Attenuation and preserved immunogenic potential of Yersinia pseudotuberculosis mutant strains evidenced in oral pig model

Experimental oral infection of pigs with a parental Yersinia pseudotuberculosis strain pIB102, serotype O:3 and two mutant isogenic strains - pIB155,DeltayopK and pIB44,DeltaypkA has been carried out. Clinical findings, microbiological and immunological parameters were examined in dynamics from day 7 to day 60 post-infection (p.i.). All types of infections ran asymptomatically, without hyperthermia, loss of appetite, etc. Experiments on the blood parameters demonstrated a transient leucocytosis with lymphocytosis and monocytosis better expressed after yopK infection. Even though pig is usually known as a reservoir of yersiniae, bacterial colonization was found in mesenterial lymph nodes and tonsils on day 7, respectively 14 p.i. with parental strain, and only in tonsils on day 14 p.i. with both mutant strains. The augmented sensitivity of mutants to the bactericidal effect of leukocytes and blood sera is the characteristic feature of attenuation in their pathogenicity, compared to the parental strain. Comparative in vitro experiments on the immune response and immunostimulating capacity of Y. pseudotuberculosis mutant strains verify their preserved immunogenic potential, predominantly in case of yopK. Hyperplasia and strong activation of the lymph tissue of Peyer's patches, mesenterial lymph nodes, tonsils and spleen of pigs challenged with both mutant strains were proved as immunomorphological rearrangements. The results obtained give the reason to claim that the genetically constructed yopK null mutant strain is significantly attenuated but is still immunogenic and has the potential for a live vaccine carrier strain.

Virulence blockers as alternatives to antibiotics: type III secretion inhibitors against Gram-negative bacteria

In recent years mounting problems related to antibiotic-resistant bacteria have resulted in the prediction that we are entering the preantibiotic era. A way of preventing such a development would be to introduce novel antibacterial medicines with modes of action distinct from conventional antibiotics. Recent studies of bacterial virulence factors and toxins have resulted in increased understanding of the way in which pathogenic bacteria manipulate host cellular processes. This knowledge may now be used to develop novel antibacterial medicines that disarm pathogenic bacteria. The type III secretion system (T3SS) is known to be a potent virulence mechanism shared by a broad spectrum of pathogenic Gram-negative bacteria that interact with human, animal and plant hosts by injecting effector proteins into the cytosol of host cells. Diseases, such as bubonic plague, shigellosis, salmonellosis, typhoid fever, pulmonary infections, sexually transmitted chlamydia and diarrhoea largely depend on the bacterial proteins injected by the T3SS machinery. Recently a number of T3SS inhibitors have been identified using screening-based approaches. One class of inhibitors, the salicylidene acylhydrazides, has been subjected to chemical optimization and evaluation in several in vitro and ex vivo assays in multiple bacterial species including Yersinia spp., Chlamydia spp., Salmonella spp. and Pseudotuberculosis aeruginosa. Reports published up to date indicate that T3SS inhibitors have the potential to be developed into novel antibacterial therapeutics.

Small-molecule inhibitors specifically targeting type III secretion

The type III secretion (TTS) system is used by several animal and plant pathogens to deliver effector proteins into the cytosol of the eukaryotic target cell as a strategy to evade the defense reactions elicited by the infected organism. The fact that these systems are highly homologous implies that novel antibacterial agents that chemically attenuate the pathogens via a specific interaction with the type III secretion mechanism can be identified. A number of small organic molecules having this potential have recently been identified (A. M. Kauppi, R. Nordfelth, H. Uvell, H. Wolf-Watz, and M. Elofsson, Chem. Biol. 10:241-249, 2003). Using different reporter gene constructs, we showed that compounds that belong to a class of acylated hydrazones of different salicylaldehydes target the TTS system of Yersinia pseudotuberculosis. One of these compounds, compound 1, was studied in detail and was found to specifically block Yop effector secretion under in vitro conditions by targeting the TTS system. In this respect the drug mimics the well-known effect of calcium on Yop secretion. In addition, compound 1 inhibits Yop effector translocation after infection of HeLa cells without affecting the eukaryotic cells or the bacteria. A HeLa cell model that mimics in vivo conditions showed that compound 1 chemically attenuates the pathogen to the advantage of the eukaryotic cell. Thus, our results show proof of concept, i.e., that small compounds targeting the TTS system can be identified, and they point to the possible use of TTS inhibitors as a novel class of antibacterial agents.

Experimental infections with wild and mutant Yersinia pseudotuberculosis strains in rabbits

Experimental oral infections of rabbits with a wild-type Yersinia pseudotuberculosis strain (pIB102), and two null-mutants (yopK and ypkA) were carried out with the aim to explore the possibility to use mutant strains of Y. pseudotuberculosis as live carrier vaccine strains. The infectious process of the three strains proceed with passing hyperthermia, leucocytosis with granulocytosis, moderate monocytosis and a transient lymphopenia, better demonstrated at mutant strain infections. Short-term bacterial dissemination into the brain and viscera was observed at yopK infection. An augmented resistance to bactericidal activity of leucocytes at the initial phase of infection was followed by an increased sensitivity discovered earlier in case of yopK strain accompanied by at least 70- and 20-fold, respectively, for ypkA lower virulence for mice. The level of attenuation of yopK was accompanied by significant Yersinia specific IgG and IgM antibody response. Inflammatory foci were found by morphological examination in brain, lung and small intestines after infection with the wild-type strain, while such foci were only observed in brain and mesenterial lymph nodes after infection with the yopK mutant. After infection with the ypkA mutant foci were found in brain and spleen of the infected animals. Morphological changes in the lymphatic tissue of rabbits infected with mutant strains were consistent with induction of immunogenesis. The data suggest that genetically constructed yopK null-mutant exhibits characteristics that makes the strain suitable to be used as a live carrier vaccine to deliver heterologous antigens.

The type III secretion chaperone LcrH co-operates with YopD to establish a negative, regulatory loop for control of Yop synthesis in Yersinia pseudotuberculosis

The enteropathogen Yersinia pseudotuberculosis is a model system used to study the molecular mechanisms by which Gram-negative pathogens secrete and subsequently translocate antihost effector proteins into target eukaryotic cells by a common type III secretion system (TTSS). In this process, YopD (Yersinia outer protein D) is essential to establish regulatory control of Yop synthesis and the ensuing translocation process. YopD function depends upon the non-secreted TTSS chaperone LcrH (low-calcium response H), which is required for presecretory stabilization of YopD. However, as a new role for TTSS chaperones in virulence gene regulation has been proposed recently, we undertook a detailed analysis of LcrH. A lcrH null mutant constitutively produced Yops, even when this strain was engineered to produce wild-type levels of YopD. Furthermore, the YopD-LcrH interaction was necessary to regain the negative regulation of virulence associated genes yops). This finding was used to investigate the biological significance of several LcrH mutants with varied YopD binding potential. Mutated LcrH alleles were introduced in trans into a lcrH null mutant to assess their impact on yop regulation and the subsequent translocation of YopE, a Rho-GTPase activating protein, across the plasma membrane of eukaryotic cells. Two mutants, LcrHK20E, E30G, I31V, M99V, D136G and LcrHE30G lost all regulatory control, even though YopD binding and secretion and the subsequent translocation of YopE was indistinguishable from wild type. Moreover, these regulatory deficient mutants showed a reduced ability to bind YscY in the two-hybrid assay. Collectively, these findings confirm that LcrH plays an active role in yop regulation that might be mediated via an interaction with the Ysc secretion apparatus. This chaperone-substrate interaction presents an innovative means to establish a regulatory hierarchy in Yersinia infections. It also raises the question as to whether or not LcrH is a true chaperone involved in stabilization and secretion of YopD or a regulatory protein responsible for co-ordinating synthesis of Yersinia virulence determinants. We suggest that LcrH can exhibit both of these activities.

Targeting exported substrates to the Yersinia TTSS: different functions for different signals?

Many Gram-negative pathogens utilize a type III secretion system (TTSS) to inject toxins into the cytosol of eukaryotic cells. Previous studies have indicated that exported substrates are targeted to the Yersinia TTSS via the coding regions of their 5' mRNA sequences, as well as by their cognate chaperones. However, recent results from our laboratory have challenged the role of mRNA targeting signals, as we have shown that the amino termini of exported substrates are crucial for type III secretion. Here, we discuss the nature of these amino-terminal secretion signals and propose a model for the secretion of exported substrates by amino-terminal and chaperone-mediated signals. In addition, we discuss the roles of chaperones as regulators of virulence gene expression and present models suggesting that such regulation can occur independently of the delivery of their substrates to the secretion apparatus.

YopB of Yersinia enterocolitica is essential for YopE translocation

A previous study has shown that YopB of Yersinia spp. is essential for translocation of Yop effectors across the eucaryotic plasma membrane (M.-P. Sory and G. R. Cornelis, Mol. Microbiol. 14:583--594, 1994). However, this role was recently challenged (V. T. Lee and O. Schneewind, Mol. Microbiol. 31:1619--1629, 1999). Using protease protection and digitonin extraction, we reconfirm that YopB of Yersinia enterocolitica is essential for the translocation of YopE into HeLa cell monolayers.

LcrV is a channel size-determining component of the Yop effector translocon of Yersinia

Delivery of Yop effector proteins by pathogenic Yersinia across the eukaryotic cell membrane requires LcrV, YopB and YopD. These proteins were also required for channel formation in infected erythrocytes and, using different osmolytes, the contact-dependent haemolysis assay was used to study channel size. Channels associated with LcrV were around 3 nm, whereas the homologous PcrV protein of Pseudomonas aeruginosa induced channels of around 2 nm in diameter. In lipid bilayer membranes, purified LcrV and PcrV induced a stepwise conductance increase of 3 nS and 1 nS, respectively, in 1 M KCl. The regions important for channel size were localized to amino acids 127-195 of LcrV and to amino acids 106-173 of PcrV. The size of the channel correlated with the ability to translocate Yop effectors into host cells. We suggest that LcrV is a size-determining structural component of the Yop translocon.

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.

The Yersinia protein kinase A is a host factor inducible RhoA/Rac-binding virulence factor

The pathogenic yersiniae inject proteins directly into eukaryotic cells that interfere with a number of cellular processes including phagocytosis and inflammatory-associated host responses. One of these injected proteins, the Yersinia protein kinase A (YpkA), has previously been shown to affect the morphology of cultured eukaryotic cells as well as to localize to the plasma membrane following its injection into HeLa cells. Here it is shown that these activities are mediated by separable domains of YpkA. The amino terminus, which contains the kinase domain, is sufficient to localize YpkA to the plasma membrane while the carboxyl terminus of YpkA is required for YpkAs morphological effects. YpkAs carboxyl-terminal region was found to affect the levels of actin-containing stress fibers as well as block the activation of the GTPase RhoA in Yersinia-infected cells. We show that the carboxyl-terminal region of YpkA, which contains sequences that bear similarity to the RhoA-binding domains of several eukaryotic RhoA-binding kinases, directly interacts with RhoA as well as Rac (but not Cdc42) and displays a slight but measurable binding preference for the GDP-bound form of RhoA. Surprisingly, YpkA binding to RhoA(GDP) affected neither the intrinsic nor guanine nucleotide exchange factor-mediated GDP/GTP exchange reaction suggesting that YpkA controls activated RhoA levels by a mechanism other than by simply blocking guanine nucleotide exchange factor activity. We go on to show that YpkAs kinase activity is neither dependent on nor promoted by its interaction with RhoA and Rac but is, however, entirely dependent on heat-sensitive eukaryotic factors present in HeLa cell extracts and fetal calf serum. Collectively, our data show that YpkA possesses both similarities and differences with the eukaryotic RhoA/Rac-binding kinases and suggest that the yersiniae utilize the Rho GTPases for unique activities during their interaction with eukaryotic cells.

A study of the YopD-lcrH interaction from Yersinia pseudotuberculosis reveals a role for hydrophobic residues within the amphipathic domain of YopD

The enteropathogen Yersinia pseudotuberculosis is a model system used to study the molecular mechanisms by which Gram-negative pathogens translocate effector proteins into target eukaryotic cells by a common type III secretion machine. Of the numerous proteins produced by Y. pseudotuberculosis that act in concert to establish an infection, YopD (Yersinia outer protein D) is a crucial component essential for yop regulation and Yop effector translocation. In this study, we describe the mechanisms by which YopD functions to control these processes. With the aid of the yeast two-hybrid system, we investigated the interaction between YopD and the cognate chaperone LcrH. We confirmed that non-secreted LcrH is necessary for YopD stabilization before secretion, presumably by forming a complex with YopD in the bacterial cytoplasm. At least in yeast, this complex depends upon the N-terminal domain and a C-terminal amphipathic alpha-helical domain of YopD. Introduction of amino acid substitutions within the hydrophobic side of the amphipathic alpha-helix abolished the YopD-LcrH interaction, indicating that hydrophobic, as opposed to electrostatic, forces of attraction are important for this process. Suppressor mutations isolated within LcrH could compensate for defects in the amphipathic domain of YopD to restore binding. Isolation of LcrH mutants unable to interact with wild-type YopD revealed no single domain responsible for YopD binding. The YopD and LcrH mutants generated in this study will be relevant tools for understanding YopD function during a Yersinia infection.

The bacterial protein YopJ abrogates multiple signal transduction pathways that converge on the transcription factor CREB

Bacterially encoded proteins are known to affect eukaryotic signalling pathways and thus cell growth and differentiation. The enteric pathogen Yersinia pseudotuberculosis (YP) can translocate Yersinia outer proteins (Yops) into eukaryotic cells. Recently, MKK proteins have been identified as tentative targets of YopJ-mediated inhibition of ligand receptor-dependent signal transduction in mammalian cells. These results prompted us to assess whether multiple signal transduction pathways and their downstream target genes would also be subject to regulation by YopJ. Here, we show that YopJ effectively blocks the lipopolysaccharide (LPS) receptor, the interleukin (IL)-1beta receptor and the UVC-induced activation of the transcription receptor cAMP response element-binding protein (CREB). In addition, by abrogating the phosphorylation of CREB and thus activating protein (AP)-1-dependent transcription, YopJ can block LPS-induced clonal expansion that is associated with an adaptive immune response. Thus, YopJ interferes with multiple pathways converging on the transcription factor CREB. Our data are discussed in the context of YopJ acting as an antagonist to circumvent innate and adaptive immune responses at multiple levels.

GAP activity of the Yersinia YopE cytotoxin specifically targets the Rho pathway: a mechanism for disruption of actin microfilament structure

The YopE cytotoxin of Yersinia pseudotuberculosis is an essential virulence determinant that is injected into the eukaryotic target cell via a plasmid-encoded type III secretion system. Injection of YopE into eukaryotic cells induces depolymerization of actin stress fibres. Here, we show that YopE exhibits a GTPase-activating protein (GAP) activity and that the presence of YopE stimulates downregulation of Rho, Rac and Cdc42 activity. YopE has an arginine finger motif showing homology with those found in other GAP proteins. Exchange of arginine 144 with alanine, located in this arginine finger motif, results in an inactive form of YopE that can no longer stimulate GTP hydrolysis by the GTPase. Furthermore, a yopE(R144A) mutant is unable to induce cytotoxicity on cultured HeLa cells in contrast to the corresponding wild-type strain. Expression of wild-type YopE in cells of Saccharomyces cerevisiae inhibits growth, while in contrast, expression of the inactive form of YopE, YopE(R144A), does not affect the yeast cells. Co-expression of proteins belonging to the Rho1 pathway of yeast, Rho1, Rom2p, Bck1 and Ste20, suppressed the growth phenotype of YopE in yeast cells. These results provide evidence that YopE exhibits a GAP activity to inactivate RhoGTPases, leading to depolymerization of the actin stress fibres in eukaryotic cells and growth inhibition in yeast.

Ras effector pathway activation by epidermal growth factor is inhibited in vivo by exoenzyme S ADP-ribosylation of Ras

We have examined the functional consequences of ADP-ribosyltransferase modification of Ras by the exoenzyme S (ExoS) protein of Pseudomonas aeruginosa. ExoS has been shown previously to ADP-ribosylate a number of proteins, including members of the Ras superfamily, which play an essential role in the processes of cell proliferation, differentiation, motility and cell division. HeLa and NIH3T3 cells were infected with ExoS protein, which was delivered via the type III secretion system of the heterologous host Yersinia pseudotuberculosis. Infection of mammalian cells with ExoS results in a change in the ratio of GTP/GDP bound directly to Ras in vivo. This ADP-ribosylation of Ras in vivo is mediated by the C-terminal domain of ExoS. Further, ExoS ADP-ribosylation of Ras in vivo inhibits activation of Ras and the ability to interact with the Ras binding domain of Raf upon stimulation with epidermal growth factor (EGF). In the present study, we show that ExoS activity does not interfere with EGF receptor phosphorylation itself, nor with the formation of a Grb2-activated Shc complex upon EGF stimulation, consistent with ExoS blockage of this mitogenic signalling pathway at the level of Ras. This is further supported by our observation of a substantial inhibition of extracellular signal-regulated kinase and protein kinase B/Akt kinase activation in response to EGF upon ExoS infection. In conclusion, in the present study, the consequences of ExoS infection on Ras effector pathway in vivo have been defined.

Localization of the Yersinia PTPase to focal complexes is an important virulence mechanism

The protein tyrosine phosphatase YopH, produced by the pathogen Yersinia pseudotuberculosis, is an essential virulence determinant involved in antiphagocytosis. Upon infection, YopH is translocated into the target cell, where it recognizes focal complexes. Genetic analysis revealed that YopH harbours a region that is responsible for specific localization of this PTPase to focal complexes in HeLa cells and professional phagocytes. This region is a prerequisite for blocking an immediate-early Yersinia-induced signal within target cells. The region is also essential for antiphagocytosis and virulence, illustrating the biological significance of localization of YopH to focal complexes during Yersinia infection. These results also indicate that focal complexes play a role in the general phagocytic process.

The chemotactic response of Vibrio anguillarum to fish intestinal mucus is mediated by a combination of multiple mucus components

Chemotactic motility has previously been shown to be essential for the virulence of Vibrio anguillarum in waterborne infections of fish. To investigate the mechanisms by which chemotaxis may function during infection, mucus was isolated from the intestinal and skin epithelial surfaces of rainbow trout. Chemotaxis assays revealed that V. anguillarum swims towards both types of mucus, with a higher chemotactic response being observed for intestinal mucus. Work was performed to examine the basis, in terms of mucus composition, of this chemotactic response. Intestinal mucus was analyzed by using chromatographic and mass spectrometric techniques, and the compounds identified were tested in a chemotaxis assay to determine the attractants present. A number of mucus-associated components, in particular, amino acids and carbohydrates, acted as chemoattractants for V. anguillarum. Importantly, only upon combination of these attractants into a single mixture were levels of chemotactic activity similar to those of intestinal mucus generated. A comparative analysis of skin mucus revealed its free amino acid and carbohydrate content to be considerably lower than that of the more chemotactically active intestinal mucus. To study whether host specificity exists in relation to vibrio chemotaxis towards mucus, comparisons with a human Vibrio pathogen were made. A cheR mutant of a Vibrio cholerae El Tor strain was constructed, and it was found that V. cholerae and V. anguillarum exhibit a chemotactic response to mucus from several animal sources in addition to that from the human jejunum and fish epithelium, respectively.

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.

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.

The yopJ locus is required for Yersinia-mediated inhibition of NF-kappaB activation and cytokine expression: YopJ contains a eukaryotic SH2-like domain that is essential for its repressive activity

Upon exposure to bacteria, eukaryotic cells activate signalling pathways that result in the increased expression of several defence-related genes. Here, we report that the yopJ locus of the enteropathogen Yersinia pseudotuberculosis encodes a protein that inhibits the activation of NF-kappaB transcription factors by a mechanism(s), which prevents the phosphorylation and subsequent degradation of the inhibitor protein IkappaB. Consequently, eukaryotic cells infected with YopJ-expressing Yersinia become impaired in NF-kappaB-dependent cytokine expression. In addition, the blockage of inducible cytokine production coincides with yopJ-dependent induction of apoptosis. Interestingly, the YopJ protein contains a region that resembles a src homology domain 2 (SH2), and we show that a wild-type version of this motif is required for YopJ activity in suppressing cytokine expression and inducing apoptosis. As SH2 domains are found in several eukaryotic signalling proteins, we propose that YopJ, which we show is delivered into the cytoplasm of infected cells, interacts directly with signalling proteins involved in inductive cytokine expression. The repressive activity of YopJ on the expression of inflammatory mediators may account for the lack of an inflammatory host response observed in experimental yersiniosis. YopJ-like activity may also be a common feature of commensal bacteria that, like Yersinia, do not provoke a host inflammatory response.

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.

Yersinia invasin, a bacterial beta1-integrin ligand, is a potent inducer of lymphocyte motility and migration to collagen type IV and fibronectin

The Yersinia pseudotuberculosis invasin protein was found to be a potent inducer of pseudopodia formation and chemotactic and haptotactic migration in human T lymphocytes. Checkerboard analysis confirmed that migration was directional. The Yersinia invasin triggered migration of otherwise poorly migratory normal T cells on fibronectin and in particular on collagen type IV, and augmented the migration of leukemic T cell lines on these components. Invasin-induced lymphocyte migration was inhibited by staurosporin that selectively prevented pseudopodia formation but, noteworthy, augmented adhesion. The motogenic and attractant properties of invasin (Inv) were mediated via beta1-integrins, as shown by lack of effect of Inv on the motility of a beta1-integrin-negative lymphoid cell line and inhibition of invasin-induced lymphocyte motility by anti-beta1 Abs. Inv was markedly more effective than the extracellular matrix components fibronectin, collagen type IV, and laminin, which also interact with lymphocyte beta1-integrins, with respect to induction of pseudopodia, chemotaxis, and haptotaxis. Thus, Yersinia invasin is a model ligand for induction of lymphocyte motility via beta1-integrins. The extraordinary capacity of Inv to trigger and guide T lymphocyte motility and potentiate lymphocyte migration to extracellular matrix components may be of pathogenetic significance for the movement of lymphocytes to extraintestinal sites secondary to Yersinia infection.

Yersinia invasin, a bacterial beta1-integrin ligand, is a potent inducer of lymphocyte motility and migration to collagen type IV and fibronectin

The Yersinia pseudotuberculosis invasin protein was found to be a potent inducer of pseudopodia formation and chemotactic and haptotactic migration in human T lymphocytes. Checkerboard analysis confirmed that migration was directional. The Yersinia invasin triggered migration of otherwise poorly migratory normal T cells on fibronectin and in particular on collagen type IV, and augmented the migration of leukemic T cell lines on these components. Invasin-induced lymphocyte migration was inhibited by staurosporin that selectively prevented pseudopodia formation but, noteworthy, augmented adhesion. The motogenic and attractant properties of invasin (Inv) were mediated via beta1-integrins, as shown by lack of effect of Inv on the motility of a beta1-integrin-negative lymphoid cell line and inhibition of invasin-induced lymphocyte motility by anti-beta1 Abs. Inv was markedly more effective than the extracellular matrix components fibronectin, collagen type IV, and laminin, which also interact with lymphocyte beta1-integrins, with respect to induction of pseudopodia, chemotaxis, and haptotaxis. Thus, Yersinia invasin is a model ligand for induction of lymphocyte motility via beta1-integrins. The extraordinary capacity of Inv to trigger and guide T lymphocyte motility and potentiate lymphocyte migration to extracellular matrix components may be of pathogenetic significance for the movement of lymphocytes to extraintestinal sites secondary to Yersinia infection.

The PTPase YopH inhibits uptake of Yersinia, tyrosine phosphorylation of p130Cas and FAK, and the associated accumulation of these proteins in peripheral focal adhesions

Pathogenic Yersinia resist uptake by eukaryotic cells by a mechanism involving the virulence protein YopH, a protein tyrosine phosphatase. We show that p130Cas and FAK are phosphorylated and recruited to peripheral focal complexes during bacterial uptake in HeLa cells. The inactive form of YopH interacts with the tyrosine phosphorylated forms of FAK and p130Cas and co-localizes with these proteins in focal adhesions. On the other hand, the presence of active YopH results in inhibition of uptake, dephosphorylation of p130Cas and FAK, and disruption of peripheral focal complexes. We suggest that p130Cas and FAK are substrates for YopH and that the dephosphorylation of these proteins impairs the uptake of Yersinia pseudotuberculosis into HeLa cells.

YopK of Yersinia pseudotuberculosis controls translocation of Yop effectors across the eukaryotic cell membrane

Introduction of anti-host factors into eukaryotic cells by extracellular bacteria is a strategy evolved by several Gram-negative pathogens. In these pathogens, the transport of virulence proteins across the bacterial membranes is governed by closely related type III secretion systems. For pathogenic Yersinia, the protein transport across the eukaryotic cell membrane occurs by a polarized mechanism requiring two secreted proteins, YopB and YopD. YopB was recently shown to induce the formation of a pore in the eukaryotic cell membrane, and through this pore, translocation of Yop effectors is believed to occur (Håkansson et al., 1996b). We have previously shown that YopK of Yersinia pseudotuberculosis is required for the development of a systemic infection in mice. Here, we have analysed the role of YopK in the virulence process in more detail. A yopK-mutant strain was found to induce a more rapid YopE-mediated cytotoxic response in HeLa cells as well as in MDCK-1 cells compared to the wild-type strain. We found that this was the result of a cell-contact-dependent increase in translocation of YopE into HeLa cells. In contrast, overexpression of YopK resulted in impaired translocation. In addition, we found that YopK also influenced the YopB-dependent lytic effect on sheep erythrocytes as well as on HeLa cells. A yopK-mutant strain showed a higher lytic activity and the induced pore was larger compared to the corresponding wild-type strain, whereas a strain overexpressing YopK reduced the lytic activity and the apparent pore size was smaller. The secreted YopK protein was found not to be translocated but, similar to YopB, localized to cell-associated bacteria during infection of HeLa cells. Based on these results, we propose a model where YopK controls the translocation of Yop effectors into eukaryotic cells.

The Yersinia Yop virulon: a bacterial system for subverting eukaryotic cells

The Yop virulon enables Yersinia spp. (Y. 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 the injection of effector proteins. It is composed of the following four elements. (I) A contact or type III secretion system called Ysc, which is devoted to the secretion of Yop proteins. This secretion apparatus, comprising some 22 proteins recognizes the Yops by a short N-terminal signal that is not cleaved off during secretion. (II) A system designed to deliver bacterial proteins into eukaryotic target cells. This system is made of YopB, YopD and possibly other Yops such as LcrV. (III) A control element (YopN). (IV) A set of effector Yop proteins designed to disarm these cells or disrupt their communications (YopE, YopH, YpkA/YopO, and YopM). The whole virulon is encoded by a 70 kb plasmid designated pYV. Transcription of the genes is controlled both by temperature and by contact with a eukaryotic cell.

Delineation and mutational analysis of the Yersinia pseudotuberculosis YopE domains which mediate translocation across bacterial and eukaryotic cellular membranes

Pathogenic yersiniae deliver a number of different effector molecules, which are referred to as Yops, into the cytosol of eukaryotic cells via a type III secretion system. To identify the regions of YopE from Yersinia pseudotuberculosis that are necessary for its translocation across the bacterial and eukaryotic cellular membranes, we constructed a series of hybrid genes which consisted of various amounts of yopE fused to the adenylate cyclase-encoding domain of the cyclolysin gene (cyaA) of Bordetella pertussis. By assaying intact cells for adenylate cyclase activity, we show that a YopE-Cya protein containing just the 11 amino-terminal residues of YopE is efficiently exported to the exterior surface of the bacterial cell. Single amino acid replacements of the first seven YopE residues significantly decreased the amount of reporter protein detected on the cell surface, suggesting that the extreme amino-terminal region of YopE is recognized by the secretion machinery. As has recently been shown for the Y. enterocolitica YopE protein (M.-P. Sory, A. Boland, I. Lambermont, and G. R. Cornelis, Proc. Natl. Acad. Sci. USA 92:11998-12002, 1995), we found that export to the cell surface was not sufficient for YopE-Cya proteins to be delivered into the eukaryotic cytoplasm. For traversing the HeLa cell membrane, at least 49 yopE-encoded residues were required. Replacement of leucine 43 of YopE with glycine severely affected the delivery of the reporter protein into HeLa cells. Surprisingly, export from the bacterial cell was also not sufficient for YopE-Cya proteins to be released from the bacterial cell surface into the culture supernatant. At least 75 residues of YopE were required to detect activity of the corresponding reporter protein in the culture supernatant, suggesting that a release domain exists in this region of YopE. We also show that the chaperone-like protein YerA required at least 75 YopE residues to form a stable complex in vitro with YopE-Cya proteins and, furthermore, that YerA is not required to target YopE-Cya proteins to the secretion complex. Taken together, our results suggest that traversing the bacterial and eukaryotic membranes occurs by separate processes that recognize distinct domains of YopE and that these processes are not dependent on YerA activity.

The YopB protein of Yersinia pseudotuberculosis is essential for the translocation of Yop effector proteins across the target cell plasma membrane and displays a contact-dependent membrane disrupting activity

During infection of cultured epithelial cells, surface-located Yersinia pseudotuberculosis deliver Yop (Yersinia outer protein) virulence factors into the cytoplasm of the target cell. A non-polar yopB mutant strain displays a wild-type phenotype with respect to in vitro Yop regulation and secretion but fails to elicit a cytotoxic response in cultured HeLa cells and is unable to inhibit phagocytosis by macrophage-like J774 cells. Additionally, the yopB mutant strain was avirulent in the mouse model. No YopE or YopH protein were observed within HeLa cells infected with the yopB mutant strain, suggesting that the loss of virulence of the mutant strain was due to its inability to translocate Yop effector proteins through the target cell plasma membrane. Expression of YopB is necessary for Yersinia-induced lysis of sheep erythrocytes. Purified YopB was shown to have membrane disruptive activity in vitro. YopB-dependent haemolytic activity required cell contact between the bacteria and the erythrocytes and could be inhibited by high, but not low, molecular weight carbohydrates. Similarly, expression of YopE reduced haemolytic activity. Therefore, we propose that YopB is essential for the formation of a pore in the target cell membrane that is required for the cell-to-cell transfer of Yop effector proteins.

Modulation of virulence factor expression by pathogen target cell contact

Upon contact with the eukaryotic cell, Yersinia pseudotuberculosis increased the rate of transcription of virulence genes (yop), as determined by in situ monitoring of light emission from individual bacteria expressing luciferase under the control of the yopE promoter. The microbe-host interaction triggered export of LcrQ, a negative regulator of Yop expression, via the Yop-type III secretion system. The intracellular concentration of LcrQ was thereby lowered, resulting in increased expression of Yops. These results suggest a key role for the type III secretion system of pathogenic bacteria to coordinate secretion with expression of virulence factors after physical contact with the target cell.

YopH of Yersinia pseudotuberculosis interrupts early phosphotyrosine signalling associated with phagocytosis

The PTPase YopH of Yersinia is essential to the ability of these bacteria to block phagocytosis. Wild-type Yersinia pseudotuberculosis, but not the yopH mutant strain, resisted phagocytosis by J774 cells. Ingestion of a yopH mutant was dependent on tyrosine kinase activity. Transcomplementation with wild-type yopH restored the anti-phagocytic effect, whereas introduction of the gene encoding the catalytically inactive yopHC403A was without effect. The PTPase inhibitor orthovanadate impaired the anti-phagocytic effect of the wild-type strain, further demonstrating the importance of bacteria-derived PTPase activity for this event. The ability to resist phagocytosis indicates that the effect of the bacterium is immediately exerted when it becomes associated with the phagocyte. Within 30 s after the onset of infection, wild-type Y. pseudotuberculosis caused a YopH-dependent dephosphorylation of phosphotyrosine proteins in J774 cells. Furthermore, interaction of the cells with phagocytosable strains led to a rapid and transient increase in tyrosine phosphorylation of paxillin and some other proteins, an event dependent on the presence of the bacterial surface-located protein invasin. Co-infection with the phagocytosable strain and the wild-type strain abolished the induction of tyrosine phosphorylation. Taken together, the present findings demonstrate an immediate YopH-mediated dephosphorylation of macrophage phosphotyrosine proteins, suggesting that this PTPase acts by preventing early phagocytosis-linked signalling in the phagocyte.

The Yersinia YpkA Ser/Thr kinase is translocated and subsequently targeted to the inner surface of the HeLa cell plasma membrane

Multiple yop mutant strains of Yersinia pseudotuberculosis not expressing several virulence effector Yop proteins (YopH, M, E, K and YpkA) were engineered. When high-copy-number plasmids carrying the ypkA or the yopE gene with their endogenous promoters were introduced into the engineered strains, the corresponding Yop protein was secreted at high levels in vitro. These multiple yop mutant strains, when harbouring the yopE gene in trans, behaved as the wild-type strain with respect to YopB-dependent translocation of YopE through the HeLa cell plasma membrane. Using these multiple yop mutant strains, it was demonstrated that the YpkA Ser/Thr protein kinase mediates morphological alterations of infected cultured HeLa cells different from those mediated by YopE and YopH. Furthermore, YpkA is shown to be translocated by a YopB-dependent translocation mechanism from surface-located bacteria and subsequently targeted to the inner surface of the target-cell plasma membrane. The pattern of YpkA localization after infection suggests that this Yop effector is involved in interference with signal transduction.

Flagellin A is essential for the virulence of Vibrio anguillarum

A flagellin gene from the fish pathogen Vibrio anguillarum was cloned, sequenced, and mutagenized. The DNA sequence suggests that the flaA gene encodes a 40.1-kDa protein and is a single transcriptional unit. A polar mutation and four in-frame deletion mutations (180 bp deleted from the 5' end of the gene, 153 bp deleted from the 3' end of the gene, a double deletion of both the 180- and 153-bp deletions, and 942 bp deleted from the entire gene) were made. Compared with the wild type, all mutants were partially motile, and a shortening of the flagellum was seen by electron microscopy. Wild-type phenotypes were regained when the mutations were transcomplemented with the flaA gene. Protein analysis indicated that the flaA gene corresponds to a 40-kDa protein and that the flagellum consists of three additional flagellin proteins with molecular masses of 41, 42, and 45 kDa. N-terminal sequence analysis confirmed that the additional proteins were flagellins with N termini that are 82 to 88% identical to the N terminus of FlaA. Virulence studies showed that the N terminal deletion, the double deletion, and the 942-bp deletion increased the 50% lethal dose between 70- and 700-fold via immersion infection, whereas infection via intraperitoneal injection showed no loss in virulence. In contrast, the polar mutant and the carboxy-terminal deletion mutant showed approximately a 10(4)-fold increase in the 50% lethal dose by both immersion and intraperitoneal infection. In summary, FlaA is needed for crossing the fish integument and may play a role in virulence after invasion of the host.

Invasin of Yersinia pseudotuberculosis activates human peripheral B cells

The Yersinia pseudotuberculosis cell surface-located protein invasin was found to promote binding between the pathogen and resting peripheral B cells via beta 1 integrin receptors (CD29). B cells responded by expressing several activation markers and by growing, In contrast, T cells did not react, although these cells express CD29. An isogenic invA mutant failed to activate B cells. The mutation could be complemented by providing the invA+ gene in trans. Purified invasin alone did not activate B cells, although it was able to block the binding of bacteria to the cells.

Chemotactic motility is required for invasion of the host by the fish pathogen Vibrio anguillarum

The role of the flagellum and motility in the virulence of the marine fish pathogen Vibrio anguillarum was examined. Non-motile mutants were generated by transposon mutagenesis. Infectivity studies revealed that disruption of the flagellum and subsequent loss of motility correlated with an approximate 500-fold decrease in virulence when fish were inoculated by immersion in bacteria-containing water. However, the flagellar filament and motility were not required for pathogenicity following intraperitoneal injection of fish. The transposon-insertion site for six mutants was determined by cloning and sequencing of the Vibrio DNA flanking the transposon. V. anguillarum genes whose products showed strong homology to proteins with an established role in flagellum biosynthesis were identified. One of the aflagellate mutants had a transposon insertion in the rpoN gene of V. anguillarum. This rpoN mutant failed to grow at low concentrations of available iron and was avirulent by both the immersion and intraperitoneal modes of inoculation. A chemotaxis gene, cheR, was located upstream of one transposon insertion and an in-frame deletion was constructed in the coding region of this gene. The resulting non-chemotactic mutant exhibited wild-type pathogenicity when injected intra-peritoneally into fish but showed a decrease in virulence similar to that seen for the non-motile aflagellate mutants following immersion infection. Hence, chemotactic motility is a required function of the flagellum for the virulence of V. anguillarum.

Serological crossreactivity between Brucella abortus and Yersinia enterocolitica 0:9 II the use of Yersinia outer proteins for the specific detection of Yersinia enterocolitica infections in ruminants

Yersinia outer protein (YOP) preparations from Y. enterocolitica and Y. pseudotuberculosis were used as antigens in immunoblots for the detection of Yersinia infections in experimentally and naturally infected ruminants. Sera from 9 groups of animals were used: (1) 51 sera from cattle which were false-positive in the standard brucellosis serological tests, (2) 52 sera from brucellosis-negative cattle, (3) 51 sera from a deer herd in which 16 animals were positive in the brucellosis tests and Yersina species were isolated from 5 animals, (4) 50 sera from a deer herd in which sera from all animals were negative in the brucellosis tests, (5) 107 sera from brucellosis-negative cattle which were received from throughout New Zealand, (6) 30 sera from cattle naturally infected with B. abortus and from which B. abortus was isolated, (7) 55 sera from cattle naturally infected with B. abortus, (8) 26 sera from cattle experimentally infected with B. abortus, with mostly high titres in the conventional brucellosis tests, and (9) sera taken weekly from 3 cattle experimentally infected with Y. enterocolitica 0:9. In all 3 Y. enterocolitica 0:9 experimentally infected animals the antibody reactivity against major YOPs in the Y. enterocolitica and in the Y. pseudotuberculosis YOP preparation correlated well with the strength in the classical brucellosis tests and with the staining of smooth lipopolysaccharides (SLPS) in blots, thus confirming the usefulness of YOPs for the detection of Yersinia infections. Sera from naturally infected cattle and deer herds, regardless of whether they were false positive or negative in the brucellosis tests, showed high frequencies of staining in YOP blots (53-58% in cattle and 80-100% in deer), indicating a high prevalence of field infections with Yersinia species in New Zealand. In two of the three sera groups from B. abortus infected animals, antibodies against YOPs were detected with high frequency, showing that dual infections may be common and may interfere with differential serological testing.

Sequence of a novel virulence-mediating gene, virC, from Vibrio anguillarum

Previously, the double-transposon (Tn) mutant VAN20 of Vibrio anguillarum (Va) 775.17B was isolated. This mutant lacked a major surface antigen (MSA) suggested to be a lipopolysaccharide (LPS) and showed a 10(5)-fold increase in the 50% lethal dose (LD50) when fish were infected intraperitoneally. In this study, the two Tn insertion sites within the chromosome were identified, a plasmid insertion mutation was made at each locus in a more virulent strain of Va, NB10, and the virulence was analyzed. One mutant displayed a 10(4)-fold increase in LD50, whereas the second mutant showed the wild-type (wt) phenotype. However, both mutants still expressed the MSA, suggesting that there may be more than two Tn insertions in VAN20 or that a double mutation is required to prevent production of the MSA. The DNA locus for the virulent phenotype was cloned and sequenced. A potential transcriptional unit consisting of three putative open reading frames (ORFs) was identified. The Tn was located in the second ORF, virC (virulence). The first ORF (34.8 kDa) showed 30% homology to the Escherichia coli and Salmonella typhimurium cysG (cysteine) genes. The virC gene (51.4 kDa) and the third ORF (24 kDa) showed no homology to other proteins in GenBank. Plasmid insertion mutants were made within each of these ORFs and the virulence was assayed. Only the virC mutant showed a loss in virulence, indicating that virC is a novel gene that is essential for the virulence of Va.

Cell-surface-bound Yersinia translocate the protein tyrosine phosphatase YopH by a polarized mechanism into the target cell

YopH is translocated by cell-surface-bound bacteria through the plasma membrane to the cytosol of the HeLa cell. The transfer mechanism is contact dependent and polarizes the translocation to only occur at the contact zone between the bacterium and the target cell. More than 99% of the PTPase activity is associated with the HeLa cells. In contrast to the wild-type strain, the yopBD mutant cannot deliver YopH to the cytosol. Instead YopH is deposited in localized areas in the proximity of cell-associated bacteria. A yopN mutant secretes 40% of the total amount of YopH to the culture medium, suggesting a critical role of YopN in regulation of the polarized translocation. Evidence for a region in YopH important for its translocation through the plasma membrane of the target cell but not for secretion from the pathogen is provided.

Functional conservation of the secretion and translocation machinery for virulence proteins of yersiniae, salmonellae and shigellae

Virulent bacteria of the genera Yersinia, Shigella and Salmonella secrete a number of virulence determinants, Yops, Ipas and Sips respectively, by a type III secretion pathway. The IpaB protein of Shigella flexneri was expressed in Yersinia pseudotuberculosis and found to be secreted under the same conditions required for Yop secretion. Likewise, YopE was secreted by the wild-type strain LT2 of Salmonella typhimurium, but YopE was not secreted by the isogenic invA mutant. Secretion of both IpaB and YopE required their respective chaperones, IpgC and YerA. In addition, yopE-containing S. typhimurium expressed a YopE-mediated cytotoxicity on cultured HeLa cells. YopE was detected in the cytosol of the infected HeLa cells and the amount of translocated YopE correlated with the degree of cytotoxicity. Both translocation and cytotoxicity were prevented by the addition of gentamicin. Treatment of HeLa cells with cytochalasin D prior to infection prevented internalization of bacteria, but translocation of YopE was still observed. These results favour the hypothesis that YopE is translocated through the plasma membrane by surface-located bacteria. We propose that virulent Salmonella and Shigella deliver virulence effector molecules into the target cell through the utilization of a functionally conserved secretion/translocation machinery similar to that shown for Yersinia.

Yersinia pseudotuberculosis inhibits Fc receptor-mediated phagocytosis in J774 cells

Nonopsonized as well as immunoglobulin-G (IgG)-opsonized Yersinia pseudotuberculosis resists phagocytic uptake by the macrophage-like cell line J774 by a mechanism involving the plasmid-encoded proteins Yops. The tyrosine phosphatase YopH was of great importance for the antiphagocytic effect of the bacteria. YopH-negative mutants did not induce antiphagocytosis; instead, they were readily ingested, almost to the same extent as that of the translocation mutants YopB and YopD and the plasmid-cured strain. The bacterial determinant invasin was demonstrated to mediate phagocytosis of nonopsonized bacteria by these cells. In addition to inhibiting uptake of itself, Y. pseudotuberculosis also interfered with the phagocytic uptake of other types of prey: J774 cells that had been exposed to virulent Y. pseudotuberculosis exhibited a reduced capacity to ingest IgG-opsonized yeast particles. This effect was impaired when the bacterium-phagocyte interaction occurred in the presence of gentamicin, indicating a requirement for in situ bacterial protein synthesis. The Yersinia-mediated antiphagocytic effect on J774 cells was reversible: after 18 h in the presence of gentamicin, the phagocytic capacity of Yersinia-exposed J774 cells was completely restored. Inhibition of the uptake of IgG-opsonized yeast particles was dependent on the Yops in a manner similar to that seen for blockage of Yersinia phagocytosis. This similarity suggests that the pathogen affected a general phagocytic mechanism. Despite a marked reduction in the capacity to ingest IgG-opsonized yeast particles, no effect was observed on the binding of the prey. Taken together, these results demonstrate that Yop-mediated antiphagocytosis by Y. pseudotuberculosis affects regulatory functions downstream of the phagocytic receptor and thereby extends to other types of phagocytosis.

Virulence plasmid-encoded YopK is essential for Yersinia pseudotuberculosis to cause systemic infection in mice

The virulence plasmid common to pathogenic Yersinia species encodes a number of secreted proteins denoted Yops (Yersinia outer proteins). Here, we identify and characterize a novel plasmid-encoded virulence determinant of Yersinia pseudotuberculosis, YopK. The yopK gene was found to be conserved among the three pathogenic Yersinia species and to be homologous to the previously described yopQ and yopK genes of Y. enterocolitica and Y. pestis, respectively. Similar to the other Yops, YopK expression and secretion were shown to be regulated by temperature and by the extracellular Ca2+ concentration; thus, yopK is part of the yop regulon. In addition, YopK secretion was mediated by the specific Yop secretion system. In Y. pseudotuberculosis, YopK was shown neither to have a role in this bacterium's ability to resist phagocytosis by macrophages nor to cause cytotoxicity in HeLa cells. YopK was, however, shown to be required for the bacterium to cause a systemic infection in both intraperitoneally and orally infected mice. Characterization of the infection kinetics showed that, similarly to the wild-type strain, the yopK mutant strain colonized and persisted in the Peyer's patches of orally infected mice. A yopE mutant which is impaired in cytotoxicity and in antiphagocytosis was, however, found to be rapidly cleared from these lymphoid organs. Neither the yopK nor the yopE mutant strain could overcome the primary host defense and reach the spleen. This finding implies that YopK acts at a different level during the infections process than the antiphagocytic YopE cytotoxin does.

Characterization of the operon encoding the YpkA Ser/Thr protein kinase and the YopJ protein of Yersinia pseudotuberculosis

The Ser/Thr protein kinase YpkA, encoded by the virulence plasmid pIB1, is an indispensable virulence determinant of Yersinia pseudotuberculosis [E. E. Galyov, S. Håkansson, A. Forsberg, and H. Wolf-Watz, Nature (London) 361:730-732, 1993]. In this study, the organization of the ypkA-containing operon and the in vitro regulation of this transcriptional unit were characterized. The operon contains two structural genes, ypkA and yopJ, and is regulated by temperature and the extracellular concentration of Ca2+, as are the yop genes. The two proteins were secreted without posttranslational processing, showing that YpkA and YopJ belong to the Yop family. Mutational analysis revealed that, in contrast to all other Yop proteins so far studied, the YopJ protein was dispensable for virulence of Y. pseudotuberculosis.

The lcrB (yscN/U) gene cluster of Yersinia pseudotuberculosis is involved in Yop secretion and shows high homology to the spa gene clusters of Shigella flexneri and Salmonella typhimurium

Virulent bacteria of the genus Yersinia secrete a number of virulence determinants called Yops. These proteins lack typical signal sequences and are not posttranslationally processed. Two gene loci have been identified as being involved in the specific Yop secretion system (G. Cornelis, p. 231-265, In C. E. Hormache, C. W. Penn, and C. J. Smythe, ed., Molecular Biology of Bacterial Infection, 1992; S. C. Straley, G. V. Plano, E. Skrzypek, P. L. Haddix, and K. A. Fields, Mol. Microbiol. 8:1005-1010, 1993). Here, we have shown that the lcrB/virB locus (yscN to yscU) encodes gene products essential for Yop secretion. As in previously described secretion apparatus mutants, expression of the Yop proteins was decreased in the yscN/U mutants. An lcrH yscR double mutant expressed the Yops at an increased level but did not secrete Yops into the culture supernatant. The block in Yop expression of the ysc mutants was also circumvented by overexpression of the activator LcrF in trans. Although the Yops were expressed in elevated amounts, the Yops were still not exported. This analysis showed that the ysc mutants were unable to secrete Yops and that they were also affected in the negative Ca(2+)-regulated loop. The yscN/U genes showed remarkably high homology to the spa genes of Shigella flexneri and Salmonella typhimurium with respect to both individual genes and gene organization. These findings indicate that the genes originated from a common ancestor.

Target cell contact triggers expression and polarized transfer of Yersinia YopE cytotoxin into mammalian cells

Pathogenic bacteria of the species Yersinia, including Yersinia pestis, block phagocytosis by macrophages. This process involves the YopE protein, which induces disruption of the host cell actin microfilament structure. Here, we show that the contact between the pathogen and the mammalian cell induces expression and then polarized transfer of YopE into the eukaryotic cell. While the bacteria remain at the surface of the target cell, the YopE cytotoxin is transferred through the host cell plasma membrane and YopE is only recovered within the cytosol of the target cell. The results suggest that the pathogen senses cell structures and focuses the transfer of YopE to occur solely at the interaction zone between the bacterium and the eukaryotic cell. The regulation of this process is shown to involve surface-located YopN sensor protein of the bacterium.

Regulation and polarized transfer of the Yersinia outer proteins (Yops) involved in antiphagocytosis

Pathogenic Yersinia express a number of strictly regulated, plasmid-encoded virulence determinants (Yops), some of which are important in enabling the pathogen to block phagocytosis. The events mediating antiphagocytosis and the regulation of this process are becoming increasingly well understood.

Characterization of a novel chromosomal virulence locus involved in expression of a major surface flagellar sheath antigen of the fish pathogen Vibrio anguillarum

The fish pathogenic bacterium Vibrio anguillarum 775.17B was mutated by the use of transposon Tn5-132. Two hundred independent exconjugants were isolated and screened for a reduction of virulence in experimental infections of rainbow trout (Onchorhynchus mykiss). Two of these exconjugants, VAN20 and VAN70, showed a significant reduction in virulence after both intraperitoneal and immersion infections. The avirulent mutants showed no loss of any previously suggested virulence determinants of V. anguillarum. One of the mutants (VAN70) was further characterized. DNA sequence analysis revealed two open reading frames, the gene into which Tn5-132 had been inserted (virA) and a closely linked upstream gene (virB). A virB mutant of 775.17B, NQ706, was isolated and also shown to be avirulent. The deduced amino acid sequences of virA and virB correspond to proteins with molecular weights of 36,000 and 42,000, respectively. Insertional mutagenesis of the corresponding virA and virB genes of a clinical isolate of V. anguillarum, serotype O1, also resulted in avirulence. In immunoblot experiments, the total cell lysates of VAN70 (virA) and NQ706 (virB) did not respond to a rabbit polyclonal antiserum directed against whole cells of 775.17B (wild type). This suggests that virA and virB are involved in the biosynthesis of a major surface antigen important for the virulence of V. anguillarum. Immunogold electron microscopy showed that a constituent of the flagellar sheath was expressed by 775.17B (wild type) but not by VAN70 (virA) and NQ706 (virB), suggesting that the major surface antigen lacking in VAN70 and NQ706 is located on the outer sheath of the flagellum. Analysis of this major surface antigen revealed it likely to be lipopolysaccharide. Further analysis showed that the flagellum and the major surface antigen were expressed in vivo during fish infections.

A secreted protein kinase of Yersinia pseudotuberculosis is an indispensable virulence determinant

Phosphorylation of proteins catalysed by protein kinases is associated with central functions in growth and proliferation of the eukaryotic cell, and kinases are particularly important in the signal transduction pathways. Enterobacterial protein kinases are structurally and functionally different from eukaryotic protein kinases, and no prokaryotic kinase has so far been described implicating a direct role for this activity in virulence. Virulent Yersinia possess a common virulence plasmid that encodes a number of secreted proteins (Yops), of which YopH has protein-tyrosine phosphatase activity with a key function in the block of phagocytosis by the pathogen. Here we report that the virulence plasmid of Yersinia pseudotuberculosis encodes a secreted protein kinase (YpkA) with extensive homology to eukaryotic Ser/Thr protein kinases. Specific mutants of ypkA resulted in avirulent strains. Thus, YpkA is, to our knowledge, the first reported prokaryotic secreted protein kinase involved in pathogenicity, presumably by interfering with the signal transduction pathways of the target cell.

YopB and YopD constitute a novel class of Yersinia Yop proteins

Virulent Yersinia species harbor a common plasmid that encodes essential virulence determinants (Yersinia outer proteins [Yops]), which are regulated by the extracellular stimuli Ca2+ and temperature. The V-antigen-encoding operon has been shown to be involved in the Ca(2+)-regulated negative pathway. The genetic organization of the V-antigen operon and the sequence of the lcrGVH genes were recently presented. The V-antigen operon was shown to be a polycistronic operon having the gene order lcrGVH-yopBD (T. Bergman, S. Håkansson, A. Forsberg, L. Norlander, A. Macellaro, A. Bäckman, I. Bölin, and H. Wolf-Watz, J. Bacteriol. 173:1607-1616, 1991; S. B. Price, K. Y. Leung, S. S. Barve, and S. C. Straley, J. Bacteriol. 171:5646-5653, 1989). We present here the sequence of the distal part of the V-antigen operons of Yersinia pseudotuberculosis and Yersinia enterocolitica. The sequence information encompasses the yopB and yopD genes and a downstream region in both species. We conclude that the V-antigen operon ends with the yopD gene. This conclusion is strengthened by the observation of an insertion-like element downstream of the yopD gene. The translational start codons of YopB and YopD have been identified by N-terminal amino acid sequencing. By computer analysis, the yopB and yopD gene products were found to be possible transmembrane proteins, and YopD was shown to contain an amphipathic alpha-helix in its carboxy terminus. These findings contrast with the general globular pattern observed for other Yops. Homology between Yersinia LcrH and Shigella flexneri IppI and between Yersinia YopB and S. flexneri IpaB was found, suggesting conservation of this locus between these two genera. YopB was also found to have a moderate level of homology, especially within the hydrophobic regions, to members of the RTX protein family of alpha-hemolysins and leukotoxins, indicating that YopB might exhibit a similar function.