Pseudomonas aeruginosa exoenzyme S requires a eukaryotic protein for ADP-ribosyltransferase activity

Evidence for intracellular Pseudomonas aeruginosa

Pseudomonas aeruginosa is a significant cause of global morbidity and mortality. Although it is often regarded as an extracellular pathogen toward human cells, numerous investigations report its ability to survive and replicate within host cells, and additional studies demonstrate specific mechanisms enabling it to adopt an intracellular lifestyle. This ability of P. aeruginosa remains less well-investigated than that of other intracellular bacteria, although it is currently gaining attention. If intracellular bacteria are not killed after entering host cells, they may instead receive protection from immune recognition and experience reduced exposure to antibiotic therapy, among additional potential advantages shared with other facultative intracellular pathogens. For this review, we compiled studies that observe intracellular P. aeruginosa across strains, cell types, and experimental systems in vitro, as well as contextualize these findings with the few studies that report similar observations in vivo. We also seek to address key findings that drove the perception that P. aeruginosa remains extracellular in order to reconcile what is currently understood about intracellular pathogenesis and highlight open questions regarding its contribution to disease.

ADP-ribosyl transferase activity and gamma radiation cytotoxicity of Pseudomonas aeruginosa exotoxin A

This work explores the ADP-ribosyltransferase activity of Pseudomonas (P.) aeruginosa exotoxin A using the guanyl hydrazone derivative, nitrobenzylidine aminoguanidine (NBAG) and the impact of gamma radiation on its efficacy. Unlike the conventional detection methods, NBAG was used as the acceptor of ADP ribose moiety instead of wheat germ extract elongation factor 2. Exotoxin A was extracted from P. aeruginosa clinical isolates and screened for toxA gene using standard PCR. NBAG was synthesized using aminoguanidine bicarbonate and 4-nitrobenzaldehyde and its identity has been confirmed by UV, FTIR, Mass and ¹³C-NMR spectroscopy. The ADP-ribosyl transferase activity of exotoxin A on NBAG in the presence of Nicotinamide adenine dinucleotide (NAD⁺) was recorded using UV spectroscopy and HPLC. In vitro ADP-ribosyl transferase activity of exotoxin A protein extract was also explored by monitoring its cytotoxicity on Hep-2 cells using sulforhodamine B cytotoxicity assay. Bacterial broths were irradiated at 5, 10, 15, 24 Gy and exotoxin A protein extract activity were assessed post exposure. Exotoxin A extract exerted an ADP-ribosyltransferase ability which was depicted by the appearance of a new ʎmax after the addition of exotoxin A to NBAG/NAD⁺ mixture, fragmentation of NAD⁺ and development of new peaks in HPLC chromatograms. Intracellular enzyme activity was confirmed by the prominent cytotoxic effects of exotoxin A extract on cultured cells. In conclusion, the activity of Exotoxin A can be monitored via its ADP-ribosyltransferase activity and low doses of gamma radiation reduced its activity. Therefore, coupling radiotherapy with exotoxin A in cancer therapy should be carefully monitored.

Identification of Inhibitors of Pseudomonas aeruginosa Exotoxin-S ADP-Ribosyltransferase Activity

The gram-negative bacterium Pseudomonas aeruginosa is an opportunistic pathogen associated with drug resistance complications and, as such, an important object for drug discovery efforts. One attractive target for development of therapeutics is the ADP-ribosyltransferase Exotoxin-S (ExoS), an early effector of the type III secretion system that is delivered into host cells to affect their transcription pattern and cytoskeletal dynamics. The purpose of this study was to formulate a real-time assay of purified recombinant ExoS activity for high-throughput application. We characterized the turnover kinetics of the fluorescent dinucleotide 1,N(6)-etheno-NAD+ as co-substrate for ExoS. Further, we found that the toxin relied on any of five tested isoforms of human 14-3-3 to modify vH-Ras and the Rho-family GTPases Rac1, -2, and -3 and RhoC. We then used 14-3-3β-stimulated ExoS modification of vH-Ras to screen a collection of low-molecular-weight compounds selected to target the poly-ADP ribose polymerase family and identified 3-(4-oxo-3,5,6,7-tetrahydro-4H-cyclopenta[4,5]thieno[2,3-d]pyrimidin-2-yl)propanoic acid as an ExoS inhibitor with micromolar potency. Thus, we present an optimized method to screen for inhibitors of ExoS activity that is amenable to high-throughput format and an intermediate affinity inhibitor that can serve both as assay control and as a starting point for further development.

Association between Pseudomonas aeruginosa type III secretion, antibiotic resistance, and clinical outcome: a review

Pseudomonas aeruginosa uses a complex type III secretion system to inject the toxins ExoS, ExoT, ExoU, and ExoY into the cytosol of target eukaryotic cells. This system is regulated by the exoenzyme S regulon and includes the transcriptional activator ExsA. Of the four toxins, ExoU is characterized as the major virulence factor responsible for alveolar epithelial injury in patients with P. aeruginosa pneumonia. Virulent strains of P. aeruginosa possess the exoU gene, whereas non-virulent strains lack this particular gene. The mechanism of virulence for the exoU⁺ genotype relies on the presence of a pathogenic gene cluster (PAPI-2) encoding exoU and its chaperone, spcU. The ExoU toxin has a patatin-like phospholipase domain in its N-terminal, exhibits phospholipase A₂ activity, and requires a eukaryotic cell factor for activation. The C-terminal of ExoU has a ubiquitinylation mechanism of activation. This probably induces a structural change in enzymatic active sites required for phospholipase A₂ activity. In P. aeruginosa clinical isolates, the exoU⁺ genotype correlates with a fluoroquinolone resistance phenotype. Additionally, poor clinical outcomes have been observed in patients with pneumonia caused by exoU⁺-fluoroquinolone-resistant isolates. Therefore, the potential exists to improve clinical outcomes in patients with P. aeruginosa pneumonia by identifying virulent and antimicrobial drug-resistant strains through exoU genotyping or ExoU protein phenotyping or both.

Bacillus cereus Certhrax ADP-ribosylates vinculin to disrupt focal adhesion complexes and cell adhesion

Bacillus cereus is often associated with mild to moderate gastroenteritis; however, some recent isolates cause inhalational anthrax-like diseases and death. These potential emerging human pathogens express multiple virulence factors. B. cereus strain G9241 expresses anthrax toxin, several polysaccharide capsules, and the novel ADP-ribosyltransferase, Certhrax. In this study, we show that Certhrax ADP-ribosylates Arg-433 of vinculin, a protein that coordinates actin cytoskeleton and extracellular matrix interactions. ADP-ribosylation of vinculin disrupted focal adhesion complexes and redistributed vinculin to the cytoplasm. Exogenous vinculin rescued these phenotypes. This provides a mechanism for strain G9241 to breach host barrier defenses and promote bacterial growth and spread. Certhrax is the first bacterial toxin to add a post-translational modification to vinculin to disrupt the actin cytoskeleton.

The molecular mechanism of acute lung injury caused by Pseudomonas aeruginosa: from bacterial pathogenesis to host response

Pseudomonas aeruginosa is the most common gram-negative pathogen causing pneumonia in immunocompromised patients. Acute lung injury induced by bacterial exoproducts is associated with a poor outcome in P. aeruginosa pneumonia. The major pathogenic toxins among the exoproducts of P. aeruginosa and the mechanism by which they cause acute lung injury have been investigated: exoenzyme S and co-regulated toxins were found to contribute to acute lung injury. P. aeruginosa secretes these toxins through the recently defined type III secretion system (TTSS), by which gram-negative bacteria directly translocate toxins into the cytosol of target eukaryotic cells. TTSS comprises the secretion apparatus (termed the injectisome), translocators, secreted toxins, and regulatory components. In the P. aeruginosa genome, a pathogenic gene cluster, the exoenzyme S regulon, encodes genes underlying the regulation, secretion, and translocation of TTSS. Four type III secretory toxins, namely ExoS, ExoT, ExoU, and ExoY, have been identified in P. aeruginosa. ExoS is a 49-kDa form of exoenzyme S, a bifunctional toxin that exerts ADP-ribosyltransferase and GTPase-activating protein (GAP) activity to disrupt endocytosis, the actin cytoskeleton, and cell proliferation. ExoT, a 53-kDa form of exoenzyme S with 75% sequence homology to ExoS, also exerts GAP activity to interfere with cell morphology and motility. ExoY is a nucleotidal cyclase that increases the intracellular levels of cyclic adenosine and guanosine monophosphates, resulting in edema formation. ExoU, which exhibits phospholipase A2 activity activated by host cell ubiquitination after translocation, is a major pathogenic cytotoxin that causes alveolar epithelial injury and macrophage necrosis. Approximately 20% of clinical isolates also secrete ExoU, a gene encoded within an insertional pathogenic gene cluster named P. aeruginosa pathogenicity island-2. The ExoU secretory phenotype is associated with a poor clinical outcome in P. aeruginosa pneumonia. Blockade of translocation by TTSS or inhibition of the enzymatic activity of translocated toxins has the potential to decrease acute lung injury and improve clinical outcome.

In vitro assays to monitor the activity of Pseudomonas aeruginosa Type III secreted proteins

Pseudomonas aeruginosa secretes numerous toxins and destructive enzymes that play distinct roles in pathogenesis. The Type III secretion system (T3SS) of Pseudomonas is a system that delivers a subset of toxins directly into the cytoplasm of eukaryotic cells. The secreted effectors include ExoS, ExoT, ExoU, and ExoY. In this chapter, we describe methods to induce T3S expression and measure the enzymatic activities of each effector in in vitro assays. ExoU is a phospholipase and its activity can be measured in a fluorescence-based assay monitoring the cleavage of the fluorogenic substrate, PED6. ExoS and ExoT both possess ADP-ribosyltransferase (ADPRT) and GTPase-activating protein (GAP) activity. ADPRT activity can be assessed by using radiolabeled nicotinamide adenine dinucleotide (NAD(+)) and measuring the covalent incorporation of ADP-ribose into a target protein. GAP activity is measured by the release of radiolabeled phosphate from [γ-(32)P]GTP-bound target proteins. In accordance with recent trends towards reducing the use of radioactivity in the laboratory, alternative assays using fluorescent or biotin-labeled reagents are described. ExoY is a nucleotidyl cyclase; cAMP production stimulated by ExoY can be monitored using reverse-phase HPLC or with commercially available immunological assays.

Exoenzyme S ADP-ribosylates Rab5 effector sites to uncouple intracellular trafficking

Pseudomonas aeruginosa exoenzyme S (ExoS) ADP-ribosylates multiple eukaryotic targets to promote cytopathology and bacterial colonization. ADP-ribosylation of the small GTPase Rab5 has previously been shown to block fluid-phase endocytosis and trafficking of plasma membrane receptors to the early endosomes as well as inhibit phagocytosis of the bacterium. In this study, ExoS is shown to be capable of ADP-ribosylating 6 candidate arginine residues that are located in the effector binding region or in the C terminus of Rab5. Two Rab5 derivatives were engineered, which contained Arg→Ala mutations at four Arg residues within the effector binding region (EF) or two Arg residues within the C-terminal tail (TL). Expression of Rab5(TL) does not affect the ability of ExoS to modify intracellular trafficking, while expression of Rab5(EF) rescued the ability of ExoS to inhibit intracellular trafficking. ADP-ribosylation of effector arginines likely uncouples Rab5 signaling to downstream effectors. This is a different mechanism for inhibition than observed for the ADP-ribosylation of Ras by ExoS, where ADP-ribosylated Ras loses the ability to bind guanine nucleotide exchange factor (GEF). Other experiments showed that expression of dominant negative Rab5(Ser34Asn) does not inhibit ExoS trafficking to the perinuclear region of intoxicated cells. This study provides insight into a mechanism for how ExoS ADP-ribosylation of Rab5 inhibits Rab5 function.

Role of the membrane localization domain of the Pseudomonas aeruginosa effector protein ExoU in cytotoxicity

ExoU is a potent effector protein that causes rapid host cell death upon injection by the type III secretion system of Pseudomonas aeruginosa. The N-terminal half of ExoU contains a patatin-like phospholipase A(2) (PLA(2)) domain that requires the host cell cofactor superoxide dismutase 1 (SOD1) for activation, while the C-terminal 137 amino acids constitute a membrane localization domain (MLD). Previous studies had utilized insertion and deletion mutations to show that portions of the MLD are required for membrane localization and catalytic activity. Here we further characterize this domain by identifying six residues that are essential for ExoU activity. Substitutions at each of these positions resulted in abrogation of membrane targeting, decreased ExoU-mediated cytotoxicity, and reductions in PLA(2) activity. Likewise, each of the six MLD residues was necessary for full virulence in cell culture and murine models of acute pneumonia. Purified recombinant ExoU proteins with substitutions at five of the six residues were not activated by SOD1, suggesting that these five residues are critical for activation by this cofactor. Interestingly, these same five ExoU proteins were partially activated by HeLa cell extracts, suggesting that a host cell cofactor other than SOD1 is capable of modulating the activity of ExoU. These findings add to our understanding of the role of the MLD in ExoU-mediated virulence.

The type III secretion system of Pseudomonas aeruginosa: infection by injection

The Gram-negative bacterium Pseudomonas aeruginosa uses a complex type III secretion apparatus to inject effector proteins into host cells. The configuration of this secretion machinery, the activities of the proteins that are injected by it and the consequences of this process for infection are now being elucidated. This Review summarizes our current knowledge of P. aeruginosa type III secretion, including the secretion and translocation machinery, the regulation of this machinery, and the associated chaperones and effector proteins. The features of this interesting secretion system have important implications for the pathogenesis of P. aeruginosa infections and for other type III secretion systems.

Pseudomonas aeruginosa virulence and therapy: evolving translational strategies

OBJECTIVE

Although most reviews of Pseudomonas aeruginosa therapeutics focus on antibiotics currently in use or in the pipeline, we review evolving translational strategies aimed at using virulence factor antagonists as adjunctive therapies.

DATA SOURCE

Current literature regarding P. aeruginosa virulence determinants and approaches that target them, with an emphasis on type III secretion, quorum-sensing, biofilms, and flagella.

DATA EXTRACTION AND SYNTHESIS

P. aeruginosa remains one of the most important pathogens in nosocomial infections, with high associated morbidity and mortality. Its predilection to develop resistance to antibiotics and expression of multiple virulence factors contributes to the frequent ineffectiveness of current therapies. Among the many P. aeruginosa virulence determinants that impact infections, type III secretion, quorum sensing, biofilm formation, and flagella have been the focus on much recent investigation. Here we review how increased understanding of these important bacterial structures and processes has enabled the development of novel approaches to inhibit each. These promising translational strategies may lead to the development of adjunctive therapies capable of improving outcomes.

CONCLUSIONS

Adjuvant therapies directed against virulence factors have the potential to improve outcomes in P. aeruginosa infections.

Host factor reveals the intrinsic enzymatic activity of a bacterial effector

Evaluation of: Lossi NS, Rolhion N, Magee AI, Boyle C, Holden DW: The Salmonella SPI-2 effector SseJ exhibits eukaryotic activator-dependent phospholipase A and glycerophospholipid:cholesterol acyltransferase activity. Microbiology 154, 2680–2688 (2008). The integral role that pathogenic bacterial effectors play during infection is well accepted; however, the biochemical activities of many pathogenic bacterial effectors remain unknown. Complicating the matter is the interplay between the bacterial effector and the host-cell environment. By way of example Lossi et al. demonstrate that recombinant SseJ is catalytically inactive in vitro but can be activated by a eukaryotic proteinaceous factor that remains to be identified and characterized.

Molecular mechanisms of the cytotoxicity of ADP-ribosylating toxins

Bacterial pathogens utilize toxins to modify or kill host cells. The bacterial ADP-ribosyltransferases are a family of protein toxins that covalently transfer the ADP-ribose portion of NAD to host proteins. Each bacterial ADP-ribosyltransferase toxin modifies a specific host protein(s) that yields a unique pathology. These toxins possess the capacity to enter a host cell or to use a bacterial Type III apparatus for delivery into the host cell. Advances in our understanding of bacterial toxin action parallel the development of biophysical and structural biology as well as our understanding of the mammalian cell. Bacterial toxins have been utilized as vaccines, as tools to dissect host cell physiology, and more recently for the development of novel therapies to treat human disease.

Identification of superoxide dismutase as a cofactor for the pseudomonas type III toxin, ExoU

Pseudomonas aeruginosa is an opportunistic pathogen that uses a type III secretion system and four effector proteins to avoid innate immune responses. ExoS, ExoT, ExoY, and ExoU all possess enzymatic activities that disrupt host cellular physiology and prevent bacterial clearance by host defense mechanisms. The specificity of these toxins for eukaryotic cells depends on the presence of substrate targets and eukaryotic cofactors responsible for effector activation. We used a combined biochemical and proteomic approach to identify Cu(2+), Zn(2+)-superoxide dismutase (SOD1) as a cofactor that activates the phospholipase activity of ExoU. Recombinant ExoU (rExoU) was activated in a dose-dependent manner by either bovine liver SOD1 or the yeast ortholog, Sod1p, but not by either Fe or Mn-containing SODs from E. coli or small molecule SOD mimetics. Inhibitor studies indicated that SOD enzymatic activity was not required for the activation of rExoU. The physical interaction between rExoU and SOD was demonstrated by capture techniques using either of the two proteins immobilized onto the solid phase. Identification of SOD as a cofactor allowed us to develop a new assay using a fluorescent substrate to measure the phospholipase activity of rExoU. The ability of SOD to act as a cytoplasmic cofactor stimulating ExoU phospholipase activity has significant implications for the biological activity of the toxin. Further elucidation of the structural mechanism of ExoU activation by this eukaryotic cofactor may provide a rational approach to the design of inhibitors that can diminish tissue damage during infection by ExoU-producing strains of P. aeruginosa.

Eukaryotic localization, activation and ubiquitinylation of a bacterial type III secreted toxin

Type III secretion is a widespread method whereby Gram-negative bacteria introduce toxins into eukaryotic cells. These toxins mimic or subvert a normal cellular process by interacting with a specific target, although how toxins reach their site of action is unclear. We set out to investigate the intracellular localization of a type III toxin of Pseudomonas aeruginosa called ExoU, which has phospholipase activity and requires a eukaryotic factor for activity. We found that ExoU is localized to the plasma membrane and undergoes modification within the cell by addition of two ubiquitin molecules at lysine-178. A region of five amino acids at position 679-683 near the C-terminus of the ExoU protein controls both membrane localization and ubiquitinylation. Site-directed mutagenesis identified a tryptophan at position 681 as crucial for these effects. We found that the same region at position 679-683 was also required for cell toxicity produced by ExoU as well as in vitro phospholipase activity. Localization of the phospholipase ExoU to the plasma membrane is thus required for activation and allows efficient utilization of adjacent substrate phospholipids.

Delineation of exoenzyme S residues that mediate the interaction with 14-3-3 and its biological activity

14-3-3 proteins belong to a family of conserved molecules expressed in all eukaryotic cells, which play an important role in a multitude of signaling pathways. 14-3-3 proteins bind to phosphoserine/phosphothreonine motifs in a sequence-specific manner. More than 200 14-3-3 binding partners have been found that are involved in cell cycle regulation, apoptosis, stress responses, cell metabolism and malignant transformation. A phosphorylation-independent interaction has been reported to occur between 14-3-3 and a C-terminal domain within exoenzyme S (ExoS), a bacterial ADP-ribosyltransferase toxin from Pseudomonas aeruginosa. In this study, we have investigated the effect of amino acid mutations in this C-terminal domain of ExoS on ADP-ribosyltransferase activity and the 14-3-3 interaction. Our results suggest that leucine-428 of ExoS is the most critical residue for ExoS enzymatic activity, as cytotoxicity analysis reveals that substitution of this leucine significantly weakens the ability of ExoS to mediate cell death. Leucine-428 is also required for the ability of ExoS to modify the eukaryotic endogenous target Ras. Finally, single amino acid substitutions of positions 426-428 reduce the interaction potential of 14-3-3 with ExoS in vitro.

Examination of the coordinate effects of Pseudomonas aeruginosa ExoS on Rac1

Exoenzyme S (ExoS) is a bifunctional toxin directly translocated into eukaryotic cells by the Pseudomonas aeruginosa type III secretory (TTS) process. The amino-terminal GTPase-activating (GAP) activity and the carboxy-terminal ADP-ribosyltransferase (ADPRT) activity of ExoS have been found to target but exert opposite effects on the same low-molecular-weight G protein, Rac1. ExoS ADP-ribosylation of Rac1 is cell line dependent. In HT-29 human epithelial cells, where Rac1 is ADP-ribosylated by TTS-ExoS, Rac1 was activated and relocalized to the membrane fraction. Arg66 and Arg68 within the GTPase-binding region of Rac1 were identified as preferred sites of ExoS ADP-ribosylation. The modification of these residues by ExoS would be predicted to interfere with Rac1 inactivation and explain the increase in active Rac1 caused by ExoS ADPRT activity. Using ExoS-GAP and ADPRT mutants to examine the coordinate effects of the two domains on Rac1 function, limited effects of ExoS-GAP on Rac1 inactivation were evident in HT-29 cells. In J774A.1 macrophages, where Rac1 was not ADP-ribosylated, ExoS caused a decrease in the levels of active Rac1, and this decrease was linked to ExoS-GAP. Using immunofluorescence staining of Rac1 to understand the cellular basis for the targeting of ExoS ADPRT activity to Rac1, an inverse relationship was observed between Rac1 plasma membrane localization and Rac1 ADP-ribosylation. The results obtained from these studies have allowed the development of a model to explain the differential targeting and coordinate effects of ExoS GAP and ADPRT activity on Rac1 within the host cell.

Uncoupling Crk signal transduction by Pseudomonas exoenzyme T

Exoenzyme T (ExoT) is a bifunctional type III cytotoxin of Pseudomonas aeruginosa that possesses both Rho GTPase-activating protein and ADP-ribosyltransferase activities. The ADP-ribosyltransferase activity of ExoT stimulated depolymerization of the actin cytoskeleton independent of Rho GTPase-activating protein function, and ExoT was subsequently shown to ADP-ribosylate Crk (CT10 regulator of kinase)-I and Crk-II. Crk proteins are eukaryotic adaptor proteins comprising SH2 and SH3 domains that are components of the integrin signaling pathway leading to Rac1 and Rap1 functions. Mass spectroscopic analysis identified Arg20 as the site of ADP-ribosylation by ExoT. Arg20 is a conserved residue located within the SH2 domain that is required for interactions with upstream signaling molecules such as paxillin and p130cas. Glutathione S-transferase pull-down and far Western assays showed that ADP-ribosylated Crk-I or Crk-I(R20K) failed to bind p130cas or paxillin. This indicates that ADP-ribosylation inhibited the direct interaction of Crk with these focal adhesion proteins. Overexpression of wild-type Crk-I reduced cell rounding by ExoT, whereas expression of dominant-active Rac1 interfered with the ability of ExoT to round cells. Thus, the ADP-ribosylation of Crk uncouples integrin signaling by direct inhibition of the binding of Crk to focal adhesion proteins.

Characterization of phospholipase activity of the Pseudomonas aeruginosa type III cytotoxin, ExoU

Recombinant ExoU (rExoU) and yeast extract were used to optimize an in vitro phospholipase assay as a basis for identifying the mechanism for enzyme activation and substrate specificity. Our results support a model in which a eukaryotic protein cofactor or complex facilitates the interaction of rExoU with phospholipid substrates.

ExoU is a potent intracellular phospholipase

The combination of a large genome encoding metabolic versatility and conserved secreted virulence determinants makes Pseudomonas aeruginosa a model pathogen that can be used to study host-parasite interactions in many eukaryotic hosts. One of the virulence regulons that likely plays a role in the ability of P. aeruginosa to avoid innate immune clearance in mammals is a type III secretion system (TTSS). Upon cellular contact, the P. aeruginosa TTSS is capable of delivering a combination of at least four different effector proteins, exoenzyme S (ExoS), ExoT, ExoU, and ExoY. Two of the four translocated proteins, ExoS and ExoU, are cytotoxic to cells during infection and transfection. The mechanism of cytotoxicity of ExoS is unclear. ExoU, however, has recently been characterized as a member of the phospholipase A family of enzymes, possessing at least phospholipase A2 activity. Similar to ExoS, ExoT and ExoY, ExoU requires either a eukaryotic-specific modification or cofactor for its activity in vitro. The biologic effects of minimal expression of ExoU in yeast can be visualized by membrane damage to different organelles and fragmentation of the vacuole. In mammalian cells, the direct injection of ExoU causes irreversible damage to cellular membranes and rapid necrotic death. ExoU likely represents a unique enzyme and is the first identified phopholipase virulence factor that is translocated into the cytosol by TTSS.

Pseudomonas aeruginosa ExoS and ExoT

ExoS and ExoT are bi-functional type-III cytotoxins of Pseudomonas aeruginosa that share 76% primary amino acid homology and contain N-terminal RhoGAP domains and C-terminal ADP-ribosylation domains. The Rho GAP activities of ExoS and ExoT appear to be biochemically and biologically identical, targeting Rho, Rac, and Cdc42. Expression of the RhoGAP domain in mammalian cells results in the disruption of the actin cytoskeleton and interference of phagocytosis. Expression of the ADP-ribosyltransferase domain of ExoS elicits a cytotoxic phenotype in cultured cells, while expression of ExoT appears to interfere with host cell phagocytic activity. Recent studies showed that ExoS and ExoT ADP-ribosylate different substrates. While ExoS has poly-substrate specificity and can ADP-ribosylate numerous host proteins, ExoT ADP-ribosylates a more restricted subset of host proteins including the Crk proteins. Protein modeling predicts that electrostatic interactions contribute to the substrate specificity of the ADP-ribosyltransferase domains of ExoS and ExoT.

How bacterial ADP-ribosylating toxins recognize substrates

ExoS and ExoT are bifunctional type III cytotoxins of Pseudomonas aeruginosa that contain an N-terminal RhoGAP domain and a C-terminal ADP-ribosylation domain. Although they share 76% amino acid identity, ExoS and ExoT ADP-ribosylate different substrates. Using protein modeling and site-directed mutagenesis, the regions of ExoS and ExoT that define substrate specificity were determined. Regions B (active site loop), C (ARTT motif) and E (PN loop) on ExoS are necessary and sufficient to recognize ExoS targets, whereas regions B, C and E on ExoT are necessary but not sufficient to recognize ExoT targets, such as the Crk proteins. A specific Crk recognition motif on ExoT was defined as region A (helix alpha1). The electrostatic properties of regions A, B, C and E define the substrate specificity of ExoS and ExoT and these interactions can explain how other bacterial ADP-ribosylating toxins recognize their unique substrates.

Ezrin/radixin/moesin proteins are high affinity targets for ADP-ribosylation by Pseudomonas aeruginosa ExoS

Pseudomonas aeruginosa ExoS is a bifunctional type III-secreted cytotoxin. The N terminus (amino acids 96-233) encodes a GTPase-activating protein activity, whereas the C terminus (amino acids 234-453) encodes a factor-activating ExoS-dependent ADP-ribosyltransferase activity. The GTPase-activating protein activity inactivates the Rho GTPases Rho, Rac, and Cdc42 in cultured cells and in vitro, whereas the ADP-ribosylation by ExoS is poly-substrate-specific and includes Ras as an early target for ADP-ribosylation. Infection of HeLa cells with P. aeruginosa producing a GTPase-activating protein-deficient form of ExoS rounded cells, indicating the ADP-ribosyltransferase domain alone is sufficient to elicit cytoskeletal changes. Examination of substrates modified by type III-delivered ExoS identified a 70-kDa protein as an early and predominant target for ADP-ribosylation. Matrix-assisted laser desorption ionization mass spectroscopy identified this protein as moesin, a member of the ezrin/radixin/moesin (ERM) family of proteins. ExoS ADP-ribosylated recombinant moesin at a linear velocity that was 5-fold faster and with a K(m) that was 2 orders of magnitude lower than Ras. Moesin homologs ezrin and radixin were also ADP-ribosylated, indicating the ERMs collectively represent high affinity targets of ExoS. Type III delivered ExoS ADP-ribosylated moesin and ezrin (and/or radixin) in cultured HeLa cells. The ERM proteins contribute to cytoskeleton dynamics, and the ability of ExoS to ADP-ribosylate the ERM proteins links ADP-ribosylation with the cytoskeletal changes associated with ExoS intoxication.

Lysophospholipase A activity of Pseudomonas aeruginosa type III secretory toxin ExoU

Acute lung injury in Pseudomonas aeruginosa pneumonia depends primarily on ExoU that is delivered directly into the eukaryotic cell via the type III secretion system. Recent studies demonstrated that ExoU has lipase activity, and that the cytotoxicity of ExoU is dependent on its patatin-like phospholipase domain. We investigated the phospholipase A (PLA) activity of ExoU. ExoU, but not non-catalytic ExoU-S142A, preincubated with the BEAS-2B cell lysate showed a weak increase of Ca(2+)-independent PLA(2) activity. When activated ExoU was mixed with secretory type PLA(2), more phospholipase activity was observed, suggesting that ExoU has lysophospholipase A (lysoPLA) activity. A significant increase in lysoPLA activity was also observed. Glycerol enhanced this activity and inhibitors of iPLA(2) suppressed ExoU's lysoPLA activity. Our results suggest that ExoU has a potent lysoPLA activity that requires the presence of the catalytically active site Ser(142) with an unknown eukaryotic cell factor(s) for its activation.

Molecular heterogeneity of a type III cytotoxin, Pseudomonas aeruginosa exoenzyme S

Pseudomonas aeruginosa ExoS is a bifunctional type III cytotoxin. The N-terminus (residues 1-232) is a Rho GTPase activating protein (GAP) domain, while the C-terminus (residues 233-453) is a FAS-dependent ADP-ribosyltransferase domain that targets Ras and Ras-like GTPases. A membrane localization domain (residues 51-72) localizes ExoS to a perinuclear region within eukaryotic cells. Recent studies observed that ExoS is auto-ADP-ribosylated upon delivery into eukaryotic cells. Auto-ADP-ribosylated ExoS analyzed from eukaryotic cells displayed pI heterogeneity and prompted an analysis of this heterogeneity. Bacterial-associated ExoS and ExoS that had been secreted by P. aeruginosa also showed pI heterogeneity with five charge forms ranging in pI from 5.1 to 5.9. The pI heterogeneity of ExoS was independent of a mass change and thus represented molecular charge conformers. Urea was not required to observe the pI conformers of ExoS; it enhanced the resolution and formation of pI conformers during the focusing component of the analysis. ExoS(E381D), a mutant deficient in ADP-ribosyltransferase activity, isolated from cultured cells showed charge forms that migrated to a more acidic pI than type III secreted ExoS but more basic than auto-ADP-ribosylated ExoS. Incubation of cell lysates with Mn(2+) shifted the pI of ExoS(E381D) to a pI identical to secreted ExoS. This indicates that within the mammalian cells ExoS undergoes a negatively charged modification, in addition to auto-ADP-ribosylation observed for wild-type ExoS. ExoT, ExoU, and YopE also focus into multiple pI forms, suggesting that this is a common property of type III cytotoxins.

Pseudomonas aeruginosa ExoT ADP-ribosylates CT10 regulator of kinase (Crk) proteins

Pseudomonas aeruginosa ExoT is a type III cytotoxin that functions as an anti-internalization factor with an N-terminal RhoGAP domain and a C-terminal ADP-ribosyltransferase domain. Although ExoT RhoGAP stimulates actin reorganization through the inactivation of Rho, Rac, and Cdc42, the function of the ADP-ribosylation domain is unknown. The present study characterized the mammalian proteins that are ADP-ribosylated by ExoT, using two-dimensional SDS-PAGE and matrix-assisted laser desorption ionization/time of flight (MALDI-TOF) analysis. ExoT ADP-ribosylated two cytosolic proteins in cell lysates upon type III delivery into cultured HeLa cells. MALDI-TOF mass spectrometry analysis identified the two proteins as Crk-I and Crk-II that are Src homology 2-3 domains containing adaptor proteins, which mediate signal pathways involving focal adhesion and phagocytosis. ExoT ADP-ribosylated recombinant Crk-I at a rate similar to the ADP-ribosylation of soybean trypsin inhibitor by ExoS. ExoS did not ADP-ribosylate Crk-I. ADP-ribosylation of Crk-I may be responsible for the anti-phagocytosis phenotype elicited by ExoT in mammalian cells.

Single-nucleotide-polymorphism mapping of the Pseudomonas aeruginosa type III secretion toxins for development of a diagnostic multiplex PCR system

We mapped the coding single nucleotide polymorphisms in four toxin genes-exoS, exoT, exoU, and exoY-of the Pseudomonas aeruginosa type III secretion system among several clinical isolates. We then used this information to design a multiplex PCR assay based on the simultaneous amplification of fragments of these genes. Eight strains of known genotype were used to test our multiplex PCR method, which showed 100% sensitivity and specificity in this small sample size. This assay appears to be promising for the rapid and accurate genotyping of the presence of these genes in clinical strains of P. aeruginosa.

Cell line differences in bacterially translocated ExoS ADP-ribosyltransferase substrate specificity

Exoenzyme S (ExoS) is an ADP-ribosyltransferase (ADPRT) directly translocated into eukaryotic cells by the type III secretory (TTS) process of Pseudomonas aeruginosa. Comparisons of the functional effects of ExoS on human epithelial and murine fibroblastic cells showed that human epithelial cells exhibited an overall increased sensitivity to the effects of bacterially translocated ExoS on cell proliferation, morphology and re-adherence. ExoS was also found to ADP-ribosylate a greater number of low-molecular-mass G (LMMG) proteins in human epithelial cells, as compared to murine fibroblasts. Examination of the cellular mechanism for differences in ExoS ADPRT substrate modification found that the more restricted pattern of substrate modification in murine fibroblasts was not linked to the efficiency of bacterial adherence nor to the efficiency of ExoS internalization by the TTS process. In exploring the cellular nature of patterns of substrate modification, more extensive substrate modification was detected in human and simian cell lines, while rodent cell lines, including rat, mouse and hamster lines, consistently exhibited the more limited pattern of LMMG protein ADP-ribosylation. Patterns of substrate modification were not altered by cellular transformation and occurred independently of cell type. These studies suggest that eukaryotic cell properties, as recognized through studies of cells of different animal origins, affect the substrate targeting of ExoS ADPRT activity, and that this in turn can influence the severity of effects of ExoS on host-cell function.

Inactivation of the elongation factor Tu by mosquitocidal toxin-catalyzed mono-ADP-ribosylation

The mosquitocidal toxin (MTX) produced by Bacillus sphaericus strain SSII-1 is an approximately 97-kDa single-chain toxin which contains a 27-kDa enzyme domain harboring ADP-ribosyltransferase activity and a 70-kDa putative binding domain. Due to cytotoxicity toward bacterial cells, the 27-kDa enzyme fragment cannot be produced in Escherichia coli expression systems. However, a nontoxic 32-kDa N-terminal truncation of MTX can be expressed in E. coli and subsequently cleaved to an active 27-kDa enzyme fragment. In vitro the 27-kDa enzyme fragment of MTX ADP-ribosylated numerous proteins in E. coli lysates, with dominant labeling of an approximately 45-kDa protein. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry combined with peptide mapping identified this protein as the E. coli elongation factor Tu (EF-Tu). ADP ribosylation of purified EF-Tu prevented the formation of the stable ternary EF-Tuaminoacyl-tRNAGTP complex, whereas the binding of GTP to EF-Tu was not altered. The inactivation of EF-Tu by MTX-mediated ADP-ribosylation and the resulting inhibition of bacterial protein synthesis are likely to play important roles in the cytotoxicity of the 27-kDa enzyme fragment of MTX toward E. coli.

Cellular hijacking: a common strategy for microbial infection

Eukaryotic cells are under constant attack from microbial intruders seeking a selective advantage for survival, propagation and dissemination. Microbial infections can often result in disease and might even be lethal to the host if they are not combatted effectively. Studies of host-pathogen interactions have revealed that virulence often requires the usurpation of existing cell signaling pathways or membrane traffic machinery of the host. Such studies provide a rich source of cell biological data that will probably prove essential for future efforts designed to either thwart these attacks or learn from them.

Comparison of the exoS gene and protein expression in soil and clinical isolates of Pseudomonas aeruginosa

Exoenzyme S (ExoS) is translocated into eukaryotic cells by the type III secretory process and has been hypothesized to function in conjunction with other virulence factors in the pathogenesis of Pseudomonas aeruginosa. To gain further understanding of how ExoS might contribute to P. aeruginosa survival and virulence, ExoS expression and the structural gene sequence were determined in P. aeruginosa soil isolates and compared with ExoS of clinical isolates. Significantly higher levels of ExoS ADP-ribosyltransferase (ADPRT) activity were detected in culture supernatants of soil isolates compared to those of clinical isolates. The higher levels of ADPRT activity of soil isolates reflected both the increased production of ExoS and the production of ExoS having a higher specific activity. ExoS structural gene sequence comparisons found the gene to be highly conserved among soil and clinical isolates, with the greatest number of nonsynonymous substitutions occurring within the region of ExoS encoding GAP function. The lack of amino acid changes in the ADPRT region in association with a higher specific activity implies that other factors produced by P. aeruginosa or residues outside the ADPRT region are affecting ExoS ADPRT activity. The data are consistent with ExoS being integral to P. aeruginosa survival in the soil and suggest that, in the transition of P. aeruginosa from the soil to certain clinical settings, the loss of ExoS expression is favored.

Exoenzyme T of Pseudomonas aeruginosa elicits cytotoxicity without interfering with Ras signal transduction

One virulence strategy used by the opportunistic pathogen Pseudomonas aeruginosa is to target toxic proteins into eukaryotic cells by a type III secretion mechanism. Two of these proteins, ExoS and ExoT, show 75% homology on amino acid level. However, compared with ExoS, ExoT exhibits highly reduced ADP-ribosylating activity and the role of ExoT in pathogenesis is poorly understood. To study the biological effect of ExoT, we used a strategy by which ExoT was delivered into host cells by the heterologous type III secretion system of Yersinia pseudotuberculosis. ExoT was found to induce a rounded cell morphology and to mediate disruption of actin microfilaments, similar to that induced by an ADP-ribosylation defective ExoS (E381A) and the related cytotoxin YopE of Y. pseudotuberculosis. In contrast to ExoS, ExoT had no major effect on cell viability and did not modify or inactivate Ras by ADP-ribosylation in vivo. However, similar to ExoS and YopE, ExoT exhibited GAP (GTPase activating protein) activity on RhoA GTPase in vitro. Interestingly, ExoT(R149K), deficient for GAP activity, still caused a morphological change of HeLa cells. Based on our findings, we suggest that the ADP-ribosylating activity of ExoT target another, as yet unidentified, host protein that is distinct from Ras.

ADP ribosylation of Arg41 of Rap by ExoS inhibits the ability of Rap to interact with its guanine nucleotide exchange factor, C3G

ExoS is a bifunctional type III cytotoxin that is secreted by Pseudomonas aeruginosa. The N-terminal domain comprises a RhoGAP activity, while the C-terminal domain comprises a ADP-ribosyltransferase activity. Previous studies showed that ExoS ADP ribosylated Ras at Arg41 which interfered with the ability of Ras to interact with its guanine nucleotide exchange factor. Rap and Ras share considerable primary amino acid homology, including Arg41. In this study, we report that ExoS ADP ribosylates Rap1b at Arg41 and that ADP ribosylation of Arg41 inhibits the ability of C3G to stimulate guanine nucleotide exchange. The mechanism responsible for this inhibition is one in which ADP-ribosylated Rap binds inefficiently to C3G, relative to wild type Rap. This identifies a second member of the Ras GTPase subfamily that can be ADP ribosylated by ExoS and indicates that ExoS can inhibit both Ras and Rap signaling pathways in eukaryotic cells.

Pseudomonas aeruginosa exoenzyme S, a bifunctional type-III secreted cytotoxin

Our recent studies have shown ExoS to be a bifunctional type-III secreted cytotoxin. Intracellular expression of the amino terminus of ExoS (C234) in eukaryotic cells stimulates actin reorganization without cytotoxicity, which involves small-molecular-weight GTPases of the Rho subfamily. Expression of the carboxyl terminus of ExoS comprises an ADP-ribosyltransferase domain, which is cytotoxic when expressed in cultured cells (Pederson and Barbieri, 1998). Rho and Ras are molecular switches, which control numerous cellular processes. Recent signaling studies suggest that there is crosstalk between Rho and Ras (Keely et al, 1997). Ras and Rho also contribute to wound healing processes and tissue regeneration. Recent studies have shown that microinjection of endothelial cells with activated Ras stimulated their motility, while microinjection of Ras-blocking antibodies inhibited cellular motility that is a component of the wound healing process (Fox et al., 1994). In addition, hepatocyte growth factor/scatter factor (HGF/ SF) and epidermal growth factor stimulate cellular motility through the Ras signal transduction pathway (Ridley et al., 1995). Rac and Rho are also involved in motility and tissue regeneration, since dominant negative Rac inhibits the cellular motility stimulated by HGF/SF (Santos et al., 1997) and inhibition of Rho by either C. difficile ToxA and ToxB or the C. botulinum C3 transferase inhibits wound healing (Santos et al., 1997). Inhibition of tissue regeneration and wound healing appear to play a role in the pathogenesis of C. difficile, since treatment of gastrointestinal mucosa with C. difficile ToxA and ToxB alone inhibits regeneration of the gastric mucosa. Thus, ExoS may contribute to the establishment of P. aeruginosa infections by inhibiting wound healing and tissue regeneration by two mechanisms. The amino terminus of ExoS could inhibit Rho function and wound healing in a manner similar to C. difficile. Alternatively, ExoS could inhibit the cellular motility and angiogenesis required for wound healing by ADP-ribosylating Ras. Through the inhibition of tissue regeneration and wound healing, ExoS may play a pivotal role in chronic disease by maintaining sites of colonization. Inhibition of Ras or Rho signaling may also interfere with both innate and acquired immunity. Small-molecular-weight GTP-binding proteins of the Ras superfamily are required for cellular processes, such as phagocytosis, as Rho proteins contribute to phagocytosis (Caron and Hall, 1998). Since Ras functions upstream of Rho in cellular signaling processes (Ridley et al., 1995), ADP-ribosylation of Ras by ExoS or the inhibition of Rho function by C234 may inhibit phagocytosis of P. aeruginosa by macrophages. Other studies indicate that Ras plays a role in T cell activation (Cantrell, 1994). Thus, ExoS may inhibit acquired immunity by inhibiting T-cell activation.

Residues of 14-3-3 zeta required for activation of exoenzyme S of Pseudomonas aeruginosa

Exoenzyme S (ExoS) is a mono-ADP-ribosyltransferase secreted by the opportunistic pathogen Pseudomonas aeruginosa. ExoS requires a eukaryotic factor, the 14-3-3 protein, for enzymatic activity. Here, two aspects of the activation of the ADP-ribosyltransferase activity of ExoS by 14-3-3 proteins are examined. Initial studies showed that several isoforms of 14-3-3, including beta, zeta, eta, sigma, and tau, activated ExoS with similar efficiency. This implicates a conserved structure in 14-3-3 that contributes to the interaction between 14-3-3 and ExoS. One candidate structure is the conserved amphipathic groove that mediates the 14-3-3/Raf-1 interaction. The next series of experiments examined the role of individual amino acids of the amphipathic groove of 14-3-3 zeta in ExoS activation and showed that ExoS activation required the basic residues lining the amphipathic groove of 14-3-3 zeta without extensive involvement of the hydrophobic residues. Strikingly, mutations of Val-176 of 14-3-3 zeta that disrupted its interaction with Raf-1 did not affect the binding and activation of ExoS by 14-3-3. Thus, ExoS selectively employs residues in the Raf-binding groove for its association with 14-3-3 proteins.

Pseudomonas aeruginosa exoenzyme S disrupts Ras-mediated signal transduction by inhibiting guanine nucleotide exchange factor-catalyzed nucleotide exchange

Pseudomonas aeruginosa exoenzyme S double ADP-ribosylates Ras at Arg(41) and Arg(128). Since Arg(41) is adjacent to the switch 1 region of Ras, ADP-ribosylation could interfere with Ras-mediated signal transduction via several mechanisms, including interaction with Raf, or guanine nucleotide exchange factor-stimulated or intrinsic nucleotide exchange. Initial experiments showed that ADP-ribosylated Ras (ADP-r-Ras) and unmodified Ras (Ras) interacted with Raf with equal efficiencies, indicating that ADP-ribosylation did not interfere with Ras-Raf interactions. While ADP-r-Ras and Ras possessed equivalent intrinsic nucleotide exchange rates, guanine nucleotide exchange factor (Cdc25) stimulated the nucleotide exchange of ADP-r-Ras at a 3-fold slower rate than Ras. ADP-r-Ras did not affect the nucleotide exchange of Ras, indicating that the ADP-ribosylation of Ras was not a dominant negative phenotype. Ras-R41K and ADP-r-Ras R41K possessed similar exchange rates as Ras, indicating that ADP-ribosylation at Arg(128) did not inhibit Cdc25-stimulated nucleotide exchange. Consistent with the slower nucleotide exchange rate of ADP-r-Ras as compared with Ras, ADP-r-Ras bound its guanine nucleotide exchange factor (Cdc25) less efficiently than Ras in direct binding experiments. Together, these data indicate that ADP-ribosylation of Ras at Arg(41) disrupts Ras-Cdc25 interactions, which inhibits the rate-limiting step in Ras signal transduction, the activation of Ras by its guanine nucleotide exchange factor.

Interruption of multiple cellular processes in HT-29 epithelial cells by Pseudomonas aeruginosa exoenzyme S

Exoenzyme S (ExoS), an ADP-ribosylating enzyme produced by the opportunistic pathogen Pseudomonas aeruginosa, is directly translocated into eukaryotic cells by bacterial contact. Within the cell, ExoS ADP-ribosylates the cell signaling protein Ras and causes inhibition of DNA synthesis and alterations in cytoskeletal structure. To further understand the interrelationship of the different cellular effects of ExoS, functional analyses were performed on HT-29 epithelial cells after exposure to ExoS-producing P. aeruginosa 388 and the non-ExoS-producing strain 388DeltaS. Two different mechanisms of morphological alteration were identified: (i) a more-transient and less-severe cell rounding caused by the non-ExoS-producing strain 388DeltaS and (ii) a more-severe, long-term cell rounding caused by ExoS-producing strain 388. Long-term effects of ExoS on cell morphology occurred in conjunction with ExoS-mediated inhibition of DNA synthesis and the ADP-ribosylation of Ras. ExoS was also found to cause alterations in HT-29 cell function, leading to the loss of cell adhesion and microvillus effacement. Nonadherent ExoS-treated cells remained viable but had a high proportion of modified Ras. While microvillus effacement was detected in both 388- and 388DeltaS-treated cells, effacement was more prevalent and rapid in cells exposed to strain 388. We conclude from these studies that ExoS can have multiple effects on epithelial cell function, with more severe cellular alterations associated with the enzymatic modification of Ras. The finding that ExoS had greater effects on cell growth and adherence than on cell viability suggests that ExoS may contribute to the P. aeruginosa infectious process by rendering cells nonfunctional.

ADP-ribosylation of oncogenic Ras proteins by pseudomonas aeruginosa exoenzyme S in vivo

The exoenzyme S (ExoS)-producing Pseudomonas aeruginosa strain, 388, and corresponding ExoS knock-out strain, 388deltaexoS, were used in a bacterial and mammalian co-culture system as a model for the contact-dependent delivery of ExoS into host cells. Examination of DNA synthesis and Ras ADP-ribosylation in tumour cell lines expressing normal and mutant Ras revealed a decrease in DNA synthesis concomitant with ADP-ribosylation of Ras proteins after exposure to ExoS-producing bacteria, but not after exposure to non-ExoS-producing bacteria. Examination of normal H-Ras, K-Ras and N-Ras by two-dimensional electrophoresis after exposure to bacteria revealed differences in the degree of ADP-ribosylation by ExoS, with H-Ras being modified most extensively. ADP-ribosylation of oncogenic forms of Ras was examined in vivo using cancer lines expressing mutant forms of H-, N- or K-Ras. The mutant Ras proteins were modified in a manner qualitatively similar to their normal counterparts. Using Ras/Raf-1 co-immunoprecipitation after co-culture, it was found that exposure to ExoS-producing bacteria caused a decrease in the amount of Raf-1 associated with EGF-activated Ras and oncogenic Ras. The results from this study indicate that ExoS ADP-ribosylates both normal and mutant Ras proteins in vivo and inhibits signalling through Ras.

Expression of FAS-independent ADP-ribosyltransferase activity by a catalytic deletion peptide of Pseudomonas aeruginosa exoenzyme S

Earlier studies reported that Pseudomonas aeruginosa exoenzyme S (ExoS) possessed an absolute requirement for the eukaryotic protein factor activating exoenzyme S (FAS) for expressing ADP-ribosyltransferase activity. During the characterization of a serum-derived FAS-like activity, we observed the ability of a catalytic deletion peptide of ExoS (DeltaN222) to ADP-ribosylate target proteins in the absence of FAS. Characterization of the activation of DeltaN222 by FAS provided an opportunity to gain insight into the mechanism of ExoS activation by FAS. Under standard enzyme assay conditions, the initial rate of FAS-independent ADP-ribosyltransferase activity of DeltaN222 was not linear with time and rapidly approached zero. Dilution into high-ionic strength buffers stabilized DeltaN222 so it could express FAS-independent ADP-ribosyltransferase activity at a linear rate. This stabilization was a general salt effect, since dilution into a 1.0 M solution of either NaCH3COOH, NaCl, or KCl stabilized the ADP-ribosyltransferase activity of DeltaN222. Kinetic analysis in a high-ionic strength buffer showed that FAS enhanced the catalytic activity of DeltaN222 by increasing the affinity for NAD and stimulating the turnover rate. Velocity experiments indicated that the stabilization of DeltaN222 by high salt was not functionally identical to stabilization by FAS. Together, these data implicate a dual role for FAS in the allosteric activation of ExoS, involving both substrate binding and catalysis.

Regulation of ExoS production and secretion by Pseudomonas aeruginosa in response to tissue culture conditions

This study was initiated to characterize the regulation and secretion of ExoS by Pseudomonas aeruginosa during contact with eukaryotic cells. The production of ExoS was monitored by a sensitive ADP-ribosyltransferase activity assay, and specific activities were calculated for supernatant and cell-associated fractions. Time course analysis indicated that ExoS was produced after a lag period, suggesting that induction of the regulon is necessary for the expression of detectable amounts of enzyme activity. Under tissue culture growth conditions, ExoS was induced when P. aeruginosa was in contact with Chinese hamster ovary (CHO) cells or after growth in tissue culture medium with serum. The serum induction of ExoS appeared to result in generalized type III secretion, while induction by contact with CHO cells appeared to result in polarized type III secretion. Mutants in the type III secretory system that express a null phenotype for ExoS production in bacteriological medium produced but did not secrete the enzyme when P. aeruginosa was grown under inducing conditions in tissue culture medium. These results suggest that both induction and secretion of ExoS may differ when the bacteria are exposed to different growth environments. The putative type III translocation proteins and secretion apparatus of P. aeruginosa were required for translocation of bacterial factors that mediate changes in CHO cell morphology during infection.

Intracellular expression of the ADP-ribosyltransferase domain of Pseudomonas exoenzyme S is cytotoxic to eukaryotic cells

Exoenzyme S of Pseudomonas aeruginosa is an ADP-ribosyltransferase, which is secreted via a type III-dependent secretion mechanism and has been demonstrated to exert cytotoxic effects on eukaryotic cells. Alignment studies predict that the amino-terminus of exoenzyme S has limited primary amino acid homology with the YopE cytotoxin of Yersinia, while biochemical studies have localized the FAS-dependent ADP-ribosyltransferase activity to the carboxyl-terminus. Thus, exoenzyme S could interfere with host cell physiology via several independent mechanisms. The goal of this study was to define the role of the ADP-ribosyltransferase domain in the modulation of eukaryotic cell physiology. The carboxyl-terminal 222 amino acids of exoenzyme S, which represent the FAS-dependent ADP-ribosyltransferase domain (termed deltaN222), and a point mutant, deltaN222-E381A, which possesses a 2000-fold reduction in the capacity to ADP-ribosylate, were transiently expressed in eukaryotic cells under the control of the immediate early CMV promoter. Lysates from cells transfected with deltaN222 expressed ADP-ribosyltransferase activity. Co-transfection of deltaN222, but not deltaN222-E381A, resulted in a decrease in the steady-state levels of two reporter proteins, green fluorescent protein and luciferase, in both CHO and Vero cells. In addition, transfection with deltaN222 resulted in a greater percentage of cells staining with trypan blue than when cells were transfected with either deltaN222-E381A or control plasmid. Together, these data indicate that expression of the ADP-ribosyltransferase domain of exoenzyme S is cytotoxic to eukaryotic cells.

Modification of Ras in eukaryotic cells by Pseudomonas aeruginosa exoenzyme S

Genetic and functional data suggest that Pseudomonas aeruginosa exoenzyme S (ExoS), an ADP-ribosyltransferase, is translocated into eukaryotic cells by a bacterial type III secretory mechanism activated by contact between bacteria and host cells. Although purified ExoS is not toxic to eukaryotic cells, ExoS-producing bacteria cause reduced proliferation and viability, possibly mediated by bacterially translocated ExoS. To investigate the activity of translocated ExoS, we examined in vivo modification of Ras, a preferred in vitro substrate. The ExoS-producing strain P. aeruginosa 388 and an isogenic mutant strain, 388DeltaexoS, which fails to produce ExoS, were cocultured with HT29 colon carcinoma cells. Ras was found to be ADP-ribosylated during coculture with 388 but not with 388DeltaexoS, and Ras modification by 388 corresponded with reduction in HT29 cell DNA synthesis. Active translocation by bacteria was found to be required, since exogenous ExoS, alone or in the presence of 388DeltaexoS, was unable to modify intracellular Ras. Other ExoS-producing strains caused modification of Ras, indicating that this is not a strain-specific event. ADP-ribosylation of Rap1, an additional Ras family substrate for ExoS in vitro, was not detectable in vivo under conditions sufficient for Ras modification, suggesting possible ExoS substrate preference among Ras-related proteins. These results confirm that intracellular Ras is modified by bacterially translocated ExoS and that the inhibition of target cell proliferation correlates with the efficiency of Ras modification.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

Exoenzyme S from P. aeruginosa ADP ribosylates rab4 and inhibits transferrin recycling in SLO-permeabilized reticulocytes

ADP-ribosylation of rab proteins by exoenzyme S (Exo S) of P. aeruginosa was studied using reticulocytes. 14-3-3 protein, the eukaryotic cofactor that is obligatory for Exo S activity, was found in association with reticulocyte endocytic vesicles and exosomes, vesicles previously shown to be enriched with rab4. Incubation of purified endocytic vesicles with Exo S triggered rab4 ADP-ribosylation. Transferrin recycling in SLO-permeabilized reticulocytes was highly impaired when Exo S was added to the cells, suggesting that ADP-ribosylation affected rab4 function. Moreover, in vitro ADP-ribosylation of different rab proteins was studied using the cofactor activity extracted from reticulocytes.

Functional analysis of exsC and exsB in regulation of exoenzyme S production by Pseudomonas aeruginosa

Expression of ExsC, ExsB, and ExsA (the exoenzyme S trans-regulatory locus) of Pseudomonas aeruginosa was analyzed by using complementation, RNase protection, translational fusion, and T7-directed protein expression analyses. T7 expression analyses in E. coli hosts demonstrated that ExsC, ExsA, and a truncated form of ExsD (a partial open reading frame located 3' of ExsA) were translated; however, a product corresponding to ExsB was undetectable. T7-mediated transcription and translation of the antisense strand resulted in production of a 18.5-kDa product, termed ExsB', which overlapped the predicted ExsB product. In complementation experiments, deletion of the region encoding ExsB and most of ExsB' severely reduced exoenzyme S production. Site-specific mutagenesis of the start codons for ExsB and ExsB', however, did not affect exoenzyme S production. RNase protection studies were initiated to examine the hypothesis that RNA encoded within the ExsB/ExsB' region exerted a regulatory effect. RNA encoding ExsB' was not detectable from chromosomal genes or complementation constructs, indicating that ExsB' was not expressed in P. aeruginosa. To determine the pattern of translation, a chloramphenicol acetyltransferase gene (cat) reporter was fused in frame with ExsB and with ExsA in the context of the entire locus or in the absence of the exsB region. These experiments indicated that exsB was not translated but that deletion of the exsB region affected the translation of ExsA-CAT. RNase protection assays further suggested that deletion of exsB resulted in a processing of ExsA mRNA. Our data indicate that the untranslated exsB region of the trans-regulatory locus mRNA mediates either the stability or the translation of exsA. Complementation analysis further suggests that ExsC may play a role in the translation or stability of ExoS.

Biochemical relationships between the 53-kilodalton (Exo53) and 49-kilodalton (ExoS) forms of exoenzyme S of Pseudomonas aeruginosa

Genetic studies have shown that the 53-kDa (Exo53) and 49-kDa (ExoS) forms of exoenzyme S of Pseudomonas aeruginosa are encoded by separate genes, termed exoT and exoS, respectively. Although ExoS and Exo53 possess 76% primary amino acid homology, Exo53 has been shown to express ADP-ribosyltransferase activity at about 0.2% of the specific activity of ExoS. The mechanism for the lower ADP-ribosyltransferase activity of Exo53 relative to ExoS was analyzed by using a recombinant deletion protein which contained the catalytic domain of Exo53, comprising its 223 carboxyl-terminal residues (termed N223-53). N223-53 was expressed in Escherichia coli as a stable, soluble fusion protein which was purified to >80% homogeneity. Under linear velocity conditions, N223-53 catalyzed the FAS (for factor activating exoenzyme S)-dependent ADP-ribosylation of soybean trypsin inhibitor (SBTI) at 0.4% and of the Ras protein at 1.0% of the rates of catalysis by N222-49. N222-49 is a protein comprising the 222 carboxyl-terminal residues of ExoS, which represent its catalytic domain. N223-53 possessed binding affinities for NAD and SBTI similar to those of N222-49 (less than fivefold differences in Kms) but showed a lower velocity rate for the ADP-ribosylation of SBTI. This indicated that the primary defect for ADP-ribosylation by Exo53 resided within its catalytic capacity. Analysis of hybrid proteins, composed of reciprocal halves of N223-53 and N222-49, localized the catalytic defect to residues between positions 235 and 349 of N223-53. E385 was also identified as a potential active site residue of Exo53.

Effects of differential expression of the 49-kilodalton exoenzyme S by Pseudomonas aeruginosa on cultured eukaryotic cells

Production of the ADP-ribosylating enzyme exoenzyme S (ExoS) by Pseudomonas aeruginosa has been associated with increased virulence. Previous studies, however, have been unable to confirm an effect of soluble ExoS in cell culture or animal model systems. To determine if bacteria must come in contact with target cells in order for an effect of ExoS to be observed, coculture systems were developed to compare the effects of ExoS- and non-ExoS-producing bacteria on eukaryotic cell function. The two P. aeruginosa strains used in these studies, 388 and 388delta exoS, maintained genetic identity, with the exception that strain 388delta exoS lacked production of the 49-kDa form of ExoS. When bacteria were cocultured with Detroit 532 fibroblastic cells, ExoS-producing 388 bacteria caused a significant decrease in DNA synthesis and viability compared to the decrease caused by non-ExoS-producing 388delta exoS bacteria. Maximal differences between the two strains were observed when 10(4) to 10(7) CFU of bacteria/ml were cocultured with Detroit cells for 4 or 6 h. Both strains were effective in eliminating Detroit cell DNA synthesis after a 20-h coculture period. Secreted ExoS had no effect on Detroit cell growth and viability, indicating that bacteria must have contact with target cells for the effect of ExoS on cellular function to be observed. Similar effects on cell proliferation and viability were observed when the two strains were cocultured with the KB epithelioid cell line. ExoS-associated decreases in eukaryotic cell viability were not found to be mediated by an inhibition of protein synthesis. These studies confirm that the 49-kDa ExoS contributes to the cellular pathogenesis of P. aeruginosa by interfering with eukaryotic cell growth and viability. In addition, the coculture system developed which recognizes this effect should provide a means for defining the function of ExoS in vivo.