YopD self-assembly and binding to LcrV facilitate type III secretion activity by Yersinia pseudotuberculosis

Distinct Mechanisms of Type 3 Secretion System Recognition Control LTB4 Synthesis in Neutrophils versus Macrophages

Leukotriene B4 (LTB4) is critical for initiating the inflammatory cascade in response to infection. However, Yersinia pestis colonizes the host by inhibiting the timely synthesis of LTB4 and inflammation. Here, we show that the bacterial type 3 secretion system (T3SS) is the primary pathogen associated molecular pattern (PAMP) responsible for LTB4 production by leukocytes in response to Yersinia and Salmonella, but synthesis is inhibited by the Yop effectors during Yersinia interactions. Moreover, we unexpectedly discovered that T3SS-mediated LTB4 synthesis by neutrophils and macrophages require two distinct host signaling pathways. We show that the SKAP2/PLC signaling pathway is essential for LTB4 production by neutrophils but not macrophages. Instead, phagocytosis and the NLRP3/CASP1 inflammasome are needed for LTB4 synthesis by macrophages. Finally, while recognition of the T3SS is required for LTB4 production, we also discovered a second unrelated PAMP-mediated signal independently activates the MAP kinase pathway needed for LTB4 synthesis. Together, these data demonstrate significant differences in the signaling pathways required by macrophages and neutrophils to quickly respond to bacterial infections.

Macrophage innate immune responses delineate between defective translocon assemblies produced by Yersinia pseudotuberculosis YopD mutants

Translocon pores formed in the eukaryotic cell membrane by a type III secretion system facilitate the translocation of immune-modulatory effector proteins into the host cell interior. The YopB and YopD proteins produced and secreted by pathogenic Yersinia spp. harboring a virulence plasmid-encoded type III secretion system perform this pore-forming translocator function. We had previously characterized in vitro T3SS function and in vivo pathogenicity of a number of strains encoding sited-directed point mutations in yopD. This resulted in the classification of mutants into three different classes based upon the severity of the phenotypic defects. To investigate the molecular and functional basis for these defects, we explored the effectiveness of RAW 264.7 cell line to respond to infection by representative YopD mutants of all three classes. Signature cytokine profiles could separate the different YopD mutants into distinct categories. The activation and suppression of certain cytokines that function as central innate immune response modulators correlated well with the ability of mutant bacteria to alter anti-phagocytosis and programmed cell death pathways. These analyses demonstrated that sub-optimal translocon pores impact the extent and magnitude of host cell responsiveness, and this limits the capacity of pathogenic Yersinia spp. to fortify against attack by both early and late arms of the host innate immune response.

Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens

Gram-negative bacteria are known to subvert eukaryotic cell physiological mechanisms using a wide array of virulence factors, among which the type three-secretion system (T3SS) is often one of the most important. The T3SS constitutes a needle-like apparatus that the bacterium uses to inject a diverse set of effector proteins directly into the cytoplasm of the host cells where they can hamper the host cellular machinery for a variety of purposes. While the structure of the T3SS is somewhat conserved and well described, effector proteins are much more diverse and specific for each pathogen. The T3SS can remodel the cytoskeleton integrity to promote intracellular invasion, as well as silence specific eukaryotic cell signals, notably to hinder or elude the immune response and cause apoptosis. This is also the case in aquatic bacterial pathogens where the T3SS can often play a central role in the establishment of disease, although it remains understudied in several species of important fish pathogens, notably in Yersinia ruckeri. In the present review, we summarise what is known of the T3SS, with a special focus on aquatic pathogens and suggest some possible avenues for research including the potential to target the T3SS for the development of new anti-virulence drugs.

The type III secretion system needle, tip, and translocon

Many Gram-negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.

The Tip Complex: From Host Cell Sensing to Translocon Formation

Type III secretion systems are used by some Gram-negative bacteria to inject effector proteins into targeted eukaryotic cells for the benefit of the bacterium. The type III secretion injectisome is a complex nanomachine comprised of four main substructures including a cytoplasmic sorting platform, an envelope-spanning basal body, an extracellular needle and an exposed needle tip complex. Upon contact with a host cell, secretion is induced, resulting in the formation of a translocon pore in the host membrane. Translocon formation completes the conduit needed for effector secretion into the host cell. Control of type III secretion occurs in response to environmental signals, with the final signal being host cell contact. Secretion control occurs primarily at two sites-the cytoplasmic sorting platform, which determines secretion hierarchy, and the needle tip complex, which is critical for sensing and responding to environmental signals. The best-characterized injectisomes are those from Yersinia, Shigella and Salmonella species where there is a wealth of information on the tip complex and the two translocator proteins. Of these systems, the best characterized from a secretion regulation standpoint is Shigella. In the Shigella system, the tip complex and the first secreted translocon both contribute to secretion control and, thus, both are considered components of the tip complex. In this review, all three of these type III secretion systems are described with discussion focused on the structure and formation of the injectisome tip complex and what is known of the transition from nascent tip complex to assembled translocon pore.

Site-Directed Mutagenesis and Its Application in Studying the Interactions of T3S Components

Type III secretion systems are a prolific virulence determinant among Gram-negative bacteria. They are used to paralyze the host cell, which enables bacterial pathogens to establish often fatal infections-unless an effective therapeutic intervention is available. However, as a result of a catastrophic rise in infectious bacteria resistant to conventional antibiotics, these bacteria are again a leading cause of worldwide mortality. Hence, this report describes a pDM4-based site-directed mutagenesis strategy that is assisting in our foremost objective to better understand the fundamental workings of the T3SS, using Yersinia as a model pathogenic bacterium. Examples are given that clearly document how pDM4-mediated site-directed mutagenesis has been used to establish clean point mutations and in-frame deletion mutations that have been instrumental in identifying and understanding the molecular interactions between components of the Yersinia type III secretion system.

The Amino-Terminal Part of the Needle-Tip Translocator LcrV of Yersinia pseudotuberculosis Is Required for Early Targeting of YopH and In vivo Virulence

Type III secretion systems (T3SS) are dedicated to targeting anti-host effector proteins into the cytosol of the host cell to promote bacterial infection. Delivery of the effectors requires three specific translocator proteins, of which the hydrophilic translocator, LcrV, is located at the tip of the T3SS needle and is believed to facilitate insertion of the two hydrophobic translocators into the host cell membrane. Here we used Yersinia as a model to study the role of LcrV in T3SS mediated intracellular effector targeting. Intriguingly, we identified N-terminal lcrV mutants that, similar to the wild-type protein, efficiently promoted expression, secretion and intracellular levels of Yop effectors, yet they were impaired in their ability to inhibit phagocytosis by J774 cells. In line with this, the YopH mediated dephosphorylation of Focal Adhesion Kinase early after infection was compromised when compared to the wild type strain. This suggests that the mutants are unable to promote efficient delivery of effectors to their molecular targets inside the host cell upon host cell contact. The significance of this was borne out by the fact that the mutants were highly attenuated for virulence in the systemic mouse infection model. Our study provides both novel and significant findings that establish a role for LcrV in early targeting of effectors in the host cell.

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.

Type III secretion systems (T3SSs) are molecular syringes used by a wide variety of Gram-negative pathogens to directly deliver proteins (effectors) into host cells, allowing the bacteria to cause disease. Injection of proteins is triggered by host-cell contact, but how the machinery to deliver effectors is assembled (the translocon), or indeed, how cell contact is perceived, is unclear. Here we identify protein-protein contacts that are critical for translocon function. Our analysis sheds light on the organization of the translocon, and reveals that host cell contact is perceived by a change in the structure of the translocation pore. This signal is then transmitted to the tip of the T3SS needle, and down to the base of the apparatus.

Structural analysis of inter-genus complexes of V-antigen and its regulator and their stabilization by divalent metal ions

Gram-negative bacteria like Yersinia, Pseudomonas, and Aeromonas need type III secretion system (T3SS) for their pathogenicity. V-antigen and its regulator are essential for functioning of T3SS. There is significant functional conservation amongst V-antigen and its regulator belonging to the Ysc family. In this study, we have structurally characterized the inter-genus complexes of V-antigen and its regulator. ConSurf analysis demonstrates that V-antigens belonging to the Ysc family show high structural identity predominantly confined to the two long helical regions. The regulator of V-antigen shows high conservation in its first intramolecular coiled-coil domain, responsible for interaction with V-antigen. ∆LcrG(1-70) localizes within the groove formed by long helices of LcrV, as observed in PcrV-∆PcrG(13-72) interaction. Inter-genus complexes of LcrV-PcrG and PcrV-LcrG exhibited elongated conformation and 1:1 heterodimeric state like the native complex of PcrV-PcrG and LcrV-LcrG. Both native and inter-genus complexes showed rigid tertiary structure, solvent-exposed hydrophobic patches, and cooperative melting behavior with high melting temperature. LcrV-PcrG and PcrV-LcrG showed nanomolar affinity of interaction, identical to PcrV-PcrG interaction, but stronger than LcrV-LcrG interaction. Calcium (a secretion blocker of T3SS) propels all the complexes towards a highly monodisperse form. Calcium and magnesium increase the helicity of the native and inter-genus complexes, and causes helix-helix stabilization. Stabilization of helices leads to a slight increase in the melting temperature by 1.5-2.0 °C. However, calcium does not alter the affinity of interaction of V-antigen and its regulator, emphasizing the effect of divalent of cations at the structural level without any regulatory implications. Therefore, the structural conservation of these inter-genus complexes could be the basis for their functional complementation.

Genetic Adaptation of Achromobacter sp. during Persistence in the Lungs of Cystic Fibrosis Patients

Achromobacter species are increasingly isolated from the respiratory tract of cystic fibrosis patients and often a chronic infection is established. How Achromobacter sp. adapts to the human host remains uncharacterised. By comparing longitudinally collected isolates of Achromobacter sp. isolated from five CF patients, we have investigated the within-host evolution of clonal lineages. The majority of identified mutations were isolate-specific suggesting co-evolution of several subpopulations from the original infecting isolate. The largest proportion of mutated genes were involved in the general metabolism of the bacterium, but genes involved in virulence and antimicrobial resistance were also affected. A number of virulence genes required for initiation of acute infection were selected against, e.g. genes of the type I and type III secretion systems and genes related to pilus and flagellum formation or function. Six antimicrobial resistance genes or their regulatory genes were mutated, including large deletions affecting the repressor genes of an RND-family efflux pump and a beta-lactamase. Convergent evolution was observed for five genes that were all implicated in bacterial virulence. Characterisation of genes involved in adaptation of Achromobacter to the human host is required for understanding the pathogen-host interaction and facilitate design of future therapeutic interventions.

Random mutagenesis identifies a C-terminal region of YopD important for Yersinia type III secretion function

A common virulence mechanism among bacterial pathogens is the use of specialized secretion systems that deliver virulence proteins through a translocation channel inserted in the host cell membrane. During Yersinia infection, the host recognizes the type III secretion system mounting a pro-inflammatory response. However, soon after they are translocated, the effectors efficiently counteract that response. In this study we sought to identify YopD residues responsible for type III secretion system function. Through random mutagenesis, we identified eight Y. pseudotuberculosis yopD mutants with single amino acid changes affecting various type III secretion functions. Three severely defective mutants had substitutions in residues encompassing a 35 amino acid region (residues 168-203) located between the transmembrane domain and the C-terminal putative coiled-coil region of YopD. These mutations did not affect regulation of the low calcium response or YopB-YopD interaction but markedly inhibited MAPK and NFκB. [corrected] activation. When some of these mutations were introduced into the native yopD gene, defects in effector translocation and pore formation were also observed. We conclude that this newly identified region is important for YopD translocon function. The role of this domain in vivo remains elusive, as amino acid substitutions in that region did not significantly affect virulence of Y. pseudotuberculosis in orogastrically-infected mice.

Yersinia pseudotuberculosis YopD mutants that genetically separate effector protein translocation from host membrane disruption

The Yersinia type III secretion system (T3SS) translocates Yop effector proteins into host cells to manipulate immune defenses such as phagocytosis and reactive oxygen species (ROS) production. The T3SS translocator proteins YopB and YopD form pores in host membranes, facilitating Yop translocation. While the YopD amino and carboxy termini participate in pore formation, the role of the YopD central region between amino acids 150-227 remains unknown. We assessed the contribution of this region by generating Y. pseudotuberculosis yopD(Δ150-170) and yopD(Δ207-227) mutants and analyzing their T3SS functions. These strains exhibited wild-type levels of Yop secretion in vitro and enabled robust pore formation in macrophages. However, the yopDΔ150-170 and yopD(Δ207-227) mutants were defective in Yop translocation into CHO cells and splenocyte-derived neutrophils and macrophages. These data suggest that YopD-mediated host membrane disruption and effector Yop translocation are genetically separable activities requiring distinct protein domains. Importantly, the yopD(Δ150-170) and yopD(Δ207-227) mutants were defective in Yop-mediated inhibition of macrophage cell death and ROS production in neutrophil-like cells, and were attenuated in disseminated Yersinia infection. Therefore, the ability of the YopD central region to facilitate optimal effector protein delivery into phagocytes, and therefore robust effector Yop function, is important for Yersinia virulence.

LcrQ blocks the role of LcrF in regulating the Ysc-Yop type III secretion genes in Yersinia pseudotuberculosis

Pathogenic Yersinia species employ the Ysc-Yop type III secretion system (T3SS) encoded by a highly conserved pYV virulence plasmid to export the virulence effectors into host cells. The Ysc-Yop T3SS is tightly regulated by multiple contributing proteins that function at different levels. However, systematic transcriptional regulation analysis of Ysc-Yop T3SS is lacking and the detailed mechanism under this regulation process is still elusive. Aimed at systematically characterizing transcriptional regulations of all T3SS genes in Y. pseudotuberculosis, we amplified 97 non-coding fragments from the pYV plasmid and analyzed transcriptional responses of the T3SS genes under different growth conditions. Transcriptions of T3SS genes were induced at 37°C and genes encoding T3SS effectors were highly induced by further depletion of Ca2+. The temperature induced gene transcription process is mediated by modules encoded on the chromosome, while the Ca2+ depletion-induced process is controlled by the positive regulatory protein LcrF as well as the negative regulatory protein LcrQ. In this process, LcrQ shares the same targets with LcrF and the effect of LcrQ is dependent on the presence of LcrF. Furthermore, over-expression of LcrF showed the same phenotype as that of the lcrQ mutant strain and intracellular amount balance of LcrQ and LcrF is important in T3SS regulation. When the expression level of LcrF exceeds LcrQ, expression of the Ysc-Yop T3SS genes is activated and vice versa. Together, these data support a model in which LcrQ blocks the activation role of LcrF in regulating the transcription of T3SS genes in Yersinia.

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.

Membrane and chaperone recognition by the major translocator protein PopB of the type III secretion system of Pseudomonas aeruginosa

The type III secretion system is a widespread apparatus used by pathogenic bacteria to inject effectors directly into the cytoplasm of eukaryotic cells. A key component of this highly conserved system is the translocon, a pore formed in the host membrane that is essential for toxins to bypass this last physical barrier. In Pseudomonas aeruginosa the translocon is composed of PopB and PopD, both of which before secretion are stabilized within the bacterial cytoplasm by a common chaperone, PcrH. In this work we characterize PopB, the major translocator, in both membrane-associated and PcrH-bound forms. By combining sucrose gradient centrifugation experiments, limited proteolysis, one-dimensional NMR, and β-lactamase reporter assays on eukaryotic cells, we show that PopB is stably inserted into bilayers with its flexible N-terminal domain and C-terminal tail exposed to the outside. In addition, we also report the crystal structure of the complex between PcrH and an N-terminal region of PopB (residues 51-59), which reveals that PopB lies within the concave face of PcrH, employing mostly backbone residues for contact. PcrH is thus the first chaperone whose structure has been solved in complex with both type III secretion systems translocators, revealing that both molecules employ the same surface for binding and excluding the possibility of formation of a ternary complex. The characterization of the major type III secretion system translocon component in both membrane-bound and chaperone-bound forms is a key step for the eventual development of antibacterials that block translocon assembly.

Translational regulation of Yersinia enterocolitica mRNA encoding a type III secretion substrate

Yersinia enterocolitica type III secretion machines transport YopQ and other Yop effectors into host immune cells. YopD and its chaperone LcrH are essential components of the Yersinia type III pathway, enabling effector translocation into host cells. YopD, LcrH, and YscM1 also regulate yop expression post-transcriptionally in response to environmental signals; however, the molecular mechanisms for this regulation and Yop secretion are unknown. We show here that YopD associates with 30 S ribosomal particles in a manner requiring LcrH. When added to ribosomes, YopD, LcrH, and YscM1 block the translation of yopQ mRNA. We propose a model whereby LcrH-dependent association of YopD with 30 S ribosomal particles enables YscM1 to block yopQ translation unless type III machines are induced to secrete the effector.

A mutant with aberrant extracellular LcrV-YscF interactions fails to form pores and translocate Yop effector proteins but retains the ability to trigger Yop secretion in response to host cell contact

The plasmid-encoded type three secretion system (TTSS) of Yersinia spp. is responsible for the delivery of effector proteins into cells of the innate immune system, where these effectors disrupt the target cells' activity. Successful translocation of effectors into mammalian cells requires Yersinia to both insert a translocon into the host cell membrane and sense contact with host cells. To probe the events necessary for translocation, we investigated protein-protein interactions among TTSS components of the needle-translocon complex using a chemical cross-linking-based approach. We detected extracellular protein complexes containing YscF, LcrV, and YopD that were dependent upon needle formation. The formation of these complexes was evaluated in a secretion-competent but translocation-defective mutant, the YscFD28AD46A strain (expressing YscF with the mutations D28A and D46A). We found that one of the YscF and most of the LcrV and YopD cross-linked complexes were nearly absent in this mutant. Furthermore, the YscFD28AD46A strain did not support YopB insertion into mammalian membranes, supporting the idea that the LcrV tip complex is required for YopB insertion and translocon formation. However, the YscFD28AD46A strain did secrete Yops in the presence of host cells, indicating that a translocation-competent tip complex is not required to sense contact with host cells to trigger Yop secretion. In conclusion, in the absence of cross-linkable LcrV-YscF interactions, translocon insertion is abolished, but Yersinia still retains the ability to sense cell contact.

Type III secretion translocon assemblies that attenuate Yersinia virulence

Type III secretion enables bacteria to intoxicate eukaryotic cells with anti-host effectors. A class of secreted cargo are the two hydrophobic translocators that form a translocon pore in the host cell plasma membrane through which the translocated effectors may gain cellular entry. In pathogenic Yersinia, YopB and YopD shape this translocon pore. Here, four in cis yopD mutations were constructed to disrupt a predicted α-helix motif at the C-terminus. Mutants YopD(I262P) and YopD(K267P) poorly localized Yop effectors into target eukaryotic cells and failed to resist uptake and killing by immune cells. These defects were due to deficiencies in host-membrane insertion of the YopD-YopB translocon. Mutants YopDA(263P) and YopD(A270P) had no measurable in vitro translocation defect, even though they formed smaller translocon pores in erythrocyte membranes. Despite this, all four mutants were attenuated in a mouse infection model. Hence, YopD variants have been generated that can spawn translocons capable of targeting effectors in vitro, yet were bereft of any lethal effect in vivo. Therefore, Yop translocators may possess other in vivo functions that extend beyond being a portal for effector delivery into host cells.

Elevated CpxR~P levels repress the Ysc-Yop type III secretion system of Yersinia pseudotuberculosis

One way that Gram-negative bacteria respond to extracytoplasmic stress is through the CpxA-CpxR system. An activated CpxA sensor kinase phosphorylates the CpxR response regulator to instigate positive auto-amplification of Cpx pathway activation, as well as synthesis of various bacterial survival factors. In the absence of CpxA, human enteropathogenic Yersinia pseudotuberculosis accumulates high CpxR~P levels aided by the action of low molecular weight phosphodonors such as acetyl~P. Critically, these bacteria are also defective for plasmid-encoded Ysc-Yop-dependent type III synthesis and secretion, an essential determinant of virulence. Herein, we investigated whether elevated CpxR~P levels account for lost Ysc-Yop function. Decisively, reducing CpxR∼P in Yersinia defective for CpxA phosphatase activity - through incorporating second-site suppressor mutations in ackA-pta or cpxR - dramatically restored Ysc-Yop T3S function. Moreover, the repressive effect of accumulated CpxR∼P is a direct consequence of binding to the promoter regions of the T3S genes. Thus, Cpx pathway activation has two consequences in Yersinia; one, to maintain quality control in the bacterial envelope, and the second, to restrict ysc-yop gene expression to those occasions where it will have maximal effect.

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

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

Membrane targeting and pore formation by the type III secretion system translocon

The type III secretion system (T3SS) is a complex macromolecular machinery employed by a number of Gram-negative species to initiate infection. Toxins secreted through the system are synthesized in the bacterial cytoplasm and utilize the T3SS to pass through both bacterial membranes and the periplasm, thus being introduced directly into the eukaryotic cytoplasm. A key element of the T3SS of all bacterial pathogens is the translocon, which comprises a pore that is inserted into the membrane of the target cell, allowing toxin injection. Three macromolecular partners associate to form the translocon: two are hydrophobic and one is hydrophilic, and the latter also associates with the T3SS needle. In this review, we discuss recent advances on the biochemical and structural characterization of the proteins involved in translocon formation, as well as their participation in the modification of intracellular signalling pathways upon infection. Models of translocon assembly and regulation are also discussed.