Cycle inhibiting factors (CIFs) are a growing family of functional cyclomodulins present in invertebrate and mammal bacterial pathogens

Contribution of pks+ Escherichia coli (E. coli) to Colon Carcinogenesis

Colorectal cancer (CRC) stands as a significant global health concern, ranking second in mortality and third in frequency among cancers worldwide. While only a small fraction of CRC cases can be attributed to inherited genetic mutations, the majority arise sporadically due to somatic mutations. Emerging evidence reveals gut microbiota dysbiosis to be a contributing factor, wherein polyketide synthase-positive Escherichia coli (pks+ E. coli) plays a pivotal role in CRC pathogenesis. pks+ bacteria produce colibactin, a genotoxic protein that causes deleterious effects on DNA within host colonocytes. In this review, we examine the role of the gut microbiota in colon carcinogenesis, elucidating how colibactin-producer bacteria induce DNA damage, promote genomic instability, disrupt the gut epithelial barrier, induce mucosal inflammation, modulate host immune responses, and influence cell cycle dynamics. Collectively, these actions foster a microenvironment conducive to tumor initiation and progression. Understanding the mechanisms underlying pks+ bacteria-mediated CRC development may pave the way for mass screening, early detection of tumors, and therapeutic strategies such as microbiota modulation, bacteria-targeted therapy, checkpoint inhibition of colibactin production and immunomodulatory pathways.

The Role of Cyclomodulins and Some Microbial Metabolites in Bacterial Microecology and Macroorganism Carcinogenesis

A number of bacteria that colonize the human body produce toxins and effectors that cause changes in the eukaryotic cell cycle—cyclomodulins and low-molecular-weight compounds such as butyrate, lactic acid, and secondary bile acids. Cyclomodulins and metabolites are necessary for bacteria as adaptation factors—which are influenced by direct selection—to the ecological niches of the host. In the process of establishing two-way communication with the macroorganism, these compounds cause limited damage to the host, despite their ability to disrupt key processes in eukaryotic cells, which can lead to pathological changes. Possible negative consequences of cyclomodulin and metabolite actions include their potential role in carcinogenesis, in particular, with the ability to cause DNA damage, increase genome instability, and interfere with cancer-associated regulatory pathways. In this review, we aim to examine cyclomodulins and bacterial metabolites as important factors in bacterial survival and interaction with the host organism to show their heterogeneous effect on oncogenesis depending on the surrounding microenvironment, pathological conditions, and host genetic background.

Cycle-Inhibiting Factor Is Associated with Burkholderia pseudomallei Invasion in Human Neuronal Cells

Burkholderia pseudomallei is a pathogenic bacterium that causes human melioidosis, which is associated with a high mortality rate. However, the underlying mechanisms of B. pseudomallei pathogenesis are largely unknown. In this study, we examined the infection of human neuronal SH-Sy5y cells by several clinically relevant B. pseudomallei strains. We found that all tested B. pseudomallei strains can invade SH-Sy5y cells, undergo intracellular replication, cause actin-tail formation, and form multinucleated giant cells. Additionally, a deletion mutant of B. pseudomallei cycle-inhibiting factor (cif) was constructed that exhibited reduced invasion in SH-Sy5y cells. Complementation of cif restored invasion of the B. pseudomallei cif-deleted mutant. Our findings enhance understanding of B. pseudomallei pathogenicity in terms of the virulence factor Cif and demonstrate the function of Cif in neurological melioidosis. This may eventually lead to the discovery of novel targets for treatment and a strategy to control the disease.

Bacterial cyclomodulins: types and roles in carcinogenesis

Various studies confirmed that bacterial infections contribute to carcinogenesis through the excessive accumulation of reactive oxygen species (ROS) and the expression of toxins that disrupt the cell cycle phases, cellular regulatory mechanisms and stimulate the production of tumorigenic inflammatory mediators. These toxins mimic carcinogens which act upon key cellular targets and result in mutations and genotoxicities. The cyclomodulins are bacterial toxins that incur cell cycle modulating effects rendering the expressing bacterial species of high carcinogenic potentiality. They are either cellular proliferating or cell cycle arrest cyclomodulins. Notably, cyclomodulins expressing bacterial species have been linked to different human carcinomas. For instance, Escherichia coli species producing the colibactin were highly prevalent among colorectal carcinoma patients, CagA⁺ Helicobacter pylori species were associated with MALT lymphomas and gastric carcinomas and Salmonella species producing CdtB were linked to hepatobiliary carcinomas. These species stimulated the overgrowth of pre-existing carcinomas and induced hyperplasia in in vivo animal models suggesting a role for the cyclomodulins in carcinogenesis. Wherefore, the prevalence and mode of action of these toxins were the focus of many researchers and studies. This review discusses different types of bacterial cyclomodulins highlighting their mode of action and possible role in carcinogenesis.

Ubiquitin-related processes and innate immunity in C. elegans

Innate immunity is an evolutionary ancient defence strategy that serves to eliminate infectious agents while maintaining host health. It involves a complex network of sensors, signaling proteins and immune effectors that detect the danger, then relay and execute the immune programme. Post-translational modifications relying on conserved ubiquitin and ubiquitin-like proteins are an integral part of the system. Studies using invertebrate models of infection, such as the nematode Caenorhabditis elegans, have greatly contributed to our understanding of how ubiquitin-related processes act in immune sensing, regulate immune signaling pathways, and participate to host defence responses. This review highlights the interest of working with a genetically tractable model organism and illustrates how C. elegans has been used to identify ubiquitin-dependent immune mechanisms, discover novel ubiquitin-based resistance strategies that mediate pathogen clearance, and unravel the role of ubiquitin-related processes in tolerance, preserving host fitness during pathogen attack. Special emphasis is placed on processes that are conserved in mammals.

From Gene to Protein-How Bacterial Virulence Factors Manipulate Host Gene Expression During Infection

Bacteria evolved many strategies to survive and persist within host cells. Secretion of bacterial effectors enables bacteria not only to enter the host cell but also to manipulate host gene expression to circumvent clearance by the host immune response. Some effectors were also shown to evade the nucleus to manipulate epigenetic processes as well as transcription and mRNA procession and are therefore classified as nucleomodulins. Others were shown to interfere downstream with gene expression at the level of mRNA stability, favoring either mRNA stabilization or mRNA degradation, translation or protein stability, including mechanisms of protein activation and degradation. Finally, manipulation of innate immune signaling and nutrient supply creates a replicative niche that enables bacterial intracellular persistence and survival. In this review, we want to highlight the divergent strategies applied by intracellular bacteria to evade host immune responses through subversion of host gene expression via bacterial effectors. Since these virulence proteins mimic host cell enzymes or own novel enzymatic functions, characterizing their properties could help to understand the complex interactions between host and pathogen during infections. Additionally, these insights could propose potential targets for medical therapy.

Antitumor Effect of Cycle Inhibiting Factor Expression in Colon Cancer via Salmonella VNP20009

BACKGROUND

Colon cancer is one of the major causes of morbidity and mortality worldwide. Cycle inhibiting factors (Cifs) have been shown to deamidate Nedd8, resulting in cell cycle arrest.

OBJECTIVE

To determine the antitumor effect of Cifs on colon cancer by using attenuated Salmonella typhimurium VNP20009.

METHODS

The VNP-SOPE2-cif and VNP-SOPE2-cif-C/A plasmids were transfected into attenuated Salmonella typhimurium VNP20009. The efficiency and specificity of the Cif promoter were validated in colon cancer SW480 cell lines. Western blotting was subsequently performed to evaluate cell cycle regulators, including P21, P27 and Wee1. In vivo, the antitumor effect of VNP20009 was evaluated in a colon cancer xenograft model.

RESULTS

Firstly, VNP-SOPE2-cif and VNP-SOPE2-cif-C/A were selectively expressed both in the bacterial and colon cancer cells. Cif expression in SW480 cells via the VNP tumor-targeted expression system induced the accumulation of Wee1, p21 and p27 expression. Moreover, tumor growth was significantly inhibited in the mice with VNP-SOPE2-cif compared to the mice with VNP with the empty construct.

CONCLUSION

These results suggest that Cif gene delivered by VNP20009 is a promising approach for the treatment of colon cancer.

Bacterial Factors Targeting the Nucleus: The Growing Family of Nucleomodulins

Pathogenic bacteria secrete a variety of proteins that manipulate host cell function by targeting components of the plasma membrane, cytosol, or organelles. In the last decade, several studies identified bacterial factors acting within the nucleus on gene expression or other nuclear processes, which has led to the emergence of a new family of effectors called “nucleomodulins”. In human and animal pathogens, Listeria monocytogenes for Gram-positive bacteria and Anaplasma phagocytophilum, Ehrlichia chaffeensis, Chlamydia trachomatis, Legionella pneumophila, Shigella flexneri, and Escherichia coli for Gram-negative bacteria, have led to pioneering discoveries. In this review, we present these paradigms and detail various mechanisms and core elements (e.g., DNA, histones, epigenetic regulators, transcription or splicing factors, signaling proteins) targeted by nucleomodulins. We particularly focus on nucleomodulins interacting with epifactors, such as LntA of Listeria and ankyrin repeat- or tandem repeat-containing effectors of Rickettsiales, and nucleomodulins from various bacterial species acting as post-translational modification enzymes. The study of bacterial nucleomodulins not only generates important knowledge about the control of host responses by microbes but also creates new tools to decipher the dynamic regulations that occur in the nucleus. This research also has potential applications in the field of biotechnology. Finally, this raises questions about the epigenetic effects of infectious diseases.

Potential Use of Biotherapeutic Bacteria to Target Colorectal Cancer-Associated Taxa

The role of the gut microbiome in human health and disease is the focus of much attention. It has been widely agreed upon that our gut bacteria play a role in host immunity, nutrient absorption, digestion, metabolism, and other key drivers of health. Furthermore, certain microbial signatures and specific taxa have also been associated with the development of diseases, such as obesity; inflammatory bowel disease; and, indeed, colorectal cancer (CRC), which is the focus of this review. By extension, such taxa represent potential therapeutic targets. In particular, the emerging human pathogen Fusobacterium nucleatum represents an important agent in CRC development and its control within the gastrointestinal tract is desirable. This paper reviews the principal bacterial pathogens that have been associated with CRC to date and discusses the in vitro and human studies that have shown the potential use of biotherapeutic strains as a means of targeting CRC-associated bacteria.

Discovery of Ubiquitin Deamidases in the Pathogenic Arsenal of Legionella pneumophila

Legionella pneumophila translocates the largest known arsenal of over 330 pathogenic factors, called "effectors," into host cells during infection, enabling L. pneumophila to establish a replicative niche inside diverse amebas and human macrophages. Here, we reveal that the L. pneumophila effectors MavC (Lpg2147) and MvcA (Lpg2148) are structural homologs of cycle inhibiting factor (Cif) effectors and that the adjacent gene, lpg2149, produces a protein that directly inhibits their activity. In contrast to canonical Cifs, both MavC and MvcA contain an insertion domain and deamidate the residue Gln40 of ubiquitin but not Gln40 of NEDD8. MavC and MvcA are functionally diverse, with only MavC interacting with the human E2-conjugating enzyme UBE2N (Ubc13). MavC deamidates the UBE2N∼Ub conjugate, disrupting Lys63 ubiquitination and dampening NF-κB signaling. Combined, our data reveal a molecular mechanism of host manipulation by pathogenic bacteria and highlight the complex regulatory mechanisms integral to L. pneumophila's pathogenic strategy.

Probiotics: If It Does Not Help It Does Not Do Any Harm. Really?

Probiotics per definition should have beneficial effects on human health, and their consumption has tremendously increased in the last decades. In parallel, the amount of published material and claims for their beneficial efficacy soared continuously. Recently, multiple systemic reviews, meta-analyses, and expert opinions expressed criticism on their claimed effects and safety. The present review describes the dark side of the probiotics, in terms of problematic research design, incomplete reporting, lack of transparency, and under-reported safety. Highlighted are the potential virulent factors and the mode of action in the intestinal lumen, risking the physiological microbiome equilibrium. Finally, regulatory topics are discussed to lighten the heterogeneous guidelines applied worldwide. The shift in the scientific world towards a better understanding of the human microbiome, before consumption of the probiotic cargo, is highly endorsed. It is hoped that better knowledge will extend the probiotic repertoire, re-confirm efficacy or safety, establish their efficacy and substantiate their beneficial effects.

Revisiting Bacterial Ubiquitin Ligase Effectors: Weapons for Host Exploitation

Protein ubiquitylation plays a central role in eukaryotic cell physiology. It is involved in several regulatory processes, ranging from protein folding or degradation, subcellular localization of proteins, vesicular trafficking and endocytosis to DNA repair, cell cycle, innate immunity, autophagy, and apoptosis. As such, it is reasonable that pathogens have developed a way to exploit such a crucial system to enhance their virulence against the host. Hence, bacteria have evolved a wide range of effectors capable of mimicking the main players of the eukaryotic ubiquitin system, in particular ubiquitin ligases, by interfering with host physiology. Here, we give an overview of this topic and, in particular, we detail and discuss the mechanisms developed by pathogenic bacteria to hijack the host ubiquitination system for their own benefit.

Characterisation of cellular effects of Burkholderia pseudomallei cycle inhibiting factor (Cif)

Cycle inhibiting factors (Cifs) are type III secretion system effectors produced by some Gram-negative pathogenic bacteria including Burkholderia pseudomallei. Through their deamidase activity, Cifs inhibit the activity of Cullin RING E3 ubiquitin ligases (CRL). CRL inhibition induces the accumulation of cell cycle inhibitors p21 and p27, thereby leading to host cell cycle arrest. However, whether Cif exerts additional effects on host cells that are important in bacterial pathogenesis is currently poorly understood. In this study, we found that Cif exerts a bimodal effect on NF-κB signalling. Cif increases basal NF-κB activity. This effect is dependent on Cif-mediated activation of ERK MAPK. On the other hand, Cif inhibits NF-κB activation by TNFα and Burkholderia thailandensis infection. This inhibitory effect on NF-κB activity is partially mediated by Cif-dependent inhibition of CRLs. We also found that Cif only has a modest effect in stimulating the intracellular replication of the B. pseudomallei surrogate, B. thailandensis. The observed Cif-dependent stimulation of B. thailandensis intracellular replication was not, or was only partially, due to CRL inhibition. Furthermore, the increased B. thailandensis replication induced by Cif was independent of ERK MAPK activation. Our findings suggest that Cif likely exerts additional cellular effects through novel targets.

Summary: Cycle inhibiting factor (Cif) is a Burkholderia pseudomallei virulence factor and is shown to exert both Cullin RING E3 ligase dependent and independent effects on host cells.

Secretion Systems and Secreted Proteins in Gram-Negative Entomopathogenic Bacteria: Their Roles in Insect Virulence and Beyond

Many Gram-negative bacteria have evolved insect pathogenic lifestyles. In all cases, the ability to cause disease in insects involves specific bacterial proteins exported either to the surface, the extracellular environment, or the cytoplasm of the host cell. They also have several distinct mechanisms for secreting such proteins. In this review, we summarize the major protein secretion systems and discuss examples of secreted proteins that contribute to the virulence of a variety of Gram-negative entomopathogenic bacteria, including Photorhabdus, Xenorhabdus, Serratia, Yersinia, and Pseudomonas species. We also briefly summarize two classes of exported protein complexes, the PVC-like elements, and the Tc toxin complexes that were first described in entomopathogenic bacteria.

Heterogeneous Family of Cyclomodulins: Smart Weapons That Allow Bacteria to Hijack the Eukaryotic Cell Cycle and Promote Infections

Some bacterial pathogens modulate signaling pathways of eukaryotic cells in order to subvert the host response for their own benefit, leading to successful colonization and invasion. Pathogenic bacteria produce multiple compounds that generate favorable conditions to their survival and growth during infection in eukaryotic hosts. Many bacterial toxins can alter the cell cycle progression of host cells, impairing essential cellular functions and impeding host cell division. This review summarizes current knowledge regarding cyclomodulins, a heterogeneous family of bacterial effectors that induce eukaryotic cell cycle alterations. We discuss the mechanisms of actions of cyclomodulins according to their biochemical properties, providing examples of various cyclomodulins such as cycle inhibiting factor, γ-glutamyltranspeptidase, cytolethal distending toxins, shiga toxin, subtilase toxin, anthrax toxin, cholera toxin, adenylate cyclase toxins, vacuolating cytotoxin, cytotoxic necrotizing factor, Panton-Valentine leukocidin, phenol soluble modulins, and mycolactone. Special attention is paid to the benefit provided by cyclomodulins to bacteria during colonization of the host.

Activation of MAPK/ERK signaling by Burkholderia pseudomallei cycle inhibiting factor (Cif)

Cycle inhibiting factors (Cifs) are virulence proteins secreted by the type III secretion system of some Gram-negative pathogenic bacteria including Burkholderia pseudomallei. Cif is known to function to deamidate Nedd8, leading to inhibition of Cullin E3 ubiquitin ligases (CRL) and consequently induction of cell cycle arrest. Here we show that Cif can function as a potent activator of MAPK/ERK signaling without significant activation of other signaling pathways downstream of receptor tyrosine kinases. Importantly, we found that the ability of Cif to activate ERK is dependent on its deamidase activity, but independent of Cullin E3 ligase inhibition. This suggests that apart from Nedd8, other cellular targets of Cif-dependent deamidation exist. We provide evidence that the mechanism involved in Cif-mediated ERK activation is dependent on recruitment of the Grb2-SOS1 complex to the plasma membrane. Further investigation revealed that Cif appears to modify the phosphorylation status of SOS1 in a region containing the CDC25-H and proline-rich domains. It is known that prolonged Cullin E3 ligase inhibition leads to cellular apoptosis. Therefore, we hypothesize that ERK activation is an important mechanism to counter the pro-apoptotic effects of Cif. Indeed, we show that Cif dependent ERK activation promotes phosphorylation of the proapoptotic protein Bim, thereby potentially conferring a pro-survival signal. In summary, we identified a novel deamidation-dependent mechanism of action of the B. pseudomallei virulence factor Cif/CHBP to activate MAPK/ERK signaling. Our study demonstrates that bacterial proteins such as Cif can serve as useful molecular tools to uncover novel aspects of mammalian signaling pathways.

Relationship between intestinal microbiota and colorectal cancer

The human gastrointestinal tract hosts a complex and vast microbial community with up to 10¹¹-10¹² microorganisms colonizing the colon. The gut microbiota has a serious effect on homeostasis and pathogenesis through a number of mechanisms. In recent years, the relationship between the intestinal microbiota and sporadic colorectal cancer has attracted much scientific interest. Mechanisms underlying colonic carcinogenesis include the conversion of procarcinogenic diet-related factors to carcinogens and the stimulation of procarcinogenic signaling pathways in luminal epithelial cells. Understanding each of these mechanisms will facilitate future studies, leading to the development of novel strategies for the diagnosis, treatment, and prevention of colorectal cancer. In this review, we discuss the relationship between colorectal cancer and the intestinal microbiota.

Enteric pathogens deploy cell cycle inhibiting factors to block the bactericidal activity of Perforin-2

Perforin-2 (MPEG1) is an effector of the innate immune system that limits the proliferation and spread of medically relevant Gram-negative, -positive, and acid fast bacteria. We show here that a cullin-RING E3 ubiquitin ligase (CRL) complex containing cullin-1 and βTrCP monoubiquitylates Perforin-2 in response to pathogen associated molecular patterns such as LPS. Ubiquitylation triggers a rapid redistribution of Perforin-2 and is essential for its bactericidal activity. Enteric pathogens such as Yersinia pseudotuberculosis and enteropathogenic Escherichia coli disarm host cells by injecting cell cycle inhibiting factors (Cifs) into mammalian cells to deamidate the ubiquitin-like protein NEDD8. Because CRL activity is dependent upon NEDD8, Cif blocks ubiquitin dependent trafficking of Perforin-2 and thus, its bactericidal activity. Collectively, these studies further underscore the biological significance of Perforin-2 and elucidate critical molecular events that culminate in Perforin-2-dependent killing of both intracellular and extracellular, cell-adherent bacteria.

DOI:http://dx.doi.org/10.7554/eLife.06505.001

A wide range of bacteria and other microbes can infect animals and cause disease. Throughout evolution, these microbes and their hosts have been fighting never ending arms races in which the microbes deploy ever more elaborate weapons, while the hosts adapt to defend themselves. An animal's first line of defense is provided by its ‘innate’ immune system. This system is activated by the general features of microbial cells; for example, the molecules that make up the walls surrounding most bacteria. Microbes must defeat the innate immune system in order to cause disease, and ultimately to spread from one host to the next.

One component of innate immunity is a protein called Perforin-2 that is present in most, if not all, animal cells. This protein forms pores on bacterial cells, causing them to split open and die. However, it was not clear how Perforin-2 is switched on and what, if anything, bacteria do to counteract it. To address these questions, McCormack et al. infected human and mice cells with bacteria that cause serious diseases of the digestive tract.

The experiments show that when animal cells detect bacteria, or merely a fragment of their cell wall, a specific group of proteins, called the CRL complex, attaches a molecule called ubiquitin to Perforin-2. Ubiquitin works much like the shipping label of a package, enabling the efficient targeting of Perforin-2 to the invading bacteria. McCormack et al. also show that some bacteria use a protein called a cell cycle inhibiting factor (or Cif for short) to inhibit the CRL complex. This blocks the ubiquitin labeling of Perforin-2, which renders it a useless weapon that can no longer be directed towards bacteria.

Mice that are infected with a bacterium called Yersinia pseudotuberculosis become seriously unwell and often die. However, McCormack et al. found that mice infected with mutant Y. pseudotuberculosis that lacked Cif remained healthy. Also, mice that lacked Perforin-2 are highly susceptible to infectious diseases. McCormack et al.'s findings reveal how Perforin-2 is activated during the innate immune response and how some bacteria can defeat this pivotal defense. In the current age of antibiotic resistant bacteria, these studies may spur the development of new drugs that restore or increase the activity of Perforin-2.

DOI:http://dx.doi.org/10.7554/eLife.06505.002

Manual classification strategies in the ECOD database

ECOD (Evolutionary Classification Of protein Domains) is a comprehensive and up-to-date protein structure classification database. The majority of new structures released from the PDB (Protein Data Bank) each week already have close homologs in the ECOD hierarchy and thus can be reliably partitioned into domains and classified by software without manual intervention. However, those proteins that lack confidently detectable homologs require careful analysis by experts. Although many bioinformatics resources rely on expert curation to some degree, specific examples of how this curation occurs and in what cases it is necessary are not always described. Here, we illustrate the manual classification strategy in ECOD by example, focusing on two major issues in protein classification: domain partitioning and the relationship between homology and similarity scores. Most examples show recently released and manually classified PDB structures. We discuss multi-domain proteins, discordance between sequence and structural similarities, difficulties with assessing homology with scores, and integral membrane proteins homologous to soluble proteins. By timely assimilation of newly available structures into its hierarchy, ECOD strives to provide a most accurate and updated view of the protein structure world as a result of combined computational and expert-driven analysis.

Melioidosis: molecular aspects of pathogenesis

Burkholderia pseudomallei is a gram-negative bacterium that causes melioidosis, a multifaceted disease that is highly endemic in southeast Asia and northern Australia. This facultative intracellular pathogen possesses a large genome that encodes a wide array of virulence factors that promote survival in vivo by manipulating host cell processes and disarming elements of the host immune system. Antigens and systems that play key roles in B. pseudomallei virulence include capsular polysaccharide, lipopolysaccharide, adhesins, specialized secretion systems, actin-based motility and various secreted factors. This review provides an overview of the current and steadily expanding knowledge regarding the molecular mechanisms used by this organism to survive within a host and their contribution to the pathogenesis of melioidosis.

Analysis of the prevalence, secretion and function of a cell cycle-inhibiting factor in the melioidosis pathogen Burkholderia pseudomallei

Enteropathogenic and enterohaemorrhagic Escherichia coli express a cell cycle-inhibiting factor (Cif), that is injected into host cells via a Type III secretion system (T3SS) leading to arrest of cell division, delayed apoptosis and cytoskeletal rearrangements. A homologue of Cif has been identified in Burkholderia pseudomallei (CHBP; Cif homologue in B. pseudomallei; BPSS1385), which shares catalytic activity, but its prevalence, secretion and function are ill-defined. Among 43 available B. pseudomallei genome sequences, 33 genomes (76.7%) harbor the gene encoding CHBP. Western blot analysis using antiserum raised to a synthetic CHBP peptide detected CHBP in 46.6% (7/15) of clinical B. pseudomallei isolates from the endemic area. Secretion of CHBP into bacterial culture supernatant could not be detected under conditions where a known effector (BopE) was secreted in a manner dependent on the Bsa T3SS. In contrast, CHBP could be detected in U937 cells infected with B. pseudomallei by immunofluorescence microscopy and Western blotting in a manner dependent on bsaQ. Unlike E. coli Cif, CHBP was localized within the cytoplasm of B. pseudomallei-infected cells. A B. pseudomallei chbP insertion mutant showed a significant reduction in cytotoxicity and plaque formation compared to the wild-type strain that could be restored by plasmid-mediated trans-complementation. However, there was no defect in actin-based motility or multinucleated giant cell formation by the chbP mutant. The data suggest that the level or timing of CHBP secretion differs from a known Bsa-secreted effector and that CHBP is required for selected virulence-associated phenotypes in vitro.

What a difference a Dalton makes: bacterial virulence factors modulate eukaryotic host cell signaling systems via deamidation

Pathogenic bacteria commonly deploy enzymes to promote virulence. These enzymes can modulate the functions of host cell targets. While the actions of some enzymes can be very obvious (e.g., digesting plant cell walls), others have more subtle activities. Depending on the lifestyle of the bacteria, these subtle modifications can be crucially important for pathogenesis. In particular, if bacteria rely on a living host, subtle mechanisms to alter host cellular function are likely to dominate. Several bacterial virulence factors have evolved to use enzymatic deamidation as a subtle posttranslational mechanism to modify the functions of host protein targets. Deamidation is the irreversible conversion of the amino acids glutamine and asparagine to glutamic acid and aspartic acid, respectively. Interestingly, all currently characterized bacterial deamidases affect the function of the target protein by modifying a single glutamine residue in the sequence. Deamidation of target host proteins can disrupt host signaling and downstream processes by either activating or inactivating the target. Despite the subtlety of this modification, it has been shown to cause dramatic, context-dependent effects on host cells. Several crystal structures of bacterial deamidases have been solved. All are members of the papain-like superfamily and display a cysteine-based catalytic triad. However, these proteins form distinct structural subfamilies and feature combinations of modular domains of various functions. Based on the diverse pathogens that use deamidation as a mechanism to promote virulence and the recent identification of multiple deamidases, it is clear that this enzymatic activity is emerging as an important and widespread feature in bacterial pathogenesis.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Mining locus tags in PubMed Central to improve microbial gene annotation

BACKGROUND

The scientific literature contains millions of microbial gene identifiers within the full text and tables, but these annotations rarely get incorporated into public sequence databases. We propose to utilize the Open Access (OA) subset of PubMed Central (PMC) as a gene annotation database and have developed an R package called pmcXML to automatically mine and extract locus tags from full text, tables and supplements.

RESULTS

We mined locus tags from 1835 OA publications in ten microbial genomes and extracted tags mentioned in 30,891 sentences in main text and 20,489 rows in tables. We identified locus tag pairs marking the start and end of a region such as an operon or genomic island and expanded these ranges to add another 13,043 tags. We also searched for locus tags in supplementary tables and publications outside the OA subset in Burkholderia pseudomallei K96243 for comparison. There were 168 publications containing 48,470 locus tags and 83% of mentions were from supplementary materials and 9% from publications outside the OA subset.

CONCLUSIONS

B. pseudomallei locus tags within the full text and tables of OA publications represent only a small fraction of the total mentions in the literature. For microbial genomes with very few functionally characterized proteins, the locus tags mentioned in supplementary tables and within ranges like genomic islands contain the majority of locus tags. Significantly, the functions in the R package provide access to additional resources in the OA subset that are not currently indexed or returned by searching PMC.

The genetic basis of the symbiosis between Photorhabdus and its invertebrate hosts

Photorhabdus is a pathogen of insects that also maintains a mutualistic association with nematodes from the family Heterorhabditis. Photorhabdus colonizes the gut of the infective juvenile (IJ) stage of the nematode. The IJ infects an insect and regurgitates the bacteria and the bacteria reproduce to kill the insect. The nematodes feed on the resulting bacterial biomass until a new generation of IJs emerges from the insect cadaver. Therefore, during its life cycle, Photorhabdus must (1) kill the insect host, (2) support nematode growth and development, and (3) be able to colonize the new generation of IJs. In this review, functional genomic studies that have been aimed at understanding the molecular mechanisms underpinning each of these roles will be discussed. These studies have begun to reveal that distinct gene sets may be required for each of these interactions, suggesting that there is only a minimal genetic overlap between pathogenicity and mutualism in Photorhabdus.

Nonstructural protein σ1s mediates reovirus-induced cell cycle arrest and apoptosis

Reovirus nonstructural protein σ1s is implicated in cell cycle arrest at the G2/M boundary and induction of apoptosis. However, the contribution of σ1s to these effects in an otherwise isogenic viral background has not been defined. To evaluate the role of σ1s in cell cycle arrest and apoptosis, we used reverse genetics to generate a σ1s-null reovirus. Following infection with wild-type virus, we observed an increase in the percentage of cells in G2/M, whereas the proportion of cells in G2/M following infection with the σ1s-null mutant was unaffected. Similarly, we found that the wild-type virus induced substantially greater levels of apoptosis than the σ1s-null mutant. These data indicate that σ1s is required for both reovirus-induced cell cycle arrest and apoptosis. To define sequences in σ1s that mediate these effects, we engineered viruses encoding C-terminal σ1s truncations by introducing stop codons in the σ1s open reading frame. We also generated viruses in which charged residues near the σ1s amino terminus were replaced individually or as a cluster with nonpolar residues. Analysis of these mutants revealed that amino acids 1 to 59 and the amino-terminal basic cluster are required for induction of both cell cycle arrest and apoptosis. Remarkably, viruses that fail to induce cell cycle arrest and apoptosis also are attenuated in vivo. Thus, identical sequences in σ1s are required for reovirus-induced cell cycle arrest, apoptosis, and pathogenesis. Collectively, these findings provide evidence that the σ1s-mediated properties are genetically linked and suggest that these effects are mechanistically related.

Actin cytoskeleton manipulation by effector proteins secreted by diarrheagenic Escherichia coli pathotypes

The actin cytoskeleton is a dynamic structure necessary for cell and tissue organization, including the maintenance of epithelial barriers. Disruption of the epithelial barrier coincides with alterations of the actin cytoskeleton in several disease states. These disruptions primarily affect the paracellular space, which is normally regulated by tight junctions. Thereby, the actin cytoskeleton is a common and recurring target of bacterial virulence factors. In order to manipulate the actin cytoskeleton, bacteria secrete and inject toxins and effectors to hijack the host cell machinery, which interferes with host-cell pathways and with a number of actin binding proteins. An interesting model to study actin manipulation by bacterial effectors is Escherichia coli since due to its genome plasticity it has acquired diverse genetic mobile elements, which allow having different E. coli varieties in one bacterial species. These E. coli pathotypes, including intracellular and extracellular bacteria, interact with epithelial cells, and their interactions depend on a specific combination of virulence factors. In this paper we focus on E. coli effectors that mimic host cell proteins to manipulate the actin cytoskeleton. The study of bacterial effector-cytoskeleton interaction will contribute not only to the comprehension of the molecular causes of infectious diseases but also to increase our knowledge of cell biology.

The molecular basis of ubiquitin-like protein NEDD8 deamidation by the bacterial effector protein Cif

The cycle inhibiting factors (Cifs) are a family of translocated effector proteins, found in diverse pathogenic bacteria, that interfere with the host cell cycle by catalyzing the deamidation of a specific glutamine residue (Gln40) in NEDD8 and the related protein ubiquitin. This modification prevents recycling of neddylated cullin-RING ligases, leading to stabilization of various cullin-RING ligase targets, and also prevents polyubiquitin chain formation. Here, we report the crystal structures of two Cif/NEDD8 complexes, revealing a conserved molecular interface that defines enzyme/substrate recognition. Mutation of residues forming the interface suggests that shape complementarity, rather than specific individual interactions, is a critical feature for complex formation. We show that Cifs from diverse bacteria bind NEDD8 in vitro and conclude that they will all interact with their substrates in the same way. The "occluding loop" in Cif gates access to Gln40 by forcing a conformational change in the C terminus of NEDD8. We used native PAGE to follow the activity of Cif from the human pathogen Yersinia pseudotuberculosis and selected variants, and the position of Gln40 in the active site has allowed us to propose a catalytic mechanism for these enzymes.

Strategies for Intracellular Survival of Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a disease with high mortality that is prevalent in tropical regions of the world. A key component of the pathogenesis of melioidosis is the ability of B. pseudomallei to enter, survive, and replicate within mammalian host cells. For non-phagocytic cells, bacterial adhesins have been identified both on the bacterial surface and associated with Type 4 pili. Cell invasion involves components of one or more of the three Type 3 Secretion System clusters, which also mediate, at least in part, the escape of bacteria from the endosome into the cytoplasm, where bacteria move by actin-based motility. The mechanism of actin-based motility is not clearly understood, but appears to differ from characterized mechanisms in other bacterial species. A small proportion of intracellular bacteria is targeted by host cell autophagy, involving direct recruitment of LC3 to endosomes rather than through uptake by canonical autophagosomes. However, the majority of bacterial cells are able to circumvent autophagy and other intracellular defense mechanisms such as the induction of inducible nitric oxide synthase, and then replicate in the cytoplasm and spread to adjacent cells via membrane fusion, resulting in the formation of multi-nucleated giant cells. A potential role for host cell ubiquitin in the autophagic response to bacterial infection has recently been proposed.

Promotion of cytoplasmic mislocalization of p27 by Helicobacter pylori in gastric cancer

The cyclin-dependent kinase inhibitor p27 plays an important role in cell cycle regulation. Reduced expression of p27 is commonly associated with poor prognosis in many malignancies, including gastric cancer. Cytoplasmic p27 mislocalization may be an additional indicator of high-grade tumors and poor prognosis in cancer. Since chronic infection by Helicobacter pylori is the most important risk factor for gastric cancer development, we evaluated the effects of H. pylori on p27 expression and localization in gastric cancer cells. Co-culture of gastric cells with H. pylori induced cytoplasmic p27 expression and reduced nuclear p27 expression in vitro. Cytoplasmic p27 expression was associated with and dependent upon phosphorylation of p27 at T157 and T198: wild type p27 accumulated in the cytoplasm, but non-phosphorylatable mutants affecting T157 or T198 were nuclear in H. pylori infected cells. These post-translational p27 changes were secondary to activation of cellular PI3K and AKT signaling pathways and dependent upon a functional H. pylori cag pathogenicity island. We investigated the clinical significance of cytoplasmic p27 mislocalization in 164 cases of resected gastric cancer in tissue microarrays. In 97 cases (59%) cytoplasmic p27 mislocalization was observed, and this was associated with increased mortality in multivariate analysis. These results show that H. pylori infection induces AKT/PI3K-mediated phosphorylation of p27 at T157 and T198 to cause cytoplasmic p27 mislocalization in gastric cancer, and that p27 mislocalization is an adverse prognostic feature in gastric cancer. This is the first demonstration of the translocation of a specific bacterial virulence factor that post-translationally regulates a host cell cyclin-dependent kinase inhibitor. This is of particular significance because p27 has both tumor-suppressive and oncogenic activities, depending upon its subcellular localization. Cytoplasmic mislocalization of p27 induced by H. pylori may be an important mechanistic link between H. pylori infection and gastric carcinogenesis.

Cycle inhibiting factors (cifs): cyclomodulins that usurp the ubiquitin-dependent degradation pathway of host cells

Cycle inhibiting factors (Cifs) are type III secreted effectors produced by diverse pathogenic bacteria. Cifs are "cyclomodulins" that inhibit the eukaryotic host cell cycle and also hijack other key cellular processes such as those controlling the actin network and apoptosis. This review summarizes current knowledge on Cif since its first characterization in enteropathogenic Escherichia coli, the identification of several xenologues in distant pathogenic bacteria, to its structure elucidation and the recent deciphering of its mode of action. Cif impairs the host ubiquitin proteasome system through deamidation of ubiquitin or the ubiquitin-like protein NEDD8 that regulates Cullin-Ring-ubiquitin Ligase (CRL) complexes. The hijacking of the ubiquitin-dependent degradation pathway of host cells results in the modulation of various cellular functions such as epithelium renewal, apoptosis and immune response. Cif is therefore a powerful weapon in the continuous arm race that characterizes host-bacteria interactions.

The struggle within: microbial influences on colorectal cancer

Recently, an unprecedented effort has been directed at understanding the interplay between chronic inflammation and development of cancer, with the case of inflammatory bowel disease (IBD)-associated colorectal cancer at the forefront of this research endeavor. The last decade has been particularly fertile, with the discovery of numerous innovative paradigms linking various inflammatory, proliferative, and innate and adaptive immune signaling pathways to the development of colorectal cancer. Because of the preponderant role of the intestinal microbiota in the initiation and progression of IBD, recent efforts have been directed at understanding the relationship between bacteria and colorectal cancer. The microbiota and its collective genome, the microbiome, form a diverse and complex ecological community that profoundly impacts intestinal homeostasis and disease states. This review will discuss the differential influence of the microbiota on the development of IBD-associated colorectal cancer and highlight the role of innate immune sensor-dependent as well as -independent mechanisms in this pathology.

Molecular insights into Burkholderia pseudomallei and Burkholderia mallei pathogenesis

Burkholderia pseudomallei and Burkholderia mallei are closely related gram-negative bacteria that can cause serious diseases in humans and animals. This review summarizes the current and rapidly expanding knowledge on the specific virulence factors employed by these pathogens and their roles in the pathogenesis of melioidosis and glanders. In particular, the contributions of recently identified virulence factors are described in the context of the intracellular lifestyle of these pathogens. Throughout this review, unique and shared virulence features of B. pseudomallei and B. mallei are discussed.

Units of plasticity in bacterial genomes: new insight from the comparative genomics of two bacteria interacting with invertebrates, Photorhabdus and Xenorhabdus

Background

Flexible genomes facilitate bacterial evolution and are classically organized into polymorphic strain-specific segments called regions of genomic plasticity (RGPs). Using a new web tool, RGPFinder, we investigated plasticity units in bacterial genomes, by exhaustive description of the RGPs in two Photorhabdus and two Xenorhabdus strains, belonging to the Enterobacteriaceae and interacting with invertebrates (insects and nematodes).

Results

RGPs account for about 60% of the genome in each of the four genomes studied. We classified RGPs into genomic islands (GIs), prophages and two new classes of RGP without the features of classical mobile genetic elements (MGEs) but harboring genes encoding enzymes catalyzing DNA recombination (RGP_mob), or with no remarkable feature (RGP_none). These new classes accounted for most of the RGPs and are probably hypervariable regions, ancient MGEs with degraded mobilization machinery or non canonical MGEs for which the mobility mechanism has yet to be described. We provide evidence that not only the GIs and the prophages, but also RGP_mob and RGP_none, have a mosaic structure consisting of modules. A module is a block of genes, 0.5 to 60 kb in length, displaying a conserved genomic organization among the different Enterobacteriaceae. Modules are functional units involved in host/environment interactions (22-31%), metabolism (22-27%), intracellular or intercellular DNA mobility (13-30%), drug resistance (4-5%) and antibiotic synthesis (3-6%). Finally, in silico comparisons and PCR multiplex analysis indicated that these modules served as plasticity units within the bacterial genome during genome speciation and as deletion units in clonal variants of Photorhabdus.

Conclusions

This led us to consider the modules, rather than the entire RGP, as the true unit of plasticity in bacterial genomes, during both short-term and long-term genome evolution.

Pathogenic bacteria target NEDD8-conjugated cullins to hijack host-cell signaling pathways

The cycle inhibiting factors (Cif), produced by pathogenic bacteria isolated from vertebrates and invertebrates, belong to a family of molecules called cyclomodulins that interfere with the eukaryotic cell cycle. Cif blocks the cell cycle at both the G₁/S and G₂/M transitions by inducing the stabilization of cyclin-dependent kinase inhibitors p21^waf1 and p27^kip1. Using yeast two-hybrid screens, we identified the ubiquitin-like protein NEDD8 as a target of Cif. Cif co-compartmentalized with NEDD8 in the host cell nucleus and induced accumulation of NEDD8-conjugated cullins. This accumulation occurred early after cell infection and correlated with that of p21 and p27. Co-immunoprecipitation revealed that Cif interacted with cullin-RING ubiquitin ligase complexes (CRLs) through binding with the neddylated forms of cullins 1, 2, 3, 4A and 4B subunits of CRL. Using an in vitro ubiquitylation assay, we demonstrate that Cif directly inhibits the neddylated CUL1-associated ubiquitin ligase activity. Consistent with this inhibition and the interaction of Cif with several neddylated cullins, we further observed that Cif modulates the cellular half-lives of various CRL targets, which might contribute to the pathogenic potential of diverse bacteria.

Among the arsenal of virulence factors used by bacterial pathogens to infect and manipulate their hosts, cyclomodulins are a growing family of bacterial toxins that interfere with the eukaryotic cell-cycle. Cif is one of these cyclomodulins produced by both mammalian and invertebrate pathogenic bacteria. Cif blocks the host cell cycle by inducing the accumulation of two regulators of cell cycle progression: the cyclin-dependent kinase inhibitors p21 and p27. To decipher the mode of action of Cif, we performed yeast two-hybrid screenings. We show that Cif binds to NEDD8 and induce accumulation of neddylated cullins early after infection. Cullins are scaffold components of cullin-RING ubiquitin ligases (CRLs), which ubiquitinate proteins and target them for degradation by the 26S proteasome. We demonstrate that Cif directly inhibits the ubiquitin ligase activity of these CRLs and consequently the targeting of p21 and p27 for ubiquitin-dependent degradation. Targeting at NEDD8 represents a novel strategy for modulation of host cell functions by bacterial pathogens. By inhibiting the most prominent class of ubiquitin-ligases, Cif controls the stability of a cohort of key regulators and impinge on not only cell cycle progression but also on many cellular and biological processes such as immunity, development, transcription, and cell signaling.

The cyclomodulin Cif of Photorhabdus luminescens inhibits insect cell proliferation and triggers host cell death by apoptosis

Cycle inhibiting factors (Cif) constitute a broad family of cyclomodulins present in bacterial pathogens of invertebrates and mammals. Cif proteins are thought to be type III effectors capable of arresting the cell cycle at G(2)/M phase transition in human cell lines. We report here the first direct functional analysis of Cif(Pl), from the entomopathogenic bacterium Photorhabdus luminescens, in its insect host. The cif(Pl) gene was expressed in P. luminescens cultures in vitro. The resulting protein was released into the culture medium, unlike the well characterized type III effector LopT. During locust infection, cif(Pl) was expressed in both the hemolymph and the hematopoietic organ, but was not essential for P. luminescens virulence. Cif(Pl) inhibited proliferation of the insect cell line Sf9, by blocking the cell cycle at the G(2)/M phase transition. It also triggered host cell death by apoptosis. The integrity of the Cif(Pl) catalytic triad is essential for the cell cycle arrest and pro-apoptotic activities of this protein. These results highlight, for the first time, the dual role of Cif in the control of host cell proliferation and apoptotic death in a non-mammalian cell line.

Glutamine deamidation and dysfunction of ubiquitin/NEDD8 induced by a bacterial effector family

A family of bacterial effectors including Cif homolog from Burkholderia pseudomallei (CHBP) and Cif from Enteropathogenic Escherichia coli (EPEC) adopt a functionally important papain-like hydrolytic fold. We show here that CHBP was a potent inhibitor of the eukaryotic ubiquitination pathway. CHBP acted as a deamidase that specifically and efficiently deamidated Gln40 in ubiquitin and ubiquitin-like protein NEDD8 both in vitro and during Burkholderia infection. Deamidated ubiquitin was impaired in supporting ubiquitin-chain synthesis. Cif selectively deamidated NEDD8, which abolished the activity of neddylated Cullin-RING ubiquitin ligases (CRLs). Ubiquitination and ubiquitin-dependent degradation of multiple CRL substrates were impaired by Cif in EPEC-infected cells. Mutations of substrate-contacting residues in Cif abolished or attenuated EPEC-induced cytopathic phenotypes of cell cycle arrest and actin stress fiber formation.

Genetic reprogramming of host cells by bacterial pathogens

During the course of infection, pathogens often induce changes in gene expression in host cells and these changes can be long lasting and global or transient and of limited amplitude. Defining how, when, and why bacterial pathogens reprogram host cells represents an exciting challenge that opens up the opportunity to grasp the essence of pathogenesis and its molecular details.

Cif type III effector protein: a smart hijacker of the host cell cycle

During coevolution with their hosts, bacteria have developed functions that allow them to interfere with the mechanisms controlling the proliferation of eukaryotic cells. Cycle inhibiting factor (Cif) is one of these cyclomodulins, the family of bacterial effectors that interfere with the host cell cycle. Acquired early during evolution by bacteria isolated from vertebrates and invertebrates, Cif is an effector protein of type III secretion machineries. Cif blocks the host cell cycle in G1 and G2 by inducing the accumulation of the cyclin-dependent kinase inhibitors p21(waf1/cip1) and p27(kip1). The x-ray crystal structure of Cif reveals it to be a divergent member of a superfamily of enzymes including cysteine proteases and acetyltransferases. This review summarizes and discusses what we know about Cif, from the bacterial gene to the host target.

The enteropathogenic Escherichia coli effector Cif induces delayed apoptosis in epithelial cells

The cycle inhibiting factor (Cif) belongs to a family of bacterial toxins, the cyclomodulins, which modulate the host cell cycle. Upon injection into the host cell by the type III secretion system of enteropathogenic Escherichia coli (EPEC), Cif induces both G(2) and G(1) cell cycle arrests. The cell cycle arrests correlate with the accumulation of p21(waf1) and p27(kip1) proteins that inhibit CDK-cyclin complexes, whose activation is required for G(1)/S and G(2)/M transitions. Increases of p21 and p27 levels are independent of p53 transcriptional induction and result from protein stabilization through inhibition of the ubiquitin/proteasome degradation pathway. In this study, we show that Cif not only induces cell cycle arrest but also eventually provokes a delayed cell death. Indeed, 48 h after infection with EPEC expressing Cif, cultured IEC-6 intestinal cells were positive for extracellular binding of annexin V and exhibited high levels of cleaved caspase-3 and lactate dehydrogenase release, indicating evidence of apoptosis. Cif was necessary and sufficient for inducing this late apoptosis, and the cysteine residue of the catalytic site was required for Cif activity. These results highlight a more complex role of Cif than previously thought, as a cyclomodulin but also as an apoptosis inducer.

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.

Enterohaemorrhagic Escherichia coli serogroup O111 inhibits NF-(kappa)B-dependent innate responses in a manner independent of a type III secreted OspG orthologue

Enterohaemorrhagic and enteropathogenic Escherichia coli (EHEC and EPEC) inject a repertoire of effector proteins into host cells via a type III secretion system (T3SS) encoded by the locus of enterocyte effacement (LEE). OspG is an effector protein initially identified in Shigella that was shown to inhibit the host innate immune response. In this study, we found ospG homologues in EHEC (mainly of serogroup O111) and in Yersinia enterocolitica. The T3SS encoded by the LEE was able to inject these different OspG homologues into host cells. Infection of HeLa cells with EHEC O111 inhibited the NF-kappaB-dependent innate immune response via a T3SS-dependent mechanism. However, an EHEC O111 ospG mutant was still able to inhibit NF-kappaB p65 transfer to the nucleus in infected cells stimulated by tumour necrosis factor alpha (TNF-alpha). In addition, no difference in the inflammatory response was observed between wild-type EHEC O111 and the isogenic ospG mutant in the rabbit ligated intestinal loop model. These results suggest that OspG is not the sole effector protein involved in the inactivation of the host innate immune system during EHEC O111 infection.

Crystal structures of Cif from bacterial pathogens Photorhabdus luminescens and Burkholderia pseudomallei

A pre-requisite for bacterial pathogenesis is the successful interaction of a pathogen with a host. One mechanism used by a broad range of Gram negative bacterial pathogens is to deliver effector proteins directly into host cells through a dedicated type III secretion system where they modulate host cell function. The cycle inhibiting factor (Cif) family of effector proteins, identified in a growing number of pathogens that harbour functional type III secretion systems and have a wide host range, arrest the eukaryotic cell cycle. Here, the crystal structures of Cifs from the insect pathogen/nematode symbiont Photorhabdus luminescens (a gamma-proteobacterium) and human pathogen Burkholderia pseudomallei (a beta-proteobacterium) are presented. Both of these proteins adopt an overall fold similar to the papain sub-family of cysteine proteases, as originally identified in the structure of a truncated form of Cif from Enteropathogenic E. coli (EPEC), despite sharing only limited sequence identity. The structure of an N-terminal region, referred to here as the 'tail-domain' (absent in the EPEC Cif structure), suggests a surface likely to be involved in host-cell substrate recognition. The conformation of the Cys-His-Gln catalytic triad is retained, and the essential cysteine is exposed to solvent and addressable by small molecule reagents. These structures and biochemical work contribute to the rapidly expanding literature on Cifs, and direct further studies to better understand the molecular details of the activity of these proteins.