The Pseudomonas syringae HrpJ protein controls the secretion of type III translocator proteins and has a virulence role inside plant cells

The bacterial plant pathogen Pseudomonas syringae injects effector proteins into plant cells via a type III secretion system (T3SS), which is required for pathogenesis. The protein HrpJ is secreted by P. syringae and is required for a fully functional T3SS. A hrpJ mutant is non-pathogenic and cannot inject effectors into plant cells or secrete the harpin HrpZ1. Here we show that the hrpJ mutant also cannot secrete the harpins HrpW1 and HopAK1 or the translocator HrpK1, suggesting that these proteins are required in the translocation (injection) of effectors into plant cells. Complementation of the hrpJ mutant with secretion incompetent HrpJ derivatives restores the secretion of HrpZ1 and HrpW1 and the ability to elicit a hypersensitive response, a measure of translocation. However, growth in planta and disease symptom production is only partially restored, suggesting that secreted HrpJ may have a direct role in virulence. Transgenic Arabidopsis plants expressing HrpJ-HA complemented the virulence phenotype of the hrpJ mutant expressing a secretion incompetent HrpJ derivative and were reduced in their immune responses. Collectively, these data indicate that HrpJ has a dual role in P. syringae: inside bacterial cells HrpJ controls the secretion of translocator proteins and inside plant cells it suppresses plant immunity.     

The Shigella T3SS needle transmits a signal for MxiC release, which controls secretion of effectors

Type III secretion systems (T3SSs) are key determinants of virulence in many Gram-negative bacteria, including animal and plant pathogens. They inject 'effector' proteins through a 'needle' protruding from the bacterial surface directly into eukaryotic cells after assembly of a 'translocator' pore in the host plasma membrane. Secretion is a tightly regulated process, which is blocked until physical contact with a host cell takes place. Host cell sensing occurs through a distal needle 'tip complex' and translocators are secreted before effectors. MxiC, a Shigella T3SS substrate, prevents premature effector secretion. Here, we examine how the different parts of T3SSs work together to allow orderly secretion. We show that T3SS assembly and needle tip composition are not altered in an mxiC mutant. We find that MxiC not only represses effector secretion but that it is also required for translocator release. We provide genetic evidence that MxiC acts downstream of the tip complex and then the needle during secretion activation. Finally, we show that the needle controls MxiC release. Therefore, for the first time, our data allow us to propose a model of secretion activation that goes from the tip complex to cytoplasmic MxiC via the needle.     

Key steps in type III secretion system (T3SS) towards translocon assembly with potential sensor at plant plasma membrane

Many plant‐ and animal‐pathogenic Gram‐negative bacteria employ the type III secretion system (T3SS) to translocate effector proteins from bacterial cells into the cytosol of eukaryotic host cells. The effector translocation occurs through an integral component of T3SS, the channel‐like translocon, assembled by hydrophilic and hydrophobic proteinaceous translocators in a two‐step process. In the first, hydrophilic translocators localize to the tip of a proteinaceous needle in animal pathogens, or a proteinaceous pilus in plant pathogens, and associate with hydrophobic translocators, which insert into host plasma membranes in the second step. However, the pilus needs to penetrate plant cell walls in advance. All hydrophilic translocators so far identified in plant pathogens are characteristic of harpins: T3SS accessory proteins containing a unitary hydrophilic domain or an additional enzymatic domain. Two‐domain harpins carrying a pectate lyase domain potentially target plant cell walls and facilitate the penetration of the pectin‐rich middle lamella by the bacterial pilus. One‐domain harpins target plant plasma membranes and may play a crucial role in translocon assembly, which may also involve contrapuntal associations of hydrophobic translocators. In all cases, sensory components in the target plasma membrane are indispensable for the membrane recognition of translocators and the functionality of the translocon. The conjectural sensors point to membrane lipids and proteins, and a phosphatidic acid and an aquaporin are able to interact with selected harpin‐type translocators. Interactions between translocators and their sensors at the target plasma membrane are assumed to be critical for translocon assembly.     

The Type III Secretion Translocation Pore Senses Host Cell Contact

Type III secretion systems (T3SS) are nano-syringes used by a wide range of Gram-negative pathogens to promote infection by directly injecting effector proteins into targeted host cells. Translocation of effectors is triggered by host-cell contact and requires assembly of a pore in the host-cell plasma membrane, which consists of two translocator proteins. Our understanding of the translocation pore, how it is assembled in the host cell membrane and its precise role in effector translocation, is extremely limited. Here we use a genetic technique to identify protein-protein contacts between pore-forming translocator proteins, as well as the T3SS needle-tip, that are critical for translocon function. The data help establish the orientation of the translocator proteins in the host cell membrane. Analysis of translocon function in mutants that break these contacts demonstrates that an interaction between the pore-forming translocator PopD and the needle-tip is required for sensing host cell contact. Moreover, tethering PopD at a dimer interface also specifically prevents host-cell sensing, arguing that the translocation pore is actively involved in detecting host cell contact. The work presented here therefore establishes a signal transduction pathway for sensing host cell contact that is initiated by a conformational change in the translocation pore, and is subsequently transmitted to the base of the apparatus via a specific contact between the pore and the T3SS needle-tip.     

HpaA from Xanthomonas is a regulator of type III secretion

The Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to inject effector proteins into the host cell cytoplasm. Efficient secretion of several effector proteins depends on the cytoplasmic global T3S chaperone HpaB. In this study, we show that HpaB interacts with the virulence factor HpaA, which is secreted by the T3S system and translocated into the plant cell. HpaA promotes secretion of pilus, translocon and effector proteins and therefore appears to be an important control protein of the T3S system. Protein-protein interaction studies and the analysis of HpaA deletion derivatives revealed that the C-terminal protein region, which contains a HpaB binding site, is crucial for the contribution of HpaA to T3S. Secretion of pilus and translocon proteins is not affected when HpaA is expressed as an N-terminal deletion derivative that lacks the secretion and translocation signal. Our data suggest that binding of HpaA to HpaB within the bacterial cell favours secretion of extracellular components of the secretion apparatus. Secretion of HpaA presumably liberates HpaB and thus promotes effector protein secretion after assembly of the T3S apparatus.     

Scc1 (CP0432) and Scc4 (CP0033) function as a type III secretion chaperone for CopN of Chlamydia pneumoniae

The Chlamydia pneumoniae CopN protein is a member of the YopN/TyeA/InvE/MxiC family of secreted proteins that function to regulate the secretion of type III secretion system (T3SS) translocator and effector proteins. In this study, the Scc1 (CP0432) and Scc4 (CP0033) proteins of C. pneumoniae AR-39 were demonstrated to function together as a type III secretion chaperone that binds to an N-terminal region of CopN. The Scc1/Scc4 chaperone promoted the efficient secretion of CopN via a heterologous T3SS, whereas, the Scc3 chaperone, which binds to a C-terminal region of CopN, reduced CopN secretion.     

Analyses of the secretomes of Erwinia amylovora and selected hrp mutants reveal novel type III secreted proteins and an effect of HrpJ on extracellular harpin levels

Erwinia amylovora is a plant pathogenic enterobacterium that causes fire blight disease of apple, pear and other rosaceous plants. A type III (T3) secretion system, encoded by clustered, chromosomal hrp genes (hypersensitive response and pathogenicity), is essential for infection, but only a few proteins are known that are secreted through this pathway (the T3 'secretome'). We developed an efficient protocol for purification and concentration of extracellular proteins and used it to characterize the T3 secretome of E. amylovora Ea273 by comparing preparations from the wild-type strain with those from mutants defective in hrp secretion, regulation, or in genes encoding putative T3-secreted proteins. Proteins were resolved by gel electrophoresis and identified using mass spectrometry and a draft sequence of the E. amylovora genome. Twelve T3-secreted proteins were identified, including homologues of known effector and helper proteins, and HrpJ, a homologue of YopN of Yersinia pestis . Several previously uncharacterized T3-secreted proteins were designated as Eops for Erwinia outer proteins. Analysis of the secretome of a non-polar hrpJ mutant demonstrated that HrpJ is required for accumulation of wild-type levels of secreted harpins. HrpJ was found to be essential for pathogenesis, and to play a major role in elicitation of the hypersensitive reaction in tobacco.     

VirB2 and VirB5 proteins: specialized adhesins in bacterial type-IV secretion systems?

Many type-IV secretion systems (T4SSs) of plant and human pathogens assemble a pilus used to inject virulence molecules (effectors) into host target cells. The T4SS of Agrobacterium tumefaciens consists of VirB1-VirB11 and VirD4 proteins. Whether targeting of T4SSs to the host requires a T4SS-adhesin that specifically engages host receptors for delivery of effectors has, until recently, remained unclear. Recent data of Agrobacterium and Helicobacter indicate that two classes of T4SS components, VirB2 and VirB5, might function as adhesins that mediate host-cell targeting through binding to specific host receptors. Here, we discuss this important issue and recent progress in the field.     

Assembly and structure of the T3SS

The Type III Secretion System (T3SS) is a multi-mega Dalton apparatus assembled from more than twenty components and is found in many species of animal and plant bacterial pathogens. The T3SS creates a contiguous channel through the bacterial and host membranes, allowing injection of specialized bacterial effector proteins directly to the host cell. In this review, we discuss our current understanding of T3SS assembly and structure, as well as highlight structurally characterized Salmonella effectors. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.     

Interactions and predicted host membrane topology of the enteropathogenic Escherichia coli translocator protein EspB

Type 3 secretion systems (T3SSs) are critical for the virulence of numerous deadly Gram-negative pathogens. T3SS translocator proteins are required for effector proteins to traverse the host cell membrane and perturb cell function. Translocator proteins include two hydrophobic proteins, represented in enteropathogenic Escherichia coli (EPEC) by EspB and EspD, which are thought to interact and form a pore in the host membrane. Here we adapted a sequence motif recognized by a host kinase to demonstrate that residues on the carboxyl-terminal side of the EspB transmembrane domain are localized to the host cell cytoplasm. Using functional internal polyhistidine tags, we confirm an interaction between EspD and EspB, and we demonstrate, for the first time, an interaction between EspD and the hydrophilic translocator protein EspA. Using a panel of espB insertion mutations, we describe two regions on either side of a putative transmembrane domain that are required for the binding of EspB to EspD. Finally, we demonstrate that EspB variants incapable of binding EspD fail to adopt the proper host cell membrane topology. These results provide new insights into interactions between translocator proteins critical for virulence.     

农杆菌T-复合物的形成与转运研究进展

在简要介绍农杆菌T-DNA转运全过程的基础上,结合作者近年的工作,重点对T-复合物的形成和T-复合物在农杆菌细胞内的转运机理的最新进展进行归纳和评述．农杆菌能够将其Ti质粒上的一段DNA以单链DNA-蛋白质复合物(简称T-复合物)的形式,通过其细胞两端的四型分泌系统(typeⅣ secretion system,T4SS)转运到宿主植物中,并使宿主发生遗传转化,因而农杆菌介导的T-DNA转运技术已成为应用最广泛的植物转基因技术,同时,由于转运T-复合物的T4SS也是某些质粒接合转移和许多病源微生物分泌致病效应蛋白的通道,因此,农杆菌T-DNA转运机理的研究受到了广泛的重视和关注,使得这方面的研究进展非常迅速．     

Type III protein secretion in Pseudomonas syringae

The type III secretion system is an essential virulence system used by many Gram-negative bacterial pathogens to deliver effector proteins into host cells. This review summarizes recent advancements in the understanding of the type III secretion system of Pseudomonas syringae, including regulation of the type III secretion genes, assembly of the Hrp pilus, secretion signals, the putative type III effectors identified to date, and their virulence action after translocation into plant cells.     

Pseudomonas syringae HrpP Is a Type III Secretion Substrate Specificity Switch Domain Protein That Is Translocated into Plant Cells but Functions Atypically for a Substrate-Switching Protein

Pseudomonas syringae delivers virulence effector proteins into plant cells via an Hrp1 type III secretion system (T3SS). P. syringae pv. tomato DC3000 HrpP has a C-terminal, putative T3SS substrate specificity switch domain, like Yersinia YscP. A ΔhrpP DC3000 mutant could not cause disease in tomato or elicit a hypersensitive response (HR) in tobacco, but the HR could be restored by expression of HrpP in trans. Though HrpP is a relatively divergent protein in the T3SS of different P. syringae pathovars, hrpP from P. syringae pv. syringae 61 and P. syringae pv. phaseolicola 1448A restored HR elicitation and pathogenicity to DC3000 ΔhrpP. HrpP was translocated into Nicotiana benthamiana cells via the DC3000 T3SS when expressed from its native promoter, but it was not secreted in culture. N- and C-terminal truncations of HrpP were tested for their ability to be translocated and to restore HR elicitation activity to the ΔhrpP mutant. No N-terminal truncation completely abolished translocation, implying that HrpP has an atypical T3SS translocation signal. Deleting more than 20 amino acids from the C terminus abolished the ability to restore HR elicitation. HrpP fused to green fluorescent protein was no longer translocated but could restore HR elicitation activity to the ΔhrpP mutant, suggesting that translocation is not essential for the function of HrpP. No T3SS substrates were detectably secreted by DC3000 ΔhrpP except the pilin subunit HrpA, which unexpectedly was secreted poorly. HrpP may function somewhat differently than YscP because the P. syringae T3SS pilus likely varies in length due to differing plant cell walls.Many proteobacterial pathogens use a type III secretion system (T3SS) as their primary mechanism to overcome and infect eukaryotic hosts. T3SSs are complex macromolecular machines that span both the bacterial cell envelope and host cell barriers to deliver proteins, commonly termed effectors, from the bacterial cytoplasm into the host cytoplasm (13, 19). After delivery into the host, effector proteins manipulate host cell function and suppress host defenses, allowing bacterial proliferation and disease development (6, 20). Bacteria that rely on T3SS to cause disease include plant pathogens such as Pseudomonas syringae, Ralstonia solanacearum, Erwinia and Xanthomonas species and animal pathogens in the genera Yersinia, Salmonella, Shigella, Escherichia, and Pseudomonas. While the repertoire of effectors delivered by a given T3SS is unique, the T3SS machinery is more universal (13). T3SS includes a core set of eight conserved proteins. These proteins, which are also conserved in bacterial flagellar biogenesis machines, make up the multiringed base structure, or basal body, that spans the bacterial membranes and cell wall. T3SS machines are also comprised of less-conserved and unique proteins that vary between systems. These include regulatory proteins that orchestrate construction of the machine and the extracellular components that function to translocate effectors across host barriers.The extracellular portion of the T3SS is comprised of the pilus or needle appendage (in plant or animal pathogens, respectively), which acts as a conduit for effector delivery, and the translocon complex, which creates the pore in the host cell membrane. These substructures vary between different T3SSs; presumably these external structures have adapted to allow different bacteria to infect different types of host cells. For Yersinia enterocolitica to infect macrophage cells, the T3SS needle must be a particular length (∼58 nm) to bridge the lipopolysaccharides extending from the bacterial outer membrane and reach the host cell membrane (35). Several other animal pathogens have T3SS needles of a defined length (48). Enteropathogenic Escherichia coli also has an additional extension beyond the needle called the EspA filament that functions to span the mucous layer found outside enterocyte cells (13). In plant pathogens, however, the extracellular gap between a bacterium and a plant cell includes a thick plant cell wall that is variable in width between plant species. Consequently, plant pathogenic Pseudomonas syringae has a pilus that can measure over 1 μm in vitro (25).Another major difference between the T3SS machineries of animal and plant pathogens is their translocon complexes. In animal pathogens, these are typically comprised of three essential proteins, but there is growing evidence that plant pathogen translocons employ diverse, functionally redundant components (28). There is growing interest in understanding the regulatory players that orchestrate the construction of diverse machinery. It is hypothesized that the assembly of the T3SS must involve several tightly regulated steps that allow secretion of the required components, followed by that of effectors upon completion. Of particular interest here is the control of pilus/needle subunit secretion, which is necessary when the pilus/needle is being constructed but would presumably compete with translocon and effector secretion after the T3SS is complete.We study the model plant pathogen P. syringae pv. tomato (Pto) DC3000, the causal agent of bacterial speck of tomato and Arabidopsis thaliana (8). DC3000 has a T3SS that delivers ca. 28 effectors and is essential for pathogenesis (11, 12, 30, 43). The P. syringae T3SS is encoded by hrp and hrc genes (hypersensitive response and pathogenicity/conserved), which are located in a pathogenicity island on the chromosome (4). hrc genes encode the conserved core components present in every T3SS. hrp genes encode T3SS components that are divergent or unique to P. syringae and enterobacterial plant pathogens, which also possess Hrp1 class T3SS (13). In contrast, plant pathogenic Ralstonia and Xanthomonas spp. have Hrp2 class T3SS, as indicated by several different Hrp proteins and distinct regulatory systems.To better understand the T3SS machinery, we previously conducted a survey of the hrp genes of P. syringae pv. syringae (Psy) 61 to complete the inventory of all those encoding proteins capable of traveling the T3SS into plant cells when expressed from a constitutive promoter (39). We hypothesized that these proteins might aid in pilus or translocon construction or regulate the construction process. HrpP was one protein found to be a T3SS substrate and important for secretion and translocation of the model effector AvrPto. Importantly, HrpP is related to a well-studied protein from Yersinia enterocolitica, YscP, which is a T3SS-secreted protein and a regulator responsible for switching the T3SS from secreting needle subunits to secreting effector proteins (15, 38, 47). It has also been shown that secretion of YscP into the culture medium is not essential for the switch function and that there may be two type III secretion signals embedded in YscP (2).The phenotype of a yscP mutant is unregulated secretion of the needle subunit, no secretion of effectors, and production of needles of indeterminate length. The switching phenotype requires a domain at the C terminus of YscP called the type III secretion substrate specificity switch (T3S4) domain, which is a conserved feature unifying its homologs (1). YscP has been proposed to act as a molecular ruler because the length of the YscP protein is directly correlated with the length of the Ysc needle (26). According to this model, when the needle has reached its proper length, YscP signals to the T3SS machinery to stop secreting needle subunits and begin secreting effector proteins. However, other functional models have been hypothesized for homologs of YscP. A recent study of the Salmonella enterica serovar Typhimurium YscP homolog InvJ showed that an invJ mutant lacked an inner rod. When the inner rod protein PrgJ was overexpressed, the length of the needle decreased relative to that of the wild type, leading the researchers to conclude that InvJ controls the inner rod, which in turn controls needle length (33). Recent evidence in Yersinia has lent more support to this model. YscP was found to negatively control secretion of YscI, the inner rod protein (51). Also, certain YscI mutations affected needle assembly but not effector secretion, implying that YscI may be a key player in substrate switching. Little is known about HrpB, the inner rod homolog in P. syringae (22), other than that the protein can be translocated into plant cells and is essential for T3SS function (39).Other models for length control/substrate switching have been proposed, such as the “C-ring cup model” in flagella, which was based on the observation that certain mutations in proteins that make up the inner membrane C ring of the basal body lead to shorter hooks (the flagellar equivalent of the needle), thus suggesting that C-ring capacity controls hook length (32). A more recent, flagellar “molecular-clock” model suggests that because overexpression of hook subunits leads to longer hooks and hook polymerization-defective mutants make shorter hooks, hook polymerization initiates a countdown, and the timing, in cooperation with the YscP homolog FliK, determines final hook length (34).HrpP is considered a member of the YscP/FliK family due mostly to the presence of a T3S4 domain at its C terminus. HrpP is also proline rich (10.6%), which is considered a characteristic of the family. The most striking feature of HrpP is its small size; the protein is 189 amino acids, compared with YscP from Y. enterocolitica, which is 453 amino acids and 8.4% proline. We were intrigued by how HrpP functions in P. syringae to regulate a pilus that can measure several hundred nanometers in length. Also, unlike animal pathogen needles and flagellar hooks, the pilus of P. syringae is predicted to be indeterminate in length, based on the fact that plant cell walls vary in width between species (40).We hypothesized that HrpP would be a main player in regulating pilus construction in P. syringae by allowing the system to make the transition between secretion of pilus subunits and secretion of translocon or effector proteins, though perhaps by a novel mechanism. In this study, we more precisely define the role of HrpP in the P. syringae T3SS. We show that HrpP is a T3SS substrate in DC3000, is translocated into plant cells at levels equivalent to those of effectors, and is essential for the function of the T3SS. Though it is highly translocated and variable, we found that HrpP from different P. syringae pathovars could complement the DC3000 hrpP mutant. Analysis of truncations of HrpP and an impassible HrpP-green fluorescent protein (GFP) fusion suggests that it has structural similarities to YscP, but surprisingly, HrpP was found to be required for full secretion of the pilus subunit HrpA as well as for translocation of HrpB.     

Using disruptive insertional mutagenesis to identify the in situ structure‐function landscape of the Shigella translocator protein IpaB

Bacterial type III secretion systems (T3SS) are used to inject proteins into mammalian cells to subvert cellular functions. The Shigella T3SS apparatus (T3SA) is comprised of a basal body, cytoplasmic sorting platform and exposed needle with needle “tip complex” (TC). TC maturation occurs when the translocator protein IpaB is recruited to the needle tip where both IpaD and IpaB control secretion induction. IpaB insertion into the host membrane is the first step of translocon pore formation and secretion induction. We employed disruptive insertional mutagenesis, using bacteriophage T4 lysozyme (T4L), within predicted IpaB loops to show how topological features affect TC functions (secretion control, translocon formation and effector secretion). Insertions within the N‐terminal half of IpaB were most likely to result in a loss of steady‐state secretion control, however, all but the two that were not recognized by the T3SA retained nearly wild‐type hemolysis (translocon formation) and invasiveness levels (effector secretion). In contrast, all but one insertion in the C‐terminal half of IpaB maintained secretion control but were impaired for hemolysis and invasion. These nature of the data suggest the latter mutants are defective in a post‐secretion event, most likely due to impaired interactions with the second translocator protein IpaC. Intriguingly, only two insertion mutants displayed readily detectable T4L on the bacterial surface. The data create a picture in which the makeup and structure of a functional T3SA TC is highly amenable to physical perturbation, indicating that the tertiary structure of IpaB within the TC is more plastic than previously realized.     

The cell death response to enteropathogenic Escherichia coli infection

Given the critical roles of inflammation and programmed cell death in fighting infection, it is not surprising that many bacterial pathogens have evolved strategies to inactivate these defences. The causative agent of infant diarrhoea, enteropathogenic Escherichia coli (EPEC), is an extracellular, intestinal pathogen that blocks both inflammation and programmed cell death. EPEC attaches to enterocytes, remains in the gut lumen and utilizes a type III secretion system (T3SS) to inject multiple virulence effector proteins directly into the infected cell, many of which subvert host antimicrobial processes through the disruption of signalling pathways. Recently, T3SS effector proteins from EPEC have been identified that inhibit death receptor‐induced apoptosis. Here we review the mechanisms used by EPEC T3SS effectors to manipulate apoptosis and promote host cell survival and discuss the role of these activities during infection.     

The Hrp pilus of Pseudomonas syringae elongates from its tip and acts as a conduit for translocation of the effector protein HrpZ

The type III secretion system (TTSS) is an essential requirement for the virulence of many Gram-negative bacteria infecting plants, animals and man. Pathogens use the TTSS to deliver effector proteins from the bacterial cytoplasm to the eukaryotic host cell, where the effectors subvert host defences. Plant pathogens have to translocate their effector proteins through the plant cell wall barrier. The best candidates for directing effector protein traffic are bacterial appendages attached to the membrane-bound components of the TTSS. We have investigated the protein secretion route in relation to the TTSS appendage, termed the Hrp pilus, of the plant pathogen Pseudomonas syringae pv. tomato. By pulse expression of proteins combined with immunoelectron microscopy, we show that the Hrp pilus elongates by the addition of HrpA pilin subunits at the distal end, and that the effector protein HrpZ is secreted only from the pilus tip. Our results indicate that both HrpA and HrpZ travel through the Hrp pilus, which functions as a conduit for the long-distance translocation of effector proteins.