The Salmonella Typhi SPI-2 injectisome enigma

The Salmonella pathogenicity island 2 (SPI-2)-encoded type III secretion system (injectisome) is assembled following uptake of bacteria into vacuoles in mammalian cells. The injectisome translocates virulence proteins (effectors) into infected cells. Numerous studies have established the requirement for a functional SPI-2 injectisome for growth of Salmonella Typhimurium in mouse macrophages, but the results of similar studies involving Salmonella Typhi and human-derived macrophages are not consistent. It is important to clarify the functions of the S. Typhi SPI-2 injectisome, not least because an inactivated SPI-2 injectisome forms the basis for live attenuated S. Typhi vaccines that have undergone extensive trials in humans. Intracellular expression of injectisome genes and effector delivery take longer in the S. Typhi/human macrophage model than for S. Typhimurium and we propose that this could explain the conflicting results. Furthermore, strains of both S. Typhimurium and S. Typhi contain intact genes for several 'core' effectors. In S. Typhimurium these cooperate to regulate the vacuole membrane and contribute to intracellular bacterial replication; similar functions are therefore likely in S. Typhi.

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.

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.

Differential regulation of Shigella Spa47 ATPase activity by a native C-terminal product of Spa33

Shigella is a Gram-negative bacterial pathogen that relies on a single type three secretion system (T3SS) as its primary virulence factor. The T3SS includes a highly conserved needle-like apparatus that directly injects bacterial effector proteins into host cells, subverting host cell function, initiating infection, and circumventing resulting host immune responses. Recent findings have located the T3SS ATPase Spa47 to the base of the Shigella T3SS apparatus and have correlated its catalytic function to apparatus formation, protein effector secretion, and overall pathogen virulence. This critical correlation makes Spa47 ATPase activity regulation a likely point of native control over Shigella virulence and a high interest target for non-antibiotic- based therapeutics. Here, we provide a detailed characterization of the natural 11.6 kDa C-terminal translation product of the Shigella T3SS protein Spa33 (Spa33C), showing that it is required for proper virulence and that it pulls down with several known T3SS proteins, consistent with a structural role within the sorting platform of the T3SS apparatus. In vitro binding assays and detailed kinetic analyses suggest an additional role, however, as Spa33C differentially regulates Spa47 ATPase activity based on Spa47s oligomeric state, downregulating Spa47 monomer activity and upregulating activity of both homo-oligomeric Spa47 and the hetero-oligomeric MxiN2Spa47 complex. These findings identify Spa33C as only the second known differential T3SS ATPase regulator to date, with the Shigella protein MxiN representing the other. Describing this differential regulatory protein pair begins to close an important gap in understanding of how Shigella may modulate virulence through Spa47 activity and T3SS function.

Transmembrane substrates of type three secretion system injectisomes

The type three secretion system injectisome of Gram-negative bacterial pathogens injects virulence proteins, called effectors, into host cells. Effectors of mammalian pathogens carry out a range of functions enabling bacterial invasion, replication, immune suppression and transmission. The injectisome secretes two translocon proteins that insert into host cell membranes to form a translocon pore, through which effectors are delivered. A subset of effectors also integrate into infected cell membranes, enabling a unique range of biochemical functions. Both translocon proteins and transmembrane effectors avoid cytoplasmic aggregation and integration into the bacterial inner membrane. Translocated transmembrane effectors locate and integrate into the appropriate host membrane. In this review, we focus on transmembrane translocon proteins and effectors of bacterial pathogens of mammals. We discuss what is known about the mechanisms underlying their membrane integration, as well as the functions conferred by the position of injectisome effectors within membranes.

Structural and Functional Analysis of SsaV Cytoplasmic Domain and Variable Linker States in the Context of the InvA-SsaV Chimeric Protein

The type III secretion (T3S) injectisome is a syringe-like protein-delivery nanomachine widely utilized by Gram-negative bacteria. It can deliver effector proteins directly from bacteria into eukaryotic host cells, which is crucial for the bacterial-host interaction. Intracellular pathogen Salmonella enterica serovar Typhimurium encodes two sets of T3S injectisomes from Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2), which are critical for its host invasion and intracellular survival, respectively. The inner membrane export gate protein, SctV (InvA in SPI-1 and SsaV in SPI-2), is the largest component of the injectisome and is essential for assembly and function of T3SS. Here, we report the 2.11 Å cryo-EM structure of the SsaV cytoplasmic domain (SsaVC) in the context of a full-length SctV chimera consisting of the transmembrane region of InvA, the linker of SsaV (SsaVL) and SsaVC. The structural analysis shows that SsaVC exists in a semi-open state and SsaVL exhibits two major orientations, implying a highly dynamic process of SsaV for the substrate selection and secretion in a full-length context. A biochemical assay indicates that SsaVL plays an essential role in maintaining the nonameric state of SsaV. This study offers near atomic-level insights into how SsaVC and SsaVL facilitate the assembly and function of SsaV and may lead to the development of potential anti-virulence therapeutics against T3SS-mediated bacterial infection. IMPORTANCE Type III secretion system (T3SS) is a multicomponent nanomachine and a critical virulence factor for a wide range of Gram-negative bacterial pathogens. It can deliver numbers of effectors into the host cell to facilitate the bacterial host infection. Export gate protein SctV, as one of the engines of T3SS, is at the center of T3SS assembly and function. In this study, we show the high-resolution atomic structure of the cytosolic domain of SctV in the nonameric state with variable linker conformations. Our first observation of conformational changes of the linker region of SctV and the semi-open state of the cytosolic domain of SctV in the full-length context further support that the substrate selection and secretion process of SctV is highly dynamic. These findings have important implications for the development of therapeutic strategies targeting SctV to combat T3SS-mediated bacterial infection.

Conserved Salt Bridges Facilitate Assembly of the Helical Core Export Apparatus of a Salmonella enterica Type III Secretion System

Virulence-associated type III secretion systems (T3SS) are utilized by Gram negative bacterial pathogens for injection of effector proteins into eukaryotic host cells. The transmembrane export apparatus at the core of T3SS is composed of a unique helical complex of the hydrophobic proteins SctR, SctS, SctT, and SctU. These components comprise a number of highly conserved charged residues within their hydrophobic domains. The structure of the closed state of the core complex SctR₅S₄T₁ revealed that several of these residues form inter- and intramolecular salt bridges, some of which have to be broken for pore opening. Mutagenesis of individual residues was shown to compromise assembly or secretion of both, the virulence-associated and the related flagellar T3SS. However, the exact role of these conserved charged residues in the assembly and function of T3SS remains elusive. Here we performed an in-depth mutagenesis analysis of these residues in the T3SS of Salmonella Typhimurium, coupled to blue native PAGE, in vivo photocrosslinking and luciferase-based secretion assays. Our data show that these conserved salt bridges are not critical for assembly of the respective protein but rather facilitate the incorporation of the following subunit into the assembling complex. Our data also indicate that these conserved charged residues are critical for type III-dependent secretion and reveal a functional link between SctS_E44 and SctT_R204 and the cytoplasmic domain of SctU in gating the T3SS injectisome. Overall, our analysis provides an unprecedented insight into the delicate requirements for the assembly and function of the machinery at the core of T3SS.

Composition and Biophysical Properties of the Sorting Platform Pods in the Shigella Type III Secretion System

Shigella flexneri, causative agent of bacillary dysentery (shigellosis), uses a type III secretion system (T3SS) as its primary virulence factor. The T3SS injectisome delivers effector proteins into host cells to promote entry and create an important intracellular niche. The injectisome's cytoplasmic sorting platform (SP) is a critical assembly that contributes to substrate selection and energizing secretion. The SP consists of oligomeric Spa33 "pods" that associate with the basal body via MxiK and connect to the Spa47 ATPase via MxiN. The pods contain heterotrimers of Spa33 with one full-length copy associated with two copies of a C-terminal domain (Spa33^C). The structure of Spa33^C is known, but the precise makeup and structure of the pods in situ remains elusive. We show here that recombinant wild-type Spa33 can be prepared as a heterotrimer that forms distinct stable complexes with MxiK and MxiN. In two-hybrid analyses, association of the Spa33 complex with these proteins occurs via the full-length Spa33 component. Furthermore, these complexes each have distinct biophysical properties. Based on these properties, new high-resolution cryo-electron tomography data and architectural similarities between the Spa33 and flagellar FliM-FliN complexes, we provide a preliminary model of the Spa33 heterotrimers within the SP pods. From these findings and evolving models of SP interfaces and dynamics in the Yersinia and Salmonella T3SS, we suggest a model for SP function in which two distinct complexes come together within the context of the SP to contribute to form the complete pod structures during the recruitment of T3SS secretion substrates.

The Shigella Type III Secretion System: An Overview from Top to Bottom

Shigella comprises four species of human-restricted pathogens causing bacillary dysentery. While Shigella possesses multiple genetic loci contributing to virulence, a type III secretion system (T3SS) is its primary virulence factor. The Shigella T3SS nanomachine consists of four major assemblies: the cytoplasmic sorting platform; the envelope-spanning core/basal body; an exposed needle; and a needle-associated tip complex with associated translocon that is inserted into host cell membranes. The initial subversion of host cell activities is carried out by the effector functions of the invasion plasmid antigen (Ipa) translocator proteins, with the cell ultimately being controlled by dedicated effector proteins that are injected into the host cytoplasm though the translocon. Much of the information now available on the T3SS injectisome has been accumulated through collective studies on the T3SS from three systems, those of Shigella flexneri, Salmonella typhimurium and Yersinia enterocolitica/Yersinia pestis. In this review, we will touch upon the important features of the T3SS injectisome that have come to light because of research in the Shigella and closely related systems. We will also briefly highlight some of the strategies being considered to target the Shigella T3SS for disease prevention.

The T3SS of Shigella: Expression, Structure, Function, and Role in Vacuole Escape

Shigella spp. are one of the leading causes of infectious diarrheal diseases. They are Escherichia coli pathovars that are characterized by the harboring of a large plasmid that encodes most virulence genes, including a type III secretion system (T3SS). The archetypal element of the T3SS is the injectisome, a syringe-like nanomachine composed of approximately 20 proteins, spanning both bacterial membranes and the cell wall, and topped with a needle. Upon contact of the tip of the needle with the plasma membrane, the injectisome secretes its protein substrates into host cells. Some of these substrates act as translocators or effectors whose functions are key to the invasion of the cytosol and the cell-to-cell spread characterizing the lifestyle of Shigella spp. Here, we review the structure, assembly, function, and methods to measure the activity of the injectisome with a focus on Shigella, but complemented with data from other T3SS if required. We also present the regulatory cascade that controls the expression of T3SS genes in Shigella. Finally, we describe the function of translocators and effectors during cell-to-cell spread, particularly during escape from the vacuole, a key element of Shigella’s pathogenesis that has yet to reveal all of its secrets.

Characterization of the Pilotin-Secretin Complex from the Salmonella enterica Type III Secretion System Using Hybrid Structural Methods

The type III secretion system (T3SS) is a multi-membrane-spanning protein channel used by Gram-negative pathogenic bacteria to secrete effectors directly into the host cell cytoplasm. In the many species reliant on the T3SS for pathogenicity, proper assembly of the outer membrane secretin pore depends on a diverse family of lipoproteins called pilotins. We present structural and biochemical data on the Salmonella enterica pilotin InvH and the S domain of its cognate secretin InvG. Characterization of InvH by X-ray crystallography revealed a dimerized, α-helical pilotin. Size-exclusion-coupled multi-angle light scattering and small-angle X-ray scattering provide supporting evidence for the formation of an InvH homodimer in solution. Structures of the InvH-InvG heterodimeric complex determined by X-ray crystallography and NMR spectroscopy indicate a predominantly hydrophobic interface. Knowledge of the interaction between InvH and InvG brings us closer to understanding the mechanisms by which pilotins assemble the secretin pore.

Novel insight into an energy transduction mechanism of the bacterial flagellar type III protein export

Type III secretion system (T3SS) is a protein translocator complex family including pathogenic injectisome or bacterial flagellum. The inejectisomal T3SS serves to deliver virulence proteins into host cell and the flagellar T3SS constructs the flagellar axial structure. Although earlier studies have provided many findings on the molecular mechanism of the Type III protein export, they were not sufficient to reveal energy transduction mechanism due to difficulties in controlling measurement conditions in vivo. Recently, we developed an in vitro flagellar Type III protein transport assay system using inverted membrane vesicles (IMVs), and analyzed protein export by using the in vitro method. We reproduced protein export of the flagellar T3SS, hook assembly and substrate specificity switch in IMV to a similar extent to what is seen in living cell. Furthermore, we demonstrated that ATP-hydrolysis energy can drive protein transport even in the absence of proton-motive force (PMF). In this mini-review, we will summarize our new in vitro Type III transport assay method and our findings on the molecular mechanism of Type III protein export.

Novel insights into the mechanism of SepL-mediated control of effector secretion in enteropathogenic Escherichia coli

Type three secretion systems (T3SSs) are virulence determinants employed by several pathogenic bacteria as molecular syringes to inject effector proteins into host cells. Diarrhea‐producing enteropathogenic Escherichia coli (EPEC) uses a T3SS to colonize the intestinal tract. T3S is a highly coordinated process that ensures hierarchical delivery of three classes of substrates: early (inner rod and needle subunits), middle (translocators), and late (effectors). Translocation of effectors is triggered upon host‐cell contact in response to different environmental cues, such as calcium levels. The T3S substrate specificity switch from middle to late substrates in EPEC is regulated by the SepL and SepD proteins, which interact with each other and form a trimeric complex with the chaperone CesL. In this study, we investigated the link between calcium concentration and secretion regulation by the gatekeeper SepL. We found that calcium depletion promotes late substrate secretion in a translocon‐independent manner. Furthermore, the stability, formation, and subcellular localization of the SepL/SepD/CesL regulatory complex were not affected by the absence of calcium. In addition, we demonstrate that SepL interacts in a calcium‐independent manner with the major export gate component EscV, which in turn interacts with both middle and late secretion substrates, providing a docking site for T3S. These results suggest that EscV serves as a binding platform for both the SepL regulatory protein and secreted substrates during the ordered assembly of the T3SS.

Functional Characterization of EscK (Orf4), a Sorting Platform Component of the Enteropathogenic Escherichia coli Injectisome

The type III secretion system (T3SS) is a supramolecular machine used by many bacterial pathogens to translocate effector proteins directly into the eukaryotic host cell cytoplasm. Enteropathogenic Escherichia coli (EPEC) is an important cause of infantile diarrheal disease in underdeveloped countries. EPEC virulence relies on a T3SS encoded within a chromosomal pathogenicity island known as the locus of enterocyte effacement (LEE). In this work, we pursued the functional characterization of the LEE-encoded protein EscK (previously known as Orf4). We provide evidence indicating that EscK is crucial for efficient T3S and belongs to the SctK (OrgA/YscK/MxiK) protein family, whose members have been implicated in the formation of a sorting platform for secretion of T3S substrates. Bacterial fractionation studies showed that EscK localizes to the inner membrane independently of the presence of any other T3SS component. Combining yeast two-hybrid screening and pulldown assays, we identified an interaction between EscK and the C-ring/sorting platform component EscQ. Site-directed mutagenesis of conserved residues revealed amino acids that are critical for EscK function and for its interaction with EscQ. In addition, we found that T3S substrate overproduction is capable of compensating for the absence of EscK. Overall, our data suggest that EscK is a structural component of the EPEC T3SS sorting platform, playing a central role in the recruitment of T3S substrates for boosting the efficiency of the protein translocation process.

IMPORTANCE

The type III secretion system (T3SS) is an essential virulence determinant for enteropathogenic Escherichia coli (EPEC) colonization of intestinal epithelial cells. Multiple EPEC effector proteins are injected via the T3SS into enterocyte cells, leading to diarrheal disease. The T3SS is encoded within a genomic pathogenicity island termed the locus of enterocyte effacement (LEE). Here we unravel the function of EscK, a previously uncharacterized LEE-encoded protein. We show that EscK is central for T3SS biogenesis and function. EscK forms a protein complex with EscQ, the main component of the cytoplasmic sorting platform, serving as a docking site for T3S substrates. Our results provide a comprehensive functional analysis of an understudied component of T3SSs.

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.

The extended structure of the periplasmic region of CdsD, a structural protein of the type III secretion system of Chlamydia trachomatis

The type III secretion system (T3SS) is required for the virulence of many gram-negative bacterial human pathogens. It is composed of several structural proteins, forming the secretion needle and its basis, the basal body. In Chlamydia spp., the T3SS inner membrane ring (IM-ring) of the basal body is formed by the periplasmic part of CdsD (outer ring) and CdsJ (inner ring). Here we describe the crystal structure of the C-terminal, periplasmic part of CdsD, not including the last 60 residues. Two crystal forms were obtained, grown in three different crystallization conditions. In both crystal forms there is one molecule per asymmetric unit adopting a similar extended structure. The structures consist of three periplasmic domains (PDs) of similar αββαβ topology as seen also in the structures of the homologous PrgH (Salmonella typhimurium) and YscD (Yersinia enterocolitica). Only in the C2 crystal form, there is a C-terminal additional helix after the PD3 domain. The relative orientation of the three subsequent CdsD PD domains with respect to each other is more extended than in PrgH but less extended than in YscD. Small-angle X-ray scattering data show that also in solution this CdsD construct adopts the same elongated shape. In both crystal forms the CdsD molecules are packed in a parallel fashion, using translational crystallographic symmetry. The most extensive crystal contacts are preserved in both crystal forms, suggesting a possible mode of assembly of the CdsD periplasmic part into a 24-mer complex forming the outer ring of the IM-ring of the T3SS.

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.

Molecular ruler determines needle length for the Salmonella Spi-1 injectisome

The type-III secretion (T3S) systems of bacteria are part of self-assembling nanomachines: the bacterial flagellum that enables cells to propel themselves through liquid and across hydrated surfaces, and the injectisome that delivers pathogenic effector proteins into eukaryotic host cells. Although the flagellum and injectisome serve different purposes, they are evolutionarily related and share many structural similarities. Core features to these T3S systems are intrinsic length control mechanisms for external cellular projections: the hook of the flagellum and the injectisome needle. We present evidence that the Spi-1 injectisome, like the Salmonella flagellar hook, uses a secreted molecular ruler, InvJ, to determine needle length. This result supports a universal length control mechanism using molecular rulers for T3S systems.

Visualization of the type III secretion sorting platform of Shigella flexneri

Bacterial type III secretion machines are widely used to inject virulence proteins into eukaryotic host cells. These secretion machines are evolutionarily related to bacterial flagella and consist of a large cytoplasmic complex, a transmembrane basal body, and an extracellular needle. The cytoplasmic complex forms a sorting platform essential for effector selection and needle assembly, but it remains largely uncharacterized. Here we use high-throughput cryoelectron tomography (cryo-ET) to visualize intact machines in a virulent Shigella flexneri strain genetically modified to produce minicells capable of interaction with host cells. A high-resolution in situ structure of the intact machine determined by subtomogram averaging reveals the cytoplasmic sorting platform, which consists of a central hub and six spokes, with a pod-like structure at the terminus of each spoke. Molecular modeling of wild-type and mutant machines allowed us to propose a model of the sorting platform in which the hub consists mainly of a hexamer of the Spa47 ATPase, whereas the MxiN protein comprises the spokes and the Spa33 protein forms the pods. Multiple contacts among those components are essential to align the Spa47 ATPase with the central channel of the MxiA protein export gate to form a unique nanomachine. The molecular architecture of the Shigella type III secretion machine and its sorting platform provide the structural foundation for further dissecting the mechanisms underlying type III secretion and pathogenesis and also highlight the major structural distinctions from bacterial flagella.

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.

In situ structural analysis of the Yersinia enterocolitica injectisome

Injectisomes are multi-protein transmembrane machines allowing pathogenic bacteria to inject effector proteins into eukaryotic host cells, a process called type III secretion. Here we present the first three-dimensional structure of Yersinia enterocolitica and Shigella flexneri injectisomes in situ and the first structural analysis of the Yersinia injectisome. Unexpectedly, basal bodies of injectisomes inside the bacterial cells showed length variations of 20%. The in situ structures of the Y. enterocolitica and S. flexneri injectisomes had similar dimensions and were significantly longer than the isolated structures of related injectisomes. The crystal structure of the inner membrane injectisome component YscD appeared elongated compared to a homologous protein, and molecular dynamics simulations documented its elongation elasticity. The ring-shaped secretin YscC at the outer membrane was stretched by 30–40% in situ, compared to its isolated liposome-embedded conformation. We suggest that elasticity is critical for some two-membrane spanning protein complexes to cope with variations in the intermembrane distance.

DOI:http://dx.doi.org/10.7554/eLife.00792.001

Humans and other animals can use the five senses—touch, taste, sight, smell, and hearing—to interpret the world around them. Single-celled organisms, however, must rely on molecular cues to understand their immediate surroundings. In particular, bacteria gather information about external conditions, including potential hosts nearby, by secreting protein sensors that can relay messages back to the cell.

Bacteria export these sensors via secretion systems that enable the organism both to receive information about the environment and to invade a host cell. A total of seven separate secretion systems, known as types I–VII, have been identified. These different secretion systems handle distinct cargoes, allowing the bacterial cell to respond to a range of feedback from the external milieu.

The type III secretion system, also known as the ‘injectisome’, is found in bacterial species that are enclosed by two membranes separated by a periplasmic space. The injectisome comprises different components that combine to form the basal body, which spans the inner and outer membranes, and a projection from the basal body, called the hollow needle, that mediates the export of cargo from a bacterium to its host or the local environment.

The distance between the inner and outer membranes may vary across species or according to environmental conditions, so the basal body must be able to accommodate these changes. However, no mechanism has yet been established that might introduce such elasticity into the injectisome. Now, Kudryashev et al. have generated three-dimensional structures for the injectisomes of two species of bacteria, Shigella flexneri and Yersinia enterocolitica, and shown that the size of the basal body can fluctuate by up to 20%.

Kudryashev et al. imaged whole injectisomes in these two species and found that the height of the basal body was proportional to the distance between the inner and outer membranes. To probe how this could occur, the properties of two proteins that are important components of the basal body were studied in greater detail. YscD, a protein that extends across the periplasmic space, was crystallized and its structure was then determined and used to develop a computer model to assess its compressibility: this model indicated that YscD could stretch or contract by up to 50% of its total length. The outer membrane component YscC also appeared elastic: when the protein was isolated and introduced into synthetic membranes, its length was reduced 30–40% relative to that observed in intact bacterial membranes.

A further experiment confirmed the adaptability of the basal body: when the separation of the membranes was deliberately increased by placing bacteria in a high-salt medium, the basal body extended approximately 10% in length. Cumulatively, therefore, these experiments suggest that the in-built flexibility of the basal body of the injectisome allows bacteria to adjust to environmental changes while maintaining their sensory abilities and host-invasion potential.

DOI:http://dx.doi.org/10.7554/eLife.00792.002

Regulation of the Yersinia type III secretion system: traffic control

Yersinia species, as well as many other Gram-negative pathogens, use a type III secretion system (T3SS) to translocate effector proteins from the bacterial cytoplasm to the host cytosol. This T3SS resembles a molecular syringe, with a needle-like shaft connected to a basal body structure, which spans the inner and outer bacterial membranes. The basal body of the injectisome shares a high degree of homology with the bacterial flagellum. Extending from the T3SS basal body is the needle, which is a polymer of a single protein, YscF. The distal end of the needle serves as a platform for the assembly of a tip complex composed of LcrV. Though never directly observed, prevailing models assume that LcrV assists in the insertion of the pore-forming proteins YopB and YopD into the host cell membrane. This completes a bridge between the bacterium and host cell to provide a continuous channel through which effectors are delivered. Significant effort has gone into understanding how the T3SS is assembled, how its substrates are recognized and how substrate delivery is controlled. Arguably the latter topic is the least understood; however, recent advances have provided new insight, and therefore, this review will focus primarily on summarizing the current state of knowledge regarding the control of substrate delivery by the T3SS. Specifically, we will discuss the roles of YopK, as well as YopN and YopE, which have long been linked to regulation of translocation. We also propose models whereby the YopK regulator communicates with the basal body of the T3SS to control translocation.

Intrinsic and Extrinsic Regulation of Type III Secretion Gene Expression in Pseudomonas Aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that is particularly problematic in the healthcare setting where it is a frequent cause of pneumonia, bloodstream, and urinary tract infections. An important determinant of P. aeruginosa virulence is a type III secretion system (T3SS). T3SS-dependent intoxication is a complex process that minimally requires binding of P. aeruginosa to host cells, injection of the cytotoxic effector proteins through the host cell plasma membrane, and induction of T3SS gene expression. The latter process, referred to as contact-dependent expression, involves a well-characterized regulatory cascade that activates T3SS gene expression in response to host cell contact. Although host cell contact is a primary activating signal for T3SS gene expression, the involvement of multiple membrane-bound regulatory systems indicates that additional environmental signals also play a role in controlling expression of the T3SS. These regulatory systems coordinate T3SS gene expression with many other cellular activities including motility, mucoidy, polysaccharide production, and biofilm formation. The signals to which the organism responds are poorly understood but many seem to be coupled to the metabolic state of the cell and integrated within a master circuit that assimilates informational signals from endogenous and exogenous sources. Herein we review progress toward unraveling this complex circuitry, provide analysis of the current knowledge gaps, and highlight potential areas for future studies. Complete understanding of the regulatory networks that control T3SS gene expression will maximize opportunities for the development of strategies to treat P. aeruginosa infections.