The type III secretion injectisome

Yersinia enterocolitica type III secretion of YopR requires a structure in its mRNA

Yersinia type III secretion machines transport substrate proteins into the extracellular medium or into the cytoplasm of host cells. Translational hybrids, involving genes that encode substrates as well as reporter proteins that otherwise cannot travel the type III pathway, identified signals that promote transport of effector Yops into host cells. Signals for the secretion of substrates into high calcium media were hitherto unknown. By exploiting attributes of translational hybrids between yopR, whose product is secreted, and genes that encode impassable proteins that jam the secretion machine, we isolated yopR mutations that abolish substrate recognition. Similar to effector Yops, an N-terminal or 5' signal in codons 1-11 is required to initiate YopR into the type III pathway. YopR secretion cannot be completed and translational hybrids cannot impose a block without a second signal, positioned at codons 131-149. Silent mutations in the second signal abrogate function and the phenotype of other mutations can be suppressed by secondary mutations predicted to restore base complementary in a 3' stem-loop structure of the yopR mRNA.

Mechanisms of type III protein export for bacterial flagellar assembly

Flagellar type III protein export is highly organized and well controlled in a timely manner by dynamic, specific and cooperative interactions among components of the export apparatus, allowing the huge and complex macromolecular assembly to be built efficiently. The bacterial flagellum, which is required for motility, consists of a rotary motor, a universal joint and a helical propeller. Most of the flagellar components are translocated to the distal, growing end of the flagellum for assembly through the central channel of the flagellum itself by the flagellar type III protein export apparatus, which is postulated to be located on the cytoplasmic side of the flagellar basal body. The export specificity switching machinery, which consists of at least two proteins that function as a molecular ruler and an export switch, respectively, monitors the state of hook-basal body assembly in the cell exterior and switches export specificity, thereby coupling sequential flagellar gene expression with the flagellar assembly process. The export ATPase complex composed of an ATPase and its regulator acts as a pilot to deliver its export substrate to the export gate and helps initial entry of the substrate N-terminal chain into a narrow pore of the export gate. The energy of ATP hydrolysis appears to be used to disassemble and release the ATPase complex from the protein about to be exported, and the rest of the successive unfolding/translocation process of the long polypeptide chain is driven solely by proton motive force (PMF), perhaps through biased one-dimensional Brownian diffusion. Interestingly, the subunits of the ATPase complex have significant sequence similarities to subunits of F(0)F(1)-ATP synthase, a rotary motor that drives the chemical reaction of ATP synthesis using PMF, and the entire crystal structure of the export ATPase is extremely similar to the alpha/beta subunits of F(0)F(1)-ATP synthase, suggesting that the flagellar export apparatus and F(0)F(1)-ATP synthase share the mechanism for their two distinct functions.

A channel connecting the mother cell and forespore during bacterial endospore formation

At an early stage during Bacillus subtilis endospore development the bacterium divides asymmetrically to produce two daughter cells. The smaller cell (forespore) differentiates into the endospore, while the larger cell (mother cell) becomes a terminally differentiated cell that nurtures the developing forespore. During development the mother cell engulfs the forespore to produce a protoplast, surrounded by two bilayer membranes, which separate it from the cytoplasm of the mother cell. The activation of sigma(G), which drives late gene expression in the forespore, follows forespore engulfment and requires expression of the spoIIIA locus in the mother cell. One of the spoIIIA-encoded proteins SpoIIIAH is targeted specifically to the membrane surrounding the forespore, through an interaction of its C-terminal extracellular domain with the C-terminal extracellular domain of the forespore membrane protein SpoIIQ. We identified a homologous relationship between the C-terminal domain of SpoIIIAH and the YscJ/FliF protein family, members of which form multimeric rings involved in type III secretion systems and flagella. If SpoIIIAH forms a similar ring structure, it may also form a channel between the mother cell and forespore membranes. To test this hypothesis we developed a compartmentalized biotinylation assay, which we used to show that the C-terminal extracellular domain of SpoIIIAH is accessible to enzymatic modification from the forespore cytoplasm. These and other results lead us to suggest that SpoIIIAH forms part of a channel between the forespore and mother cell that is required for the activation of sigma(G).

Structural dissection of the extracellular moieties of the type III secretion apparatus

Many Gram-negative bacterial pathogens use type III secretion systems (TTSSs) for subverting the normal cellular functions of their target eukaryotic cells. The type III secretion apparatus (TTSA) functions like a syringe to inject proteins through an external needle and into a target cell's membrane and cytosol. The TTSA basal body spans the bacterial inner and outer membranes, and the external needle is topped with a tip complex that controls the secretion and delivery of translocator and effector proteins. Recently solved structures of TTSA proteins have greatly advanced our understanding of shared themes in apparatus assembly and function. In this highlight, the structure-function of TTSA needle and tip complex proteins are described and common themes discussed.

Vaccination of mice with a Yop translocon complex elicits antibodies that are protective against infection with F1- Yersinia pestis

Yersinia pestis, the bacterial agent of plague, secretes several proteins important for pathogenesis or host protection. The F1 protein forms a capsule on the bacterial cell surface and is a well-characterized protective antigen but is not essential for virulence. A type III secretion system that is essential for virulence exports Yop proteins, which function as antiphagocytic or anti-inflammatory factors. Yop effectors (e.g., YopE) are delivered across the host cell plasma membrane by a translocon, composed of YopB and YopD. Complexes of YopB, YopD, and YopE (BDE) secreted by Yersinia pseudotuberculosis were purified by affinity chromatography and used as immunogens to determine if antibodies to the translocon could provide protection against Y. pestis in mice. Mice vaccinated with BDE generated high-titer immunoglobulin G antibodies specific for BDE, as shown by enzyme-linked immunosorbent assay and immunoblotting, and were protected against lethal intravenous challenge with F1(-) but not F1(+) Y. pestis. Mice passively immunized with anti-BDE serum were protected from lethal challenge with F1(-) Y. pestis. The YopB protein or a complex of YopB and YopD (BD) was purified and determined by vaccination to be immunogenic in mice. Mice actively vaccinated with BD or passively vaccinated with anti-BD serum were protected against lethal challenge with F1(-) Y. pestis. These results indicate that anti-translocon antibodies can be used as immunotherapy to treat infections by F1(-) Y. pestis.

Functional analysis of the Salmonella pathogenicity island 2-mediated inhibition of antigen presentation in dendritic cells

Salmonella enterica is a facultative intracellular pathogen that is able to modify host cell functions by means of effector proteins translocated by the type III secretion system (T3SS) encoded by Salmonella Pathogenicity Island 2 (SPI2). The SPI2-T3SS is also active in Salmonella after uptake by murine bone marrow-derived dendritic cells (BM-DC). We have previously shown that intracellular Salmonella interfere with the ability of BM-DC to stimulate antigen-dependent T-cell proliferation in an SPI2-T3SS-dependent manner. We observed that Salmonella-mediated inhibition of antigen presentation could be restored by external addition of peptides on major histocompatibility complex class II (MHC-II). The processing of antigens in Salmonella-infected cells was not altered; however, the intracellular loading of peptides on MHC-II was reduced as a function of the SPI2-T3SS. We set out to identify the effector proteins of the SPI2-T3SS involved in inhibition of antigen presentation and demonstrated that effector proteins SifA, SspH2, SlrP, PipB2, and SopD2 were equally important for the interference with antigen presentation, whereas SseF and SseG contributed to a lesser extent to this phenotype. These observations indicate the presence of a host cell-specific virulence function of a novel subset of SPI2-effector proteins.

Characterization of the putative type III secretion ATPase CdsN (Cpn0707) of Chlamydophila pneumoniae

Type III secretion (T3S) is utilized by a wide range of gram-negative bacterial pathogens to allow the efficient delivery of effector proteins into the host cell cytoplasm through the use of a syringe-like injectisome. Chlamydophila pneumoniae is a gram-negative, obligate intracellular pathogen that has the structural genes coding for a T3S system, but the functionality of the system has not yet been demonstrated. T3S is dependent on ATPase activity, which catalyzes the unfolding of proteins and the secretion of effector proteins through the injectisome. CdsN (Cpn0707) is predicted to be the T3S ATPase of C. pneumoniae based on sequence similarity to other T3S ATPases. Full-length CdsN and a C-terminal truncation of CdsN were cloned as glutathione S-transferase (GST)-tagged constructs and expressed in Escherichia coli. The GST-tagged C-terminal truncation of CdsN possessed ATPase activity, catalyzing the release of ADP and P(i) from ATP at a rate of 0.55 +/- 0.07 micromol min(-1) mg(-1) in a time- and dose-dependent manner. CdsN formed oligomers and high-molecular-weight multimers, as assessed by formaldehyde fixation and nondenaturing polyacrylamide gel electrophoresis. Using bacterial two-hybrid and GST pull-down assays, CdsN was shown to interact with CdsD, CdsL, CdsQ, and CopN, four putative structural components of the C. pneumoniae T3S system. CdsN also interacted with an unannotated protein, Cpn0706, a putative CdsN chaperone. Interactions between CdsN, CdsD, and CopN represent novel interactions not previously reported for other bacterial T3S systems and may be important in the localization and/or function of the ATPase at the inner membrane of C. pneumoniae.

DsbB is required for the pathogenesis process of Xanthomonas campestris pv. campestris

The DsbA/DsbB oxidation pathway is one of the two pathways that catalyze disulfide bond formation of proteins in the periplasm of gram-negative bacteria. It has been demonstrated that DsbA is essential for multiple virulence factors of several animal bacterial pathogens. In this article, we present genetic evidence to show that the open reading frame XC_3314 encodes a DsbB protein that is involved in disulfide bond formation in periplasm of Xanthomonas campestris pv. campestris, the causative agent of crucifer black rot disease. The dsbB mutant of X. campestris pv. campestris exhibited attenuation in virulence, hypersensitive response, cell motility, and bacterial growth in planta. Furthermore, mutation in the dsbB gene resulted in ineffective type II and type III secretion systems as well as flagellar assembly. These findings reveal that DsbB is required for the pathogenesis process of X. campestris pv. campestris.

Down-modulation of TCR expression by Salmonella enterica serovar Typhimurium

T cell-mediated adaptive immunity is required to help clear infection with the facultative intracellular bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), yet development of T cell-mediated adaptive immunity to S. Typhimurium has been described as slow and inefficient. A key step in inducing T cell-mediated adaptive immunity is T cell priming; the activation, proliferation, and differentiation of naive T cells following initial encounter with Ag. We previously demonstrated that S. Typhimurium had a direct inhibitory effect on naive T cells from mouse, blocking their proliferation. In this study, we show that S. Typhimurium down-modulates expression of the TCR beta-chain, a molecule that is essential for Ag recognition and T cell function. Specifically, we demonstrate that reduced amounts of surface and intracellular TCR-beta protein and decreased levels of tcrbeta transcript are expressed by T cells cultured in the presence of S. Typhimurium. We further show that the down-modulation of TCR-beta expression requires contact between S. Typhimurium and the T cells and that once contact occurs, a factor capable of reducing TCR-beta expression is secreted. These results provide new insight into the mechanism by which S. Typhimurium may inhibit T cell priming and avoid clearance by the adaptive immune system.

Crystal structure of Spa40, the specificity switch for the Shigella flexneri type III secretion system

The pathogenic bacterium Shigella flexneri uses a type III secretion system to inject virulence factors from the bacterial cytosol directly into host cells. The machinery that identifies secretion substrates and controls the export of extracellular components and effector proteins consists of several inner-membrane and cytoplasmic proteins. One of the inner membrane components, Spa40, belongs to a family of proteins proposed to regulate the switching of substrate specificity of the export apparatus. We show that Spa40 is cleaved within the strictly conserved amino acid sequence NPTH and substitution of the proposed autocatalytic residue abolishes cleavage. Here we also report the crystal structure of the cytoplasmic complex Spa40(C) and compare it with the recent structures of the homologues from Escherichia coli and Salmonella typhimurium. These structures reveal the tight association of the cleaved fragments and show that the conserved NPTH sequence lies on a loop which, when cleaved, swings away from the catalytic N257 residue, resulting in different surface features in this region. This structural rearrangement suggests a mechanism by which non-cleaving forms of these proteins interfere with correct substrate switching of the apparatus.

Type III secretion system 1 genes in Vibrio parahaemolyticus are positively regulated by ExsA and negatively regulated by ExsD

Vibrio parahaemolyticus harbours two distinct type III secretion systems (T3SS1 and T3SS2). A subset of 10 T3SS1 genes are transcribed when V. parahaemolyticus is grown in tissue culture medium [Dulbecco's modified Eagle's medium (DMEM)], while transcription of these genes (except exsD) is minimal upon growth in Luria-Bertani-Salt (LB-S). Transcription of T3SS1 genes and cytotoxicity towards HeLa cells was prevented by deletion of exsA while complementation with exsA restored these traits. Overexpression of ExsA in the wild-type strain, NY-4, activated the transcription of T3SS1 genes when bacteria were grown in LB-S. Thus, ExsA is necessary and sufficient to induce the transcription of T3SS1 genes. Deletion of the exsD permitted the transcription of T3SS1 genes when bacteria were grown in the LB-S medium and complementation with the wild-type exsD gene-blocked transcription of T3SS1 genes. Overexpression of ExsD in NY-4 prevented the transcription of T3SS1 gene when bacteria were grown in DMEM. A gel mobility shift assay demonstrated that purified ExsA protein binds a novel motif in the upstream region of vp1668 and vp1687, indicating that ExsA interacts directly with the promoter sequences of T3SS1 genes. ExsA positively regulates the expression and secretion of Vp1656 while ExsD negatively regulates the expression and secretion of Vp1656.

Oligomerization of PcrV and LcrV, protective antigens of Pseudomonas aeruginosa and Yersinia pestis

Protective antigens of Pseudomonas aeruginosa (PcrV) and Yersinia pestis (LcrV) are key elements of specialized machinery, the type III secretion system (T3SS), which enables the injection of effector molecules into eukaryotic cells. Being positioned at the injectisome extremity, V proteins participate in the translocation process across the host cell plasma membrane. In this study, we demonstrate the assembly of V proteins into oligomeric doughnut-like complexes upon controlled refolding of the proteins in vitro. The oligomeric nature of refolded PcrV was revealed by size exclusion chromatography, native gel electrophoresis, and native mass spectrometry, which ascertain the capacity of the protein to multimerize into higher-order species. Furthermore, transmission electron microscopy performed on oligomers of both PcrV and LcrV revealed the presence of distinct structures with approximate internal and external diameters of 3-4 and 8-10 nm, respectively. The C-terminal helix, alpha12, of PcrV and notably the hydrophobic residues Val(255), Leu(262), and Leu(276) located within this helix, were shown to be crucial for oligomerization. Moreover, the corresponding mutant proteins produced in P. aeruginosa were found to be non-functional in in vivo type III-dependent cytotoxicity assays by directly affecting the correct assembly of PopB/D translocon within the host cell membranes. The detailed understanding of structure-function relationships of T3SS needle tip proteins will be of value in further developments of new vaccines and antimicrobials.

What transmission electron microscopes can visualize now and in the future

Our review concentrates on the progress made in high-resolution transmission electron microscopy (TEM) in the past decade. This includes significant improvements in sample preparation by quick-freezing aimed at preserving the specimen in a close-to-native state in the high vacuum of the microscope. Following advances in cold stage and TEM vacuum technology systems, the observation of native, frozen hydrated specimens has become a widely used approach. It fostered the development of computer guided, fully automated low-dose data acquisition systems allowing matched pairs of images and diffraction patterns to be recorded for electron crystallography, and the collection of entire tilt-series for electron tomography. To achieve optimal information transfer to atomic resolution, field emission electron guns combined with acceleration voltages of 200-300 kV are now routinely used. The outcome of these advances is illustrated by the atomic structure of mammalian aquaporin-O and by the pore-forming bacterial cytotoxin ClyA resolved to 12 A. Further, the Yersinia injectisome needle, a bacterial pseudopilus and the binding of phalloidin to muscle actin filaments were chosen to document the advantage of the high contrast offered by dedicated scanning transmission electron microscopy (STEM) and/or the STEM's ability to measure the mass of protein complexes and directly link this to their shape. Continued progress emerging from leading research laboratories and microscope manufacturers will eventually enable us to determine the proteome of a single cell by electron tomography, and to more routinely solve the atomic structure of membrane proteins by electron crystallography.

Coordinating assembly of a bacterial macromolecular machine

The assembly of large and complex organelles, such as the bacterial flagellum, poses the formidable problem of coupling temporal gene expression to specific stages of the organelle-assembly process. The discovery that levels of the bacterial flagellar regulatory protein FlgM are controlled by its secretion from the cell in response to the completion of an intermediate flagellar structure (the hook-basal body) was only the first of several discoveries of unique mechanisms that coordinate flagellar gene expression with assembly. In this Review, we discuss this mechanism, together with others that also coordinate gene regulation and flagellar assembly in Gram-negative bacteria.

Delivery of biologically active anti-inflammatory cytokines IL-10 and IL-1ra in vivo by the Shigella type III secretion apparatus

Pathogenicity of many Gram-negative bacteria relies on a type III secretion (T3S) apparatus, which is used for delivery of bacterial effectors into the host cell cytoplasm allowing the bacteria to manipulate host cell cytoskeleton network as well as to interfere with intracellular signaling pathways. In this study, we investigated the potential of the Shigella flexneri T3SA as an in vivo delivery system for biologically active molecules such as cytokines. The anti-inflammatory cytokines IL-10 and IL-1 receptor antagonist (IL-1ra) were genetically fused to the first 30 or 60 residues of the Shigella T3S effector IpaH9.8 or to the first 50 residues of the Yersinia enterocolitica effector YopE and the recombinant fusion proteins were expressed in S. flexneri. YopE(50)-IL-10, IpaH(60)-IL-10, and IpaH(60)-IL-1ra were efficiently secreted via the T3S apparatus of Shigella. Moreover, these recombinant proteins did not impair the invasive ability of the bacteria in vitro. In a murine model, Shigella strains expressing YopE(50)-IL-10, IpaH(60)-IL-10, and IpaH(60)-IL-1ra induced a lower mortality in mice that was associated with reduced inflammation and a restricted localization of bacteria within the lung tissues as compared with wild-type Shigella. Moreover, the level of TNF-alpha and IL-1beta mRNA were reduced in the lungs following infection by IL-10- and IL-1ra-secreting Shigella, respectively. These findings demonstrate that the Shigella T3S apparatus can deliver biologically active cytokines in vivo, thus opening new avenues for the use of attenuated bacteria to deliver proteins for immunomodulation or gene therapy purposes.

The type III secretion chaperone SycE promotes a localized disorder-to-order transition in the natively unfolded effector YopE

Many virulence-related, bacterial effector proteins are translocated directly into the cytosol of host cells by the type III secretion (TTS) system. Translocation of most TTS effectors requires binding by specific chaperones in the bacterial cytosol, although how chaperones promote translocation is unclear. To provide insight into the action of such chaperones, we studied the consequences of binding by the Yersinia chaperone SycE to the effector YopE by NMR. These studies examined the intact form of the effector, whereas prior studies have been limited to well ordered fragments. We found that YopE had the characteristics of a natively unfolded protein, with its N-terminal 100 residues, including its chaperone-binding (Cb) region, flexible and disordered in the absence of SycE. SycE binding caused a pronounced disorder-to-order transition in the Cb region of YopE. The effect of SycE was strictly localized to the Cb region, with other portions of YopE being unperturbed. These results provide stringent limits on models of chaperone action and are consistent with the chaperone promoting formation of a three-dimensional targeting signal in the Cb region of the effector. The target of this putative signal is unknown but appears to be a bacterial component other than the TTS ATPase YscN.

Site-directed mutagenesis of the Pseudomonas aeruginosa type III secretion system protein PscJ reveals an essential role for surface-localized residues in needle complex function

The Pseudomonas aeruginosa type III secretion system (TTSS) protein PscJ belongs to the PrgK family of TTSS proteins. These proteins are predicted to form one of the inner membrane localized ring substructures of the TTSS needle complex. To determine which amino acid residues of PscJ are important for its function, the pscJ gene was subjected to site-directed mutagenesis. Fifteen individual PscJ amino acid residues that are located in conserved regions of the PrgK family were targeted for mutagenesis. Eight of these residues could be subjected to non-conservative substitution mutagenesis without affecting the function of the resultant mutant protein. Substitution of the other 7 residues (E26, K52, E105, A107, G126, H133, and V189) resulted in either a non-functional protein or the loss of detectable protein. When the essential residues were mapped on to the crystal structure of the E. coli PrgK homolog EscJ, the majority appeared to localize to surface-exposed regions of the protein suggesting a role for these regions in the assembly of the PscJ ring structure.

Function and molecular architecture of the Yersinia injectisome tip complex

By quantitative immunoblot analyses and scanning transmission electron microscopy (STEM), we determined that the needle of the Yersinia enterocolitica E40 injectisome consists of 139 +/- 19 YscF subunits and that the tip complex is formed by three to five LcrV monomers. A pentamer represented the best fit for an atomic model of this complex. The N-terminal globular domain of LcrV forms the base of the tip complex, while the central globular domain forms the head. Hybrids between LcrV and its orthologues PcrV (Pseudomonas aeruginosa) or AcrV (Aeromonas salmonicida) were engineered and recombinant Y. enterocolitica expressing the different hybrids were tested for their capacity to form the translocation pore by a haemolysis assay. There was a good correlation between haemolysis, insertion of YopB into erythrocyte membranes and interaction between YopB and the N-terminal globular domain of the tip complex subunit. Hence, the base of the tip complex appears to be critical for the functional insertion of YopB into the host cell membrane.

Coordinating assembly of a bacterial macromolecular machine

The assembly of large and complex organelles, such as the bacterial flagellum, poses the formidable problem of coupling temporal gene expression to specific stages of the organelle-assembly process. The discovery that levels of the bacterial flagellar regulatory protein FlgM are controlled by its secretion from the cell in response to the completion of an intermediate flagellar structure (the hook-basal body) was only the first of several discoveries of unique mechanisms that coordinate flagellar gene expression with assembly. In this Review, we discuss this mechanism, together with others that also coordinate gene regulation and flagellar assembly in Gram-negative bacteria.

Thermal unfolding simulations of bacterial flagellin: insight into its refolding before assembly

Flagellin is the subunit of the bacterial filament, the micrometer-long propeller of a bacterial flagellum. The protein is believed to undergo unfolding for transport through the channel of the filament and to refold in a chamber at the end of the channel before being assembled into the growing filament. We report a thermal unfolding simulation study of S. typhimurium flagellin in aqueous solution as an attempt to gain atomic-level insight into the refolding process. Each molecule comprises two filament-core domains {D0, D1} and two hypervariable-region domains {D2, D3}. D2 can be separated into subdomains D2a and D2b. We observed a similar unfolding order of the domains as reported in experimental thermal denaturation. D2a and D3 exhibited high thermal stability and contained persistent three-stranded beta-sheets in the denatured state which could serve as folding cores to guide refolding. A recent mutagenesis study on flagellin stability seems to suggest the importance of the folding cores. Using crude size estimates, our data suggests that the chamber might be large enough for either denatured hypervariable-region domains or filament-core domains, but not whole flagellin; this implicates a two-staged refolding process.

Hierarchical effector protein transport by the Salmonella Typhimurium SPI-1 type III secretion system

Background

Type III secretion systems (TTSS) are employed by numerous pathogenic and symbiotic bacteria to inject a cocktail of different “effector proteins” into host cells. These effectors subvert host cell signaling to establish symbiosis or disease.

Methodology/Principal Findings

We have studied the injection of SipA and SptP, two effector proteins of the invasion-associated Salmonella type III secretion system (TTSS-1). SipA and SptP trigger different host cell responses. SipA contributes to triggering actin rearrangements and invasion while SptP reverses the actin rearrangements after the invasion has been completed. Nevertheless, SipA and SptP were both pre-formed and stored in the bacterial cytosol before host cell encounter. By time lapse microscopy, we observed that SipA was injected earlier than SptP. Computer modeling revealed that two assumptions were sufficient to explain this injection hierarchy: a large number of SipA and SptP molecules compete for transport via a limiting number of TTSS; and the TTSS recognize SipA more efficiently than SptP.

Conclusions/Significance

This novel mechanism of hierarchical effector protein injection may serve to avoid functional interference between SipA and SptP. An injection hierarchy of this type may be of general importance, allowing bacteria to precisely time the host cell manipulation by type III effectors.

What's the point of the type III secretion system needle?

Recent work by several groups has significantly expanded our knowledge of the structure, regulation of assembly, and function of components of the extracellular portion of the type III secretion system (T3SS) of Gram-negative bacteria. This perspective presents a structure-informed analysis of functional data and discusses three nonmutually exclusive models of how a key aspect of T3SS biology, the sensing of host cells, may be performed.

Enterohemorrhagic Escherichia coli exploits EspA filaments for attachment to salad leaves

Enterohemorrhagic Escherichia coli (EHEC) strains are important food-borne pathogens that use a filamentous type III secretion system (fT3SS) for colonization of the gut epithelium. In this study we have shown that EHEC O157 and O26 strains use the fT3SS apparatus for attachment to leaves. Leaf attachment was independent of effector protein translocation.

Pseudomonas aeruginosa induces membrane blebs in epithelial cells, which are utilized as a niche for intracellular replication and motility

Pseudomonas aeruginosa is known to invade epithelial cells during infection and in vitro. However, little is known of bacterial or epithelial factors modulating P. aeruginosa intracellular survival or replication after invasion, except that it requires a complete lipopolysaccharide core. In this study, real-time video microscopy revealed that invasive P. aeruginosa isolates induced the formation of membrane blebs in multiple epithelial cell types and that these were then exploited for intracellular replication and rapid real-time motility. Further studies revealed that the type three secretion system (T3SS) of P. aeruginosa was required for blebbing. Mutants lacking either the entire T3SS or specific T3SS components were instead localized to intracellular perinuclear vacuoles. Most T3SS mutants that trafficked to perinuclear vacuoles gradually lost intracellular viability, and vacuoles containing those bacteria were labeled by the late endosomal marker lysosome-associated marker protein 3 (LAMP-3). Interestingly, mutants deficient only in the T3SS translocon structure survived and replicated within the vacuoles that did not label with LAMP-3. Taken together, these data suggest two novel roles of the P. aeruginosa T3SS in enabling bacterial intracellular survival: translocon-dependent formation of membrane blebs, which form a host cell niche for bacterial growth and motility, and effector-dependent bacterial survival and replication within intracellular perinuclear vacuoles.

YscU cleavage and the assembly of Yersinia type III secretion machine complexes

YscU, a component of the Yersinia type III secretion machine, promotes auto-cleavage at asparagine 263 (N263). Mutants with an alanine substitution at yscU codon 263 displayed secretion defects for some substrates (LcrV, YopB and YopD); however, transport of effector proteins into host cells (YopE, YopH, YopM) continued to occur. Two yscU mutations were isolated that, unlike N263A, completely abolished type III secretion; YscU(G127D) promoted auto-cleavage at N263, whereas YscU(G270N) did not. When fused to glutathione S-transferase (Gst), the YscU C-terminal cytoplasmic domain promoted auto-cleavage and Gst-YscU(C) also exerted a dominant-negative phenotype by blocking type III secretion. Gst-YscU(C/N263A) caused a similar blockade and Gst-YscU(C/G270N) reduced secretion. Gst-YscU(C) and Gst-YscU(C/N263A) bound YscL, the regulator of the ATPase YscN, whereas Gst-YscU(C/G270N) did not. When isolated from Yersinia, Gst-YscU(C) and Gst-YscU(C/N263A) associated with YscK-YscL-YscQ; however, Gst-YscU(C/G270N) interacted predominantly with the machine component YscO, but not with YscK-YscL-YscQ. A model is proposed whereby YscU auto-cleavage promotes interaction with YscL and recruitment of ATPase complexes that initiate type III secretion.

The type III secretion system tip complex and translocon

The type III secretion machinery of Gram-negative bacteria, also known as the injectisome or needle complex, is composed of a basal body spanning both bacterial membranes and the periplasm, and an external needle protruding from the bacterial surface. A set of three proteins, two hydrophobic and one hydrophilic, are required to allow translocation of proteins from the bacterium to the host cell cytoplasm. These proteins are involved in the formation of a translocation pore, the translocon, in the host cell membrane. Exciting progress has recently been made on the interaction between the translocators and the injectisome needle and the assembly of the translocon in the host cell membrane. As expected, the two hydrophobic translocators insert into the target cell membrane. Unexpectedly, the third, hydrophilic translocator, forms a complex on the distal end of the injectisome needle, the tip complex, and serves as an assembly platform for the two hydrophobic translocators.

Control of gene expression by type III secretory activity

The bacterial flagellum and the highly related injectisome (or needle complex) are among the most complicated multi-protein structures found in Gram-negative microorganisms. The assembly of both structures is dependent upon a type III secretion system. An interesting regulatory feature unique to these systems is the coordination of gene expression with type III secretory activity. This means of regulation ensures that secretion substrates are expressed only when required during the assembly process or upon completion of the fully functional structure. Prominent within the regulatory scheme are secreted proteins and type III secretion chaperones that exert effects on gene expression at the transcriptional and post-transcriptional levels. Although the major structural components of the flagellum and injectisome systems are highly conserved, recent studies reveal diversity in the mechanisms used by secretion substrates and chaperones to control gene expression.

Role of Porphyromonas gingivalis SerB in gingival epithelial cell cytoskeletal remodeling and cytokine production

The SerB protein of Porphyromonas gingivalis is a HAD family serine phosphatase that plays a critical role in entry and survival of the organism in gingival epithelial cells. SerB is secreted by P. gingivalis upon contact with epithelial cells. Here it is shown by microarray analysis that SerB impacts the transcriptional profile of gingival epithelial cells, with pathways involving the actin cytoskeleton and cytokine production among those significantly overpopulated with differentially regulated genes. Consistent with the transcriptional profile, a SerB mutant of P. gingivalis exhibited defective remodeling of actin in epithelial cells. Interaction between gingival epithelial cells and isolated SerB protein resulted in actin rearrangement and an increase in the F/G actin ratio. SerB protein was also required for P. gingivalis to antagonize interleukin-8 accumulation following stimulation of epithelial cells with Fusobacterium nucleatum. SerB is thus capable of modulating host cell signal transduction that impacts the actin cytoskeleton and cytokine production.

High-yield production of secreted active proteins by the Pseudomonas aeruginosa type III secretion system

The Escherichia coli system is the system of choice for recombinant protein production because it is possible to obtain a high protein yield in inexpensive media. The accumulation of protein in an insoluble form in inclusion bodies remains a major disadvantage. Use of the Pseudomonas aeruginosa type III secretion system can avoid this problem, allowing the production of soluble secreted proteins.

Extracellular-loop peptide antibodies reveal a predominant hemichannel organization of connexins in polarized intestinal cells

Shigella, the causative agent of bacillary dysentery, invades colonic epithelial cells to elicit an intense inflammatory reaction leading to destruction of the mucosa. ATP-dependent paracrine signalling induced by connexin (Cx) hemichannel opening was previously shown to favor Shigella flexneri invasion and dissemination in transfectants of HeLa cells [G. Tran Van Nhieu, C. Clair, R. Bruzzone, M. Mesnil, P. Sansonetti and L. Combettes. (2003). Connexin-dependent intercellular communication increases invasion and dissemination of Shigella in epithelial cells. Nat. Cell Biol. 5, 720-726.]. However, although Cxs have been described in polarized epithelial cells, little is known about their structural organization and the role of hemichannels during S. flexneri invasion. We show here that polarized Caco-2/TC7 cells express significant amounts of Cx26, Cx32 and Cx43, but that unexpectedly, cell-cell coupling assessed by dye-transfer experiments is inefficient. Consistent with a predominant Cx organization in hemichannels, dye loading induced by low calcium was readily observed, with preferential loading at the basolateral side. Antibodies (Abs) against connexin extracellular loop peptides (CELAbs) demonstrated the importance of hemichannel signalling since they inhibited dye uptake at low calcium and at physiological calcium concentrations during S. flexneri invasion. Importantly, CELAbs allowed the visualization of hemichannels at the surface of epithelial cells, as structures distinct from gap intercellular junctions.

Assessment of the genetic diversity of Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans as a basis to identify putative pathogenicity genes and a type III secretion system of the SPI-1 family by multiple suppression subtractive hybridizations

Fluorescent amplified fragment length polymorphism revealed that strains of Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans are genetically distinct and can be grouped into four genetic lineages. Four suppression subtractive hybridizations were then performed to isolate DNA fragments present in these bean pathogens and absent from closely related xanthomonads. Virulence gene candidates were identified such as homologs of hemagglutinins, TonB-dependent receptors, zinc-dependent metalloproteases, type III effectors, and type IV secretion system components. Unexpectedly, homologs of the type III secretion apparatus components (SPI-1 family), usually reported in animal pathogens and insect symbionts, were also detected.

Pili in Gram-positive bacteria: assembly, involvement in colonization and biofilm development

Various cell-surface multisubunit protein polymers, known as pili or fimbriae, have a pivotal role in the colonization of specific host tissues by many pathogenic bacteria. In contrast to Gram-negative bacteria, Gram-positive bacteria assemble pili by a distinct mechanism involving a transpeptidase called sortase. Sortase crosslinks individual pilin monomers and ultimately joins the resulting covalent polymer to the cell-wall peptidoglycan. Here we review current knowledge of this mechanism and the roles of Gram-positive pili in the colonization of specific host tissues, modulation of host immune responses and the development of bacterial biofilms.

Energy source of flagellar type III secretion

Bacterial flagella contain a specialized secretion apparatus that functions to deliver the protein subunits that form the filament and other structures to outside the membrane. This apparatus is related to the injectisome used by many gram-negative pathogens and symbionts to transfer effector proteins into host cells; in both systems this export mechanism is termed 'type III' secretion. The flagellar secretion apparatus comprises a membrane-embedded complex of about five proteins, and soluble factors, which include export-dedicated chaperones and an ATPase, FliI, that was thought to provide the energy for export. Here we show that flagellar secretion in Salmonella enterica requires the proton motive force (PMF) and does not require ATP hydrolysis by FliI. The export of several flagellar export substrates was prevented by treatment with the protonophore CCCP, with no accompanying decrease in cellular ATP levels. Weak swarming motility and rare flagella were observed in a mutant deleted for FliI and for the non-flagellar type-III secretion ATPases InvJ and SsaN. These findings show that the flagellar secretion apparatus functions as a proton-driven protein exporter and that ATP hydrolysis is not essential for type III secretion.

Distinct roles of the FliI ATPase and proton motive force in bacterial flagellar protein export

Translocation of many soluble proteins across cell membranes occurs in an ATPase-driven manner. For construction of the bacterial flagellum responsible for motility, most of the components are exported by the flagellar protein export apparatus. The FliI ATPase is required for this export, and its ATPase activity is regulated by FliH; however, it is unclear how the chemical energy derived from ATP hydrolysis is used for the export process. Here we report that flagellar proteins of Salmonella enterica serovar Typhimurium are exported even in the absence of FliI. A fliH fliI double null mutant was weakly motile. Certain mutations in FlhA or FlhB, which form the core of the export gate, substantially improved protein export and motility of the double null mutant. Furthermore, proton motive force was essential for the export process. These results suggest that the FliH-FliI complex facilitates only the initial entry of export substrates into the gate, with the energy of ATP hydrolysis being used to disassemble and release the FliH-FliI complex from the protein about to be exported. The rest of the successive unfolding/translocation process of the substrates is driven by proton motive force.

Structural and functional properties of chimeric EspA-FliCi filaments of EPEC

Enteropathogenic Escherichia coli utilise a filamentous type III secretion system to translocate effector proteins into host gut epithelial cells. The primary constituent of the extracellular component of the filamentous type III secretion system is EspA. This forms a long flexible helical conduit between the bacterium and host and has a structure almost identical to that of the flagella filament. We have inserted the D3 domain of FliCi (from Salmonella typhimurium) into the outer domain of EspA and have studied the structure and function of modified filaments when expressed in an enteropathogenic E. coli espA mutant. We found that the chimeric protein EspA-FliCi filaments were biologically active as they supported protein secretion and translocation [assessed by their ability to trigger actin polymerisation beneath adherent bacteria (fluorescent actin staining test)]. The expressed filaments were recognised by both EspA and FliCi antisera. Visualisation and analysis of the chimeric filaments by electron microscopy after negative staining showed that, remarkably, EspA filaments are able to tolerate a large protein insertion without a significant effect on their helical architecture.

A novel pathway for exotoxin delivery by an intracellular pathogen

Fundamental to the biology of many bacterial pathogens are bacterial proteins with the capacity to modulate host cellular functions. These bacterial proteins are delivered to the host's molecular targets by a great diversity of mechanisms of varying complexity. The different delivery mechanisms are adapted to the specific biology of the pathogen. Here we focus our attention on a recently described delivery pathway adapted to the biology of an intracellular pathogen, in which an exotoxin is delivered from an intracellular location to its molecular target through autocrine and paracrine pathways.

Interactions between CdsD, CdsQ, and CdsL, three putative Chlamydophila pneumoniae type III secretion proteins

Chlamydophila pneumoniae is a gram-negative obligate intracellular bacterial pathogen that causes pneumonia and bronchitis and may contribute to atherosclerosis. The developmental cycle of C. pneumoniae includes a morphological transition from an infectious extracellular elementary body (EB) to a noninfectious intracellular reticulate body (RB) that divides by binary fission. The C. pneumoniae genome encodes a type III secretion (T3S) apparatus that may be used to infect eukaryotic cells and to evade the host immune response. In the present study, Cpn0712 (CdsD), Cpn0704 (CdsQ), and Cpn0826 (CdsL), three C. pneumoniae genes encoding yersiniae T3S YscD, YscQ, and YscL homologs, respectively, were cloned and expressed as histidine- and glutathione S-transferase (GST)-tagged proteins in Escherichia coli. Purified recombinant proteins were used to raise hyper-immune polyclonal antiserum and were used in GST pull-down and copurification assays to identify protein-protein interactions. CdsD was detected in both EB and RB lysates by Western blot analyses, and immunofluorescent staining demonstrated the presence of CdsD within inclusions. Triton X-114 solubilization and phase separation of chlamydial EB proteins indicated that CdsD partitions with cytoplasmic proteins, suggesting it is not an integral membrane protein. GST pull-down assays indicated that recombinant CdsD interacts with CdsQ and CdsL, and copurification assays with chlamydial lysates confirmed that native CdsD interacts with CdsQ and CdsL. To the best of our knowledge, this is the first report demonstrating interactions between YscD, YscQ, and YscL homologs of bacterial T3S systems. These novel protein interactions may play important roles in the assembly or function of the chlamydial T3S apparatus.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Piecing together the type III injectisome of bacterial pathogens

The Type III secretion system is a bacterial 'injectisome' which allows Gram-negative bacteria to shuttle virulence proteins directly into the host cells they infect. This macromolecular assembly consists of more than 20 different proteins put together to collectively span three biological membranes. The recent T3SS crystal structures of the major oligomeric inner membrane ring, the helical needle filament, needle tip protein, the associated ATPase, and outer membrane pilotin together with electron microscopy reconstructions have dramatically furthered our understanding of how this protein translocator functions. The crucial details that describe how these proteins assemble into this oligomeric complex will need a hybrid of structural methodologies including EM, crystallography, and NMR to clarify the intra- and inter-molecular interactions between different structural components of the apparatus.

Cytoplasmic targeting of IpaC to the bacterial pole directs polar type III secretion in Shigella

Type III secretion (T3S) systems are largely used by pathogenic gram-negative bacteria to inject multiple effectors into eukaryotic cells. Upon cell contact, these bacterial microinjection devices insert two T3S substrates into host cell membranes, forming a so-called 'translocon' that is required for targeting of type III effectors in the cell cytosol. Here, we show that secretion of the translocon component IpaC of invasive Shigella occurs at the level of one bacterial pole during cell invasion. Using IpaC fusions with green fluorescent protein variants (IpaCi), we show that the IpaC cytoplasmic pool localizes at an old or new bacterial pole, where secretion occurs upon T3S activation. Deletions in ipaC identified domains implicated in polar localization. Only polar IpaCi derivatives inhibited T3S, while IpaCi fusions with diffuse cytoplasmic localization had no detectable effect on T3S. Moreover, the deletions that abolished polar localization led to secretion defects when introduced in ipaC. These results indicate that cytoplasmic polar localization directs secretion of IpaC at the pole of Shigella, and may represent a mandatory step for T3S.

The biology and future prospects of antivirulence therapies

The emergence and increasing prevalence of bacterial strains that are resistant to available antibiotics demand the discovery of new therapeutic approaches. Targeting bacterial virulence is an alternative approach to antimicrobial therapy that offers promising opportunities to inhibit pathogenesis and its consequences without placing immediate life-or-death pressure on the target bacterium. Certain virulence factors have been shown to be potential targets for drug design and therapeutic intervention, whereas new insights are crucial for exploiting others. Targeting virulence represents a new paradigm to empower the clinician to prevent and treat infectious diseases.

Structural characterization of the Yersinia pestis type III secretion system needle protein YscF in complex with its heterodimeric chaperone YscE/YscG

The plague-causing bacterium Yersinia pestis utilizes a type III secretion system to deliver effector proteins into mammalian cells where they interfere with signal transduction pathways that mediate phagocytosis and the inflammatory response. Effector proteins are injected through a hollow needle structure composed of the protein YscF. YscG and YscE act as "chaperones" to prevent premature polymerization of YscF in the cytosol of the bacterium prior to assembly of the needle. Here, we report the crystal structure of the YscEFG protein complex at 1.8 A resolution. Overall, the structure is similar to that of the analogous PscEFG complex from the Pseudomonas aeruginosa type III secretion system, but there are noteworthy differences. The structure confirms that, like PscG, YscG is a member of the tetratricopeptide repeat family of proteins. YscG binds tightly to the C-terminal half of YscF, implying that it is this region of YscF that controls its polymerization into the needle structure. YscE interacts with the N-terminal tetratricopeptide repeat motif of YscG but makes very little direct contact with YscF. Its function may be to stabilize the structure of YscG and/or to participate in recruiting the complex to the secretion apparatus. No electron density could be observed for the 49 N-terminal residues of YscF. This and additional evidence suggest that the N-terminus of YscF is disordered in the complex with YscE and YscG. As expected, conserved residues in the C-terminal half of YscF mediate important intra- and intermolecular interactions in the complex. Moreover, the phenotypes of some previously characterized mutations in the C-terminal half of YscF can be rationalized in terms of the structure of the heterotrimeric YscEFG complex.

Flagellar motility in bacteria structure and function of flagellar motor

Bacterial flagella are filamentous organelles that drive cell locomotion. They thrust cells in liquids (swimming) or on surfaces (swarming) so that cells can move toward favorable environments. At the base of each flagellum, a reversible rotary motor, which is powered by the proton- or the sodium-motive force, is embedded in the cell envelope. The motor consists of two parts: the rotating part, or rotor, that is connected to the hook and the filament, and the nonrotating part, or stator, that conducts coupling ion and is responsible for energy conversion. Intensive genetic and biochemical studies of the flagellum have been conducted in Salmonella typhimurium and Escherichia coli, and more than 50 gene products are known to be involved in flagellar assembly and function. The energy-coupling mechanism, however, is still not known. In this chapter, we survey our current knowledge of the flagellar system, based mostly on studies from Salmonella, E. coli, and marine species Vibrio alginolyticus, supplemented with distinct aspects of other bacterial species revealed by recent studies.

A Legionella pneumophila peptidyl-prolyl cis-trans isomerase present in culture supernatants is necessary for optimal growth at low temperatures

Several Legionella pneumophila proteins were highly expressed in low-temperature supernatants. One of these proteins was the peptidyl-prolyl isomerase PpiB. Mutants lacking ppiB exhibited reduced growth at 17 degrees C. Since PpiB lacked a signal sequence and was present in 17 degrees C supernatants of type II and type IV secretion mutants, this protein may be secreted by a novel mechanism.