Crystal structure of the Yersinia enterocolitica type III secretion chaperone SycT

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.

Molecular basis of binding between the global post-transcriptional regulator CsrA and the T3SS chaperone CesT

The T3SS chaperone CesT is recently shown to interact with the post-transcriptional regulator CsrA to modulate post-attachment signaling in enteropathogenic and enterohemorrhagic Escherichia coli. The molecular basis of the CesT/CsrA binding, however, remains elusive. Here, we show that CesT and CsrA both created two ligand binding sites in their homodimers, forming irregular multimeric complexes in solution. Through construction of a recombinant CsrA-dimer (Re-CsrA) that contains a single CesT binding site, the atomic binding features between CesT and CsrA are delineated via the structure of the CesT/Re-CsrA complex. In contrast to a previously reported N-terminally swapped dimer-form, CesT adopts a dimeric architecture with a swapped C-terminal helix for CsrA engagement. In CsrA, CesT binds to a surface patch that extensively overlaps with its mRNA binding site. The binding mode therefore justifies a mechanism of CsrA-modulation by CesT via competitive inhibition of the CsrA/mRNA interactions.

CesT is a type III secretion system chaperone that interacts with the post-transcriptional regulator CsrA, which is important for the modulation of post-attachment signaling in enteropathogenic and enterohemorrhagic Escherichia coli. Here the authors present the structure of the CsrA/CesT complex and propose a mechanism for CsrA-modulation by CesT.

Novel T3SS effector EseK in Edwardsiella piscicida is chaperoned by EscH and EscS to express virulence

Bacterium usually utilises type III secretion systems (T3SS) to deliver effectors directly into host cells with the aids of chaperones. Hence, it is very important to identify bacterial T3SS effectors and chaperones for better understanding of host-pathogen interactions. Edwardsiella piscicida is an invasive enteric bacterium, which infects a wide range of hosts from fish to human. Given E. piscicida encodes a functional T3SS to promote infection, very few T3SS effectors and chaperones have been identified in this bacterium so far. Here, we reported that EseK is a new T3SS effector protein translocated by E. piscicida. Bioinformatic analysis indicated that escH and escS encode two putative class I T3SS chaperones. Further investigation indicated that EscH and EscS can enhance the secretion and translocation of EseK. EscH directly binds EseK through undetermined binding domains, whereas EscS binds EseK via its N-terminal α-helix. We also found that EseK has an N-terminal chaperone-binding domain, which binds EscH and EscS to form a ternary complex. Zebrafish infection experiments showed that EseK and its chaperones EscH and EscS are necessary for bacterial colonisation in zebrafish. This work identified a new T3SS effector, EseK, and its two T3SS chaperones, EscH and EscS, in E. piscicida, which enriches our knowledge of bacterial T3SS effector-chaperone interaction and contributes to our understanding of bacterial pathogenesis.

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.

BtcA, A class IA type III chaperone, interacts with the BteA N-terminal domain through a globular/non-globular mechanism

Bordetella pertussis, the etiological agent of "whooping cough" disease, utilizes the type III secretion system (T3SS) to deliver a 69 kDa cytotoxic effector protein, BteA, directly into the host cells. As with other T3SS effectors, prior to its secretion BteA binds BtcA, a 13.9 kDa protein predicted to act as a T3SS class IA chaperone. While this interaction had been characterized for such effector-chaperone pairs in other pathogens, it has yet to be fully investigated in Bordetella. Here we provide the first biochemical proof that BtcA is indeed a class IA chaperone, responsible for the binding of BteA's N-terminal domain. We bring forth extensive evidence that BtcA binds its substrate effector through a dual-interface binding mechanism comprising of non-globular and bi-globular interactions at a moderate micromolar level binding affinity. We demonstrate that the non-globular interactions involve the first 31 N-terminal residues of BteA287 and their removal leads to destabilization of the effector-chaperone complex and lower binding affinities to BtcA. These findings represent an important first step towards a molecular understanding of BteA secretion and cell entry.

The Deinococcus radiodurans DR1245 protein, a DdrB partner homologous to YbjN proteins and reminiscent of type III secretion system chaperones

The bacterium Deinococcus radiodurans exhibits an extreme resistance to ionizing radiation. A small subset of Deinococcus genus-specific genes were shown to be up-regulated upon exposure to ionizing radiation and to play a role in genome reconstitution. These genes include an SSB-like protein called DdrB. Here, we identified a novel protein encoded by the dr1245 gene as an interacting partner of DdrB. A strain devoid of the DR1245 protein is impaired in growth, exhibiting a generation time approximately threefold that of the wild type strain while radioresistance is not affected. We determined the three-dimensional structure of DR1245, revealing a relationship with type III secretion system chaperones and YbjN family proteins. Thus, DR1245 may display some chaperone activity towards DdrB and possibly other substrates.

Recombinant production of Yersinia enterocolitica pyruvate kinase isoenzymes PykA and PykF

The glycolytic enzyme pyruvate kinase (PK) generates ATP from ADP through substrate-level phosphorylation powered by the conversion of phosphoenolpyruvate to pyruvate. In contrast to other bacteria, Enterobacteriaceae, such as pathogenic yersiniae, harbour two pyruvate kinases encoded by pykA and pykF. The individual roles of these isoenzymes are poorly understood. In an attempt to make the Yersinia enterocolitica pyruvate kinases PykA and PykF amenable to structural and functional characterisation, we produced them untagged in Escherichia coli and purified them to near homogeneity through a combination of ion exchange and size exclusion chromatography, yielding more than 180 mg per litre of batch culture. The solution structure of PykA and PykF was analysed through small angle X-ray scattering which revealed the formation of PykA and PykF tetramers and confirmed the binding of the allosteric effector fructose-1,6-bisphosphate (FBP) to PykF but not to PykA.

Yersinia enterocolitica and Photorhabdus asymbiotica β-lactamases BlaA are exported by the twin-arginine translocation pathway

In general, β-lactamases of medically important Gram-negative bacteria are Sec-dependently translocated into the periplasm. In contrast, β-lactamases of Mycobacteria spp. (BlaC, BlaS) and the Gram-negative environmental bacteria Stenotrophomonas maltophilia (L2) and Xanthomonas campestris (Bla(XCC-1)) have been reported to be secreted by the twin-arginine translocation (Tat) system. Yersinia enterocolitica carries 2 distinct β-lactamase genes (blaA and blaB) encoding BlaA(Ye) and the AmpC-like β-lactamase BlaB, respectively. By using the software PRED-TAT for prediction and discrimination of Sec from Tat signal peptides, we identified a functional Tat signal sequence for Yersinia BlaA(Ye). The Tat-dependent translocation of BlaA(Ye) could be clearly demonstrated by using a Y. enterocolitica tatC-mutant and cell fractionation. Moreover, we could demonstrate a unique unusual temperature-dependent activity profile of BlaA(Ye) ranging from 15 to 60 °C and a high 'melting temperature' (T(M)=44.3°) in comparison to the related Sec-dependent β-lactamase TEM-1 (20-50°C, T(M)=34.9 °C). Strikingly, the blaA gene of Y. enterocolitica is present in diverse environmental Yersinia spp. and a blaA homolog gene could be identified in the closely related Photorhabdus asymbiotica (BlaA(Pa); 69% identity to BlaA(Ye)). For BlaA(Pa) of P. asymbiotica, we could also demonstrate Tat-dependent secretion. These results suggest that Yersinia BlaA-related β-lactamases may be the prototype of a large Tat-dependent β-lactamase family, which originated from environmental bacteria.

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

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

Analysis of the crystal structure of the ExsC.ExsE complex reveals distinctive binding interactions of the Pseudomonas aeruginosa type III secretion chaperone ExsC with ExsE and ExsD

Pseudomonas aeruginosa, like many Gram-negative bacterial pathogens, requires its type III secretion system (T3SS) to facilitate acute infections. In P. aeruginosa, the expression of all T3SS-related genes is regulated by the transcriptional activator ExsA. A signaling cascade involving ExsA and three additional proteins, ExsC, ExsD, and ExsE, directly ties the upregulation of ExsA-mediated transcription to the activation of the type III secretion apparatus. In order to characterize the events underlying the signaling process, the crystal structure of the T3SS chaperone ExsC in complex with its cognate effector ExsE has been determined. The structure reveals critical contacts that mediate the interactions between these two proteins. Particularly striking is the presence of two Arg-X-Val-X-Arg motifs in ExsE that form identical interactions along opposite sides of an ExsC dimer. The structure also provides insights into the interactions of ExsC with the antiactivator protein ExsD. It was shown that the amino-terminal 46 residues of ExsD are sufficient for ExsC binding. On the basis of these findings, a new model for the ExsC.ExsD complex is proposed to explain its distinctive 2:2 stoichiometry and why ExsC displays a weaker affinity for ExsD than for ExsE.

A solvent-exposed patch in chaperone-bound YopE is required for translocation by the type III secretion system

Most effector proteins of bacterial type III secretion (T3S) systems require chaperone proteins for translocation into host cells. Such effectors are bound by chaperones in a conserved and characteristic manner, with the chaperone-binding (Cb) region of the effector wound around the chaperone in a highly extended conformation. This conformation has been suggested to serve as a translocation signal in promoting the association between the chaperone-effector complex and a bacterial component required for translocation. We sought to test a prediction of this model by identifying a potential association site for the Yersinia pseudotuberculosis chaperone-effector pair SycE-YopE. We identified a set of residues in the YopE Cb region that are required for translocation but are dispensable for expression, SycE binding, secretion into the extrabacterial milieu, and stability in mammalian cells. These residues form a solvent-exposed patch on the surface of the chaperone-bound Cb region, and thus their effect on translocation is consistent with the structure of the chaperone-bound Cb region serving as a signal for translocation.

Bacterial toxins induce sustained mRNA expression of the silencing transcription factor klf2 via inactivation of RhoA and Rhophilin 1

Yersiniae bearing the Yersinia virulence plasmid pYV impact the transcriptome of J774A.1 macrophage-like cells in two distinct ways: (i) by suppressing, in a Yersinia outer protein P (YopP)-dependent manner, the induction of inflammatory response genes and (ii) by mRNA induction of the silencing transcription factor klf2. Here we show that klf2 induction by Yersinia enterocolitica occurs in several cell lines of macrophage and squamous and upper gastrointestinal epithelial origin as well as in bone marrow-derived dendritic cells. Several strains of Pseudomonas aeruginosa and Staphylococcus aureus are equally effective as Y. enterocolitica in inducing klf2 expression. Screening of mutant strains or incubation with recombinant toxins identified the rho-inactivating toxins YopT from Yersinia spp., ExoS from Pseudomonas aeruginosa, EDIN-B from Staphylococcus aureus, and C3bot from Clostridium botulinum as bacterial inducers of klf2 mRNA. klf2 mRNA induction by these toxins does not require de novo protein synthesis. Serum response factor or actin depolymerization does not seem to be involved in regulating klf2 expression in response to bacterial infection. Instead, short hairpin RNA-mediated inactivation of RhoA and its effector rhophilin 1 is sufficient to induce long-term klf2 expression. Thus, bacteria exploit the RhoA-rhophilin signaling cascade to mediate sustained expression of the immunosuppressive transcription factor klf2.

Cross-talk between type three secretion system and metabolism in Yersinia

Pathogenic yersiniae utilize a type three secretion system (T3SS) to inject Yop proteins into host cells in order to undermine their immune response. YscM1 and YscM2 proteins have been reported to be functionally equivalent regulators of the T3SS in Yersinia enterocolitica. Here, we show by affinity purification, native gel electrophoresis and small angle x-ray scattering that both YscM1 and YscM2 bind to phosphoenolpyruvate carboxylase (PEPC) of Y. enterocolitica. Under in vitro conditions, YscM1, but not YscM2, was found to inhibit PEPC with an apparent IC(50) of 4 mum (K(i) = 1 mum). To analyze the functional roles of PEPC, YscM1, and YscM2 in Yop-producing bacteria, cultures of Y. enterocolitica wild type and mutants defective in the formation of PEPC, YscM1, or YscM2, respectively, were grown under low calcium conditions in the presence of [U-(13)C(6)]glucose. The isotope compositions of secreted Yop proteins and nine amino acids from cellular proteins were analyzed by mass spectrometry. The data indicate that a considerable fraction of oxaloacetate used as precursor for amino acids was derived from [(13)C(3)]phosphoenolpyruvate by the catalytic action of PEPC in the wild-type strain but not in the PEPC(-) mutant. The data imply that PEPC is critically involved in replenishing the oxaloacetate pool in the citrate cycle under virulence conditions. In the YscM1(-) and YscM2(-) mutants, increased rates of pyruvate formation via glycolysis or the Entner-Doudoroff pathway, of oxaloacetate formation via the citrate cycle, and of amino acid biosynthesis suggest that both regulators trigger the central metabolism of Y. enterocolitica. We propose a "load-and-shoot cycle" model to account for the cross-talk between T3SS and metabolism in pathogenic yersiniae.

Essential role of the SycP chaperone in type III secretion of the YspP effector

The Ysa type III secretion (T3S) system enhances gastrointestinal infection by Yersinia enterocolitica bv. 1B. One effector protein targeted into host cells is YspP, a protein tyrosine phosphatase. It was determined in this study that the secretion of YspP requires a chaperone, SycP. Genetic analysis showed that deletion of sycP completely abolished the secretion of YspP without affecting the secretion of other Ysps by the Ysa T3S system. Analysis of the secretion and translocation signals of YspP defined the first 73 amino acids to form the minimal region of YspP necessary to promote secretion and translocation by the Ysa T3S system. Function of the YspP secretion/translocation signals was dependent on SycP. Curiously, when YspP was constitutively expressed in Y. enterocolitica bv. 1B, it was recognized and secreted by the Ysc T3S system and the flagellar T3S system. In these cases, the first 21 amino acids were sufficient to promote secretion, and while SycP did enhance secretion, it was not essential. However, neither the Ysc T3S system nor the flagellar T3S system translocated YspP into mammalian cells. This supports a model where SycP confers secretion/translocation specificities for YspP by the Ysa T3S system. A series of biochemical approaches further established that SycP specifically interacts with YspP and protected YspP degradation in the cell prior to secretion. Collectively, the evidence suggests that YspP secretion by the Ysa T3S system is a posttranslational event.

The Helicobacter pylori CagD (HP0545, Cag24) protein is essential for CagA translocation and maximal induction of interleukin-8 secretion

Pathogenic strains of Helicobacter pylori use a type IV secretion system (T4SS) to deliver the toxin CagA into human host cells. The T4SS, along with the toxin itself, is coded into a genomic insert, which is termed the cag pathogenicity island. The cag pathogenicity island contains about 30 open-reading frames, for most of which the exact function is not well characterized or totally unknown. We have determined the crystal structure of one of the proteins coded by the cag genes, CagD, in two crystal forms. We show that the protein is a covalent dimer in which each monomer folds as a single domain that is composed of five beta-strands and three alpha-helices. Our data show that in addition to a cytosolic pool, CagD partially associates with the inner membrane, where it may be exposed to the periplasmic space. Furthermore, CagA tyrosine phosphorylation and interleukin-8 assays identified CagD as a crucial component of the T4SS that is involved in CagA translocation into host epithelial cells; however, it does not seem absolutely necessary for pilus assembly. We have also identified significant amounts of CagD in culture supernatants, which are not a result of general bacterial lysis. Since this localization was independent of the various tested cag mutants, our findings may indicate that CagD is released into the supernatant during host cell infection and then binds to the host cell surface or is incorporated in the pilus structure. Overall, our results suggest that CagD may serve as a unique multifunctional component of the T4SS that may be involved in CagA secretion at the inner membrane and may localize outside the bacteria to promote additional effects on the host cell.

The type III secretion chaperone SycE promotes a localized disorder-to-order transition in the natively unfolded effector YopE

Many virulence-related, bacterial effector proteins are translocated directly into the cytosol of host cells by the type III secretion (TTS) system. Translocation of most TTS effectors requires binding by specific chaperones in the bacterial cytosol, although how chaperones promote translocation is unclear. To provide insight into the action of such chaperones, we studied the consequences of binding by the Yersinia chaperone SycE to the effector YopE by NMR. These studies examined the intact form of the effector, whereas prior studies have been limited to well ordered fragments. We found that YopE had the characteristics of a natively unfolded protein, with its N-terminal 100 residues, including its chaperone-binding (Cb) region, flexible and disordered in the absence of SycE. SycE binding caused a pronounced disorder-to-order transition in the Cb region of YopE. The effect of SycE was strictly localized to the Cb region, with other portions of YopE being unperturbed. These results provide stringent limits on models of chaperone action and are consistent with the chaperone promoting formation of a three-dimensional targeting signal in the Cb region of the effector. The target of this putative signal is unknown but appears to be a bacterial component other than the TTS ATPase YscN.

The Yersinia enterocolitica type three secretion chaperone SycO is integrated into the Yop regulatory network and binds to the Yop secretion protein YscM1

Background

Pathogenic yersiniae (Y. pestis, Y. pseudotuberculosis, Y. enterocolitica) share a virulence plasmid encoding a type three secretion system (T3SS). This T3SS comprises more than 40 constituents. Among these are the transport substrates called Yops (Yersinia outer proteins), the specific Yop chaperones (Sycs), and the Ysc (Yop secretion) proteins which form the transport machinery. The effectors YopO and YopP are encoded on an operon together with SycO, the chaperone of YopO. The characterization of SycO is the focus of this study.

Results

We have established the large-scale production of recombinant SycO in its outright form. We confirm that Y. enterocolitica SycO forms homodimers which is typical for Syc chaperones. SycO overproduction in Y. enterocolitica decreases secretion of Yops into the culture supernatant suggesting a regulatory role of SycO in type III secretion. We demonstrate that in vitro SycO interacts with YscM1, a negative regulator of Yop expression in Y. enterocolitica. However, the SycO overproduction phenotype was not mediated by YscM1, YscM2, YopO or YopP as revealed by analysis of isogenic deletion mutants.

Conclusion

We present evidence that SycO is integrated into the regulatory network of the Yersinia T3SS. Our picture of the Yersinia T3SS interactome is supplemented by identification of the SycO/YscM1 interaction. Further, our results suggest that at least one additional interaction partner of SycO has to be identified.

The weak interaction of LcrV and TLR2 does not contribute to the virulence of Yersinia pestis

Yersinia pestis and the enteropathogenic Yersinia pseudotuberculosis and Yersinia enterocolitica share the virulence-antigen LcrV. Previously, using reverse genetics we have proven that LcrV contributes to the virulence of Y. enterocolitica serotype O:8 by inducing IL-10 via Toll-like receptor 2 (TLR2). However, both the ability of Y. pestis LcrV to activate TLR2 and a possible role of TLR2-dependent IL-10 induction by LcrV in Y. pestis are not yet known. To eliminate interference from additional protein sequences, we produced LcrVs without affinity tags from Y. pestis and from Y. enterocolitica O:8 (LcrVO:8). LcrVO:8 was much more potent in TLR2-activity than Y. pestis LcrV. To analyse the role of TLR2 in plague, we infected both wild-type and TLR2-/- mice subcutaneously with Y. pestis GB. While TLR2-/- mice exhibited lower blood levels of IL-10 (day 2 post-infection) and of the pro-inflammatory cytokines TNF-alpha, IFN-gamma and MCP-1 (day 4) than wild-type mice, there was no significant difference in survival. The low TLR2-activity of Y. pestis LcrV and associated cytokine expression might explain why - in contrast to Y. enterocolitica O:8 infection - TLR2-deficient mice are not more resistant than wild-type mice in a bubonic plague model.

The type III secretion injectisome

The type III secretion injectisome is a complex nanomachine that allows bacteria to deliver protein effectors across eukaryotic cellular membranes. In recent years, significant progress has been made in our understanding of its structure, assembly and mode of operation. The principal structural components of the injectisome, from the base located in the bacterial cytosol to the tip of the needle protruding from the cell surface, have been investigated in detail. The structures of several constituent proteins were solved at the atomic level and important insights into the assembly process have been gained. However, despite the ongoing concerted efforts of molecular and structural biologists, the role of many of the constituent components of this nanomachine remain unknown.

Structure-based mutagenesis of SigE verifies the importance of hydrophobic and electrostatic residues in type III chaperone function

Despite sharing little sequence identity, most type III chaperones display a similar homodimeric structure characterized by negative charges distributed broadly over their entire surface, interspersed with hydrophobic patches. Here we have used SigE from Salmonella as a model for class IA type III chaperones to investigate the role of these surface-exposed residues in chaperone function. SigE is essential for the stability, secretion and translocation of its cognate effector, SopB (SigD). We analysed the effect of mutating nine conserved hydrophobic and electronegative surface-exposed amino acids of SigE on SopB binding, stability, secretion and translocation. Six of these mutations affected some aspect of SigE function (Leu14, Asp20, Leu22, Leu23, Ile25 and Asp51) and three were without effect (Leu54, Glu92 and Glu99). Our results highlight that both hydrophobic and electronegative surfaces are required for the function of SigE and provide an important basis for the prediction of side-chain requirements for other chaperone-effector pairs.

Yersinia enterocolitica type III secretion chaperone SycD: Recombinant expression, purification and characterization of a homodimer

Yersinia species pathogenic to human benefit from a protein transport machinery, a type three secretion system (T3SS), which enables the bacteria to inject effector proteins into host cells. Several of the transport substrates of the Yersinia T3SS, called Yops (Yersinia outer proteins), are assisted by specific chaperones (Syc for specific Yop chaperone) prior to transport. Yersinia enterocolitica SycD (LcrH in Yersinia pestis and Yersinia pseudotuberculosis) is a chaperone dedicated to the assistance of the translocator proteins YopB and YopD, which are assumed to form a pore in the host cell membrane. In an attempt to make SycD amenable to structural investigations we recombinantly expressed SycD with a hexahistidine tag in Escherichia coli. Combining immobilized nickel affinity chromatography and gel filtration we obtained purified SycD with an exceptional yield of 120mg per liter of culture and homogeneity above 95%. Analytical gel filtration and cross-linking experiments revealed the formation of homodimers in solution. Secondary structure analysis based on circular dichroism suggests that SycD is mainly composed of alpha-helical elements. To prove functionality of purified SycD previously suggested interactions of SycD with Yop secretion protein M2 (YscM2), and low calcium response protein V (LcrV), respectively, were reinvestigated.

The discovery of SycO highlights a new function for type III secretion effector chaperones

Bacterial injectisomes deliver effector proteins straight into the cytosol of eukaryotic cells (type III secretion, T3S). Many effectors are associated with a specific chaperone that remains inside the bacterium when the effector is delivered. The structure of such chaperones and the way they interact with their substrate is well characterized but their main function remains elusive. Here, we describe and characterize SycO, a new chaperone for the Yersinia effector kinase YopO. The chaperone-binding domain (CBD) within YopO coincides with the membrane localization domain (MLD) targeting YopO to the host cell membrane. The CBD/MLD causes intrabacterial YopO insolubility and the binding of SycO prevents this insolubility but not folding and activity of the kinase. Similarly, SycE masks the MLD of YopE and SycT covers an aggregation-prone domain of YopT, presumably corresponding to its MLD. Thus, SycO, SycE and most likely SycT mask, inside the bacterium, a domain needed for proper localization of their cognate effector in the host cell. We propose that covering an MLD might be an essential function of T3S effector chaperones.

Tetratricopeptide repeats in the type III secretion chaperone, LcrH: their role in substrate binding and secretion

Non-flagellar type III secretion systems (T3SSs) transport proteins across the bacterial cell and into eukaryotic cells. Targeting of proteins into host cells requires a dedicated translocation apparatus. Efficient secretion of the translocator proteins that make up this apparatus depends on molecular chaperones. Chaperones of the translocators (also called class-II chaperones) are characterized by the possession of three tandem tetratricopeptide repeats (TPRs). We wished to dissect the relations between chaperone structure and function and to validate a structural model using site-directed mutagenesis. Drawing on a number of experimental approaches and focusing on LcrH, a class-II chaperone from the Yersinia Ysc-Yop T3SS, we examined the contributions of different residues, residue classes and regions of the protein to chaperone stability, chaperone-substrate binding, substrate stability and secretion and regulation of Yop protein synthesis. We confirmed the expected role of the conserved canonical residues from the TPRs to chaperone stability and function. Eleven mutations specifically abrogated YopB binding or secretion while three mutations led to a specific loss of YopD secretion. These are the first mutations described for any class-II chaperone that allow interactions with one translocator to be dissociated from interactions with the other. Strikingly, all mutations affecting the interaction with YopB mapped to residues with side chains projecting from the inner, concave surface of the modelled TPR structure, defining a YopB interaction site. Conversely, all mutations preventing YopD secretion affect residues that lie on the outer, convex surface of the triple-TPR cluster in our model, suggesting that this region of the molecule represents a distinct interaction site for YopD. Intriguingly, one of the LcrH double mutants, Y40A/F44A, was able to maintain stable substrates inside bacteria, but unable to secrete them, suggesting that these two residues might influence delivery of substrates to the secretion apparatus.

The type III needle and the damage done

Many Gram-negative pathogens translocate virulence proteins directly into host cells using a type III secretion system. This complex secretion machinery is composed of approximately 25 different proteins that assemble to span both bacterial membranes, and contact the host cell to form a direct channel between the bacterial and host cell cytoplasms. Assembly of the system and efficient secretion of virulence proteins through this apparatus require specific chaperones. Although the machinery is morphologically conserved among all bacteria, the secreted proteins vary widely and are responsible for the range of diseases caused by bacterial pathogens. Recent structures have given insights into important chaperone and effector proteins, as well as revealing the first atomic structures of portions of the secretion machinery itself.