LcrD, a membrane-bound regulator of the Yersinia pestis low-calcium response

Type III Secretion in Chlamydia

Type III secretion systems (T3SSs) are utilized by Gram-negative pathogens to enhance their pathogenesis. This secretion system is associated with the delivery of effectors through a needle-like structure from the bacterial cytosol directly into a target eukaryotic cell. These effector proteins then manipulate specific eukaryotic cell functions to benefit pathogen survival within the host. The obligate intracellular pathogens of the family Chlamydiaceae have a highly evolutionarily conserved nonflagellar T3SS that is an absolute requirement for their survival and propagation within the host with about one-seventh of the genome dedicated to genes associated with the T3SS apparatus, chaperones, and effectors. Chlamydiae also have a unique biphasic developmental cycle where the organism alternates between an infectious elementary body (EB) and replicative reticulate body (RB). T3SS structures have been visualized on both EBs and RBs. And there are effector proteins that function at each stage of the chlamydial developmental cycle, including entry and egress. This review will discuss the history of the discovery of chlamydial T3SS and the biochemical characterization of components of the T3SS apparatus and associated chaperones in the absence of chlamydial genetic tools. These data will be contextualized into how the T3SS apparatus functions throughout the chlamydial developmental cycle and the utility of heterologous/surrogate models to study chlamydial T3SS. Finally, there will be a targeted discussion on the history of chlamydial effectors and recent advances in the field.

The Regulatory Circuit Underlying Downregulation of a Type III Secretion System in Yersinia enterocolitica by Transcription Factor OmpR

In a previous study, differential proteomic analysis was used to identify membrane proteins of the human enteropathogen Yersinia enterocolitica, whose levels are influenced by OmpR, the transcriptional regulator in the two-component EnvZ/OmpR system. Interestingly, this analysis demonstrated that at 37 °C, OmpR negatively affects the level of over a dozen Ysc-Yop proteins, which constitute a type III secretion system (T3SS) that is essential for the pathogenicity of Y. enterocolitica. Here, we focused our analysis on the role of OmpR in the expression and secretion of Yops (translocators and effectors). Western blotting with anti-Yops antiserum and specific anti-YopD, -YopE and -YopH antibodies, confirmed that the production of Yops is down-regulated by OmpR with the greatest negative effect on YopD. The RT-qPCR analysis demonstrated that, while OmpR had a negligible effect on the activity of regulatory genes virF and yscM1, it highly repressed the expression of yopD. OmpR was found to bind to the promoter of the lcrGVsycD-yopBD operon, suggesting a direct regulatory effect. In addition, we demonstrated that the negative regulatory influence of OmpR on the Ysc-Yop T3SS correlated with its positive role in the expression of flhDC, the master regulator of the flagellar-associated T3SS.

The Yersinia pestis type III secretion system: expression, assembly and role in the evasion of host defenses

Yersinia pestis, the etiologic agent of plague, utilizes a type III secretion system (T3SS) to subvert the defenses of its mammalian hosts. T3SSs are complex nanomachines that allow bacterial pathogens to directly inject effector proteins into eukaryotic cells. The Y. pestis T3SS is not expressed during transit through the flea vector, but T3SS gene expression is rapidly thermoinduced upon entry into a mammalian host. Assembly of the T3S apparatus is a highly coordinated process that requires the homo- and hetero-oligomerization over 20 Yersinia secretion (Ysc) proteins, several assembly intermediates and the T3S process to complete the assembly of the rod and external needle structures. The activation of effector secretion is controlled by the YopN/TyeA/SycN/YscB complex, YscF and LcrG in response to extracellular calcium and/or contact with a eukaryotic cell. Cell contact triggers the T3S process including the secretion and assembly of a pore-forming translocon complex that facilitates the translocation of effector proteins, termed Yersinia outer proteins (Yops), across the eukaryotic membrane. Within the host cell, the Yop effector proteins function to inhibit bacterial phagocytosis and to suppress the production of pro-inflammatory cytokines.

Bacterial type III secretion systems: specialized nanomachines for protein delivery into target cells

One of the most exciting developments in the field of bacterial pathogenesis in recent years is the discovery that many pathogens utilize complex nanomachines to deliver bacterially encoded effector proteins into target eukaryotic cells. These effector proteins modulate a variety of cellular functions for the pathogen's benefit. One of these protein-delivery machines is the type III secretion system (T3SS). T3SSs are widespread in nature and are encoded not only by bacteria pathogenic to vertebrates or plants but also by bacteria that are symbiotic to plants or insects. A central component of T3SSs is the needle complex, a supramolecular structure that mediates the passage of the secreted proteins across the bacterial envelope. Working in conjunction with several cytoplasmic components, the needle complex engages specific substrates in sequential order, moves them across the bacterial envelope, and ultimately delivers them into eukaryotic cells. The central role of T3SSs in pathogenesis makes them great targets for novel antimicrobial strategies.

Assembly of the bacterial type III secretion machinery

Many bacteria that live in contact with eukaryotic hosts, whether as symbionts or as pathogens, have evolved mechanisms that manipulate host cell behaviour to their benefit. One such mechanism, the type III secretion system, is employed by Gram-negative bacterial species to inject effector proteins into host cells. This function is reflected by the overall shape of the machinery, which resembles a molecular syringe. Despite the simplicity of the concept, the type III secretion system is one of the most complex known bacterial nanomachines, incorporating one to more than hundred copies of up to twenty different proteins into a multi-MDa transmembrane complex. The structural core of the system is the so-called needle complex that spans the bacterial cell envelope as a tripartite ring system and culminates in a needle protruding from the bacterial cell surface. Substrate targeting and translocation are accomplished by an export machinery consisting of various inner membrane embedded and cytoplasmic components. The formation of such a multimembrane-spanning machinery is an intricate task that requires precise orchestration. This review gives an overview of recent findings on the assembly of type III secretion machines, discusses quality control and recycling of the system and proposes an integrated assembly model.

Yersinia Ysc-Yop type III secretion feedback inhibition is relieved through YscV-dependent recognition and secretion of LcrQ

Human pathogenic Yersinia species share a virulence plasmid encoding the Ysc-Yop type III secretion system (T3SS). A plasmid-encoded anti-activator, LcrQ, negatively regulates the expression of this secretion system. Under inducible conditions, LcrQ is secreted outside of bacterial cells and this activates the T3SS, but the mechanism of targeting LcrQ for type III secretion remains largely unknown. In this study, we characterized the regulatory role of the export apparatus component YscV. Depletion or overexpression of YscV compromised Yop synthesis and this primarily prevented secretion of LcrQ. It followed that a lcrQ deletion reversed the repressive effects of excessive YscV. Further characterization demonstrated that the YscV residues 493-511 located within the C-terminal soluble cytoplasmic domain directly bound with LcrQ. Critically, YscV-LcrQ complex formation was a requirement for LcrQ secretion, since YscVΔ493-511 failed to secrete LcrQ. This forced a cytoplasmic accumulation of LcrQ, which predictably caused the feedback inhibition of Yops synthesis. Based on these observations, we proposed a model for the YscV-dependent secretion of LcrQ and its role in regulating Yop synthesis in Yersinia.

The inner membrane protein HrcV from Xanthomonas spp. is involved in substrate docking during type III secretion

Pathogenicity of the gram-negative plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria depends on a membrane-spanning type III secretion (T3S) system, which translocates effector proteins into eukaryotic host cells. In this study, we characterized the T3S system component HrcV, which is a member of the YscV/FlhA family of inner membrane proteins. HrcV consists of eight transmembrane helices and a cytoplasmic region (HrcVC). Mutant and protein-protein interaction studies showed that HrcVC is essential for protein function and binds to T3S substrates, including the early substrate HrpB2, the pilus protein HrpE, and effector proteins. Furthermore, HrcVC interacts with itself and with components and control proteins of the T3S apparatus. The interaction of HrcVC with HrpB2, HrpE, and T3S system components depends on amino acid residues in a conserved motif, designated flagella/hypersensitive response/invasion proteins export pore (FHIPEP), which is located in a cytoplasmic loop between transmembrane helix four and five of HrcV. Mutations in the FHIPEP motif abolish HrcV function but do not affect the interaction of HrcVC with effector proteins.

RfaL is required for Yersinia pestis type III secretion and virulence

Yersinia pestis, the causative agent of plague, uses a type III secretion system (T3SS) to inject cytotoxic Yop proteins directly into the cytosol of mammalian host cells. The T3SS can also be activated in vitro at 37°C in the absence of calcium. The chromosomal gene rfaL (waaL) was recently identified as a virulence factor required for proper function of the T3SS. RfaL functions as a ligase that adds the terminal N-acetylglucosamine to the lipooligosaccharide core of Y. pestis. We previously showed that deletion of rfaL prevents secretion of Yops in vitro. Here we show that the divalent cations calcium, strontium, and magnesium can partially or fully rescue Yop secretion in vitro, indicating that the secretion phenotype of the rfaL mutant may be due to structural changes in the outer membrane and the corresponding feedback inhibition on the T3SS. In support of this, we found that the defect can be overcome by deleting the regulatory gene lcrQ. Consistent with a defective T3SS, the rfaL mutant is less virulent than the wild type. We show here that the virulence defect of the mutant correlates with a decrease in both T3SS gene expression and ability to inject innate immune cells, combined with an increased sensitivity to cationic antimicrobial peptides.

Identification of novel protein-protein interactions of Yersinia pestis type III secretion system by yeast two hybrid system

Type III secretion system (T3SS) of the plague bacterium Y. pestis encodes a syringe-like structure consisting of more than 20 proteins, which can inject virulence effectors into host cells to modulate the cellular functions. Here in this report, interactions among the possible components in T3SS of Yersinia pestis were identified using yeast mating technique. A total of 57 genes, including all the pCD1-encoded genes except those involved in plasmid replication and partition, pseudogenes, and the putative transposase genes, were subjected to yeast mating analysis. 21 pairs of interaction proteins were identified, among which 9 pairs had been previously reported and 12 novel pairs were identified in this study. Six of them were tested by GST pull down assay, and interaction pairs of YscG-SycD, YscG-TyeA, YscI-YscF, and YopN-YpCD1.09c were successfully validated, suggesting that these interactions might play potential roles in function of Yersinia T3SS. Several potential new interactions among T3SS components could help to understand the assembly and regulation of Yersinia T3SS.

Virulence properties and integron-associated antibiotic resistance of Klebsiella mobilis strains isolated from clinical specimens

This study examined Klebsiella mobilis isolates cultured from clinical specimens for virulence-associated properties and antibiotic resistance. The strains produced a number of siderophores, including enterobactin, aerobactin and yersiniabactin. All isolates were able to adhere to and invade epithelial cells. They had cytotoxic activity, which caused destruction of human laryngeal epithelial HEp-2 cells and evoked lysis of murine macrophage J774 cells. Analyses of HEp-2 and J774 cellular morphology and DNA fragmentation in the cells showed features typical of cells undergoing apoptosis. Some K. mobilis strains harboured class 1 integrons carrying the aadA1 gene encoding an aminoglycoside adenyltransferase.

Membrane topology of conserved components of the type III secretion system from the plant pathogen Xanthomonas campestris pv. vesicatoria

Type III secretion (T3S) systems play key roles in the assembly of flagella and the translocation of bacterial effector proteins into eukaryotic host cells. Eleven proteins which are conserved among Gram-negative plant and animal pathogenic bacteria have been proposed to build up the basal structure of the T3S system, which spans both inner and outer bacterial membranes. We studied six conserved proteins, termed Hrc, predicted to reside in the inner membrane of the plant pathogen Xanthomonas campestris pv. vesicatoria. The membrane topology of HrcD, HrcR, HrcS, HrcT, HrcU and HrcV was studied by translational fusions to a dual alkaline phosphatase–β-galactosidase reporter protein. Two proteins, HrcU and HrcV, were found to have the same membrane topology as the Yersinia homologues YscU and YscV. For HrcR, the membrane topology differed from the model for the homologue from Yersinia, YscR. For our data on three other protein families, exemplified by HrcD, HrcS and HrcT, we derived the first topology models. Our results provide what is believed to be the first complete model of the inner membrane topology of any bacterial T3S system and will aid in elucidating the architecture of T3S systems by ultrastructural analysis.

Beta1 integrin-dependent engulfment of Yersinia enterocolitica by macrophages is coupled to the activation of autophagy and suppressed by type III protein secretion

Autophagy is a central lysosomal degradation process that is essential for the maintenance of cellular homeostasis. Autophagy has furthermore emerged as integral part of the host immune response. Autophagic processes promote the separation and degradation of intracellular microorganisms which contributes to the development of innate and adaptive immunity. Some pathogenic microbes have therefore evolved mechanisms to evade or impede autophagy. We analyzed the effects of the enteropathogenic bacterium Yersinia enterocolitica on autophagy in macrophages. Yersiniae use a number of defined adhesins and secreted proteins to manipulate host immune responses. Our results showed that Y. enterocolitica defective in type III protein secretion efficiently activated autophagy in macrophages. Autophagy was mediated by the Yersinia adhesins invasin and YadA and particularly depended on the engagement of beta(1) integrin receptors. Several autophagy-related events followed beta(1) integrin-mediated engulfment of the bacteria including the formation of autophagosomes, processing of the marker protein LC3, redistribution of GFP-LC3 to bacteria-containing vacuoles, and the segregation of intracellular bacteria by autophagosomal compartments. These results provide direct evidence for the linkage of beta(1) integrin-mediated phagocytosis and autophagy induction. Multiple microbes signal through integrin receptors, and our results suggest a general principle by which the sensing of an extracellular microbe triggers autophagy. Owing to the importance of autophagy as host defense response, wild-type Y. enterocolitica suppressed autophagy by mobilizing type III protein secretion. The subversion of autophagy may be part of the Y. enterocolitica virulence strategy that supports bacterial survival when beta(1) integrin-dependent internalization and autophagy activation by macrophages are deleterious for the pathogen.

Growth of calcium-blind mutants of Yersinia pestis at 37 degrees C in permissive Ca2+-deficient environments

Cells of wild-type Yersinia pestis exhibit a low-calcium response (LCR) defined as bacteriostasis with expression of a pCD-encoded type III secretion system (T3SS) during cultivation at 37 degrees C without added Ca(2+) versus vegetative growth with downregulation of the T3SS with Ca(2+) (>or=2.5 mM). Bacteriostasis is known to reflect cumulative toxicity of Na(+), l-glutamic acid and culture pH; control of these variables enables full-scale growth ('rescue') in the absence of Ca(2+). Several T3SS regulatory proteins modulate the LCR, because their absence promotes a Ca(2+)-blind phenotype in which growth at 37 degrees C ceases and the T3SS is constitutive even with added Ca(2+). This study analysed the connection between the LCR and Ca(2+) by determining the response of selected Ca(2+)-blind mutants grown in Ca(2+)-deficient rescue media containing Na(+) plus l-glutamate (pH 5.5), where the T3SS is not expressed, l-glutamate alone (pH 6.5), where l-aspartate is fully catabolized, and Na(+) alone (pH 9.0), where the electrogenic sodium pump NADH : ubiquinone oxidoreductase becomes activated. All three conditions supported essentially full-scale Ca(2+)-independent growth at 37 degrees C of wild-type Y. pestis as well as lcrG and yopN mutants (possessing a complete but dysregulated T3SS), indicating that bacteriostasis reflects a Na(+)-dependent lesion in bioenergetics. In contrast, mutants lacking the negative regulator YopD or the YopD chaperone (LcrH) failed to grow in any rescue medium and are therefore truly temperature-sensitive. The Ca(2+)-blind yopD phenotype was fully suppressed in a Ca(2+)-independent background lacking the injectisome-associated inner-membrane component YscV but not peripheral YscK, suggesting that the core translocon energizes YopD.

Analysis of proteins in Chlamydia trachomatis L2 outer membrane complex, COMC

The protein composition and N-terminal sequences of proteins in the outer membrane of Chlamydia trachomatis L2 were analysed following isolation of N-terminal peptides using combined fractional diagonal chromatography and identification by liquid chromatography tandem MS. Acetylation of primary amino groups of in vivo generated proteolytic cleavage sites facilitated identification of such sites in known outer membrane proteins (MOMPs). Our results further support a proposed prediction of the topology of the MOMPs. Furthermore, a previously unknown MOMP, CTL0626 (Ct372), was assigned as an MOMP with a carbohydrate-selective porin (OprB) family motif, and the presence of CTL0626 was confirmed using antibodies raised against the protein.

Deletion of the genes encoding the type III secretion system and cytotoxic enterotoxin alters host responses to Aeromonas hydrophila infection

In our previous study, we deleted the gene encoding Aeromonas outer membrane protein B (AopB), a structural component of the type III secretion system (T3SS) from a cytotoxic enterotoxin gene (act)-minus diarrheal isolate SSU of Aeromonas hydrophila. Our laboratory also molecularly characterized the cytotoxic enterotoxin (Act), which is secreted by the bacterium utilizing the type II secretion system (T2SS). The act/aopB mutant exhibited significantly reduced cytotoxicity to cultured cells (e.g. RAW 264.7 murine macrophages and HT-29 human colonic epithelial cells) and was avirulent in mice. In this study, we developed additional A. hydrophila mutants in which T3SS-associated ascV and acrV genes were deleted, either individually or in combination with that of the act gene, to examine host-pathogen interactions. A significant reduction in the induction of inflammatory cytokines and chemokines was noted in the sera of mice infected with these mutants when compared to animals infected with wild-type (WT) A. hydrophila. After infection with the WT and act/aopB mutant, we performed microarray analyses on RNA from the above-mentioned murine macrophages and human colonic epithelial cells to examine global cellular transcriptional responses. Based on three independent experiments, WT A. hydrophila altered the expression of 434 genes in RAW 264.7 cells and 80 genes in HT-29 cells. Alteration in the expression of 209 macrophage and 32 epithelial cell genes was reduced when the act/aopB mutant was used, compared to when cells were infected with the WT bacterium, indicating the involvement of Act and/or AopB in transcriptional regulation of these genes. We verified up-regulation of 15 genes by real-time reverse transcriptase-polymerase chain reaction and confirmed A. hydrophila WT-versus mutant-induced production of cytokines/chemokines in supernatants from RAW 264.7 and HT-29 cells. This is the first description of host cell transcriptional responses to A. hydrophila infection.

Targeting of two effector protein classes to the type III secretion system by a HpaC- and HpaB-dependent protein complex fromXanthomonas campestrispv.vesicatoria

The Gram-negative plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria translocates effector proteins via a specialized type III secretion (TTS) system into the host cell cytosol. The efficient secretion of many effector proteins depends on the global TTS chaperone HpaB. Here, we identified a novel export control protein, HpaC, which significantly contributes to bacterial pathogenicity. Deletion of hpaC leads to a severe reduction in secretion of effector proteins and the putative type III translocon proteins HrpF and XopA. By contrast, secretion of the TTS pilus protein HrpE is not affected. We provide experimental evidence that HpaC differentiates between two classes of effector proteins. Using an in vivo reporter assay, we found that HpaC specifically promotes the translocation of the effector proteins XopJ and XopF1 into the plant cell, whereas AvrBs3 and XopC are efficiently translocated even in the absence of HpaC. Similar findings were obtained for HpaB. Inhibition of protein synthesis suggests that HpaB is involved in the secretion of stored effector proteins. Furthermore, protein-protein interaction studies revealed that HpaB and HpaC form an oligomeric protein complex and that they interact with members of both effector protein classes and the conserved TTS system component HrcV. Taken together, our data indicate that HpaB and HpaC play a central role in recruiting TTS substrates to the secretion apparatus.

The ATP-dependent ClpXP and Lon proteases regulate expression of the Yersinia pestis type III secretion system via regulated proteolysis of YmoA, a small histone-like protein

The Yersinia pestis plasmid pCD1-encoded type III secretion system (T3SS) is essential for the pathogenicity of Y. pestis in mammalian hosts. T3SS-associated genes are maximally expressed at 37 degrees C in the absence of extracellular calcium. Expression of T3SS genes requires LcrF, an AraC-like transcriptional activator, and is repressed by YmoA, a small histone-like protein. The mechanism by which temperature regulates T3SS gene expression has not been determined; however, changes in DNA topology have been implicated in this process. We report here that a Y. pestis strain deficient in production of the ClpXP and Lon proteases does not express a functional T3SS partly because of high cytosolic levels of YmoA. YmoA is rapidly degraded at 37 degrees C in wild-type Y. pestis, but remains stable in a clpXPlon deletion mutant. The stability of YmoA in wild-type Y. pestis increased as the growth temperature of the culture decreased; in contrast, YmoA was stable at all temperatures examined in the clpXPlon deletion mutant. These results indicate that the ClpXP and Lon proteases contribute to the environmental regulation of the Y. pestis T3SS system through regulated proteolysis of YmoA.

Three-dimensional structure of a macromolecular assembly that regulates type III secretion in Yersinia pestis

Yersinia pestis, the causative agent of plague, utilizes a type III secretion system (T3SS) to inject effector proteins directly into the cytosol of mammalian cells where they interfere with signal transduction pathways that regulate actin cytoskeleton dynamics and inflammation, thereby enabling the bacterium to avoid engulfment and destruction by macrophages. Type III secretion normally does not occur in the absence of close contact with eukaryotic cells. Negative regulation is mediated in part by a multiprotein complex that has been proposed to act as a physical impediment to type III secretion by blocking the entrance to the secretion apparatus prior to contact with mammalian cells. This complex is composed of YopN, its heterodimeric secretion chaperone SycN-YscB, and TyeA. Here, we report two crystal structures of YopN in complex with its heterodimeric secretion chaperone SycN-YscB and the co-regulatory protein TyeA, respectively. By merging these two overlapping structures, it was possible to construct a credible theoretical model of the YopN-SycN-YscB-TyeA complex. The modeled assembly features the secretion signaling elements of YopN at one end of an elongated structure and the secretion regulating TyeA binding site at the other. A patch of highly conserved residues on the surface of the C-terminal alpha-helix of TyeA may mediate its interaction with structural components of the secretion apparatus. Conserved arginine residues that reside inside a prominent cavity at the dimer interface of SycN-YscB were mutated in order to investigate whether they play a role in targeting the YopN-chaperone complex to the type III secretion apparatus. One of the mutants exhibited a phenotype that is consistent with this hypothesis.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Process of protein transport by the type III secretion system

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

Acidic pH is required for the functional assembly of the type III secretion system encoded bySalmonellapathogenicity island 2

Salmonella enterica employs two type III secretion systems (T3SS) for interactions with host cells during pathogenesis. The T3SS encoded by Salmonella pathogenicity island 2 (SPI2) is required for the intracellular replication of Salmonella and the survival inside phagocytes. During growth in vitro, acidic pH is a signal that promotes secretion of proteins by this T3SS. We analyzed protein levels and subcellular localization of various T3SS subunits under in vitro conditions at acidic or neutral pH, inducing or ablating secretion, respectively. Growth at acidic pH resulted in higher levels of SsaC, a protein forming the outer membrane secretin, without increasing expression of the operon containing ssaC. Acidic pH also induced oligomerization of SsaC subunits, a prerequisite for a functional secretin pore. It has previously been described that environmental stimuli resembling the intraphagosomal habitat of Salmonella control the expression of SPI2 genes. Here we propose that such stimuli also modulate the assembly of a functional T3SS that is capable of translocation of effector proteins into the host cell.

Secretin of the enteropathogenic Escherichia coli type III secretion system requires components of the type III apparatus for assembly and localization

At least 16 proteins are thought to be involved in forming the enteropathogenic Escherichia coli (EPEC) type III translocation apparatus which delivers virulence factors into host cells, yet their function and location have not been determined. A biochemical analysis was performed on three components: EscN, a predicted cytoplasmic ATPase; EscV, a predicted inner membrane protein; and EscC, a predicted outer membrane secretin. Wild-type EPEC and mutants constructed in these genes were fractionated by lysozyme treatment, ultracentrifugation, and selective detergent extraction. Fractionation revealed that the type III effectors Tir and EspB required a complete type III apparatus for any degree of export by EPEC, suggesting a continuous channel. Epitope-tagged EscC, EscV, and EscN were localized by fractionation, confirming computer modeling predictions for their location. Transcomplementation experiments revealed that localization of EscV and EscN was unaffected by mutations in other examined type III components. Remarkably, localization of EscC was altered in escV or escN mutants, where EscC accumulated in the periplasm. EscC was correctly localized in the escF needle component mutant, indicating that secretin localization is independent of needle formation. These data indicate that, contrary to previous indications, correct insertion and function of EscC secretin in the outer membrane depends not only on the sec-dependent secretion pathway but also on other type III apparatus components.

Type iii protein secretion in yersinia species

The type III mechanism of protein secretion is a pathogenic strategy shared by a number of gram-negative pathogens of plants and animals that has evolved in order to inject virulence proteins into the cytosol of target eukaryotic cells. The pathogens of the Yersinia genus represent a model system where much progress has been made in understanding this secretion pathway. Herein, we review what has been recently learned in yersiniae about the various environmental signals that induce type III secretion, how the synthesis of secretion substrates is regulated, and how such a diverse group of proteins is recognized as a substrate for secretion.

Bordetella type III secretion induces caspase 1-independent necrosis

The Bordetella bronchiseptica type III (TIII) secretion system induces cytotoxicity in infected macrophages and epithelial cells. In this report we characterize the cell death phenotype and compare it to the TIII-dependent cytotoxicity induced by Yersinia enterocolitica and Shigella flexneri. Bordetella bronchiseptica strain RB58 was able to induce cell death in J774A.1 macrophages with the same efficiency as Shigella and Yersinia, but only B. bronchiseptica was able to kill epithelial cells in a TIII-dependent manner. Primary macrophages from caspase 1-/- mice were susceptible to RB58-mediated killing, suggesting that unlike Shigella and Salmonella, caspase 1 does not mediate cell death. RB58-induced cytotoxicity was not inhibited by addition of the pan-caspase inhibitor zVAD, and Western blot analyses of RB58-infected HeLa cells indicated that neither caspase 3 nor 7 was cleaved and PARP remained in its full-length active form. Morphologically the RB58-infected HeLa cells resembled necrotic rather than apoptotic cells, exhibiting cytoplasmic swelling and extensive membrane blebbing in the absence of nuclear changes. The addition of exogenous glycine, which has been shown to prevent necrotic cell death by blocking non-specific ion fluxes across the plasma membrane, blocked RB58-induced cytotoxicity. Addition of cyclosporin A which prevents the opening of the mitochondrial permeability pore, had no effect on RB58-infected cells. We conclude that the B. bronchiseptica TIII secretion system induces a mode of cell death consistent with necrosis that is distinct from that of Yersinia and Shigella.

Detection of type III secretion genes as a general indicator of bacterial virulence

Type III secretion systems of Gram-negative bacteria are specific export machineries for virulence factors which allow their translocation to eukaryotic cells. Since they correlate with bacterial pathogenicity, their presence is used as a general indicator of bacterial virulence. By comparing the genetic relationship of the major type III secretion systems we found the family of genes encoding the inner-membrane channel proteins represented by the Yersinia enterocolitica lcrD (synonym yscV) and its homologous genes from other species an ideal component for establishing a general detection approach for type III secretion systems. Based on the genes of the lcrD family we developed gene probes for Gram-negative human, animal and plant pathogens. The probes comprise lcrD from Y. enterocolitica, sepA from enteropathogenic Escherichia coli, invA from Salmonella typhimurium, mxiA from Shigella sonnei, as well as hrcV from Erwinia amylovora. In addition we included as a control probe the flhA gene from E. coli K-12 to validate our approach. FlhA is part of the flagellar export apparatus which shows a high degree of similarity with type III secretions systems, but is not involved in pathogenicity. The probes were evaluated by screening a series of pathogenic as well as non-pathogenic bacteria. The probes detected type III secretion in pathogens where such systems were either known or were expected to be present, whereas no positive hybridization signals could be found in non-pathogenic Gram-negative bacteria. Gram-positive bacteria were devoid of known type III secretion systems. No interference due to the genetic similarity between the type III secretion system and the flagellar export apparatus was observed. However, potential type III secretion systems could be detected in bacteria where no such systems have been described yet. The presented approach provides therefore a useful tool for the assessment of the virulence potential of bacterial isolates of human, animal and plant origin. Moreover, it is a powerful means for a first safety assessment of poorly characterized strains intended to be used in biotechnological applications.

Translocation of YopE and YopN into eukaryotic cells by Yersinia pestis yopN, tyeA, sycN, yscB and lcrG deletion mutants measured using a phosphorylatable peptide tag and phosphospecific antibodies

Yersinia pestis, the causative agent of plague, exports a set of virulence proteins called Yops upon contact with eukaryotic cells. A subset of these Yops is translocated directly into the cytosol of host cells. In this study, a novel protein tag-based reporter system is used to measure the translocation of Yops into cultured eukaryotic cells. The reporter system uses a small bipartite phosphorylatable peptide tag, termed the Elk tag. Translocation of an Elk-tagged protein into eukaryotic cells results in host cell protein kinase-dependent phosphorylation of the tag at a specific serine residue, which can subsequently be detected with phosphospecific antibodies. The YopN, TyeA, SycN, YscB and LcrG proteins function to prevent Yop secretion before host cell contact. The role of these proteins was investigated in the translocation of Elk-tagged YopE (YopE129-Elk) and YopN (YopN293-Elk) into HeLa cells. Y. pestis yopN, tyeA, sycN and yscB deletion mutants showed reduced levels of YopE129-Elk phosphorylation compared with the parent strain, indicating that these mutants translocate reduced amounts of YopE. We also demonstrate that YopN293-Elk is translocated into HeLa cells and that this process is more efficient in a Yersinia yop polymutant strain lacking the six translocated effector Yops. Y. pestis sycN and yscB mutants translocated reduced amounts of YopN293-Elk; however, tyeA and lcrG mutants translocated higher amounts of YopN293-Elk compared with the parent strain. These data suggest that TyeA and LcrG function to suppress the secretion of YopN before host cell contact, whereas SycN and YscB facilitate YopN secretion and subsequent translocation.

The Helicobacter pylori flbA flagellar biosynthesis and regulatory gene is required for motility and virulence and modulates urease of H. pylori and Proteus mirabilis

Helicobacter pylori and Proteus mirabilis ureases are nickel-requiring metallo-enzymes that hydrolyse urea to NH3 and CO2. In both H. pylori and in an Escherichia coli model of H. pylori urease activity, a high affinity nickel transporter, NixA, is required for optimal urease activity, whereas the urea-dependent UreR positive transcriptional activator governs optimal urease expression in P. mirabilis. The H. pylori flbA gene is a flagellar biosynthesis and regulatory gene that modulates urease activity in the E. coli model of H. pylori urease activity. All flbA mutants of eight strains of H. pylori were non-motile and five had a strain-dependent alteration in urease activity. The flbA gene decreased urease activity 15-fold when expressed in E. coli containing the H. pylori urease locus and the nixA gene; this was reversed by disruption of flbA. The flbA gene decreased nixA transcription. flbA also decreased urease activity three-fold in E. coli containing the P. mirabilis urease locus in a urea- and UreR-dependent fashion. Here the flbA gene repressed the P. mirabilis urease promoter. Thus, FlbA decreased urease activity of both H. pylori and P. mirabilis, but through distinct mechanisms. H. pylori wild-type strain SS1 colonised gerbils at a mean of 5.4 x 10(6) cfu/g of antrum and caused chronic gastritis and lesions in the antrum. In contrast, the flbA mutant did not colonise five of six gerbils and caused no lesions, indicating that motility mediated by flbA was required for colonisation. Because FlbA regulates flagellar biosynthesis and secretion, as well as forming a structural component of the flagellar secretion apparatus, two seemingly unrelated virulence attributes, motility and urease, may be coupled in H. pylori and P. mirabilis and possibly also in other motile, ureolytic bacteria.

Evidence for a type III secretion system in Aeromonas salmonicida subsp. salmonicida

Aeromonas salmonicida subsp. salmonicida, the etiological agent of furunculosis, is an important fish pathogen. We have screened this bacterium with a broad-host-range probe directed against yscV, the gene that encodes the archetype of a highly conserved family of inner membrane proteins found in every known type III secretion system. This has led to the identification of seven open reading frames that encode homologues to proteins functioning within the type III secretion systems of Yersinia species. Six of these proteins are encoded by genes comprising a virA operon. The A. salmonicida subsp. salmonicida yscV homologue, ascV, was inactivated by marker replacement mutagenesis and used to generate an isogenic ascV mutant. Comparison of the extracellular protein profiles from the ascV mutant and the wild-type strain indicates that A. salmonicida subsp. salmonicida secretes proteins via a type III secretion system. The recently identified ADP-ribosylating toxin AexT was identified as one such protein. Finally, we have compared the toxicities of the wild-type A. salmonicida subsp. salmonicida strain and the ascV mutant against RTG-2 rainbow trout gonad cells. While infection with the wild-type strain results in significant morphological changes, including cell rounding, infection with the ascV mutant has no toxic effect, indicating that the type III secretion system we have identified plays an important role in the virulence of this pathogen.

Yersinia enterocolitica type III secretion: mutational analysis of the yopQ secretion signal

Pathogenic Yersinia spp. secrete Yop proteins via the type III pathway. yopQ codons 1 to 15 were identified as a signal necessary and sufficient for the secretion of a fused reporter protein. Frameshift mutations that alter codons 2 to 15 with little alteration of yopQ mRNA sequence do not abolish type III transport, suggesting a model in which yopQ mRNA may provide a signal for secretion (D. M. Anderson and O. Schneewind, Mol. Microbiol. 31:1139-1148, 2001). In a recent study, the yopE signal was truncated to codons 1 to 12. All frameshift mutations introduced within the first 12 codons of yopE abolished secretion. Also, multiple synonymous mutations that changed the mRNA sequence of yopE codons 1 to 12 without altering the amino acid sequence did not affect secretion. These results favor a model whereby an N-terminal signal peptide initiates YopE into the type III pathway (S. A. Lloyd et al., Mol. Microbiol. 39:520-531, 2001). It is reported here that codons 1 to 10 of yopQ act as a minimal secretion signal. Further truncation of yopQ, either at codon 10 or at codon 2, abolished secretion. Replacement of yopQ AUG with either of two other start codons, UUG or GUG, did not affect secretion. However, replacement of AUG with CUG or AAA and initiating translation at the fusion site with npt did not permit Npt secretion, suggesting that the translation of yopQ codons 1 to 15 is a prerequisite for secretion. Frameshift mutations of yopQ codons 1 to 10, 1 to 11, and 1 to 12 abolished secretion signaling, whereas frameshift mutations of yopQ codons 1 to 13, 1 to 14, and 1 to 15 did not. Codon changes at yopQ positions 2 and 10 affected secretion signaling when placed within the first 10 codons but had no effect when positioned in the larger fusion of yopQ codons 1 to 15. An mRNA mutant of yopQ codons 1 to 10, generated by a combination of nine synonymous mutations, was defective in secretion signaling, suggesting that the YopQ secretion signal is not proteinaceous. A model is discussed whereby the initiation of YopQ polypeptide into the type III pathway is controlled by properties of yopQ mRNA.

Roles of LcrG and LcrV during type III targeting of effector Yops by Yersinia enterocolitica

Yersinia enterocolitica target effector Yop proteins into the cytosol of eukaryotic cells by a mechanism requiring the type III machinery. LcrG and LcrV have been suggested to fulfill essential functions during the type III targeting of effector Yops. It is reported here that knockout mutations of lcrG caused mutant yersiniae to prematurely secrete Yops into the extracellular medium without abolishing the type III targeting mechanism (Los phenotype [loss of type III targeting specificity]). Knockout mutations in lcrV reduced type III targeting of mutant yersiniae but did not promote secretion into the extracellular medium (Not [no type III targeting]). However, knockout mutations in both genes caused DeltalcrGV yersiniae to display a Los phenotype similar to that of strains carrying knockout mutations in lcrG alone. LcrG binding to LcrV resulted in the formation of soluble LcrGV complexes in the bacterial cytoplasm. Membrane-associated, bacterial-surface-displayed or -secreted LcrG could not be detected. Most of LcrV was located in the bacterial cytoplasm; however, small amounts were secreted into the extracellular medium. These data support a model whereby LcrG may act as a negative regulator of type III targeting in the bacterial cytoplasm, an activity that is modulated by LcrG binding to LcrV. No support could be gathered for the hypothesis whereby LcrG and LcrV may act as a bacterial surface receptor for host cells, allowing effector Yop translocation across the eukaryotic plasma membrane.

Characterization of the type III secretion locus of Bordetella pertussis

Multiple sequence comparisons of proteins of the LcrD/FlbF family allowed the design of primers that specifically amplify sequences coding for type III secretion components. Amplification of Bordetella pertussis DNA with these primers yielded a fragment that was further used as a probe for screening a genomic library. The nucleotide sequence of a positive clone revealed a 2100-bp gene, called bcrD, which specifies a 75-kDa polypeptide homologous to the Yersinia LcrD protein. Chromosome walking allowed the characterization of a 35-kb DNA segment that contains the entire locus and flanking housekeeping genes. The B. pertussis type III secretion locus consists of more than 30 open reading frames (ORFs), most of which are identical to annotated genes of Bordetella spp and share similarities with known type III secretion genes of related bacteria. In order to assess the function of this locus, we engineered a bcrD null mutant. However, none of the tested phenotypes, such as protein secretion, cellular invasion, cytotoxicity or mouse lung colonization, differentiated the mutant from its parental strain. Studies of bcrD and bscN expressions indicated that, under our experimental conditions, these genes are not expressed in vitro. Restriction analyses on pulsed-field gel electrophoresis allowed the type III locus mapping at coordinate position 1,590 kb on the Tohama I strain chromosome.

Yersinia YopE is targeted for type III secretion by N-terminal, not mRNA, signals

Pathogenic Yersinia species inject virulence proteins, known as Yops, into the cytosol of eukaryotic cells. The injection of Yops is mediated via a type III secretion system. Previous studies have suggested that YopE is targeted for secretion by two signals. One is mediated by its cognate chaperone YerA, whereas the other consists of either the 5' end of yopE mRNA or the N-terminus of YopE. In order to characterize the YopE N-terminal/5' mRNA secretion signal, the first 11 codons of yopE were systematically mutagenized. Frameshift mutations, which completely alter the amino acid sequence of residues 2-11 but leave the mRNA sequence essentially intact, drastically reduce the secretion of YopE in a yerA mutant. In contrast, a mutation that alters the yopE mRNA sequence, while leaving the amino acid sequence of YopE unchanged, does not impair the secretion of YopE. Therefore, the N-terminus of YopE, and not the 5' end of yopE mRNA, serves as a targeting signal for type III secretion. In addition, the chaperone YerA can target YopE for type III secretion in the absence of a functional N-terminal signal. Mutational analysis of the YopE N-terminus revealed that a synthetic amphipathic sequence of eight residues is sufficient to serve as a targeting signal. YopE is also secreted rapidly upon a shift to secretion-permissive conditions. This 'rapid secretion' of YopE does not require de novo protein synthesis and is dependent upon YerA. Furthermore, this burst of YopE secretion can induce a cytotoxic response in infected HeLa cells.

Triggering the ExoS regulon of Pseudomonas aeruginosa: A GFP-reporter analysis of exoenzyme (Exo) S, ExoT and ExoU synthesis

The ExoS regulon of Pseudomonas aeruginosa encodes diverse type III secreted effector proteins which have been shown to exert cytotoxic effects in cell culture experiments. However, little information exists about the environmental conditions and stimuli for upregulation of the ExoS regulon. Translational reporter fusion proteins of exoenzyme (Exo) S, ExoT and ExoU, as well as the type II secreted exotoxin A (ETA) to the green fluorescent protein (GFP), were constructed in order to compare exoprotein production under diverse growth conditions. Reporter protein activity was recorded by FACS-analysis and by conventional and confocal laser scanning microscopy. Low ion concentration induced co-ordinated upregulation of ExoS, ExoT and ExoU with a maximum effect at 37 degrees C. A dose-dependent upregulation was seen with human serum or increasing NaCl concentrations. A type III secretion-negative pcrD mutant of P. aeruginosa showed a weak ExoS response to environmental stimuli, compared with the parental strain, suggesting a negative regulatory mechanism. Co-culture with the mammalian cell lines J774A.1 or HeLa led to rapid upregulation of ExoS, ExoT and ExoU synthesis. These data suggest that the ExoS regulon of P. aeruginosa can be triggered by a variety of environmental signals as well as by cell contact with eukaryotic cells.

On the presence and organization of open reading frames of the nonmotile pathogen Brucella abortus similar to class II, III, and IV flagellar genes and to LcrD virulence superfamily

Brucellae are pathogenic, nonmotile bacteria that are facultative intracellular parasites. Little is known about the genetics of these bacteria. Open reading frames from Brucella abortus with similarity to the flagellin, M-ring, and hook of related bacteria were discovered. The open reading frames encode proteins of three of the four flagellum gene classes, namely II, III, and IV. A homolog of the LcrD virulence superfamily was also found. This superfamily is involved in type III protein secretion. B. abortus has the potential for motility and type III secretion.

YscP, a Yersinia protein required for Yop secretion that is surface exposed, and released in low Ca2+

The Yersinia Ysc apparatus is made of more than 20 proteins, 11 of which have homologues in many type III systems. Here, we characterize YscP from Yersinia enterocolitica. This 515-residue protein has a high proline content, a large tandem repetition and a slow migration in SDS-PAGE. Unlike the products of neighbouring genes, it has a counterpart only in Pseudomonas aeruginosa and it varies even between Yersinia Ysc machineries. An yscPDelta97-465 mutant was unable to secrete any Yop, even under conditions overcoming feedback inhibition of Yop synthesis. Interestingly, a cloned yscPDelta57-324 from Yersinia pestis introduced in the yscPDelta97-465 mutant can sustain a significant Yop secretion and thus partially complemented the mutation. This explains the leaky phenotype observed with the yscP mutant of Y. pestis. In accordance with this secretion deficiency, YscP is required for the delivery of Yop effectors into macrophages. Mechanical shearing, immunolabelling and electron microscopy experiments showed that YscP is exposed at the bacterial surface when bacteria are incubated at 37 degrees C in the presence of Ca2+ and thus do not secrete Yops. At 37 degrees C, when Ca2+ ions are chelated, YscP is released like a Yop protein. We conclude that YscP is a part of the Ysc injectisome which is localized at the bacterial surface and is destabilized by Ca2+ chelation.

A chromosomally encoded type III secretion pathway in Yersinia enterocolitica is important in virulence

Numerous Gram-negative bacteria use a type III, or contact dependent, secretion system to deliver proteins into the cytosol of host cells. All of these systems identified to date have been shown to have a role in pathogenesis. We have identified 13 genes on the Yersinia enterocolitica chromosome that encode a type III secretion apparatus plus two associated putative regulatory genes. In order to determine the function of this chromosomally-encoded secretion apparatus, we created an in frame deletion of a gene that has homology to the hypothesized inner membrane pore, ysaV. The ysaV mutant strain failed to secrete eight proteins, called Ysps, normally secreted by the parental strain when grown at 28 degrees C in Luria-Bertani (LB) broth supplemented with 0.4 M NaCl. Disruption of the ysaV gene had no effect on motility or phospholipase activity, suggesting this chromosomally encoded type III secretion pathway is distinct from the flagella secretion pathway of Y. enterocolitica. Deletion of the ysaV gene in a virulence plasmid positive strain had no effect on in vitro secretion of Yops by the plasmid-encoded type III secretion apparatus. Secretion of the Ysps was unaffected by the presence or absence of the virulence plasmid, suggesting the chromosomally encoded and plasmid-encoded type III secretion pathways act independently. Y. enterocolitica thus has three type III secretion pathways that appear to act independently. The ysaV mutant strain was somewhat attenuated in virulence compared with the wild type in the mouse oral model of infection (an approximately 0.9 log difference in LD50). The ysaV mutant strain was nearly as virulent as the wild type when inoculated intraperitoneally in the mouse model. A ysaV probe hybridized to sequences in other Yersinia spp. and homologues were found in the incomplete Y. pestis genome sequence, indicating a possible role for this system throughout the genus.

The Bpel locus encodes type III secretion machinery in Bordetella pertussis

Type III secretory genes(Bscl, J, K, L, N and O) have recently been identified in Bordetella bronchiseptica and shown to be under the control of the BvgAS locus. We examined a 35 616 byte DNA sequence amplified from Bordetella pertussis Tohama I for homology with known type III secretory genes in Yersinia spp. and Pseudomonas sppand a total of 20 homologous open reading frames were detected. Putative type III secretion proteins in B. pertussis were designated according to their homology with type III secretion proteins in B. bronchiseptica, Yersinia and Pseudomonas. These ORFs were arranged in two putative operons, which together we have designated as the BpeI locus. The first spans nucleotides 23385-7888 and encodes the putative proteins LcrH1, BopD, BopB, LcfH2, BscI, BscJ, BscK, BscL, BscN, BscO, BscQ, BscR, BscS, BscT, BscU, and BscC, in this order. The second spans nucleotides 23580-29863 and encodes the putative proteins LcrE, LcrD, BscD and BscF, in this order. The homology of these proteins to type III secretory proteins was B. bronchiseptica (73-99%), Yersinia spp. (17-65%), Pseudomonas spp. (18-64%). The B. pertussis proteins were similar to their homologues in B. bronchiseptica, Yersinia and Pseudomonas in terms of length, molecular weight and isoelectric point. Coiled-coil domains were detected in putative translocation proteins, BopB and BopD. BopB and BopD were similar to each other, to the RTX toxin family and to cyaA, cyaB, cyaD and cyaE. The percentage G+C content of the sequence analysed was 66.16%, which is similar to the published percentage G+C (67-70%) for the B. pertussis chromosome.

Virulence role of V antigen of Yersinia pestis at the bacterial surface

Yersinia pestis, the etiologic agent of plague, secretes a set of environmentally regulated, plasmid pCD1-encoded virulence proteins termed Yops and V antigen (LcrV) by a type III secretion mechanism (Ysc). LcrV is a multifunctional protein that has been shown to act at the level of secretion control by binding the Ysc inner-gate protein LcrG and to modulate the host immune response by altering cytokine production. LcrV also is essential for the unidirectional targeting of Yops to the cytosol of infected eukaryotic cells. In this study, we constructed an in-frame deletion within lcrG (DeltalcrG3) to further analyze the requirement of LcrV in Yop targeting. We confirmed the essentiality of LcrV and found that LcrG may have a facilitative role, perhaps by promoting efficient secretion of LcrV. We also constructed mutants of lcrV expressing LcrV truncated at the N or C terminus. Both the N and C termini of LcrV were required for the secretion of LcrV into the medium and targeting of Yops. LcrV was detected in punctate zones on the surface of fixed Y. pestis by laser-scanning confocal microscopy, and this localization required a functional Ysc. However, the truncated LcrV proteins were not found on the bacterial surface. Finally, we tested the ability of LcrV-specific Fab antibody fragments or full-length antibody to interfere with Yop targeting and found no interference, even though this antibody protects mice against plague. These results indicate that LcrV may function in Yop targeting at the extracellular surface of yersiniae and that the protective efficacy of LcrV-specific antibodies can be manifested without blocking Yop targeting.

LcrV of Yersinia pestis enters infected eukaryotic cells by a virulence plasmid-independent mechanism

Yersinia pestis is the causative agent of bubonic plague and possesses a set of plasmid-encoded, secretable virulence proteins termed LcrV and Yops which are essential for survival in mammalian hosts. Yops and LcrV are secreted by a type III mechanism (Ysc), and Yops are unidirectionally targeted into the cytosol of associated eukaryotic cells in a tissue culture infection model. LcrV is required for Yops targeting, and recent findings have revealed that it can localize to the bacterial surface; however, its fate in this infection model has not been investigated in detail. In this study, we compared the localization of LcrV to that of the targeted proteins YopE and YopM by immunoblot analysis of fractions of Yersinia-infected HeLa cultures or by laser-scanning confocal microscopy of infected monolayers. Both LcrV and YopE were secreted by contact-activated, extracellularly localized yersiniae and were targeted to the HeLa cell cytosol. Although a significant amount of LcrV partitioned to the culture medium (unlike YopE), this extracellular pool of LcrV was not the source of the LcrV that entered HeLa cells. Unlike targeting of YopE and YopM, targeting of LcrV occurred in the absence of a functional Ysc apparatus and other virulence plasmid (pCD1)-expressed proteins. However, the Ysc is necessary for LcrV to be released into the medium, and our recent work has shown that localization of LcrV on the bacterial surface requires the Ysc. These results indicate that two mechanisms exist for the secretion of LcrV by Y. pestis, both of which are activated by contact with eukaryotic cells. LcrV secreted by the Ysc reaches the bacterial surface and the surrounding medium, whereas the second is a novel, Ysc-independent pathway which results in localization of LcrV in the cytosol of infected cells but not the surrounding medium.

The Avr (effector) proteins HrmA (HopPsyA) and AvrPto are secreted in culture from Pseudomonas syringae pathovars via the Hrp (type III) protein secretion system in a temperature- and pH-sensitive manner

We present here data showing that the Avr proteins HrmA and AvrPto are secreted in culture via the native Hrp pathways from Pseudomonas syringae pathovars that produce these proteins. Moreover, their secretion is strongly affected by the temperature and pH of the culture medium. Both HrmA and AvrPto were secreted at their highest amounts when the temperature was between 18 and 22 degrees C and when the culture medium was pH 6.0. In contrast, temperature did not affect the secretion of HrpZ. pH did affect HrpZ secretion, but not as strongly as it affected the secretion of HrmA. This finding suggests that there are at least two classes of proteins that travel the P. syringae pathway: putative secretion system accessory proteins, such as HrpZ, which are readily secreted in culture; and effector proteins, such as HrmA and AvrPto, which apparently are delivered inside plant cells and are detected in lower amounts in culture supernatants under the appropriate conditions. Because HrmA was shown to be a Hrp-secreted protein, we have changed the name of hrmA to hopPsyA to reflect that it encodes a Hrp outer protein from P. syringae pv. syringae. The functional P. syringae Hrp cluster encoded by cosmid pHIR11 conferred upon P. fluorescens but not Escherichia coli the ability to secrete HopPsyA in culture. The use of these optimized conditions should facilitate the identification of additional proteins traveling the Hrp pathway and the signals that regulate this protein traffic.

Yersinia enterocolitica type III secretion. On the role of SycE in targeting YopE into HeLa cells

Yersinia enterocolitica inject toxic proteins (effector Yops) into the cytosol of eukaryotic cells by a mechanism requiring the type III machinery. Previous work mapped a signal sufficient for the targeting of fused reporter proteins to amino acids 1-100 of YopE. Targeting requires the binding of SycE to YopE residues 15-100 in the bacterial cytoplasm. We asked whether SycE functions only to stabilize YopE in the bacterial cytoplasm, or whether the secretion chaperone itself contributes to substrate recognition by the type III machinery. Fusions of glutathione S-transferase to either the N or C terminus of SycE resulted in hybrid proteins that bound YopE but prevented targeting of the export substrate into HeLa cells. As compared with wild-type SycE, glutathione S-transferase-SycE bound and stabilized YopE in the bacterial cytoplasm but failed to release the polypeptide for export by the type III machinery. Thus, it appears that SycE functions to deliver YopE to the type III secretion machinery. A model is presented that accounts for substrate recognition of effector Yops, a group of proteins that do not share amino acid sequence or physical similarities.