The YopD translocator of Yersinia pseudotuberculosis is a multifunctional protein comprised of discrete domains

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.

Calcium-responsive plasmid copy number regulation is dependent on discrete YopD domains in Yersinia pseudotuberculosis

Yersinia pathogenicity depends mainly on a Type III Secretion System (T3SS) responsible for translocating effector proteins into the eukaryotic target cell cytosol. The T3SS is encoded on a 70 kb, low copy number virulence plasmid, pYV. A key T3SS regulator, YopD, is a multifunctional protein and consists of discrete modular domains that are essential for pore formation and translocation of Yop effectors. In Y. pseudotuberculosis, the temperature-dependent plasmid copy number increase that is essential for elevated T3SS gene dosage and virulence is also affected by YopD. Here, we found that the presence of intracellular YopD results in increased levels of the CopA-RNA and CopB, two inhibitors of plasmid replication. Secretion of YopD leads to decreased expression of copA and copB, resulting in increased plasmid copy number. Moreover, using a systematic mutagenesis of YopD mutants, we demonstrated that the same discrete modular domains important for YopD translocation are also necessary for both the regulation of plasmid copy number as well as copA and copB expression. Hence, Yersinia has evolved a mechanism coupling active secretion of a plasmid-encoded component of the T3SS, YopD, to the regulation of plasmid replication. Our work provides evidence for the cross-talk between plasmid-encoded functions with the IncFII replicon.

A bacterial secreted translocator hijacks riboregulators to control type III secretion in response to host cell contact

Numerous Gram-negative pathogens use a Type III Secretion System (T3SS) to promote virulence by injecting effector proteins into targeted host cells, which subvert host cell processes. Expression of T3SS and the effectors is triggered upon host cell contact, but the underlying mechanism is poorly understood. Here, we report a novel strategy of Yersinia pseudotuberculosis in which this pathogen uses a secreted T3SS translocator protein (YopD) to control global RNA regulators. Secretion of the YopD translocator upon host cell contact increases the ratio of post-transcriptional regulator CsrA to its antagonistic small RNAs CsrB and CsrC and reduces the degradosome components PNPase and RNase E levels. This substantially elevates the amount of the common transcriptional activator (LcrF) of T3SS/Yop effector genes and triggers the synthesis of associated virulence-relevant traits. The observed hijacking of global riboregulators allows the pathogen to coordinate virulence factor expression and also readjusts its physiological response upon host cell contact.

Guanylate Binding Proteins Regulate Inflammasome Activation in Response to Hyperinjected Yersinia Translocon Components

Gram-negative bacterial pathogens utilize virulence-associated secretion systems to inject, or translocate, effector proteins into host cells to manipulate cellular processes and promote bacterial replication. However, translocated bacterial products are sensed by nucleotide binding domain and leucine-rich repeat-containing proteins (NLRs), which trigger the formation of a multiprotein complex called the inflammasome, leading to secretion of interleukin-1 (IL-1) family cytokines, pyroptosis, and control of pathogen replication. Pathogenic Yersinia bacteria inject effector proteins termed Yops, as well as pore-forming proteins that comprise the translocon itself, into target cells. The Yersinia translocation regulatory protein YopK promotes bacterial virulence by limiting hyperinjection of the translocon proteins YopD and YopB into cells, thereby limiting cellular detection of Yersinia virulence activity. How hyperinjection of translocon proteins leads to inflammasome activation is currently unknown. We found that translocated YopB and YopD colocalized with the late endosomal/lysosomal protein LAMP1 and that the frequency of YopD and LAMP1 association correlated with the level of caspase-1 activation in individual cells. We also observed colocalization between YopD and Galectin-3, an indicator of endosomal membrane damage. Intriguingly, YopK limited the colocalization of Galectin-3 with YopD, suggesting that YopK limits the induction or sensing of endosomal membrane damage by components of the type III secretion system (T3SS) translocon. Furthermore, guanylate binding proteins (GBPs) encoded on chromosome 3 (Gbp^Chr3 ), which respond to pathogen-induced damage or alteration of host membranes, were necessary for inflammasome activation in response to hyperinjected YopB/-D. Our findings indicate that lysosomal damage by Yersinia translocon proteins promotes inflammasome activation and implicate GBPs as key regulators of this process.

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.

Temperature-regulated expression of type VI secretion systems in fish pathogen Pseudomonas plecoglossicida revealed by comparative secretome analysis

Pseudomonas plecoglossicida is a facultative fish pathogen. Recent studies showed that P. plecoglossicida infection in fish was associated with temperature. The aim of this study was to compare the secretomes of P. plecoglossicida cultured in vitro at representative temperatures for pathogenic (20°C) and less pathogenic (30°C) phenotypes. Thirteen proteins in the culture supernatants of P. plecoglossicida showed significant difference in abundance at 20 vs. 30°C. Four proteins were strongly increased at 20°C, including two hemolysin co-regulated proteins (Hcp) that are part of the bacterial type VI secretion system (T6SS), flagellin and an unknown protein. Immunoblot analysis verified the induced secretion of Hcps at 20°C. Furthermore, the upregulation of Hcps at 20°C was confirmed at transcriptional level by RT-qPCR analysis, which also demonstrated the induction of expression of other T6SS-related genes at 20°C. Taken together, we demonstrate the presence of two functionally active T6SS proteins in fish pathogenic P. plecoglossicida strains, as evidenced by the secretion of the T6SS substrate Hcp, the production of which were found to be controlled by temperature. Our findings also support efforts to develop vaccines targeting secreted virulence factors as prophylactic strategies for diseases in fish caused by P. plecoglossicida.

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.

Expression and Association of the Yersinia pestis Translocon Proteins, YopB and YopD, Are Facilitated by Nanolipoprotein Particles

Yersinia pestis enters host cells and evades host defenses, in part, through interactions between Yersinia pestis proteins and host membranes. One such interaction is through the type III secretion system, which uses a highly conserved and ordered complex for Yersinia pestis outer membrane effector protein translocation called the injectisome. The portion of the injectisome that interacts directly with host cell membranes is referred to as the translocon. The translocon is believed to form a pore allowing effector molecules to enter host cells. To facilitate mechanistic studies of the translocon, we have developed a cell-free approach for expressing translocon pore proteins as a complex supported in a bilayer membrane mimetic nano-scaffold known as a nanolipoprotein particle (NLP) Initial results show cell-free expression of Yersinia pestis outer membrane proteins YopB and YopD was enhanced in the presence of liposomes. However, these complexes tended to aggregate and precipitate. With the addition of co-expressed (NLP) forming components, the YopB and/or YopD complex was rendered soluble, increasing the yield of protein for biophysical studies. Biophysical methods such as Atomic Force Microscopy and Fluorescence Correlation Spectroscopy were used to confirm that the soluble YopB/D complex was associated with NLPs. An interaction between the YopB/D complex and NLP was validated by immunoprecipitation. The YopB/D translocon complex embedded in a NLP provides a platform for protein interaction studies between pathogen and host proteins. These studies will help elucidate the poorly understood mechanism which enables this pathogen to inject effector proteins into host cells, thus evading host defenses.

Bacterial Control of Pores Induced by the Type III Secretion System: Mind the Gap

Type III secretion systems (T3SSs) are specialized secretion apparatus involved in the virulence of many Gram-negative pathogens, enabling the injection of bacterial type III effectors into host cells. The T3SS-dependent injection of effectors requires the insertion into host cell membranes of a pore-forming "translocon," whose effects on cell responses remain ill-defined. As opposed to pore-forming toxins that damage host cell plasma membranes and induce cell survival mechanisms, T3SS-dependent pore formation is transient, being regulated by cell membrane repair mechanisms or bacterial effectors. Here, we review host cell responses to pore formation induced by T3SSs associated with the loss of plasma membrane integrity and regulation of innate immunity. We will particularly focus on recent advances in mechanisms controlling pore formation and the activity of the T3SS linked to type III effectors or bacterial proteases. The implications of the regulation of the T3SS translocon activity during the infectious process will be discussed.

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.

Inflammasome activation in response to the Yersinia type III secretion system requires hyperinjection of translocon proteins YopB and YopD

Type III secretion systems (T3SS) translocate effector proteins into target cells in order to disrupt or modulate host cell signaling pathways and establish replicative niches. However, recognition of T3SS activity by cytosolic pattern recognition receptors (PRRs) of the nucleotide-binding domain leucine rich repeat (NLR) family, either through detection of translocated products or membrane disruption, induces assembly of multiprotein complexes known as inflammasomes. Macrophages infected with Yersinia pseudotuberculosis strains lacking all known effectors or lacking the translocation regulator YopK induce rapid activation of both the canonical NLRP3 and noncanonical caspase-11 inflammasomes. While this inflammasome activation requires a functional T3SS, the precise signal that triggers inflammasome activation in response to Yersinia T3SS activity remains unclear. Effectorless strains of Yersinia as well as ΔyopK strains translocate elevated levels of T3SS substrates into infected cells. To dissect the contribution of pore formation and translocation to inflammasome activation, we took advantage of variants of YopD and LcrH that separate these functions of the T3SS. Notably, YopD variants that abrogated translocation but not pore-forming activity failed to induce inflammasome activation. Furthermore, analysis of individual infected cells revealed that inflammasome activation at the single-cell level correlated with translocated levels of YopB and YopD themselves. Intriguingly, LcrH mutants that are fully competent for effector translocation but produce and translocate lower levels of YopB and YopD also fail to trigger inflammasome activation. Our findings therefore suggest that hypertranslocation of YopD and YopB is linked to inflammasome activation in response to the Yersinia T3SS.

The innate immune response is critical to effective clearance of pathogens. Recognition of conserved virulence structures and activities by innate immune receptors such as NLRs constitute one of the first steps in mounting the innate immune response. However, pathogens such as Yersinia actively evade or subvert components of host defense, such as inflammasomes. The T3SS-secreted protein YopK is an essential virulence factor that limits translocation of other Yops, thereby limiting T3SS-induced inflammasome activation. However, what triggers inflammasome activation in cells infected by YopK-deficient Yersinia is not clear. Our findings indicate that hypertranslocation of pore complex proteins promotes inflammasome activation and that YopK prevents inflammasome activation by the T3SS by limiting translocation of YopD and YopB themselves.

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.

Identification of novel type III secretion chaperone-substrate complexes of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterial pathogen of humans that uses a type III secretion (T3S) system to manipulate host cells through the delivery of effector proteins into their cytosol and membranes. The function of T3S systems depends on small bacterial cytosolic chaperone-like proteins, which bind T3S substrates and ensure their appropriate secretion. To find novel T3S chaperone-substrate complexes of C. trachomatis we first searched its genome for genes encoding proteins with features of T3S chaperones. We then systematically tested for interactions between candidate chaperones and chlamydial T3S substrates by bacterial two-hybrid. This revealed interactions between Slc1 (a known T3S chaperone) or CT584 and several T3S substrates. Co-immunoprecipation after protein expression in Yersinia enterocolitica and protein overlay binding assays indicated that Slc1 interacted with the N-terminal region of the known T3S substrates Tarp (a previously described substrate of Slc1), CT694, and CT695, and that CT584 interacted with a central region of CT082, which we identified as a C. trachomatis T3S substrate using Y. enterocolitica as a heterologous system. Further T3S assays in Yersinia indicated that Slc1 or CT584 increased the amount of secreted Tarp, CT694, and CT695, or CT082, respectively. Expression of CT584 increased the intra-bacterial stability of CT082, while Slc1 did not affect the stability of its substrates. Overall, this indicated that in C. trachomatis Slc1 is a chaperone of multiple T3S substrates and that CT584 is a chaperone of the newly identified T3S substrate CT082.

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.

Impact of host membrane pore formation by the Yersinia pseudotuberculosis type III secretion system on the macrophage innate immune response

Type III secretion systems (T3SSs) are used by Gram-negative pathogens to form pores in host membranes and deliver virulence-associated effector proteins inside host cells. In pathogenic Yersinia, the T3SS pore-forming proteins are YopB and YopD. Mammalian cells recognize the Yersinia T3SS, leading to a host response that includes secretion of the inflammatory cytokine interleukin-1β (IL-1β), Toll-like receptor (TLR)-independent expression of the stress-associated transcription factor Egr1 and the inflammatory cytokine tumor necrosis factor alpha (TNF-α), and host cell death. The known Yersinia T3SS effector proteins are dispensable for eliciting these responses, but YopB is essential. Three models describe how the Yersinia T3SS might trigger inflammation: (i) mammalian cells sense YopBD-mediated pore formation, (ii) innate immune stimuli gain access to the host cytoplasm through the YopBD pore, and/or (iii) the YopB-YopD translocon itself or its membrane insertion is proinflammatory. To test these models, we constructed a Yersinia pseudotuberculosis mutant expressing YopD devoid of its predicted transmembrane domain (YopD(ΔTM)) and lacking the T3SS cargo proteins YopHEMOJTN. This mutant formed pores in macrophages, but it could not mediate translocation of effector proteins inside host cells. Importantly, this mutant did not elicit rapid host cell death, IL-1β secretion, or TLR-independent Egr1 and TNF-α expression. These data suggest that YopBD-mediated translocation of unknown T3SS cargo leads to activation of host pathways influencing inflammation, cell death, and response to stress. As the YopD(ΔTM) Y. pseudotuberculosis mutant formed somewhat smaller pores with delayed kinetics, an alternative model is that the wild-type YopB-YopD translocon is specifically sensed by host cells.

LcrV mutants that abolish Yersinia type III injectisome function

LcrV, the type III needle cap protein of pathogenic Yersinia, has been proposed to function as a tether between YscF, the needle protein, and YopB-YopD to constitute the injectisome, a conduit for the translocation of effector proteins into host cells. Further, insertion of LcrV-capped needles from a calcium-rich environment into host cells may trigger the low-calcium signal for effector translocation. Here, we used a genetic approach to test the hypothesis that the needle cap responds to the low-calcium signal by promoting injectisome assembly. Growth restriction of Yersinia pestis in the absence of calcium (low-calcium response [LCR(+)] phenotype) was exploited to isolate dominant negative lcrV alleles with missense mutations in its amber stop codon (lcrV(*327)). The addition of at least four amino acids or the eight-residue Strep tag to the C terminus was sufficient to generate an LCR(-) phenotype, with variant LcrV capping type III needles that cannot assemble the YopD injectisome component. The C-terminal Strep tag appears buried within the cap structure, blocking effector transport even in Y. pestis yscF variants that are otherwise calcium blind, a constitutive type III secretion phenotype. Thus, LcrV(*327) mutants arrest the needle cap in a state in which it cannot respond to the low-calcium signal with either injectisome assembly or the activation of type III secretion. Insertion of the Strep tag at other positions of LcrV produced variants with wild-type LCR(+), LCR(-), or dominant negative LCR(-) phenotypes, thereby allowing us to identify discrete sites within LcrV as essential for its attributes as a secretion substrate, needle cap, and injectisome assembly factor.

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.

Expression hierarchy in the Yersinia type III secretion system established through YopD recognition of RNA

The Yersinia type III secretion system (T3SS) is environmentally responsive to enable its rapid induction upon contact with host cells and is necessary for Yersiniae to establish a replicative niche and cause disease. YopD, a translocator protein, represses the expression of T3SS genes until signalled by environmental cues, a mechanism known as the low calcium response. In this work, we investigated recognition of target genes by Yersinia pestis YopD. Expression of all genes of the T3SS was induced in a yopD mutant, though not to the same degree, with effector Yops most affected. Two, short AU-rich sequence elements up- and downstream of start codons of target genes were necessary but not sufficient for YopD mediated repression. Purified YopD-LcrH bound specifically to target RNAs in vitro with different relative affinities, with effector Yops having greater affinity. Together, the data suggest YopD binds to T3SS transcripts where it may prevent ribosome binding causing accelerated mRNA degradation. This regulatory mechanism may ensure an expression hierarchy during the low calcium response as low affinity YopD targets such as chaperones would be translated prior to high affinity targets such as effector Yops allowing the bacteria another layer of control over Yop translocation during infection.

The RACK1 signaling scaffold protein selectively interacts with Yersinia pseudotuberculosis virulence function

Many gram-negative bacteria use type III secretion systems to translocate effector proteins into host cells. These effectors interfere with cellular functions in a highly regulated manner resulting in effects that are beneficial for the bacteria. The pathogen Yersinia can resist phagocytosis by eukaryotic cells by translocating Yop effectors into the target cell cytoplasm. This is called antiphagocytosis, and constitutes an important virulence feature of this pathogen since it allows survival in immune cell rich lymphoid organs. We show here that the virulence protein YopK has a role in orchestrating effector translocation necessary for productive antiphagocytosis. We present data showing that YopK influences Yop effector translocation by modulating the ratio of the pore-forming proteins YopB and YopD in the target cell membrane. Further, we show that YopK that can interact with the translocators, is exposed inside target cells and binds to the eukaryotic signaling protein RACK1. This protein is engaged upon Y. pseudotuberculosis-mediated β1-integrin activation and localizes to phagocytic cups. Cells with downregulated RACK1 levels are protected from antiphagocytosis. This resistance is not due to altered levels of translocated antiphagocytic effectors, and cells with reduced levels of RACK1 are still sensitive to the later occurring cytotoxic effect caused by the Yop effectors. Further, a yopK mutant unable to bind RACK1 shows an avirulent phenotype during mouse infection, suggesting that RACK1 targeting by YopK is a requirement for virulence. Together, our data imply that the local event of Yersinia-mediated antiphagocytosis involves a step where YopK, by binding RACK1, ensures an immediate specific spatial delivery of antiphagocytic effectors leading to productive inhibition of phagocytosis.

EseD, a putative T3SS translocon component of Edwardsiella tarda, contributes to virulence in fish and is a candidate for vaccine development

Edwardsiella tarda has a type III secretion system (T3SS) essential for pathogenesis. EseD, together with EseB and EseC, has been suggested to form a putative T3SS translocon complex, although its further function is unclear. To investigate the physiological role of EseD, a mutant strain of E. tarda was constructed with an in-frame deletion of the entire eseD gene. One finding was that the ∆eseD mutant decreased the secretion levels of EseC and EseB proteins. Additionally, the ∆eseD mutant showed attenuated swarming and contact-hemolysis abilities. However, the ∆eseD mutant showed increased biofilm formation. Complementation of the mutant strain with eseD restored these phenotypes to those similar to the wild-type strain. Furthermore, infection experiments in fish showed that the ∆eseD mutant exhibited slower proliferation and a tenfold decrease in virulence in fish. These results indicate a specific role of EseD in the pathogenesis of E. tarda. Finally, recombinant EseD protein elicited high antibody titers in immunized fish and various levels of protection against lethal challenge with the wild-type strain. These results indicate that EseD protein may be a candidate antigen for development of a subunit vaccine against Edwardsiellosis.

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

YopD-like translocator proteins encoded by several Gram-negative bacteria are important for type III secretion-dependent delivery of anti-host effectors into eukaryotic cells. This probably depends on their ability to form pores in the infected cell plasma membrane, through which effectors may gain access to the cell interior. In addition, Yersinia YopD is a negative regulator essential for the control of effector synthesis and secretion. As a prerequisite for this functional duality, YopD may need to establish molecular interactions with other key T3S components. A putative coiled-coil domain and an alpha-helical amphipathic domain, both situated in the YopD C terminus, may represent key protein-protein interaction domains. Therefore, residues within the YopD C terminus were systematically mutagenized. All 68 mutant bacteria were first screened in a variety of assays designed to identify individual residues essential for YopD function, possibly by providing the interaction interface for the docking of other T3S proteins. Mirroring the effect of a full-length yopD gene deletion, five mutant bacteria were defective for both yop regulatory control and effector delivery. Interestingly, all mutations clustered to hydrophobic amino acids of the amphipathic domain. Also situated within this domain, two additional mutants rendered YopD primarily defective in the control of Yop synthesis and secretion. Significantly, protein-protein interaction studies revealed that functionally compromised YopD variants were also defective in self-oligomerization and in the ability to engage another translocator protein, LcrV. Thus, the YopD amphipathic domain facilitates the formation of YopD/YopD and YopD/LcrV interactions, two critical events in the type III secretion process.

Seroepidemiological survey of pathogenic Yersinia in breeding squirrel monkeys in Japan

To investigate the prevalence of antibodies to pathogenic Yersinia in breeding squirrel monkeys, the serum samples of 252 squirrel monkeys from 9 zoological gardens in Japan were tested by ELISA using plasmid-encoded Yersinia outer membrane protein (Yops) as the antigen. The cutoff value was calculated by using the serum samples of the squirrel monkeys from Suriname, where no prevalence of pathogenic Yersinia have been reported. According to the cutoff value, 164 of 252 (65.1%) squirrel monkeys were considered positive against pathogenic Yersinia. These positive monkeys belonged to 8 of the 9 zoological gardens, and the percentage of the seropositive monkeys ranged from 22.2 to 89.4%. Furthermore, in one zoological garden, the positive rate of the squirrel monkeys which were over 1 year old (95.7%) was significantly higher than those which were under 1 year old (23.3%). These results suggested that pathogenic Yersinia is highly prevalent among breeding monkeys in Japan.

Mapping of the chaperone AcrH binding regions of translocators AopB and AopD and characterization of oligomeric and metastable AcrH-AopB-AopD complexes in the type III secretion system of Aeromonas hydrophila

In the type III secretion system (T3SS) of Aeromonas hydrophila, AcrH acts as a chaperone for translocators AopB and AopD. AcrH forms a stable 1:1 monomeric complex with AopD, whereas the 1:1 AcrH-AopB complex exists mainly as a metastable oligomeric form and only in minor amounts as a stable monomeric form. Limited protease digestion shows that these complexes contain highly exposed regions, thus allowing mapping of intact functional chaperone binding regions of AopB and AopD. AopD uses the transmembrane domain (DF1, residues 16-147) and the C-terminal amphipathic helical domain (DF2, residues 242-296) whereas AopB uses a discrete region containing the transmembrane domain and the putative N-terminal coiled coil domain (BF1, residues 33-264). Oligomerization of the AcrH-AopB complex is mainly through the C-terminal coiled coil domain of AopB, which is dispensable for chaperone binding. The three proteins, AcrH, AopB, and AopD, can be coexpressed to form an oligomeric and metastable complex. These three proteins are also oligomerized mainly through the C-terminal domain of AopB. Formation of such an oligomeric and metastable complex may be important for the proper formation of translocon of correct topology and stoichiometry on the host membrane.

The role of relA and spoT in Yersinia pestis KIM5 pathogenicity

The ppGpp molecule is part of a highly conserved regulatory system for mediating the growth response to various environmental conditions. This mechanism may represent a common strategy whereby pathogens such as Yersinia pestis, the causative agent of plague, regulate the virulence gene programs required for invasion, survival and persistence within host cells to match the capacity for growth. The products of the relA and spoT genes carry out ppGpp synthesis. To investigate the role of ppGpp on growth, protein synthesis, gene expression and virulence, we constructed a ΔrelA ΔspoT Y. pestis mutant. The mutant was no longer able to synthesize ppGpp in response to amino acid or carbon starvation, as expected. We also found that it exhibited several novel phenotypes, including a reduced growth rate and autoaggregation at 26°C. In addition, there was a reduction in the level of secretion of key virulence proteins and the mutant was>1,000-fold less virulent than its wild-type parent strain. Mice vaccinated subcutaneously (s.c.) with 2.5×10⁴ CFU of the ΔrelA ΔspoT mutant developed high anti-Y. pestis serum IgG titers, were completely protected against s.c. challenge with 1.5×10⁵ CFU of virulent Y. pestis and partially protected (60% survival) against pulmonary challenge with 2.0×10⁴ CFU of virulent Y. pestis. Our results indicate that ppGpp represents an important virulence determinant in Y. pestis and the ΔrelA ΔspoT mutant strain is a promising vaccine candidate to provide protection against plague.

Investigation of EscA as a chaperone for the Edwardsiella tarda type III secretion system putative translocon component EseC

Edwardsiella tarda is an important Gram-negative enteric pathogen affecting both animals and humans. It possesses a type III secretion system (T3SS) essential for pathogenesis. EseB, EseC and EseD have been shown to form a translocon complex after secretion, while EscC functions as a T3SS chaperone for EseB and EseD. In this paper we identify EscA, a protein required for accumulation and proper secretion of another translocon component, EseC. The escA gene is located upstream of eseC and the EscA protein has the characteristics of T3SS chaperones. Cell fractionation experiments indicated that EscA is located in the cytoplasm and on the cytoplasmic membrane. Mutation with in-frame deletion of escA greatly decreased the secretion of EseC, while complementation of escA restored the wild-type secretion phenotype. The stabilization and accumulation of EseC in the cytoplasm were also affected in the absence of EscA. Mutation of escA did not affect the transcription of eseC but reduced the accumulation level of EseC as measured by using an EseC-LacZ fusion protein in Ed. tarda. Co-purification and co-immunoprecipitation studies demonstrated a specific interaction between EscA and EseC. Further analysis showed that residues 31-137 of EseC are required for EseC-EscA interaction. Mutation of EseC residues 31-137 reduced the secretion and accumulation of EseC in Ed. tarda. Finally, infection experiments showed that mutations of EscA and residues 31-137 of EseC increased the LD(50) by approximately 10-fold in blue gourami fish. These results indicated that EscA functions as a specific chaperone for EseC and contributes to the virulence of Ed. tarda.

The type III secretion system tip complex and translocon

The type III secretion machinery of Gram-negative bacteria, also known as the injectisome or needle complex, is composed of a basal body spanning both bacterial membranes and the periplasm, and an external needle protruding from the bacterial surface. A set of three proteins, two hydrophobic and one hydrophilic, are required to allow translocation of proteins from the bacterium to the host cell cytoplasm. These proteins are involved in the formation of a translocation pore, the translocon, in the host cell membrane. Exciting progress has recently been made on the interaction between the translocators and the injectisome needle and the assembly of the translocon in the host cell membrane. As expected, the two hydrophobic translocators insert into the target cell membrane. Unexpectedly, the third, hydrophilic translocator, forms a complex on the distal end of the injectisome needle, the tip complex, and serves as an assembly platform for the two hydrophobic translocators.

Control of gene expression by type III secretory activity

The bacterial flagellum and the highly related injectisome (or needle complex) are among the most complicated multi-protein structures found in Gram-negative microorganisms. The assembly of both structures is dependent upon a type III secretion system. An interesting regulatory feature unique to these systems is the coordination of gene expression with type III secretory activity. This means of regulation ensures that secretion substrates are expressed only when required during the assembly process or upon completion of the fully functional structure. Prominent within the regulatory scheme are secreted proteins and type III secretion chaperones that exert effects on gene expression at the transcriptional and post-transcriptional levels. Although the major structural components of the flagellum and injectisome systems are highly conserved, recent studies reveal diversity in the mechanisms used by secretion substrates and chaperones to control gene expression.

Yersinia controls type III effector delivery into host cells by modulating Rho activity

Yersinia pseudotuberculosis binds to beta1 integrin receptors, and uses the type III secretion proteins YopB and YopD to introduce pores and to translocate Yop effectors directly into host cells. Y. pseudotuberculosis lacking effectors that inhibit Rho GTPases, YopE and YopT, have high pore forming activity. Here, we present evidence that Y. pseudotuberculosis selectively modulates Rho activity to induce cellular changes that control pore formation and effector translocation. Inhibition of actin polymerization decreased pore formation and YopE translocation in HeLa cells infected with Y. pseudotuberculosis. Inactivation of Rho, Rac, and Cdc42 by treatment with Clostridium difficile toxin B inhibited pore formation and YopE translocation in infected HeLa cells. Expression of a dominant negative form of Rac did not reduce the uptake of membrane impermeable dyes in HeLa cells infected with a pore forming strain YopEHJT(-). Similarly, the Rac inhibitor NSC23766 did not decrease pore formation or translocation, although it efficiently hindered Rac-dependent bacterial uptake. In contrast, C. botulinum C3 potently reduced pore formation and translocation, implicating Rho A, B, and/or C in the control of the Yop delivery. An invasin mutant (Y. pseudotuberculosis invD911E) that binds to beta1 integrins, but inefficiently transduces signals through the receptors, was defective for YopE translocation. Interfering with the beta1 integrin signaling pathway, by inhibiting Src kinase activity, negatively affected YopE translocation. Additionally, Y. pseudotuberculosis infection activated Rho by a mechanism that was dependent on YopB and on high affinity bacteria interaction with beta1 integrin receptors. We propose that Rho activation, mediated by signals triggered by the YopB/YopD translocon and from engagement of beta1 integrin receptors, stimulates actin polymerization and activates the translocation process, and that once the Yops are translocated, the action of YopE or YopT terminate delivery of Yops and prevents pore formation.

Diminished LcrV secretion attenuates Yersinia pseudotuberculosis virulence

Many gram-negative bacterial pathogenicity factors that function beyond the outer membrane are secreted via a contact-dependent type III secretion system. Two types of substrates are predestined for this mode of secretion, namely, antihost effectors that are translocated directly into target cells and the translocators required for targeting of the effectors across the host cell membrane. N-terminal secretion signals are important for recognition of the protein cargo by the type III secretion machinery. Even though such signals are known for several effectors, a consensus signal sequence is not obvious. One of the translocators, LcrV, has been attributed other functions in addition to its role in translocation. These functions include regulation, presumably via interaction with LcrG inside bacteria, and immunomodulation via interaction with Toll-like receptor 2. Here we wanted to address the significance of the specific targeting of LcrV to the exterior for its function in regulation, effector targeting, and virulence. The results, highlighting key N-terminal amino acids important for LcrV secretion, allowed us to dissect the role of LcrV in regulation from that in effector targeting/virulence. While only low levels of exported LcrV were required for in vitro effector translocation, as deduced by a cell infection assay, fully functional export of LcrV was found to be a prerequisite for its role in virulence in the systemic murine infection model.

Inhibition of type III secretion in Salmonella enterica serovar Typhimurium by small-molecule inhibitors

Type III secretion systems (T3SS) are conserved in many pathogenic gram-negative bacteria. Small molecules that specifically target T3SS in Yersinia and Chlamydia spp. have recently been identified. Here we show that two such compounds inhibit Salmonella T3SS-1, preventing secretion of T3SS-1 effectors, invasion of cultured epithelial cells, and enteritis in vivo.

Tetratricopeptide repeats are essential for PcrH chaperone function in Pseudomonas aeruginosa type III secretion

The type III secretion system (T3SS) is a specialized apparatus evolved by Gram-negative bacteria to deliver effector proteins into host cells, thus facilitating the establishment of an infection. Effector translocation across the target cell plasma membrane is believed to occur via pores formed by at least two secreted translocator proteins, the functions of which are dependent upon customized class II T3SS chaperones. Recently, three internal tetratricopeptide repeats (TPRs) were identified in this class of chaperones. Here, defined mutagenesis of the class II chaperone PcrH of Pseudomonas aeruginosa revealed these TPRs to be essential for chaperone activity towards the translocator proteins PopB and PopD and subsequently for the translocation of exoenzymes into host cells.

Protein Secretion and Membrane Insertion Systems in Gram-Negative Bacteria

In contrast to other organisms, gram-negative bacteria have evolved numerous systems for protein export. Eight types are known that mediate export across or insertion into the cytoplasmic membrane, while eight specifically mediate export across or insertion into the outer membrane. Three of the former secretory pathway (SP) systems, type I SP (ISP, ABC), IIISP (Fla/Path) and IVSP (Conj/Vir), can export proteins across both membranes in a single energy-coupled step. A fourth generalized mechanism for exporting proteins across the two-membrane envelope in two distinct steps (which we here refer to as type II secretory pathways [IISP]) utilizes either the general secretory pathway (GSP or Sec) or the twin-arginine targeting translocase for translocation across the inner membrane, and either the main terminal branch or one of several protein-specific export systems for translocation across the outer membrane. We here survey the various well-characterized protein translocation systems found in living organisms and then focus on the systems present in gram-negative bacteria. Comparisons between these systems suggest specific biogenic, mechanistic and evolutionary similarities as well as major differences.

Extracytoplasmic-stress-responsive pathways modulate type III secretion in Yersinia pseudotuberculosis

Three signal transduction pathways, the two-component systems CpxRA and BaeSR and the alternative sigma factor sigma(E), respond to extracytoplasmic stress that facilitates bacterial adaptation to changing environments. At least the CpxRA and sigma(E) pathways control the production of protein-folding and degradation factors that counter the effects of protein misfolding in the periplasm. This function also influences the biogenesis of multicomponent extracellular appendages that span the bacterial envelope, such as various forms of pili. Herein, we investigated whether any of these regulatory pathways in the enteropathogen Yersinia pseudotuberculosis affect the functionality of the Ysc-Yop type III secretion system. This is a multicomponent molecular syringe spanning the bacterial envelope used to inject effector proteins directly into eukaryotic cells. Disruption of individual components revealed that the Cpx and sigma(E) pathways are important for Y. pseudotuberculosis type III secretion of Yops (Yersinia outer proteins). In particular, a loss of CpxA, a sensor kinase, reduced levels of structural Ysc (Yersinia secretion) components in bacterial membranes, suggesting that these mutant bacteria are less able to assemble a functional secretion apparatus. Moreover, these bacteria were no longer capable of localizing Yops into the eukaryotic cell interior. In addition, a cpxA lcrQ double mutant engineered to overproduce and secrete Yops was still impaired in intoxicating cells. Thus, the Cpx pathway might mediate multiple influences on bacterium-target cell contact that modulate Yersinia type III secretion-dependent host cell cytotoxicity.

Analysis of the function of enteropathogenic Escherichia coli EspB by random mutagenesis

Enteropathogenic Escherichia coli (EPEC) is an important cause of infantile diarrhea, especially in developing countries. EspB, a key virulence factor of EPEC, is required for the attaching and effacing effect characteristic of EPEC and enterohemorrhagic E. coli and has been posited to play several functions in the process of infection. Attaching and effacing activity is associated with the accumulation of filamentous actin beneath adherent bacteria as measured in the fluorescence actin staining (FAS) test. To determine whether different domains of EspB are responsible for different functions, 42 plasmids carrying mutated espB were introduced into an espB deletion mutant. Two major groups of espB mutants were identified. One group of 17 mutants exhibited positive FAS results and normal levels of hemolytic activity. Another group of 22 mutants exhibited negative FAS results and low levels of hemolytic activity. Three mutants were exceptional. One mutant was FAS positive but had significantly reduced hemolytic activity. Conversely, a second mutant was FAS negative but had full hemolytic activity. A third mutant had a significantly reduced FAS level compared to the wild type but full hemolytic activity. The results of EspF and Tir translocation assays confirmed that FAS-negative insertions disrupt effector translocation and mutants with FAS-positive insertions retain protein translocation activity. These results suggest that EspB has distinct domain functions involved in effector translocation that can be distinguished from its role as a component of the translocation pore.

BopC is a novel type III effector secreted by Bordetella bronchiseptica and has a critical role in type III-dependent necrotic cell death

In Bordetella bronchiseptica, the functional type III secretion system (TTSS) is required for the induction of necrotic cell death in infected mammalian cells. To identify the factor(s) involved in necrotic cell death, type III-secreted proteins from B. bronchiseptica were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electrospray ionization tandem mass spectrometry. We identified a 69-kDa secreted protein designated BopC. The gene encoding BopC is located outside of the TTSS locus and is also highly conserved in both Bordetella parapertussis and Bordetella pertussis. The results of a lactate dehydrogenase release assay and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling assay demonstrated that BopC is required for necrotic cell death. It has been reported that tyrosine-phosphorylated proteins (PY) of host cells are dephosphorylated during B. bronchiseptica infection in a TTSS-dependent manner. We found that BopC is also involved in PY dephosphorylation in infected host cells. It appears that the necrotic cell death triggered by BopC occurs prior to the PY reduction in host cells, because Bordetella-induced cell death was not affected even in the presence of a dephosphorylation inhibitor. Furthermore, a translocation assay showed that the signal sequence for both secretion into culture supernatant and translocation into the host cell is located in 48 amino acid residues of the BopC N terminus. This report reveals for the first time that a novel type III effector, BopC, is required for the induction of necrotic cell death during Bordetella infection.

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.

Mapping of a YscY binding domain within the LcrH chaperone that is required for regulation of Yersinia type III secretion

Type III secretion systems are used by many animal and plant interacting bacteria to colonize their host. These systems are often composed of at least 40 genes, making their temporal and spatial regulation very complex. Some type III chaperones of the translocator class are important regulatory molecules, such as the LcrH chaperone of Yersinia pseudotuberculosis. In contrast, the highly homologous PcrH chaperone has no regulatory effect in native Pseudomonas aeruginosa or when produced in Yersinia. In this study, we used LcrH-PcrH chaperone hybrids to identify a discrete region in the N terminus of LcrH that is necessary for YscY binding and regulatory control of the Yersinia type III secretion machinery. PcrH was unable to bind YscY and the homologue Pcr4 of P. aeruginosa. YscY and Pcr4 were both essential for type III secretion and reciprocally bound to both substrates YscX of Yersinia and Pcr3 of P. aeruginosa. Still, Pcr4 was unable to complement a DeltayscY null mutant defective for type III secretion and yop-regulatory control in Yersinia, despite the ability of YscY to function in P. aeruginosa. Taken together, we conclude that the cross-talk between the LcrH and YscY components represents a strategic regulatory pathway specific to Yersinia type III secretion.

Genetic and Molecular Analysis of GogB, a Phage-encoded Type III-secreted Substrate in Salmonella enterica Serovar Typhimurium with Autonomous Expression from its Associated Phage

Salmonella enterica serovar Typhimurium is lysogenized by several temperate bacteriophages that encode lysogenic conversion genes, which can act as virulence factors during infection and contribute to the genetic diversity and pathogenic potential of the lysogen. We have investigated the temperate bacteriophage called Gifsy-1 in S.enterica serovar Typhimurium and show here that the product of the gogB gene encoded within this phage shares similarity with proteins from other Gram-negative pathogens. The amino-terminal portion of GogB shares similarity with leucine-rich repeat-containing virulence-associated proteins from other Gram-negative pathogens, whereas the carboxyl-terminal portion of GogB shares similarity with uncharacterized proteins in other pathogens. We show that GogB is secreted by both type III secretion systems encoded in Salmonella Pathogenicity Island-1 (SPI-1) and SPI-2 but translocation into host cells is a SPI-2-mediated process. Once translocated, GogB localizes to the cytoplasm of infected host cells. The genetic regulation of gogB in Salmonella is influenced by the transcriptional activator, SsrB, under SPI-2-inducing conditions, but the modular nature of the gogB gene allows for autonomous expression and type III secretion following horizontal gene transfer into a heterologous pathogen. These data define the first autonomously expressed lysogenic conversion gene within Gifsy-1 that acts as a modular and promiscuous type III-secreted substrate of the infection process.

The hrpK operon of Pseudomonas syringae pv. tomato DC3000 encodes two proteins secreted by the type III (Hrp) protein secretion system: HopB1 and HrpK, a putative type III translocator

Pseudomonas syringae is a gram-negative bacterial plant pathogen that is dependent on a type III protein secretion system (TTSS) and the effector proteins it translocates into plant cells for pathogenicity. The P. syringae TTSS is encoded by hrp-hrc genes that reside in a central region of a pathogenicity island (Pai). Flanking one side of this Pai is the exchangeable effector locus (EEL). We characterized the transcriptional expression of the open reading frames (ORFs) within the EEL of P. syringae pv. tomato DC3000. One of these ORFs, PSPTO1406 (hopB1) is expressed in the same transcriptional unit as hrpK. Both HopB1 and HrpK were secreted in culture and translocated into plant cells via the TTSS. However, the translocation of HrpK required its C-terminal half. HrpK shares low similarity with a putative translocator, HrpF, from Xanthomonas campestris pv. vesicatoria. DC3000 mutants lacking HrpK were significantly reduced in disease symptoms and multiplication in planta, whereas DC3000 hopB1 mutants produced phenotypes similar to the wild type. Additionally, hrpK mutants were reduced in their ability to elicit the hypersensitive response (HR), a programmed cell death associated with plant defense. The reduced HR phenotype exhibited by hrpK mutants was complemented by hrpK expressed in bacteria but not by HrpK transgenically expressed in tobacco, suggesting that HrpK does not function inside plant cells. Further experiments identified a C-terminal transmembrane domain within HrpK that is required for HrpK translocation. Taken together, HopB1 is a type III effector and HrpK plays an important role in the TTSS and is a putative type III translocator.