The surface-located YopN protein is involved in calcium signal transduction in Yersinia pseudotuberculosis

Type III export: new uses for an old pathway

Gram-negative bacteria use type III secretion (TTS) systems to translocate proteins into the extracellular environment or directly into eukaryotic cells. These complex secretory systems are assembled from over 20 different structural proteins, including 10 that have counterparts in the flagellar export pathway. Secretion substrates are directed to the TTS machinery via mRNA and/or amino acid secretion signals. TTS chaperones bind to select secretion substrates and assist in the export process. Recent progress in the understanding of TTS is reviewed.

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.

Evidence for the secretion of Chlamydia trachomatis CopN by a type III secretion mechanism

The medically significant, obligate intracellular pathogen Chlamydia trachomatis replicates within vacuoles termed inclusions. A developmental cycle is initiated after entry into a host cell and is manifested by the transformation of infectious elementary bodies (EBs) to larger, non-infectious reticulate bodies (RBs). Analysis of the C. trachomatis genome has revealed that chlamydiae possess genes that may encode a type III secretion apparatus. In other Gram-negative pathogens, the type III secretion mechanism is used to target virulence factors directly to the host cell cytoplasm and is essential for full virulence. To evaluate the possibility of a functional type III secretion mechanism in C. trachomatis, we initially focused on a locus containing genes encoding products with similarity to chaperones (Scc1), secretion pore components (Cds1 and Cds2) and secreted proteins (CopN) from other type III systems. Gene expression was tested by reverse transcriptase-polymerase chain reaction (RT-PCR) of total RNA extracted from infected HeLa cell monolayers at 2, 6, 12 and 20 h after infection and normalized for the number of C. trachomatis genomes present. Message was detected for Scc1 at all times, whereas message for all other tested genes was detected in significant amounts at 12 h and 20 h. Immunoblot analysis with Scc1- and CopN-specific antibodies revealed that CopN and Scc1 were present in EBs, RBs and whole-culture extracts harvested 20 h after infection. CopN is homologous to the secreted protein YopN of Yersinia sp., and analysis of monolayers 20 h after infection via indirect immunofluorescence showed specific labelling of inclusion membranes when probed with CopN-specific antibodies but not with Scc1-specific antibodies. His-tagged CopN and a chlamydial cytoplasmic control protein (NrdB) were expressed in Yersinia enterocolitica containing or lacking the virulence plasmid pYV. CopN, but not NrdB, was secreted by Y. enterocolitica in a Ca2+- and pYV-dependent fashion. These data indicate that components of the putative type III apparatus of C. trachomatis are expressed and that at least one of these products is secreted by chlamydiae to the inclusion membrane. The observation that CopN is also secreted by the Yersinia type III apparatus provides support for the notion that chlamydiae secrete proteins via a type III mechanism.

Yersinia enterocolitica can deliver Yop proteins into a wide range of cell types: development of a delivery system for heterologous proteins

Y. enterocolitica translocates virulence proteins, called Yop effectors, into the cytosol of eukaryotic cells. Here we investigated whether Y. enterocolitica could translocate Yops into a range of eukaryotic cells including neurons and insect cells. Y. enterocolitica translocated the hybrid reporter protein YopE-Cya into each of the eukaryotic cell types tested. In addition, Y. enterocolitica was cytotoxic for each of the adherent cell types. Thus we detected no limit to the range of eukaryotic cells into which Y. enterocolitica can translocate Yops. The Yop effectors YopE, YopH and YopT were each cytotoxic for the adherent cell types tested, showing that not only is Y. enterocolitica not selective in its translocation of particular Yop effectors into each cell type, but also that the action of these Yop effectors is not cell type specific. Invasin and/or YadA, two powerful adhesins were required for translocation of Yop into non-phagocytic cells but not for translocation into macrophages. To use the Yersinia translocation system for broad applications, a Y. enterocolitica translocation strain and vector for the delivery of heterologous proteins into eukaryotic cells was constructed. This strain + vector combination lacks the translocated Yop effectors and allows delivery into eukaryotic cells of heterologous proteins fused to the minimal N-terminal secretion/translocation signal of YopE. Using this strategy translocation of a YopE-Diphtheria toxin subunit A hybrid protein into several cell types has been shown.

How to survive in the host: the Yersinia lesson

The Yop virulon allows Yersinia spp. to resist the immune response of the host by injecting harmful proteins into host cells. It is composed of four elements: (i) type III secretion machinery called Ysc; (ii) a set of proteins required to translocate the effector proteins inside the eukaryotic cells; (iii) a control system, and (iv) six Yop effector proteins.

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.

LcrQ/YscM1, regulators of the Yersinia yop virulon, are injected into host cells by a chaperone-dependent mechanism

Pathogenic Yersinia species employ type III machines to secrete YopBDR into the extracellular milieu. After attaching to host cells, yersiniae transform the type III machinery into an injection device and target YopEHMNOPT into eukaryotic cells. Yersinia pseudotuberculosis LcrQ is a transcriptional regulator that prevents the expression of yop genes. We report that LcrQ is injected into eukaryotic cells. YscM1, the transciptional regulator of Yersinia enterocolitica, is also injected into eukaryotic cells, whereas the related YscM2 protein remains associated with bacterial cells. Type III targeting of YscM1 requires binding to the SycH chaperone. Chaperone binding as well as depletion of YscM1 and YscM2 from the cytoplasm of Y. enterocolitica causes an increase in yop expression, whereas a block in regulator export reduces expression. We propose a model whereby the chaperone-mediated injection of LcrQ/YscM1 functions as a regulatory switch for bacteria that are attached to host cells, triggering the expression of Yops that travel the type III targeting pathway.

Yersinia enterocolitica TyeA, an intracellular regulator of the type III machinery, is required for specific targeting of YopE, YopH, YopM, and YopN into the cytosol of eukaryotic cells

Pathogenic Yersinia species employ type III machines to target effector Yops into the cytosol of eukaryotic cells. Yersinia tyeA mutants are thought to be defective in the targeting of YopE and YopH without affecting the injection of YopM, YopN, YopO, YopP, and YopT into the cytosol of eukaryotic cells. One model suggests that TyeA may form a tether between YopN (LcrE) and YopD on the bacterial surface, a structure that may translocate YopE and YopH across the plasma membrane of eukaryotic cells (M. Iriarte, M. P. Sory, A. Boland, A. P. Boyd, S. D. Mills, I. Lambermont, and G. R. Cornelis, EMBO J. 17:1907-1918, 1998). We have examined the injection of Yop proteins by tyeA mutant yersiniae with the digitonin fractionation technique. We find that tyeA mutant yersiniae not only secreted YopE, YopH, YopM, and YopN into the extracellular medium but also targeted these polypeptides into the cytosol of HeLa cells. Protease protection, cell fractionation, and affinity purification experiments suggest that TyeA is located intracellularly and binds to YopN or YopD. We propose a model whereby TyeA functions as a negative regulator of the type III targeting pathway in the cytoplasm of yersiniae, presumably by preventing the export of YopN.

Type III machines of Gram-negative bacteria: delivering the goods

Many Gram-negative pathogens use a type III secretion machine to translocate protein toxins across the bacterial cell envelope. Pathogenic Yersinia spp. export at least 14 Yop proteins via a type III machine, which recognizes secretion substrates by signals encoded in yop mRNA or chaperones bound to unfolded Yop proteins. During infection, substrate recognition appears to be regulated in a manner that allows the Yersinia type III pathway to direct Yops to the bacterial envelope, the extracellular medium or into the cytosol of host cells.

The Yersinia pestis YscY protein directly binds YscX, a secreted component of the type III secretion machinery

Human pathogenic yersiniae organisms export and translocate the Yop virulence proteins and V antigen upon contact with a eukaryotic cell. Yersinia pestis mutants defective for production of YscX or YscY were unable to export the Yops and V antigen. YscX and YscY were both present in the Y. pestis cell pellet fraction; however, YscX was also found in the culture supernatant. YscY showed structural and amino acid sequence similarities to the Syc family of proteins. YscY specifically recognized and bound to a region of YscX that included a predicted coiled-coil region. These data suggest that YscY may function as a chaperone for YscX in Y. pestis.

Modulation of host immune responses, induction of apoptosis and inhibition of NF-kappaB activation by the Bordetella type III secretion system

Bordetella bronchiseptica establishes respiratory tract infections in laboratory animals with high efficiency. Colonization persists for the life of the animal and infection is usually asymptomatic in immunocompetent hosts. We hypothesize that this reflects a balance between immunostimulatory events associated with infection and immunomodulatory events mediated by the bacteria. We have identified 15 loci that are part of a type III secretion apparatus in B. bronchiseptica and three secreted proteins. The functions of the type III secretion system were investigated by comparing the phenotypes of wild-type bacteria with two strains that are defective in type III secretion using in vivo and in vitro infection models. Type III secretion mutants were defective in long-term colonization of the trachea in immunocompetent mice. The mutants also elicited higher titres of anti-Bordetella antibodies upon infection compared with wild-type bacteria. Type III secretion mutants also showed increased lethal virulence in immunodeficient SCID-beige mice. These observations suggest that type III-secreted products of B. bronchiseptica interact with components of both innate and adaptive immune systems of the host. B. bronchiseptica induced apoptosis in macrophages in vitro and inflammatory cells in vivo and type III secretion was required for this process. Infection of an epithelial cell line with high numbers of wild type, but not type III deficient B. bronchiseptica resulted in rapid aggregation of NF-kappaB into large complexes in the cytoplasm. NF-kappaB aggregation was dependent on type III secretion and aggregated NF-kappaB did not respond to TNFalpha activation, suggesting B. bronchiseptica may modulate host immunity by inactivating NF-kappaB. Based on these in vivo and in vitro results, we hypothesize that the Bordetella type III secretion system functions to modulate host immune responses during infection.

The gene coding for the Hrp pilus structural protein is required for type III secretion of Hrp and Avr proteins in Pseudomonas syringae pv. tomato

Bacterial surface appendages called pili often are associated with DNA and/or protein transfer between cells. The exact function of pili in the transfer process is not understood and is a matter of considerable debate. The Hrp pilus is assembled by the Hrp type III protein secretion system of Pseudomonas syringae pv. tomato (Pst) strain DC3000. In this study, we show that the hrpA gene, which encodes the major subunit of the Hrp pilus, is required for secretion of putative virulence proteins, such as HrpW and AvrPto. In addition, the hrpA gene is required for full expression of genes that encode regulatory, secretion, and effector proteins of the type III secretion system. hrpA-mediated gene regulation apparently is through effect on the mRNA level of two previously characterized regulatory genes, hrpR and hrpS. Ectopic expression of the hrpRS gene operon restored gene expression, but not protein secretion, in the hrpA mutant. Three single amino acid mutations at the HrpA carboxyl terminus were identified that affect the secretion or regulatory function of the HrpA protein. These results define an essential role of the Hrp pilus structural gene in protein secretion and coordinate regulation of the type III secretion system in Pst DC3000.

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.

DsbA is required for stable expression of outer membrane protein YscC and for efficient Yop secretion in Yersinia pestis

The role of the periplasmic disulfide oxidoreductase DsbA in Yop secretion was investigated in Yersinia pestis. A Y. pestis dsbA mutant secreted reduced amounts of the V antigen and Yops and expressed reduced amounts of the full-sized YscC protein. Site-directed mutagenesis of the four cysteine residues present in the YscC protein resulted in defects similar to those found in the dsbA mutant. These results suggest that YscC contains at least one disulfide bond that is essential for the function of this protein in Yop secretion.

Molecular basis of the interaction of Salmonella with the intestinal mucosa

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed.

The V-antigen of Yersinia is surface exposed before target cell contact and involved in virulence protein translocation

Type III-mediated translocation of Yop effectors is an essential virulence mechanism of pathogenic Yersinia. LcrV is the only protein secreted by the type III secretion system that induces protective immunity. LcrV also plays a significant role in the regulation of Yop expression and secretion. The role of LcrV in the virulence process has, however, remained elusive on account of its pleiotropic effects. Here, we show that anti-LcrV antibodies can block the delivery of Yop effectors into the target cell cytosol. This argues strongly for a critical role of LcrV in the Yop translocation process. Additional evidence supporting this role was obtained by genetic analysis. LcrV was found to be present on the bacterial surface before the establishment of bacteria target cell contact. These findings suggest that LcrV serves an important role in the initiation of the translocation process and provides one possible explanation for the mechanism of LcrV-induced protective immunity.

YscP of Yersinia pestis is a secreted component of the Yop secretion system

The Yersinia pestis low-Ca2+ response stimulon is responsible for the environmentally regulated expression and secretion of antihost proteins (V antigen and Yops). We have previously shown that yscO encodes a secreted core component of the Yop secretion (Ysc) mechanism. In this study, we constructed and characterized in-frame deletions in the adjacent gene, yscP, in the yscN-yscU operon. The DeltaP1 mutation, which removed amino acids 246 to 333 of YscP, had no effect on Yop expression or secretion, and the mutant protein, YscP1, was secreted, as was YscP in the parent. In contrast, the DeltaP2 strain expressed and secreted less of each Yop than did the parent under the inductive conditions of 37 degrees C and the absence of Ca2+, with an exception being YopE, which was only minimally affected by the mutation. The YscP2 protein, missing amino acids 57 to 324 of YscP, was expressed but not secreted by the DeltaP2 mutant. The effect of the DeltaP2 mutation was at the level of Yop secretion because YopM and V antigen still showed limited secretion when overproduced in trans. Excess YscP also affected secretion: overexpression of YscP in the parent, in either yscP mutant, or in an lcrG mutant effectively shut off secretion. However, co-overexpression of YscO and YscP had no effect on secretion, and YscP overexpression in an lcrE mutant had little effect on Yop secretion, suggesting that YscP acts, in conjunction with YscO, at the level of secretion control of LcrE at the bacterial surface. These findings place YscP among the growing family of mobile Ysc components that both affect secretion and themselves are secreted by the Ysc.

Immune response to Yersinia outer proteins and other Yersinia pestis antigens after experimental plague infection in mice

There is limited information concerning the nature and extent of the immune response to the virulence determinants of Yersinia pestis during the course of plague infection. In this study, we evaluated the humoral immune response of mice that survived lethal Y. pestis aerosol challenge after antibiotic treatment. Such a model may replicate the clinical situation in humans and indicate which virulence determinants are expressed in vivo. Immunoglobulin G enzyme-linked immunosorbent assay and immunoblotting were performed by using purified, recombinant antigens including F1, V antigen, YpkA, YopH, YopM, YopB, YopD, YopN, YopE, YopK, plasminogen activator protease (Pla), and pH 6 antigen as well as purified lipopolysaccharide. The major antigens recognized by murine convalescent sera were F1, V antigen, YopH, YopM, YopD, and Pla. Early treatment with antibiotics tended to reduce the immune response and differences between antibiotic treatment regimens were noted. These results may indicate that only some virulence factors are expressed and/or immunogenic during infection. This information may prove useful for selecting potential vaccine candidates and for developing improved serologic diagnostic assays.

Type III secretion machines and the pathogenesis of enteric infections caused by Yersinia and Salmonella spp

Salmonella and Yersinia spp. infect the intestinal tract of humans. Although these organisms cause fundamentally different diseases, each pathogen relies on type III secretion machines to either inject virulence factors into the cytosol of eukaryotic cells or release toxins into the extracellular milieu. Type III secretion machines are composed of many different subunits and export several polypeptides with unique substrate requirements. During Salmonella pathogenesis, the type III machine encoded by the Salmonella pathogenicity island (SPI)-1 genetic element functions to cause invasion of the intestinal epithelium, whereas another type III machine (SPI-2) is required for survival in macrophages. Yersinia enterocolitica and Yersinia pseudotuberculosis employ type III machines to resist macrophage phagocytosis and to manipulate the host's immune response, thereby colonizing intestinal lymphoid tissues. We describe what is known about the pathogenic functions of virulence factors secreted by type III machines. Furthermore, type III secretion machines may be exploited for the injection of recombinant proteins, a strategy that has already been successfully employed to elicit a cell-mediated immune response.

Protective efficacy of recombinant Yersinia outer proteins against bubonic plague caused by encapsulated and nonencapsulated Yersinia pestis

To evaluate the role of Yersinia outer proteins (Yops) in conferring protective immunity against plague, six yop loci from Yersinia pestis were individually amplified by PCR, cloned, and expressed in Escherichia coli. The recombinant proteins were purified and injected into mice. Most Yop-vaccinated animals succumbed to infection with either wild-type encapsulated Y. pestis or a virulent, nonencapsulated isogenic variant. Vaccination with YpkA significantly prolonged mean survival time but did not increase overall survival of mice infected with the nonencapsulated strain. The only significant protection against death was observed in YopD-vaccinated mice challenged with the nonencapsulated strain.

Yersinia enterocolitica type III secretion: an mRNA signal that couples translation and secretion of YopQ

Pathogenic Yersinia species export Yop proteins via a type III machinery to escape their phagocytic killing during animal infections. Here, we reveal the type III export mechanism of YopQ. In the presence of calcium, when type III secretion was blocked, yopQ mRNA was not translated. The signal of YopQ sufficient for the secretion of translationally fused reporter proteins was contained within the first 10 codons of its open reading frame. Some frameshift mutations that completely altered the peptide sequence specified by this signal did not impair secretion of the reporter protein. Exchanging the upstream untranslated mRNA leader of yopQ for that of E. coli lacZ also did not affect secretion. However, removal of the first 15 codons abolished YopQ export. Pulse-labelled YopE, but not YopQ, could be secreted after the polypeptide had been synthesized within the cytoplasm of Yersinia (post-translational secretion). Thus, YopQ appears to be exported by a mechanism that couples yopQ mRNA translation with the type III secretion of the encoded polypeptide.

Type III machines of Gram-negative pathogens: injecting virulence factors into host cells and more

Many Gram-negative bacteria that cause disease in either mammals or plants share a strategy of delivering toxic proteins into the cytoplasm of host cells known as type III secretion. Recent advances have provided a glimpse at the molecular nature of these lethal injection machines. Several groups have reported fibrous structures on bacterial surfaces that appear to be extensions of type III machines and necessary for toxin injection into host cells. Other research revealed complex mechanisms of secretion substrate recognition that presumably function to direct toxins to different locations during infection.

Identification of SycN, YscX, and YscY, three new elements of the Yersinia yop virulon

The Yop virulon allows Yersinia spp. to resist the immune response of the host by injecting harmful proteins into host cells. We identified three new elements of the Yop virulon: SycN, required for normal secretion of YopN, and YscX and YscY, two new components of the secretion machinery.

Targeting of the Yersinia pestis YopM protein into HeLa cells and intracellular trafficking to the nucleus

The YopM virulence protein of Yersinia pestis has been described as binding human alpha-thrombin and inhibiting thrombin-induced platelet aggregation in vitro. However, recent studies have shown that a YopM-CyaA fusion protein could be targeted vectorially into eukaryotic cells through the Yersinia type III secretion system. In this study, our objective was to characterize YopM's fate in more detail. We followed YopM in the culture medium and inside infected HeLa cells. We confirmed that the native YopM is targeted into HeLa cells, where it is insensitive to exogenous trypsin. The bacteria must be surface located to target YopM, and YopB and YopD are necessary, whereas the LcrE protein (called also YopN) makes this process more efficient. Immunofluorescence localization revealed that YopM, in contrast to YopE, is not only targeted to the cytoplasm but also trafficks to the cell's nucleus by means of a vesicle-associated pathway that is strongly inhibited by brefeldin A, perturbed by monensin or bafilomycin A1 and dependent upon microtubules (decreased by colchicine and nocodazole). These findings revealed a novel interaction of Yersinia pestis with its eukaryotic host.

The virulence plasmid of Yersinia, an antihost genome

The 70-kb virulence plasmid enables Yersinia spp. (Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It encodes the Yop virulon, an integrated system allowing extracellular bacteria to disarm the cells involved in the immune response, to disrupt their communications, or even to induce their apoptosis by the injection of bacterial effector proteins. This system consists of the Yop proteins and their dedicated type III secretion apparatus, called Ysc. The Ysc apparatus is composed of some 25 proteins including a secretin. Most of the Yops fall into two groups. Some of them are the intracellular effectors (YopE, YopH, YpkA/YopO, YopP/YopJ, YopM, and YopT), while the others (YopB, YopD, and LcrV) form the translocation apparatus that is deployed at the bacterial surface to deliver the effectors into the eukaryotic cells, across their plasma membrane. Yop secretion is triggered by contact with eukaryotic cells and controlled by proteins of the virulon including YopN, TyeA, and LcrG, which are thought to form a plug complex closing the bacterial secretion channel. The proper operation of the system also requires small individual chaperones, called the Syc proteins, in the bacterial cytosol. Transcription of the genes is controlled both by temperature and by the activity of the secretion apparatus. The virulence plasmid of Y. enterocolitica and Y. pseudotuberculosis also encodes the adhesin YadA. The virulence plasmid contains some evolutionary remnants including, in Y. enterocolitica, an operon encoding resistance to arsenic compounds.

A complex composed of SycN and YscB functions as a specific chaperone for YopN in Yersinia pestis

Human pathogenic Yersinia resist host defences, in part through the expression and delivery of a set of plasmid-encoded virulence proteins termed Yops. A number of these Yops are exported from the bacteria directly into the cytoplasm of their eukaryotic host's cells upon contact with these cells. The secreted YopN protein (also known as LcrE) is required to block Yop secretion in the presence of calcium in vitro or before contact with a eukaryotic cell in vivo. In this study, we characterize the role of the tyeA, sycN and yscB gene products in the regulation of Yop secretion in Yersinia pestis. Mutants specifically defective in the expression of TyeA, SycN or YscB were no longer able to block Yop secretion in the presence of calcium. In addition, the secretion of YopN was specifically reduced in both the sycN and the yscB deletion mutants. Protein cross-linking and immunoprecipitation studies in conjunction with yeast two-hybrid analyses showed that SycN and YscB interact with one another to form a SycN/YscB complex. Yeast three-hybrid analyses demonstrated that the SycN/YscB complex, but not SycN or YscB alone, specifically associates with YopN. SycN and YscB share amino acid sequence similarity and structural similarities with the specific Yop chaperones SycE and SycH. Together, these results indicate that a complex composed of SycN and YscB functions as a specific chaperone for YopN in Y. pestis.

DNA sequencing and analysis of the low-Ca2+-response plasmid pCD1 of Yersinia pestis KIM5

The low-Ca2+-response (LCR) plasmid pCD1 of the plague agent Yersinia pestis KIM5 was sequenced and analyzed for its genetic structure. pCD1 (70,509 bp) has an IncFIIA-like replicon and a SopABC-like partition region. We have assigned 60 apparently intact open reading frames (ORFs) that are not contained within transposable elements. Of these, 47 are proven or possible members of the LCR, a major virulence property of human-pathogenic Yersinia spp., that had been identified previously in one or more of Y. pestis or the enteropathogenic yersiniae Yersinia enterocolitica and Yersinia pseudotuberculosis. Of these 47 LCR-related ORFs, 35 constitute a continuous LCR cluster. The other LCR-related ORFs are interspersed among three intact insertion sequence (IS) elements (IS100 and two new IS elements, IS1616 and IS1617) and numerous defective or partial transposable elements. Regional variations in percent GC content and among ORFs encoding effector proteins of the LCR are additional evidence of a complex history for this plasmid. Our analysis suggested the possible addition of a new Syc- and Yop-encoding operon to the LCR-related pCD1 genes and gave no support for the existence of YopL. YadA likely is not expressed, as was the case for Y. pestis EV76, and the gene for the lipoprotein YlpA found in Y. enterocolitica likely is a pseudogene in Y. pestis. The yopM gene is longer than previously thought (by a sequence encoding two leucine-rich repeats), the ORF upstream of ypkA-yopJ is discussed as a potential Syc gene, and a previously undescribed ORF downstream of yopE was identified as being potentially significant. Eight other ORFs not associated with IS elements were identified and deserve future investigation into their functions.

YscB of Yersinia pestis functions as a specific chaperone for YopN

Following contact with a eucaryotic cell, Yersinia species pathogenic for humans (Y. pestis, Y. pseudotuberculosis, and Y. enterocolitica) export and translocate a distinct set of virulence proteins (YopE, YopH, YopJ, YopM, and YpkA) from the bacterium into the eucaryotic cell. During in vitro growth at 37 degrees C in the presence of calcium, Yop secretion is blocked; however, in the absence of calcium, Yop secretion is triggered. Yop secretion occurs via a plasmid-encoded type III, or "contact-dependent," secretion system. The secreted YopN (also known as LcrE), TyeA, and LcrG proteins are necessary to prevent Yop secretion in the presence of calcium and prior to contact with a eucaryotic cell. In this paper we characterize the role of the yscB gene product in the regulation of Yop secretion in Y. pestis. A yscB deletion mutant secreted YopM and V antigen both in the presence and in the absence of calcium; however, the export of YopN was specifically reduced in this strain. Complementation with a functional copy of yscB in trans completely restored the wild-type secretion phenotype for YopM, YopN, and V antigen. The YscB amino acid sequence showed significant similarities to those of SycE and SycH, the specific Yop chaperones for YopE and YopH, respectively. Protein cross-linking and immunoprecipitation studies demonstrated a specific interaction between YscB and YopN. In-frame deletions in yopN eliminating the coding region for amino acids 51 to 85 or 6 to 100 prevented the interaction of YopN with YscB. Taken together, these results indicate that YscB functions as a specific chaperone for YopN in Y. pestis.

YopD of Yersinia pseudotuberculosis is translocated into the cytosol of HeLa epithelial cells: evidence of a structural domain necessary for translocation

Yersinia pseudotuberculosis YopB and YopD proteins are essential for translocation of Yop effector proteins into the target cell cytosol. YopB is suggested to mediate pore formation in the target cell plasma membrane, allowing translocation of Yop effector proteins, although the function of YopD is unclear. To investigate the role in translocation for YopD, a mutant strain in Y. pseudotuberculosis was constructed containing an in frame deletion of essentially the entire yopD gene. As shown recently for the Y. pestis YopD protein, we found that the in vitro low calcium response controlling virulence gene expression was negatively regulated by YopD. This yopD null mutant (YPIII/pIB621) was also non-cytotoxic towards HeLa cell monolayers, supporting the role for YopD in the translocation process. Although other constituents of the Yersinia translocase apparatus (YopB, YopK and YopN) are not translocated into the host cell cytosol, fractionation of infected HeLa cells allowed us to identify the cytosolic localization of YopD by the wild-type strain (YPIII/pIB102), but not by strains defective in either YopD or YopB. YopD was also identified by immunofluorescence in the cytoplasm of HeLa cell monolayers infected with a multiple yop mutant strain (YPIII/pIB29MEKA). These results demonstrate a dual function for YopD in negative regulation of Yop production and Yop effector translocation, including the YopD protein itself. To investigate whether an amphipathic domain near the C-terminus of YopD is involved in the translocation process, a mutant strain (YPIII/pIB155deltaD278-292) was constructed that is devoid of this region. Phenotypically, this small in frame deltayopD278-292 deletion mutant was indistinguishable from the yopD null mutant. The truncated YopD protein and Yop effectors were not translocated into the cytosol of HeLa cell monolayers infected with this mutant. The comparable regulatory and translocation phenotypes displayed by the small in frame deltayopD278-292 deletion and deltayopD null mutants suggest that regulation of Yop synthesis and Yop translocation are intimately coupled. We present an intriguing scenario to the Yersinia infection process that highlights the need for polarized translocation of YopD to specifically establish translocation of Yop effectors. These observations are contrary to previous suggestions that members of the translocase apparatus were not translocated into the host cell cytosol.

YscO of Yersinia pestis is a mobile core component of the Yop secretion system

The Yersinia pestis low-Ca2+ response stimulon is responsible for the temperature- and Ca2+-regulated expression and secretion of plasmid pCD1-encoded antihost proteins (V antigen and Yops). We have previously shown that lcrD, yscC, yscD, yscG, and yscR encode proteins that are essential for high-level expression and secretion of V antigen and Yops at 37 degreesC in the absence of Ca2+. In this study, we characterized yscO of the Yop secretion (ysc) operon that contains yscN through yscU by determining the localization of its gene product and the phenotype of an in-frame deletion. The yscO mutant grew and expressed the same levels of Yops as the parent at 37 degreesC in the presence of Ca2+. In the absence of Ca2+, the mutant grew independently of Ca2+, expressed only basal levels of V antigen and Yops, and failed to secrete these. These defects could be partially complemented by providing yscO in trans in the yscO mutant. Overexpression of YopM and V antigen in the mutant failed to restore the export of either protein, showing that the mutation had a direct effect on secretion. These results indicated that the yscO gene product is required for high-level expression and secretion of V antigen and Yops. YscO was found by immunoblot analysis in the soluble and membrane fractions of bacteria growing at 37 degreesC irrespective of the presence of Ca2+ and in the culture medium in the absence of Ca2+. YscO is the only mobile protein identified so far in the Yersinia species that is required for secretion of V antigen and Yops.

The V antigen of Yersinia pestis regulates Yop vectorial targeting as well as Yop secretion through effects on YopB and LcrG

Yersinia pestis expresses a set of secreted proteins called Yops and the bifunctional LcrV, which has both regulatory and antihost functions. Yops and LcrV expression and the activity of the type III mechanism for their secretion are coordinately regulated by environmental signals such as Ca2+ concentration and eukaryotic cell contact. In vitro, Yops and LcrV are secreted into the culture medium in the absence of Ca2+ as part of the low-Ca2+ response (LCR). The LCR is induced in a tissue culture model by contact with eukaryotic cells that results in Yop translocation into cells and subsequent cytotoxicity. The secretion mechanism is believed to indirectly regulate expression of lcrV and yop operons by controlling the intracellular concentration of a secreted negative regulator. LcrG, a secretion-regulatory protein, is thought to block secretion of Yops and LcrV, possibly at the inner face of the inner membrane. A recent model proposes that when the LCR is induced, the increased expression of LcrV yields an excess of LcrV relative to LcrG, and this is sufficient for LcrV to bind LcrG and unblock secretion. To test this LcrG titration model, LcrG and LcrV were expressed alone or together in a newly constructed lcrG deletion strain, a delta lcrG2 mutant, of Y. pestis that produces low levels of LcrV and constitutively expresses and secretes Yops. Overexpression of LcrG in this mutant background was able to block secretion and depress expression of Yops in the presence of Ca2+ and to dramatically decrease Yop expression and secretion in growth medium lacking Ca2+. Overexpression of both LcrG and LcrV in the delta lcrG2 strain restored wild-type levels of Yop expression and Ca2+ control of Yop secretion. Surprisingly, when HeLa cells were infected with the delta lcrG2 strain, no cytotoxicity was apparent and translocation of Yops was abolished. This correlated with an altered distribution of YopB as measured by accessibility to trypsin. These effects were not due to the absence of LcrG, because they were alleviated by restoration of LcrV expression and secretion alone. LcrV itself was found to enter HeLa cells in a nonpolarized manner. These studies supported the LcrG titration model of LcrV's regulatory effect at the level of Yop secretion and revealed a further role of LcrV in the deployment of YopB, which in turn is essential for the vectorial translocation of Yops into eukaryotic cells.

Type III protein secretion systems in bacterial pathogens of animals and plants

Various gram-negative animal and plant pathogens use a novel, sec-independent protein secretion system as a basic virulence mechanism. It is becoming increasingly clear that these so-called type III secretion systems inject (translocate) proteins into the cytosol of eukaryotic cells, where the translocated proteins facilitate bacterial pathogenesis by specifically interfering with host cell signal transduction and other cellular processes. Accordingly, some type III secretion systems are activated by bacterial contact with host cell surfaces. Individual type III secretion systems direct the secretion and translocation of a variety of unrelated proteins, which account for species-specific pathogenesis phenotypes. In contrast to the secreted virulence factors, most of the 15 to 20 membrane-associated proteins which constitute the type III secretion apparatus are conserved among different pathogens. Most of the inner membrane components of the type III secretion apparatus show additional homologies to flagellar biosynthetic proteins, while a conserved outer membrane factor is similar to secretins from type II and other secretion pathways. Structurally conserved chaperones which specifically bind to individual secreted proteins play an important role in type III protein secretion, apparently by preventing premature interactions of the secreted factors with other proteins. The genes encoding type III secretion systems are clustered, and various pieces of evidence suggest that these systems have been acquired by horizontal genetic transfer during evolution. Expression of type III secretion systems is coordinately regulated in response to host environmental stimuli by networks of transcription factors. This review comprises a comparison of the structure, function, regulation, and impact on host cells of the type III secretion systems in the animal pathogens Yersinia spp., Pseudomonas aeruginosa, Shigella flexneri, Salmonella typhimurium, enteropathogenic Escherichia coli, and Chlamydia spp. and the plant pathogens Pseudomonas syringae, Erwinia spp., Ralstonia solanacearum, Xanthomonas campestris, and Rhizobium spp.

LcrG is required for efficient translocation of Yersinia Yop effector proteins into eukaryotic cells

Extracellular Yersinia disables the immune system of its host by injecting effector Yop proteins into host cells. We show that a Yersinia enterocolitica nonpolar lcrG mutant is severely impaired in the translocation of YopE, YopH, YopM, YpkA/YopO, and YopP into eukaryotic cells. LcrG is thus required for efficient internalization of all the known Yop effectors.

Symbiotic implications of type III protein secretion machinery in Rhizobium

The symbiotic plasmid of Rhizobium sp. NGR234 carries a cluster of genes that encodes components of a bacterial type III secretion system (TTSS). In both animal and plant pathogens, the TTSS is an essential component of pathogenicity. Here, we show that secretion of at least two proteins (y4xL and NolX) is controlled by the TTSS of NGR234 and occurs after the induction with flavonoids. Polar mutations in two TTSS genes, rhcN and the nod-box controlled regulator of transcription y4xl, block the secretion of both proteins and strongly affect the ability of NGR234 to nodulate a variety of tropical legumes including Pachyrhizus tuberosus and Tephrosia vogelii.

Targeting of Yersinia Yop proteins into the cytosol of HeLa cells: one-step translocation of YopE across bacterial and eukaryotic membranes is dependent on SycE chaperone

Pathogenic Yersiniae adhere to and kill macrophages by targeting some of their Yop proteins into the eukaryotic cytosol. There is debate about whether YopE targeting proceeds as a direct translocation of polypeptide between cells or in two distinct steps, each requiring specific signals for YopE secretion across the bacterial envelope and for translocation into the eukaryotic cytosol. Here, we used the selective solubilization of the eukaryotic plasma membrane with digitonin to measure Yop targeting during Yersinia infections of HeLa cells. YopE, YopH, YopM and YopN were found in the eukaryotic cytosol but not in the extracellular medium. When bound to SycE chaperone in the Yersinia cytoplasm, YopE residues 1-100 are necessary and sufficient for the targeting of hybrid neomycin phosphotransferase. Electron microscopic analysis failed to detect an extracellular intermediate of YopE targeting, suggesting a one-step translocation mechanism.

Role of YopP in suppression of tumor necrosis factor alpha release by macrophages during Yersinia infection

The Yersinia plasmid-encoded Yop virulon enables extracellular adhering bacteria to deliver toxic effector proteins inside their target cells. It includes a type III secretion system (Ysc), at least two translocator proteins (YopB, YopD), and a set of intracellular Yop effectors (YopE, YopH, YopO, YopM, and YopP). Infection of macrophages with a wild-type strain leads to low levels of tumor necrosis factor alpha (TNF-alpha) release compared to infection with plasmid-cured strains, suggesting that the virulence plasmid encodes a factor impairing the normal TNF-alpha response of infected macrophages. This effect is correlated with the inhibition of the macrophage mitogen-activated protein kinase (MAPK) activities. To identify the Yop protein responsible for the suppression of TNF-alpha release, we infected J774A.1 and PU5-1.8 macrophages with a battery of knockout Yersinia enterocolitica mutants and we quantified the TNF-alpha released. Mutants affected in secretion (yscN), in translocation (yopB and yopD), or in synthesis of all the known Yop effectors (yopH, yopO, yopP, yopE, and yopM polymutants) were unable to block the TNF-alpha response of the macrophages. In contrast, single yopE, yopH, yopO, and yopM mutants behaved like the wild-type strain. A yopP mutant elicited elevated TNF-alpha release, and complementation of the yopP mutant or the yop effector polymutant strain with yopP alone led to a drop in TNF-alpha release. In addition, YopP was also responsible for the inhibition of the extracellular signal-regulated kinase2 (ERK2) and p38 MAPK activities. These results show that YopP is the Yop effector responsible for the Yersinia-induced suppression of TNF-alpha release by infected macrophages.

TyeA, a protein involved in control of Yop release and in translocation of Yersinia Yop effectors

Extracellular Yersinia spp. disarm the immune system by injecting the effector Yersinia outer proteins (Yops) into the target cell. Yop secretion is triggered by contact with eukaryotic cells or by Ca2+ chelation. Two proteins, YopN and LcrG, are known to be involved in Yop-secretion control. Here we describe TyeA, a third protein involved in the control of Yop release. Like YopN, TyeA is localized at the bacterial surface. A tyeA knock-out mutant secreted Yops in the presence of Ca2+ and in the absence of eukaryotic cells. Unlike a yopN null mutant, the tyeA mutant was defective for translocation of YopE and YopH, but not YopM, YopO and YopP, into eukaryotic cells. This is the first observation suggesting that Yop effectors can be divided into two sets for delivery into eukaryotic cells. TyeA was found to interact with the translocator YopD and with residues 242-293 of YopN. In contrast with a yopN null mutant, a yopNDelta248-272 mutant was also unable to translocate YopE and YopH. Our results suggest that TyeA forms part of the translocation-control apparatus together with YopD and YopN, and that the interaction of these proteins is required for selective translocation of Yops inside eukaryotic cells.

The Yersinia Yop virulon: LcrV is required for extrusion of the translocators YopB and YopD

LcrV, an essential piece of the Yop virulon, is encoded by the large lcrGVsycDyopBD operon. In spite of repeated efforts, the role of LcrV in the Yop virulon remains elusive. In an attempt to clarify this, we engineered a complete deletion of lcrV in the pYV plasmid of Yersinia enterocolitica E40 and characterized the phenotype of the mutant. Complementation experiments showed that the mutation was not polar with regard to yopB and yopD. Nevertheless the mutation abolished secretion of YopB and YopD, while secretion of the other Yops was unaffected or even increased. Northern blot analysis showed that transcription of yopD was not affected. YopD could be detected inside the bacteria, showing that the lack of its secretion was not due to a lack of translation or to proteolysis. This indicated that LcrV is specifically involved in the process of release of YopB and YopD. We then investigated the possible interactions between LcrV and YopB or YopD. We constructed a glutathione S-transferase-LcrV hybrid protein, and we observed that either YopB or YopD could be copurified with it. The same approach showed that LcrV also interacts with LcrG but not with the chaperone SycD. Using deletants of lcrV, we then identified a definite LcrG-binding domain in the C terminus of LcrV.

Heparin interferes with translocation of Yop proteins into HeLa cells and binds to LcrG, a regulatory component of the Yersinia Yop apparatus

Yersiniae are equipped with the Yop virulon, an apparatus that allows extracellular bacteria to deliver toxic Yop proteins inside the host cell cytosol in order to sabotage the communication networks of the host cell or even to cause cell death. LcrG is a component of the Yop virulon involved in the regulation of secretion of the Yops. In this paper, we show that LcrG can bind HeLa cells, and we analyse the role of proteoglycans in this phenomenon. Treatment of the HeLa cells with heparinase I, but not chondroitinase ABC, led to inhibition of binding. Competition assays indicated that heparin and dextran sulphate strongly inhibited binding, but that other glycosaminoglycans did not. This demonstrated that binding of HeLa cells to purified LcrG is caused by heparan sulphate proteoglycans. LcrG could bind directly to heparin-agarose beads and, in agreement with these results, analysis of the protein sequence of Yersinia enterocolitica LcrG revealed heparin-binding motifs. In vitro production and secretion by Y. enterocolitica of the Yops was unaffected by the addition of heparin. However, the addition of exogenous heparin decreased the level of YopE-Cya translocation into HeLa cells. A similar decrease was seen with dextran sulphate, whereas the other glycosaminoglycans tested had no significant effect. Translocation was also decreased by treatment of HeLa cells with heparinitase, but not with chondroitinase. Thus, heparan sulphate proteoglycans have an important role to play in translocation. The interaction between LcrG and heparan sulphate anchored at the surface of HeLa cells could be a signal triggering deployment of the Yop translocation machinery. This is the first report of a eukaryotic receptor interacting with the type III secretion and associated translocation machinery of Yersinia or of other bacteria.

Bidirectional signaling between Yersinia and its target cell

Preventing the early host immune defense allows pathogenic Yersinia to proliferate in lymphatic tissue. This ability depends on signaling that occurs between the bacteria and the host cells. Following intimate contact with the target cell a signal is generated within the bacterium that results in increased expression of virulence-associated proteins that are subsequently delivered into the infected cell. These proteins, designated Yops, interfere with the host-cell signaling pathways that are normally activated to eliminate infectious agents.

YopD of Yersinia pestis plays a role in negative regulation of the low-calcium response in addition to its role in translocation of Yops

Yersinia pestis produces a set of virulence proteins (Yops and LcrV) that are expressed at high levels and secreted by a type III secretion system (Ysc) upon bacterium-host cell contact, and four of the Yops are vectorially translocated into eukaryotic cells. YopD, YopB, and YopK are required for the translocation process. In vitro, induction and secretion occur at 37 degrees C in the absence of calcium. LcrH (also called SycD), a protein required for the stability and secretion of YopD, had initially been identified as a negative regulator of Yop expression. In this study, we constructed a yopD mutation in both wild-type and secretion-defective (ysc) Y. pestis to determine if the lcrH phenotype could be attributed to the decreased stability of YopD. These mutants were constitutively induced for expression of Yops and LcrV, despite the presence of the secreted negative regulator LcrQ, demonstrating that YopD is involved in negative regulation, regardless of a functioning Ysc system. Normally, secretion of Yops and LcrV is blocked in the presence of calcium. The single yopD mutant was not completely effective in blocking secretion: LcrV was secreted equally well in the presence and absence of calcium, while there was partial secretion of Yops in the presence of calcium. YopD is probably not rate limiting for negative regulation, as increasing levels of YopD did not result in decreased Yop expression. Overexpression of LcrQ in the yopD mutant had no significant effect on Yop expression, whereas increased levels of LcrQ in the parent resulted in decreased levels of Yops. These results indicate that LcrQ requires YopD to function as a negative regulator.

The Yersinia Yop virulon, a bacterial system to subvert cells of the primary host defense

The Yop virulon enables yersinias (Yersinia pestis, Y. pseudotuberculosis and Y. enterocolitica) to survive and multiply in the lymphoid tissues of their host. It is an integrated system allowing extracellular bacteria to communicate with the host cell's cytosol by injection of effector proteins. It is composed of four elements: (1) a contact or type III secretion system called Ysc, devoted to the secretion of Yop proteins. This secretion apparatus, made of some 22 proteins, recognizes the Yops by a short N-terminal signal that is not cleaved off during secretion; (2) a system designed to deliver bacterial proteins into eukaryotic target cells. This system is made of YopB, YopD and LcrV; (3) a control element (YopN) and (4) a set of effector Yop proteins designed to disarm these cells or disrupt their communications (YopE, YopH, YopM, YpkA/YopO, YopP). The whole virulon is encoded by a 70-kb plasmid called pYV. Transcription of the genes is controlled both by temperature and by contact with a eukaryotic cell.

Hrp-controlled interkingdom protein transport: learning from flagellar assembly?

Plant pathogenic bacteria appear to deliver avirulence and virulence proteins through the cell wall and into the host cells via an Hrp (hypersensitive reaction and pathogenicity)-encoded type III secretion system. Recent results suggest that there is a similarity between this secretion system and the flagellum assembly apparatus.

Identification of type III secreted products of the Pseudomonas aeruginosa exoenzyme S regulon

Extracellular protein profiles from wild-type and regulatory or secretory isogenic mutants of the Pseudomonas aeruginosa exoenzyme S regulon were compared to identify proteins coordinately secreted with ExoS. Data from amino-terminal sequence analysis of purified extracellular proteins were combined with data from nucleotide sequence analysis of loci linked to exoenzyme S production. We report the identification of P. aeruginosa homologs to proteins of Yersinia spp. that function as regulators of the low calcium response, regulators of secretion, and mediators of the type III translocation mechanism.

The invasion-associated type-III protein secretion system in Salmonella--a review

The genetic determinants that confer upon Salmonella the ability to enter non-phagocytic cells are largely encoded in a pathogenicity island located at centisome 63 of the bacterial chromosome. Molecular genetic analysis has revealed that this region encodes a specialized protein secretion system that mediates the export and/or translocation of putative signaling proteins into the host cell. This protein secretion system, which has been termed type III or contact-dependent, has also been identified in other plant and animal pathogens that have, in common, the ability to interact with eukaryotic host cells in an intimate manner.