N-terminal type III secretion signal of enteropathogenic Escherichia coli translocator proteins

Secretion of functional interferon by the type 3 secretion system of enteropathogenic Escherichia coli

BACKGROUND

Type I interferons (IFN-I)-a group of cytokines with immunomodulatory, antiproliferative, and antiviral properties-are widely used as therapeutics for various cancers and viral diseases. Since IFNs are proteins, they are highly susceptible to degradation by proteases and by hydrolysis in the strong acid environment of the stomach, and they are therefore administered parenterally. In this study, we examined whether the intestinal bacterium, enteropathogenic Escherichia coli (EPEC), can be exploited for oral delivery of IFN-Is. EPEC survives the harsh conditions of the stomach and, upon reaching the small intestine, expresses a type III secretion system (T3SS) that is used to translocate effector proteins across the bacterial envelope into the eukaryotic host cells.

RESULTS

In this study, we developed an attenuated EPEC strain that cannot colonize the host but can secrete functional human IFNα2 variant through the T3SS. We found that this bacteria-secreted IFN exhibited antiproliferative and antiviral activities similar to commercially available IFN.

CONCLUSION

These findings present a potential novel approach for the oral delivery of IFN via secreting bacteria.

Substrate recruitment mechanism by gram-negative type III, IV, and VI bacterial injectisomes

Bacteria use a wide arsenal of macromolecular substrates (DNA and proteins) to interact with or infect prokaryotic and eukaryotic cells. To do so, they utilize substrate-injecting secretion systems or injectisomes. However, prior to secretion, substrates must be recruited to specialized recruitment platforms and then handed over to the secretion apparatus for secretion. In this review, we provide an update on recent advances in substrate recruitment and delivery by gram-negative bacterial recruitment platforms associated with Type III, IV, and VI secretion systems.

Recruitment of heterologous substrates by bacterial secretion systems for transkingdom translocation

Bacterial secretion systems mediate the selective exchange of macromolecules between bacteria and their environment, playing a pivotal role in processes such as horizontal gene transfer or virulence. Among the different families of secretion systems, Type III, IV and VI (T3SS, T4SS and T6SS) share the ability to inject their substrates into human cells, opening up the possibility of using them as customized injectors. For this to happen, it is necessary to understand how substrates are recruited and to be able to engineer secretion signals, so that the transmembrane machineries can recognize and translocate the desired substrates in place of their own. Other factors, such as recruiting proteins, chaperones, and the degree of unfolding required to cross through the secretion channel, may also affect transport. Advances in the knowledge of the secretion mechanism have allowed heterologous substrate engineering to accomplish translocation by T3SS, and to a lesser extent, T4SS and T6SS into human cells. In the case of T4SS, transport of nucleoprotein complexes adds a bonus to its biotechnological potential. Here, we review the current knowledge on substrate recognition by these secretion systems, the many examples of heterologous substrate translocation by engineering of secretion signals, and the current and future biotechnological and biomedical applications derived from this approach.

Cryoelectron-microscopy structure of the enteropathogenic Escherichia coli type III secretion system EspA filament

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic Escherichia coli (EHEC) utilize a macromolecular type III secretion system (T3SS) to inject effector proteins into eukaryotic cells. This apparatus spans the inner and outer bacterial membranes and includes a helical needle protruding into the extracellular space. Thus far observed only in EPEC and EHEC and not found in other pathogenic Gram-negative bacteria that have a T3SS is an additional helical filament made by the EspA protein that forms a long extension to the needle, mediating both attachment to eukaryotic cells and transport of effector proteins through the intestinal mucus layer. Here, we present the structure of the EspA filament from EPEC at 3.4 Å resolution. The structure reveals that the EspA filament is a right-handed 1-start helical assembly with a conserved lumen architecture with respect to the needle to ensure the seamless transport of unfolded cargos en route to the target cell. This functional conservation is despite the fact that there is little apparent overall conservation at the level of sequence or structure with the needle. We also unveil the molecular details of the immunodominant EspA epitope that can now be exploited for the rational design of epitope display systems.

Fluorescence Detection of Type III Secretion Using a Glu-CyFur Reporter System in Citrobacter rodentium

Enteropathogenic Escherichia coli (EPEC) is a major cause of infantile diarrhea worldwide. EPEC and the closely related murine model of EPEC infection, Citrobacter rodentium, utilize a type III secretion system (T3SS) to propagate the infection. Since the T3SS is not essential for the bacteria to survive or propagate, inhibiting the virulence factor with a therapeutic would treat the infection without causing harm to commensal bacteria. Studying inhibitors of the T3SS usually requires a BSL-2 laboratory designation and eukaryotic host cells while not indicating the mechanism of inhibition. We have designed a BSL-1 assay using the murine model C. rodentium that does not require mammalian cell culture. This CPG2-reporter assay allows for more rapid analysis of secretion efficiency than Western blotting and is sensitive enough to differentiate between partial and total inhibition of the T3SS. Here we present our method and the results of a small collection of compounds we have screened, including known T3SS inhibitors EGCG, regacin, and aurodox and related quorum sensing inhibitors tannic acid and ellagic acid. We have further characterized EGCG as a T3SS inhibitor and established its IC₅₀ of 1.8 ± 0.4 μM. We also establish tannic acid as a potent inhibitor of the T3SS with an IC₅₀ of 0.65 ± 0.09 μM.

A Reporter System for Fast Quantitative Monitoring of Type 3 Protein Secretion in Enteropathogenic E. coli

The type 3 secretion system is essential for pathogenesis of several human and animal Gram-negative bacterial pathogens. The T3SS comprises a transmembrane injectisome, providing a conduit from the bacterial cytoplasm to the host cell cytoplasm for the direct delivery of effectors (including toxins). Functional studies of T3SS commonly monitor the extracellular secretion of proteins by SDS-PAGE and western blot analysis, which are slow and semi-quantitative in nature. Here, we describe an enzymatic reporter-based quantitative and rapid in vivo assay for T3SS secretion studies in enteropathogenic E. coli (EPEC). The assay monitors the secretion of the fusion protein SctA-PhoA through the injectisome based on a colorimetric assay that quantifies the activity of alkaline phosphatase. We validated the usage of this reporter system by following the secretion in the absence of various injectisome components, including domains of the gatekeeper essential for T3SS function. This platform can now be used for the isolation of mutations, functional analysis and anti-virulence compound screening.

Mitochondrial Targeting of the Enteropathogenic Escherichia coli Map Triggers Calcium Mobilization, ADAM10-MAP Kinase Signaling, and Host Cell Apoptosis

Enteropathogenic E. coli (EPEC) is an important human diarrhea-causing bacterium. The pathogenic effects of EPEC largely depend upon its ability to inject a series of proteins, termed effectors, into the host cells. One such effector is the mitochondrion-associated protein (Map). Map has been shown to induce actin-rich projections (i.e., filopodia) on the infected cell surface and activate a Rho GTPase enzyme termed Cdc42. Nonetheless, although most injected Map localizes to host mitochondria, its functions in the mitochondria remain unknown. Here, we show that Map targeting of mitochondria stimulates the disruption of mitochondrial membrane potential to induce Ca²⁺ efflux into the host cytoplasm. The efflux stimulates the activity of a protein termed ADAM10, which induces activation of a mitogen-activated protein kinase cascade leading to host cell apoptosis. As apoptosis plays a central role in host-pathogen interactions, our findings provide novel insights into the functions of mitochondrial Map in promoting the EPEC disease.

The ability of diarrheagenic bacterial pathogens, such as enteropathogenic Escherichia coli (EPEC), to modulate the activity of mitogen-activated protein kinases (MAPKs) and cell survival has been suggested to benefit bacterial colonization and infection. However, our understanding of the mechanisms by which EPEC modulate these functions is incomplete. In this study, we show that the EPEC type III secreted effector Map stimulates the sheddase activity of the disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) and the ERK and p38 MAPK signaling cascades. Remarkably, all these activities were dependent upon the ability of Map to target host mitochondria, mainly via its mitochondrial toxicity region (MTR). Map targeting of mitochondria disrupted the mitochondrial membrane potential, causing extrusion of mitochondrial Ca²⁺ into the host cell cytoplasm. We also found that Map targeting of mitochondria is essential for triggering host cell apoptosis. Based on these findings, we propose a model whereby Map imported into mitochondria causes mitochondrial dysfunction and Ca²⁺ efflux into the host cytoplasm. Since Ca²⁺ has been reported to promote ADAM10 activation, the acute elevation of Ca²⁺ in the cytoplasm may stimulate the ADAM10 sheddase activity, resulting in the release of epidermal growth factors that stimulate the ERK signaling cascade. As p38 activity is also Ca²⁺ sensitive, elevation in cytoplasmic Ca²⁺ may independently also activate p38. We hypothesize that Map-dependent MAPK activation, combined with Map-mediated mitochondrial dysfunction, evokes mitochondrial host cell apoptosis, potentially contributing to EPEC colonization and infection of the gut.

Delivery of Heterologous Proteins, Enzymes, and Antigens via the Bacterial Type III Secretion System

The Type III Secretion System (T3SS) is a multimeric protein complex composed of over 20 different proteins, utilized by Gram-negative bacteria to infect eukaryotic host cells. The T3SS has been implicated as a virulence factor by which pathogens cause infection and has recently been characterized as a communication tool between bacteria and plant cells in the rhizosphere. The T3SS has been repurposed to be used as a tool for the delivery of non-native or heterologous proteins to eukaryotic cells or the extracellular space for a variety of purposes, including drug discovery and drug delivery. This review covers the methodology of heterologous protein secretion as well as multiple cases of utilizing the T3SS to deliver heterologous proteins or artificial materials. The research covered in this review will serve to outline the scope and limitations of utilizing the T3SS as a tool for protein delivery.

Natural Product Type III Secretion System Inhibitors

Many known inhibitors of the bacterial type III secretion system (T3SS), a virulence factor used by pathogenic bacteria to infect host cells, are natural products. These compounds, produced by bacteria, fungi, and plants, may have developed as prophylactic treatments for potential attack by bacterial pathogens or as an attempt by symbiotic organisms to protect their hosts. Regardless, better understanding of the structures and mechanisms of action of these compounds may open opportunities for drug development against diseases caused by pathogens utilizing the T3SS. This review will cover selected known natural products of the T3SS and detail what is known of their origin and mechanism of action. These inhibitors highlight nature’s ability to modulate interactions between organisms at a cellular level.

Transposon-insertion sequencing screens unveil requirements for EHEC growth and intestinal colonization

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is an important food-borne pathogen that colonizes the colon. Transposon-insertion sequencing (TIS) was used to identify genes required for EHEC and E. coli K-12 growth in vitro and for EHEC growth in vivo in the infant rabbit colon. Surprisingly, many conserved loci contribute to EHEC's but not to K-12's growth in vitro. There was a restrictive bottleneck for EHEC colonization of the rabbit colon, which complicated identification of EHEC genes facilitating growth in vivo. Both a refined version of an existing analytic framework as well as PCA-based analysis were used to compensate for the effects of the infection bottleneck. These analyses confirmed that the EHEC LEE-encoded type III secretion apparatus is required for growth in vivo and revealed that only a few effectors are critical for in vivo fitness. Over 200 mutants not previously associated with EHEC survival/growth in vivo also appeared attenuated in vivo, and a subset of these putative in vivo fitness factors were validated. Some were found to contribute to efficient type-three secretion while others, including tatABC, oxyR, envC, acrAB, and cvpA, promote EHEC resistance to host-derived stresses. cvpA is also required for intestinal growth of several other enteric pathogens, and proved to be required for EHEC, Vibrio cholerae and Vibrio parahaemolyticus resistance to the bile salt deoxycholate, highlighting the important role of this previously uncharacterized protein in pathogen survival. Collectively, our findings provide a comprehensive framework for understanding EHEC growth in the intestine.

Genome-wide prediction of bacterial effector candidates across six secretion system types using a feature-based statistical framework

Gram-negative bacteria are responsible for hundreds of millions infections worldwide, including the emerging hospital-acquired infections and neglected tropical diseases in the third-world countries. Finding a fast and cheap way to understand the molecular mechanisms behind the bacterial infections is critical for efficient diagnostics and treatment. An important step towards understanding these mechanisms is the discovery of bacterial effectors, the proteins secreted into the host through one of the six common secretion system types. Unfortunately, current prediction methods are designed to specifically target one of three secretion systems, and no accurate "secretion system-agnostic" method is available. Here, we present PREFFECTOR, a computational feature-based approach to discover effector candidates in Gram-negative bacteria, without prior knowledge on bacterial secretion system(s) or cryptic secretion signals. Our approach was first evaluated using several assessment protocols on a manually curated, balanced dataset of experimentally determined effectors across all six secretion systems, as well as non-effector proteins. The evaluation revealed high accuracy of the top performing classifiers in PREFFECTOR, with the small false positive discovery rate across all six secretion systems. Our method was also applied to six bacteria that had limited knowledge on virulence factors or secreted effectors. PREFFECTOR web-server is freely available at: http://korkinlab.org/preffector .

Comparative genome and evolution analysis of the locus of enterocyte effacement from enteropathogenic Escherichia coli Deng and its transcriptional response to ciprofloxacin

PURPOSE

In this study, we aimed to investigate the genomic characteristics and evolution of pathogenicity islands of an enteropathogenic Escherichia coli (EPEC) strain, and to obtain a transcriptional profile of EPEC under different concentrations of ciprofloxacin using microarray analysis.

METHODOLOGY

The complete EPEC Deng genome was sequenced and compared to genomes of 12 previously sequenced E. coli strains. A 180 min time course experiment was performed in which the effect of ciprofloxacin on EPEC Deng growth was evaluated. Microarray profiling was used to study the effect of varying ciprofloxacin pressure on genome-wide transcriptional expression. Differential expression of the genes identified using microarray data was confirmed using real-time quantitative reverse transcriptase PCR (RTQ). Target gene-defective recombineering strains were created to investigate the influence of the grlA gene on ciprofloxacin susceptibility.

RESULTS

Genomic comparisons revealed a close phylogenic relationship between EPEC Deng and E. coli strains O111_H_11128 and O26_H11_11368, with low genetic diversity among their type III secretion system genes and typically genetic variation in the map, tir, eae and espA genes of EPEC. It is noteworthy that 21 genes were down-regulated at all time points examined in the group exposed to 2 µg ml^-1 of ciprofloxacin. A grlA-mutant derivative with increased susceptibility to ciprofloxacin was discovered.

CONCLUSIONS

The present findings provide an overview of the phylogenetic characteristics of EPEC Deng and its transcriptional response to ciprofloxacin, further suggesting that GrlA may play a clinically important role in EPEC responses to ciprofloxacin.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

YopN and TyeA Hydrophobic Contacts Required for Regulating Ysc-Yop Type III Secretion Activity by Yersinia pseudotuberculosis

Yersinia bacteria target Yop effector toxins to the interior of host immune cells by the Ysc-Yop type III secretion system. A YopN-TyeA heterodimer is central to controlling Ysc-Yop targeting activity. A + 1 frameshift event in the 3-prime end of yopN can also produce a singular secreted YopN-TyeA polypeptide that retains some regulatory function even though the C-terminal coding sequence of this YopN differs greatly from wild type. Thus, this YopN C-terminal segment was analyzed for its role in type III secretion control. Bacteria producing YopN truncated after residue 278, or with altered sequence between residues 279 and 287, had lost type III secretion control and function. In contrast, YopN variants with manipulated sequence beyond residue 287 maintained full control and function. Scrutiny of the YopN-TyeA complex structure revealed that residue W₂₇₉ functioned as a likely hydrophobic contact site with TyeA. Indeed, a YopN_W279G mutant lost all ability to bind TyeA. The TyeA residue F₈ was also critical for reciprocal YopN binding. Thus, we conclude that specific hydrophobic contacts between opposing YopN and TyeA termini establishes a complex needed for regulating Ysc-Yop activity.

Type III-Dependent Translocation of HrpB2 by a Nonpathogenic hpaABC Mutant of the Plant-Pathogenic Bacterium Xanthomonas campestris pv. vesicatoria

The plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to translocate effector proteins into plant cells. The T3S apparatus spans both bacterial membranes and is associated with an extracellular pilus and a channel-like translocon in the host plasma membrane. T3S is controlled by the switch protein HpaC, which suppresses secretion and translocation of the predicted inner rod protein HrpB2 and promotes secretion of translocon and effector proteins. We previously reported that HrpB2 interacts with HpaC and the cytoplasmic domain of the inner membrane protein HrcU (C. Lorenz, S. Schulz, T. Wolsch, O. Rossier, U. Bonas, and D. Büttner, PLoS Pathog 4:e1000094, 2008, http://dx.doi.org/10.1371/journal.ppat.1000094). However, the molecular mechanisms underlying the control of HrpB2 secretion are not yet understood. Here, we located a T3S and translocation signal in the N-terminal 40 amino acids of HrpB2. The results of complementation experiments with HrpB2 deletion derivatives revealed that the T3S signal of HrpB2 is essential for protein function. Furthermore, interaction studies showed that the N-terminal region of HrpB2 interacts with the cytoplasmic domain of HrcU, suggesting that the T3S signal of HrpB2 contributes to substrate docking. Translocation of HrpB2 is suppressed not only by HpaC but also by the T3S chaperone HpaB and its secreted regulator, HpaA. Deletion of hpaA, hpaB, and hpaC leads to a loss of pathogenicity but allows the translocation of fusion proteins between the HrpB2 T3S signal and effector proteins into leaves of host and non-host plants.

IMPORTANCE

The T3S system of the plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria is essential for pathogenicity and delivers effector proteins into plant cells. T3S depends on HrpB2, which is a component of the predicted periplasmic inner rod structure of the secretion apparatus. HrpB2 is secreted during the early stages of the secretion process and interacts with the cytoplasmic domain of the inner membrane protein HrcU. Here, we localized the secretion and translocation signal of HrpB2 in the N-terminal 40 amino acids and show that this region is sufficient for the interaction with the cytoplasmic domain of HrcU. Our results suggest that the T3S signal of HrpB2 is required for the docking of HrpB2 to the secretion apparatus. Furthermore, we provide experimental evidence that the N-terminal region of HrpB2 is sufficient to target effector proteins for translocation in a nonpathogenic X. campestris pv. vesicatoria strain.

Engineering the Controlled Assembly of Filamentous Injectisomes in E. coli K-12 for Protein Translocation into Mammalian Cells

Bacterial pathogens containing type III protein secretion systems (T3SS) assemble large needle-like protein complexes in the bacterial envelope, called injectisomes, for translocation of protein effectors into host cells. The application of these “molecular syringes” for the injection of proteins into mammalian cells is hindered by their structural and genomic complexity, requiring multiple polypeptides encoded along with effectors in various transcriptional units (TUs) with intricate regulation. In this work, we have rationally designed the controlled expression of the filamentous injectisomes found in enteropathogenic Escherichia coli (EPEC) in the nonpathogenic strain E. coli K-12. All structural components of EPEC injectisomes, encoded in a genomic island called the locus of enterocyte effacement (LEE), were engineered in five TUs (eLEEs) excluding effectors, promoters and transcriptional regulators. These eLEEs were placed under the control of the IPTG-inducible promoter Ptac and integrated into specific chromosomal sites of E. coli K-12 using a marker-less strategy. The resulting strain, named synthetic injector E. coli (SIEC), assembles filamentous injectisomes similar to those in EPEC. SIEC injectisomes form pores in the host plasma membrane and are able to translocate T3-substrate proteins (e.g., translocated intimin receptor, Tir) into the cytoplasm of HeLa cells reproducing the phenotypes of intimate attachment and polymerization of actin-pedestals elicited by EPEC bacteria. Hence, SIEC strain allows the controlled expression of functional filamentous injectisomes for efficient translocation of proteins with T3S-signals into mammalian cells.

SepD/SepL-dependent secretion signals of the type III secretion system translocator proteins in enteropathogenic Escherichia coli

The type III protein secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE) is essential for the pathogenesis of attaching/effacing bacterial pathogens, including enteropathogenic Escherichia coli (EPEC), enterohemorrhagic E. coli (EHEC), and Citrobacter rodentium. These pathogens use the T3SS to sequentially secrete three categories of proteins: the T3SS needle and inner rod protein components; the EspA, EspB, and EspD translocators; and many LEE- and non-LEE-encoded effectors. SepD and SepL are essential for translocator secretion, and mutations in either lead to hypersecretion of effectors. However, how SepD and SepL control translocator secretion and secretion hierarchy between translocators and effectors is poorly understood. In this report, we show that the secreted T3SS components, the translocators, and both LEE- and non-LEE-encoded effectors all carry N-terminal type III secretion and translocation signals. These signals all behave like those of the effectors and are sufficient for mediating type III secretion and translocation by wild-type EPEC and hypersecretion by the sepD and sepL mutants. Our results extended previous observations and suggest that the secretion hierarchy of the different substrates is determined by a signal other than the N-terminal secretion signal. We identified a domain located immediately downstream of the N-terminal secretion signal in the translocator EspB that is required for SepD/SepL-dependent secretion. We further demonstrated that this EspB domain confers SepD/SepL- and CesAB-dependent secretion on the secretion signal of effector EspZ. Our results thus suggest that SepD and SepL control and regulate secretion hierarchy between translocators and effectors by recognizing translocator-specific export signals.

IMPORTANCE

Many bacterial pathogens use a syringe-like protein secretion apparatus, termed the type III protein secretion system (T3SS), to secrete and inject numerous proteins directly into the host cells to cause disease. The secreted proteins perform different functions at various stages during infection and are classified into three substrate categories (T3SS components, translocators, and effectors). They all contain secretion signals at their N termini, but how their secretion hierarchy is determined is poorly understood. Here, we show that the N-terminal secretion signals from different substrate categories all behave the same and do not confer substrate specificity. We further characterize the secretion signals of the translocators and identify a translocator-specific signal, demonstrating that substrate-specific secretion signals are required in regulating T3SS substrate hierarchy.

Enterohaemorrhagic Escherichia coli inhibits recycling endosome function and trafficking of surface receptors

Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC/EHEC) manipulate many cell processes by injecting effector proteins from the bacteria into the host cell via a Type III secretion system. In this paper we report that the effector protein EspG disrupts recycling endosome function. In particular, we found that following transferrin binding and endocytosis EspG reduces recycling of the transferrin receptor (TfR), the prototypical recycling protein, from an intracellular location to the cell surface, resulting in an accumulation of TfR within the cell. The surface levels of three receptors [TfR, epidermal growth factor receptor (EGFR) and β1 integrin] were tested and found to be reduced dependent on EspG translocation. Furthermore, disruption of recycling endosome function and the reduced surface presentation of receptors was dependent on the previously reported RabGAP activity and ARF binding ability of EspG. This paper therefore supports the previous hypothesis that EspG acts as an enzyme scaffold perturbing cell signalling events, in this case altering recycling endosome function and cell surface receptor levels during infection.

Substrate-activated conformational switch on chaperones encodes a targeting signal in type III secretion

The targeting of type III secretion (TTS) proteins at the injectisome is an important process in bacterial virulence. Nevertheless, how the injectisome specifically recognizes TTS substrates among all bacterial proteins is unknown. A TTS peripheral membrane ATPase protein located at the base of the injectisome has been implicated in the targeting process. We have investigated the targeting of the EspA filament protein and its cognate chaperone, CesAB, to the EscN ATPase of the enteropathogenic E. coli (EPEC). We show that EscN selectively engages the EspA-loaded CesAB but not the unliganded CesAB. Structure analysis revealed that the targeting signal is encoded in a disorder-order structural transition in CesAB that is elicited only upon the binding of its physiological substrate, EspA. Abrogation of the interaction between the CesAB-EspA complex and EscN resulted in severe secretion and infection defects. Additionally, we show that the targeting and secretion signals are distinct and that the two processes are likely regulated by different mechanisms.

Identification of novel type III secretion chaperone-substrate complexes of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterial pathogen of humans that uses a type III secretion (T3S) system to manipulate host cells through the delivery of effector proteins into their cytosol and membranes. The function of T3S systems depends on small bacterial cytosolic chaperone-like proteins, which bind T3S substrates and ensure their appropriate secretion. To find novel T3S chaperone-substrate complexes of C. trachomatis we first searched its genome for genes encoding proteins with features of T3S chaperones. We then systematically tested for interactions between candidate chaperones and chlamydial T3S substrates by bacterial two-hybrid. This revealed interactions between Slc1 (a known T3S chaperone) or CT584 and several T3S substrates. Co-immunoprecipation after protein expression in Yersinia enterocolitica and protein overlay binding assays indicated that Slc1 interacted with the N-terminal region of the known T3S substrates Tarp (a previously described substrate of Slc1), CT694, and CT695, and that CT584 interacted with a central region of CT082, which we identified as a C. trachomatis T3S substrate using Y. enterocolitica as a heterologous system. Further T3S assays in Yersinia indicated that Slc1 or CT584 increased the amount of secreted Tarp, CT694, and CT695, or CT082, respectively. Expression of CT584 increased the intra-bacterial stability of CT082, while Slc1 did not affect the stability of its substrates. Overall, this indicated that in C. trachomatis Slc1 is a chaperone of multiple T3S substrates and that CT584 is a chaperone of the newly identified T3S substrate CT082.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

EspZ of enteropathogenic and enterohemorrhagic Escherichia coli regulates type III secretion system protein translocation

Translocation of effector proteins via a type III secretion system (T3SS) is a widespread infection strategy among Gram-negative bacterial pathogens. Each pathogen translocates a particular set of effectors that subvert cell signaling in a way that suits its particular infection cycle. However, as effector unbalance might lead to cytotoxicity, the pathogens must employ mechanisms that regulate the intracellular effector concentration. We present evidence that the effector EspZ controls T3SS effector translocation from enteropathogenic (EPEC) and enterohemorrhagic (EHEC) Escherichia coli. Consistently, an EPEC espZ mutant is highly cytotoxic. Following ectopic expression, we found that EspZ inhibited the formation of actin pedestals as it blocked the translocation of Tir, as well as other effectors, including Map and EspF. Moreover, during infection EspZ inhibited effector translocation following superinfection. Importantly, while EspZ of EHEC O157:H7 had a universal “translocation stop” activity, EspZ of EPEC inhibited effector translocation from typical EPEC strains but not from EHEC O157:H7 or its progenitor, atypical EPEC O55:H7. We found that the N and C termini of EspZ, which contains two transmembrane domains, face the cytosolic leaflet of the plasma membrane at the site of bacterial attachment, while the extracellular loop of EspZ is responsible for its strain-specific activity. These results show that EPEC and EHEC acquired a sophisticated mechanism to regulate the effector translocation.

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC) are important diarrheal pathogens responsible for significant morbidity and mortality in developing countries and the developed world, respectively. The virulence strategy of EPEC and EHEC revolves around a conserved type III secretion system (T3SS), which translocates bacterial proteins known as effectors directly into host cells. Previous studies have shown that when cells are infected in two waves with EPEC, the first wave inhibits effector translocation by the second wave in a T3SS-dependent manner, although the factor involved was not known. Importantly, we identified EspZ as the effector responsible for blocking protein translocation following a secondary EPEC infection. Interestingly, we found that while EspZ of EHEC can block protein translocation from both EPEC and EHEC strains, EPEC EspZ cannot block translocation from EHEC. These studies show that EPEC and EHEC employ a novel infection strategy to regulate T3SS translocation.

Recruitment and membrane interactions of host cell proteins during attachment of enteropathogenic and enterohaemorrhagic Escherichia coli

EPEC (enteropathogenic Escherichia coli) and EHEC (enterohaemorrhagic Escherichia coli) are attaching and effacing pathogens frequently associated with infectious diarrhoea. EPEC and EHEC use a T3SS (type III secretion system) to translocate effectors that subvert different cellular processes to sustain colonization and multiplication. The eukaryotic proteins NHERF2 (Na(+)/H(+) exchanger regulatory factor 2) and AnxA2 (annexin A2), which are involved in regulation of intestinal ion channels, are recruited to the bacterial attachment sites. Using a stable HeLa-NHERF2 cell line, we found partial co-localization of AnxA2 and NHERF2; in EPEC-infected cells, AnxA2 and NHERF2 were extensively recruited to the site of bacterial attachment. We confirmed that NHERF2 dimerizes and found that NHERF2 interacts with AnxA2. Moreover, we found that AnxA2 also binds both the N- and C-terminal domains of the bacterial effector Tir through its C-terminal domain. Immunofluorescence of HeLa cells infected with EPEC showed that AnxA2 is recruited to the site of bacterial attachment in a Tir-dependent manner, but independently of Tir-induced actin polymerization. Our results suggest that AnxA2 and NHERF2 form a scaffold complex that links adjacent Tir molecules at the plasma membrane forming a lattice that could be involved in retention and dissemination of other effectors at the bacterial attachment site.

Impact of the N-terminal secretor domain on YopD translocator function in Yersinia pseudotuberculosis type III secretion

Type III secretion systems (T3SSs) secrete needle components, pore-forming translocators, and the translocated effectors. In part, effector recognition by a T3SS involves their N-terminal amino acids and their 5' mRNA. To investigate whether similar molecular constraints influence translocator secretion, we scrutinized this region within YopD from Yersinia pseudotuberculosis. Mutations in the 5' end of yopD that resulted in specific disruption of the mRNA sequence did not affect YopD secretion. On the other hand, a few mutations affecting the protein sequence reduced secretion. Translational reporter fusions identified the first five codons as a minimal N-terminal secretion signal and also indicated that the YopD N terminus might be important for yopD translation control. Hybrid proteins in which the N terminus of YopD was exchanged with the equivalent region of the YopE effector or the YopB translocator were also constructed. While the in vitro secretion profile was unaltered, these modified bacteria were all compromised with respect to T3SS activity in the presence of immune cells. Thus, the YopD N terminus does harbor a secretion signal that may also incorporate mechanisms of yopD translation control. This signal tolerates a high degree of variation while still maintaining secretion competence suggestive of inherent structural peculiarities that make it distinct from secretion signals of other T3SS substrates.

Enteropathogenic and enterohemorrhagic Escherichia coli type III secretion effector EspV induces radical morphological changes in eukaryotic cells

Enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic Escherichia coli (EHEC) are important human pathogens that rely on translocation of type III secretion system (T3SS) effectors for subversion of signal transduction pathways and colonization of the mammalian gut mucosa. While a core set of effectors is conserved between EPEC and EHEC strains, a growing number of accessory effectors that were found at various frequencies in clinical and environmental isolates have been recently identified. Recent genome projects identified espV as a pseudogene in EHEC but a putative functional gene in EPEC strains E110019 and E22 and the closely related mouse pathogen Citrobacter rodentium. The aim of this study was to determine the distribution of espV among clinical EPEC and EHEC strains and to investigate its function and role in pathogenesis. espV was found in 16% of the tested strains. While deletion of espV from C. rodentium did not affect colonization dynamics or fitness in mixed infections, expression of EspV in mammalian cells led to drastic morphological alterations, which were characterized by nuclear condensation, cell rounding, and formation of dendrite-like projections. Expression of EspV in yeast resulted in a dramatic increase in cell size and irreversible growth arrest. Although the role of EspV in infection and its target host cell protein(s) require further investigation, the data point to a novel mechanism by which the T3SS subverts cell signaling.

Direct injection of functional single-domain antibodies from E. coli into human cells

Intracellular proteins have a great potential as targets for therapeutic antibodies (Abs) but the plasma membrane prevents access to these antigens. Ab fragments and IgGs are selected and engineered in E. coli and this microorganism may be also an ideal vector for their intracellular delivery. In this work we demonstrate that single-domain Ab (sdAbs) can be engineered to be injected into human cells by E. coli bacteria carrying molecular syringes assembled by a type III protein secretion system (T3SS). The injected sdAbs accumulate in the cytoplasm of HeLa cells at levels ca. 10⁵–10⁶ molecules per cell and their functionality is shown by the isolation of sdAb-antigen complexes. Injection of sdAbs does not require bacterial invasion or the transfer of genetic material. These results are proof-of-principle for the capacity of E. coli bacteria to directly deliver intracellular sdAbs (intrabodies) into human cells for analytical and therapeutic purposes.