Heterogeneity of the Yersinia YopM protein

Bimolecular fluorescence complementation assay to explore protein-protein interactions of the Yersinia virulence factor YopM

Yersinia outer protein M (YopM) is one of the effector proteins and essential for virulence. YopM is delivered by the Yersinia type III secretion system (T3SS) into the host cell, where it shows immunosuppressive effect through interaction with host proteins. Therefore, protein-protein interactions of YopM is significant to understand its molecular mechanism. In this study, we aimed to explore protein-protein interactions of YopM with the two components of T3SS, namely LcrV and LcrG. We used bimolecular fluorescence complementation (BiFC) assay and monitored the reassembly of green fluorescence protein in Escherichia coli. As an indicator of the protein-protein interaction, we monitored the in vivo reconstitution of fluorescence by measuring fluorescence intensity and imaging the cells under fluorescence microscope. We showed, for the first time, that YopM interacts with LcrG, but not with LcrV. Here, we propose BiFC assay as a simple method to screen novel interaction partners of YopM.

The cell-penetrating YopM protein-functionalized quantum dot-plasmid DNA conjugate as a novel gene delivery vector

Non-viral gene delivery systems have great potential for safe and efficient gene therapy, while inefficient cellular and nuclear uptake remain as the major hurdles. Novel approaches are needed to enhance the transfection efficiency of non-viral vectors. In accordance with this need, the objective of this study was to construct a non-viral vector that could achieve gene delivery without using additional lipid-based transfection agent. We aimed to impart self-delivery property to a non-viral vector by using the cell and nucleus penetrating properties of YopM proteins from the three Yersinia spp. (Y. pestis, Y. enterocolotica and Y. pseudotuberculosis). Plasmid DNA (pDNA) encoding green fluorescent protein (GFP) was labeled with quantum dots (QDs) via peptide-nucleic acid (PNA) recognition site. Recombinant YopM protein was then attached to the conjugate via a second PNA recognition site. The YopM ̶ QDs ̶ pDNA conjugate was transfected into HeLa cells without using additional transfection reagent. All three conjugates produced GFP fluorescence, indicating that the plasmid was successfully delivered to the nucleus. As control, naked pDNA was transfected into the cells by using a commercial transfection reagent. The Y. pseudotuberculosis YopM-functionalized conjugate achieved the highest GFP expression, compared to other two YopM proteins and the transfection reagent. To the best of our knowledge, YopM protein was used for the first time in a non-viral gene delivery vector.

Distribution and Evolution of Yersinia Leucine-Rich Repeat Proteins

Leucine-rich repeat (LRR) proteins are widely distributed in bacteria, playing important roles in various protein-protein interaction processes. In Yersinia, the well-characterized type III secreted effector YopM also belongs to the LRR protein family and is encoded by virulence plasmids. However, little has been known about other LRR members encoded by Yersinia genomes or their evolution. In this study, the Yersinia LRR proteins were comprehensively screened, categorized, and compared. The LRR proteins encoded by chromosomes (LRR1 proteins) appeared to be more similar to each other and different from those encoded by plasmids (LRR2 proteins) with regard to repeat-unit length, amino acid composition profile, and gene expression regulation circuits. LRR1 proteins were also different from LRR2 proteins in that the LRR1 proteins contained an E3 ligase domain (NEL domain) in the C-terminal region or an NEL domain-encoding nucleotide relic in flanking genomic sequences. The LRR1 protein-encoding genes (LRR1 genes) varied dramatically and were categorized into 4 subgroups (a to d), with the LRR1a to -c genes evolving from the same ancestor and LRR1d genes evolving from another ancestor. The consensus and ancestor repeat-unit sequences were inferred for different LRR1 protein subgroups by use of a maximum parsimony modeling strategy. Structural modeling disclosed very similar repeat-unit structures between LRR1 and LRR2 proteins despite the different unit lengths and amino acid compositions. Structural constraints may serve as the driving force to explain the observed mutations in the LRR regions. This study suggests that there may be functional variation and lays the foundation for future experiments investigating the functions of the chromosomally encoded LRR proteins of Yersinia.

Immunosuppressive Yersinia Effector YopM Binds DEAD Box Helicase DDX3 to Control Ribosomal S6 Kinase in the Nucleus of Host Cells

Yersinia outer protein M (YopM) is a crucial immunosuppressive effector of the plaque agent Yersinia pestis and other pathogenic Yersinia species. YopM enters the nucleus of host cells but neither the mechanisms governing its nucleocytoplasmic shuttling nor its intranuclear activities are known. Here we identify the DEAD-box helicase 3 (DDX3) as a novel interaction partner of Y. enterocolitica YopM and present the three-dimensional structure of a YopM:DDX3 complex. Knockdown of DDX3 or inhibition of the exportin chromosomal maintenance 1 (CRM1) increased the nuclear level of YopM suggesting that YopM exploits DDX3 to exit the nucleus via the CRM1 export pathway. Increased nuclear YopM levels caused enhanced phosphorylation of Ribosomal S6 Kinase 1 (RSK1) in the nucleus. In Y. enterocolitica infected primary human macrophages YopM increased the level of Interleukin-10 (IL-10) mRNA and this effect required interaction of YopM with RSK and was enhanced by blocking YopM's nuclear export. We propose that the DDX3/CRM1 mediated nucleocytoplasmic shuttling of YopM determines the extent of phosphorylation of RSK in the nucleus to control transcription of immunosuppressive cytokines.

LMOh7858_0369, a gene encoding a putative leucine-rich repeat-containing protein, is required for virulence of Listeria monocytogenes

Listeria monocytogenes possesses the highest number of leucine-rich repeat (LRR)-containing proteins among all Gram-positive bacteria; these LRR-containing molecules are known as the 'internalin' family. To understand the functions of largely uncharacterized LRR-containing molecules, we constructed seven deletion mutants in the L. monocytogenes H7858 strain targeting genes in this family and tested their virulence. Among the seven mutants, the ΔLMOh7858_0369 strain and the ΔLMOh7858_2546 strain showed significantly impaired invasiveness of HepG2 cells. We further tested the virulence of these two strains in the intravascular sepsis model using BALB/c mice. Interestingly, the ΔLMOh7858_0369 strain showed significant reduction in organ colonization, bacteremia and invasion of the brain compared with the parental wild-type strain. Host immune responses to listerial intravascular infection were measured at 24 and 72 h post-infection. Transcript levels of several proinflammatory cytokines and chemokines were significantly lower when induced by the ΔlmOh7858_0369 strain than when induced by the wild type. These results suggest that the putative LRR-containing protein encoded by LMOh7858_0369 might be a novel virulence factor of the L. monocytogenes H7858 strain.

Manipulation of pro-inflammatory cytokine production by the bacterial cell-penetrating effector protein YopM is independent of its interaction with host cell kinases RSK1 and PRK2

The effector protein Yersinia outer protein M (YopM) of Yersinia enterocolitica has previously been identified and characterized as the first bacterial cell-penetrating protein (CPP). We found that recombinant YopM (rYopM) enters different eukaryotic cell types and downregulates the expression of several pro-inflammatory cytokines (e.g., tumor necrosis factor-α [TNF-α]) after autonomous translocation. After infection with Y. enterocolitica or transfection of host cells, YopM interacts with isoforms of the two kinases ribosomal S6 protein kinase (RSK) and protein kinase C-related kinase (PRK). This interaction caused sustained RSK activation due to interference with dephosphorylation. Here we demonstrate by co-immunoprecipitation that rYopM interacts with RSK and PRK following cell-penetration. We show that autonomously translocated rYopM forms a trimeric complex with different RSK and PRK isoforms. Furthermore, we constructed a series of truncated versions of rYopM to map the domain required for the formation of the complex. The C-terminus of rYopM was identified to be essential for the interaction with RSK1, whereas any deletion in rYopM's leucin-rich repeat domains abrogated PRK2 binding. Moreover, we found that the interaction of cell-penetrating rYopM with RSK led to enhanced autophosphorylation of this kinase at serine 380. Finally, we investigated whether downstream signaling of the trimeric rYopM-RSK/PRK complex modulates the expression of pro-inflammatory TNF-α. Here, we could exclude that interaction with RSK1 and PRK2 is essential for the anti-inflammatory effects of rYopM.

Current activities of the Yersinia effector protein YopM

Yersinia outer protein M (YopM) belongs to the group of Yop effector proteins, which are highly conserved among pathogenic Yersinia species. During infection, the effectors are delivered into the host cell cytoplasm via the type 3 secretion system to subvert the host immune response and support the survival of Yersinia. In contrast to the other Yop effectors, YopM does not possess a known enzymatic activity and its molecular mechanism(s) of action remain(s) poorly understood. However, YopM was shown to promote colonization and dissemination of Yersinia, thus being crucial for the pathogen's virulence in vivo. Moreover, YopM interacts with several host cell proteins and might utilize them to execute its anti-inflammatory activities. The results obtained so far indicate that YopM is a multifunctional protein that counteracts the host immune defense by multiple activities, which are at least partially independent of each other. Finally, its functions seem to be also influenced by differences between the specific YopM isoforms expressed by Yersinia subspecies. In this review, we focus on the global as well as more specific contribution of YopM to virulence of Yersinia during infection and point out the various extra- and intracellular molecular functions of YopM. In addition, the novel cell-penetrating ability of recombinant YopM and its potential applications as a self-delivering immunomodulatory therapeutic will be discussed.

IQGAP1 is important for activation of caspase-1 in macrophages and is targeted by Yersinia pestis type III effector YopM

YopM is a leucine-rich repeat (LRR)-containing effector in several Yersinia species, including Yersinia pestis and Y. pseudotuberculosis. Different Yersinia strains encode distinct YopM isoforms with variable numbers of LRRs but conserved C-terminal tails. A 15-LRR isoform in Y. pseudotuberculosis YPIII was recently shown to bind and inhibit caspase-1 via a YLTD motif in LRR 10, and attenuation of YopM⁻ YPIII was reversed in mice lacking caspase-1, indicating that caspase-1 inhibition is a major virulence function of YopM^YPIII. To determine if other YopM proteins inhibit caspase-1, we utilized Y. pseudotuberculosis strains natively expressing a 21-LRR isoform lacking the YLTD motif (YopM³²⁷⁷⁷) or ectopically expressing a Y. pestis 15-LRR version with a functional (YopM^KIM) or inactivated (YopM^KIM D₂₇₁A) YLTD motif. Results of mouse and macrophage infections with these strains showed that YopM³²⁷⁷⁷, YopM^KIM, and YopM^KIM D₂₇₁A inhibit caspase-1 activation, indicating that the YLTD motif is dispensable for this activity. Analysis of YopM^KIM deletion variants revealed that LRRs 6 to 15 and the C-terminal tail are required to inhibit caspase-1 activation. YopM³²⁷⁷⁷, YopM^KIM, and YopM^KIM deletion variants were purified, and binding partners in macrophage lysates were identified. Caspase-1 bound to YopM^KIM but not YopM³²⁷⁷⁷. Additionally, YopM^KIM bound IQGAP1 and the use of Iqgap1^−/− macrophages revealed that this scaffolding protein is important for caspase-1 activation upon infection with YopM⁻Y. pseudotuberculosis. Thus, while multiple YopM isoforms inhibit caspase-1 activation, their variable LRR domains bind different host proteins to perform this function and the LRRs of YopM^KIM target IQGAP1, a novel regulator of caspase-1, in macrophages.

Activation of caspase-1, mediated by macromolecular complexes termed inflammasomes, is important for innate immune defense against pathogens. Pathogens can, in turn, subvert caspase-1-dependent responses through the action of effector proteins. For example, the Yersinia effector YopM inhibits caspase-1 activation by arresting inflammasome formation. This caspase-1 inhibitory activity has been studied in a specific YopM isoform, and in this case, the protein was shown to act as a pseudosubstrate to bind and inhibit caspase-1. Different Yersinia strains encode distinct YopM isoforms, many of which lack the pseudosubstrate motif. We studied additional isoforms and found that these YopM proteins inhibit caspase-1 activation independently of a pseudosubstrate motif. We also identified IQGAP1 as a novel binding partner of the Yersinia pestis YopM^KIM isoform and demonstrated that IQGAP1 is important for caspase-1 activation in macrophages infected with Yersinia. Thus, this study reveals new insights into inflammasome regulation during Yersinia infection.

Rabbit monoclonal antibodies directed at the T3SS effector protein YopM identify human pathogenic Yersinia isolates

The Yersinia outer protein M (YopM) is a type 3 secretion system (T3SS)-dependent effector protein of Yersinia enterocolitica, Yersinia pseudotuberculosis and Yersinia pestis. Although YopM is indispensable for full virulence, its molecular functions still remain largely elusive. Recently, we could identify the recombinant YopM (rYopM) protein derived from the Y. enterocolitica strain 8081 (JB580) as a cell-penetrating protein, which down-regulates the expression of various pro-inflammatory cytokines including TNFα. In this study, we have generated rabbit monoclonal anti-YopM antibodies (RabMabs). RabMabs were characterized by SDS-PAGE and Western blotting using various truncated versions of rYopM to identify epitope-containing domains. RabMabs recognizing either the N- or C-terminus of YopM were characterized further and validated using a collection of 61 pathogenic and non-pathogenic Yersinia strains as well as exemplary strains of major intestinal bacterial pathogens such as Salmonella enterica ssp. enterica, Shigella flexneri and intestinal pathogenic Escherichia coli. RabMab 41.3 directed at the N-terminus of YopM of Y. enterocolitica strain 8081 recognized all YopM-expressing pathogenic Yersinia strains analyzed in this study but failed to recognize non-pathogenic isolates. Thus, RabMab 41.3 might be applicable for the detection of pathogenic Yersinia strains.

Autonomous translocation and intracellular trafficking of the cell-penetrating and immune-suppressive effector protein YopM

Extracellular Gram-negative pathogenic bacteria target essential cytoplasmic processes of eukaryotic cells by using effector protein delivery systems such as the type III secretion system (T3SS). These secretion systems directly inject effector proteins into the host cell cytoplasm. Among the T3SS-dependent Yop proteins of pathogenic Yersinia, the function of the effector protein YopM remains enigmatic. In a recent study, we demonstrated that recombinant YopM from Yersinia enterocolitica enters host cells autonomously without the presence of bacteria and thus identified YopM as a novel bacterial cell-penetrating protein. Following entry YopM down-regulates expression of pro-inflammatory cytokines such as tumor necrosis factor α. These properties earmark YopM for further development as a novel anti-inflammatory therapeutic. To elucidate the uptake and intracellular targeting mechanisms of this bacterial cell-penetrating protein, we analyzed possible routes of internalization employing ultra-cryo electron microscopy. Our results reveal that under physiological conditions, YopM enters cells predominantly by exploiting endocytic pathways. Interestingly, YopM was detected free in the cytosol and inside the nucleus. We could not observe any colocalization of YopM with secretory membranes, which excludes retrograde transport as the mechanism for cytosolic release. However, our findings indicate that direct membrane penetration and/or an endosomal escape of YopM contribute to the cytosolic and nuclear localization of the protein. Surprisingly, even when endocytosis is blocked, YopM was found to be associated with endosomes. This suggests an intracellular endosome-associated transport of YopM.

Interleukin-10 induction is an important virulence function of the Yersinia pseudotuberculosis type III effector YopM

Pathogenic Yersinia species modulate host immune responses through the activity of a plasmid-encoded type III secretion system and its associated effector proteins. One effector, YopM, is a leucine-rich-repeat-containing protein that is important for virulence in murine models of Yersinia infection. Although the mechanism by which YopM promotes virulence is unknown, we previously demonstrated that YopM was required for the induction of high levels of the immunosuppressive cytokine interleukin-10 (IL-10) in sera of C57BL/6J mice infected with Yersinia pseudotuberculosis. To determine if IL-10 production is important for the virulence function of YopM, C57BL/6J or congenic IL-10⁻/⁻ mice were infected intravenously with wild-type or yopM mutant Y. pseudotuberculosis strains. Analysis of cytokine levels in serum and bacterial colonization in the spleen and liver showed that YopM is required for IL-10 induction in C57BL/6J mice infected with either the IP32953 or the 32777 strain of Y. pseudotuberculosis, demonstrating that the phenotype is conserved in the species. In single-strain infections, the ability of the 32777ΔyopM mutant to colonize the liver was significantly increased by the delivery of exogenous IL-10 to C57BL/6J mice. In mixed infections, the competitive advantage of a yopM⁺ 32777 strain over an isogenic yopM mutant to colonize spleen and liver, as observed for C57BL/6J mice, was significantly reduced in IL-10⁻/⁻ animals. Thus, by experimentally controlling IL-10 levels in a mouse infection model, we obtained evidence that the induction of this cytokine is an important mechanism by which YopM contributes to Y. pseudotuberculosis virulence.

Deletion of internal structured repeats increases the stability of a leucine-rich repeat protein, YopM

Mapping the stability distributions of proteins in their native folded states provides a critical link between structure, thermodynamics, and function. Linear repeat proteins have proven more amenable to this kind of mapping than globular proteins. C-terminal deletion studies of YopM, a large, linear leucine-rich repeat (LRR) protein, show that stability is distributed quite heterogeneously, yet a high level of cooperativity is maintained [1]. Key components of this distribution are three interfaces that strongly stabilize adjacent sequences, thereby maintaining structural integrity and promoting cooperativity. To better understand the distribution of interaction energy around these critical interfaces, we studied internal (rather than terminal) deletions of three LRRs in this region, including one of these stabilizing interfaces. Contrary to our expectation that deletion of structured repeats should be destabilizing, we find that internal deletion of folded repeats can actually stabilize the native state, suggesting that these repeats are destabilizing, although paradoxically, they are folded in the native state. We identified two residues within this destabilizing segment that deviate from the consensus sequence at a position that normally forms a stacked leucine ladder in the hydrophobic core. Replacement of these nonconsensus residues with leucine is stabilizing. This stability enhancement can be reproduced in the context of nonnative interfaces, but it requires an extended hydrophobic core. Our results demonstrate that different LRRs vary widely in their contribution to stability, and that this variation is context-dependent. These two factors are likely to determine the types of rearrangements that lead to folded, functional proteins, and in turn, are likely to restrict the pathways available for the evolution of linear repeat proteins.

YopK regulates the Yersinia pestis type III secretion system from within host cells

The pathogenic Yersinia species share a conserved type III secretion system, which delivers cytotoxic effectors known as Yops into target mammalian cells. In all three species, YopK (also called YopQ) plays an important role in regulating this process. In cell culture infections, yopK mutants inject higher levels of Yops, leading to increase cytotoxicity; however, in vivo the same mutants are highly attenuated. In this work, we investigate the mechanism behind this paradox. Using a β-lactamase reporter assay to directly measure the effect of YopK on translocation, we demonstrated that YopK controls the rate of Yop injection. Furthermore, we find that YopK cannot regulate effector Yop translocation from within the bacterial cytosol. YopE is also injected into host cells and was previously shown to contribute to regulation of the injectisome. In this work we show that YopK and YopE work at different steps to regulate Yop injection, with YopK functioning independently of YopE. Finally, by expressing YopK within tissue culture cells, we confirm that YopK regulates translocation from inside the host cell, and we show that cells pre-loaded with YopK are resistant to Yop injection. These results suggest a novel role for YopK in controlling the Yersinia type III secretion system.

Destabilization of YopE by the ubiquitin-proteasome pathway fine-tunes Yop delivery into host cells and facilitates systemic spread of Yersinia enterocolitica in host lymphoid tissue

Pathogenic Yersinia species inject a panel of Yop virulence proteins by type III protein secretion into host cells to modulate cellular defense responses. This enables the survival and dissemination of the bacteria in the host lymphoid tissue. We have previously shown that YopE of the Y. enterocolitica serogroup O8 is degraded in the host cell through the ubiquitin-proteasome pathway. YopE normally manipulates rearrangements of the actin cytoskeleton and triggers phagocytosis resistance. To shed light into the physiological role of YopE inactivation, we mutagenized the lysine polyubiquitin acceptor sites of YopE in the Y. enterocolitica serogroup O8 virulence plasmid. The resulting mutant strain escaped polyubiquitination and degradation of YopE and displayed increased intracellular YopE levels, which was accompanied by a pronounced cytotoxic effect on infected cells. Despite its intensified activity on cultured cells, the Yersinia mutant with stabilized YopE showed reduced dissemination into liver and spleen following enteral infection of mice. Furthermore, the accumulation of degradation-resistant YopE was accompanied by the diminished delivery of YopP and YopH into cultured, Yersinia-infected cells. A role of YopE in the regulation of Yop translocation has already been described. Our results imply that the inactivation of YopE by the proteasome could be a tool to ensure intermediate intracellular YopE levels, which may effectuate optimized Yop injection into host cells. In this regard, Y. enterocolitica O8 appears to exploit the host ubiquitin proteasome system to destabilize YopE and to fine-tune the activities of the Yop virulence arsenal on the infected host organism.

Yersinia virulence factor YopM induces sustained RSK activation by interfering with dephosphorylation

Background

Pathogenic yersiniae inject several effector proteins (Yops) into host cells, which subverts immune functions and enables the bacteria to survive within the host organism. YopM, whose deletion in enteropathogenic yersiniae results in a dramatic loss of virulence, has previously been shown to form a complex with and activate the multifunctional kinases PKN2 and RSK1 in transfected cells.

Methodology/Principal Findings

In a near physiological approach with double-affinity-tagged YopM being translocated into the macrophage cell line J774A.1 via the natural type three secretion system of Yersinia we verified the interaction of YopM with PKN2 and RSK1 and detected association with additional PKN and RSK isoforms. In transfected and infected cells YopM induced sustained phosphorylation of RSK at its activation sites serine-380 and serine-221 even in the absence of signalling from its upstream kinase ERK1/2, suggesting inhibition of dephosphorylation. ATP-depletion and in vitro assays using purified components directly confirmed that YopM shields RSK isoforms from phosphatase activity towards serines 380 and 221.

Conclusions/Significance

Our study suggests that during Yersinia infection YopM induces sustained activation of RSK by blocking dephosphorylation of its activatory phosphorylation sites. This may represent a novel mode of action of a bacterial virulence factor.

Delineation of regions of the Yersinia YopM protein required for interaction with the RSK1 and PRK2 host kinases and their requirement for interleukin-10 production and virulence

The YopM protein of Yersinia sp. is a type III secreted effector that is required for virulence in murine models of infection. YopM has previously been shown to contain leucine-rich repeats (LRRs) and to interact with two host kinases, RSK1 and PRK2, although the consequence of these interactions is unknown. A series of YopM proteins missing different numbers of LRRs or a C-terminal domain were produced and used for in vitro binding reactions to map domains required for interaction with RSK1 and PRK2. A C-terminal domain of YopM (from LRR12 to the C terminus) was shown to be required for interaction with RSK1, while an internal portion encompassing LRR6 to LRR15 was shown to be required for interaction with PRK2. The virulence of a Yersinia pseudotuberculosis Delta yopM mutant in mice via an intravenous route of infection was significantly attenuated. At day 4 postinfection, there were significantly increased levels of gamma interferon and reduced levels of interleukin-18 (IL-18) and IL-10 in the serum of the Delta yopM-infected mice compared to that of mice infected with the wild type, suggesting that YopM action alters the balance of these key cytokines to promote virulence. The PRK2 and RSK1 interaction domains of YopM were both required for IL-10 induction in vivo, irrespective of splenic colonization levels. In an orogastric model of Y. pseudotuberculosis infection, a Delta yopM mutant was defective in dissemination from the intestine to the spleen and significantly reduced in virulence. In addition, Y. pseudotuberculosis mutants expressing YopM proteins unable to interact with either RSK1 (YopM Delta 12-C) or PRK2 (YopM Delta 6-15) were defective for virulence in this assay, indicating that both interaction domains are important for YopM to promote pathogenesis.

Production and characterization of monoclonal antibodies against YopM effector protein of Yersinia pestis

The YopM is an essential virulence effector produced by the bubonic plague bacterium. Yersinia pestis specific PCR gene was developed using 780 bp fragment of yopM gene. The PCR product was further cloned (in pUC57) an subcloned (pQE32 expression vector) and transformed in SG13009 E. coli host cells. The IPTG-induced recombinant protein was expressed at approximately 32 kDa region by SDS-PAGE. The recombinant protein was with 80% purity and 3mg/mL of concentration. Polyclonal and monoclonal antibodies (MAb) were generated. A total number of nine specific monoclonal antibodies obtained reacted at 43 kDa native protein of Y. pestis. Both the PCR-based assay and immunoassays were evaluated on Indian Y. pestis strains. Isolates recovered from outbreak region were positive, whereas isolates recovered from the surveillance region were negative (except one) by yopM gene PCR- and MAb-based dot-ELISA. The PCR- and ELISA-based systems developed in the present study might be utilized for detection or strain typing of Y. pestis alone or in conjunction with virulence markers such as F1 (fraction 1) and Pla (plasminogen activator).

YopM of Yersinia enterocolitica specifically interacts with alpha1-antitrypsin without affecting the anti-protease activity

It was previously shown that alpha1-antitrypsin (AAT) interacts with the type III secreted (T3S) EspB and EspD proteins of enteropathogenic Escherichia coli (EPEC), resulting in reduced functionality of the proteins. To determine if AAT is also able to interact with T3S proteins of other pathogens, the binding of AAT to Yop proteins of Yersinia enterocolitica was analysed. AAT did not interact with YopB or YopD, which have functions in type III translocation similar to EspB and EspD in EPEC, but specifically interacts with YopM, a member of the leucine-rich repeat (LRR) family of proteins, in overlay and pull-down assays. To determine regions of YopM involved in AAT binding, various N- and C-terminally truncated versions of YopM were recombinantly expressed, and their ability to interact with AAT analysed. All versions tested were able to bind AAT, indicating that at least eight LRR of YopM are sufficient for AAT interaction. The main physiological role of AAT is to inhibit neutrophil elastase; however, elastase was efficiently inhibited by AAT in the presence and absence of YopM, indicating that YopM does not interfere with the anti-protease inhibition activity of AAT, and that the domain of AAT interacting with YopM is not identical to AAT's protease interaction domain. Furthermore, it was shown that elastase efficiently degrades YopM and other Yop proteins. The data suggest that AAT has additional functions in the host response against bacterial infections that are not related to its anti-protease activity.

Virulence markers of LCR plasmid in Indian isolates of Yersinia pestis

Presence of 10 important yop genes in Yersinia pestis isolates (18 in number) of Indian origin from 1994 plague outbreak regions of Maharashtra (6 Rattus rattus & Tetera indica rodents) and Gujarat (11 from human patients, 1 from R. rattus) and from plague endemic regions of the Deccan plateau (8 from T. indica) was located by PCR and specific enzyme immunoassay. PCRs were standardized for six effector yops (YopE, YopH, YopJ, YopM, YopO and YopT), three translocator yops (YopB, YopD and YopK) and a regulator LcrV gene. Amplification of all the 10 yop genes was observed in isolates recovered from pneumonic patients and in 5 of 7 rodents from outbreak regions. Among these, amplification of the yopD gene was absent in all eight isolates, and that of yopM in all except one (10R). One of the isolates from rodents of the Deccan plateau (24H) was consistently negative for all the yops. Cloning and expression of truncated yopM (780 bp), yopB (700 bp) and lcrV (796 bp) genes in pQE vectors with SG13009 host cells yielded recombinant proteins for generation of monoclonal antibodies for further use in enzyme immunoassay. Ten stable reactive clones for YopB, nine for YopM and six for LcrV were obtained, all of them exhibiting specific reactions only to Y. pestis. Testing of 26 Y. pestis isolates by monoclonal antibody dot-ELISA and Western blotting provided results identical to PCR, suggesting that the isolates that failed to show PCR amplification also had no expression of their respective proteins. The Y. pestis isolates of outbreak regions had their virulence factors intact in the LCR plasmid. Yersinia pestis isolates recovered from rodents of the Deccan plateau were relatively heterogeneous. It appears that a long residency of Y. pestis of nearly 100 years in the enzootic plague foci has resulted in shedding of virulence genes in the LCR plasmid region in a fairly large proportion of the organisms, possibly due to natural recombination.

Insights into the evolution of Yersinia pestis through whole-genome comparison with Yersinia pseudotuberculosis

Yersinia pestis, the causative agent of plague, is a highly uniform clone that diverged recently from the enteric pathogen Yersinia pseudotuberculosis. Despite their close genetic relationship, they differ radically in their pathogenicity and transmission. Here, we report the complete genomic sequence of Y. pseudotuberculosis IP32953 and its use for detailed genome comparisons with available Y. pestis sequences. Analyses of identified differences across a panel of Yersinia isolates from around the world reveal 32 Y. pestis chromosomal genes that, together with the two Y. pestis-specific plasmids, to our knowledge, represent the only new genetic material in Y. pestis acquired since the the divergence from Y. pseudotuberculosis. In contrast, 149 other pseudogenes (doubling the previous estimate) and 317 genes absent from Y. pestis were detected, indicating that as many as 13% of Y. pseudotuberculosis genes no longer function in Y. pestis. Extensive insertion sequence-mediated genome rearrangements and reductive evolution through massive gene loss, resulting in elimination and modification of preexisting gene expression pathways, appear to be more important than acquisition of genes in the evolution of Y. pestis. These results provide a sobering example of how a highly virulent epidemic clone can suddenly emerge from a less virulent, closely related progenitor.

Identification of a nuclear targeting signal in YopM from Yersinia spp

YopM is a type III secretion effector from Yersinia which contributes to pathogenicity but whose action still remains unclear. It is an acidic, leucine-rich repeats (LRR) containing protein which migrates to the nucleus of target cells in spite of the fact that it does not contain any classical nuclear localization signal (NLS). Using a yeast approach, we observed that the three first LRRs (LRR1-3) and the 32 C-terminal residues of YopM (YopMC-ter) act as NLSs in yeast. Furthermore, by transfection of HEK293T cells, we observed that YopMC-ter could direct large recombinant EGFP-LexA-AD proteins into the nucleus of mammalian cells confirming that it contains a NLS. Critical residues for nuclear targeting were identified by site-directed mutagenesis in YopMC-ter. In addition, we show that YopMC-ter NLS is crucial for the nuclear targeting of an EGFP-YopM fusion protein.

DNA sequence and analysis of the pYVa127/90 virulence plasmid of Yersinia enterocolitica strain A127/90

The complete nucleotide sequence and organization of the Yersinia enterocolitica strain A127/90 serotype O:8 biotype 1B virulence plasmid (pYVa127/90) were determined. In contrast to other biotype 1B strains, usually isolated in the USA, strain A127/90 was isolated in Japan. The organization of the genes carried by the 66591-bp pYVa127/90 plasmid is globally similar to that of 67720-bp pYVe8081, the virulence plasmid from 8081, an American isolate of serotype O:8 biotype 1B. Differences are noticeable in the amino acid sequences of the SycH chaperone, the YopM cytotoxin and the plague-protective antigen LcrV. Interestingly, pYVa127/90 contains the entire coding sequence of the YlpA lipoprotein, which is missing from pYVe8081. In addition, we identified five potential new genes. Several vestigial insertion sequence elements appeared to be different between the two plasmids. These observations suggest that both pYV plasmids are from a common ancestor but may have evolved independently following the conquest of two distant ecological niches.

The yersinia virulence factor YopM forms a novel protein complex with two cellular kinases

Pathogenic Yersinia contain a virulence plasmid that encodes genes for intracellular effectors, which neutralize the host immune response. One effector, YopM, is necessary for Yersinia virulence, but its function in host cells is unknown. To identify potential cellular pathways affected by YopM, proteins that co-immunoprecipitate with YopM in mammalian cells were isolated and identified by mass spectrometry. Results demonstrate that two kinases, protein kinase C-like 2 (PRK2) and ribosomal S6 protein kinase 1 (RSK1), interact directly with YopM. These two kinases associate only when YopM is present, and expression of YopM in cells stimulates the activity of both kinases. RSK1 is activated directly by interaction with YopM, and RSK1 kinase activity is required for YopM-stimulated PRK2 activity. YopM activation of RSK1 occurs independently of the actions of YopJ on the MAPK pathway. YopM is also required for Yersinia-induced changes in RSK1 mobility in infected macrophage cells. These results identify the first intracellular targets of YopM and suggest YopM acts to stimulate the activity of PRK2 and RSK1.

Application of a Saccharomyces cerevisiae model to study requirements for trafficking of Yersinia pestis YopM in eucaryotic cells

YopM is a leucine-rich repeat (LRR) virulence protein that is delivered into host cells when any of the three human-pathogenic species of Yersinia binds to mammalian cells. It exhibits heterogeneity of size and sequence among the yersiniae, but the functional consequences of this variability are not yet known. Yersinia pestis YopM was previously shown to accumulate in the nuclei of infected HeLa cells by a mechanism that requires vesicular trafficking. In this study, we characterized the trafficking of Y. pestis YopM in a Saccharomyces cerevisiae model previously found to support nuclear localization of YopM from an enteropathogenic Yersinia strain (C. F. Lesser and S. I. Miller, EMBO J. 20:1840-1849, 2001). Y. pestis YopM was N-terminally fused to the yeast enhanced green fluorescent protein (yEGFP) and inducibly expressed in the cytoplasm. yEGFP-YopM localized to the yeast nucleus, showing that this property is conserved for YopMs so far tested and that infection and the presence of other Yops are not required for its trafficking. When expressed in S. cerevisiae that is temperature sensitive for vesicular transport, YopM failed to accumulate in the nucleus at the nonpermissive temperature but did accumulate when the permissive temperature was restored. This shows that vesicular trafficking also is required in yeast for normal localization of YopM. YopM consists of a 71-residue leader sequence, 15 LRRs, and a 32-residue tail. Deletion analysis revealed that the leader sequence or tail is alone insufficient to direct YopM to the nucleus, showing that the LRR structure is required. Both the N-terminal and C-terminal halves of YopM localized to the nucleus, indicating the possible presence of two nuclear localization signals (NLSs) in YopM or domains in YopM where an NLS-containing protein might bind; this fits with the presence of two highly conserved regions among Yersinia YopMs. yEGFP-YopM lacking LRRs 4 to 7 or 7 to 10 accumulated in the nucleus in yeast, and YopM lacking these LRRs concentrated normally in the HeLa cell nucleus after delivery by Yersinia infection, showing that these LRRs are not essential for YopM trafficking in eucaryotic cells. However, because Y. pestis carrying either of these YopMs is strongly compromised in virulence in mice, these findings revealed that LRRs 4 to 10 map a region of YopM or support a conformation of YopM that is necessary for a pathogenic effect.

Unusual molecular architecture of the Yersinia pestis cytotoxin YopM: a leucine-rich repeat protein with the shortest repeating unit

Many Gram-negative bacterial pathogens employ a contact-dependent (type III) secretion system to deliver effector proteins into the cytosol of animal or plant cells. Collectively, these effectors enable the bacteria to evade the immune response of the infected organism by modulating host-cell functions. YopM, a member of the leucine-rich repeat protein superfamily, is an effector produced by the bubonic plague bacterium, Yersinia pestis, that is essential for virulence. Here, we report crystal structures of YopM at 2.4 and 2.1 A resolution. Among all leucine-rich repeat family members whose atomic coordinates have been reported, the repeating unit of YopM has the least canonical secondary structure. In both crystals, four YopM monomers form a hollow cylinder with an inner diameter of 35 A. The domain that targets YopM for translocation into eukaryotic cells adopts a well-ordered, alpha-helical conformation that packs tightly against the proximal leucine-rich repeat module. A similar alpha-helical domain can be identified in virulence-associated leucine-rich repeat proteins produced by Salmonella typhimurium and Shigella flexneri, and in the conceptual translation products of several open reading frames in Y. pestis.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Structure-function analysis of Yersinia pestis YopM's interaction with alpha-thrombin to rule on its significance in systemic plague and to model YopM's mechanism of binding host proteins

The plague virulence protein YopM of Yersinia pestis KIM5 belongs to the large family of leucine-rich repeat (LRR) proteins. The only activity demonstrated so far for YopM is thrombin-binding, which could be a function of the small amount of YopM that is released into surrounding tissues by the bacteria. This study combined deletional and mutational analysis, chemical crosslinking assays, and in vitro functional tests with molecular modelling to identify key features of YopM necessary for interacting with thrombin. Two Y. pestis strains expressing YopM variants that differed in thrombin binding were used to assess the importance of thrombin-binding for lethality of plague. Both strains suffered a similar decrease in virulence by three orders of magnitude, indicating that thrombin-binding per se was not the major deficiency for lethality in the systemic disease model employed. It remains possible that extracellular YopM could contribute to plague pathology and to early events in peripheral tissues. The structural studies provided a model for how YopM may interact with thrombin and an insight into how YopM's LRR structure may assemble distinct regions for binding different targets.

Shigella flexneri IpaH(7.8) facilitates escape of virulent bacteria from the endocytic vacuoles of mouse and human macrophages

The behavior of Shigella flexneri ipaH mutants was studied in human monocyte-derived macrophages (HMDM), in 1-day-old human monocytes, and in J774 mouse macrophage cell line. In HMDM, strain pWR700, an ipaH(7.8) deletion mutant of S. flexneri 2a strain 2457T, behaved like the wild-type strain 2457T. This strain caused rapid host cell death by oncosis, and few bacterial CFU were recovered after incubation in the presence of gentamicin as previously described for 2457T-infected HMDM. However, analysis of bacterial compartmentalization within endocytic vacuoles with gentamicin and chloroquine indicated that more pWR700 than 2457T was present within the endocytic vacuoles of HMDM, suggesting that ipaH(7.8) deletion mutant transited more slowly from the vacuoles to the cytoplasm. In contrast to findings with HMDM, CFU recovered from pWR700-infected mouse J774 cells were 2 to 3 logs higher than CFU from 2457T-infected J774 cells. These values exceeded CFU recovered after infection of J774 cells with plasmid-cured avirulent strain M4243A1. Incubation with gentamicin and chloroquine clearly showed that pWR700 within J774 cells was mostly present within the endocytic vacuoles. This distribution pattern was similar to that seen with M4243A1 and contrasted with the pattern seen with 2457T. Complementation of pWR700 with a recombinant clone expressing ipaH(7. 8) restored the intracellular distribution of bacteria to that seen with the wild-type strain. Strains with deletions in ipaH(4.5) or ipaH(9.8), however, behaved like 2457T in both HMDM and J774 cells. The distribution profile of pWR700 in 1-day-old monocytes was similar to that seen in J774 cells. Like infected J774 cells, 1-day-old human monocytes demonstrated apoptosis upon infection with virulent Shigella. These results suggest that a role of the ipaH(7. 8) gene product is to facilitate the escape of the virulent bacteria from the phagocytic vacuole of monocytes and macrophages.

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.