Crystal structures reveal a thiol protease-like catalytic triad in the C-terminal region of Pasteurella multocida toxin

Cellular and molecular action of the mitogenic protein-deamidating toxin from Pasteurella multocida

The mitogenic toxin from Pasteurella multocida (PMT) is a member of the dermonecrotic toxin family, which includes toxins from Bordetella, Escherichia coli and Yersinia. Members of the dermonecrotic toxin family modulate G-protein targets in host cells through selective deamidation and/or transglutamination of a critical active site Gln residue in the G-protein target, which results in the activation of intrinsic GTPase activity. Structural and biochemical data point to the uniqueness of PMT among these toxins in its structure and action. Whereas the other dermonecrotic toxins act on small Rho GTPases, PMT acts on the α subunits of heterotrimeric G(q) -, G(i) - and G(12/13) -protein families. To date, experimental evidence supports a model in which PMT potently stimulates various mitogenic and survival pathways through the activation of G(q) and G(12/13) signaling, ultimately leading to cellular proliferation, whilst strongly inhibiting pathways involved in cellular differentiation through the activation of G(i) signaling. The resulting cellular outcomes account for the global physiological effects observed during infection with toxinogenic P. multocida, and hint at potential long-term sequelae that may result from PMT exposure.

Plasma membrane association of three classes of bacterial toxins is mediated by a basic-hydrophobic motif

Plasma membrane targeting is essential for the proper function of many bacterial toxins. A conserved fourhelical bundle membrane localization domain (4HBM) was recently identified within three diverse families of toxins: clostridial glucosylating toxins, MARTX toxins and Pasteurella multocida-like toxins. When expressed in tissue culture cells or in yeast, GFP fusions to at least one 4HBM from each toxin family show significant peripheral membrane localization but with differing profiles. Both in vivo expression and in vitro binding studies reveal that the ability of these domains to localize to the plasma membrane and bind negatively charged phospholipids requires a basic-hydrophobic motif formed by the L1 and L3 loops. The different binding capacity of each 4HBM is defined by the hydrophobicity of an exposed residue within the motif. This study establishes that bacterial effectors utilize a normal host cell mechanism to locate the plasma membrane where they can then access their intracellular targets.

Bacterial protein toxins that modify host regulatory GTPases

Many bacterial pathogens produce protein toxins to outmanoeuvre the immune system of the host. Some of these proteins target regulatory GTPases such as those belonging to the RHO family, which control the actin cytoskeleton of the host cell. In this Review, I discuss a diversity of mechanisms that are used by bacterial effectors and toxins to modulate the activity of host GTPases, with a focus on covalent modifications such as ADP-ribosylation, glucosylation, adenylylation, proteolysis, deamidation and transglutamination.

Enzymatic actions of Pasteurella multocida toxin detected by monoclonal antibodies recognizing the deamidated α subunit of the heterotrimeric GTPase Gq

Pasteurella multocida toxin (PMT) is a virulence factor responsible for the pathogenesis of some Pasteurellosis. PMT exerts its toxic effects through the activation of heterotrimeric GTPase (G(q), G(12/13) and G(i))-dependent pathways, by deamidating a glutamine residue in the α subunit of these GTPases. However, the enzymatic characteristics of PMT are yet to be analyzed in detail because the deamidation has only been observed in cell-based assays. In the present study, we developed rat monoclonal antibodies, specifically recognizing the deamidated Gα(q), to detect the actions of PMT by immunological techniques such as western blotting. Using the monoclonal antibodies, we found that the toxin deamidated Gα(q) only under reducing conditions. The C-terminal region of PMT, C-PMT, was more active than the full-length PMT. The C3 domain possessing the enzyme core catalyzed the deamidation in vitro without any other domains. These results not only support previous observations on toxicity, but also provide insights into the enzymatic nature of PMT. In addition, we present several lines of evidence that Gα(11), as well as Gα(q), could be a substrate for PMT.

Arf6-dependent intracellular trafficking of Pasteurella multocida toxin and pH-dependent translocation from late endosomes

The potent mitogenic toxin from Pasteurella multocida (PMT) is the major virulence factor associated with a number of epizootic and zoonotic diseases caused by infection with this respiratory pathogen. PMT is a glutamine-specific protein deamidase that acts on its intracellular G-protein targets to increase intracellular calcium, cytoskeletal, and mitogenic signaling. PMT enters cells through receptor-mediated endocytosis and then translocates into the cytosol through a pH-dependent process that is inhibited by NH₄Cl or bafilomycin A1. However, the detailed mechanisms that govern cellular entry, trafficking, and translocation of PMT remain unclear. Co-localization studies described herein revealed that while PMT shares an initial entry pathway with transferrin (Tfn) and cholera toxin (CT), the trafficking pathways of Tfn, CT, and PMT subsequently diverge, as Tfn is trafficked to recycling endosomes, CT is trafficked retrograde to the ER, and PMT is trafficked to late endosomes. Our studies implicate the small regulatory GTPase Arf6 in the endocytic trafficking of PMT. Translocation of PMT from the endocytic vesicle occurs through a pH-dependent process that is also dependent on both microtubule and actin dynamics, as evidenced by inhibition of PMT activity in our SRE-based reporter assay, with nocodazole and cytochalasin D, respectively, suggesting that membrane translocation and cytotoxicity of PMT is dependent on its transfer to late endosomal compartments. In contrast, disruption of Golgi-ER trafficking with brefeldin A increased PMT activity, suggesting that inhibiting PMT trafficking to non-productive compartments that do not lead to translocation, while promoting formation of an acidic tubulovesicle system more conducive to translocation, enhances PMT translocation and activity.

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

Characterization of the membrane-targeting C1 domain in Pasteurella multocida toxin

Pasteurella multocida toxin (PMT) is a virulence factor responsible for the pathogenesis of some forms of pasteurellosis. The toxin activates G(q)- and G(12/13)-dependent pathways through the deamidation of a glutamine residue in the alpha-subunit of heterotrimeric GTPases. We recently reported the crystal structure of the C terminus (residues 575-1285) of PMT (C-PMT), which is composed of three domains (C1, C2, and C3), and that the C1 domain is involved in the localization of C-PMT to the plasma membrane, and the C3 domain possesses a cysteine protease-like catalytic triad. In this study, we analyzed the membrane-targeting function of the C1 domain in detail. The C1 domain consists of seven helices of which the first four (residues 590-670), showing structural similarity to the N terminus of Clostridium difficile toxin B, were found to be involved in the recruitment of C-PMT to the plasma membrane. C-PMT lacking these helices (C-PMT DeltaC1(4H)) neither localized to the plasma membrane nor stimulated the G(q/12/13)-dependent signaling pathways. When the membrane-targeting property was complemented by a peptide tag with an N-myristoylation motif, C-PMT DeltaC1(4H) recovered the PMT activity. Direct binding between the C1 domain and liposomes containing phospholipids was evidenced by surface plasmon resonance analyses. These results indicate that the C1 domain of C-PMT functions as a targeting signal for the plasma membrane.

Identification of a conserved membrane localization domain within numerous large bacterial protein toxins

Vibrio cholerae is the causative agent of the diarrheal disease cholera. Many virulence factors contribute to intestinal colonization and disease including the Multifunctional Autoprocessing RTX toxin (MARTX(Vc)). The Rho-inactivation domain (RID) of MARTX(Vc) is responsible for inactivating the Rho-family of small GTPases, which leads to depolymerization of the actin cytoskeleton. Based on a deletion analysis of RID to determine the minimal functional domain, we have identified a subdomain at the N terminus of RID that is homologous to the membrane targeting C1 domain of Pasteurella multocida toxin. A GFP fusion to this subdomain from RID colocalized with a plasma membrane marker when transiently expressed within HeLa cells and can be found in the membrane fraction following subcellular fractionation. This C1-like subdomain is present in multiple families of bacterial toxins, including all of the clostridial glucosyltransferase toxins and various MARTX toxins. GFP-fusions to these homologous domains are also membrane associated, indicating that this is a conserved membrane localization domain (MLD). We have identified three residues (Y23, S68, R70) as necessary for proper localization of one but not all MLDs. In addition, we found that substitution of the RID MLD with the MLDs from two different effector domains from the Vibrio vulnificus MARTX toxin restored RID activity, indicating that there is functional overlap between these MLDs. This study describes the initial recognition of a family of conserved plasma membrane-targeting domains found in multiple large bacterial toxins.

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 Cβ, 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.

The Salmonella SPI2 effector SseI mediates long-term systemic infection by modulating host cell migration

Host-adapted strains of Salmonella enterica cause systemic infections and have the ability to persist systemically for long periods of time despite the presence of a robust immune response. Chronically infected hosts are asymptomatic and transmit disease to naïve hosts via fecal shedding of bacteria, thereby serving as a critical reservoir for disease. We show that the bacterial effector protein SseI (also called SrfH), which is translocated into host cells by the Salmonella Pathogenicity Island 2 (SPI2) type III secretion system (T3SS), is required for Salmonella typhimurium to maintain a long-term chronic systemic infection in mice. SseI inhibits normal cell migration of primary macrophages and dendritic cells (DC) in vitro, and such inhibition requires the host factor IQ motif containing GTPase activating protein 1 (IQGAP1), an important regulator of cell migration. SseI binds directly to IQGAP1 and co-localizes with this factor at the cell periphery. The C-terminal domain of SseI is similar to PMT/ToxA, a bacterial toxin that contains a cysteine residue (C1165) that is critical for activity. Mutation of the corresponding residue in SseI (C178A) eliminates SseI function in vitro and in vivo, but not binding to IQGAP1. In addition, infection with wild-type (WT) S. typhimurium suppressed DC migration to the spleen in vivo in an SseI-dependent manner. Correspondingly, examination of spleens from mice infected with WT S. typhimurium revealed fewer DC and CD4⁺ T lymphocytes compared to mice infected with ΔsseI S. typhimurium. Taken together, our results demonstrate that SseI inhibits normal host cell migration, which ultimately counteracts the ability of the host to clear systemic bacteria.

Bacteria belonging to the genus Salmonella are capable of causing long-term chronic systemic infections, and bacteria primarily reside within macrophages in lymphoid tissues and sporadically are shed in the feces. These persistently infected individuals serve as a significant reservoir for disease transmission. Despite the importance of Salmonella as a human pathogen, relatively little is known about the host immune response or virulence mechanisms of long-term systemic infections. Host-adapted Salmonella strains invade and manipulate host cells by releasing specialized bacterial effector proteins into the host cell. We show that one of these bacterial effector proteins, SseI (SrfH), is required for Salmonella to maintain a long-term chronic systemic infection in mice. SseI is able to block the migration of host immune cells and consequentially attenuate the host's ability to clear systemic bacteria. SseI accomplishes this inhibitory activity in part by associating with the host protein IQGAP1, an important regulator of cell migration. The amino acid sequence of SseI is similar to several other protein sequences of known bacterial pathogens, including PMT/ToxA, a toxin, indicating that these factors may function similarly to one another and may comprise a new family of bacterial effector proteins.

The Rho GTPase inactivation domain in Vibrio cholerae MARTX toxin has a circularly permuted papain-like thiol protease fold

A Rho GTPase inactivation domain (RID) has been discovered in the multifunctional, autoprocessing RTX toxin RtxA from Vibrio cholerae. The RID domain causes actin depolymerization and rounding of host cells through inactivation of the small Rho GTPases Rho, Rac, and Cdc42. With only a few toxin proteins containing RID domains in the current sequence database, the structure and molecular mechanisms of this domain are unknown. Using comparative sequence and structural analyses, we report homology inference, fold recognition, and active site prediction for RID domains. Remote homologs of RID domains were identified in two other experimentally characterized bacterial virulence factors: IcsB of Shigella flexneri and BopA of Burkholderia pseudomallei, as well as in a group of uncharacterized bacterial membrane proteins. IcsB plays an important role in helping Shigella to evade the host autophagy defense system. RID domain homologs share a conserved diad of cysteine and histidine residues, and are predicted to adopt a circularly permuted papain-like thiol protease fold. RID domains from MARTX toxins and virulence factors IcsB and BopA thus could function as proteases or acyltransferases acting on host molecules. Our results provide structural and mechanistic insights into several important proteins functioning in bacterial pathogenesis.

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.

A bacterial type III effector family uses the papain-like hydrolytic activity to arrest the host cell cycle

Pathogenic bacteria deliver effector proteins into host cells through the type III secretion apparatus to modulate the host function. We identify a family of proteins, homologous to the type III effector Cif from enteropathogenic Escherichia coli, in pathogens including Yersinia, Photorhabdus, and Burkholderia that contain functional type III secretion systems. Like Cif, this family of proteins is capable of arresting the host cell cycle at G(2)/M. Structure of one of the family members, Cif homolog in Burkholderia pseudomallei (CHBP), reveals a papain-like fold and a conserved Cys-His-Gln catalytic triad despite the lack of primary sequence identity. For CHBP and Cif, only the putative catalytic Cys is susceptible to covalent modification by E-64, a specific inhibitor of papain-like cysteine proteases. Unlike papain-like enzymes where the S2 site is the major determinant of cleavage-site specificity, CHBP has a characteristic negatively charged pocket occupying surface areas corresponding to the S1/S1' site in papain-like proteases. The negative charge is provided by a conserved aspartate, and the pocket best fits an arginine, as revealed by molecular docking analysis. Mutation analysis establishes the essential role of the catalytic triad and the negatively charged pocket in inducing cell cycle arrest in host cells. Our results demonstrate that bacterial pathogens have evolved a unique papain-like hydrolytic activity to block the normal host cell cycle progression.

Activation of Galpha (i) and subsequent uncoupling of receptor-Galpha(i) signaling by Pasteurella multocida toxin

Bacterial protein toxins are powerful tools for elucidating signaling mechanisms in eukaryotic cells. A number of bacterial protein toxins, e.g. cholera toxin, pertussis toxin (PTx), or Pasteurella multocida toxin (PMT), target heterotrimeric G proteins and have been used to stimulate or block specific signaling pathways or to demonstrate the contribution of their target proteins in cellular effects. PMT is a major virulence factor of P. multocida causing pasteurellosis in man and animals and is responsible for atrophic rhinitis in pigs. PMT modulates various signaling pathways, including phospholipase Cbeta and RhoA, by acting on the heterotrimeric G proteins Galpha(q) and Galpha(12/13), respectively. Here we report that PMT is a powerful activator of G(i) protein. We show that PMT decreases basal isoproterenol and forskolin-stimulated cAMP accumulation in intact Swiss 3T3 cells, inhibits adenylyl cyclase activity in cell membrane preparations, and enhances the inhibition of cAMP accumulation caused by lysophosphatidic acid via endothelial differentiation gene receptors. PMT-mediated inhibition of cAMP production is independent of toxin activation of Galpha(q) and/or Galpha(12/13). Although the effects of PMT are not inhibited by PTx, PMT blocks PTx-catalyzed ADP-ribosylation of G(i). PMT also inhibits steady-state GTPase activity and GTP binding of G(i) in Swiss 3T3 cell membranes stimulated by lysophosphatidic acid. The data indicate that PMT is a novel activator of G(i), modulating its GTPase activity and converting it into a PTx-insensitive state.

Structure and mode of action of clostridial glucosylating toxins: the ABCD model

Toxins A and B, which are the major virulence factors of antibiotic-associated diarrhea and pseudomembranous colitis caused by Clostridium difficile, are the prototypes of the family of clostridial glucosylating toxins. The toxins inactivate Rho and Ras proteins by glucosylation. Recent findings on the autocatalytic processing of the toxins and analysis of the crystal structures of their domains have made a revision of the current model of their actions on the eukaryotic target cells necessary.

The C3 domain of Pasteurella multocida toxin is the minimal domain responsible for activation of Gq-dependent calcium and mitogenic signaling

The large 1285-amino-acid protein toxin from Pasteurella multocida (PMT) is a multifunctional single-chain polypeptide that binds to and enters eukaryotic cells and acts intracellularly to promote G(q) and G(12/13) protein-dependent calcium and mitogenic signal transduction. Previous studies indicated that the intracellular activity domain responsible for PMT action was located within the C-terminal 600-700 amino acids. In this study, we have exogenously expressed a series of N- and C-terminal PMT fragments directly in mammalian cells and have used the dual luciferase reporter system to assay for toxin-mediated activation of calcium-calcineurin-NFAT signaling (NFAT-luciferase) and mitogenic serum response signaling (SRE-luciferase). Using this approach, we have defined the last 180 amino acids, which encompass the C3 domain in the crystal structure, as the minimum domain sufficient to activate both NFAT and SRE signaling pathways.

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.

Histidine Residues at the Active Site of the Pasteurella multocida Toxin

We have investigated histidine residues near the active site of the mitogenic Pasteurella multocida toxin. Mutation of H1202 or H1228 had little effect, while the effect of mutation on H1223 depended on the amino acid substituted. Mutation of H1205 caused complete loss of activity, indicating its importance in PMT activity.