+-dependent manner at a calmodulin binding domain identified near its C-terminal region (Wang, K. L., Khan, M. T., and Roufogalis, B. D. (1997) J. Biol. Chem. 272, 16002-16009). In this study we show the enhancement of GTP binding to Ral-A by Ca2+/calmodulin. The stimulation up to 3-fold by calmodulin was Ca2+-dependent, with half-maximum activation occurring at 180 nM calmodulin and 80 nM free Ca2+ concentration. The present work supports a regulatory role of Ca2+/calmodulin for the activation of Ral-A and suggests a possible direct link between signal transduction pathways of Ca2+/calmodulin and Ral-A proteins.

Identification and characterization of potential effector molecules of the Ras-related GTPase Rap2

In search for effectors of the Ras-related GTPase Rap2, we used the yeast two-hybrid method and identified the C-terminal Ras/Rap interaction domain of the Ral exchange factors (RalGEFs) Ral GDP dissociation stimulator (RalGDS), RalGDS-like (RGL), and RalGDS-like factor (Rlf). These proteins, which also interact with activated Ras and Rap1, are effectors of Ras and mediate the activation of Ral in response to the activation of Ras. Here we show that the full-length RalGEFs interact with the GTP-bound form of Rap2 in the two-hybrid system as well as in vitro. When co-transfected in HeLa cells, an activated Rap2 mutant (Rap2Val-12) but not an inactive protein (Rap2Ala-35) co-immunoprecipitates with RalGDS and Rlf; moreover, Rap2-RalGEF complexes can be isolated from the particulate fraction of transfected cells and were localized by confocal microscopy to the resident compartment of Rap2, i.e. the endoplasmic reticulum. However, the overexpression of activated Rap2 neither leads to the activation of the Ral GTPase via RalGEFs nor inhibits Ras-dependent Ral activation in vivo. Several hypotheses that could explain these results, including compartmentalization of proteins involved in signal transduction, are discussed. Our results suggest that in cells, the interaction of Rap2 with RalGEFs might trigger other cellular responses than activation of the Ral GTPase.

Ral-specific guanine nucleotide exchange factor activity opposes other Ras effectors in PC12 cells by inhibiting neurite outgrowth

Ras proteins can activate at least three classes of downstream target proteins: Raf kinases, phosphatidylinositol-3 phosphate (PI3) kinase, and Ral-specific guanine nucleotide exchange factors (Ral-GEFs). In NIH 3T3 cells, activated Ral-GEFs contribute to Ras-induced cell proliferation and oncogenic transformation by complementing the activities of Raf and PI3 kinases. In PC12 cells, activated Raf and PI3 kinases mediate Ras-induced cell cycle arrest and differentiation into a neuronal phenotype. Here, we show that in PC12 cells, Ral-GEF activity acts opposite to other Ras effectors. Elevation of Ral-GEF activity induced by transfection of a mutant Ras protein that preferentially activates Ral-GEFs, or by transfection of the catalytic domain of the Ral-GEF Rgr, suppressed cell cycle arrest and neurite outgrowth induced by nerve growth factor (NGF) treatment. In addition, Rgr reduced neurite outgrowth induced by a mutant Ras protein that preferentially activates Raf kinases. Furthermore, inhibition of Ral-GEF activity by expression of a dominant negative Ral mutant accelerated cell cycle arrest and enhanced neurite outgrowth in response to NGF treatment. Ral-GEF activity may function, at least in part, through inhibition of the Rho family GTPases, CDC42 and Rac. In contrast to Ras, which was activated for hours by NGF treatment, Ral was activated for only approximately 20 min. These findings suggest that one function of Ral-GEF signaling induced by NGF is to delay the onset of cell cycle arrest and neurite outgrowth induced by other Ras effectors. They also demonstrate that Ras has the potential to promote both antidifferentiation and prodifferentiation signaling pathways through activation of distinct effector proteins. Thus, in some cell types the ratio of activities among Ras effectors and their temporal regulation may be important determinants for cell fate decisions between proliferation and differentiation.

Plasma membrane recruitment of RalGDS is critical for Ras-dependent Ral activation

In COS cells, Ral GDP dissociation stimulator (RalGDS)-induced Ral activation was stimulated by RasG12V or a Rap1/Ras chimera in which the N-terminal region of Rap1 was ligated to the C-terminal region of Ras but not by Rap1G12V or a Ras/Rap1 chimera in which the N-terminal region of Ras was ligated to the C-terminal region of Rap1, although RalGDS interacted with these small GTP-binding proteins. When RasG12V, Ral and the Rap1/Ras chimera were individually expressed in NIH3T3 cells, they localized to the plasma membrane. Rap1Q63E and the Ras/Rap1 chimera were detected in the perinuclear region. When RalGDS was expressed alone, it was abundant in the cytoplasm. When coexpressed with RasG12V or the Rap1/Ras chimera, RalGDS was detected at the plasma membrane, whereas when coexpressed with Rap1Q63E or the Ras/Rap1 chimera, RalGDS was observed in the perinuclear region. RalGDS which was targeted to the plasma membrane by the addition of Ras farnesylation site (RalGDS-CAAX) activated Ral in the absence of RasG12V. Although RalGDS did not stimulate the dissociation of GDP from Ral in the absence of the GTP-bound form of Ras in a reconstitution assay using the liposomes, RalGDS-CAAX could stimulate it without Ras. RasG12V activated Raf-1 when they were coexpressed in Sf9 cells, whereas RasG12V did not affect the RalGDS activity. These results indicate that Ras recruits RalGDS to the plasma membrane and that the translocated RalGDS induces the activation of Ral, but that Rap1 does not activate Ral due to distinct subcellular localization.

Ras caught in another affair: the exchange factors for Ral

The Ral guanine nucleotide exchange factors are direct targets of Ras, providing a mechanism for Ral activation by extracellular signals. In addition, Ral can be activated by a Ras-independent pathway. Ral guanine nucleotide exchange factors contribute to cellular transformation induced by oncogenic Ras through an Erk-independent mechanism which may involve activation of transcription.

The Ras/Erk pathway induces primitive endoderm but prevents parietal endoderm differentiation of F9 embryonal carcinoma cells

The formation of parietal endoderm (PE) is one of the first differentiation processes during mouse development and can be studied in vitro using F9 embryonal carcinoma (EC) cells. Treatment of F9 EC cells with retinoic acid (RA) induces differentiation toward primitive endoderm (PrE), while differentiation toward PE is induced by subsequent addition of parathyroid hormone (PTH) or PTH-related peptide (PTHrP). The signal transduction mechanisms involved in this two-step process are largely unclear. We show that the RA-induced differentiation toward PrE is accompanied by a sustained increase in Ras activity and that ectopic expression of oncogenic Ha-Ras is sufficient to induce PrE differentiation. Ras activity subsequently decreases upon PTH-induced differentiation toward PE. This is a necessary event, since expression of oncogenic Ha-Ras in PrE-like cells prevents PTH-induced PE differentiation. Expression of active PKA in PrE-like F9 cells mimics PTH-induced PE differentiation and is again prevented by oncogenic Ha-Ras. The effect of oncogenic Ras on both differentiation steps is abolished by the MEK inhibitor PD98059 and can be mimicked by constitutively active forms of Raf and MEK. In conclusion, our data suggest that activation of the Ras/Erk is sufficient to induce differentiation to PrE and to prevent subsequent differentiation toward PE. Activation of PKA down-regulates Ras activity, resulting in disappearance of this blockade and transmission of signal(s) triggering PE differentiation.

Low density lipoprotein phosphorylates the focal adhesion-associated kinase p125(FAK) in human platelets independent of integrin alphaIIb beta3

Low density lipoprotein (LDL) is known to sensitize platelets to agonists via integrin mediated outside-in signaling (Hackeng, C. M., Huigsloot, M., Pladet, M. W., Nieuwenhuis, H. K., Rijn, H. J. M. v., and Akkerman, J. W. N. (1999) Arterioscler. Thromb. Vasc. Biol., in press). As outside in signaling is associated with phosphorylation of p125(FAK), the effect of LDL on p125(FAK) phosphorylation in platelets was investigated. LDL induced p125(FAK) phosphorylation in a dose- and time- dependent manner. The phosphorylation was independent of ligand binding to integrin alphaIIbbeta3 and aggregation, such in contrast to alpha-thrombin-induced p125(FAK) phosphorylation, that critically depended on platelet aggregation. Platelets from patients with Glanzmann's thrombastenia showed the same LDL- induced phos- phorylation of p125(FAK) as control platelets, whereas alpha-thrombin completely failed to phosphorylate the kinase in the patients platelets. LDL signaling to p125(FAK) was independent of integrin alpha2 beta1, the FcgammaRII receptor, and the lysophosphatidic acid receptor and not affected by inhibitors of cyclooxygenase, protein kinase C, ERK1/2 or p38(MAPK). Phosphorylation of p125(FAK) by LDL was strongly inhibited by cyclic AMP. These observations indicate that LDL is a unique platelet agonist, as it phosphorylates p125(FAK) in platelet suspensions, under unstirred conditions and independent of integrin alphaIIb beta3.

Epac is a Rap1 guanine-nucleotide-exchange factor directly activated by cyclic AMP

Rap1 is a small, Ras-like GTPase that was first identified as a protein that could suppress the oncogenic transformation of cells by Ras. Rap1 is activated by several extracellular stimuli and may be involved in cellular processes such as cell proliferation, cell differentiation, T-cell anergy and platelet activation. At least three different second messengers, namely diacylglycerol, calcium and cyclic AMP, are able to activate Rap1 by promoting its release of the guanine nucleotide GDP and its binding to GTP. Here we report that activation of Rap1 by forskolin and cAMP occurs independently of protein kinase A (also known as cAMP-activated protein kinase). We have cloned the gene encoding a guanine-nucleotide-exchange factor (GEF) which we have named Epac (exchange protein directly activated by cAMP). This protein contains a cAMP-binding site and a domain that is homologous to domains of known GEFs for Ras and Rap1. Epac binds cAMP in vitro and exhibits in vivo and in vitro GEF activity towards Rap1. cAMP strongly induces the GEF activity of Epac towards Rap1 both in vivo and in vitro. We conclude that Epac is a GEF for Rap1 that is regulated directly by cAMP and that Epac is a new target protein for cAMP.

All in the family? New insights and questions regarding interconnectivity of Ras, Rap1 and Ral

Ras, Rap1 and Ral are related small GTPases. While the function of Ras in signal transduction is well established, it has been recognized only recently that Rap1 and Ral also are activated rapidly in response to a large variety of extracellular signals. Between the three GTPase an intriguing interconnectivity exists, in that guanine nucleotide exchange factors for Ral associate with the GTP-bound form of both Ras and Rap1. Furthermore, Rap1 is considered to function as an antagonist of Ras signalling by trapping Ras effectors in an inactive complex. Here, I summarize the recent developments in understanding the functional relationship between these three GTPase and argue that Rap1 functions in a signalling pathway distinct from Ras, while using similar or identical effectors.

Extracellular signal-regulated activation of Rap1 fails to interfere in Ras effector signalling

The small GTPase Rap1 has been implicated in both negative and positive control of Ras-mediated signalling events. We have investigated which extracellular signals can activate Rap1 and whether this activation leads to a modulation of Ras effector signalling, i.e. the activation of ERK and the small GTPase Ral. We found that Rap1 is rapidly activated following stimulation of a large variety of growth factor receptors. These receptors include receptor tyrosine kinases for platelet-derived growth factor (PDGF) and epithelial growth factor (EGF), and G protein-coupled receptors for lysophosphatidic acid (LPA), thrombin and endothelin. At least three distinct pathways may transduce a signal towards Rap1 activation: increase in intracellular calcium, release of diacylglycerol and cAMP synthesis. Surprisingly, activation of endogenous Rap1 fails to affect Ras-dependent ERK activation. In addition, we found that although overexpression of active Rap1 is able to activate the Ral pathway, activation of endogenous Rap1 in fibroblasts does not result in Ral activation. Rap1 also does not negatively influence Ras-mediated Ral activation. We conclude that activation of Rap1 is a common event upon growth factor treatment and that the physiological function of Rap1 is likely to be different from modulation of Ras effector signalling.

Identification and characterization in Xenopus of XsmgGDS, a RalB binding protein

We have previously isolated a cDNA coding for a RalB protein from a Xenopus maternal library. By mutagenesis of RalB and microinjection of its mRNA in embryos we show that RalB is involved in Xenopus early development. We have used the yeast two-hybrid system to screen a Xenopus maternal cDNA library in order to isolate proteins interacting to RalB. We have identified a full-length cDNA encoding a protein homologous to mammalian smgGDS. The Xenopus gene shows 84.6% identity with the mammalian counterpart and contains one additional armadillo repeat. The XsmgGDS mRNA is expressed from oogenesis up to late embryogenesis at a higher level than that in adult tissues. Thus RalB is another protein that interacts with smgGDS which suggests that RalB may be activated by a Ras independent pathway.

Activation of the Small GTPase Rap1 in Human Neutrophils

The small GTPase Rap1 is highly expressed in human neutrophils, but its function is largely unknown. Using the Rap1-binding domain of RalGDS (RalGDS-RBD) as an activation-specific probe for Rap1, we have investigated the regulation of Rap1 activity in primary human neutrophils. We found that a variety of stimuli involved in neutrophil activation, including fMet-Leu-Phe (fMLP), platelet-activating factor (PAF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IgG-coated particles, induce a rapid and transient Rap1 activation. In addition, we found that Rap1 is normally activated in neutrophils from chronic granulomatous disease patients that lack cytochrome b558 or p47phox and have a defective NADPH oxidase system. From these results we conclude that in neutrophils Rap1 is activated independently of respiratory burst induction. Finally, we found that Rap1 is activated by both the Ca2+ ionophore ionomycin and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), indicating that phospholipase C (PLC) activation leading to elevated levels of intracellular free Ca2+ and diacylglycerol (DAG) can mediate Rap1 activation. However, inhibition of PLC and Ca2+ depletion only marginally affected fMLP-induced Rap1 activation, suggesting that additional pathways may control Rap1 activation.

© 1998 by The American Society of Hematology.

Activation of the Small GTPase Rap1 in Human Neutrophils

The small GTPase Rap1 is highly expressed in human neutrophils, but its function is largely unknown. Using the Rap1-binding domain of RalGDS (RalGDS-RBD) as an activation-specific probe for Rap1, we have investigated the regulation of Rap1 activity in primary human neutrophils. We found that a variety of stimuli involved in neutrophil activation, including fMet-Leu-Phe (fMLP), platelet-activating factor (PAF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IgG-coated particles, induce a rapid and transient Rap1 activation. In addition, we found that Rap1 is normally activated in neutrophils from chronic granulomatous disease patients that lack cytochrome b558 or p47phox and have a defective NADPH oxidase system. From these results we conclude that in neutrophils Rap1 is activated independently of respiratory burst induction. Finally, we found that Rap1 is activated by both the Ca2+ ionophore ionomycin and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), indicating that phospholipase C (PLC) activation leading to elevated levels of intracellular free Ca2+ and diacylglycerol (DAG) can mediate Rap1 activation. However, inhibition of PLC and Ca2+ depletion only marginally affected fMLP-induced Rap1 activation, suggesting that additional pathways may control Rap1 activation.

© 1998 by The American Society of Hematology.

Ras-independent activation of Ral by a Ca(2+)-dependent pathway

The RalA and RalB proteins comprise a distinct family of small GTPases [1]. Ral-specific guanine-nucleotide exchange factors such as RalGDS, Rlf and RGL interact with activated Ras and cooperate with Ras in the transformation of murine fibroblasts [2-5]. Thus, the interaction of RalGDS with Ras and the subsequent activation of Ral are thought to constitute a distinct Ras-dependent signaling pathway. The function of Ral is largely unknown. There is circumstantial evidence that Ral may have a function in regulating the cytoskeleton through its interaction with RIP1 (also known as RLIP or RalBP1), a GTPase-activating protein specific for the small GTPases Cdc42 and Rac [6-8]. Ral also binds to phospholipase D (PLD) and thus may play a role in signaling through phospholipids [9]. We have examined endogenous levels of activated, GTP-bound Ral (Ral-GTP) in Rat-2 fibroblasts stimulated with various mitogens. Lysophosphatidic acid (LPA) and epidermal growth factor (EGF), which activate both Ras-dependent and Ras-independent signaling pathways [10,11], rapidly activated Ral. Inhibition of Ras activation by dominant-negative Ras (RasS17N) or pertussis toxin had little effect on Ral-GTP levels, however. Ral was activated by the Ca2+ ionophore ionomycin, and activation by LPA or EGF could be blocked by a phospholipase C (PLC) inhibitor. The results presented here demonstrate a Ca(2+)-dependent mechanism for the activation of Ral.