The Helicobacter pylori CagD (HP0545, Cag24) protein is essential for CagA translocation and maximal induction of interleukin-8 secretion. J Mol Biol. 2009;386:204-217. [PMID: 19109970 DOI: 10.1016/j.jmb.2008.12.018]

The role of CEACAMs versus integrins in Helicobacter pylori CagA translocation: a systematic review

The delivery of Helicobacter pylori CagA into host cells was long believed to occur through the integrin cell surface receptors. However, the role of CEACAM receptors has recently been highlighted, instead. Here, we have categorized the existing experimental evidence according to whether deletion, upregulation, downregulation, or inhibition of the target ligands (T4SS or HopQ) or receptors (integrins or CEACAMs), result in alterations in CagA phosphorylation, cell elongation, or IL-8 production. According to our analysis, the statistics favor the essence of most of the T4SS constituents and the involvement of HopQ adhesin in all three functions. Concerning the integrin family, the collected data is controversial, but yielding towards it being dispensable or involved in CagA translocation. Yet, regarding cell elongation, more events are showing β1 integrin being involved, than αvβ4 being inhibitory. Concerning IL-8 secretion, again there are more events showing α5, β1 and β6 integrins to be involved, than those showing inhibitory roles for β1, β4 and β6 integrins. Finally, CEACAM 1, 3, and 5 are identified as mostly essential or involved in CagA phosphorylation, whereasCEACAM 4, 7, and 8 are found dispensable and CEACAM6 is under debate. Conversely, CEACAM1, 5 and 6 appear mostly dispensable for cell elongation. Noteworthy is the choice of cell type, bacterial strain, multiplicity and duration of infection, as well as the sensitivity of the detection methods, all of which can affect the variably obtained results.

Antibodies towards TVLLPVIFF Amino Acid Sequence of TNF Receptor Induced by Helicobacter pylori in Patients with Coronary Heart Disease

Background: Molecular mimicry between Helicobacter pylori (Hp) and the host components resulting in induction of cross-reacting antibodies has been suggested as accessory mechanism in the development of coronary heart disease (CHD). A potential target for antibodies induced during Hp infection by the components of these bacteria might be amino acid sequence TVLLPVIFF (P1) of tumor necrosis factor receptor (TNFR), which is exposed on vascular endothelium and immunocompetent cells, driving inflammation. Aim: To examine whether anti-P1 IgG are induced during Hp infection in CHD patients. Methods: Sera from CHD patients infected with Hp (54) vs. sera of uninfected healthy donors (22) were tested by the ELISA for anti-H. pylori antibodies, anti-P1 IgG, and for antibodies towards control sequence IAKEGFEKIS (P2). Sera of Caviae porcellus infected experimentally with Hp (30) or uninfected (10) were included into this study. The same serum samples, which were positive for anti-P1 IgG, were adsorbed with Hp and then subjected to the ELISA. The biological activity of anti-P1 IgG was assessed in complement (C1q) binding assay. Results: Sera of 43 CHD patients seropositive for anti-Hp IgG contained anti-P1 IgG binding C1q. Additionally, 10 serum samples of animals seropositive for anti-Hp IgG contained anti-P1 IgG. Anti-P1 IgG in tested sera were neutralized by their adsorption with Hp. Conclusion: In CHD patients infected with Hp, antibodies cross-reacting with TNFR common sequence are produced. Further studies are necessary to define immunogenic Hp determinants and to confirm possible cellular effects of cross-reacting antibodies.

In Silico Protein-Protein Interaction of Pterois volitans Venom with Cancer Inducers of Helicobacter pylori

Gastric cancer is a pathological condition induced by the bacteria Helicobacter pylori. Targeting the key virulence factors of H. pylori causing gastric cancer is a promising method for treating gastric cancer. Recently, research has been focused on analyzing the adrenergic, cholinergic, and anti-cancer properties of their venom proteins. Testing the anti-cancer activity of the lethal proteins in the venom of P. volitans provides a bioactive compound for cancer treatment. Still, it is also helpful to eliminate the ecological imbalance caused by these fish in the marine environment. This study focuses on an in silico approach using Z-dock to analyze the bioactive prospective of the venom proteins of P. volitans against the essential virulence proteins of H. pylori responsible for inducing cancer. Our in silico docking study using a computational model of the venom proteins and H. pylori proteins has displayed the possible interactions between these proteins. The results revealed that P. volitans hyaluronidase and PV toxin's venom proteins effectively interact with H. pylori proteins Cag A, Cag L, GGT, Cag D, and urease that may be promising proteins in cancer therapy.

Identification of Pathogenicity Island Genes Associated with Loss of Type IV Secretion Function during Murine Infection with Helicobacter pylori

Chronic Helicobacter pylori colonization in animal models often leads to downregulation of the type IV secretion system (T4SS), typically by recombination in cagY, which is an essential T4SS gene. However, 17 other cag pathogenicity island (cagPAI) genes, as well as some non-cagPAI genes, are also essential for T4SS function. To get a more complete picture of how H. pylori regulates the T4SS during animal colonization, we examined cagY in 534 mouse-passaged isolates that lost T4SS function, defined as a normalized interleukin-8 (IL-8) value of <0.3 relative to the input H. pylori strain PMSS1. In order to analyze the genetic changes in the strains with unchanged cagY, we sequenced the entire pathogenicity island of 60 such isolates using single-molecule, real-time (SMRT) sequencing technology (PacBio, Menlo Park, CA), and we compared the results to the PMSS1 wild type (WT). Of the 534 strains, 271 (51%) showed evidence of recombination in cagY, but we also found indels or nonsynonymous changes in 13 other essential cagPAI genes implicated in H. pylori T4SS function, most commonly cag5, cag10, and cagA While cagY recombination is the most common mechanism by which H. pylori downregulates T4SS function during murine infection, loss of function is also associated with changes in other essential cagPAI genes.

Acid-regulated gene expression of Helicobacter pylori: Insight into acid protection and gastric colonization

BACKGROUND

The pathogen Helicobacter pylori encounters many stressors as it transits to and infects the gastric epithelium. Gastric acidity is the predominate stressor encountered by the bacterium during initial infection and establishment of persistent infection. H. pylori initiates a rapid response to acid to maintain intracellular pH and proton motive force appropriate for a neutralophile. However, acid sensing by H. pylori may also serve as a transcriptional trigger to increase the levels of other pathogenic factors needed to subvert host defenses such as acid acclimation, antioxidants, flagellar synthesis and assembly, and CagA secretion.

MATERIALS AND METHODS

Helicobacter pylori were acid challenged at pH 3.0, 4.5, 6.0 vs nonacidic pH for 4 hours in the presence of urea, followed by RNA-seq analysis and qPCR. Cytoplasmic pH was monitored under the same conditions.

RESULTS

About 250 genes were induced, and an equal number were repressed at acidic pHs. Genes encoding for antioxidant proteins, flagellar structural proteins, particularly class 2 genes, T4SS/Cag-PAI, F_o F₁ -ATPase, and proteins involved in acid acclimation were highly expressed at acidic pH. Cytoplasmic pH decreased from 7.8 at pH_out of 8.0 to 6.0 at pH_out of 3.0.

CONCLUSIONS

These results suggest that increasing extracellular or intracellular acidity or both are detected by the bacterium and serve as a signal to initiate increased production of protective and pathogenic factors needed to counter host defenses for persistent infection. These changes are dependent on degree of acidity and time of acid exposure, triggering a coordinated response to the environment required for colonization.

Composition, structure and function of the Helicobacter pylori cag pathogenicity island encoded type IV secretion system

Many Gram-negative pathogens harbor type IV secretion systems (T4SS) that translocate bacterial virulence factors into host cells to hijack cellular processes. The pathology of the gastric pathogen Helicobacter pylori strongly depends on a T4SS encoded by the cag pathogenicity island. This T4SS forms a needle-like pilus, and its assembly is accomplished by multiple protein-protein interactions and various pilus-associated factors that bind to integrins followed by delivery of the CagA oncoprotein into gastric epithelial cells. Recent studies revealed the crystal structures of six T4SS proteins and pilus formation is modulated by iron and zinc availability. All these T4SS interactions are crucial for deregulating host signaling events and disease progression. New developments in T4SS functions and their importance for pathogenesis are discussed.

Structural Biology of Bacterial Type IV Secretion Systems

Type IV secretion systems (T4SSs) are large multisubunit translocons, found in both gram-negative and gram-positive bacteria and in some archaea. These systems transport a diverse array of substrates from DNA and protein-DNA complexes to proteins, and play fundamental roles in both bacterial pathogenesis and bacterial adaptation to the cellular milieu in which bacteria live. This review describes the various biochemical and structural advances made toward understanding the biogenesis, architecture, and function of T4SSs.

Patterns of EPIYA motifs among cagA-positive Helicobacter pylori strains: a case-control study in a Turkish population with Eurasian geographical features

Geographical variation in the frequency of various gastroduodenal pathologies was shown to be related to the geographical diversity of H. pylori CagA Glu-Pro-Ile-Tyr-Ala (EPIYA) patterns. We examined the EPIYA patterns of H. pylori and the association of EPIYA patterns with gastric cancer (GC) for the first time, to the best of our knowledge, in Turkey. The patient group (PG) contained 60 patients [38 GC and 22 duodenal ulcer (DU) patients]. The control group (CG) was 110 individuals [94 gastritis patients and 16 persons with a normal gastrointestinal system (NGIS)]. Specific primers were used for the detection of cagA including empty-site-positive and EPIYA-A, -B, -C, -D PCR. Bands of EPIYA-A, -B, -C were confirmed by DNA sequencing. One hundred and forty-two (83.5 %) strains [60 in the PG (38 GC, 22 DU), 82 in the CG (72 gastritis, 10 NGIS)] were positive for the cagA gene. EPIYA-C with multiple repeats was detected in 34 (23.9 %) strains, and 22 (64.7 %) were from GC patients. EPIYA-C with one repeat was detected in 89 (62.7 %) strains, and 54 (60.7 %) were from gastritis patients. EPIYT was detected in 10 strains, and EPIYA-D was not detected. The number of EPIYA-C with multiple repeats was significantly higher for the PG than for the CG (P < 0.0001). In GC patients, the number of EPIYA-C with multiple repeats was significantly higher than one repeat (P < 0.0001). In conclusion, our study showed that multiple EPIYA-C repeats increases the GC risk by 30.6-fold and the DU risk by 8.9-fold versus the CG. This indicates that Western-type H. pylori strains in Turkey have similar EPIYA motifs to those of neighbouring countries and Western populations.

Structural and functional aspects of the Helicobacter pylori secretome

Proteins secreted by Helicobacter pylori (H. pylori), an important human pathogen responsible for severe gastric diseases, are reviewed from the point of view of their biochemical characterization, both functional and structural. Despite the vast amount of experimental data available on the proteins secreted by this bacterium, the precise size of the secretome remains unknown. In this review, we consider as secreted both proteins that contain a secretion signal for the periplasm and proteins that have been detected in the external medium in in vitro experiments. In this way, H. pylori’s secretome appears to be composed of slightly more than 160 proteins, but this number must be considered very cautiously, not only because the definition of secretome itself is ambiguous but also because the included proteins were observed as secreted in in vitro experiments that were not representative of the environmental situation in vivo. The proteins that appear to be secreted can be grouped into different classes: enzymes (48 proteins), outer membrane proteins (43), components of flagella (11), members of the cytotoxic-associated genes pathogenicity island or other toxins (8 and 5, respectively), binding and transport proteins (9), and others (11). A final group, which includes 28 members, is represented by hypothetical uncharacterized proteins. Despite the large amount of data accumulated on the H. pylori secretome, a considerable amount of work remains to reach a full comprehension of the system at the molecular level.

Molecular characterization and polyclonal antibody generation against core component CagX protein of Helicobacter pylori type IV secretion system

Gram-negative bacteria Helicobacter pylori cause gastric ulcer, duodenal cancer, and found in almost half of the world's residents. The protein responsible for this disease is secreted through type IV secretion system (TFSS) of H. pylori. TFSS is encoded by 40-kb region of chromosomal DNA known as cag-pathogenicity island (PAI). TFSS comprises of three major components: cytoplasmic/inner membrane ATPase, transmembrane core-complex and outer membranous pilli, and associated subunits. Core complex consists of CagX, CagT, CagM, and Cag3(δ) proteins as per existing knowledge. In this study, we have characterized one of the important component of core-complex forming sub-unit protein, i.e., CagX. Complete ORF of CagX except signal peptide coding region was cloned and expressed in pET28a vector. Purification of CagX protein was performed, and polyclonal anti-sera against full-length recombinant CagX were raised in rabbit model. We obtained a very specific and high titer, CagX anti-sera that were utilized to characterize endogenous CagX. Surface localization of CagX was also seen by immunofluorescence microscopy. In short for the first time a full-length CagX was characterized, and we showed that CagX is the part of high molecular weight core complex, which is important for assembly and function of H. pylori TFSS.

Evaluation of the effect of cagPAI genes of Helicobacter pylori on AGS epithelial cell morphology and IL-8 secretion

Helicobacter pylori cagPAI genes play an important role in pathogenesis, however little is known about their functions in isolates from Turkish patients. We aimed to evaluate the intactness and the effect of the cagPAI genes (cagT, cagM, cagE, cagA) and cagA EPIYA motifs on the AGS morphological changes and IL-8 induction. Of 53 patients 38 were found infected with H. pylori. PCR amplification of the cagPAI genes showed 42.1 % intact, 39.5 % partially deleted and 18.4 % with complete deletions. Isolates from gastritis, duodenal and gastric ulcer patients with intact and partially deleted cagPAI genes induced higher IL-8 secretion than those with complete deletions. Isolates from gastritis patients had higher deletion frequencies of the cagT and cagM genes than the other two genes. Infection of AGS cells with isolates that possess intact cagPAI and EPIYA-ABC resulted in the formation of the hummingbird phenotype. The cagA positive isolates induced higher IL-8 secretion than cagA negative isolates. Isolates from DU patients with more than one EPIYA-C motif induced higher concentrations of IL-8 than those with EPIYA-ABC. In conclusion, the intactness of the cagPAI in our isolates from different patients was not conserved. An intact cagPAI was found to play an important role in the pathogenesis of DU but not GU or gastritis. The cagA gene, but not other cagPAI genes, was associated with the induction of IL-8 and the morphological changes of the AGS cells. An increase in the number of EPIYA-C motifs had noticeable effect on the formation of the hummingbird phenotype.

Cag type IV secretion system: CagI independent bacterial surface localization of CagA

Helicobacter pylori Cag type IV secretion system (Cag-T4SS) is a multi-component transporter of oncoprotein CagA across the bacterial membranes into the host epithelial cells. To understand the role of unique Cag-T4SS component CagI in CagA translocation, we have characterized it by biochemical and microscopic approaches. We observed that CagI is a predominantly membrane attached periplasmic protein partially exposed to the bacterial surface especially on the pili. The association of the protein with membrane appeared to be loose as it could be easily recovered in soluble fraction. We documented that the stability of the protein is dependent on several key components of the secretion system and it has multiple interacting partners including a non-cag-PAI protein HP1489. Translocation of CagA across the bacterial membranes to cell surface is CagI-independent process. The observations made herein are expected to assist in providing an insight into the substrate translocation by the Cag-T4SS system and Helicobacter pylori pathogenesis.

Protein HP1028 from the human pathogen Helicobacter pylori belongs to the lipocalin family

Helicobacter pylori is a bacterial pathogen that causes severe diseases, including gastritis, ulcers and gastric cancer. Although this bacterium has been extensively studied, the physiological functions of a large number of the proteins encoded by its genome are unknown. HP1028 is a protein that is relevant to colonization and to the survival of the bacterium in the stomach, but its function is not clearly understood. Bioinformatics studies suggest that HP1028 is a monomeric protein that is secreted in the H. pylori periplasm. The crystal structure of HP1028 has been determined at 2.6 Å resolution using the SAD method. The three-dimensional structure of the protein reveals that it belongs to the lipocalin family, a group of proteins that bind and transport (often hydrophobic) small molecules. The structure of HP1028, together with the possible localization of the mature protein in the bacterial periplasm and the position of the hp1028 gene in the bacterial genome, point to a role in H. pylori chemotaxis.

The Helicobacter pylori protein CagM is located in the transmembrane channel that is required for CagA translocation

Helicobacter pylori (H. pylori) is a human gastric pathogen that colonizes the stomach in more than 50 % of the world's human population. Infection with this bacterium can induce several gastric diseases ranging from gastritis to peptic ulcer and gastric cancer. Virulent H. pylori isolates harboring the cag pathogenicity island (cag PAI), which encodes a Type IV Secretion System (T4SS), form a pilus for the injection of its major virulence protein CagA into gastric cells. Several cag PAI genes have been identified as homologues of T4SS genes from Agrobacterium tumefaciens, while the other members in cag PAI still have no known function. We studied one of such proteins with unknown function, CagM, which was predicted to have a putative N-terminal signal sequence and at least three transmembrane helices. To determine the subcellular localization of CagM, we performed a cell fractionation procedure and produced rabbit anti-CagM polyclonal antibodies for immunoblotting assays. Furthermore, we generated an isogenic ΔcagM mutant to investigate the ability of CagA translocation compared with the wild-type NCTC 11637 strain using GES-1 and MKN-45 cell infection experiments. Our results indicated that CagM was mainly located in the bacterial membrane, partially located in the periplasm, and essential for CagA translocation both in GES-1 and MKN-45 cells, which suggested that CagM was one of the core members of Cag T4SS and localized in the transmembrane channel.

CagI is an essential component of the Helicobacter pylori Cag type IV secretion system and forms a complex with CagL

Helicobacter pylori, the causative agent of type B gastritis, peptic ulcers, gastric adenocarcinoma and MALT lymphoma, uses the Cag type IV secretion system to induce a strong proinflammatory response in the gastric mucosa and to inject its effector protein CagA into gastric cells. CagA translocation results in altered host cell gene expression profiles and cytoskeletal rearrangements, and it is considered as a major bacterial virulence trait. Recently, it has been shown that binding of the type IV secretion apparatus to integrin receptors on target cells is a crucial step in the translocation process. Several bacterial proteins, including the Cag-specific components CagL and CagI, have been involved in this interaction. Here, we have examined the localization and interactions of CagI in the bacterial cell. Since the cagI gene overlaps and is co-transcribed with the cagL gene, the role of CagI for type IV secretion system function has been difficult to assess, and conflicting results have been reported regarding its involvement in the proinflammatory response. Using a marker-free gene deletion approach and genetic complementation, we show now that CagI is an essential component of the Cag type IV secretion apparatus for both CagA translocation and interleukin-8 induction. CagI is distributed over soluble and membrane-associated pools and seems to be partly surface-exposed. Deletion of several genes encoding essential Cag components has an impact on protein levels of CagI and CagL, suggesting that both proteins require partial assembly of the secretion apparatus. Finally, we show by co-immunoprecipitation that CagI and CagL interact with each other. Taken together, our results indicate that CagI and CagL form a functional complex which is formed at a late stage of secretion apparatus assembly.

In vivo expression of Helicobacter pylori virulence genes in patients with gastritis, ulcer, and gastric cancer

The best-studied Helicobacter pylori virulence factor associated with development of peptic ulcer disease or gastric cancer (GC) rather than asymptomatic nonatrophic gastritis (NAG) is the cag pathogenicity island (cagPAI), which encodes a type IV secretion system (T4SS) that injects the CagA oncoprotein into host epithelial cells. Here we used real-time reverse transcription-PCR (RT-PCR) to measure the in vivo expression of genes on the cagPAI and of other virulence genes in patients with NAG, duodenal ulcer (DU), or GC. In vivo expression of H. pylori virulence genes was greater overall in gastric biopsy specimens of patients with GC than in those of patients with NAG or DU. However, since in vitro expression of cagA was not greater in H. pylori strains from patients with GC than in those from patients with NAG or DU, increased expression in GC in vivo is likely a result of environmental conditions in the gastric mucosa, though it may in turn cause more severe pathology. Increased expression of virulence genes in GC may represent a stress response to elevated pH or other environmental conditions in the stomach of patients with GC, which may be less hospitable to H. pylori colonization than the acidic environment in patients with NAG or DU.

Characterization of CagI in the cag pathogenicity island of Helicobacter pylori

Helicobacter pylori is a highly successful human-specific gastric pathogen that infects up to 50% of the world's population. Virulent H. pylori isolates harbor the cytotoxin-associated genes pathogenicity island (cag-PAI), which encodes a type IV secretion system that translocates bacterial effector (e.g., CagA oncoprotein) molecules into host cells. Although some cag-PAI genes are shown to be required for CagA delivery or localization, the majority have no known function. In the current study, the authors performed a cell components fractionation assay and showed that CagI, one of the cag-PAI proteins located in the bacterial membrane, was not translocated into host cells. The homologous recombination method then was used to construct the isogenic mutant of H. pylori cagI, and the translocation assay was performed. The results showed that the isogenic mutant of H. pylori NCTC 11637 cagI could cause a reduction in the degree of CagA translocation. Overall, the results suggested that CagI might be an accessory component of the CagA secretion system not translocated into host cells and that it is located in the bacterial membrane.

Molecular mechanisms of gastric epithelial cell adhesion and injection of CagA by Helicobacter pylori

Helicobacter pylori is a highly successful pathogen uniquely adapted to colonize humans. Gastric infections with this bacterium can induce pathology ranging from chronic gastritis and peptic ulcers to gastric cancer. More virulent H. pylori isolates harbour numerous well-known adhesins (BabA/B, SabA, AlpA/B, OipA and HopZ) and the cag (cytotoxin-associated genes) pathogenicity island encoding a type IV secretion system (T4SS). The adhesins establish tight bacterial contact with host target cells and the T4SS represents a needle-like pilus device for the delivery of effector proteins into host target cells such as CagA. BabA and SabA bind to blood group antigen and sialylated proteins respectively, and a series of T4SS components including CagI, CagL, CagY and CagA have been shown to target the integrin β₁ receptor followed by injection of CagA across the host cell membrane. The interaction of CagA with membrane-anchored phosphatidylserine may also play a role in the delivery process. While substantial progress has been made in our current understanding of many of the above factors, the host cell receptors for OipA, HopZ and AlpA/B during infection are still unknown. Here we review the recent progress in characterizing the interactions of the various adhesins and structural T4SS proteins with host cell factors. The contribution of these interactions to H. pylori colonization and pathogenesis is discussed.

Helicobacter pylori with stronger intensity of CagA phosphorylation lead to an increased risk of gastric intestinal metaplasia and cancer

Background

Nearly all Taiwanese H. pylori stains are cagA-genopositive and encode CagA protein. In this study, we evaluated whether different intensity of tyrosine phosphorylated-CagA (p-CagA) had an impact on the clinical diseases and histological outcomes in this area.

Results

We enrolled 469 dyspeptic patients and prospectively obtained the gastric biopsy specimens and the H. pylori isolates. These patients were categorized according to the clinical diseases, such as duodenal ulcer, gastric ulcer, gastric cancer, and gastritis with or without intestinal metaplasia. Their gastric specimens were reviewed by the updated Sydney's system. Furthermore, a total of 146 patients were randomly selected from each clinical category for evaluation of their isolates' p-CagA intensity by in vitro AGS cells co-culture. The p-CagA was sparse in 30 (20.5%), weak in 59 (40.5%), and strong in 57 (39%) isolates. The isolates from the patients of gastric cancer or gastritis with intestinal metaplasia had stronger p-CagA intensity than those of gastritis without intestinal metaplasia (p ≤ 0.002). Moreover, the patients infected with isolates with strong or weak p-CagA intensity had a higher risk of gastric intestinal metaplasia (p < 0.05, odds ratio 3.09~15.26) than those infected with sparse p-CagA isolates.

Conclusions

Infection with H. pylori stains with stronger p-CagA intensity may lead to an increased risk of gastric intestinal metaplasia and cancer.

Structural and functional aspects of unique type IV secretory components in the Helicobacter pylori cag-pathogenicity island

Helicobacter pylori cytotoxin-associated gene-pathogenicity island (cagPAI) is responsible for the secretion of the CagA effector through a type IV secretion system (T4SS) apparatus, as well as of peptidoglycan and possibly other not yet identified factors. Twenty-nine different polypeptide chains are encoded by this cluster of genes, although only some of them show a significant similarity with the constitutive elements of well characterized secretion systems from other bacteria. The other cagPAI components represent almost unique proteins in this scenario. The majority of the T4SS include approximately fifteen components, taking into account either the transmembrane complex subunits, ATPases or substrate factors. The composition of the cagPAI is very complex: it includes proteins most likely involved at different levels in the pilus assembly, stabilization and processing of secreted substrate, as well as regulatory particles possibly involved in the control of the entire apparatus. Despite recent findings with respect to components that play a role in the interaction with the host cell, the function of several cagPAI proteins remains unclear or unknown. This is particularly true for those that represent unique members with no clear similarity to those of other T4SS and no obvious evidence of involvement in the secretion of CagA or induction of pro-inflammatory responses. We summarize what is known about these accessory components, both from a molecular and structural point of view, as well as their putative physiological role.

Architecture of the Helicobacter pylori Cag-type IV secretion system

Type IV secretion systems (T4SS) are macromolecular assemblies used by bacteria to transport material across their membranes. T4SS are generally composed of a set of twelve proteins (VirB1-11 and VirD4). This represents a dynamic machine powered by three ATPases. T4SS are widespread in pathogenic bacteria where they are often used to deliver effectors into host cells. For example, the human pathogen Helicobacter pylori encodes a T4SS, the Cag-T4SS, which mediates the injection of the toxin CagA. We review the progress made in the past decade in our understanding of T4SS architecture. We translate this new knowledge to derive an understanding of the structure of the H. pylori Cag system, and use recent protein-protein interaction data to refine this model.

Assembly and molecular mode of action of the Helicobacter pylori Cag type IV secretion apparatus

Bacterial type IV secretion systems (T4SS) form supramolecular protein complexes that are capable of transporting DNA or protein substrates across the bacterial cell envelope and, in many cases, also across eukaryotic target cell membranes. Because of these characteristics, they are often used by pathogenic bacteria for the injection of host cell-modulating virulence factors. One example is the human pathogen Helicobacter pylori, which uses the Cag-T4SS to induce a pro-inflammatory response and multiple cytoskeletal and gene regulatory effects in gastric epithelial cells. Work in recent years has shown that the Cag-T4SS exhibits marked differences in relation to other systems, both with respect to the composition of its secretion apparatus and the molecular details of its secretion mechanisms. This review describes the molecular properties of the Cag-T4SS and compares these with prototypical systems of this family of protein transporters.

The coupling protein Cagbeta and its interaction partner CagZ are required for type IV secretion of the Helicobacter pylori CagA protein

Bacterial type IV secretion systems are macromolecule transporters with essential functions for horizontal gene transfer and for symbiotic and pathogenic interactions with eukaryotic host cells. Helicobacter pylori, the causative agent of type B gastritis, peptic ulcers, gastric adenocarcinoma, and mucosa-associated lymphoid tissue (MALT) lymphoma, uses the Cag type IV secretion system to inject its effector protein CagA into gastric cells. This protein translocation results in altered host cell gene expression profiles and cytoskeletal rearrangements, and it has been linked to cancer development. Interactions of CagA with host cell proteins have been studied in great detail, but little is known about the molecular details of CagA recognition as a type IV secretion substrate or of the translocation process. Apart from components of the secretion apparatus, we previously identified several CagA translocation factors that are either required for or support CagA translocation. To identify protein-protein interactions between these translocation factors, we used a yeast two-hybrid approach comprising all cag pathogenicity island genes. Among several other interactions involving translocation factors, we found a strong interaction between the coupling protein homologue Cagβ (HP0524) and the Cag-specific translocation factor CagZ (HP0526). We show that CagZ has a stabilizing effect on Cagβ, and we demonstrate protein-protein interactions between the cytoplasmic part of Cagβ and CagA and between CagZ and Cagβ, using immunoprecipitation and pull-down assays. Together, our data suggest that these interactions represent a substrate-translocation factor complex at the bacterial cytoplasmic membrane.

Helicobacter pylori lipopolysaccharide is synthesized via a novel pathway with an evolutionary connection to protein N-glycosylation

Lipopolysaccharide (LPS) is a major component on the surface of Gram negative bacteria and is composed of lipid A-core and the O antigen polysaccharide. O polysaccharides of the gastric pathogen Helicobacter pylori contain Lewis antigens, mimicking glycan structures produced by human cells. The interaction of Lewis antigens with human dendritic cells induces a modulation of the immune response, contributing to the H. pylori virulence. The amount and position of Lewis antigens in the LPS varies among H. pylori isolates, indicating an adaptation to the host. In contrast to most bacteria, the genes for H. pylori O antigen biosynthesis are spread throughout the chromosome, which likely contributed to the fact that the LPS assembly pathway remained uncharacterized. In this study, two enzymes typically involved in LPS biosynthesis were found encoded in the H. pylori genome; the initiating glycosyltransferase WecA, and the O antigen ligase WaaL. Fluorescence microscopy and analysis of LPS from H. pylori mutants revealed that WecA and WaaL are involved in LPS production. Activity of WecA was additionally demonstrated with complementation experiments in Escherichia coli. WaaL ligase activity was shown in vitro. Analysis of the H. pylori genome failed to detect a flippase typically involved in O antigen synthesis. Instead, we identified a homolog of a flippase involved in protein N-glycosylation in other bacteria, although this pathway is not present in H. pylori. This flippase named Wzk was essential for O antigen display in H. pylori and was able to transport various glycans in E. coli. Whereas the O antigen mutants showed normal swimming motility and injection of the toxin CagA into host cells, the uptake of DNA seemed to be affected. We conclude that H. pylori uses a novel LPS biosynthetic pathway, evolutionarily connected to bacterial protein N-glycosylation.

Bacterial surfaces are decorated with glycans. The human stomach pathogen Helicobacter pylori exposes lipopolysaccharide (LPS) containing Lewis antigens that mimic human glycan structures. H. pylori alters its Lewis antigen display in adaptation to the individual host. Lewis antigens can interact with human dendritic cells, thereby inducing a suppression of the immune response and facilitating a chronic H. pylori infection. Whereas three general LPS biosynthesis pathways are known, the route of LPS assembly in H. pylori remained to be elucidated. We identified and characterized two components of the H. pylori LPS pathway, WecA and WaaL, which demonstrated that, as in other bacteria, the glycan is initially assembled onto a polyprenoid lipid carrier. This intermediate then has to cross a membrane barrier, requiring specialized translocases. H. pylori does not employ a translocase from common LPS pathways. We show that instead H. pylori uses a translocase named Wzk, which is involved in protein N-glycosylation in other bacteria. Wzk was able to translocate various glycan structures. The identification of Wzk as the H. pylori translocase involved in LPS biosynthesis indicates an evolutionary connection between LPS and glycoprotein biosynthesis pathways.

Cag3 is a novel essential component of the Helicobacter pylori Cag type IV secretion system outer membrane subcomplex

Helicobacter pylori strains harboring the cag pathogenicity island (PAI) have been associated with more severe gastric disease in infected humans. The cag PAI encodes a type IV secretion (T4S) system required for CagA translocation into host cells as well as induction of proinflammatory cytokines, such as interleukin-8 (IL-8). cag PAI genes sharing sequence similarity with T4S components from other bacteria are essential for Cag T4S function. Other cag PAI-encoded genes are also essential for Cag T4S, but lack of sequence-based or structural similarity with genes in existing databases has precluded a functional assignment for the encoded proteins. We have studied the role of one such protein, Cag3 (HP0522), in Cag T4S and determined Cag3 subcellular localization and protein interactions. Cag3 is membrane associated and copurifies with predicted inner and outer membrane Cag T4S components that are essential for Cag T4S as well as putative accessory factors. Coimmunoprecipitation and cross-linking experiments revealed specific interactions with HpVirB7 and CagM, suggesting Cag3 is a new component of the Cag T4S outer membrane subcomplex. Finally, lack of Cag3 lowers HpVirB7 steady-state levels, further indicating Cag3 makes a subcomplex with this protein.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori induces chronic inflammation of the gastric mucosa, but only a proportion of infected individuals develop peptic ulcer disease or gastric carcinoma. Reasons underlying these observations include differences in bacterial pathogenicity as well as in host susceptibility. Numerous studies published in the last year provided new insight into H. pylori virulence factors, their interaction with the host and consequences in pathogenesis. These include the role of bacterial genetic diversity in host colonization and persistence, outer membrane proteins and modulation of adhesin expression, new aspects of VacA functions, and CagA and its phosphorylation-dependent and -independent cellular effects. This article will also review the recent novel findings on the interactions of H. pylori with diverse host epithelial signaling pathways and events involved in the initiation of carcinogenesis, including genetic instability and dysregulation of DNA repair.