The type III secretion injectisome

Induction and relaxation dynamics of the regulatory network controlling the type III secretion system encoded within Salmonella pathogenicity island 1

Bacterial pathogenesis requires the precise spatial and temporal control of gene expression, the dynamics of which are controlled by regulatory networks. A network encoded within Salmonella Pathogenicity Island 1 controls the expression of a type III protein secretion system involved in the invasion of host cells. The dynamics of this network are measured in single cells using promoter-green fluorescent protein (gfp) reporters and flow cytometry. During induction, there is a temporal order of gene expression, with transcriptional inputs turning on first, followed by structural and effector genes. The promoters show varying stochastic properties, where graded inputs are converted into all-or-none and hybrid responses. The relaxation dynamics are measured by shifting cells from inducing to noninducing conditions and by measuring fluorescence decay. The gfp expressed from promoters controlling the transcriptional inputs (hilC and hilD) and structural genes (prgH) decay exponentially, with a characteristic time of 50-55 min. In contrast, the gfp expressed from a promoter controlling the expression of effectors (sicA) persists for 110+/-9 min. This promoter is controlled by a genetic circuit, formed by a transcription factor (InvF), a chaperone (SicA), and a secreted protein (SipC), that regulates effector expression in response to the secretion capacity of the cell. A mathematical model of this circuit demonstrates that the delay is due to a split positive feedback loop. This model is tested in a DeltasicA knockout strain, where sicA is complemented with and without the feedback loop. The delay is eliminated when the feedback loop is deleted. Furthermore, a robustness analysis of the model predicts that the delay time can be tuned by changing the affinity of SicA:InvF multimers for an operator in the sicA promoter. This prediction is used to construct a targeted library, which contains mutants with both longer and shorter delays. This combination of theory and experiments provides a platform for predicting how genetic perturbations lead to changes in the global dynamics of a regulatory network.

IpgB1 and IpgB2, two homologous effectors secreted via the Mxi-Spa type III secretion apparatus, cooperate to mediate polarized cell invasion and inflammatory potential of Shigella flexenri

Type III secretion systems (T3SS) are present in many pathogenic gram-negative bacteria and mediate the translocation of bacterial effector proteins into host cells. Here, we report the phenotypic characterization of S. flexneri ipgB1 and ipgB2 mutants, in which the genes encoding the IpgB1 and IpgB2 effectors have been inactivated, either independently or simultaneously. Like IpgB1, we found that IpgB2 is secreted by the T3SS and its secretion requires the Spa15 chaperone. Upon infection of semi-confluent HeLa cells, the ipgB2 mutant exhibited the same invasive capacity as the wild-type strain and the ipgB1 mutant was 50% less invasive. Upon infection of polarised Caco2-cells, the ipgB2 mutant did not show a significant defect in invasion and the ipgB1 mutant was slightly more invasive than the wild-type strain. Entry of the ipgB1 ipgB2 mutant in polarized cells was reduced by 70% compared to the wild-type strain. Upon infection of the cornea in Guinea pigs, the ipgB2 mutant exhibited a wild-type phenotype, the ipgB1 mutant was hypervirulent and elicited a more pronounced proinflammatory response, while the ipgB1 ipgB2 mutant was highly attenuated. The attenuated phenotype of the ipgB1 ipgB2 mutant was confirmed using a murine pulmonary model of infection and histopathology and immunochemistry studies.

The type III secretion system of Proteus mirabilis HI4320 does not contribute to virulence in the mouse model of ascending urinary tract infection

The Gram-negative enteric bacterium Proteus mirabilis is a frequent cause of urinary tract infections (UTIs) in individuals with long-term indwelling catheters or with complicated urinary tracts. The recent release of the P. mirabilis strain HI4320 genome sequence has facilitated identification of potential virulence factors in this organism. Genes appearing to encode a type III secretion system (TTSS) were found in a low GC-content pathogenicity island in the P. mirabilis chromosome. This island contains 24 intact genes that appear to encode all components necessary to assemble a TTSS needle complex, plus at least two putative secreted effector proteins and their chaperones. The genetic organization of the TTSS genes is very similar to that of the TTSS of Shigella flexneri. RT-PCR analysis indicated that these genes are expressed at low levels in vitro. However, insertional mutation of two putative TTSS genes, encoding the requisite ATPase and a possible negative regulator, resulted in no change in either the growth rate of the mutant or the secreted protein profile compared to wild-type. Furthermore, there was no difference in quantitative cultures of urine, bladder and kidney between the ATPase mutant and the wild-type strain in the mouse model of ascending UTI in either independent challenge or co-challenge experiments. The role of the P. mirabilis TTSS, if any, is yet to be determined.

The downy mildew effector proteins ATR1 and ATR13 promote disease susceptibility in Arabidopsis thaliana

The downy mildew (Hyaloperonospora parasitica) effector proteins ATR1 and ATR13 trigger RPP1-Nd/WsB- and RPP13-Nd-dependent resistance, respectively, in Arabidopsis thaliana. To better understand the functions of these effectors during compatible and incompatible interactions of H. parasitica isolates on Arabidopsis accessions, we developed a novel delivery system using Pseudomonas syringae type III secretion via fusions of ATRs to the N terminus of the P. syringae effector protein, AvrRPS4. ATR1 and ATR13 both triggered the hypersensitive response (HR) and resistance to bacterial pathogens in Arabidopsis carrying RPP1-Nd/WsB or RPP13-Nd, respectively, when delivered from P. syringae pv tomato (Pst) DC3000. In addition, multiple alleles of ATR1 and ATR13 confer enhanced virulence to Pst DC3000 on susceptible Arabidopsis accessions. We conclude that ATR1 and ATR13 positively contribute to pathogen virulence inside host cells. Two ATR13 alleles suppressed bacterial PAMP (for Pathogen-Associated Molecular Patterns)-triggered callose deposition in susceptible Arabidopsis when delivered by DC3000 DeltaCEL mutants. Furthermore, expression of another allele of ATR13 in plant cells suppressed PAMP-triggered reactive oxygen species production in addition to callose deposition. Intriguingly, although Wassilewskija (Ws-0) is highly susceptible to H. parasitica isolate Emco5, ATR13Emco5 when delivered by Pst DC3000 triggered localized immunity, including HR, on Ws-0. We suggest that an additional H. parasitica Emco5 effector might suppress ATR13-triggered immunity.

In vivo expression technology identifies a type VI secretion system locus in Burkholderia pseudomallei that is induced upon invasion of macrophages

The Gram-negative proteobacterium Burkholderia pseudomallei can survive and multiply within a variety of eukaryotic cells, including macrophages. This property is believed to be important for its ability to cause the disease melioidosis in a wide range of animal species, including humans. To identify determinants that are important for the ability of B. pseudomallei to survive within macrophages, in vivo expression technology (IVET) was employed. Several putative macrophage-inducible genes were identified that are likely to contribute to the virulence of B. pseudomallei, including three genes (tssH-5, tssI-5 and tssM-5) located within the same type VI secretion system cluster (tss-5), mntH, encoding a natural resistance-associated macrophage protein (NRAMP)-like manganese ion transporter, and a haem acquisition gene, bhuT. The macrophage-inducibility of the tss-5 gene cluster was confirmed by reporter gene analysis. Construction of tssH-5 and bhuT null mutants indicated that expression of the tss-5 unit and the bhu operon were not required for intramacrophage survival. A further five tss units were identified within the B. pseudomallei genome that, together with tss-5, account for approximately 2.3 % of the total genome size. The presence of six type VI secretion systems in this organism is likely to be an important factor in making this bacterium such a versatile pathogen.

Intracellular type III secretion by cytoplasmic Shigella flexneri promotes caspase-1-dependent macrophage cell death

The Gram-negative bacterium Shigella flexneri triggers pro-inflammatory apoptotic cell death in macrophages, which is crucial for the onset of an acute inflammatory diarrhoea termed bacillary dysentery. The Mxi-Spa type III secretion system promotes bacterial uptake and escape into the cytoplasm, where, dependent on the translocator/effector protein IpaB, caspase-1 [interleukin (IL)-1beta-converting enzyme] and its substrate IL-1beta are activated. Here, we show that in the course of a macrophage infection, IpaB is secreted intracellularly for more than 1 h post-infection and progressively accumulates in aggregates on the bacterial surface. Concomitantly, the bacterial pool of IpaB is gradually depleted. The protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) dose-dependently inhibited the Mxi-Spa-dependent secretion of IpaB triggered by the dye Congo red in vitro and abolished translocation of IpaB into the host-cell cytoplasm of S. flexneri-infected macrophages. CCCP specifically inhibited S. flexneri-triggered macrophage death in a dose-dependent manner, even if added up to 60 min post-infection. Addition of CCCP 15 min after infection blocked macrophage cell death, the activation of caspase-1 and the maturation of IL-1beta, without affecting uptake or escape of S. flexneri from the phagosome. By contrast, CCCP used at the same concentration had no effect on ATP-induced caspase-1 activation or staurosporine-induced apoptosis. Our results indicate that under the conditions used, CCCP rapidly and specifically blocks bacterial type III secretion, and thus, intracellular type III secretion promotes cytotoxicity of S. flexneri.

Mucosal antibody responses of colonized cattle to Escherichia coli O157-secreted proteins, flagellin, outer membrane proteins and lipopolysaccharide

The aim of this work was to characterize adaptive mucosal immune responses to Escherichia coli O157:H7 at the principal site of colonization in the bovine species. Following experimental infection, extracts from terminal rectum mucosal samples were tested for IgA antibodies by immunoblotting against different bacterial antigens including: whole-cell E. coli O157:H7 with and without proteinase treatment, outer membrane and cytoplasmic preparations, secreted protein supernatants and purified E. coli O157 lipopolysaccharide and H7 flagellin. Lipopolysaccharide and H7 flagellin preparations were also used to coat enzyme-linked immunosorbent assay plates to determine mucosal IgG1 and IgA antibody titers. In this work, evidence is presented of strong local IgA immune responses induced following infection at the bovine terminal rectal mucosa directed against multiple antigens including type III secretion-dependent proteins, O157 lipopolysaccharide, H7 flagellin and OmpC.

Roundtrip explorations of bacterial infection: from single cells to the entire host and back

Host-pathogen interactions are highly regulated, dynamic processes that take place at the molecular, cellular and organ level. Innovative imaging technologies have emerged recently to investigate the underlying mechanisms of host-pathogen interactions. Innovations in fluorescence microscopy enable functional studies on the single-cell level. New light microscopes have been developed that improve the resolution to less than 100 nm. At the other extreme, intravital microscopy enables the correlation of cellular events on the organ level. This is also achieved by alternatives to microscopy such as bioluminescence, positron-emission tomography and magnetic resonance imaging. The methodologies described here will have a tremendous effect on our understanding of host-pathogen interactions.

Membrane dynamics and spatial distribution of Salmonella-containing vacuoles

Salmonella enterica are facultative intracellular bacteria that cause intestinal and systemic diseases, and replicate within host cells in a membrane-bound compartment, the Salmonella-containing vacuole. Intravacuolar bacterial replication depends on spatiotemporal regulated interactions with host cell vesicular compartments. Recent studies have shown that type III secretion effector proteins control both the vacuolar membrane dynamics and intracellular positioning of bacterial vacuoles. The functions of these effectors, which are beginning to be understood, disclose a complex hijacking of host cell microtubule motors--kinesins and dynein--and regulators of their function, and suggest interactions with the Golgi complex. Here, we discuss current models describing the mode of action of Salmonella type III secretion effector proteins involved in these processes.

Protein-protein interactions within type III secretion system-dependent pili of Rhizobium sp. strain NGR234

Pili synthesized by the type III secretion system of Rhizobium species strain NGR234 are essential for protein secretion and thus for efficient symbiosis with many legumes. Isolation and partial purification of these pili showed that they are composed of at least three proteins, NopA, NopB, and NopX. Using biochemical assays, we show here that these proteins interact directly with one another.

Pseudomonas syringae lytic transglycosylases coregulated with the type III secretion system contribute to the translocation of effector proteins into plant cells

Pseudomonas syringae translocates virulence effector proteins into plant cells via a type III secretion system (T3SS) encoded by hrp (for hypersensitive response and pathogenicity) genes. Three genes coregulated with the Hrp T3SS system in P. syringae pv. tomato DC3000 have predicted lytic transglycosylase domains: PSPTO1378 (here designated hrpH), PSPTO2678 (hopP1), and PSPTO852 (hopAJ1). hrpH is located between hrpR and avrE1 in the Hrp pathogenicity island and is carried in the functional cluster of P. syringae pv. syringae 61 hrp genes cloned in cosmid pHIR11. Strong expression of DC3000 hrpH in Escherichia coli inhibits bacterial growth unless the predicted catalytic glutamate at position 148 is mutated. Translocation tests involving C-terminal fusions with a Cya (Bordetella pertussis adenylate cyclase) reporter indicate that HrpH and HopP1, but not HopAJ1, are T3SS substrates. Pseudomonas fluorescens carrying a pHIR11 derivative lacking hrpH is poorly able to translocate effector HopA1, and this deficiency can be restored by HopP1 and HopAJ1, but not by HrpH(E148A) or HrpH(1-241). DC3000 mutants lacking hrpH or hrpH, hopP1, and hopAJ1 combined are variously reduced in effector translocation, elicitation of the hypersensitive response, and virulence. However, the mutants are not reduced in secretion of T3SS substrates in culture. When produced in wild-type DC3000, the HrpH(E148A) and HrpH(1-241) variants have a dominant-negative effect on the ability of DC3000 to elicit the hypersensitive response in nonhost tobacco and to grow and cause disease in host tomato. The three Hrp-associated lytic transglycosylases in DC3000 appear to have overlapping functions in contributing to T3SS functions during infection.

AvrAC(Xcc8004), a type III effector with a leucine-rich repeat domain from Xanthomonas campestris pathovar campestris confers avirulence in vascular tissues of Arabidopsis thaliana ecotype Col-0

Xanthomonas campestris pathovar campestris causes black rot, a vascular disease on cruciferous plants, including Arabidopsis thaliana. The gene XC1553 from X. campestris pv. campestris strain 8004 encodes a protein containing leucine-rich repeats (LRRs) and appears to be restricted to strains of X. campestris pv. campestris. LRRs are found in a number of type III-secreted effectors in plant and animal pathogens. These prompted us to investigate the role of the XC1553 gene in the interaction between X. campestris pv. campestris and A. thaliana. Translocation assays using the hypersensitive-reaction-inducing domain of X. campestris pv. campestris AvrBs1 as a reporter revealed that XC1553 is a type III effector. Infiltration of Arabidopsis leaf mesophyll with bacterial suspensions showed no differences between the wild-type strain and an XC1553 gene mutant; both strains induced disease symptoms on Kashmir and Col-0 ecotypes. However, a clear difference was observed when bacteria were introduced into the vascular system by piercing the central vein of leaves. In this case, the wild-type strain 8004 caused disease on the Kashmir ecotype, but not on ecotype Col-0; the XC1553 gene mutant became virulent on the Col-0 ecotype and still induced disease on the Kashmir ecotype. Altogether, these data show that the XC1553 gene, which was renamed avrAC(Xcc8004), functions as an avirulence gene whose product seems to be recognized in vascular tissues.

Enzymatic characterization of the enteropathogenic Escherichia coli type III secretion ATPase EscN

Type III secretion is a transport mechanism by which bacteria secrete proteins across their cell envelope. This protein export pathway is used by two different bacterial nanomachines: the flagellum and the injectisome. An indispensable component of these secretion systems is an ATPase similar to the F1-ATPase beta subunit. Here we characterize EscN, an enteropathogenic Escherichia coli type III ATPase. A recombinant version of EscN, which was fully functional in complementation tests, was purified to homogeneity. Our results demonstrate that EscN is a Mg2+-dependent ATPase (kcat 0.35 s(-1)). We also define optimal conditions for the hydrolysis reaction. EscN displays protein concentration-dependent activity, suggesting that the specific activity changes with the oligomeric state of the protein. The presence of active oligomers was revealed by size exclusion chromatography and native gel electrophoresis.

Identification of harpins in Pseudomonas syringae pv. tomato DC3000, which are functionally similar to HrpK1 in promoting translocation of type III secretion system effectors

Harpins are a subset of type III secretion system (T3SS) substrates found in all phytopathogenic bacteria that utilize a T3SS. Pseudomonas syringae pv. tomato DC3000 was previously reported to produce two harpins, HrpZ1 and HrpW1. DC3000 was shown here to deploy two additional proteins, HopAK1 and HopP1, which have the harpin-like properties of lacking cysteine, eliciting the hypersensitive response (HR) when partially purified and infiltrated into tobacco leaves, and possessing a two-domain structure similar to that of the HrpW1 class of harpins. Unlike the single-domain harpin HrpZ1, the two-domain harpins have C-terminal enzyme-like domains: pectate lyase for HopAK1 and lytic transglycosylase for HopP1. Genetic techniques to recycle antibiotic markers were applied to DC3000 to generate a quadruple harpin gene polymutant. The polymutant was moderately reduced in the elicitation of the HR and translocation of the T3SS effector AvrPto1 fused to a Cya translocation reporter, but the mutant was unaffected in the secretion of AvrPto1-Cya. The DC3000 hrpK1 gene encodes a putative translocator in the HrpF/NopX family and was deleted in combination with the four harpin genes. The hrpK1 quadruple harpin gene polymutant was strongly reduced in HR elicitation, virulence, and translocation of AvrPto1-Cya into plant cells but not in the secretion of representative T3SS substrates in culture. HrpK1, HrpZ1, HrpW1, and HopAK1, but not HopP1, were independently capable of restoring some HR elicitation to the hrpK1 quadruple harpin gene polymutant, which suggests that a consortium of semiredundant translocators from three protein classes cooperate to form the P. syringae T3SS translocon.

Sec- and Tat-mediated protein secretion across the bacterial cytoplasmic membrane--distinct translocases and mechanisms

In bacteria, two major pathways exist to secrete proteins across the cytoplasmic membrane. The general Secretion route, termed Sec-pathway, catalyzes the transmembrane translocation of proteins in their unfolded conformation, whereupon they fold into their native structure at the trans-side of the membrane. The Twin-arginine translocation pathway, termed Tat-pathway, catalyses the translocation of secretory proteins in their folded state. Although the targeting signals that direct secretory proteins to these pathways show a high degree of similarity, the translocation mechanisms and translocases involved are vastly different.

Cloning and transfer of the Salmonella pathogenicity island 2 type III secretion system for studies of a range of gram-negative genera

The engineering of bacterial strains with specific phenotypes frequently requires the use of blocks or "cassettes" of genes that act together to perform a desired function. The potential benefits of utilizing type III secretion systems in this regard are becoming increasingly realized since these systems can be used to direct interactions with host cells for beneficial purposes such as vaccine development, anticancer therapies, and targeted protein delivery. However, convenient methods to clone and transfer type III secretion systems for studies of a range of different types of bacteria are lacking. In addition to functional applications, such methods would also reveal important information about the evolution of a given type III secretion system, such as its ability to be expressed and functional outside of the strain of origin. We describe here the cloning of the Salmonella enterica serovar Typhimurium pathogenicity island 2 (SPI-2) type III secretion system onto a vector that can be easily transferred to a range of gram-negative bacterial genera. We found that expression of the cloned SPI-2 system in different Gammaproteobacteria and Alphaproteobacteria (as monitored by SseB protein levels) is dependent on the bacterial strain and growth medium. We also demonstrate that the cloned system is functional for secretion, can direct interactions with macrophages, and can be used as a novel tool to analyze the predicted interaction of SseB with host cells. This work provides a foundation for future applications where the cloned SPI-2 region (or other cloned type III systems) can provide a desired function to an engineered gram-negative strain.

Adaptive evolution has targeted the C-terminal domain of the RXLR effectors of plant pathogenic oomycetes

Oomycete plant pathogens deliver effector proteins inside host cells to modulate plant defense circuitry and to enable parasitic colonization. These effectors are defined by a conserved motif, termed RXLR (for Arg, any amino acid, Leu, Arg), that is located downstream of the signal peptide and that has been implicated in host translocation. Because the phenotypes of RXLR effectors extend to plant cells, their genes are expected to be the direct target of the evolutionary forces that drive the antagonistic interplay between pathogen and host. We used the draft genome sequences of three oomycete plant pathogens, Phytophthora sojae, Phytophthora ramorum, and Hyaloperonospora parasitica, to generate genome-wide catalogs of RXLR effector genes and determine the extent to which these genes are under positive selection. These analyses revealed that the RXLR sequence is overrepresented and positionally constrained in the secretome of Phytophthora relative to other eukaryotes. The three examined plant pathogenic oomycetes carry complex and diverse sets of RXLR effector genes that have undergone relatively rapid birth and death evolution. We obtained robust evidence of positive selection in more than two-thirds of the examined paralog families of RXLR effectors. Positive selection has acted for the most part on the C-terminal region, consistent with the view that RXLR effectors are modular, with the N terminus involved in secretion and host translocation and the C-terminal domain dedicated to modulating host defenses inside plant cells.

A Pseudomonas syringae pv. tomato DC3000 mutant lacking the type III effector HopQ1-1 is able to cause disease in the model plant Nicotiana benthamiana

The model pathogen Pseudomonas syringae pv. tomato DC3000 causes bacterial speck in tomato and Arabidopsis, but Nicotiana benthamiana, an important model plant, is considered to be a non-host. Strain DC3000 injects approximately 28 effector proteins into plant cells via the type III secretion system (T3SS). These proteins were individually delivered into N. benthamiana leaf cells via T3SS-proficient Pseudomonas fluorescens, and eight, including HopQ1-1, showed some capacity to cause cell death in this test. Four gene clusters encoding 13 effectors were deleted from DC3000: cluster II (hopH1, hopC1), IV (hopD1, hopQ1-1, hopR1), IX (hopAA1-2, hopV1, hopAO1, hopG1), and native plasmid pDC3000A (hopAM1-2, hopX1, hopO1-1, hopT1-1). DC3000 mutants deleted for cluster IV or just hopQ1-1 acquired the ability to grow to high levels and produce bacterial speck lesions in N. benthamiana. HopQ1-1 showed other hallmarks of an avirulence determinant in N. benthamiana: expression in the tobacco wildfire pathogen P. syringae pv. tabaci 11528 rendered this strain avirulent in N. benthamiana, and elicitation of the hypersensitive response in N. benthamiana by HopQ1-1 was dependent on SGT1. DC3000 polymutants involving other effector gene clusters in a hopQ1-1-deficient background revealed that clusters II and IX contributed to the severity of lesion symptoms in N. benthamiana, as well as in Arabidopsis and tomato. The results support the hypothesis that the host ranges of P. syringae pathovars are limited by the complex interactions of effector repertoires with plant anti-effector surveillance systems, and they demonstrate that N. benthamiana can be a useful model host for DC3000.

YopJ targets TRAF proteins to inhibit TLR-mediated NF-kappaB, MAPK and IRF3 signal transduction

The Yersinia pestis virulence factor YopJ is a potent inhibitor of the NF-kappaB and MAPK signalling pathways, however, its molecular mechanism and relevance to pathogenesis are the subject of much debate. In this report, we characterize the effects of this type III effector protein on bone fide signalling events downstream of Toll-like receptors (TLRs), critical sensors in innate immunity. YopJ inhibited TLR-mediated NF-kappaB and MAP kinase activation, as suggested by previous studies. In addition, induction of the TLR-mediated interferon response was blocked by YopJ, indicating that YopJ also inhibits IRF3 signalling. Examination of the NF-kappaB signalling pathway in detail suggested that YopJ acts at the level of TAK1 (MAP3K7) activation. Further studies revealed a YopJ-dependent decrease in the ubiquitination of TRAF3 and TRAF6. These data support the hypothesis that YopJ is a deubiquitinating protease that acts on TRAF proteins to prevent or remove the K63-polymerized ubiquitin conjugates required for signal transduction. Our data do not directly address the alternative hypothesis that YopJ is an acetyltransferase that acts on the activation loop of IKK and MKK proteins, but support the conclusion that the critical function of YopJ is to deubiquinate TRAF proteins.

Type III secretion system translocator has a molten globule conformation both in its free and chaperone-bound forms

Type III secretion systems of Gram-negative pathogenic bacteria allow the injection of effector proteins into the cytosol of host eukaryotic cells. Crossing of the eukaryotic plasma membrane is facilitated by a translocon, an oligomeric structure made up of two bacterial proteins inserted into the host membrane during infection. In Pseudomonas aeruginosa, a major human opportunistic pathogen, these proteins are PopB and PopD. Their interactions with their common chaperone PcrH in the cytosol of the bacteria are essential for the proper function of the injection system. The interaction region between PopD and PcrH was identified using limited proteolysis, revealing that the putative PopD transmembrane fragment is buried within the PopD/PcrH complex. In addition, structural features of PopD and PcrH, either individually or within the binary complex, were characterized using spectroscopic methods and 1D NMR. Whereas PcrH possesses the characteristics of a folded protein, PopD is in a molten globule state either alone or in the PopD/PcrH complex. The molten globule state is known to enable the membrane insertion of translocation/pore-forming domains of bacterial toxins. Therefore, within the bacterial cytoplasm, PopD is preserved in a state that is favorable to secretion and insertion into cell membranes.

Identification of Pseudomonas syringae pv. syringae 61 type III secretion system Hrp proteins that can travel the type III pathway and contribute to the translocation of effector proteins into plant cells

Pseudomonas syringae translocates effector proteins into plant cells via an Hrp1 type III secretion system (T3SS). T3SS components HrpB, HrpD, HrpF, and HrpP were shown to be pathway substrates and to contribute to elicitation of the plant hypersensitive response and to translocation and secretion of the model effector AvrPto1.

YscU recognizes translocators as export substrates of the Yersinia injectisome

YscU is an essential component of the export apparatus of the Yersinia injectisome. It consists of an N-terminal transmembrane domain and a long cytoplasmic C-terminal domain, which undergoes auto-cleavage at a NPTH site. Substitutions N263A and P264A prevented cleavage of YscU and abolished export of LcrV, YopB and YopD but not of Yop effectors. As a consequence, yscU(N263A) mutant bacteria made needles without the LcrV tip complex and they could not form translocation pores. The graft of the export signal of the effector YopE, at the N-terminus of LcrV, restored LcrV export and assembly of the tip complex. Thus, YscU cleavage is required to acquire the conformation allowing recognition of translocators, which represent an individual category of substrates in the hierarchy of export. In addition, yscU(N263A) mutant bacteria exported reduced amounts of the YscP ruler and made longer needles. Increasing YscP export resulted in needles with normal size, depending on the length of the ruler. Hence, the effect of the yscU(N263A) mutation on needle length was the consequence of a reduced YscP export.

Differential requirement of P2X7 receptor and intracellular K+ for caspase-1 activation induced by intracellular and extracellular bacteria

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that plays an important role in host defense and inflammatory diseases. The maturation and secretion of IL-1beta are mediated by caspase-1, a protease that processes pro-IL-1beta into biologically active IL-1beta. The activity of caspase-1 is controlled by the inflammasome, a multiprotein complex formed by NLR proteins and the adaptor ASC, that induces the activation of caspase-1. The current model proposes that changes in the intracellular concentration of K(+) potentiate caspase-1 activation induced by the recognition of bacterial products. However, the roles of P2X7 receptor and intracellular K(+) in IL-1beta secretion induced by bacterial infection remain unknown. Here we show that, in response to Toll-like receptor agonists such as lipopolysaccharide or infection with extracellular bacteria Staphylococcus aureus and Escherichia coli, efficient caspase-1 activation is only triggered by addition of ATP, a signal that promotes caspase-1 activation through depletion of intracellular K(+) caused by stimulation of the purinergic P2X7 receptor. In contrast, activation of caspase-1 that relies on cytosolic sensing of flagellin or intracellular bacteria did not require ATP stimulation or depletion of cytoplasmic K(+). Consistently, caspase-1 activation induced by intracellular Salmonella or Listeria was unimpaired in macrophages deficient in P2X7 receptor. These results indicate that, unlike caspase-1 induced by Toll-like receptor agonists and ATP, activation of the inflammasome by intracellular bacteria and cytosolic flagellin proceeds normally in the absence of P2X7 receptor-mediated cytoplasmic K(+) perturbations.

Debugging how bacteria manipulate the immune response

Beyond the innate response that is elicited when tissues are infected, bacterial pathogens have evolved strategies to subvert the immune response and "recalibrate" it both qualitatively and quantitatively, thereby achieving a balance consistent with the survival of both the microbe and its infected host, a compromise that is likely the result of a long process of coevolution between pathogens and their hosts. By collaboratively studying the mechanisms employed, microbiologists and immunologists are fostering development of a renewed approach of infectious diseases that is expected to provide useful new concepts and applications for their control. In addition, the molecular strategies developed by bacteria to dampen immune mechanisms result from such strong and prolonged selective pressure for survival that they may point to original mechanisms and targets to conceive novel immunomodulatory, anti-inflammatory, and anti-infectious molecules.

Structure of the heterotrimeric complex that regulates type III secretion needle formation

Type III secretion systems (T3SS), found in several Gram-negative pathogens, are nanomachines involved in the transport of virulence effectors directly into the cytoplasm of target cells. T3SS are essentially composed of basal membrane-embedded ring-like structures and a hollow needle formed by a single polymerized protein. Within the bacterial cytoplasm, the T3SS needle protein requires two distinct chaperones for stabilization before its secretion, without which the entire T3SS is nonfunctional. The 2.0-A x-ray crystal structure of the PscE-PscF(55-85)-PscG heterotrimeric complex from Pseudomonas aeruginosa reveals that the C terminus of the needle protein PscF is engulfed within the hydrophobic groove of the tetratricopeptide-like molecule PscG, indicating that the macromolecular scaffold necessary to stabilize the T3SS needle is totally distinct from chaperoned complexes between pilus- or flagellum-forming molecules. Disruption of specific PscG-PscF interactions leads to impairment of bacterial cytotoxicity toward macrophages, indicating that this essential heterotrimer, which possesses homologs in a wide variety of pathogens, is a unique attractive target for the development of novel antibacterials.

Aspects of genome plasticity in pathogenic Escherichia coli

The species Escherichia coli comprises not only non-pathogenic or commensal variants that belong to the normal intestinal flora of most mammals, but also various pathogenic strains causing diverse intestinal and extraintestinal infections in man and animals. Virulence factors and mechanisms involved in pathogenesis have been successfully analyzed for many years resulting in a wealth of knowledge about many E. coli pathotypes. However, our knowledge on the genome content, diversity and variability between pathogenic and also non-pathogenic subtypes is only slowly accumulating. Pathotypes have been largely defined by the presence or absence of particular DNA segments that in most cases appear to have been acquired via horizontal gene transfer events. As these regions are frequently subjected to excisions, rearrangements, and transfers they contribute to the previously unexpected and underestimated rapid evolution of E. coli variants resulting in the development of novel strains and even pathotypes. In these studies various novel aspects of genome diversity and plasticity in extraintestinal and intestinal pathogenic E. coli pathotypes have been addressed and the results have been directly applied for the improvement of diagnostic methods.

Exploitation of eukaryotic ubiquitin signaling pathways by effectors translocated by bacterial type III and type IV secretion systems

The specific and covalent addition of ubiquitin to proteins, known as ubiquitination, is a eukaryotic-specific modification central to many cellular processes, such as cell cycle progression, transcriptional regulation, and hormone signaling. Polyubiquitination is a signal for the 26S proteasome to destroy earmarked proteins, but depending on the polyubiquitin chain topology, it can also result in new protein properties. Both ubiquitin-orchestrated protein degradation and modification have also been shown to be essential for the host's immune response to pathogens. Many animal and plant pathogenic bacteria utilize type III and/or type IV secretion systems to inject effector proteins into host cells, where they subvert host signaling cascades as part of their infection strategy. Recent progress in the determination of effector function has taught us that playing with the host's ubiquitination system seems a general tactic among bacteria. Here, we discuss how bacteria exploit this system to control the timing of their effectors' action by programming them for degradation, to block specific intermediates in mammalian or plant innate immunity, or to target host proteins for degradation by mimicking specific ubiquitin/proteasome system components. In addition to analyzing the effectors that have been described in the literature, we screened publicly available bacterial genomes for mimicry of ubiquitin proteasome system subunits and detected several new putative effectors. Our understanding of the intimate interplay between pathogens and their host's ubiquitin proteasome system is just beginning. This exciting research field will aid in better understanding this interplay, and may also provide new insights into eukaryotic ubiquitination processes.

Engineering bacterial vectors for delivery of genes and proteins to antigen-presenting cells

Bacterial vectors offer a biological route to gene and protein delivery with this article featuring delivery to antigen-presenting cells (APCs). Primarily in the context of immune stimulation against infectious disease or cancer, the goal of bacterially mediated delivery is to overcome the hurdles to effective macromolecule delivery. This review will present several bacterial vectors as macromolecule (protein or gene) delivery devices with both innate and acquirable (or engineered) biological features to facilitate delivery to APCs. The review will also present topics related to large-scale manufacture, storage, and distribution that must be considered if the bacterial delivery devices are ever to be used in a global market.