Expression of recombinant exoenzyme S of Pseudomonas aeruginosa

Achromobacter xylosoxidans Cellular Pathology Is Correlated with Activation of a Type III Secretion System

Achromobacter xylosoxidans is increasingly recognized as a colonizer of cystic fibrosis (CF) patients, but the role that A. xylosoxidans plays in pathology remains unknown. This knowledge gap is largely due to the lack of model systems available to study the toxic potential of this bacterium. Recently, a phospholipase A₂ (PLA₂) encoded by a majority of A. xylosoxidans genomes, termed AxoU, was identified. Here, we show that AxoU is a type III secretion system (T3SS) substrate that induces cytotoxicity to mammalian cells. A tissue culture model was developed showing that a subset of A. xylosoxidans isolates from CF patients induce cytotoxicity in macrophages, suggestive of a pathogenic or inflammatory role in the CF lung. In a toxic strain, cytotoxicity is correlated with transcriptional activation of axoU and T3SS genes, demonstrating that this model can be used as a tool to identify and track expression of virulence determinants produced by this poorly understood bacterium.

Examining the role of actin-plasma membrane association in Pseudomonas aeruginosa infection and type III secretion translocation in migratory T24 epithelial cells

The opportunistic pathogen Pseudomonas aeruginosa targets wounded epithelial barriers, but the cellular alteration that increases susceptibility to P. aeruginosa infection remains unclear. This study examined how cell migration contributes to the establishment of P. aeruginosa infections using (i) highly migratory T24 epithelial cells as a cell culture model, (ii) mutations in the type III secretion (T3S) effector ExoS to manipulate P. aeruginosa infection, and (iii) high-resolution immunofluorescent microscopy to monitor ExoS translocation. ExoS includes both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) activities, and P. aeruginosa cells expressing wild-type ExoS preferentially bound to the leading edge of T24 cells, where ExoS altered leading-edge architecture and actin anchoring in conjunction with interrupting T3S translocation. Inactivation of ExoS GAP activity allowed P. aeruginosa to be internalized and secrete ExoS within T24 cells, but as with wild-type ExoS, translocation was limited in association with disruption of actin anchoring. Inactivation of ExoS ADPRT activity resulted in significantly enhanced T3S translocation by P. aeruginosa cells that remained extracellular and in conjunction with maintenance of actin-plasma membrane association. Infection with P. aeruginosa expressing ExoS lacking both GAP and ADPRT activities resulted in the highest level of T3S translocation, and this occurred in conjunction with the entry and alignment of P. aeruginosa and ExoS along actin filaments. Collectively, in using ExoS mutants to modulate and visualize T3S translocation, we were able to (i) confirm effector secretion by internalized P. aeruginosa, (ii) differentiate the mechanisms underlying the effects of ExoS GAP and ADPRT activities on P. aeruginosa internalization and T3S translocation, (iii) confirm that ExoS ADPRT activity targeted a cellular substrate that interrupted T3S translocation, (iv) visualize the ability of P. aeruginosa and ExoS to align with actin filaments, and (v) demonstrate an association between actin anchoring at the leading edge of T24 cells and the establishment of P. aeruginosa infection. Our studies also highlight the contribution of ExoS to the opportunistic nature of P. aeruginosa infection through its ability to exert cytotoxic effects that interrupt T3S translocation and P. aeruginosa internalization, which in turn limit the P. aeruginosa infectious process.

Strain-dependent induction of neutrophil histamine production and cell death by Pseudomonas aeruginosa

Airway diseases often feature persistent neutrophilic inflammation and infection. In cystic fibrosis bronchitis, for example, Pseudomonas aeruginosa is isolated frequently. Previously, this laboratory revealed that neutrophils become major sources of histamine in mice with tracheobronchitis caused by the wall-less bacterium Mycoplasma pulmonis. To test the hypothesis that more-broadly pathogenic P. aeruginosa (which expresses cell wall-associated LPS and novel toxins) has similar effects, we incubated naïve mouse neutrophils with two strains of P. aeruginosa. Strain PAO1 greatly increased neutrophil histamine content and secretion, whereas strain PA103 depressed histamine production by killing neutrophils. The histamine-stimulating capacity of PAO1, but not PA103-mediated toxicity, persisted in heat-killed organisms. In PAO1-infected mice, lung and neutrophil histamine content increased. However, PAO1 did not alter production by mast cells (classical histamine reservoirs), which also resisted PA103 toxicity. To explore mechanisms of neutrophil-selective induction, we measured changes in mRNA encoding histidine decarboxylase (rate-limiting for histamine synthesis), probed involvement of endotoxin-TLR pathways in Myd88-deficient neutrophils, and examined contributions of pyocyanin and exotoxins. Results revealed that PAO1 increased histamine production by up-regulating histidine decarboxylase mRNA via pathways largely independent of TLR, pyocyanin, and type III secretion system exotoxins. PAO1 also increased histidine decarboxylase mRNA in neutrophils purified from infected lung. Stimulation required direct contact with neutrophils and was blocked by phagocytosis inhibitor cytochalasin D. In summary, Pseudomonas-augmented histamine production by neutrophils is strain-dependent in vitro and likely mediated by up-regulation of histidine decarboxylase. These findings raise the possibility that Pseudomonas-stimulated neutrophils can enhance airway inflammation by producing histamine.

Plasma membrane localization affects the RhoGAP specificity ofPseudomonasExoS

Pseudomonas aeruginosa ExoS (453 amino acids) is a bifunctional type III cytotoxin, comprising a Rho GTPase-activating protein domain (RhoGAP), and a 14-3-3 dependent ADP-ribosyltransferase domain. In addition, ExoS contains a membrane localization domain (termed MLD, residues 51-77) which localizes and traffics ExoS within intoxicated host cells. While membrane localization has been shown to be essential for ExoS to ADP-ribosylate Ras, the relationship between intracellular localization and expression of RhoGAP activity has not been addressed. In this study, loss of MLD function was observed to abolish expression of ExoS RhoGAP activity in HeLa cells. One mutation within the MLD (R56, R63, D70 mutated to N, RRD-->N) diminished plasma membrane localization and altered the cell rounding phenotype elicited by ExoS RhoGAP. In addition, cell rounding caused by ExoS-MLD(RRD-->N) was reversed by dominant active Rac1, but not dominant active Cdc42, indicating a switch in ExoS RhoGAP substrate specificity. Mutation of the C-terminal polybasic region abolished the ability of dominant active Rac1 to protect HeLa cells from expression of the RhoGAP activity of ExoS-MLD(RRD-->N). This study shows the importance of membrane localization in the targeting of Rho GTPases by ExoS RhoGAP.

Intracellular localization of type III-delivered Pseudomonas ExoS with endosome vesicles

ExoS (453 amino acids) is a bi-functional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96-219 include the Rho GTPase-activating protein (RhoGAP) domain, and residues 234-453 include the 14-3-3-dependent ADP-ribosyltransferase domain. Earlier studies also identified an N-terminal domain (termed the membrane localization domain) that comprises residues 51-77 and includes a novel leucine-rich motif that targets ExoS to the perinuclear region of cultured cells. There is limited information on how ExoS or other type III cytotoxins enter and target intracellular host proteins. Type III-delivered ExoS localized to both plasma membrane and perinuclear region, whereas ExoS(DeltaMLD) was localized to the cytosol. Plasma membrane localization of ExoS was transient and had a half-life of approximately 20 min. Type III-delivered ExoS co-immunoprecipitated 14-3-3 proteins and Rab9, Rab6, and Rab5. Immunofluorescence experiments showed that ExoS colocalized with Rab9, Rab6, and Rab5. Fluorescent energy transfer was detected between ExoS and 14-3-3 proteins but not between ExoS and Rabs proteins. Together, these results indicate that type III-delivered ExoS localizes on the host endosomes and utilizes multiple pathways to traffic from the plasma membrane to the perinuclear region of intoxicated host cells.

ADP-ribosylation of cyclophilin A by Pseudomonas aeruginosa exoenzyme S

The virulence of the opportunistic pathogen Pseudomonas aeruginosa (Pa) is in part mediated by the type III secretion (TTS) of bacterial proteins into eukaryotic hosts. Exoenzyme S (ExoS) is a bifunctional Pa TTS effector protein, with GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) activities. Known cellular substrates of TTS-translocated ExoS (TTS-ExoS) ADPRT activity include proteins in the Ras superfamily and ERM family proteins. This study describes the ADP-ribosylation of a non-G-protein substrate of TTS-ExoS, cyclophilin A (CpA), a peptidyl-prolyl isomerase (PPIase). Four novel 17 kDa proteins (pI 6.5-6.8) were recognized in a proteomic screen of lysates of human epithelial cells that had been exposed to ExoS-producing Pa, but not an isogenic non-ExoS producing strain. The proteins were identified as isoforms of CpA using MALDI-TOF mass spectrometry and confirmed by Western blotting. Mutagenesis analysis identified arginine 55 and 69 of CpA as sites of ExoS ADP-ribosylation. Examination of the effect of ExoS ADP-ribosylation on CpA function found a moderate (19%) decrease in prolyl isomerization of a Xaa-Pro containing peptides. In comparison, GST-CpA co-immunoprecipitation studies found ExoS ADP-ribosylation of CpA to efficiently inhibit CpA binding to calcineurin/PP2B phosphatase. Our results support that ExoS ADP-ribosylates and affects the function of the cytosolic protein, CpA, with the predominant functional effect relating to interference of CpA-cellular protein interactions.

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.

Characterization of an ExoS Type III translocation-resistant cell line

Pseudomonas aeruginosa ExoS is a type III-secreted type III-secreted, bifunctional protein that causes diverse effects on eukaryotic cell function. The coculture of P. aeruginosa strains expressing ExoS with HL-60 myeloid cells revealed the cell line to be resistant to the toxic effects of ExoS. Differentiation of HL-60 cells with phorbol 12-myristate 13-acetate (TPA) rendered the cell line sensitive to ExoS. To understand the cellular basis for the alteration in sensitivity, undifferentiated and TPA-differentiated HL-60 cells were compared for differences in bacterial adherence, type III secretion induction, and ExoS translocation. These comparisons found that ExoS was translocated more efficiently in TPA-differentiated HL-60 cells than in undifferentiated cells. The studies support the ability of eukaryotic cells to influence P. aeruginosa TTS at the level of membrane translocation.

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.

Characterization of Pseudomonas aeruginosa exoenzyme S as a bifunctional enzyme in J774A.1 macrophages

Pseudomonas aeruginosa exoenzyme S (ExoS) is a type III secretion (TTS) effector, which includes both a GTPase-activating protein (GAP) activity toward the Rho family of low-molecular-weight G (LMWG) proteins and an ADP-ribosyltransferase (ADPRT) activity that targets LMWG proteins in the Ras, Rab, and Rho families. The coordinate function of both activities of ExoS in J774A.1 macrophages was assessed by using P. aeruginosa strains expressing and translocating wild-type ExoS or ExoS defective in GAP and/or ADPRT activity. Distinct and coordinated functions were identified for both domains. The GAP activity was required for the antiphagocytic effect of ExoS and was linked to interference of lamellopodium and membrane ruffle formation. Alternatively, the ADPRT activity of ExoS altered cellular adherence and morphology and was linked to effects on filopodium formation. The cellular mechanism of ExoS GAP activity included an inactivation of Rac1 function, as determined in p21-activated kinase 1-glutathione S-transferase (GST) pull-down assays. The ADPRT activity of ExoS targeted Ras and RalA but not Rab or Rho proteins, and Ral binding protein 1-GST pull-down assays identified an effect of ExoS ADPRT activity on RalA activation. The results from these studies confirm the bifunctional nature of ExoS activity within macrophages when translocated by TTS.

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.

Intracellular localization modulates targeting of ExoS, a type III cytotoxin, to eukaryotic signalling proteins

ExoS is a bifunctional type III cytotoxin produced by Pseudomonas aeruginosa. Residues 96-232 comprise the Rho GTPase activating protein (Rho GAP) domain, whereas residues 233-453 comprise the 14-3-3-dependent ADP-ribosyltransferase domain. Earlier studies showed that the N-terminus targeted ExoS to intracellular membranes within eukaryotic cells. This N-terminal targeting region is now characterized for cellular and biological contributions to intoxications by ExoS. An ExoS(1-107)-green fluorescent protein (GFP) fusion protein co-localized with alpha-mannosidase, which indicated that the fusion protein localized near the Golgi. Residues 51-72 of ExoS (termed the membrane localization domain, MLD) were necessary and sufficient for membrane localization within eukaryotic cells. Deletion of the MLD did not inhibit type III secretion of ExoS from P. aeruginosa or type III delivery of ExoS into eukaryotic cells. Type III-delivered ExoS(DeltaMLD) localized within the cytosol of eukaryotic cells, whereas type III-delivered ExoS was membrane associated. Although type III-delivered ExoS(DeltaMLD) stimulated the reorganization of the actin cytoskeleton (a Rho GAP activity), it did not ADP-ribosylate Ras. Type III-delivered ExoS(DeltaMLD) and ExoS showed similar capacities for eliciting a cytotoxic response in CHO cells, which uncoupled the ADP-ribosylation of Ras from the cytotoxicity elicited by ExoS.

Exoenzyme S shows selective ADP-ribosylation and GTPase-activating protein (GAP) activities towards small GTPases in vivo

Intracellular targeting of the Pseudomonas aeruginosa toxins exoenzyme S (ExoS) and exoenzyme T (ExoT) initially results in disruption of the actin microfilament structure of eukaryotic cells. ExoS and ExoT are bifunctional cytotoxins, with N-terminal GTPase-activating protein (GAP) and C-terminal ADP-ribosyltransferase activities. We show that ExoS can modify multiple GTPases of the Ras superfamily in vivo. In contrast, ExoT shows no ADP-ribosylation activity towards any of the GTPases tested in vivo. We further examined ExoS targets in vivo and observed that ExoS modulates the activity of several of these small GTP-binding proteins, such as Ras, Rap1, Rap2, Ral, Rac1, RhoA and Cdc42. We suggest that ExoS is the major ADP-ribosyltransferase protein modulating small GTPase function encoded by P. aeruginosa. Furthermore, we show that the GAP activity of ExoS abrogates the activation of RhoA, Cdc42 and Rap1.

ADP-ribosylation and functional effects of Pseudomonas exoenzyme S on cellular RalA

Exoenzyme S (ExoS) is a bifunctional virulence factor directly translocated into eukaryotic cells by the type III secretory process of Pseudomonas aeruginosa. Bacterial translocation of ExoS into epithelial cells is associated with diverse effects on cell function, including inhibition of growth, alterations in cell morphology, and effects on adherence processes. Preferred substrates of the ADP-ribosyltransferase (ADPRT) portion of ExoS include low molecular weight G-proteins (LMWG-proteins) in the Ras family. In examining the ADP-ribosylation and functional effects of ExoS on RalA, ExoS was found to ADP-ribosylate endogenous RalA and recombinant RalADeltaCAAX at multiple sites, with Arg52 identified as the preferred site of ADP-ribosylation. The binding of RalA to the Ral binding domain (RBD) of its downstream effector, RalBP1, was inhibited by bacterially translocated ExoS, indicating an effect of ExoS on cellular RalA function. In vitro analyses confirmed that ADP-ribosylation of RalA directly interfered with its ability to bind to the RBD of RalBP1. The studies support the fact that RalA is a cellular substrate of bacterially translocated ExoS and that ADP-ribosylation by ExoS affects RalA interaction with its downstream effector, RalBP1.

Eukaryotic cell determination of ExoS ADP-ribosyltransferase substrate specificity

Exoenzyme S (ExoS) ADP-ribosylates multiple low-molecular-mass G- (LMMG-) proteins in vitro. Identification of the in vivo substrate specificity of ExoS has been hindered by its bacterial contact delivery into eukaryotic cells and difficulties in identifying ADP-ribosylated proteins within cells. Two-dimensional electrophoresis comparisons of substrate modifications by ExoS in vitro to that following bacterial translocation into HT-29 epithelial cells identified Ras, Ral, and Rab proteins and Rac1 as in vivo substrates of ExoS ADPRT activity. Cellular fractionation studies identified a relationship between membrane association and efficiency of substrate modification. Moreover, Rac and Cdc42 relocalized to the membrane in response to ExoS. Comparisons of substrate modification to time of exposure to ExoS identified a progression of substrate modification, with Ras, RalA, and Rab5 modified first, followed by Rab8 and 11, then Rab7 and Rac1. The data support that intrinsic properties of LMMG-proteins and their subcellular localization are determinants of bacterially translocated ExoS substrate selectivity.

Independent and coordinate effects of ADP-ribosyltransferase and GTPase-activating activities of exoenzyme S on HT-29 epithelial cell function

Type III-mediated translocation of exoenzyme S (ExoS) into HT-29 epithelial cells by Pseudomonas aeruginosa causes complex alterations in cell function, including inhibition of DNA synthesis, altered cytoskeletal structure, loss of readherence, microvillus effacement, and interruption of signal transduction. ExoS is a bifunctional protein having both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) functional domains. Comparisons of alterations in HT-29 cell function caused by P. aeruginosa strains that translocate ExoS having GAP or ADPRT mutations allowed the independent and coordinate functions of the two activities to be assessed. An E381A ADPRT mutation revealed that ExoS ADPRT activity was required for effects of ExoS on DNA synthesis and long-term cell rounding. Conversely, the R146A GAP mutation appeared to have little impact on the cellular effects of ExoS. While transient cell rounding was detected following exposure to the E381A mutant, this rounding was eliminated by an E379A-E381A ADPRT double mutation, implying that residual ADPRT activity, rather than GAP activity, was effecting transient cell rounding by the E381A mutant. To explore this possibility, E381A and R146A-E381A mutants were examined for their ability to ADP-ribosylate Ras in vitro or in vivo. While no ADP-ribosylation of Ras was detected by either mutant in vitro, both mutants were able to modify Ras when translocated by the bacteria, with the R146A-E381A mutant causing more efficient modification than the E381A mutant, in association with increased inhibition of DNA synthesis. Comparisons of Ras ADP-ribosylation by wild-type and E381A mutant ExoS by two-dimensional electrophoresis found the former to ADP-ribosylate Ras at two sites, while the latter modified Ras only once. These studies draw attention to the key role of ExoS ADPRT activity in causing the effects of bacterially translocated ExoS on DNA synthesis and cell rounding. In addition, the studies provide insight into the enhancement of ExoS ADPRT activity within the eukaryotic cell microenvironment and into possible modulatory roles that the GAP and ADPRT domains might have on the function of each other.

ADP-ribosylation of Rab5 by ExoS of Pseudomonas aeruginosa affects endocytosis

Pseudomonas aeruginosa exoenzyme S (ExoS) is an ADP-ribosyltransferase that modifies low-molecular-weight GTPases. Here we studied the effect of Rab5 ADP-ribosylation by ExoS on its cellular function, i.e., regulation of early endocytic events. Coculture of CHO cells with P. aeruginosa induced a marked decrease in horseradish peroxidase (HRP) uptake compared to noninfected cells, while coculture with a P. aeruginosa mutant strain that fails to produce ExoS did not lead to any change in HRP uptake. Microinjection of recombinant ExoS into Xenopus oocytes induced strong inhibition of basal HRP uptake by oocytes. Moreover, coinjection of recombinant ExoS with Rab5 abolished the typical stimulation of HRP uptake obtained after GTPase microinjection. Cytosols prepared from injected oocytes were used in an endosome-endosome fusion assay. Cytosol from ExoS-microinjected oocytes was ineffective in promoting endosome-endosome fusion. However, in these conditions, the addition of Rab5 to the assay led to fusion recovery. Finally, we found that the interaction of Rab5 with EEA1 was markedly diminished after Rab5 ADP-ribosylation by ExoS.

Growth phase-dependent invasion of Pseudomonas aeruginosa and its survival within HeLa cells

Clinical isolates of Pseudomonas aeruginosa are classified into invasive and noninvasive (cytolytic) strains. In a noninvasive PA103 background, ExoS and ExoT have recently been shown to function as anti-internalization factors. However, these two factors seemed not to have such a function in an invasive strain PAK background. In this study, using HeLa tissue culture cells, we observed that the internalization of invasive strain PAK is dependent on its growth phases, with the stationary-phase cells internalized about 100-fold more efficiently than the exponential-phase cells. This growth phase-dependent internalization was not observed in the noninvasive PA103 strain. Further analysis of various mutant derivatives of the invasive PAK and the noninvasive PA103 strains demonstrated that ExoS or ExoT that is injected into host cells by a type III secretion machinery functions as an anti-internalization factor in both types of strains. In correlation with the growth phase-dependent internalization, the invasive strain PAK translocates much higher amount of ExoS and ExoT into HeLa cells when it is in an exponential-growth phase than when it is in a stationary-growth phase, whereas the translocation of ExoT by the noninvasive strain PA103 is consistently high regardless of the growth phases, suggesting a difference in the regulatory mechanism of type III secretion between the two types of strains. Consistent with the invasive phenotype of the parent strain, an internalized PAK derivative survived well within the HeLa cells, whereas the viability of internalized PA103 derivative was dramatically decreased and completely cleared within 48 h. These results indicate that the invasive strains of P. aeruginosa have evolved the mechanism of intracellular survival, whereas the noninvasive P. aeruginosa strains have lost or not acquired the ability to survive within the epithelial cells.

Multiple domains are required for the toxic activity of Pseudomonas aeruginosa ExoU

Expression of ExoU by Pseudomonas aeruginosa is correlated with acute cytotoxicity in a number of epithelial and macrophage cell lines. In vivo, ExoU is responsible for epithelial injury. The absence of a known motif or significant homology with other proteins suggests that ExoU may possess a new mechanism of toxicity. To study the intracellular effects of ExoU, we developed a transient-transfection system in Chinese hamster ovary cells. Transfection with full-length but not truncated forms of ExoU inhibited reporter gene expression. Inhibition of reporter activity after cotransfection with ExoU-encoding constructs was correlated with cellular permeability and death. The toxicity of truncated versions of ExoU could be restored by coexpression of the remainder of the molecule from separate plasmids in trans. This strategy was used to map N- and C-terminal regions of ExoU that are necessary but not sufficient for toxicity. Disruption of a middle region of the protein reduces toxicity. This portion of the molecule is postulated to allow the N- and C-terminal regions to functionally complement one another. In contrast to ExoS and ExoT, native and recombinant ExoU molecules do not oligomerize or form aggregates. The complex domain structure of ExoU suggests that, like other P. aeruginosa-encoded type III effectors (ExoS and ExoT), ExoU toxicity may result from a molecule that possesses more than one activity.

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.

Sera from adult patients with cystic fibrosis contain antibodies to Pseudomonas aeruginosa type III apparatus

Expression of type III proteins of Pseudomonas aeruginosa in patients with cystic fibrosis (CF) was investigated by measuring the immune response against components of the type III pathway. Twenty-three of the 33 sera contained antibodies against PcrV, a protein involved in translocation of type III cytotoxins into eukaryotic cells, and 11 of 33 had antibodies against ExoS, while most CF sera contained antibodies against PopB and PopD, components of the type III apparatus. These data indicate that P. aeruginosa commonly expresses components of the type III translocation apparatus in adult CF patients.

Pseudomonas aeruginosa mediated apoptosis requires the ADP-ribosylating activity of exoS

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primarily infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were discovered to induce apoptosis at high frequency in HeLa and other epithelial and fibroblast cell lines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morphology, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, and (iv) activation of caspase-3 activity. Induction of the type III secretion machinery, but not invasion of P. aeruginosa is required for induction of apoptosis. The apoptosis phenotype is independent of the cytoskeletal rearrangements that occur in the host cell early after infection. Mutants in P. aeruginosa exoS fail to induce apoptosis and complementation with wild-type exoS restored the apoptosis-inducing capacity, demonstrating that ExoS is the effector molecule. Analysis of exoS activity mutants shows that the ADP-ribosylating capacity of ExoS is essential for inducing the apoptotic pathway.

Intracellular localization and processing of Pseudomonas aeruginosa ExoS in eukaryotic cells

ExoS is a type III cytotoxin of Pseudomonas aeruginosa, which modulates two eukaryotic signalling pathways. The N-terminus (residues 1-234) is a GTPase activating protein (GAP) for RhoGTPases, while the C-terminus (residues 232-453) encodes an ADP-ribosyltransferase. Utilizing a series of N-terminal deletion peptides of ExoS and an epitope-tagged full-length ExoS, two independent domains have been identified within the N-terminus of ExoS that are involved in intracellular localization and expression of GAP activity. N-terminal peptides of ExoS localized to the perinuclear region of CHO cells, and a membrane localization domain was localized between residues 36 and 78 of ExoS. The capacity to elicit CHO cell rounding and express GAP activity resided within residues 90-234 of ExoS, which showed that membrane localization was not required to elicit actin reorganization. ExoS was present in CHO cells as a full-length form, which fractionated with membranes, and as an N-terminally processed fragment, which localized to the cytosol. Thus, ExoS localizes in eukaryotic cells to the perinuclear region and is processed to a soluble fragment, which possesses both the GAP and ADP-ribosyltransferase activities.

Pseudomonas aeruginosa exoenzyme S stimulates murine lymphocyte proliferation in vitro

The exuberant immunoinflammatory response that is associated with Pseudomonas aeruginosa infection is the major source of the morbidity and mortality in cystic fibrosis (CF) patients. Previous studies have established that an exoproduct of P. aeruginosa (exoenzyme S) is a mitogen for human T lymphocytes and activates a larger percentage of T cells than most superantigens, which may contribute to the immunoinflammatory response. An animal model would facilitate studies of the pathophysiologic consequences of this activation. As a first step toward developing an animal model, the murine lymphocyte response to exoenzyme S was examined. When stimulated with exoenzyme S, splenocytes isolated from naive mice entered S phase and proliferated. The optimum response occurred after 2 to 3 days in culture, at 4 x 10(5) cells per well and 5.0 micrograms of exoenzyme S per ml. The response was not due to lipopolysaccharide, since Rhodobacter sphaeroides lipid A antagonist did not block the response. Other preparations of exoenzyme S stimulated lymphocyte proliferation, since the response to recombinant exoenzyme S (rHisExo S) cloned from strain 388 was similar to the response to exoenzyme S from strain DG1. There was evidence that genetic variability influenced the response, since A/J, CBA/J, and C57BL/6 mice were high responders and BALB/cJ mice were low responders following stimulation with exoenzyme S. Both splenic T and B lymphocytes entered the cell cycle in response to exoenzyme S. Thus, murine lymphocytes, like human lymphocytes, respond to P. aeruginosa exoenzyme S, which supports the development of a murine model that may facilitate our understanding of the role that exoenzyme S plays in the pathogenesis of P. aeruginosa infections in CF patients.

RecombinantPseudomonasexoenzyme S and exoenzyme S fromPseudomonas aeruginosaDG1 share the ability to stimulate T lymphocyte proliferation

Exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 have distinct characteristics, which has led to a controversy about their homology and their pathophysiologic consequences. We have been investigating the ability of exoenzyme S to activate T lymphocytes, and therefore performed studies to determine whether exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 and expressed in Pseudomonas aeruginosa PA103 or in E. coli BL21(DE3), could induce T lymphocyte activation and proliferation. Both preparations were able to activate T cells and induce lymphocyte proliferation at similar levels as measured by flow cytometry of surface-activation markers and DNA synthesis, respectively. Further, a monoclonal antibody raised against exoenzyme S from strain DG1 partially neutralized T cell activation induced by recombinant exoenzyme S and bound to it in an immunoblot suggesting that the epitope responsible for T cell activation is shared by exoenzyme S from strain DG1 and recombinant exoenzyme S. These data suggest that the two different preparations of exoenzyme S, despite biochemical differences, share the characteristic that is responsible for T lymphocyte activation.Key words: exoenzyme S, Pseudomonas aeruginosa, T lymphocyte, cystic fibrosis.

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.

Differential sensitivity of human epithelial cells to Pseudomonas aeruginosa exoenzyme S

Exoenzyme S (ExoS) is an ADP-ribosyltransferase produced and directly translocated into eukaryotic cells by the opportunistic pathogen Pseudomonas aeruginosa. Model systems that allow bacterial translocation of ExoS have found ExoS to have multiple effects on eukaryotic cell function, affecting DNA synthesis, actin cytoskeletal structure, and cell matrix adherence. To understand mechanisms underlying differences observed in cell sensitivities to ExoS, we examined the effects of bacterially translocated ExoS on multiple human epithelial cell lines. Of the cell lines examined, confluent normal kidney (NK) epithelial cells were most resistant to ExoS, while tumor-derived cell lines were highly sensitive to ExoS. Analysis of the mechanisms of resistance indicated that cell association as well as an intrinsic resistance to morphological alterations were associated with increased resistance to ExoS. Conversely, increased sensitivity to ExoS appeared to be linked to epithelial cell growth, with tumor cells capable of undergoing non-contact-inhibited, anchorage-independent growth all being sensitive to ExoS, and NK cells becoming sensitive to ExoS when subconfluent and growing. Consistent with the possibility that growth-related, actin-based structures are involved in sensitivity to ExoS, scanning electron microscopy revealed cellular extensions from sensitive, growing cells to bacteria, which were not readily evident in resistant cells. In all studies, the severity of effects of ExoS on cell function directly correlated with the degree of Ras modification, indicating that sensitivity to ExoS in some manner related to the efficiency of ExoS translocation and its ADP-ribosylation of Ras. Our results suggest that factors expressed by growing epithelial cells are required for the bacterial contact-dependent translocation of ExoS; as normal epithelial cells differentiate into polarized confluent monolayers, expression of these factors is altered, and cells in turn become more resistant to the effects of ExoS.

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.

Interaction of 14-3-3 with a nonphosphorylated protein ligand, exoenzyme S of Pseudomonas aeruginosa

The 14-3-3 proteins are a family of conserved, dimeric proteins that interact with a diverse set of ligands, including molecules involved in cell cycle regulation and apoptosis. It is well-established that 14-3-3 binds to many ligands through phosphoserine motifs. Here we characterize the interaction of 14-3-3 with a nonphosphorylated protein ligand, the ADP-ribosyltransferase Exoenzyme S (ExoS) from Pseudomonas aeruginosa. By using affinity chromatography and surface plasmon resonance, we show that the zeta isoform of 14-3-3 (14-3-3zeta) can directly bind a catalytically active fragment of ExoS in vitro. The interaction between ExoS and 14-3-3zeta is of high affinity, with an equilibrium dissociation constant of 7 nM. ExoS lacks any known 14-3-3 binding motif, but to address the possibility that 14-3-3 binds a noncanonical phosphoserine site, we assayed ExoS for protein-bound phosphate by using mass spectrometry. No detectable phosphoproteins were found. A phosphopeptide ligand of 14-3-3, pS-Raf-259, was capable of inhibiting the binding of 14-3-3 to ExoS, suggesting that phosphorylated and nonphosphorylated ligands may share a common binding site, the conserved amphipathic groove. It is conceivable that 14-3-3 proteins may bind both phosphoserine and nonphosphoserine ligands in cells, possibly allowing kinase-dependent as well as kinase-independent regulation of 14-3-3 binding.

Biological effects of Pseudomonas aeruginosa type III-secreted proteins on CHO cells

A strain of Pseudomonas aeruginosa that fails to express known type III-secreted effector proteins was constructed as an expression host. Individual effectors were expressed in trans, and their biological effects on CHO cells were assessed in an acute cellular infection model. Intoxication with ExoS, ExoT, or ExoY resulted in alterations in cell morphology. As shown in previous genetic studies, ExoU expression was linked to acute cytotoxicity.

Pseudomonas aeruginosa exoenzyme S is a biglutamic acid ADP-ribosyltransferase

Kinetic analysis of two mutations within Pseudomonas aeruginosa exoenzyme S (ExoS) showed that a E379D mutation inhibited expression of ADP-ribosyltransferase activity but had little effect on the expression of NAD glycohydrolase activity while a E381D mutation inhibited expression of both activities. These data identify ExoS as a biglutamic acid ADP-ribosyltransferase, where E381 is the catalytic residue and E379 contributes to the transfer of ADP-ribose to the target protein.

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.

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.

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.

Adherence to lipids and intestinal mucin by a recently recognized human pathogen, Campylobacter upsaliensis

Campylobacter upsaliensis is a recently recognized human enteric pathogen associated with enteritis, colitis, bacteremia, and sepsis. Very little is known about the mechanisms of pathogenesis of this organism. The goals of this study were to determine whether C. upsaliensis binds to epithelial cells and whether there are specific lipid molecules that might serve as cell membrane receptors. In addition, we also explored C. upsaliensis binding to purified human small-intestinal mucin, since the mucus gel overlying the epithelium provides an initial contact surface for the bacteria and must be penetrated for the organisms to reach their cell receptors. Binding of C. upsaliensis to model epithelial cells was shown by microscopy adhesion assays, and binding to lipids was detected by thin-layer chromatography-overlay assays. Bacteria bound to phosphatidylethanolamine (PE), gangliotetraosylceramide (Gg4), and, more weakly, to phosphatidylserine (PS). There was no binding to ceramide, cholesterol, phosphatidylcholine, and globosides. Using receptor-based microtiter well immunoassays, we observed binding to be equal, specific, and saturable for PE and Gg 4 but low and nonspecific for PS. At least five bacterial surface proteins (50 to 90 kDa) capable of PE binding were identified by a lipid-silica affinity column technique. In slot blot overlay assays, biotin-labeled C. upsaliensis also bound in a concentration-dependent fashion to purified human small-intestinal mucin, implying that these microorganisms also express an adhesin(s) recognizing a specific mucin epitope(s). We speculate that binding to mucins may influence access of the bacteria to cell membrane receptors and thereby influence host resistance to infection.

Genetic relationship between the 53- and 49-kilodalton forms of exoenzyme S from Pseudomonas aeruginosa

Exoenzyme S is an ADP-ribosylating extracellular protein of Pseudomonas aeruginosa that is produced as two immunologically related forms, a 49-kDa enzymatically active form and a 53-kDa inactive form. The postulated relationship between the two proteins involves a carboxy-terminal proteolytic cleavage of the 53-kDa precursor to produce an enzymatically active 49-kDa protein. To determine the genetic relationship between the two forms of exoenzyme S, exoS (encoding the 49-kDa form) was used as a probe in Southern blot analyses of P. aeruginosa chromosomal digests. Cross-hybridizing bands were detected in chromosomal digests of a strain of P. aeruginosa in which exoS had been deleted by allelic exchange. A chromosomal bank was prepared from the exoS deletion strain, 388deltaexoS::TC, and screened with a probe internal to exoS. Thirteen clones that cross-hybridized with the exoS probe were identified. One representative clone contained the open reading frame exoT; this open reading frame encoded a protein of 457 amino acids which showed 75% amino acid identity to ExoS. The exoT open reading frame, cloned into a T7 expression system, produced a 53-kDa protein in Escherichia coli, termed Exo53, which reacted to antisera against exoenzyme S. A histidine-tagged derivative of recombinant Exo53 possessed approximately 0.2% of the ADP-ribosyltransferase activity of recombinant ExoS. Inactivation of exoT in an allelic-replacement strain resulted in an Exo53-deficient phenotype without modifying the expression of ExoS. These studies prove that the 53- and 49-kDa forms of exoenzyme S are encoded by separate genes. In addition, this is the first report of the factor-activating-exoenzyme-S-dependent ADP-ribosyltransferase activity of the 53-kDa form of exoenzyme S.

Identification of glutamic acid 381 as a candidate active site residue of Pseudomonas aeruginosa exoenzyme S

Exoenzyme S of Pseudomonas aeruginosa (ExoS) is a member of the family of bacterial ADP-ribosylating exotoxins (bAREs). Site-directed mutagenesis of glutamic acids within the catalytic domain of ExoS (termed delta N222) allowed the identification of the preferential inactivation of ADP-ribosyltransferase activity by alanine substitution of E381. The specific activity of E381A mutant was 0.02% of wild-type delta N222. Delta N222(E381A) retained the requirement of factor activating exoenzyme S (FAS) activation for the expression of ADP-ribosyltransferase activity. In contrast, E387A, E399A, and E414A mutants possessed ADP-ribosyltransferase activity similar to that of wild-type delta N222. Kinetic evaluation of E381A and two other mutants, E381D and E381S, showed that their primary defect was a lower kcat in the ADP-ribosylation of soybean trypsin inhibitor (SBTI). The Km for NAD and SBTI and activation by FAS varied 2- and 10-fold relative to delta N222. In addition, the E381 mutants possessed identical protease patterns during thrombin and trypsin digestion as delta N222, which indicated that E381 mutants had retained their overall conformation. Together, these data identify E381 as contributing to the catalytic activity of exoenzyme S.

Genetic analysis of exoenzyme S expression by Pseudomonas aeruginosa

The dissemination of Pseudomonas aeruginosa to the bloodstream increases the likelihood of developing fatal sepsis. In experimental models, the ability to disseminate is linked to expression of the exoenzyme S pathway. Genetic and biochemical analysis of the pathway has led to the identification of the two structural genes encoding exoenzyme S, exoS and exoT. A key regulator of several loci of the pathway has been identified as a DNA-binding protein with transcriptional activation properties. Preliminary evidence suggests that exoenzyme S and the Yop virulence determinants of yersiniae share homology among proteins involved in their synthesis and secretion. With the addition of exoS and exoT to the molecular arsenal, questions concerning in vivo toxicity and target specificities of exoenzyme S can be directly addressed.

Functional domains of Pseudomonas aeruginosa exoenzyme S

Recombinant exoenzyme S (rHisExoS) of Pseudomonas aeruginosa was expressed in Escherichia coli as a soluble, cytosolic His fusion protein. rHisExoS was purified by Ni(2+)-affinity chromatography in the presence of protease inhibitors without detectable degradation. rHisExoS possessed a specific activity (within twofold) for the factor-activating exoenzyme S-dependent ADP-ribosylation of soybean trypsin inhibitor (SBTI) similar to that of native exoenzyme S. Analysis of several deletion peptides showed that delta N222, which encoded the carboxyl-terminal 222 amino acids of exoenzyme S, possessed factor-activating exoenzyme S-dependent ADP-ribosyltransferase activity. delta N222 catalyzed the ADP-ribosylation of SBTI at a rate sixfold greater than rHisExoS. Relative to rHisExoS, delta N222 had a similar affinity for NAD, a threefold greater affinity for SBTI, and a four- to eightfold greater kcat for the ADP-ribosylation of SBTI. Like native exoenzyme S, rHisExoS chromatographed as an aggregate with an apparent molecular mass of > 300 kDa. In contrast, delta N222 did not chromatograph as an aggregate, which showed that the amino-terminal 99 amino acids of exoenzyme S were responsible for the aggregation phenotype.

Analyses of the DNA-binding and transcriptional activation properties of ExsA, the transcriptional activator of the Pseudomonas aeruginosa exoenzyme S regulon

ExsA has been implicated as a central regulator of exoenzyme S production by Pseudomonas aeruginosa. In this study, the DNA-binding and transcriptional activation properties of ExsA were investigated. ExsA was produced and purified as a fusion protein, MALA3A2, which was shown to bind specifically to promoter regions that regulated transcription of the exoenzyme S trans-regulatory locus (pC) and a locus located directly downstream of exsA (pD). Previously, MALA3A2 was shown to bind the exoS 5' PstI-NsiI region, which contained two independent but coordinately regulated (ExsA-mediated) promoters, pS' (now termed pORF1) and pS. DNase I footprint analysis of the promoter regions bound by ExsA revealed a common protected consensus sequence of TXAAAAXA. The consensus sequence was located -51 to -52 bp upstream of the transcriptional start sites for pD, pS, and pORF1. Promoter fusion, DNA-binding, and mutagenesis analysis indicated that the consensus sequence was important for transcriptional activation. Each ExsA-controlled promoter region contained at least two consensus sites in close proximity, similar to the arrangement of half-sites seen in AraC-controlled (Escherichia coli) or VirF-controlled (Yersinia enterocolitica) promoters. However, the results of this study suggested that only one consensus site was required in the exoenzyme S (pS) or ORF1 promoter (pORF1) to initiate transcription. These data suggest that members of the exoenzyme S regulon can be defined as possessing an ExsA consensus element which maps at bp -51 or -52 relative to the transcriptional start site.

VirG, a Yersinia enterocolitica lipoprotein involved in Ca2+ dependency, is related to exsB of Pseudomonas aeruginosa

Pathogenic yersiniae require Ca2+ for growth at 37 degrees C. They harbor closely related plasmids of about 70 kb that are essential for virulence. At 37 degrees C and in the absence of Ca2+ ions, these plasmids cause a decrease in growth rate and the release of large amounts of proteins called Yops. Here we describe the virG gene of Yersinia enterocolitica; virG is located just upstream of the virF gene, which encodes the transcriptional activator of some plasmid virulence factors. Analysis of the VirG amino acid sequence suggested that virG encodes a lipoprotein, which was confirmed by [3H]palmitate labeling of VirG-PhoA fusion proteins. A nonpolar virG mutant was constructed and found to be Ca2+ independent for growth at 37 degrees C but to still secrete Yops. This phenotype was complemented by the introduction of a plasmid harboring an intact virG gene. VirG was found to be homologous to ExsB, a protein encoded by a Pseudomonas aeruginosa gene located in the locus controlling exoenzyme S synthesis. Interestingly, the exsA gene, located just downstream of exsB, is also homologous to virF.

Transcriptional analysis of the Pseudomonas aeruginosa exoenzyme S structural gene

The transcriptional regulation of the Pseudomonas aeruginosa exoS gene was investigated. Expression of exoS in P. aeruginosa PA103 was dependent upon growth in a low-cation environment and the presence of a functional exsA gene. Promoter fusion analysis indicated that a 285-bp PstI-NsiI fragment, located 5' of the exoS coding region, contained a functional promoter for exoS. Expression of the reporter gene was inducible in a low-cation growth environment and required a functional copy of exsA. Divergent promoters, coordinately regulated with exoS transcription, were identified within the PstI-NsiI fragment. A fusion derivative of ExsA, MALA3A2, was shown to bind directly to the PstI-NsiI probe. DNase I protection analysis demonstrated that MALA3A2 bound to the intergenic region between the postulated -35 boxes of each promoter region. Northern (RNA) blot analysis with probes internal to and upstream of exoS demonstrated that separate, coordinately regulated mRNAs were expressed in P. aeruginosa. These data suggested that a locus, coregulated with exoS transcription, was located upstream of exoS. DNA sequence analysis of the exoS upstream region revealed three open reading frames, ORF 1, ORF 2, and ORF 3. ORF 1 demonstrated significant homology to the SycE/YerA protein of Yersinia sp. SycE/YerA is postulated to function as a chaperone for the YopE cytotoxin. The loci encoding YopE and ExoS show similarities in genetic organization, protein composition, and regulation.