Salmonella Typhimurium effector SseI inhibits chemotaxis and increases host cell survival by deamidation of heterotrimeric Gi proteins.

Salmonella enterica serotype Typhimurium (S. Typhimurium) is one of the most frequent causes of food-borne illness in humans and usually associated with acute self-limiting gastroenteritis. However, in immunocompromised patients, the pathogen can disseminate and lead to severe systemic diseases. S. Typhimurium are facultative intracellular bacteria. For uptake and intracellular life, Salmonella translocate numerous effector proteins into host cells using two type-III secretion systems (T3SS), which are encoded within Salmonella pathogenicity islands 1 (SPI-1) and 2 (SPI-2). While SPI-1 effectors mainly promote initial invasion, SPI-2 effectors control intracellular survival and proliferation. Here, we elucidate the mode of action of Salmonella SPI-2 effector SseI, which is involved in control of systemic dissemination of S. Typhimurium. SseI deamidates a specific glutamine residue of heterotrimeric G proteins of the Gαi family, resulting in persistent activation of the G protein. Gi activation inhibits cAMP production and stimulates PI3-kinase γ by Gαi-released Gßγ subunits, resulting in activation of survival pathways by phosphorylation of Akt and mTOR. Moreover, SseI-induced deamidation leads to non-polarized activation of Gαi and, thereby, to loss of directed migration of dendritic cells.

A systemic Pasteurella multocida toxin aggravates cardiac hypertrophy and fibrosis in mice.

Pasteurella multocida toxin (PMT) persistently activates heterotrimeric G proteins of the Gαq/11 , Gα12/13 and Gαi family without interaction with G protein-coupled receptors (GPCRs). We show that PMT acts on heart tissue in vivo and on cardiomyocytes and cardiac fibroblasts in vitro by deamidation of heterotrimeric G proteins. Increased normalized ventricle weights and fibrosis were detected after intraperitoneal administration of PMT in combination with the GPCR agonist phenylephrine. In neonatal rat cardiomyocytes, PMT stimulated the mitogen-activated protein kinase pathway, which is crucial for the development of cellular hypertrophy. The toxin induced phosphorylation of the canonical phosphorylation sites of the extracellular-regulated kinase 1/2 and, additionally, caused phosphorylation of the recently recognized autophosphorylation site, which appears to be important for the development of cellular hypertrophy. Moreover, PMT stimulated the small GTPases Rac1 and RhoA. Both switch proteins are involved in cardiomyocyte hypertrophy. In addition, PMT stimulated RhoA and Rac1 in neonatal rat cardiac fibroblasts. RhoA and Rac1 have been implicated in the regulation of connective tissue growth factor (CTGF) secretion and expression. Accordingly, we show that PMT treatment increased secretion and expression of CTGF in cardiac fibroblasts. Altogether, the data indicate that PMT is an inducer of pathological remodelling of cardiac cells and identifies the toxin as a promising tool for studying heterotrimeric G protein-dependent signalling in cardiac cells.

Pasteurella multocida toxin prevents osteoblast differentiation by transactivation of the MAP-kinase cascade via the Gα(q/11)--p63RhoGEF--RhoA axis.

The 146-kDa Pasteurella multocida toxin (PMT) is the main virulence factor to induce P. multocida-associated progressive atrophic rhinitis in various animals. PMT leads to a destruction of nasal turbinate bones implicating an effect of the toxin on osteoblasts and/or osteoclasts. The toxin induces constitutive activation of Gα proteins of the G(q/11)-, G12/13- and G(i)-family by deamidating an essential glutamine residue. To study the PMT effect on bone cells, we used primary osteoblasts derived from rat calvariae and stromal ST-2 cells as differentiation model. As marker of functional osteoblasts the expression and activity of alkaline phosphatase, formation of mineralization nodules or expression of specific transcription factors as osterix was determined. Here, we show that the toxin inhibits differentiation and/or function of osteoblasts by activation of Gα(q/11). Subsequently, Gα(q/11) activates RhoA via p63RhoGEF, which specifically interacts with Gα(q/11) but not with other G proteins like Gα12/13 and Gα(i). Activated RhoA transactivates the mitogen-activated protein (MAP) kinase cascade via Rho kinase, involving Ras, MEK and ERK, resulting in inhibition of osteoblast differentiation. PMT-induced inhibition of differentiation was selective for the osteoblast lineage as adipocyte-like differentiation of ST-2 cells was not hampered. The present work provides novel insights, how the bacterial toxin PMT can control osteoblastic development by activating heterotrimeric G proteins of the Gα(q/11)-family and is a molecular pathogenetic basis for understanding the role of the toxin in bone loss during progressive atrophic rhinitis induced by Pasteurella multocida.

Substrate specificity of Pasteurella multocida toxin for α subunits of heterotrimeric G proteins.

Pasteurella multocida is the causative agent of a number of epizootic and zoonotic diseases. Its major virulence factor associated with atrophic rhinitis in animals and dermonecrosis in bite wounds is P. multocida toxin (PMT). PMT stimulates signal transduction pathways downstream of heterotrimeric G proteins, leading to effects such as mitogenicity, blockade of apoptosis, or inhibition of osteoblast differentiation. On the basis of Gα(i2), it was demonstrated that the toxin deamidates an essential glutamine residue of the Gα(i2) subunit, leading to constitutive activation of the G protein. Here, we studied the specificity of PMT for its G-protein targets by mass spectrometric analyses and by utilizing a monoclonal antibody, which recognizes specifically G proteins deamidated by PMT. The studies revealed deamidation of 3 of 4 families of heterotrimeric G proteins (Gα(q/11), Gα(i1,2,3), and Gα(12/13) of mouse or human origin) by PMT but not by a catalytic inactive toxin mutant. With the use of G-protein fragments and chimeras of responsive or unresponsive G proteins, the structural basis for the discrimination of heterotrimeric G proteins was studied. Our results elucidate substrate specificity of PMT on the molecular level and provide evidence for the underlying structural reasons of substrate discrimination.

Pasteurella multocida toxin as a transporter of non-cell-permeating proteins.

The protein toxin Pasteurella multocida toxin (PMT) is the causative agent of atrophic rhinitis in pigs, leading to atrophy of the nasal turbinate bones by affecting osteoblasts and osteoclasts. The mechanism of PMT-induced intoxication is a deamidation of α-subunits of heterotrimeric G proteins, including Gαq, Gα13, and Gαi, thereby causing persistent activation of the G proteins. Here we utilized PMT as a transporter of the non-cell-permeating A domain of diphtheria toxin (DTa). Fusion proteins of PMT and DTa ADP-ribosylated elongation factor 2, the natural target of diphtheria toxin, leading to cell toxicity. PMT-DTa effects were competed by PMT, indicating binding to the same cell surface receptor. Fluorescently labeled PMT-DTa and PMT colocalized with specific markers of early and late endosomes. Bafilomycin A, which inhibits vacuolar H(+)-ATPase, blocked PMT-DTa-induced intoxication of HEK-293 cells. By constructing various PMT-DTa chimeras, we identified a minimal region of PMT necessary for uptake of DTa. The data suggest that PMT is able to transport cargo proteins into eukaryotic cells by utilizing the PMT-specific uptake route.

Molecular biology of Pasteurella multocida toxin.

Pasteurella multocida toxin (PMT) is the causative agent of progressive atrophic rhinitis in swine. The 146 kDa single-chain toxin harbours discrete domains important for receptor binding, internalisation and biological activity. The molecular basis of the toxin's activity is the deamidation of a specific glutamine residue in the α-subunit of heterotrimeric G proteins. This results in an inhibition of the inherent GTPase activity leading to a constitutively active phenotype of the G protein. Due to the ability of the toxin to act on various families of heterotrimeric G proteins, a large subset of signal transduction pathways is stimulated.

Pasteurella multocida toxin activation of heterotrimeric G proteins by deamidation.

Pasteurella multocida toxin is a major virulence factor of Pasteurella multocida, which causes pasteurellosis in men and animals and atrophic rhinitis in rabbits and pigs. The approximately 145 kDa protein toxin stimulates various signal transduction pathways by activating heterotrimeric G proteins of the Galpha(q), Galpha(i), and Galpha(12/13) families by using an as yet unknown mechanism. Here, we show that Pasteurella multocida toxin deamidates glutamine-205 of Galpha(i2) to glutamic acid. Therefore, the toxin inhibits the intrinsic GTPase activity of Galpha(i) and causes persistent activation of the G protein. A similar modification is also evident for Galpha(q), but not for the closely related Galpha(11), which is not a substrate of Pasteurella multocida toxin. Our data identify the alpha-subunits of heterotrimeric G proteins as the direct molecular target of Pasteurella multocida toxin and indicate that the toxin does not act like a protease, which was suggested from its thiol protease-like catalytic triad, but instead causes constitutive activation of G proteins by deamidase activity.

Pasteurella multocida toxin is a potent activator of anti-apoptotic signalling pathways.

Toxigenic Pasteurella multocida strains produce a 146 kDa protein toxin (PMT) that due to its high mitogenic activity is thought to possess carcinogenic properties. PMT affects several signal transduction pathways related to cancer by constitutively stimulating heterotrimeric G proteins. Downstream of Galpha(q), Galpha(13) and Galpha(i), the toxin activates the small GTPase RhoA, MAP kinases and signal transducer and activator of transcription (STAT) proteins. PMT also stimulates Gbetagamma signalling and activates phosphoinositide 3-kinase (PI3K)-related pathways, which play a crucial role in proliferation and apoptosis. We show that treatment of HEK293 cells with PMT inhibits staurosporine-mediated apoptosis through PI3K-dependent phosphorylation of Akt and constitutive expression of Pim-1 kinase. Simultaneous activation of these survival kinases allows the activation of pro-survival pathways, such as GSK3beta, Mcl-1, Bcl-xL and Bcl-2, as well as the downregulation of apoptotic signals by Bax or Puma. Only the combined inhibition of Akt and Pim reverses the PMT-induced protection from staurosporine-induced apoptosis. In addition, we show that apoptosis induced by tumour chemotherapeutic agents is blocked by PMT in human cancer cell lines. Our data indicate that PMT is a highly potent anti-apoptotic agent, which supports the view of a carcinogenic potential of the toxin.

Activation of Gq signaling by Pasteurella multocida toxin inhibits the osteoblastogenic-like actions of Activin A in C2C12 myoblasts, a cell model of fibrodysplasia ossificans progressiva.

The human disease fibrodysplasia ossificans progressiva (FOP) is a rare and highly disabling disorder of extensive heterotopic bone growth that is caused by a point mutation (R206H) in the activation domain of Alk2, a BMP (bone morphogenic protein) type 1 receptor. The mutation leads to extensive BMP-signaling induced by Activin A, which is normally an antagonist for wildtype receptors, resulting in excessive and uncontrolled bone formation. Here, we studied the effects of Pasteurella multocida toxin (PMT), which activates osteoclasts and inhibits osteoblast activity, in C2C12 myoblasts expressing the mutant Alk2(R206H) receptor as model of FOP. In our study, we mainly used alkaline phosphatase (ALP) activity as marker to determine osteoblast differentiation. BMP-4 stimulated an increase in ALP activity in C2C12-Alk2wt and C2C12-Alk2(R206H) cells. By contrast, Activin A only induced ALP activity in C2C12-Alk2(R206H) cells. In both cases, PMT acted as a potent inhibitor of ALP activity. PMT-induced inhibition of ALP activity was paralleled by a constitutive activation of the heterotrimeric Gq protein. Expression of a permanently active Gαq blocked Activin A/Alk2(R206H)-dependent increase in ALP activity. Inactivation of Gq by specific inhibitor FR900359 blocked the PMT effect. Similarly, canonical second messengers and effectors of Gαq (e.g. ionophore A23187-induced increase in intracellular Ca2+ and activation of PKC by PMA (phorbol 12-myristate 13-acetate)) inhibited Alk2(R206H)-mediated induction of ALP activity. Notably, Activin A-induced increase in ALP activity in C2C12-Alk2(R206H) cells was also inhibited by stimulation of the α1A-adrenoceptor, which couples to Gαq, by phenylephrine. PMT did not alter tail phosphorylation of the major downstream effectors of the Alk2 receptor, Smad1/5/9; neither did the toxin affect nuclear translocation of the Smad-complex. However, PMT diminished BMP responsive element-induced gene expression. The data indicate that PMT potently inhibits the induction of osteoblast markers in a FOP model via activation of G proteins. Moreover, our findings indicate that activation of G protein-coupled receptors and of G protein signaling might be a rationale for pharmacological therapy of FOP.

Action of Pasteurella multocida toxin on Galpha(q) is persistent and independent of interaction with G-protein-coupled receptors.

Pasteurella multocida toxin (PMT) activates Galpha(q) and facilitates stimulation of inositol phosphate accumulation induced by agonists via G(q)-coupled membrane receptors. Here, we studied the effects of PMT on agonist-induced GTPgammaS binding to G(q) in cell membranes and a role of G-protein-coupled receptors in the action of PMT. Pre-treatment of Swiss 3T3 cells with PMT increased bombesin or vasopressin-induced GTPgammaS-binding in cell membranes by about 50 to 150%. Increase in agonist-stimulated GTPgammaS-binding caused by PMT pretreatment was specific for Galpha(q) and not observed with Galpha(11). PMT-induced effects on GTPgammaS-binding were persistent after removing the toxin or in the presence of anti-PMT antibody. Stimulation of agonist-induced GTPgammaS-binding by PMT was independent of phosphorylation of the C-terminal tyrosine356 of Galpha(q). Activation of phospholipase C by PMT occurred via Galpha(q) which was fused to the alpha(1b)-adrenoceptor and also with a C-terminally deleted Galpha(q), which is not able to interact with G protein-coupled membrane receptors. The data indicate that activation of Galpha(q) by PMT is persistent and independent of a functional interaction of G(q) with G-protein-coupled receptors.

Involvement of Osteocytes in the Action of Pasteurella multocida Toxin.

Pasteurella multocida toxin (PMT) causes progressive atrophic rhinitis with severe turbinate bone degradation in pigs. It has been reported that the toxin deamidates and activates heterotrimeric G proteins, resulting in increased differentiation of osteoclasts and blockade of osteoblast differentiation. So far, the action of PMT on osteocytes, which is the most abundant cell type in bone tissue, is not known. In MLO-Y4 osteocytes, PMT deamidated heterotrimeric G proteins, resulting in loss of osteocyte dendritic processes, stress fiber formation, cell spreading and activation of RhoC but not of RhoA. Moreover, the toxin caused processing of membrane-bound receptor activator of NF-κB ligand (RANKL) to release soluble RANKL and enhanced the secretion of osteoclastogenic TNF-α. In a co-culture model of osteocytes and bone marrow cells, PMT-induced osteoclastogenesis was largely increased as compared to the mono-culture model. The enhancement of osteoclastogenesis observed in the co-culture was blocked by sequestering RANKL with osteoprotegerin and by an antibody against TNF-α indicating involvement of release of the osteoclastogenic factors from osteocytes. Data support the crucial role of osteocytes in bone metabolism and osteoclastogenesis and identify osteocytes as important target cells of PMT in progressive atrophic rhinitis.

Auranofin Inhibits the Enzyme Activity of Pasteurella multocida Toxin PMT in Human Cells and Protects Cells from Intoxication.

The AB-type protein toxin from Pasteurella multocida (PMT) contains a functionally important disulfide bond within its catalytic domain, which must be cleaved in the host cell cytosol to render the catalytic domain of PMT into its active conformation. Here, we found that the reductive potential of the cytosol of target cells, and more specifically, the activity of the thioredoxin reductase (TrxR) is crucial for this process. This was demonstrated by the strong inhibitory effect of the pharmacological TrxR inhibitor auranofin, which inhibited the intoxication of target cells with PMT, as determined by analyzing the PMT-catalyzed deamidation of GTP-binding proteins (G-proteins) in the cytosol of cells. The amount of endogenous substrate levels modified by PMT in cells pretreated with auranofin was reduced compared to cells treated with PMT alone. Auranofin had no inhibitory effect on the activity of the catalytic domain of constitutively active PMT in vitro, demonstrating that auranofin did not directly inhibit PMT activity, but interferes with the mode of action of PMT in cells. In conclusion, the results show that TrxR is crucial for the mode of action of PMT in mammalian cells, and that the drug auranofin can serve as an efficient inhibitor, which might be a starting point for novel therapeutic options against toxin-associated diseases.

The two-pore channel TPC1 is required for efficient protein processing through early and recycling endosomes.

Two-pore channels (TPCs) are localized in endo-lysosomal compartments and assumed to play an important role for vesicular fusion and endosomal trafficking. Recently, it has been shown that both TPC1 and 2 were required for host cell entry and pathogenicity of Ebola viruses. Here, we investigate the cellular function of TPC1 using protein toxins as model substrates for distinct endosomal processing routes. Toxin uptake and activation through early endosomes but not processing through other compartments were reduced in TPC1 knockout cells. Detailed co-localization studies with subcellular markers confirmed predominant localization of TPC1 to early and recycling endosomes. Proteomic analysis of native TPC1 channels finally identified direct interaction with a distinct set of syntaxins involved in fusion of intracellular vesicles. Together, our results demonstrate a general role of TPC1 for uptake and processing of proteins in early and recycling endosomes, likely by providing high local Ca2+ concentrations required for SNARE-mediated vesicle fusion.

Noncanonical G-protein-dependent modulation of osteoclast differentiation and bone resorption mediated by Pasteurella multocida toxin.

UNLABELLED: Pasteurella multocida toxin (PMT) induces atrophic rhinitis in animals, which is characterized by a degradation of nasal turbinate bones, indicating an effect of the toxin on bone cells such as osteoblasts and osteoclasts. The underlying molecular mechanism of PMT was defined as a persistent activation of heterotrimeric G proteins by deamidation of a specific glutamine residue. Here, we show that PMT acts directly on osteoclast precursor cells such as bone marrow-derived CD14(+) monocytes and RAW246.7 cells to induce osteoclastogenesis as measured by expression of osteoclast-specific markers such as tartrate-resistant acid phosphatase and bone resorption activity. Treatment performed solely with PMT stimulates osteoclast differentiation, showing a receptor activator of nuclear factor-κB ligand (RANKL)-independent action of the toxin. The underlying signal transduction pathway was defined as activation of the heterotrimeric G proteins Gαq/11 leading to the transactivation of Ras and the mitogen-activated protein kinase pathway. Gαq/11 transactivates Ras via its effector phospholipase Cß-protein kinase C (PKC) involving proline-rich tyrosine kinase 2 (Pyk2). PMT-induced activation of the mitogen-activated protein kinase pathway results in stimulation of the osteoclastogenic transcription factors AP-1, NF-κB, and NFATc1. In addition, Ca(2+)-dependent calcineurin activation of NFAT is crucial for PMT-induced osteoclastogenesis. The data not only elucidate a rationale for PMT-dependent bone loss during atrophic rhinitis but also highlight a noncanonical, G-protein-dependent pathway toward bone resorption that is distinct from the RANKL-RANK pathway but mimics it. We define heterotrimeric G proteins as as-yet-underestimated entities/players in the maturation of osteoclasts which might be of pharmacological relevance. IMPORTANCE: Pasteurella multocida toxin (PMT) induces degradation of nasal turbinate bones, leading to the syndrome of atrophic rhinitis. Recently, the molecular mechanism and substrate specificity of PMT were identified. The toxin activates heterotrimeric G proteins by a covalent modification. However, the mechanism by which PMT induces bone degradation is poorly understood. Our report demonstrates a direct effect of PMT on osteoclast precursor cells, leading to maturation of bone-degrading osteoclasts. Interestingly, PMT stimulates osteoclastogenesis independently of the cytokine RANKL, which is a key factor in induction of osteoclast differentiation. This implicates a noncanonical osteoclastogenic signaling pathway induced by PMT. The elucidated Gαq/11-dependent osteoclastogenic signal transduction pathway ends in osteoclastogenic NFAT signaling. The noncanonical, heterotrimeric G protein-dependent osteoclast differentiation process may be of pharmacological relevance, as members of this pathway are highly druggable. In particular, modulation of G protein-coupled receptor activity in osteoclast progenitors by small molecules might be of specific interest.

Granzyme A produces bioactive IL-1ß through a nonapoptotic inflammasome-independent pathway.

Bacterial components are recognized by the immune system through activation of the inflammasome, eventually causing processing of the proinflammatory cytokine interleukin-1? (IL-1?), a pleiotropic cytokine and one of the most important mediators of inflammation, through the protease caspase-1. Synthesis of the precursor protein and processing into its bioactive form are tightly regulated, given that disturbed control of IL-1? release can cause severe autoinflammatory diseases or contribute to cancer development. We show that the bacterial Pasteurella multocida toxin (PMT) triggers Il1b gene transcription in macrophages independently of Toll-like receptor signaling through RhoA/Rho-kinase-mediated NF-?? activation. Furthermore, PMT mediates signal transducer and activator of transcription (STAT) protein-controlled granzyme A (a serine protease) expression in macrophages. The exocytosed granzyme A enters target cells and mediates IL-1? maturation independently of caspase-1 and without inducing cytotoxicity. These findings show that macrophages can induce an IL-1?-initiated immune response independently of inflammasome activity.

A bacterial toxin catalyzing tyrosine glycosylation of Rho and deamidation of Gq and Gi proteins.

Entomopathogenic Photorhabdus asymbiotica is an emerging pathogen in humans. Here, we identified a P. asymbiotica protein toxin (PaTox), which contains a glycosyltransferase and a deamidase domain. PaTox mono-O-glycosylates Y32 (or Y34) of eukaryotic Rho GTPases by using UDP-N-acetylglucosamine (UDP-GlcNAc). Tyrosine glycosylation inhibits Rho activation and prevents interaction with downstream effectors, resulting in actin disassembly, inhibition of phagocytosis and toxicity toward insects and mammalian cells. The crystal structure of the PaTox glycosyltransferase domain in complex with UDP-GlcNAc determined at 1.8-Å resolution represents a canonical GT-A fold and is the smallest glycosyltransferase toxin known. (1)H-NMR analysis identifies PaTox as a retaining glycosyltransferase. The glutamine-deamidase domain of PaTox blocks GTP hydrolysis of heterotrimeric Gαq/11 and Gαi proteins, thereby activating RhoA. Thus, PaTox hijacks host GTPase signaling in a bidirectional manner by deamidation-induced activation and glycosylation-induced inactivation of GTPases.

Cell-free synthesis and characterization of a novel cytotoxic pierisin-like protein from the cabbage butterfly Pieris rapae.

Pierisin-like proteins comprise a growing family of ADP-ribosyltransferases expressed in various species of white butterflies. The prototype pierisin-1 from the cabbage butterfly, Pieris rapae, was identified as a potent apoptosis-inducing agent, acting on various types of carcinoma cell lines by mono-ADP-ribosylation of DNA. The characterization of pierisin-like proteins is hampered by its potent toxicity, which prevents its expression as a recombinant protein in Escherichia coli. Here we characterized a new member of the pierisin protein family named pierisin-1b, which was cloned from P. rapae. Pierisin-1b consists of 849 amino acids residues and shares 63%-91% identity with already described pierisins. For expression of pierisin-1b a novel in vitro translation system was utilized. Obtained protein exhibits specific ADP-ribosyltransferase activity on deoxyguanosine residues of DNA leading to induction of apoptosis and cell death.

Pasteurella multocida toxin activates various heterotrimeric G proteins by deamidation.

Pasteurella multocida produces a 146-kDa protein toxin (Pasteurella multocida toxin, PMT), which stimulates diverse cellular signal transduction pathways by activating heterotrimeric G proteins. PMT deamidates a conserved glutamine residue of the α-subunit of heterotrimeric G proteins that is essential for GTP-hydrolysis, thereby arresting the G protein in the active state. The toxin substrates are Gα(q) Gα(13) and the Gα(i)-family proteins. Activation of these α-subunits causes stimulation of phospholipase Cß, Rho-guanine nucleotide exchange factors or inhibition of adenylyl cyclase. This article provides the current knowledge on PMT concerning the structure-function analysis based on the crystal structure and recently elucidated molecular mode of action. Furthermore, the impact of PMT on cellular signaling is discussed.

Pleiotropic role of Rac in mast cell activation revealed by a cell permeable Bordetella dermonecrotic fusion toxin.

To activate the GTPase Rac in rat basophilic leukemia (RBL) cells and mouse bone marrow-derived mast cells (BMMC) a TAT fusion toxin of Bordetella dermonecrotic toxin (DNT-TAT) was constructed. The fusion toxin activated Rac1 and RhoA in vitro but only Rac in RBL cells and BMMC. DNT-TAT caused an increase in inositol phosphate formation, calcium mobilization, ERK activation and degranulation of mast cells. All these effects were inhibited by the Rho GTPase-inactivating Clostridium difficile toxin B and Clostridium sordellii lethal toxin. Also the calcium ionophore A23187 caused mast cell activation, including ERK phosphorylation, by processes involving an activation of Rac. The data indicate pleiotropic functions of Rac in mast cell activation.