Yersinia enterocolitica type III secretion depends on the proton motive force but not on the flagellar motor components MotA and MotB

Quantifying Substrate Protein Secretion via the Type III Secretion System of the Bacterial Flagellum

Protein transport across the cytoplasmic membrane is coupled to energy derived from ATP hydrolysis or the proton motive force. A sophisticated, multi-component type III secretion system (T3SS) exports substrate proteins of both the bacterial flagellum and virulence-associated injectisome system of many Gram-negative pathogens. The T3SS is primarily a proton motive force-driven protein exporter. Here, we describe a method to investigate the export of substrate proteins of the flagellar T3SS into the culture supernatant under conditions that manipulate the proton motive force. Further, we describe methods to precisely quantify flagellar protein export into the culture supernatant using a split NanoLuc luciferase, and how fluorescence labeling of the extracellular flagellar filament can bring insights into the protein export rate of individual flagellar T3SS.

Monitoring Effector Translocation with the TEM-1 Beta-Lactamase Reporter System: From Endpoint to Time Course Analysis

Among the bacterial secretion systems, the Type III, IV, and VI secretion systems enable bacteria to secrete proteins directly into a target cell. This specific form of secretion, referred to as "translocation", is essential for a number of pathogens to alter and/or kill the targeted cell. The translocated proteins, called effector proteins, can directly interfere with the normal processes of the targeted cell, preventing elimination of the pathogen and promoting its multiplication. The function of the effector proteins varies greatly depending on the considered pathogen and the targeted cell. In addition, there is often no magic bullet and the number of effector proteins can range from a handful to hundreds, with, for instance, over 300 effector proteins substrate of the Icm/Dot Type IV secretion system in the human pathogen Legionella pneumophila. Identifying, detecting, and monitoring the translocation of each of the effector proteins represent an active field or research and are key to understanding the bacterial molecular weaponry. Translational fusion of the effector with a reporter protein of known activity remains the best method to monitor effector translocation. The development of a fluorescent substrate for the TEM-1 beta-lactamase has turned this antibiotic-resistance protein into a highly versatile reporter system to investigate protein transfer events associated with microbial infection of host cells. We here described a simple protocol to assay translocation of an effector protein by the Icm/Dot system of the human pathogen Legionella pneumophila. Taking advantage that the protonophore CCCP inhibits the secretion activity, this simple protocol can be derived into a time course analysis to follow the kinetic of effector translocation into target cells.

Primary architecture and energy requirements of Type III and Type IV secretion systems

Many pathogens use Type III and Type IV protein secretion systems to secrete virulence factors from the bacterial cytosol into host cells. These systems operate through a one-step mechanism. The secreted substrates (protein or nucleo-protein complexes in the case of Type IV conjugative systems) are guided to the base of the secretion channel, where they are directly delivered into the host cell in an ATP-dependent unfolded state. Despite the numerous disparities between these secretion systems, here we have focused on the structural and functional similarities between both systems. In particular, on the structural similarity shared by one of the main ATPases (EscN and VirD4 in Type III and Type IV secretion systems, respectively). Interestingly, these ATPases also exhibit a structural resemblance to F1-ATPases, which suggests a common mechanism for substrate secretion. The correlation between structure and function of essential components in both systems can provide significant insights into the molecular mechanisms involved. This approach is of great interest in the pursuit of identifying inhibitors that can effectively target these systems.

Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome

Two of the most fascinating bacterial nanomachines-the broadly disseminated rotary flagellum at the heart of cellular motility and the eukaryotic cell-puncturing injectisome essential to specific pathogenic species-utilize at their core a conserved export machinery called the type III secretion system (T3SS). The T3SS not only secretes the components that self-assemble into their extracellular appendages but also, in the case of the injectisome, subsequently directly translocates modulating effector proteins from the bacterial cell into the infected host. The injectisome is thought to have evolved from the flagellum as a minimal secretory system lacking motility, with the subsequent acquisition of additional components tailored to its specialized role in manipulating eukaryotic hosts for pathogenic advantage. Both nanomachines have long been the focus of intense interest, but advances in structural and functional understanding have taken a significant step forward since 2015, facilitated by the revolutionary advances in cryo-electron microscopy technologies. With several seminal structures of each nanomachine now captured, we review here the molecular similarities and differences that underlie their diverse functions.

SPI-1 virulence gene expression modulates motility of Salmonella Typhimurium in a proton motive force- and adhesins-dependent manner

Both the bacterial flagellum and the evolutionary related injectisome encoded on the Salmonella pathogenicity island 1 (SPI-1) play crucial roles during the infection cycle of Salmonella species. The interplay of both is highlighted by the complex cross-regulation that includes transcriptional control of the flagellar master regulatory operon flhDC by HilD, the master regulator of SPI-1 gene expression. Contrary to the HilD-dependent activation of flagellar gene expression, we report here that activation of HilD resulted in a dramatic loss of motility, which was dependent on the presence of SPI-1. Single cell analyses revealed that HilD-activation triggers a SPI-1-dependent induction of the stringent response and a substantial decrease in proton motive force (PMF), while flagellation remains unaffected. We further found that HilD activation enhances the adhesion of Salmonella to epithelial cells. A transcriptome analysis revealed a simultaneous upregulation of several adhesin systems, which, when overproduced, phenocopied the HilD-induced motility defect. We propose a model where the SPI-1-dependent depletion of the PMF and the upregulation of adhesins upon HilD-activation enable flagellated Salmonella to rapidly modulate their motility during infection, thereby enabling efficient adhesion to host cells and delivery of effector proteins.

Delineating specific regions of N- terminal domain of T3SS ATPase YsaN of Yersinia enterocolitica governing its different oligomerization states

Oligomerization of YsaN, a putative T3SS-ATPase is a necessary and crucial event for T3SS functioning in Y. enterocolitica. Different oligomeric states have been proposed for similar ATPases, yet, the true nature of its activation and formation of different oligomers is still poorly understood. In-vitro studies of YsaN reveal that its activation and oligomerization depend on its N-terminal region and occur as a result of active catalysis of ATP in an ATP concentration-dependent manner following two-step cooperative kinetics. Also, the N-terminal 83 amino acid residues of YsaN are crucial for higher-order oligomer formation while YsaN∆83 is capable of hexamer formation upon oligomerization. Enzyme kinetics study shows reduced ATPase activity of YsaN∆83 (3.19 ± 0.09 μmol/min/mg) in comparison to YsaN (9.076 ± 0.72 μmol/min/mg). Negative-TEM study of YsaN and YsaN∆83 oligomer suggests that the formation of higher-order oligomer (probably dodecamer) occurs by stacking of two hexamers through their N-terminal faces involving N-terminal 83 amino acid residues which have been further supported by the docking of two hexamers during the in-silico study. These results suggest that YsaN is an oligomerization-activated T3SS ATPase, where distinct regions of its N-terminal domain regulate its different oligomeric nature and is essential for its activation.

Listeria monocytogenes requires cellular respiration for NAD+ regeneration and pathogenesis

Cellular respiration is essential for multiple bacterial pathogens and a validated antibiotic target. In addition to driving oxidative phosphorylation, bacterial respiration has a variety of ancillary functions that obscure its contribution to pathogenesis. We find here that the intracellular pathogen Listeria monocytogenes encodes two respiratory pathways which are partially functionally redundant and indispensable for pathogenesis. Loss of respiration decreased NAD⁺ regeneration, but this could be specifically reversed by heterologous expression of a water-forming NADH oxidase (NOX). NOX expression fully rescued intracellular growth defects and increased L. monocytogenes loads >1000-fold in a mouse infection model. Consistent with NAD⁺ regeneration maintaining L. monocytogenes viability and enabling immune evasion, a respiration-deficient strain exhibited elevated bacteriolysis within the host cytosol and NOX expression rescued this phenotype. These studies show that NAD⁺ regeneration represents a major role of L. monocytogenes respiration and highlight the nuanced relationship between bacterial metabolism, physiology, and pathogenesis.

Cellular respiration is one of the main ways organisms make energy. It works by linking the oxidation of an electron donor (like sugar) to the reduction of an electron acceptor (like oxygen). Electrons pass between the two molecules along what is known as an ‘electron transport chain’. This process generates a force that powers the production of adenosine triphosphate (ATP), a molecule that cells use to store energy.

Respiration is a common way for cells to replenish their energy stores, but it is not the only way. A simpler process that does not require a separate electron acceptor or an electron transport chain is called fermentation. Many bacteria have the capacity to perform both respiration and fermentation and do so in a context-dependent manner.

Research has shown that respiration can contribute to bacterial diseases, like tuberculosis and listeriosis (a disease caused by the foodborne pathogen Listeria monocytogenes). Indeed, some antibiotics even target bacterial respiration. Despite being often discussed in the context of generating ATP, respiration is also important for many other cellular processes, including maintaining the balance of reduced and oxidized nicotinamide adenine dinucleotide (NAD) cofactors. Because of these multiple functions, the exact role respiration plays in disease is unknown.

To find out more, Rivera-Lugo, Deng et al. developed strains of the bacterial pathogen Listeria monocytogenes that lacked some of the genes used in respiration. The resulting bacteria were still able to produce energy, but they became much worse at infecting mammalian cells. The use of a genetic tool that restored the balance of reduced and oxidized NAD cofactors revived the ability of respiration-deficient L. monocytogenes to infect mammalian cells, indicating that this balance is what the bacterium requires to infect.

Research into respiration tends to focus on its role in generating ATP. But these results show that for some bacteria, this might not be the most important part of the process. Understanding the other roles of respiration could change the way that researchers develop antibacterial drugs in the future. This in turn could help with the growing problem of antibiotic resistance.

Galectin-1 Cooperates with Yersinia Outer Protein (Yop) P to Thwart Protective Immunity by Repressing Nitric Oxide Production

Yersinia enterocolitica (Ye) inserts outer proteins (Yops) into cytoplasm to infect host cells. However, in spite of considerable progress, the mechanisms implicated in this process, including the association of Yops with host proteins, remain unclear. Here, we evaluated the functional role of Galectin-1 (Gal1), an endogenous β-galactoside-binding protein, in modulating Yop interactions with host cells. Our results showed that Gal1 binds to Yops in a carbohydrate-dependent manner. Interestingly, Gal1 binding to Yops protects these virulence factors from trypsin digestion. Given that early control of Ye infection involves activation of macrophages, we evaluated the role of Gal1 and YopP in the modulation of macrophage function. Although Gal1 and YopP did not influence production of superoxide anion and/or TNF by Ye-infected macrophages, they coordinately inhibited nitric oxide (NO) production. Notably, recombinant Gal1 (rGal1) did not rescue NO increase observed in Lgals1-/- macrophages infected with the YopP mutant Ye ∆yopP. Whereas NO induced apoptosis in macrophages, no significant differences in cell death were detected between Gal1-deficient macrophages infected with Ye ∆yopP, and WT macrophages infected with Ye wt. Strikingly, increased NO production was found in WT macrophages treated with MAPK inhibitors and infected with Ye wt. Finally, rGal1 administration did not reverse the protective effect in Peyer Patches (PPs) of Lgals1-/- mice infected with Ye ∆yopP. Our study reveals a cooperative role of YopP and endogenous Gal1 during Ye infection.

Genome Scale Analysis Reveals IscR Directly and Indirectly Regulates Virulence Factor Genes in Pathogenic Yersinia

The iron-sulfur cluster coordinating transcription factor IscR is important for the virulence of Yersinia pseudotuberculosis and a number of other bacterial pathogens. However, the IscR regulon has not yet been defined in any organism. To determine the Yersinia IscR regulon and identify IscR-dependent functions important for virulence, we employed chromatin immunoprecipitation sequencing (ChIP-Seq) and RNA sequencing (RNA-Seq) of Y. pseudotuberculosis expressing or lacking iscR following iron starvation conditions, such as those encountered during infection. We found that IscR binds to the promoters of genes involved in iron homeostasis, reactive oxygen species metabolism, and cell envelope remodeling and regulates expression of these genes in response to iron depletion. Consistent with our previous work, we also found that IscR binds in vivo to the promoter of the Ysc type III secretion system (T3SS) master regulator LcrF, leading to regulation of T3SS genes. Interestingly, comparative genomic analysis suggested over 93% of IscR binding sites were conserved between Y. pseudotuberculosis and the related plague agent Yersinia pestis. Surprisingly, we found that the IscR positively regulated sufABCDSE Fe-S cluster biogenesis pathway was required for T3SS activity. These data suggest that IscR regulates the T3SS in Yersinia through maturation of an Fe-S cluster protein critical for type III secretion, in addition to its known role in activating T3SS genes through LcrF. Altogether, our study shows that iron starvation triggers IscR to coregulate multiple, distinct pathways relevant to promoting bacterial survival during infection.

Protein Export via the Type III Secretion System of the Bacterial Flagellum

The bacterial flagellum and the related virulence-associated injectisome system of pathogenic bacteria utilize a type III secretion system (T3SS) to export substrate proteins across the inner membrane in a proton motive force-dependent manner. The T3SS is composed of an export gate (FliPQR/FlhA/FlhB) located in the flagellar basal body and an associated soluble ATPase complex in the cytoplasm (FliHIJ). Here, we summarise recent insights into the structure, assembly and protein secretion mechanisms of the T3SS with a focus on energy transduction and protein transport across the cytoplasmic membrane.

Cryo-EM structure of the EspA filament from enteropathogenic Escherichia coli: Revealing the mechanism of effector translocation in the T3SS

The type III secretion system (T3SS) is a virulence mechanism employed by Gram-negative pathogens. The T3SS forms a proteinaceous channel that projects a needle into the extracellular medium where it interacts with the host cell to deliver virulence factors. Enteropathogenic Escherichia coli (EPEC) is unique in adopting a needle extension to the T3SS-a filament formed by EspA-which is absolutely required for efficient colonization of the gut. Here, we describe the cryoelectron microscopy structure of native EspA filaments from EPEC at 3.6-Å resolution. Within the filament, positively charged residues adjacent to a hydrophobic groove line the lumen of the filament in a spiral manner, suggesting a mechanism of substrate translocation mediated via electrostatics. Using structure-guided mutagenesis, in vivo studies corroborate the role of these residues in secretion and translocation function. The high-resolution structure of the EspA filament could aid in structure-guided drug design of antivirulence therapeutics.

Structural Conservation and Adaptation of the Bacterial Flagella Motor

Many bacteria require flagella for the ability to move, survive, and cause infection. The flagellum is a complex nanomachine that has evolved to increase the fitness of each bacterium to diverse environments. Over several decades, molecular, biochemical, and structural insights into the flagella have led to a comprehensive understanding of the structure and function of this fascinating nanomachine. Notably, X-ray crystallography, cryo-electron microscopy (cryo-EM), and cryo-electron tomography (cryo-ET) have elucidated the flagella and their components to unprecedented resolution, gleaning insights into their structural conservation and adaptation. In this review, we focus on recent structural studies that have led to a mechanistic understanding of flagellar assembly, function, and evolution.

Cryo-EM analysis of the SctV cytosolic domain from the enteropathogenic E. coli T3SS injectisome

The bacterial injectisome and flagella both rely on type III secretion systems for their assembly. The syringe-like injectisome creates a continuous channel between the bacterium and the host cell, through which signal-modulating effector proteins are secreted. The inner membrane pore protein SctV controls the hierarchy of substrate selection and may also be involved in energizing secretion. We present the 4.7 Å cryo-EM structure of the SctV cytosolic domain (SctV_C) from the enteropathogenic Escherichia coli injectisome. SctV_C forms a nonameric ring with primarily electrostatic interactions between its subunits. Molecular dynamics simulations show that monomeric SctV_C maintains a closed conformation, in contrast with previous studies on flagellar homologue FlhA. Comparison with substrate-bound homologues suggest that a conformational change would be required to accommodate binding partners.

Cryo-EM structure of the Shigella type III needle complex

The Type III Secretion Systems (T3SS) needle complex is a conserved syringe-shaped protein translocation nanomachine with a mass of about 3.5 MDa essential for the survival and virulence of many Gram-negative bacterial pathogens. This system is composed of a membrane-embedded basal body and an extracellular needle that deliver effector proteins into host cells. High-resolution structures of the T3SS from different organisms and infection stages are needed to understand the underlying molecular mechanisms of effector translocation. Here, we present the cryo-electron microscopy structure of the isolated Shigella T3SS needle complex. The inner membrane (IM) region of the basal body adopts 24-fold rotational symmetry and forms a channel system that connects the bacterial periplasm with the export apparatus cage. The secretin oligomer adopts a heterogeneous architecture with 16- and 15-fold cyclic symmetry in the periplasmic N-terminal connector and C-terminal outer membrane ring, respectively. Two out of three IM subunits bind the secretin connector via a β-sheet augmentation. The cryo-EM map also reveals the helical architecture of the export apparatus core, the inner rod, the needle and their intervening interfaces.

Diarrheal diseases evoke about 2.2. million dead people annually and are the second leading cause of postneonatal child mortality worldwide. Shigella causing dysentery utilizes the type 3-secretion system (T3SS) to inject virulence factors into the gut cells. The T3SS needle complex is a syringe-shaped nanomachine consisting of two membrane-embedded ring systems that sheath a central export apparatus and a hollow needle-like structure through which the virulence factors are transported. We present here the structure of the Shigella T3SS needle complex obtained by high-end electron microscopy. The outer membrane (OM) ring system adopts a mixed 15- and 16-fold cyclic symmetry and the near-atomic structure shows the connection of the inner membrane (IM) and OM rings. Conserved channels in the IM ring connect the bacterial periplasm with the central export apparatus. Similar to the Salmonella flagellar system, the export apparatus and its connected needle-like structure assemble in a helical manner. This study advances our understanding of the role of essential structural elements in the T3SS assembly and function.

Dominant negative effects by inactive Spa47 mutants inhibit T3SS function and Shigella virulence

Type three secretion systems (T3SS) are complex nano-machines that evolved to inject bacterial effector proteins directly into the cytoplasm of eukaryotic cells. Many high-priority human pathogens rely on one or more T3SSs to cause disease and evade host immune responses, underscoring the need to better understand the mechanisms through which T3SSs function and their role(s) in supporting pathogen virulence. We recently identified the Shigella protein Spa47 as an oligomerization-activated T3SS ATPase that fuels the T3SS and supports overall Shigella virulence. Here, we provide both in vitro and in vivo characterization of Spa47 oligomerization and activation in the presence and absence of engineered ATPase-inactive Spa47 mutants. The findings describe mechanistic details of Spa47-catalyzed ATP hydrolysis and uncover critical distinctions between oligomerization mechanisms capable of supporting ATP hydrolysis in vitro and those that support T3SS function in vivo. Concentration-dependent ATPase kinetics and experiments combining wild-type and engineered ATPase inactive Spa47 mutants found that monomeric Spa47 species isolated from recombinant preparations exhibit low-level ATPase activity by forming short-lived oligomers with active site contributions from at least two protomers. In contrast, isolated Spa47 oligomers exhibit enhanced ATP hydrolysis rates that likely result from multiple preformed active sites within the oligomeric complex, as is predicted to occur within the context of the type three secretion system injectisome. High-resolution fluorescence microscopy, T3SS activity, and virulence phenotype analyses of Shigella strains co-expressing wild-type Spa47 and the ATPase inactive Spa47 mutants demonstrate that the N-terminus of Spa47, not ATPase activity, is responsible for incorporation into the injectisome where the mutant strains exhibit a dominant negative effect on T3SS function and Shigella virulence. Together, the findings presented here help to close a significant gap in our understanding of how T3SS ATPases are activated and define restraints with respect to how ATP hydrolysis is ultimately coupled to T3SS function in vivo.

The cytoplasmic domain of MxiG interacts with MxiK and directs assembly of the sorting platform in the Shigella type III secretion system

Many Gram-negative bacteria use type III secretion systems (T3SSs) to inject virulence effector proteins into eukaryotic cells. The T3SS apparatus (T3SA) is structurally conserved among diverse bacterial pathogens and consists of a cytoplasmic sorting platform, an envelope-spanning basal body, and an extracellular needle with tip complex. The sorting platform is essential for effector recognition and powering secretion. Studies using bacterial "minicells" have revealed an unprecedented level of structural detail of the sorting platform; however, many of the structure-function relationships within this complex remain enigmatic. Here, we report on improved cryo-electron tomographic approaches to enhance the resolution of the Shigella T3SA sorting platform (at ≤2 nm resolution) done in concert with biochemical and genetic methods to define the sorting platform interactome and interactions with the T3SA inner membrane ring (IR). We observed that the sorting platform consists of "pods" with 6-fold symmetry that interact with the Spa47 ATPase via radial extensions comprising MxiN. Most importantly, MxiK maintained an interaction with the IR via specific interactions with the cytoplasmic domain of the IR protein MxiG (MxiG^C), which is a noncanonical forkhead-associated domain, and MxiK has an elongated structure that interacts with the IR via MxiG^C T4 lysozyme-mediated insertional mutagenesis of MxiK revealed its orientation within the sorting platform and enabled disruption of interactions with its binding partners, which abolished sorting platform assembly. Finally, a comparison with the homologous interactions in the Salmonella T3SS sorting platform revealed clear differences in their IR-sorting platform interfaces that have possible mechanistic implications.

Interfacial amino acids support Spa47 oligomerization and shigella type three secretion system activation

Like many Gram-negative pathogens, Shigella rely on a type three secretion system (T3SS) for injection of effector proteins directly into eukaryotic host cells to initiate and sustain infection. Protein secretion through the needle-like type three secretion apparatus (T3SA) requires ATP hydrolysis by the T3SS ATPase Spa47, making it a likely target for in vivo regulation of T3SS activity and an attractive target for small molecule therapeutics against shigellosis. Here, we developed a model of an activated Spa47 homo-hexamer, identifying two distinct regions at each protomer interface that we hypothesized to provide intermolecular interactions supporting Spa47 oligomerization and enzymatic activation. Mutational analysis and a series of high-resolution crystal structures confirm the importance of these residues, as many of the engineered mutants are unable to form oligomers and efficiently hydrolyze ATP in vitro. Furthermore, in vivo evaluation of Shigella virulence phenotype uncovered a strong correlation between T3SS effector protein secretion, host cell membrane disruption, and cellular invasion by the tested mutant strains, suggesting that perturbation of the identified interfacial residues/interactions influences Spa47 activity through preventing oligomer formation, which in turn regulates Shigella virulence. The most impactful mutations are observed within the conserved Site 2 interface where the native residues support oligomerization and likely contribute to a complex hydrogen bonding network that organizes the active site and supports catalysis. The critical reliance on these conserved residues suggests that aspects of T3SS regulation may also be conserved, providing promise for the development of a cross-species therapeutic that broadly targets T3SS ATPase oligomerization and activation.

Biophysical Mechanism of Protein Export by Bacterial Type III Secretion System

Type III secretion system (T3SS) is a protein injection nano-machine consisting of syringe and needle-like structure spanning bacterial inner and outer membranes. Bacteria insert the tip of T3SS needle to host cell membranes, and deliver effector proteins directly into host cells via T3SS to prime the host cell environment for infection. Thus inhibition of T3SS would be a potent strategy for suppressing bacterial infection. We previously demonstrated that T3SS needle rotates by proton-motive force (PMF) with the same mechanism as two evolutionally related rotary protein motors, flagellum and ATP synthase (FASEB J., 27, 2013, Ohgita et al.). Inhibition of needle rotation resulted in suppression of effector secretion, indicating the requirement of needle rotation for effector export. Simulation analysis of protein export by the T3SS needle suggests the importance of a hydrophobic helical groove formed by the conserved aromatic residue in the needle components. Based on these results, we have proposed a novel model of protein export by the T3SS needle, in which effector proteins are exported by PMF-dependent needle rotation oppositely to the hydrophobic helical groove in the needle. Quantitative examinations of the correlation between the speeds of T3SS rotation and the amount of effector export support this model. In this review, we summarize our current understanding of T3SS, and discuss our novel model of the protein export mechanism of T3SS based on the needle rotation.

Cryo-EM structure of the homohexameric T3SS ATPase-central stalk complex reveals rotary ATPase-like asymmetry

Many Gram-negative bacteria, including causative agents of dysentery, plague, and typhoid fever, rely on a type III secretion system - a multi-membrane spanning syringe-like apparatus - for their pathogenicity. The cytosolic ATPase complex of this injectisome is proposed to play an important role in energizing secretion events and substrate recognition. We present the 3.3 Å resolution cryo-EM structure of the enteropathogenic Escherichia coli ATPase EscN in complex with its central stalk EscO. The structure shows an asymmetric pore with different functional states captured in its six catalytic sites, details directly supporting a rotary catalytic mechanism analogous to that of the heterohexameric F₁/V₁-ATPases despite its homohexameric nature. Situated at the C-terminal opening of the EscN pore is one molecule of EscO, with primary interaction mediated through an electrostatic interface. The EscN-EscO structure provides significant atomic insights into how the ATPase contributes to type III secretion, including torque generation and binding of chaperone/substrate complexes.

Export Mechanisms and Energy Transduction in Type-III Secretion Machines

The remarkably complex architecture and organization of bacterial nanomachines originally raised the enigma to how they are assembled in a coordinated manner. Over the years, the assembly processes of the flagellum and evolutionary-related injectisome complexes have been deciphered and were shown to rely on a conserved protein secretion machine: the type-III secretion system. In this book chapter, we demonstrate how individually evolved mechanisms cooperate in highly versatile and robust secretion machinery to export and assemble the building blocks of those nanomachines.

An Experimental Pipeline for Initial Characterization of Bacterial Type III Secretion System Inhibitor Mode of Action Using Enteropathogenic Yersinia

Dozens of Gram negative pathogens use one or more type III secretion systems (T3SS) to disarm host defenses or occupy a beneficial niche during infection of a host organism. While the T3SS represents an attractive drug target and dozens of compounds with T3SS inhibitory activity have been identified, few T3SS inhibitors have been validated and mode of action determined. One issue is the lack of standardized orthogonal assays following high throughput screening. Using a training set of commercially available compounds previously shown to possess T3SS inhibitory activity, we demonstrate the utility of an experiment pipeline comprised of six distinct assays to assess the stages of type III secretion impacted: T3SS gene copy number, T3SS gene expression, T3SS basal body and needle assembly, secretion of cargo through the T3SS, and translocation of T3SS effector proteins into host cells. We used enteropathogenic Yersinia as the workhorse T3SS-expressing model organisms for this experimental pipeline, as Yersinia is sensitive to all T3SS inhibitors we tested, including those active against other T3SS-expressing pathogens. We find that this experimental pipeline is capable of rapidly distinguishing between T3SS inhibitors that interrupt the process of type III secretion at different points in T3SS assembly and function. For example, our data suggests that Compound 3, a malic diamide, blocks either activity of the assembled T3SS or alters the structure of the T3SS in a way that blocks T3SS cargo secretion but not antibody recognition of the T3SS needle. In contrast, our data predicts that Compound 4, a haloid-containing sulfonamidobenzamide, disrupts T3SS needle subunit secretion or assembly. Furthermore, we suggest that misregulation of copy number control of the pYV virulence plasmid, which encodes the Yersinia T3SS, should be considered as a possible mode of action for compounds with T3SS inhibitory activity against Yersinia.

Cryo-EM analysis of the T3S injectisome reveals the structure of the needle and open secretin

The bacterial type III secretion system, or injectisome, is a syringe shaped nanomachine essential for the virulence of many disease causing Gram-negative bacteria. At the core of the injectisome structure is the needle complex, a continuous channel formed by the highly oligomerized inner and outer membrane hollow rings and a polymerized helical needle filament which spans through and projects into the infected host cell. Here we present the near-atomic resolution structure of a needle complex from the prototypical Salmonella Typhimurium SPI-1 type III secretion system, with local masking protocols allowing for model building and refinement of the major membrane spanning components of the needle complex base in addition to an isolated needle filament. This work provides significant insight into injectisome structure and assembly and importantly captures the molecular basis for substrate induced gating in the giant outer membrane secretin portal family.

Energy Requirements for Protein Secretion via the Flagellar Type III Secretion System

Protein transport across the cytoplasmic membrane is coupled to energy derived from adenosine triphosphate hydrolysis or the protein motive force (pmf). A sophisticated, multi-component type III secretion system exports substrate proteins of both the bacterial flagellum and virulence-associated injectisome system of many Gram-negative pathogens. The type-III secretion system is primarily a pmf-driven protein exporter. Here, I describe methods to investigate the export of substrate proteins into the culture supernatant under conditions that manipulate the pmf.

Type-III secretion pore formed by flagellar protein FliP

During assembly of the bacterial flagellum, protein subunits that form the exterior structures are exported through a specialized secretion apparatus energized by the proton gradient. This category of protein transport, together with the similar process that occurs in the injectisomes of gram-negative pathogens, is termed type-III secretion. The membrane-embedded part of the flagellar export apparatus contains five essential proteins: FlhA, FlhB, FliP, FliQ and FliR. Here, we have undertaken a variety of experiments that together support the proposal that the protein-conducting conduit is formed primarily, and possibly entirely, by FliP. Chemical modification experiments demonstrate that positions near the center of certain FliP trans-membrane (TM) segments are accessible to polar reagents. FliP expression sensitizes cells to a number of chemical agents, and mutations at predicted channel-facing positions modulate this effect. Multiple assays are used to show that FliP suffices to form a channel that can conduct a variety of medium-sized, polar molecules. Conductance properties are strongly modulated by mutations in a methionine-rich loop that is predicted to lie at the inner mouth of the channel, which might form a gasket around cargo molecules undergoing export. The results are discussed in the framework of an hypothesis for the architecture and action of the cargo-conducting part of the type-III secretion apparatus.

Synthetic Cyclic Peptomers as Type III Secretion System Inhibitors

Antibiotic-resistant bacteria are an emerging threat to global public health. New classes of antibiotics and tools for antimicrobial discovery are urgently needed. Type III secretion systems (T3SS), which are required by dozens of Gram-negative bacteria for virulence but largely absent from nonpathogenic bacteria, are promising virulence blocker targets. The ability of mammalian cells to recognize the presence of a functional T3SS and trigger NF-κB activation provides a rapid and sensitive method for identifying chemical inhibitors of T3SS activity. In this study, we generated a HEK293 stable cell line expressing green fluorescent protein (GFP) driven by a promoter containing NF-κB enhancer elements to serve as a readout of T3SS function. We identified a family of synthetic cyclic peptide-peptoid hybrid molecules (peptomers) that exhibited dose-dependent inhibition of T3SS effector secretion in Yersinia pseudotuberculosis and Pseudomonas aeruginosa without affecting bacterial growth or motility. Among these inhibitors, EpD-3'N, EpD-1,2N, EpD-1,3'N, EpD-1,2,3'N, and EpD-1,2,4'N exhibited strong inhibitory effects on translocation of the Yersinia YopM effector protein into mammalian cells (>40% translocation inhibition at 7.5 μM) and showed no toxicity to mammalian cells at 240 μM. In addition, EpD-3'N and EpD-1,2,4'N reduced the rounding of HeLa cells caused by the activity of Yersinia effector proteins that target the actin cytoskeleton. In summary, we have discovered a family of novel cyclic peptomers that inhibit the injectisome T3SS but not the flagellar T3SS.

Type III Secretion in the Melioidosis Pathogen Burkholderia pseudomallei

Burkholderia pseudomallei is a Gram-negative intracellular pathogen and the causative agent of melioidosis, a severe disease of both humans and animals. Melioidosis is an emerging disease which is predicted to be vastly under-reported. Type III Secretion Systems (T3SSs) are critical virulence factors in Gram negative pathogens of plants and animals. The genome of B. pseudomallei encodes three T3SSs. T3SS-1 and -2, of which little is known, are homologous to Hrp2 secretion systems of the plant pathogens Ralstonia and Xanthomonas. T3SS-3 is better characterized and is homologous to the Inv/Mxi-Spa secretion systems of Salmonella spp. and Shigella flexneri, respectively. Upon entry into the host cell, B. pseudomallei requires T3SS-3 for efficient escape from the endosome. T3SS-3 is also required for full virulence in both hamster and murine models of infection. The regulatory cascade which controls T3SS-3 expression and the secretome of T3SS-3 have been described, as well as the effect of mutations of some of the structural proteins. Yet only a few effector proteins have been functionally characterized to date and very little work has been carried out to understand the hierarchy of assembly, secretion and temporal regulation of T3SS-3. This review aims to frame current knowledge of B. pseudomallei T3SSs in the context of other well characterized model T3SSs, particularly those of Salmonella and Shigella.

Assembly, structure, function and regulation of type III secretion systems

Type III secretion systems (T3SSs) are protein transport nanomachines that are found in Gram-negative bacterial pathogens and symbionts. Resembling molecular syringes, T3SSs form channels that cross the bacterial envelope and the host cell membrane, which enable bacteria to inject numerous effector proteins into the host cell cytoplasm and establish trans-kingdom interactions with diverse hosts. Recent advances in cryo-electron microscopy and integrative imaging have provided unprecedented views of the architecture and structure of T3SSs. Furthermore, genetic and molecular analyses have elucidated the functions of many effectors and key regulators of T3SS assembly and secretion hierarchy, which is the sequential order by which the protein substrates are secreted. As essential virulence factors, T3SSs are attractive targets for vaccines and therapeutics. This Review summarizes our current knowledge of the structure and function of this important protein secretion machinery. A greater understanding of T3SSs should aid mechanism-based drug design and facilitate their manipulation for biotechnological applications.

Mechanism of type-III protein secretion: Regulation of FlhA conformation by a functionally critical charged-residue cluster

The bacterial flagellum contains a specialized secretion apparatus in its base that pumps certain protein subunits through the growing structure to their sites of installation beyond the membrane. A related apparatus functions in the injectisomes of gram-negative pathogens to export virulence factors into host cells. This mode of protein export is termed type-III secretion (T3S). Details of the T3S mechanism are unclear. It is energized by the proton gradient; here, a mutational approach was used to identify proton-binding groups that might function in transport. Conserved proton-binding residues in all the membrane components were tested. The results identify residues R147, R154 and D158 of FlhA as most critical. These lie in a small, well-conserved cytoplasmic domain of FlhA, located between transmembrane segments 4 and 5. Two-hybrid experiments demonstrate self-interaction of the domain, and targeted cross-linking indicates that it forms a multimeric array. A mutation that mimics protonation of the key acidic residue (D158N) was shown to trigger a global conformational change that affects the other, larger cytoplasmic domain that interacts with the export cargo. The results are discussed in the framework of a transport model based on proton-actuated movements in the cytoplasmic domains of FlhA.

Piericidin A1 Blocks Yersinia Ysc Type III Secretion System Needle Assembly

The bacterial type III secretion system (T3SS) is widely used by both human and animal pathogens to cause disease yet remains incompletely understood. Deciphering how some natural products, such as the microbial metabolite piericidin, inhibit type III secretion can provide important insight into how the T3SS functions or is regulated. Taking this approach, we investigated the ability of piericidin to block T3SS function in several human pathogens. Surprisingly, piericidin selectively inhibited the Ysc family T3SS in enteropathogenic Yersinia but did not affect the function of a different T3SS within the same species. Furthermore, piericidin specifically blocked the formation of T3SS needles on the bacterial surface without altering the localization of several other T3SS components or regulation of T3SS gene expression. These data show that piericidin targets a mechanism important for needle assembly that is unique to the Yersinia Ysc T3SS.

The type III secretion system (T3SS) is a bacterial virulence factor expressed by dozens of Gram-negative pathogens but largely absent from commensals. The T3SS is an attractive target for antimicrobial agents that may disarm pathogenic bacteria while leaving commensal populations intact. We previously identified piericidin A1 as an inhibitor of the Ysc T3SS in Yersinia pseudotuberculosis. Piericidins were first discovered as inhibitors of complex I of the electron transport chain in mitochondria and some bacteria. However, we found that piericidin A1 did not alter Yersinia membrane potential or inhibit flagellar motility powered by the proton motive force, indicating that the piericidin mode of action against Yersinia type III secretion is independent of complex I. Instead, piericidin A1 reduced the number of T3SS needle complexes visible by fluorescence microscopy at the bacterial surface, preventing T3SS translocator and effector protein secretion. Furthermore, piericidin A1 decreased the abundance of higher-order YscF needle subunit complexes, suggesting that piericidin A1 blocks YscF needle assembly. While expression of T3SS components in Yersinia are positively regulated by active type III secretion, the block in secretion by piericidin A1 was not accompanied by a decrease in T3SS gene expression, indicating that piericidin A1 may target a T3SS regulatory circuit. However, piericidin A1 still inhibited effector protein secretion in the absence of the T3SS regulator YopK, YopD, or YopN. Surprisingly, while piericidin A1 also inhibited the Y. enterocolitica Ysc T3SS, it did not inhibit the SPI-1 family Ysa T3SS in Y. enterocolitica or the Ysc family T3SS in Pseudomonas aeruginosa. Together, these data indicate that piericidin A1 specifically inhibits Yersinia Ysc T3SS needle assembly.

IMPORTANCE The bacterial type III secretion system (T3SS) is widely used by both human and animal pathogens to cause disease yet remains incompletely understood. Deciphering how some natural products, such as the microbial metabolite piericidin, inhibit type III secretion can provide important insight into how the T3SS functions or is regulated. Taking this approach, we investigated the ability of piericidin to block T3SS function in several human pathogens. Surprisingly, piericidin selectively inhibited the Ysc family T3SS in enteropathogenic Yersinia but did not affect the function of a different T3SS within the same species. Furthermore, piericidin specifically blocked the formation of T3SS needles on the bacterial surface without altering the localization of several other T3SS components or regulation of T3SS gene expression. These data show that piericidin targets a mechanism important for needle assembly that is unique to the Yersinia Ysc T3SS.

The Structure and Function of Type III Secretion Systems

Type III secretion systems (T3SSs) afford Gram-negative bacteria an intimate means of altering the biology of their eukaryotic hosts--the direct delivery of effector proteins from the bacterial cytoplasm to that of the eukaryote. This incredible biophysical feat is accomplished by nanosyringe "injectisomes," which form a conduit across the three plasma membranes, peptidoglycan layer, and extracellular space that form a barrier to the direct delivery of proteins from bacterium to host. The focus of this chapter is T3SS function at the structural level; we will summarize the core findings that have shaped our understanding of the structure and function of these systems and highlight recent developments in the field. In turn, we describe the T3SS secretory apparatus, consider its engagement with secretion substrates, and discuss the posttranslational regulation of secretory function. Lastly, we close with a discussion of the future prospects for the interrogation of structure-function relationships in the T3SS.

Type III secretion systems: the bacterial flagellum and the injectisome

The flagellum and the injectisome are two of the most complex and fascinating bacterial nanomachines. At their core, they share a type III secretion system (T3SS), a transmembrane export complex that forms the extracellular appendages, the flagellar filament and the injectisome needle. Recent advances, combining structural biology, cryo-electron tomography, molecular genetics, in vivo imaging, bioinformatics and biophysics, have greatly increased our understanding of the T3SS, especially the structure of its transmembrane and cytosolic components, the transcriptional, post-transcriptional and functional regulation and the remarkable adaptivity of the system. This review aims to integrate these new findings into our current knowledge of the evolution, function, regulation and dynamics of the T3SS, and to highlight commonalities and differences between the two systems, as well as their potential applications.

MxiA, MxiC and IpaD Regulate Substrate Selection and Secretion Mode in the T3SS of Shigella flexneri

Type III secretion systems (T3SSs) are central virulence devices for many Gram-negative bacterial pathogens of humans, animals & plants. Upon physical contact with eukaryotic host cells, they translocate virulence-mediating proteins, known as effectors, into them during infection. T3SSs are gated from the outside by host-cell contact and from the inside via two cytoplasmic negative regulators, MxiC and IpaD in Shigella flexneri, which together control the effector secretion hierarchy. Their absence leads to premature and increased secretion of effectors. Here, we investigated where and how these regulators act. We demonstrate that the T3SS inner membrane export apparatus protein MxiA plays a role in substrate selection. Indeed, using a genetic screen, we identified two amino acids located on the surface of MxiA's cytoplasmic region (MxiAC) which, when mutated, upregulate late effector expression and, in the case of MxiAI674V, also secretion. The cytoplasmic region of MxiA, but not MxiAN373D and MxiAI674V, interacts directly with the C-terminus of MxiC in a two-hybrid assay. Efficient T3S requires a cytoplasmic ATPase and the proton motive force (PMF), which is composed of the ΔΨ and the ΔpH. MxiA family proteins and their regulators are implicated in utilization of the PMF for protein export. However, our MxiA point mutants show similar PMF utilisation to wild-type, requiring primarily the ΔΨ. On the other hand, lack of MxiC or IpaD, renders the faster T3S seen increasingly dependent on the ΔpH. Therefore, MxiA, MxiC and IpaD act together to regulate substrate selection and secretion mode in the T3SS of Shigella flexneri.

Environmental Regulation of Yersinia Pathophysiology

Hallmarks of Yersinia pathogenesis include the ability to form biofilms on surfaces, the ability to establish close contact with eukaryotic target cells and the ability to hijack eukaryotic cell signaling and take over control of strategic cellular processes. Many of these virulence traits are already well-described. However, of equal importance is knowledge of both confined and global regulatory networks that collaborate together to dictate spatial and temporal control of virulence gene expression. This review has the purpose to incorporate historical observations with new discoveries to provide molecular insight into how some of these regulatory mechanisms respond rapidly to environmental flux to govern tight control of virulence gene expression by pathogenic Yersinia.

Fueling type III secretion

Type III secretion systems (T3SSs) are complex nanomachines that export proteins from the bacterial cytoplasm across the cell envelope in a single step. They are at the core of the machinery used to assemble the bacterial flagellum, and the needle complex many Gram-negative pathogens use to inject effector proteins into host cells and cause disease. Several models have been put forward to explain how this export is energized, and the mechanism has been the subject of considerable debate. Here we present an overview of these models and discuss their relative merits. Recent evidence suggests that the proton motive force (pmf) is the primary energy source for type III secretion, although contribution from refolding of secreted proteins has not been ruled out. The mechanism by which the pmf is converted to protein export remains enigmatic.

Composition, formation, and regulation of the cytosolic c-ring, a dynamic component of the type III secretion injectisome

The injectisome is a membrane complex through which some bacteria can inject effector proteins into host cells. This study reveals that the cytosolic C-ring structure has a dynamic relationship to the rest of the injectisome, with implications for the regulation of secretion.

Many gram-negative pathogens employ a type III secretion injectisome to translocate effector proteins into eukaryotic host cells. While the structure of the distal “needle complex” is well documented, the composition and role of the functionally important cytosolic complex remain less well understood. Using functional fluorescent fusions, we found that the C-ring, an essential and conserved cytosolic component of the system, is composed of ~22 copies of SctQ (YscQ in Yersinia enterocolitica), which require the presence of YscQ_C, the product of an internal translation initiation site in yscQ, for their cooperative assembly. Photoactivated localization microscopy (PALM) reveals that in vivo, YscQ is present in both a free-moving cytosolic and a stable injectisome-bound state. Notably, fluorescence recovery after photobleaching (FRAP) shows that YscQ exchanges between the injectisome and the cytosol, with a t_½ of 68 ± 8 seconds when injectisomes are secreting. In contrast, the secretin SctC (YscC) and the major export apparatus component SctV (YscV) display minimal exchange. Under non-secreting conditions, the exchange rate of YscQ is reduced to t_½ = 134 ± 16 seconds, revealing a correlation between C-ring exchange and injectisome activity, which indicates a possible role for C-ring stability in regulation of type III secretion. The stabilization of the C-ring depends on the presence of the functional ATPase SctN (YscN). These data provide new insights into the formation and composition of the injectisome and present a novel aspect of type III secretion, the exchange of C-ring subunits, which is regulated with respect to secretion.

The type III secretion system, also known as the injectisome, is a key virulence factor in many gram-negative bacteria, and is responsible for the transmission of bacterial proteins directly into host cells. While some elements of the system are well characterized, the cytosolic components involved in substrate recognition and handling are not well understood. One of the major questions is the role of the C-ring, an essential yet enigmatic cytosolic injectisome member. We used fluorescence microscopy to analyze the architecture and behavior of the C-ring in live Y. enterocolitica bacteria, a human pathogen. We found that in vivo, the C-ring assembles cooperatively with the help of additional copies of its own C-terminal fragment and has a highly dynamic structure, with C-ring subunits exchanging between the working injectisomes and a cytosolic pool. The rate of exchange is different between secreting and non-secreting injectisomes and depends on the function of the type III secretion ATPase, indicating that the stability of the complex is altered when functioning. This dynamic behaviour raises the possibility that the C-ring is a regulator of targeted protein delivery by the type III secretion system and makes the C-ring a viable target for the development of novel anti-virulence drugs.

Structure of Salmonella FlhE, conserved member of a flagellar type III secretion operon

The bacterial flagellum is assembled by a multicomponent transport apparatus categorized as a type III secretion system. The secretion of proteins that assemble into the flagellum is driven by the proton motive force. The periplasmic protein FlhE is a member of the flhBAE operon in the majority of bacteria where FlhE is found. FlhA and FlhB are established components of the flagellar type III secretion system. The absence of FlhE results in a proton leak through the flagellar system, inappropriate secretion patterns, and cell death, indicating that FlhE regulates an important aspect of proper flagellar biosynthesis. We isolated FlhE from the periplasm of Salmonella and solved its structure to 1.5Å resolution. The structure reveals a β-sandwich fold, with no close structural homologs. Possible roles of FlhE, including that of a chaperone, are discussed.

Towards an integrated model of bacterial conjugation

Bacterial conjugation is one of the main mechanisms for horizontal gene transfer. It constitutes a key element in the dissemination of antibiotic resistance and virulence genes to human pathogenic bacteria. DNA transfer is mediated by a membrane-associated macromolecular machinery called Type IV secretion system (T4SS). T4SSs are involved not only in bacterial conjugation but also in the transport of virulence factors by pathogenic bacteria. Thus, the search for specific inhibitors of different T4SS components opens a novel approach to restrict plasmid dissemination. This review highlights recent biochemical and structural findings that shed new light on the molecular mechanisms of DNA and protein transport by T4SS. Based on these data, a model for pilus biogenesis and substrate transfer in conjugative systems is proposed. This model provides a renewed view of the mechanism that might help to envisage new strategies to curb the threating expansion of antibiotic resistance.

Identification of the docking site between a type III secretion system ATPase and a chaperone for effector cargo

A number of Gram-negative pathogens utilize type III secretion systems (T3SSs) to inject bacterial effector proteins into the host. An important component of T3SSs is a conserved ATPase that captures chaperone-effector complexes and energizes their dissociation to facilitate effector translocation. To date, there has been limited work characterizing the chaperone-T3SS ATPase interaction despite it being a fundamental aspect of T3SS function. In this study, we present the 2.1 Å resolution crystal structure of the Salmonella enterica SPI-2-encoded ATPase, SsaN. Our structure revealed a local and functionally important novel feature in helix 10 that we used to define the interaction domain relevant to chaperone binding. We modeled the interaction between the multicargo chaperone, SrcA, and SsaN and validated this model using mutagenesis to identify the residues on both the chaperone and ATPase that mediate the interaction. Finally, we quantified the benefit of this molecular interaction on bacterial fitness in vivo using chromosomal exchange of wild-type ssaN with mutants that retain ATPase activity but no longer capture the chaperone. Our findings provide insight into chaperone recognition by T3SS ATPases and demonstrate the importance of the chaperone-T3SS ATPase interaction for the pathogenesis of Salmonella.

Bacterial type III secretion systems: specialized nanomachines for protein delivery into target cells

One of the most exciting developments in the field of bacterial pathogenesis in recent years is the discovery that many pathogens utilize complex nanomachines to deliver bacterially encoded effector proteins into target eukaryotic cells. These effector proteins modulate a variety of cellular functions for the pathogen's benefit. One of these protein-delivery machines is the type III secretion system (T3SS). T3SSs are widespread in nature and are encoded not only by bacteria pathogenic to vertebrates or plants but also by bacteria that are symbiotic to plants or insects. A central component of T3SSs is the needle complex, a supramolecular structure that mediates the passage of the secreted proteins across the bacterial envelope. Working in conjunction with several cytoplasmic components, the needle complex engages specific substrates in sequential order, moves them across the bacterial envelope, and ultimately delivers them into eukaryotic cells. The central role of T3SSs in pathogenesis makes them great targets for novel antimicrobial strategies.

IscR is essential for yersinia pseudotuberculosis type III secretion and virulence

Type III secretion systems (T3SS) are essential for virulence in dozens of pathogens, but are not required for growth outside the host. Therefore, the T3SS of many bacterial species are under tight regulatory control. To increase our understanding of the molecular mechanisms behind T3SS regulation, we performed a transposon screen to identify genes important for T3SS function in the food-borne pathogen Yersinia pseudotuberculosis. We identified two unique transposon insertions in YPTB2860, a gene that displays 79% identity with the E. coliiron-sulfur cluster regulator, IscR. A Y. pseudotuberculosis iscR in-frame deletion mutant (ΔiscR) was deficient in secretion of Ysc T3SS effector proteins and in targeting macrophages through the T3SS. To determine the mechanism behind IscR control of the Ysc T3SS, we carried out transcriptome and bioinformatic analysis to identify Y. pseudotuberculosis genes regulated by IscR. We discovered a putative IscR binding motif upstream of the Y. pseudotuberculosis yscW-lcrF operon. As LcrF controls transcription of a number of critical T3SS genes in Yersinia, we hypothesized that Yersinia IscR may control the Ysc T3SS through LcrF. Indeed, purified IscR bound to the identified yscW-lcrF promoter motif and mRNA levels of lcrF and 24 other T3SS genes were reduced in Y. pseudotuberculosis in the absence of IscR. Importantly, mice orally infected with the Y. pseudotuberculosis ΔiscR mutant displayed decreased bacterial burden in Peyer's patches, mesenteric lymph nodes, spleens, and livers, indicating an essential role for IscR in Y. pseudotuberculosis virulence. This study presents the first characterization of Yersinia IscR and provides evidence that IscR is critical for virulence and type III secretion through direct regulation of the T3SS master regulator, LcrF.

Bacterial pathogens use regulators that sense environmental cues to enhance their fitness. Here, we identify a transcriptional regulator in the human gut pathogen, Yersinia pseudotuberculosis, which controls a specialized secretion system essential for bacterial growth in mammalian tissues. This regulator was shown in other bacterial species to alter its activity in response to changes in iron concentration and oxidative stress, but has never been studied in Yersinia. Importantly, Y. pseudotuberculosis experiences large changes in iron bioavailability upon transit from the gut to deeper tissues and iron is a critical component in Yersinia virulence, as individuals with iron overload disorders have enhanced susceptibility to systemic Yersinia infections. Our work places this iron-modulated transcriptional regulator within the regulatory network that controls virulence gene expression in Y. pseudotuberculosis, identifying it as a potential new target for antimicrobial agents.

Control of type III secretion activity and substrate specificity by the cytoplasmic regulator PcrG

Pathogenic Gram-negative bacteria use syringe-like type III secretion systems (T3SS) to inject effector proteins directly into targeted host cells. Effector secretion is triggered by host cell contact, and before contact is prevented by a set of conserved regulators. How these regulators interface with the T3SS apparatus to control secretion is unclear. We present evidence that the proton motive force (pmf) drives T3SS secretion in Pseudomonas aeruginosa, and that the cytoplasmic regulator PcrG interacts with distinct components of the T3SS apparatus to control two important aspects of effector secretion: (i) It coassembles with a second regulator (Pcr1) on the inner membrane T3SS component PcrD to prevent effectors from accessing the T3SS, and (ii) In conjunction with PscO, it controls protein secretion activity by modulating the ability of T3SS to convert pmf.

Assembly of the bacterial type III secretion machinery

Many bacteria that live in contact with eukaryotic hosts, whether as symbionts or as pathogens, have evolved mechanisms that manipulate host cell behaviour to their benefit. One such mechanism, the type III secretion system, is employed by Gram-negative bacterial species to inject effector proteins into host cells. This function is reflected by the overall shape of the machinery, which resembles a molecular syringe. Despite the simplicity of the concept, the type III secretion system is one of the most complex known bacterial nanomachines, incorporating one to more than hundred copies of up to twenty different proteins into a multi-MDa transmembrane complex. The structural core of the system is the so-called needle complex that spans the bacterial cell envelope as a tripartite ring system and culminates in a needle protruding from the bacterial cell surface. Substrate targeting and translocation are accomplished by an export machinery consisting of various inner membrane embedded and cytoplasmic components. The formation of such a multimembrane-spanning machinery is an intricate task that requires precise orchestration. This review gives an overview of recent findings on the assembly of type III secretion machines, discusses quality control and recycling of the system and proposes an integrated assembly model.

Suppression of type III effector secretion by polymers

Bacteria secrete effector proteins required for successful infection and expression of toxicity into host cells. The type III secretion apparatus is involved in these processes. Previously, we showed that the viscous polymer polyethylene glycol (PEG) 8000 suppressed effector secretion by Pseudomonas aeruginosa. We thus considered that other viscous polymers might also suppress secretion. We initially showed that PEG200 (formed from the same monomer (ethylene glycol) as PEG8000, but which forms solutions of lower viscosity than the latter compound) did not decrease effector secretion. By contrast, alginate, a high-viscous polymer formed from mannuronic and guluronic acid, unlike PEG8000, effectively inhibited secretion. The effectiveness of PEG8000 and alginate in this regard was closely associated with polymer viscosity, but the nature of viscosity dependence differed between the two polymers. Moreover, not only the natural polymer alginate, but also mucin, which protects against infection, suppressed secretion. We thus confirmed that polymer viscosity contributes to the suppression of effector secretion, but other factors (e.g. electrostatic interaction) may also be involved. Moreover, the results suggest that regulation of bacterial secretion by polymers may occur naturally via the action of components of biofilm or mucin layer.

Activation of Ran GTPase by a Legionella effector promotes microtubule polymerization, pathogen vacuole motility and infection

The causative agent of Legionnaires' disease, Legionella pneumophila, uses the Icm/Dot type IV secretion system (T4SS) to form in phagocytes a distinct "Legionella-containing vacuole" (LCV), which intercepts endosomal and secretory vesicle trafficking. Proteomics revealed the presence of the small GTPase Ran and its effector RanBP1 on purified LCVs. Here we validate that Ran and RanBP1 localize to LCVs and promote intracellular growth of L. pneumophila. Moreover, the L. pneumophila protein LegG1, which contains putative RCC1 Ran guanine nucleotide exchange factor (GEF) domains, accumulates on LCVs in an Icm/Dot-dependent manner. L. pneumophila wild-type bacteria, but not strains lacking LegG1 or a functional Icm/Dot T4SS, activate Ran on LCVs, while purified LegG1 produces active Ran(GTP) in cell lysates. L. pneumophila lacking legG1 is compromised for intracellular growth in macrophages and amoebae, yet is as cytotoxic as the wild-type strain. A downstream effect of LegG1 is to stabilize microtubules, as revealed by conventional and stimulated emission depletion (STED) fluorescence microscopy, subcellular fractionation and Western blot, or by microbial microinjection through the T3SS of a Yersinia strain lacking endogenous effectors. Real-time fluorescence imaging indicates that LCVs harboring wild-type L. pneumophila rapidly move along microtubules, while LCVs harboring ΔlegG1 mutant bacteria are stalled. Together, our results demonstrate that Ran activation and RanBP1 promote LCV formation, and the Icm/Dot substrate LegG1 functions as a bacterial Ran activator, which localizes to LCVs and promotes microtubule stabilization, LCV motility as well as intracellular replication of L. pneumophila.

A sophisticated multi-step secretion mechanism: how the type 3 secretion system is regulated

Many Gram-negative pathogens utilize type 3 secretion systems (T3SSs) for a successful infection. The T3SS is a large macromolecular complex which spans both bacterial membranes and delivers effector proteins into the host cell. The infection requires spatiotemporal control of diverse sets of secreted effectors and various mechanisms have evolved to regulate T3SS in response to external stimuli. This review will describe mechanisms that may control type 3 secretion, revealing a multi-step regulatory strategy. We then propose an updated model of T3SS that illustrates different stages of secretion and integrates the most recent structural and functional data.

Molecular motors in bacterial secretion

Secretion of effectors across bacterial membranes is usually mediated by large multisubunit complexes. In most cases, the secreted effectors are virulent factors normally associated to pathogenic diseases. The biogenesis of these secretion systems and the transport of the effectors are processes that require energy. This energy could be directly obtained by using the proton motive force, but in most cases the energy associated to these processes is derived from ATP hydrolysis. Here, a description of the machineries involved in generating the energy required for system biogenesis and substrate transport by type II, III and IV secretion systems is provided, with special emphasis on highlighting the structural similarities and evolutionary relationships among the secretion ATPases.