Interactions of bacterial effector proteins with host proteins

Brucella-driven host N-glycome remodeling controls infection

Many powerful methods have been employed to elucidate the global transcriptomic, proteomic, or metabolic responses to pathogen-infected host cells. However, the host glycome responses to bacterial infection remain largely unexplored, and hence, our understanding of the molecular mechanisms by which bacterial pathogens manipulate the host glycome to favor infection remains incomplete. Here, we address this gap by performing a systematic analysis of the host glycome during infection by the bacterial pathogen Brucella spp. that cause brucellosis. We discover, surprisingly, that a Brucella effector protein (EP) Rhg1 induces global reprogramming of the host cell N-glycome by interacting with components of the oligosaccharide transferase complex that controls N-linked protein glycosylation, and Rhg1 regulates Brucella replication and tissue colonization in a mouse model of brucellosis, demonstrating that Brucella exploits the EP Rhg1 to reprogram the host N-glycome and promote bacterial intracellular parasitism, thereby providing a paradigm for bacterial control of host cell infection.

Protein interaction mapping reveals widespread targeting of development-related host transcription factors by phytoplasma effectors

Phytoplasmas are pathogenic bacteria that reprogram plant host development for their own benefit. Previous studies have characterized a few different phytoplasma effector proteins that destabilize specific plant transcription factors. However, these are only a small fraction of the potential effectors used by phytoplasmas; therefore, the molecular mechanisms through which phytoplasmas modulate their hosts require further investigation. To obtain further insights into the phytoplasma infection mechanisms, we generated a protein-protein interaction network between a broad set of phytoplasma effectors and a large, unbiased collection of Arabidopsis thaliana transcription factors and transcriptional regulators. We found widespread, but specific, interactions between phytoplasma effectors and host transcription factors, especially those related to host developmental processes. In particular, many unrelated effectors target specific sets of TCP transcription factors, which regulate plant development and immunity. Comparison with other host-pathogen protein interaction networks shows that phytoplasma effectors have unusual targets, indicating that phytoplasmas have evolved a unique and unusual infection strategy. This study contributes a rich and solid data source that guides further investigations of the functions of individual effectors, as demonstrated for some herein. Moreover, the dataset provides insights into the underlying molecular mechanisms of phytoplasma infection.

The WxxxE proteins in microbial pathogenesis

Effector proteins secreted by pathogens modulate various host cellular processes and help in bacterial pathogenesis. Some of these proteins, injected by enteric pathogens via Type Three Secretion System (T3SS) were grouped together based on a conserved signature motif (WxxxE) present in them. The presence of WxxxE motif is not limited to effectors released by enteric pathogens or the T3SS but has been detected in non-enteric pathogens, plant pathogens and in association with Type II and Type IV secretion systems. WxxxE effectors are involved in actin organization, inflammation regulation, vacuole or tubule formation, endolysosomal signalling regulation, tight junction disruption, and apoptosis. The WxxxE sequence has also been identified in TIR [Toll/interleukin-1 (IL-1) receptor] domains of bacteria and host. In the present review, we have focussed on the established and predicted functions of WxxxE effectors secreted by several pathogens, including enteric, non-enteric, and plant pathogens.

Pan-kinome of Legionella expanded by a bioinformatics survey

The pathogenic Legionella bacteria are notorious for delivering numerous effector proteins into the host cell with the aim of disturbing and hijacking cellular processes for their benefit. Despite intensive studies, many effectors remain uncharacterized. Motivated by the richness of Legionella effector repertoires and their oftentimes atypical biochemistry, also by several known atypical Legionella effector kinases and pseudokinases discovered recently, we undertook an in silico survey and exploration of the pan-kinome of the Legionella genus, i.e., the union of the kinomes of individual species. In this study, we discovered 13 novel (pseudo)kinase families (all are potential effectors) with the use of non-standard bioinformatic approaches. Together with 16 known families, we present a catalog of effector and non-effector protein kinase-like families within Legionella, available at http://bioinfo.sggw.edu.pl/kintaro/ . We analyze and discuss the likely functional roles of the novel predicted kinases. Notably, some of the kinase families are also present in other bacterial taxa, including other pathogens, often phylogenetically very distant from Legionella. This work highlights Nature's ingeniousness in the pathogen-host arms race and offers a useful resource for the study of infection mechanisms.

SLAMF8 Downregulates Mouse Macrophage Microbicidal Mechanisms via PI3K Pathways

Signaling lymphocytic activation molecule family 8 (SLAMF8) is involved in the negative modulation of NADPH oxidase activation. However, the impact of SLAMF8 downregulation on macrophage functionality and the microbicide mechanism remains elusive. To study this in depth, we first analyzed NADPH oxidase activation pathways in wild-type and SLAMF8-deficient macrophages upon different stimulus. Herein, we describe increased phosphorylation of the Erk1/2 and p38 MAP kinases, as well as increased phosphorylation of NADPH oxidase subunits in SLAMF8-deficient macrophages. Furthermore, using specific inhibitors, we observed that specific PI3K inhibition decreased the differences observed between wild-type and SLAMF8-deficient macrophages, stimulated with either PMA, LPS, or Salmonella typhimurium infection. Consequently, SLAMF8-deficient macrophages also showed increased recruitment of small GTPases such as Rab5 and Rab7, and the p47^phox subunit to cytoplasmic Salmonella, suggesting an impairment of Salmonella-containing vacuole (SCV) progression in SLAMF8-deficient macrophages. Enhanced iNOS activation, NO production, and IL-6 expression were also observed in the absence of SLAMF8 upon Salmonella infection, either in vivo or in vitro, while overexpression of SLAMF8 in RAW264.7 macrophages showed the opposite phenotype. In addition, SLAMF8-deficient macrophages showed increased activation of Src kinases and reduced SHP-1 phosphate levels upon IFNγ and Salmonella stimuli in comparison to wild-type macrophages. In agreement with in vitro results, Salmonella clearance was augmented in SLAMF8-deficient mice compared to that in wild-type mice. Therefore, in conclusion, SLAMF8 intervention upon bacterial infection downregulates mouse macrophage activation, and confirmed that SLAMF8 receptor could be a potential therapeutic target for the treatment of severe or unresolved inflammatory conditions.

Bacterial proteomics and its application for pathogenesis studies

Bacteria build their structures by implementing several macromolecules such as proteins, polysaccharides, phospholipids, and nucleic acids, which leads to preserve their lives and play an essential role in their pathogenesis. There are two genomic and proteomic methods to study various macromolecules of bacteria, which are complementary methods and provide comprehensive information. Proteomic approaches are used to identify proteins and their cell applications. Furthermore, to study bacterial proteins, macromolecules are involved in the bacteria's structures and functions. These protein-based methods provide comprehensive information about the cells, such as the external structures, internal compositions, post-translational modifications, and mechanisms of particular actions such as biofilm formation, antibiotic resistance, and adaptation to the environment, which are helpful in promoting bacterial pathogenesis. These methods use various devices such as MALDI-TOF MS, LC-MS, and two-dimensional electrophoresis, which are valuable tools for studying different structural and functional proteins of the bacteria and their mechanisms of pathogenesis that causes rapid, easy, and accurate diagnosis of the infections.

Development and validation of a high-content screening assay for inhibitors of enteropathogenic E. coli adhesion

Enteropathogenic E. coli (EPEC) causes intestinal infections leading to severe diarrhea. EPEC attaches to the host cell causing lesions to the intestinal epithelium coupled with the effacement of microvilli. In the process, actin accumulates into a pedestal-like structure under bacterial microcolonies. We designed an automated fluorescence microscopy-based screening method for discovering compounds capable of inhibiting EPEC adhesion and virulence using aurodox, a type three secretion system (T3SS) inhibitor, as a positive control. The screening assay employs an EPEC strain (2348/69) expressing a fluorescent protein and actin staining for monitoring the bacteria and their pedestals respectively, analyzing these with a custom image analysis pipeline. The assay allows for the discovery of compounds capable of preventing the formation of pathogenic actin rearrangements. These compounds may be interfering with virulence-related molecular pathways relevant for developing antivirulence leads.

Investigating host-bacterial interactions among enteric pathogens

Background

In 2017, World Health Organization (WHO) published a catalogue of 12 families of antibiotic-resistant “priority pathogens” that are posing the greatest threats to human health. Six of these dreaded pathogens are known to infect the human gastrointestinal system. In addition to causing gastrointestinal and systemic infections, these pathogens can also affect the composition of other microbes constituting the healthy gut microbiome. Such aberrations in gut microbiome can significantly affect human physiology and immunity. Identifying the virulence mechanisms of these enteric pathogens are likely to help in developing newer therapeutic strategies to counter them.

Results

Using our previously published in silico approach, we have evaluated (and compared) Host-Pathogen Protein-Protein Interaction (HPI) profiles of four groups of enteric pathogens, namely, different species of Escherichia, Shigella, Salmonella and Vibrio. Results indicate that in spite of genus/ species specific variations, most enteric pathogens possess a common repertoire of HPIs. This core set of HPIs are probably responsible for the survival of these pathogen in the harsh nutrient-limiting environment within the gut. Certain genus/ species specific HPIs were also observed.

Conslusions

The identified bacterial proteins involved in the core set of HPIs are expected to be helpful in understanding the pathogenesis of these dreaded gut pathogens in greater detail. Possible role of genus/ species specific variations in the HPI profiles in the virulence of these pathogens are also discussed. The obtained results are likely to provide an opportunity for development of novel therapeutic strategies against the most dreaded gut pathogens.

Genome-wide prediction of bacterial effector candidates across six secretion system types using a feature-based statistical framework

Gram-negative bacteria are responsible for hundreds of millions infections worldwide, including the emerging hospital-acquired infections and neglected tropical diseases in the third-world countries. Finding a fast and cheap way to understand the molecular mechanisms behind the bacterial infections is critical for efficient diagnostics and treatment. An important step towards understanding these mechanisms is the discovery of bacterial effectors, the proteins secreted into the host through one of the six common secretion system types. Unfortunately, current prediction methods are designed to specifically target one of three secretion systems, and no accurate "secretion system-agnostic" method is available. Here, we present PREFFECTOR, a computational feature-based approach to discover effector candidates in Gram-negative bacteria, without prior knowledge on bacterial secretion system(s) or cryptic secretion signals. Our approach was first evaluated using several assessment protocols on a manually curated, balanced dataset of experimentally determined effectors across all six secretion systems, as well as non-effector proteins. The evaluation revealed high accuracy of the top performing classifiers in PREFFECTOR, with the small false positive discovery rate across all six secretion systems. Our method was also applied to six bacteria that had limited knowledge on virulence factors or secreted effectors. PREFFECTOR web-server is freely available at: http://korkinlab.org/preffector .

Expression of Francisella pathogenicity island protein intracellular growth locus E (IglE) in mammalian cells is involved in intracellular trafficking, possibly through microtubule organizing center

Francisella tularensis is the causative agent of the infectious disease tularemia and is designated a category A bioterrorism agent. The type VI secretion system encoded by the Francisella pathogenicity island (FPI) is necessary for intracellular growth; however, the functions of FPI proteins are largely unknown. In this study, we found that the FPI protein intracellular growth locus E (IglE) showed a unique localization pattern compared to other FPI proteins. Deleting iglE from Francisella tularensis subsp. novicida (F. novicida) decreased intracellular growth. Immunoprecipitation and pull‐down assays revealed that IglE was associated with β‐tubulin. Additionally, GFP‐fused IglE colocalized with microtubule organizing centers (MTOCs) in 293T cells. The iglE deletion mutant was transferred with dynein toward MTOCs and packed into lysosome‐localizing areas. Conversely, the wild‐type F. novicida exhibited intracellular growth distant from MTOCs. In addition, IglE expressed in 293T cells colocalized with dynein. These results suggest that IglE helps to prevent dynein‐ and MTOC‐mediated intracellular trafficking in host cells to inhibit the transport of F. novicida toward lysosomes.

Bacterial LPX motif-harboring virulence factors constitute a species-spanning family of cell-penetrating effectors

Effector proteins are key virulence factors of pathogenic bacteria that target and subvert the functions of essential host defense mechanisms. Typically, these proteins are delivered into infected host cells via the type III secretion system (T3SS). Recently, however, several effector proteins have been found to enter host cells in a T3SS-independent manner thereby widening the potential range of these virulence factors. Prototypes of such bacteria-derived cell-penetrating effectors (CPEs) are the Yersinia enterocolitica-derived YopM as well as the Salmonella typhimurium effector SspH1. Here, we investigated specifically the group of bacterial LPX effector proteins comprising the Shigella IpaH proteins, which constitute a subtype of the leucine-rich repeat protein family and share significant homologies in sequence and structure. With particular emphasis on the Shigella-effector IpaH9.8, uptake into eukaryotic cell lines was shown. Recombinant IpaH9.8 (rIpaH9.8) is internalized via endocytic mechanisms and follows the endo-lysosomal pathway before escaping into the cytosol. The N-terminal alpha-helical domain of IpaH9.8 was identified as the protein transduction domain required for its CPE ability as well as for being able to deliver other proteinaceous cargo. rIpaH9.8 is functional as an ubiquitin E3 ligase and targets NEMO for poly-ubiquitination upon cell penetration. Strikingly, we could also detect other recombinant LPX effector proteins from Shigella and Salmonella intracellularly when applied to eukaryotic cells. In this study, we provide further evidence for the general concept of T3SS-independent translocation by identifying novel cell-penetrating features of these LPX effectors revealing an abundant species-spanning family of CPE.

A systematic analysis of the RNA-targeting potential of secreted bacterial effector proteins

Many pathogenic bacteria utilize specialized secretion systems to deliver proteins called effectors into eukaryotic cells for manipulation of host pathways. The vast majority of known effector targets are host proteins, whereas a potential targeting of host nucleic acids remains little explored. There is only one family of effectors known to target DNA directly, and effectors binding host RNA are unknown. Here, we take a two-pronged approach to search for RNA-binding effectors, combining biocomputational prediction of RNA-binding domains (RBDs) in a newly assembled comprehensive dataset of bacterial secreted proteins, and experimental screening for RNA binding in mammalian cells. Only a small subset of effectors were predicted to carry an RBD, indicating that if RNA targeting was common, it would likely involve new types of RBDs. Our experimental evaluation of effectors with predicted RBDs further argues for a general paucity of RNA binding activities amongst bacterial effectors. We obtained evidence that PipB2 and Lpg2844, effector proteins of Salmonella and Legionella species, respectively, may harbor novel biochemical activities. Our study presenting the first systematic evaluation of the RNA-targeting potential of bacterial effectors offers a basis for discussion of whether or not host RNA is a prominent target of secreted bacterial proteins.

RalF-Mediated Activation of Arf6 Controls Rickettsia typhi Invasion by Co-Opting Phosphoinositol Metabolism

Rickettsiae are obligate intracellular pathogens that induce their uptake into nonphagocytic cells; however, the events instigating this process are incompletely understood. Importantly, diverse Rickettsia species are predicted to utilize divergent mechanisms to colonize host cells, as nearly all adhesins and effectors involved in host cell entry are differentially encoded in diverse Rickettsia species. One particular effector, RalF, a Sec7 domain-containing protein that functions as a guanine nucleotide exchange factor of ADP-ribosylation factors (Arfs), is critical for Rickettsia typhi (typhus group rickettsiae) entry but pseudogenized or absent from spotted fever group rickettsiae. Secreted early during R. typhi infection, RalF localizes to the host plasma membrane and interacts with host ADP-ribosylation factor 6 (Arf6). Herein, we demonstrate that RalF activates Arf6, a process reliant on a conserved Glu within the RalF Sec7 domain. Furthermore, Arf6 is activated early during infection, with GTP-bound Arf6 localized to the R. typhi entry foci. The regulation of phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which generates PI(4,5)P₂, by activated Arf6 is instrumental for bacterial entry, corresponding to the requirement of PI(4,5)P₂ for R. typhi entry. PI(3,4,5)P₃ is then synthesized at the entry foci, followed by the accumulation of PI(3)P on the short-lived vacuole. Inhibition of phosphoinositide 3-kinases, responsible for the synthesis of PI(3,4,5)P₃ and PI(3)P, negatively affects R. typhi infection. Collectively, these results identify RalF as the first bacterial effector to directly activate Arf6, a process that initiates alterations in phosphoinositol metabolism critical for a lineage-specific Rickettsia entry mechanism.

A review on host-pathogen interactions: classification and prediction

The research on host-pathogen interactions is an ever-emerging and evolving field. Every other day a new pathogen gets discovered, along with comes the challenge of its prevention and cure. As the intelligent human always vies for prevention, which is better than cure, understanding the mechanisms of host-pathogen interactions gets prior importance. There are many mechanisms involved from the pathogen as well as the host sides while an interaction happens. It is a vis-a-vis fight of the counter genes and proteins from both sides. Who wins depends on whether a host gets an infection or not. Moreover, a higher level of complexity arises when the pathogens evolve and become resistant to a host's defense mechanisms. Such pathogens pose serious challenges for treatment. The entire human population is in danger of such long-lasting persistent infections. Some of these infections even increase the rate of mortality. Hence there is an immediate emergency to understand how the pathogens interact with their host for successful invasion. It may lead to discovery of appropriate preventive measures, and the development of rational therapeutic measures and medication against such infections and diseases. This review, a state-of-the-art updated scenario of host-pathogen interaction research, has been done by keeping in mind this urgency. It covers the biological and computational aspects of host-pathogen interactions, classification of the methods by which the pathogens interact with their hosts, different machine learning techniques for prediction of host-pathogen interactions, and future scopes of this research field.

Identification and Characterization of Putative Translocated Effector Proteins of the Edwardsiella ictaluri Type III Secretion System

The bacterial pathogen Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), an economically significant disease of farm-raised channel catfish. Commercial catfish production accounts for the majority of the total fin fish aquaculture in the United States, with almost 300,000 tons produced annually, and ESC is the leading cause of disease loss in the industry. We have demonstrated the survival and replication of E. ictaluri within channel catfish cells and identified a secretion system that is essential for E. ictaluri intracellular replication and virulence. We have also identified nine proteins encoded in the E. ictaluri genome that we believe are actively transferred from the bacterium to the cytoplasm of the host cell and act to manipulate host cell physiology to the advantage of the bacterium. The data presented here confirm that the proteins are actually transferred during an infection, which will lead to further work on approaches to preventing or controlling ESC.

Edwardsiella ictaluri, a major pathogen in channel catfish aquaculture, encodes a type III secretion system (T3SS) that is essential for intracellular replication and virulence. Previous work identified three putative T3SS effectors in E. ictaluri, and in silico analysis of the E. ictaluri genome identified six additional putative effectors, all located on the chromosome outside the T3SS pathogenicity island. To establish active translocation by the T3SS, we constructed translational fusions of each effector to the amino-terminal adenylate cyclase (AC) domain of the Bordetella pertussis adenylate cyclase toxin CyaA. When translocated through the membrane of the Edwardsiella-containing vacuole (ECV), the cyclic AMP produced by the AC domain in the presence of calmodulin in the host cell cytoplasm can be measured. Results showed that all nine effectors were translocated from E. ictaluri in the ECV to the cytoplasm of the host cells in the wild-type strain but not in a T3SS mutant, indicating that translocation is dependent on the T3SS machinery. This confirms that the E. ictaluri T3SS is similar to the Salmonella pathogenicity island 2 T3SS in that it translocates effectors through the membrane of the bacterial vacuole directly into the host cell cytoplasm. Additional work demonstrated that both initial acidification and subsequent neutralization of the ECV were necessary for effector translocation, except for two of them that did not require neutralization. Single-gene mutants constructed for seven of the individual effectors were all attenuated for replication in CCO cells, but only three were replication deficient in head kidney-derived macrophages (HKDM).

IMPORTANCE The bacterial pathogen Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), an economically significant disease of farm-raised channel catfish. Commercial catfish production accounts for the majority of the total fin fish aquaculture in the United States, with almost 300,000 tons produced annually, and ESC is the leading cause of disease loss in the industry. We have demonstrated the survival and replication of E. ictaluri within channel catfish cells and identified a secretion system that is essential for E. ictaluri intracellular replication and virulence. We have also identified nine proteins encoded in the E. ictaluri genome that we believe are actively transferred from the bacterium to the cytoplasm of the host cell and act to manipulate host cell physiology to the advantage of the bacterium. The data presented here confirm that the proteins are actually transferred during an infection, which will lead to further work on approaches to preventing or controlling ESC.

MIB-MIP is a mycoplasma system that captures and cleaves immunoglobulin G

Mycoplasmas are "minimal" bacteria able to infect humans, wildlife, and a large number of economically important livestock species. Mycoplasma infections include a spectrum of clinical manifestations ranging from simple fever to fulminant inflammatory diseases with high mortality rates. These infections are mostly chronic, suggesting that mycoplasmas have developed means to evade the host immune response. Here we present and functionally characterize a two-protein system from Mycoplasma mycoides subspecies capri that is involved in the capture and cleavage of IgG. The first component, Mycoplasma Ig binding protein (MIB), is an 83-kDa protein that is able to tightly bind to the Fv region of a wide range of IgG. The second component, Mycoplasma Ig protease (MIP), is a 97-kDa serine protease that is able to cleave off the VH domain of IgG. We demonstrate that MIB is necessary for the proteolytic activity of MIP. Cleavage of IgG requires a sequential interaction of the different partners of the system: first MIB captures the IgG, and then MIP is recruited to the MIB-IgG complex, enabling protease activity. MIB and MIP are encoded by two genes organized in tandem, with homologs found in the majority of pathogenic mycoplasmas and often in multiple copies. Phylogenetic studies suggest that genes encoding the MIB-MIP system are specific to mycoplasmas and have been disseminated by horizontal gene transfer. These results highlight an original and complex system targeting the host immunoglobulins, playing a potentially key role in the immunity evasion by mycoplasmas.

Priming maize resistance by its neighbors: activating 1,4-benzoxazine-3-ones synthesis and defense gene expression to alleviate leaf disease

Plant disease can be effectively suppressed in intercropping systems. Our previous study demonstrated that neighboring maize plants can restrict the spread of soil-borne pathogens of pepper plants by secreting defense compounds into the soil. However, whether maize plant can receive benefits from its neighboring pepper plants in an intercropping system is little attention. We examined the effects of maize roots treated with elicitors from the pepper pathogen Phytophthora capsici and pepper root exudates on the synthesis of 1,4-benzoxazine-3-ones (BXs), the expression of defense-related genes in maize, and their ability to alleviate the severity of southern corn leaf blight (SCLB) caused by Bipolaris maydis. We found that SCLB was significantly reduced after the above treatments. The contents of 1,4-benzoxazine-3-ones (BXs: DIBOA, DIMBOA, and MBOA) and the expression levels of BX synthesis and defense genes in maize roots and shoots were up-regulated. DIMBOA and MBOA effectively inhibited the mycelium growth of Bipolaris maydis at physiological concentrations in maize shoots. Further studies suggested that the defense related pathways or genes in maize roots and shoots were activated by elicitors from the P. capsici or pepper root exudates. In conclusion, maize increased the levels of BXs and defense gene expression both in roots and shoots after being triggered by root exudates and pathogen from neighboring pepper plants, eventually enhancing its resistance.

Flagellar membrane proteins in kinetoplastid parasites

All kinetoplastid parasites, including protozoa such as Leishmania species, Trypanosoma brucei, and Trypanosoma cruzi that cause devastating diseases in humans and animals, are flagellated throughout their life cycles. Although flagella were originally thought of primarily as motility organelles, flagellar functions in other critical processes, especially in sensing and signal transduction, have become more fully appreciated in the recent past. The flagellar membrane is a highly specialized subdomain of the surface membrane, and flagellar membrane proteins are likely to be critical components for all the biologically important roles of flagella. In this review, we summarize recent discoveries relevant to flagellar membrane proteins in these parasites, including the identification of such proteins, investigation of their biological functions, and mechanisms of selective trafficking to the flagellar membrane. Prospects for future investigations and current unsolved problems are highlighted.

Which Way In? The RalF Arf-GEF Orchestrates Rickettsia Host Cell Invasion

Bacterial Sec7-domain-containing proteins (RalF) are known only from species of Legionella and Rickettsia, which have facultative and obligate intracellular lifestyles, respectively. L. pneumophila RalF, a type IV secretion system (T4SS) effector, is a guanine nucleotide exchange factor (GEF) of ADP-ribosylation factors (Arfs), activating and recruiting host Arf1 to the Legionella-containing vacuole. In contrast, previous in vitro studies showed R. prowazekii (Typhus Group) RalF is a functional Arf-GEF that localizes to the host plasma membrane and interacts with the actin cytoskeleton via a unique C-terminal domain. As RalF is differentially encoded across Rickettsia species (e.g., pseudogenized in all Spotted Fever Group species), it may function in lineage-specific biology and pathogenicity. Herein, we demonstrate RalF of R. typhi (Typhus Group) interacts with the Rickettsia T4SS coupling protein (RvhD4) via its proximal C-terminal sequence. RalF is expressed early during infection, with its inactivation via antibody blocking significantly reducing R. typhi host cell invasion. For R. typhi and R. felis (Transitional Group), RalF ectopic expression revealed subcellular localization with the host plasma membrane and actin cytoskeleton. Remarkably, R. bellii (Ancestral Group) RalF showed perinuclear localization reminiscent of ectopically expressed Legionella RalF, for which it shares several structural features. For R. typhi, RalF co-localization with Arf6 and PI(4,5)P₂ at entry foci on the host plasma membrane was determined to be critical for invasion. Thus, we propose recruitment of PI(4,5)P₂ at entry foci, mediated by RalF activation of Arf6, initiates actin remodeling and ultimately facilitates bacterial invasion. Collectively, our characterization of RalF as an invasin suggests that, despite carrying a similar Arf-GEF unknown from other bacteria, different intracellular lifestyles across Rickettsia and Legionella species have driven divergent roles for RalF during infection. Furthermore, our identification of lineage-specific Arf-GEF utilization across some rickettsial species illustrates different pathogenicity factors that define diverse agents of rickettsial diseases.

Phylogenomics analysis indicates divergent mechanisms for host cell invasion across diverse species of obligate intracellular Rickettsia. For instance, only some Rickettsia species carry RalF, the rare bacterial Arf-GEF effector utilized by Legionella pneumophila to facilitate fusion of ER-derived membranes with its host-derived vacuole. For R. prowazekii (Typhus Group, TG), prior in vitro studies suggested the Arf-GEF activity of RalF, which is absent from Spotted Fever Group species, might be spatially regulated at the host plasma membrane. Herein, we demonstrate RalF of R. typhi (TG) and R. felis (Transitional Group) localizes to the host plasma membrane, yet R. bellii (Ancestral Group) RalF shows perinuclear localization reminiscent of RalF-mediated recruitment of Arf1 by L. pneumophila to its vacuole. For R. typhi, RalF expression occurs early during infection, with RalF inactivation significantly reducing host cell invasion. Furthermore, RalF co-localization with Arf6 and the phosphoinositide PI(4,5)P₂ at the host plasma membrane was determined to be critical for R. typhi invasion. Thus, our work illustrates that different intracellular lifestyles across species of Rickettsia and Legionella have driven divergent roles for RalF during host cell infection. Collectively, we identify lineage-specific Arf-GEF utilization across diverse rickettsial species, previously unappreciated mechanisms for host cell invasion and infection.

Effector triggered immunity

Pathogenic bacteria produce virulence factors called effectors, which are important components of the infection process. Effectors aid in pathogenesis by facilitating bacterial attachment, pathogen entry into or exit from the host cell, immunoevasion, and immunosuppression. Effectors also have the ability to subvert host cellular processes, such as hijacking cytoskeletal machinery or blocking protein translation. However, host cells possess an evolutionarily conserved innate immune response that can sense the pathogen through the activity of its effectors and mount a robust immune response. This “effector triggered immunity” (ETI) was first discovered in plants but recent evidence suggest that the process is also well conserved in metazoans. We will discuss salient points of the mechanism of ETI in metazoans from recent studies done in mammalian cells and invertebrate model hosts.

A novel type 3 secretion system effector, YspI of Yersinia enterocolitica, induces cell paralysis by reducing total focal adhesion kinase

Some of the world's most important diseases are caused by bacterial pathogens that deliver toxic effector proteins directly into eukaryotic cells using type III secretion systems. The myriad of pathological outcomes caused by these pathogens is determined, in part, by the manipulation of host cell physiology due to the specific activities of individual effectors among the unique suite each pathogen employs. YspI was found to be an effector, delivered by Yersinia enterocolitica Biovar 1B, that inhibits host cell motility. The action of YspI comes about through its specific interaction with focal adhesion kinase, FAK, which is a fulcrum of focal adhesion complexes for controlling cellular motility. The interaction was defined by a specific domain of YspI that bound to the FAK kinase domain. Further examination revealed that YspI-FAK interaction leads to a reduction of FAK steady-state levels without altering its phosphorylation state. This collection of observations and results showed YspI displays unique functionality by targeting the key regulator of focal adhesion complexes to inhibit cellular movement.

Francisella novicida pathogenicity island encoded proteins were secreted during infection of macrophage-like cells

Intracellular pathogens and other organisms have evolved mechanisms to exploit host cells for their life cycles. Virulence genes of some intracellular bacteria responsible for these mechanisms are located in pathogenicity islands, such as secretion systems that secrete effector proteins. The Francisella pathogenicity island is required for phagosomal escape, intracellular replication, evasion of host immune responses, virulence, and encodes a type 6 secretion system. We hypothesize that some Francisella novicida pathogenicity island proteins are secreted during infection of host cells. To test this hypothesis, expression plasmids for all Francisella novicida FPI-encoded proteins with C-terminal and N-terminal epitope FLAG tags were developed. These plasmids expressed their respective epitope FLAG-tagged proteins at their predicted molecular weights. J774 murine macrophage-like cells were infected with Francisella novicida containing these plasmids. The FPI proteins expressed from these plasmids successfully restored the intramacrophage growth phenotype in mutants of the respective genes that were deficient for intramacrophage growth. Using these expression plasmids, the localization of the Francisella pathogenicity island proteins were examined via immuno-fluorescence microscopy within infected macrophage-like cells. Several Francisella pathogenicity island encoded proteins (IglABCDEFGHIJ, PdpACE, DotU and VgrG) were detected extracellularly and they were co-localized with the bacteria, while PdpBD and Anmk were not detected and thus remained inside bacteria. Proteins that were co-localized with bacteria had different patterns of localization. The localization of IglC was dependent on the type 6 secretion system. This suggests that some Francisella pathogenicity island proteins were secreted while others remain within the bacterium during infection of host cells as structural components of the secretion system and were necessary for secretion.

Detection and functionality of the CdtB, PltA, and PltB from Salmonella enterica serovar Javiana

Salmonella infection is one of the major foodborne illnesses in the United States. Several Gram-negative bacterial pathogens, including Salmonella Typhi, produce cytolethal distending toxin (CDT), which arrests growth, induces apoptosis of infected host cells and extends persistence of pathogenic bacteria in the host. The aim of this study was to characterize the functionality of CDT (cdtB, pltA and pltB) from nontyphoidal Salmonella isolates. Fifty Salmonella enterica serovar Javiana isolates from food, environmental, and clinical samples were screened for cdtB, pltA, and pltB genes by PCR, and all were positive for all three genes. Nucleotide sequence analysis of all amplified PCR products showed 100% identity to S. Typhi cdtB. To understand the roles of CdtB, PltA, and PltB in S. Javiana, cdtB, pltA, and pltB deletion mutants were constructed using a lambda Red-based recombination system. In vitro-cultured HeLa cell lines were infected with a wild-type strain and its isogenic ∆cdtB, ∆pltA, and ∆pltB to determine whether the strains of S. Javiana are responsible for invasion and cytolethal distending intoxication, including cell cycle arrest, cytoplasmic distension, and nuclear enlargement of host target cells. The results showed that HeLa cells infected with S. Javiana wild type were arrested in G2 /M and had distended cytoplasm and nuclei that were larger than those infected with S. Javiana ∆cdtB and ∆pltA strains. The S. Javiana ∆pltB strain retained the ability to induce cytoplasmic distension and cell cycle arrest, whereas the complemented ∆cdtB and ∆pltA S. Javiana strains showed activity like the wild-type strains. CdtB and pltA from S. Javiana had apparent effects on the distension of both cytoplasm and nucleus as well as cell cycle arrest of HeLa cell lines after 72 h of infection. Our data show a significant difference between the wild-type cdtB strain and its isogenic ∆cdtB for invasion of the cell lines. Therefore, CdtB produced from S. Javiana strains may play an important role in pathogenesis in host cells.

In vitro translocation experiments with RxLR-reporter fusion proteins of Avr1b from Phytophthora sojae and AVR3a from Phytophthora infestans fail to demonstrate specific autonomous uptake in plant and animal cells

Plant-pathogenic oomycetes have a large set of secreted effectors that can be translocated into their host cells during infection. One group of these effectors are the RxLR effectors for which it has been shown, in a few cases, that the RxLR motif is important for their translocation. It has been suggested that the RxLR-leader sequences alone are enough to translocate the respective effectors into eukaryotic cells through binding to surface-exposed phosphoinositol-3-phosphate. These conclusions were primary based on translocation experiments conducted with recombinant fusion proteins whereby the RxLR leader of RxLR effectors (i.e., Avr1b from Phytophthora sojae) were fused to the green fluorescent protein reporter-protein. However, we failed to observe specific cellular uptake for a comparable fusion protein where the RxLR leader of the P. infestans AVR3a was fused to monomeric red fluorescent protein. Therefore, we reexamined the ability of the reported P. sojae AVR1b RxLR leader to enter eukaryotic cells. Different relevant experiments were performed in three independent laboratories, using fluorescent reporter fusion constructs of AVR3a and Avr1b proteins in a side-by-side comparative study on plant tissue and human and animal cells. We report that we were unable to obtain conclusive evidence for specific RxLR-mediated translocation.

Effector-triggered versus pattern-triggered immunity: how animals sense pathogens

A fundamental question regarding any immune system is how it can discriminate between pathogens and non-pathogens. Here, we discuss how this discrimination can be mediated by a surveillance system distinct from pattern-recognition receptors that recognize conserved microbial patterns. It can be based instead on the ability of the host to sense perturbations in host cells induced by bacterial toxins or 'effectors' that are encoded by pathogenic microorganisms. Such 'effector-triggered immunity' was previously demonstrated mainly in plants, but recent data confirm that animals can also use this strategy.

Bacterial invasion factors: tools for crossing biological barriers and drug delivery?

The oral route is the preferential route of drug delivery in humans. However, effective delivery through the gastrointestinal tract is often hampered by the low permeability of the intestinal epithelium. One possibility to overcome this problem is the encapsulation of drugs inside nanoparticulate systems, containing targeting moieties with cell invasive properties. The bioinvasive features of the delivery system could be provided by the attachment of bacterial invasion factors, which promote efficient uptake into host cells and mediate rapid transcytosis of the pathogen through the intestinal epithelium. This review gives an overview of bacterial invasion systems. The molecular structure and function of suitable bacterial invasins, their relative values as targeting agents and possible pitfalls of their use are described. The potential of bioinvasive drug delivery systems is mainly presented on the basis of the well-characterized Yersinia invasin protein, which enters M cells to gain access to subepithelial layers of the gastrointestinal tract, but alternative approaches and future prospects for oral drug delivery are also discussed.

PTEN: an intercellular peacekeeper?

It is generally assumed that cells synthesize their own intracellular enzymes. Therefore, if expression of a specific gene is silenced in a potential cancer cell, it is expected that loss of protein function will follow. A provocative study indicates an unexpected mechanism of intercellular tumor suppression, showing that PTEN (phosphatase and tensin homolog deleted from chromosome 10), a cytosolic enzyme, can be transferred between cells in exosomes to suppress signaling and proliferation in target cells.

Meta-analysis of chicken--salmonella infection experiments

Background

Chicken meat and eggs can be a source of human zoonotic pathogens, especially Salmonella species. These food items contain a potential hazard for humans. Chickens lines differ in susceptibility for Salmonella and can harbor Salmonella pathogens without showing clinical signs of illness. Many investigations including genomic studies have examined the mechanisms how chickens react to infection. Apart from the innate immune response, many physiological mechanisms and pathways are reported to be involved in the chicken host response to Salmonella infection. The objective of this study was to perform a meta-analysis of diverse experiments to identify general and host specific mechanisms to the Salmonella challenge.

Results

Diverse chicken lines differing in susceptibility to Salmonella infection were challenged with different Salmonella serovars at several time points. Various tissues were sampled at different time points post-infection, and resulting host transcriptional differences investigated using different microarray platforms. The meta-analysis was performed with the R-package metaMA to create lists of differentially regulated genes. These gene lists showed many similarities for different chicken breeds and tissues, and also for different Salmonella serovars measured at different times post infection. Functional biological analysis of these differentially expressed gene lists revealed several common mechanisms for the chicken host response to Salmonella infection. The meta-analysis-specific genes (i.e. genes found differentially expressed only in the meta-analysis) confirmed and expanded the biological functional mechanisms.

Conclusions

The meta-analysis combination of heterogeneous expression profiling data provided useful insights into the common metabolic pathways and functions of different chicken lines infected with different Salmonella serovars.

The Shigella flexneri effector OspI deamidates UBC13 to dampen the inflammatory response

Many bacterial pathogens can enter various host cells and then survive intracellularly, transiently evade humoral immunity, and further disseminate to other cells and tissues. When bacteria enter host cells and replicate intracellularly, the host cells sense the invading bacteria as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) by way of various pattern recognition receptors. As a result, the host cells induce alarm signals that activate the innate immune system. Therefore, bacteria must modulate host inflammatory signalling and dampen these alarm signals. How pathogens do this after invading epithelial cells remains unclear, however. Here we show that OspI, a Shigella flexneri effector encoded by ORF169b on the large plasmid and delivered by the type ΙΙΙ secretion system, dampens acute inflammatory responses during bacterial invasion by suppressing the tumour-necrosis factor (TNF)-receptor-associated factor 6 (TRAF6)-mediated signalling pathway. OspI is a glutamine deamidase that selectively deamidates the glutamine residue at position 100 in UBC13 to a glutamic acid residue. Consequently, the E2 ubiquitin-conjugating activity required for TRAF6 activation is inhibited, allowing S. flexneri OspI to modulate the diacylglycerol-CBM (CARD-BCL10-MALT1) complex-TRAF6-nuclear-factor-κB signalling pathway. We determined the 2.0 Å crystal structure of OspI, which contains a putative cysteine-histidine-aspartic acid catalytic triad. A mutational analysis showed this catalytic triad to be essential for the deamidation of UBC13. Our results suggest that S. flexneri inhibits acute inflammatory responses in the initial stage of infection by targeting the UBC13-TRAF6 complex.

Host-targeting protein 1 (SpHtp1) from the oomycete Saprolegnia parasitica translocates specifically into fish cells in a tyrosine-O-sulphate-dependent manner

The eukaryotic oomycetes, or water molds, contain several species that are devastating pathogens of plants and animals. During infection, oomycetes translocate effector proteins into host cells, where they interfere with host-defense responses. For several oomycete effectors (i.e., the RxLR-effectors) it has been shown that their N-terminal polypeptides are important for the delivery into the host. Here we demonstrate that the putative RxLR-like effector, host-targeting protein 1 (SpHtp1), from the fish pathogen Saprolegnia parasitica translocates specifically inside host cells. We further demonstrate that cell-surface binding and uptake of this effector protein is mediated by an interaction with tyrosine-O-sulfate-modified cell-surface molecules and not via phospholipids, as has been reported for RxLR-effectors from plant pathogenic oomycetes. These results reveal an effector translocation route based on tyrosine-O-sulfate binding, which could be highly relevant for a wide range of host-microbe interactions.

Type III secretion is essential for the rapidly fatal diarrheal disease caused by non-O1, non-O139 Vibrio cholerae

Cholera is a severe diarrheal disease typically caused by O1 serogroup strains of Vibrio cholerae. The pathogenicity of all pandemic V. cholerae O1 strains relies on two critical virulence factors: cholera toxin, a potent enterotoxin, and toxin coregulated pilus (TCP), an intestinal colonization factor. However, certain non-O1, non-O139 V. cholerae strains, such as AM-19226, do not produce cholera toxin or TCP, yet they still cause severe diarrhea. The molecular basis for the pathogenicity of non-O1, non-O139 V. cholerae has not been extensively characterized, but many of these strains encode related type III secretion systems (TTSSs). Here, we used infant rabbits to assess the contribution of the TTSS to non-O1, non-O139 V. cholerae pathogenicity. We found that all animals infected with wild-type AM-19226 developed severe diarrhea even more rapidly than rabbits infected with V. cholerae O1. Unlike V. cholerae O1 strains, which do not damage the intestinal epithelium in rabbits or humans, AM-19226 caused marked disruptions of the epithelial surface in the rabbit small intestine. TTSS proved to be essential for AM-19226 virulence in infant rabbits; an AM-19226 derivative deficient for TTSS did not elicit diarrhea, colonize the intestine, or induce pathological changes in the intestine. Deletion of either one of the two previously identified or two newly identified AM-19226 TTSS effectors reduced but did not eliminate AM-19226 pathogenicity, suggesting that at least four effectors contribute to this strain’s virulence. In aggregate, our results suggest that the TTSS-dependent virulence in non-O1, non-O139 V. cholerae represents a new type of diarrheagenic mechanism.

Cholera, which is caused by Vibrio cholerae, is an important cause of diarrheal disease in many developing countries. The mechanisms of virulence of nonpandemic strains that can cause a diarrheal illness are poorly understood. AM-19226, like several other pathogenic, nonpandemic V. cholerae strains, carries genes that encode a type III secretion system (TTSS), but not cholera toxin (CT) or toxin coregulated pilus (TCP). In this study, we used infant rabbits to study AM-19226 virulence. Infant rabbits orally inoculated with this strain rapidly developed a fatal diarrheal disease, which was accompanied by marked disruptions of the intestinal epithelium. This strain’s TTSS proved essential for its pathogenicity, and there was no diarrhea, intestinal pathology, or colonization in rabbits infected with a TTSS mutant. The effector proteins translocated by the TTSS all appear to contribute to AM-19226 virulence. Thus, our study provides insight into in vivo mechanisms by which a novel TTSS contributes to diarrheal disease caused by nonpandemic strains of V. cholerae.

Shigella flexneri Spa15 crystal structure verified in solution by double electron electron resonance

Shigella flexneri Spa15 is a chaperone of the type 3 secretion system, which binds a number of effectors to ensure their stabilization prior to secretion. One of these effectors is IpgB1, a mimic of the human Ras-like Rho guanosine triphosphatase RhoG. In this study, Spa15 alone and in complex with IpgB1 has been studied by double electron electron resonance, an experiment that gives distance information showing the spacial separation of attached spin labels. This distance is explained by determining the crystal structure of the spin-labeled Spa15 where labels are seen to be buried in hydrophobic pockets. The double electron electron resonance experiment on the Spa15 complex with IpgB1 shows that IpgB1 does not bind Spa15 in the same way as is seen in the homologous Salmonella sp. chaperone:effector complex InvB:SipA.

The KdpD/KdpE two-component system of Photorhabdus asymbiotica promotes bacterial survival within M. sexta hemocytes

Many bacteria persist within phagocytes, deploying complex sets of tightly regulated virulence factors to manipulate and survive within host cells. So far, no single factor has been identified that is sufficient to allow intracellular persistence of an otherwise non-pathogenic bacterium. Here we report that the two-component KdpD/KdpE sensor kinase/response regulator of the insect and human pathogen Photorhabdus asymbiotica (Pa) is sufficient to allow a harmless laboratory strain of E. coli to resist phagocytic killing and persist within insect hemocytes, ultimately killing the insect. Screening of a cosmid library of Pa in E. coli by injection into the moth Manduca sexta, previously identified three overlapping clones which caused the insect to cease feeding and subsequently die. Transposon mutagenesis revealed a cosmid encoded kdp high affinity potassium pump regulon was responsible for this phenotype. Gentamycin protection assays and confocal microscopy revealed the cosmid clones were persisting inside insect hemocytes far longer than control bacteria. Cloning and expression of PakdpD/kdpE alone into E. coli recapitulated the phenotype. Bioassay results and transcriptional analysis of various E. coli kdp mutants harboring the Pa kdp genes confirmed that Pa KdpD/KdpE was able to induce the E. coli kdp pump structural genes in response to exposure to insect hemocytes but not blood plasma alone. The finding that Pa KdpD/KdpE can facilitate resistance of E. coli to phagocytic killing suggests a central role for potassium in this process, supporting previous work implicating potassium sensing in virulence of other bacteria and also in the normal process of protease killing of engulfed bacteria by neutrophils.

Pairwise network mechanisms in the host signaling response to coxsackievirus B3 infection

Signal transduction networks can be perturbed biochemically, genetically, and pharmacologically to unravel their functions. But at the systems level, it is not clear how such perturbations are best implemented to extract molecular mechanisms that underlie network function. Here, we combined pairwise perturbations with multiparameter phosphorylation measurements to reveal causal mechanisms within the signaling network response of cardiomyocytes to coxsackievirus B3 (CVB3) infection. Using all possible pairs of six kinase inhibitors, we assembled a dynamic nine-protein phosphorylation signature of perturbed CVB3 infectivity. Cluster analysis of the resulting dataset showed repeatedly that paired inhibitor data were required for accurate data-driven predictions of kinase substrate links in the host network. With pairwise data, we also derived a high-confidence network based on partial correlations, which identified phospho-IκBα as a central "hub" in the measured phosphorylation signature. The reconstructed network helped to connect phospho-IκBα with an autocrine feedback circuit in host cells involving the proinflammatory cytokines, TNF and IL-1. Autocrine blockade substantially inhibited CVB3 progeny release and improved host cell viability, implicating TNF and IL-1 as cell autonomous components of CVB3-induced myocardial damage. We conclude that pairwise perturbations, when combined with network-level intracellular measurements, enrich for mechanisms that would be overlooked by single perturbants.

Clearance of apoptotic cells: implications in health and disease

Recent advances in defining the molecular signaling pathways that regulate the phagocytosis of apoptotic cells have improved our understanding of this complex and evolutionarily conserved process. Studies in mice and humans suggest that the prompt removal of dying cells is crucial for immune tolerance and tissue homeostasis. Failed or defective clearance has emerged as an important contributing factor to a range of disease processes. This review addresses how specific molecular alterations of engulfment pathways are linked to pathogenic states. A better understanding of the apoptotic cell clearance process in healthy and diseased states could offer new therapeutic strategies.

The inflammatory cytokine tumor necrosis factor modulates the expression of Salmonella typhimurium effector proteins

Tumor necrosis factor alpha (TNF-alpha)is a host inflammatory factor. Bacteria increase TNF-alpha expression in a variety of human diseases including infectious diseases, inflammatory bowel diseases, and cancer. It is unknown, however, how TNF-alpha directly modulates bacterial protein expression during intestinal infection and chronic inflammation. In the current study, we hypothesize that Salmonella typhimurium senses TNF-alpha and show that TNF-alpha treatment modulates Salmonella virulent proteins (called effectors), thus changing the host-bacterial interaction in intestinal epithelial cells. We investigated the expression of 23 Salmonella effectors after TNF-alpha exposure. We found that TNF-alpha treatment led to differential effector expression: effector SipA was increased by TNF-alpha treatment, whereas the expression levels of other effectors, including gogB and spvB, decreased in the presence of TNF-alpha. We verified the protein expression of Salmonella effectors AvrA and SipA by Western blots. Furthermore, we used intestinal epithelial cells as our experimental model to explore the response of human intestinal cells to TNF-alpha pretreated Salmonella. More bacterial invasion was found in host cells colonized with Salmonella strains pretreated with TNF-alpha compared to Salmonella without TNF-alpha treatment. TNF-alpha pretreated Salmonella induced higher proinflammatory JNK signalling responses compared to the Salmonella strains without TNF-alpha exposure. Exposure to TNF-alpha made Salmonella to induce more inflammatory cytokine IL-8 in intestinal epithelial cells. JNK inhibitor treatment was able to suppress the effects of TNF-pretreated-Salmonella in enhancing expressions of phosphorylated-JNK and c-jun and secretion of IL-8. Overall, our study provides new insights into Salmonella-host interactions in intestinal inflammation.

Phosphoproteomic analysis reveals the multiple roles of phosphorylation in pathogenic bacterium Streptococcus pneumoniae

Recent phosphoproteomic characterizations of Bacillus subtilis, Escherichia coli, Lactococcus lactis, Pseudomonas putida, and Pseudomonas aeruginosa have suggested that protein phosphorylation on serine, threonine, and tyrosine residues is a major regulatory post-translational modification in bacteria. In this study, we carried out a global and site-specific phosphoproteomic analysis on the Gram-positive pathogenic bacterium Streptococcus pneumoniae. One hundred and two unique phosphopeptides and 163 phosphorylation sites with distributions of 47%/44%/9% for Ser/Thr/Tyr phosphorylations from 84 S. pneumoniae proteins were identified through the combined use of TiO(2) enrichment and LC-MS/MS determination. The identified phosphoproteins were found to be involved in various biological processes including carbon/protein/nucleotide metabolisms, cell cycle and division regulation. A striking characteristic of S. pneumoniae phosphoproteome is the large number of multiple species-specific phosphorylated sites, indicating that high level of protein phosphorylation may play important roles in regulating many metabolic pathways and bacterial virulence.

A bacterial effector targets host DH-PH domain RhoGEFs and antagonizes macrophage phagocytosis

Bacterial pathogens often harbour a type III secretion system (TTSS) that injects effector proteins into eukaryotic cells to manipulate host processes and cause diseases. Identification of host targets of bacterial effectors and revealing their mechanism of actions are crucial for understating bacterial virulence. We show that EspH, a type III effector conserved in enteric bacterial pathogens including enteropathogenic Escherichia coli (EPEC), enterohaemorrhagic E. coli and Citrobacter rodentium, markedly disrupts actin cytoskeleton structure and induces cell rounding up when ectopically expressed or delivered into HeLa cells by the bacterial TTSS. EspH inactivates host Rho GTPase signalling pathway at the level of RhoGEF. EspH directly binds the DH-PH domain in multiple RhoGEFs, which prevents their binding to Rho and thereby inhibits nucleotide exchange-mediated Rho activation. Consistently, infection of mouse macrophages with EPEC harbouring EspH attenuates phagocytosis of the bacteria as well as FcgammaR-mediated phagocytosis. EspH represents the first example of targeting RhoGEFs by bacterial effectors, and our results also reveal an unprecedented mechanism used by enteric pathogens to counteract the host defence system.

The presence of professional phagocytes dictates the number of host cells targeted for Yop translocation during infection

Type III secretion systems deliver effector proteins from Gram-negative bacterial pathogens into host cells, where they disarm host defences, allowing the pathogens to establish infection. Although Yersinia pseudotuberculosis delivers its effector proteins, called Yops, into numerous cell types grown in culture, we show that during infection Y. pseudotuberculosis selectively targets Yops to professional phagocytes in Peyer's patches, mesenteric lymph nodes and spleen, although it colocalizes with B and T cells as well as professional phagocytes. Strikingly, in the absence of neutrophils, the number of cells with translocated Yops was significantly reduced although the bacterial loads were similar, indicating that Y. pseudotuberculosis did not arbitrarily deliver Yops to the available cells. Using isolated splenocytes, selective binding and selective targeting to professional phagocytes when bacteria were limiting was also observed, indicating that tissue architecture was not required for the tropism for professional phagocytes. In isolated splenocytes, YadA and Invasin increased the number of all cells types with translocated Yops, but professional phagocytes were still preferentially translocated with Yops in the absence of these adhesins. Together these results indicate that Y. pseudotuberculosis discriminates among cells it encounters during infection and selectively delivers Yops to phagocytes while refraining from translocation to other cell types.

Pathogenic Bacterial Proteins and their Anti-Inflammatory Effects in the Eukaryotic Host

Bacteria use multiple strategies to bypass the inflammatory responses in order to survive in the host cells. In this review, we discuss the mechanism of the bacerial proteins in inhibiting inflammation. We highlight the anti-inflammatory roles of the type three secretion proteins including Salmonella AvrA, Enteropathogenic Escherichia coli Cif, and Yersinia YopJ, Staphylococcus aureus extracellular adherence protein, and Chlamydia proteins. We also discuss the research progress on the structures of these anti-inflammatory bacterial proteins. The current therapeutic methods for diseases, such as inflammatory bowel diseases, sclerosis, lack influence on the course of chronic inflammation and infection. Therefore, based on the molecular mechanism of the anti-inflammatory bacterial proteins and their 3-Dimension structure, we can design new peptides or non-peptidic molecules that serve as anti-inflammatory drugs without the possible side effect of promoting bacterial infection.

Gene Ontology for type III effectors: capturing processes at the host-pathogen interface

Disease development is determined by the interplay of host defense processes and pathogen factors that subvert defenses and remodel the host for parasitic benefit. The goal of the Plant-Associated Microbe Gene Ontology (PAMGO) interest group is the development of Gene Ontology (GO) terms that capture the range of biological processes occurring between hosts and symbionts (from mutualists to pathogens). Here, the application of the new GO terms to type III effector proteins (T3Es) from the plant pathogen Pseudomonas syringae serves as an example to systematically document the available extensive data and to reveal shared aspects of interactions with various host plants. Extending the comparison to T3Es deployed by animal pathogens further highlights how GO can uncover the common strategies employed by diverse symbionts as they exploit the host niche. Future application of GO terms to gene products mediating pathogenic or mutualistic interactions involving other microbes will enhance researchers' abilities to identify fundamental patterns among diverse systems and generate new hypotheses based on associations among annotations.

Bacterial growth restriction during host resistance to Pseudomonas syringae is associated with leaf water loss and localized cessation of vascular activity in Arabidopsis thaliana

The physiological mechanisms by which plants limit the growth of bacterial pathogens during gene-for-gene resistance are poorly understood. We characterized early events in the Arabidopsis thaliana-Pseudomonas syringae pathosystem to identify physiological changes for which the kinetics are consistent with bacterial growth restriction. Using a safranine-O dye solution to detect vascular activity, we demonstrated that A. thaliana Col-0 resistance to P. syringae pv. tomato DC3000 cells expressing avrRpm1 involved virtually complete cessation of vascular water movement into the infection site within only 3 h postinoculation (hpi), under the conditions tested. This vascular restriction preceded or was simultaneous with precipitous decreases in photosynthesis, stomatal conductance, and leaf transpiration, with the latter two remaining at detectable levels. Microscopic plant cell death was detected as early as 2 hpi. Interestingly, suppression of bacterial growth during AvrRpm1-mediated resistance was eliminated by physically blocking leaf water loss through the stomata without altering plant cell death and was nearly eliminated by incubating plants at high relative humidity. The majority of the population growth benefit from blocking leaf water loss occurred early after inoculation, i.e., between 4 and 8 hpi. Collectively, these results support a model in which A. thaliana suppresses P. syringae growth during gene-for-gene resistance, at least in part, by coupling restricted vascular flow to the infection site with water loss through partially open stomata; that is, the plants effectively starve the invading bacteria for water.

Arabidopsis actin-depolymerizing factor AtADF4 mediates defense signal transduction triggered by the Pseudomonas syringae effector AvrPphB

The actin cytoskeleton has been implicated in plant defenses against pathogenic fungi and oomycetes with limited, indirect evidence. To date, there are no reports linking actin with resistance against phytopathogenic bacteria. The dynamic behavior of actin filaments is regulated by a diverse array of actin-binding proteins, among which is the Actin-Depolymerizing Factor (ADF) family of proteins. Here, we demonstrate that actin dynamics play a role in the activation of gene-for-gene resistance in Arabidopsis (Arabidopsis thaliana) following inoculation with the phytopathogenic bacterium Pseudomonas syringae pv tomato. Using a reverse genetics approach, we explored the roles of Arabidopsis ADFs in plant defenses. AtADF4 was identified as being specifically required for resistance triggered by the effector AvrPphB but not AvrRpt2 or AvrB. Recombinant AtADF4 bound to monomeric actin (G-actin) with a marked preference for the ADP-loaded form and inhibited the rate of nucleotide exchange on G-actin, indicating that AtADF4 is a bona fide actin-depolymerizing factor. Exogenous application of the actin-disrupting agent cytochalasin D partially rescued the Atadf4 mutant in the AvrPphB-mediated hypersensitive response, demonstrating that AtADF4 mediates defense signaling through modification of the actin cytoskeleton. Unlike the mechanism by which the actin cytoskeleton confers resistance against fungi and oomycetes, AtADF4 is not involved in resistance against pathogen entry. Collectively, this study identifies AtADF4 as a novel component of the plant defense signaling pathway and provides strong evidence for actin dynamics as a primary component that orchestrates plant defenses against P. syringae.