Genes required for assembly of pili associated with the Helicobacter pylori cag type IV secretion system. Infect Immun. 2014;82:3457-3470. [PMID: 24891108 DOI: 10.1128/iai.01640-14]

Roles of the components of the cag-pathogenicity island encoded type IV secretion system in Helicobacter pylori

The Helicobacter pylori (H. pylori) cytotoxin-associated gene pathogenicity island (cagPAI) encodes 31 genes that assemble the cag type IV secretion system (T4SS) apparatus, which includes structures such as the outer membrane core complex, periplasmic ring, inner membrane complex and bacterial hairs. These proteins interact with each other to inject CagA into the host gastric epithelium. There are also individual unique functions that help H. pylori interfere with host cellular pathways, modulate the immune response and colonize the host for a long time. However, the functions of some of the proteins remain unclear. This review summarizes what is known about the structure and function of these auxiliary components and discusses their role in H. pylori pathogenesis.

Subdomains of the Helicobacter pylori Cag T4SS outer membrane core complex exhibit structural independence

The Helicobacter pylori Cag type IV secretion system (Cag T4SS) has an important role in the pathogenesis of gastric cancer. The Cag T4SS outer membrane core complex (OMCC) is organized into three regions: a 14-fold symmetric outer membrane cap (OMC) composed of CagY, CagX, CagT, CagM, and Cag3; a 17-fold symmetric periplasmic ring (PR) composed of CagY and CagX; and a stalk with unknown composition. We investigated how CagT, CagM, and a conserved antenna projection (AP) region of CagY contribute to the structural organization of the OMCC. Single-particle cryo-EM analyses showed that complexes purified from ΔcagT or ΔcagM mutants no longer had organized OMCs, but the PRs remained structured. OMCCs purified from a CagY antenna projection mutant (CagY∆AP) were structurally similar to WT OMCCs, except for the absence of the α-helical antenna projection. These results indicate that CagY and CagX are sufficient for maintaining a stable PR, but the organization of the OMC requires CagY, CagX, CagM, and CagT. Our results highlight an unexpected structural independence of two major subdomains of the Cag T4SS OMCC.

Remodeling of the gastric environment in Helicobacter pylori-induced atrophic gastritis

Helicobacter pylori colonization of the human stomach is a strong risk factor for gastric cancer. To investigate H. pylori-induced gastric molecular alterations, we used a Mongolian gerbil model of gastric carcinogenesis. Histologic evaluation revealed varying levels of atrophic gastritis (a premalignant condition characterized by parietal and chief cell loss) in H. pylori-infected animals, and transcriptional profiling revealed a loss of markers for these cell types. We then assessed the spatial distribution and relative abundance of proteins in the gastric tissues using imaging mass spectrometry and liquid chromatography with tandem mass spectrometry. We detected striking differences in the protein content of corpus and antrum tissues. Four hundred ninety-two proteins were preferentially localized to the corpus in uninfected animals. The abundance of 91 of these proteins was reduced in H. pylori-infected corpus tissues exhibiting atrophic gastritis compared with infected corpus tissues exhibiting non-atrophic gastritis or uninfected corpus tissues; these included numerous proteins with metabolic functions. Fifty proteins localized to the corpus in uninfected animals were diffusely delocalized throughout the stomach in infected tissues with atrophic gastritis; these included numerous proteins with roles in protein processing. The corresponding alterations were not detected in animals infected with a H. pylori ∆cagT mutant (lacking Cag type IV secretion system activity). These results indicate that H. pylori can cause loss of proteins normally localized to the gastric corpus as well as diffuse delocalization of corpus-specific proteins, resulting in marked changes in the normal gastric molecular partitioning into distinct corpus and antrum regions.IMPORTANCEA normal stomach is organized into distinct regions known as the corpus and antrum, which have different functions, cell types, and gland architectures. Previous studies have primarily used histologic methods to differentiate these regions and detect H. pylori-induced alterations leading to stomach cancer. In this study, we investigated H. pylori-induced gastric molecular alterations in a Mongolian gerbil model of carcinogenesis. We report the detection of numerous proteins that are preferentially localized to the gastric corpus but not the antrum in a normal stomach. We show that stomachs with H. pylori-induced atrophic gastritis (a precancerous condition characterized by the loss of specialized cell types) exhibit marked changes in the abundance and localization of proteins normally localized to the gastric corpus. These results provide new insights into H. pylori-induced gastric molecular alterations that are associated with the development of stomach cancer.

New strategies for Helicobacter pylori isolation and sequencing analysis for virulence genes contributing to its pathogenicity

BACKGROUND

Helicobacter pylori is a fastidious pathogen that is required a complicated medium for growth. Invading epithelial cells of the stomach. H. pylori virulence factors are classified by function, acidic resistivity, adhesion, chemotaxis and motility, molecular mimicry, immunological invasion and modulation, and toxins formation such as cytotoxin-associated genes A (cagA) and vacuolating cytotoxin A (vacA). This study aims to determine a simple and innovative technique to isolate H. pylori from gastric biopsies and assess pathogenicity by virulence factor gene detection.

METHODS

A total of 200 patients who were suspected of having H. pylori infection had two antral gastric biopsies undertaken. A rapid urease test (RUT) was used for one, and Brain Heart Infusion broth (BHI) was used to cultivate the other. The molecular study included diagnostics utilizing the 16sRNA housekeeping gene along with the identification of the virulence factors genes (cagA, cagT, and vacA) and sequencing, RESULT: Of the 200 antral gastric biopsies collected, 135 were positive rapid urease tests, and 17 H. pylori isolates were successfully obtained from 135 biopsies. The 16SrRNA as a housekeeping gene is confirmed, and about 53%, 70.5%, and 82.3% of the 17 isolates show carrying cagA, cagT, and vacA genes, respectively. All peptic ulcer isolates have the cagA gene, while Gastroesophageal Reflux Disease (GERD) and non-peptic ulcer disease (NPUD) isolates show the lack of the cagA gene. All bacteria, which were isolated from peptic ulcer, nodular gastritis, and gastritis patients, have a vacA gene.

CONCLUSION

The effective method for isolating H. pylori is centrifuging the transport broth after 24 h of incubation. The cagA toxin causes peptic ulcer while vacA toxin induces several histopathological changes in the stomach. Three virulence genes were present in all peptic ulcer-causing bacteria, while only one or none were present in the GERD and NPUD biopsy isolates.

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.

Essential role of Helicobacter pylori apolipoprotein N-acyltransferase (Lnt) in stomach colonization

Bacterial lipoproteins are post-translationally modified with acyl chains, anchoring these proteins to bacterial membranes. In Gram-negative bacteria, three enzymes complete the modifications. Lgt (which adds two acyl chains) and LspA (which removes the signal peptide) are essential. Lnt (which adds a third acyl chain) is not essential in certain bacteria including Francisella tularensis, Neisseria gonorrhoeae, and Acinetobacter baumannii. Deleting lnt results in mild to severe physiologic changes. We previously showed lnt is not essential for Helicobacter pylori growth in vitro. Here, the physiologic consequences of deleting lnt in H. pylori and the role of Lnt in the host response to H. pylori were examined using in vitro and in vivo models. Comparing wild-type, Δlnt, and complemented mutant H. pylori, no changes in growth rates or sensitivity to acid or antibiotics were observed. Since deleting lnt changes the number of acyl chains on lipoproteins and the number of acyl chains on lipoproteins impacts the innate immune response through Toll-like receptor 2 (TLR2) signaling, primary human gastric epithelial cells were treated with a purified lipoprotein from wild-type or lnt mutant H. pylori. Differential gene expression analysis indicated that lipoprotein from the lnt mutant induced a more robust TLR2 response. In a complementary approach, we infected wild-type and Tlr2-/- mice and found that both the wild-type and complemented mutant strains successfully colonized the animals. However, the lnt mutant strain was unable to colonize either mouse strain. These results show that lnt is essential for H. pylori colonization and identifies lipoprotein synthesis as a target for therapeutic intervention.

Architectural asymmetry enables DNA transport through the Helicobacter pylori cag type IV secretion system

Structural asymmetry within secretion system architecture is fundamentally important for apparatus diversification and biological function. However, the mechanism by which symmetry mismatch contributes to nanomachine assembly and interkingdom effector translocation are undefined. Here, we show that architectural asymmetry orchestrates dynamic substrate selection and enables trans-kingdom DNA conjugation through the Helicobacter pylori cag type IV secretion system (cag T4SS). Structural analyses of asymmetric units within the cag T4SS periplasmic ring complex (PRC) revealed intermolecular π-π stacking interactions that coordinate DNA binding and license trans-kingdom conjugation without disrupting the translocation of protein and peptidoglycan effector molecules. Additionally, we identified a novel proximal translocation channel gating mechanism that regulates cargo loading and governs substrate transport across the outer membrane. We thus propose a model whereby the organization and geometry of architectural symmetry mismatch exposes π-π interfaces within the PRC to facilitate DNA transit through the cag T4SS translocation channel.

Designed Ankyrin Repeat Proteins provide insights into the structure and function of CagI and are potent inhibitors of CagA translocation by the Helicobacter pylori type IV secretion system

The bacterial human pathogen Helicobacter pylori produces a type IV secretion system (cagT4SS) to inject the oncoprotein CagA into gastric cells. The cagT4SS external pilus mediates attachment of the apparatus to the target cell and the delivery of CagA. While the composition of the pilus is unclear, CagI is present at the surface of the bacterium and required for pilus formation. Here, we have investigated the properties of CagI by an integrative structural biology approach. Using Alpha Fold 2 and Small Angle X-ray scattering, it was found that CagI forms elongated dimers mediated by rod-shape N-terminal domains (CagIN) prolonged by globular C-terminal domains (CagIC). Three Designed Ankyrin Repeat Proteins (DARPins) K2, K5 and K8 selected against CagI interacted with CagIC with subnanomolar affinities. The crystal structures of the CagI:K2 and CagI:K5 complexes were solved and identified the interfaces between the molecules, thereby providing a structural explanation for the difference in affinity between the two binders. Purified CagI and CagIC were found to interact with adenocarcinoma gastric (AGS) cells, induced cell spreading and the interaction was inhibited by K2. The same DARPin inhibited CagA translocation by up to 65% in AGS cells while inhibition levels were 40% and 30% with K8 and K5, respectively. Our study suggests that CagIC plays a key role in cagT4SS-mediated CagA translocation and that DARPins targeting CagI represent potent inhibitors of the cagT4SS, a crucial risk factor for gastric cancer development.

Conjugative Transfer of Acute Hepatopancreatic Necrosis Disease-Causing pVA1-Type Plasmid Is Mediated by a Novel Self-Encoded Type IV Secretion System

The pathogenic pVA1-type plasmids that carry pirAB toxin genes are the genetic basis for Vibrio to cause acute hepatopancreatic necrosis disease (AHPND), a lethal shrimp disease posing an urgent threat to shrimp aquaculture. Emerging evidence also demonstrate the rapid spread of pVA1-type plasmids across Vibrio species. The pVA1-type plasmids have been predicted to encode a self-encoded type IV secretion system (T4SS). Here, phylogenetic analysis indicated that the T4SS is a novel member of Trb-type. We further confirmed that the T4SS was able to mediate the conjugation of pVA1-type plasmids. A trbE gene encoding an ATPase and a traG gene annotated as a type IV coupling protein (T4CP) were characterized as key components of the T4SS. Deleting either of these 2 genes abolished the conjugative transfer of a pVA1-type plasmid from AHPND-causing Vibrio parahaemolyticus to Vibrio campbellii, which was restored by complementation of the corresponding gene. Moreover, we found that bacterial density, temperature, and nutrient levels are factors that can regulate conjugation efficiency. In conclusion, we proved that the conjugation of pVA1-type plasmids across Vibrio spp. is mediated by a novel T4SS and regulated by environmental factors. IMPORTANCE AHPND is a global shrimp bacteriosis and was listed as a notifiable disease by the World Organization for Animal Health (WOAH) in 2016, causing losses of more than USD 7 billion each year. Several Vibrio species such as V. parahaemolyticus, V. harveyi, V. campbellii, and V. owensii harboring the virulence plasmid (designated as the pVA1-type plasmid) can cause AHPND. The increasing number of Vibrio species makes prevention and control more difficult, threatening the sustainable development of the aquaculture industry. In this study, we found that the horizontal transfer of pVA1-type plasmid is mediated by a novel type IV secretion system (T4SS). Our study explained the formation mechanism of pathogen diversity in AHPND. Moreover, bacterial density, temperature, and nutrient levels can regulate horizontal efficiency. We explore new ideas for controlling the spread of virulence plasmid and form the basis of management strategies leading to the prevention and control of AHPND.

IncFV plasmid pED208: Sequence analysis and evidence for translocation of maintenance/leading region proteins through diverse type IV secretion systems

Two phylogenetically distantly-related IncF plasmids, F and pED208, serve as important models for mechanistic and structural studies of F-like type IV secretion systems (T4SS_Fs) and F pili. Here, we present the pED208 sequence and compare it to F and pUMNF18, the closest match to pED208 in the NCBI database. As expected, gene content of the three cargo regions varies extensively, although the maintenance/leading regions (MLRs) and transfer (Tra) regions also carry novel genes or motifs with predicted modulatory effects on plasmid stability, dissemination and host range. By use of a Cre recombinase assay for translocation (CRAfT), we recently reported that pED208-carrying donors translocate several products of the MLR (ParA, ParB1, ParB2, SSB, PsiB, PsiA) intercellularly through the T4SS_F. Here, we extend these findings by reporting that pED208-carrying donors translocate 10 additional MLR proteins during conjugation. In contrast, two F plasmid-encoded toxin components of toxin-antitoxin (TA) modules, CcdB and SrnB, were not translocated at detectable levels through the T4SS_F. Remarkably, most or all of the pED208-encoded MLR proteins and CcdB and SrnB were translocated through heterologous T4SSs encoded by IncN and IncP plasmids pKM101 and RP4, respectively. Together, our sequence analyses underscore the genomic diversity of the F plasmid superfamily, and our experimental data demonstrate the promiscuous nature of conjugation machines for protein translocation. Our findings raise intriguing questions about the nature of T4SS translocation signals and of the biological and evolutionary consequences of conjugative protein transfer.

Positive Selection of Mutations in the Helicobacter pylori katA 5' Untranslated Region in a Mongolian Gerbil Model of Gastric Disease

To evaluate potential effects of gastric inflammation on Helicobacter pylori diversification and evolution within the stomach, we experimentally infected Mongolian gerbils with an H. pylori strain in which Cag type IV secretion system (T4SS) activity is controlled by a TetR/tetO system. Gerbils infected with H. pylori under conditions in which Cag T4SS activity was derepressed had significantly higher levels of gastric inflammation than gerbils infected under conditions with repressed Cag T4SS activity. Mutations in the 5' untranslated region (UTR) of katA (encoding catalase) were detected in strains cultured from 8 of the 17 gerbils infected with Cag T4SS-active H. pylori and none of the strains from 17 gerbils infected with Cag T4SS-inactive H. pylori. Catalase enzymatic activity, steady-state katA transcript levels, and katA transcript stability were increased in strains with these single nucleotide polymorphisms (SNPs) compared to strains in which these SNPs were absent. Moreover, strains harboring these SNPs exhibited increased resistance to bactericidal effects of hydrogen peroxide, compared to control strains. Experimental introduction of the SNPs into the wild-type katA 5' UTR resulted in increased katA transcript stability, increased katA steady-state levels, and increased catalase enzymatic activity. Based on site-directed mutagenesis and modeling of RNA structure, increased katA transcript levels were correlated with higher predicted thermal stability of the katA 5' UTR secondary structure. These data suggest that high levels of gastric inflammation positively select for H. pylori strains producing increased levels of catalase, which may confer survival advantages to the bacteria in an inflammatory gastric environment.

Type IV secretion systems: Advances in structure, function, and activation

Bacterial type IV secretion systems (T4SSs) are a functionally diverse translocation superfamily. They consist mainly of two large subfamilies: (i) conjugation systems that mediate interbacterial DNA transfer and (ii) effector translocators that deliver effector macromolecules into prokaryotic or eukaryotic cells. A few other T4SSs export DNA or proteins to the milieu, or import exogenous DNA. The T4SSs are defined by 6 or 12 conserved "core" subunits that respectively elaborate "minimized" systems in Gram-positive or -negative bacteria. However, many "expanded" T4SSs are built from "core" subunits plus numerous others that are system-specific, which presumptively broadens functional capabilities. Recently, there has been exciting progress in defining T4SS assembly pathways and architectures using a combination of fluorescence and cryoelectron microscopy. This review will highlight advances in our knowledge of structure-function relationships for model Gram-negative bacterial T4SSs, including "minimized" systems resembling the Agrobacterium tumefaciens VirB/VirD4 T4SS and "expanded" systems represented by the Helicobacter pylori Cag, Legionella pneumophila Dot/Icm, and F plasmid-encoded Tra T4SSs. Detailed studies of these model systems are generating new insights, some at atomic resolution, to long-standing questions concerning mechanisms of substrate recruitment, T4SS channel architecture, conjugative pilus assembly, and machine adaptations contributing to T4SS functional versatility.

Immunostimulatory membrane proteins potentiate H. pylori-induced carcinogenesis by enabling CagA translocation

Infection with Helicobacter pylori is the single greatest risk factor for developing gastric adenocarcinoma. In prospective, population-based studies, seropositivity to the uncharacterized H. pylori proteins Hp0305 and Hp1564 was significantly associated with cancer risk in East Asia. However, the mechanism underlying this observation has not been elucidated. Here, we show that Hp0305 and Hp1564 act in concert with previously ascribed H. pylori virulence mechanisms to orchestrate cellular alterations that promote gastric carcinogenesis. In samples from 546 patients exhibiting premalignant gastric lesions, seropositivity to Hp0305 and Hp1564 was significantly associated with increased gastric atrophy across all stomach conditions. In vitro, depletion of Hp0305 and Hp1564 significantly reduced levels of gastric cell-associated bacteria and markedly impaired the ability of H. pylori to stimulate pro-inflammatory cytokine production. Remarkably, our studies revealed that Hp1564 is required for translocation of the oncoprotein CagA into gastric epithelial cells. Our data provide experimental insight into the molecular mechanisms governing novel H. pylori pathogenicity factors that are strongly associated with gastric disease and highlight the potential of Hp0305 and Hp1564 as robust molecular tools that can improve identification of individuals that are highly susceptible to gastric cancer. We demonstrate that Hp0305 and Hp1564 augment H. pylori-mediated inflammation and gastric cancer risk by promoting key bacteria-gastric cell interactions that facilitate delivery of oncogenic microbial cargo to target cells. Thus, therapeutically targeting microbial interactions driven by Hp0305/Hp1564 may enable focused H. pylori eradication strategies to prevent development of gastric malignancies in high-risk populations.

Helicobacter pylori Virulence Factor Cytotoxin-Associated Gene A (CagA)-Mediated Gastric Pathogenicity

Helicobacter pylori causes persistent infection in the gastric epithelium of more than half of the world’s population, leading to the development of severe complications such as peptic ulcer diseases, gastric cancer, and gastric mucosa-associated lymphoid tissue (MALT) lymphoma. Several virulence factors, including cytotoxin-associated gene A (CagA), which is translocated into the gastric epithelium via the type 4 secretory system (T4SS), have been indicated to play a vital role in disease development. Although infection with strains harboring the East Asian type of CagA possessing the EPIYA-A, -B, and -D sequences has been found to potentiate cell proliferation and disease pathogenicity, the exact mechanism of CagA involvement in disease severity still remains to be elucidated. Therefore, we discuss the possible role of CagA in gastric pathogenicity.

Helicobacter pylori cagE, cagG, and cagM can be a prognostic marker for intestinal and diffuse gastric cancer

It is known that Helicobacter pylori is the main cause of peptic ulceration and gastric cancer. However, there is a lack of information on whether H. pylori strains may differ in gastric cancer histological subtypes. This study aimed to investigate different H. pylori strains considering six cag Pathogenicity Island - cagPAI genes (cagA, cagE, cagG, cagM, cagT, and virb11), and vacuolating cytotoxin - vacA alleles, and their relation to gastric cancer histologic subtypes. For this purpose, tumor samples from 285 patients with gastric carcinoma were used. H. pylori infection and genotypes were determined by polymerase chain reaction (PCR). H. pylori was detected in 93.9% of gastric tumors. For comparative analyzes between histopathological subtypes considering H. pylori cagPAI genes the strains were grouped according to the vacA s1/s2 alleles. In the vacAs1 group, the strains cagA(-)cagE(+), cagA(+)cagE(+)cagG(+), cagA(+)cagM(+), or only cagE(+) strains were more frequent in the intestinal subtype (P = .009; P = .024; P = .046, respectively). In contrast, cagM(+)cagG(+)cagA(-) and cagE(-) were associated with diffuse tumors (P = .036), highlighting the presence of cagE in the development of intestinal tumors, and the presence of cagG and absence of cagE in diffuse tumors. Furthermore, WEKA software and Decision Tree (CART) analyses confirmed these findings, in which cagE presence was associated with intestinal tumors, and cagE absence and cagG(+) with diffuse tumors. In conclusion our results showed that vacAs1 (cagG + cagM) strains, mainly cagG positive with cagE absence, were relevant in the studied population for the diffuse outcome, while the presence of cagE was relevant for the intestinal outcome. These findings suggest the relevance of these H. pylori genes as potential markers for gastric cancer histological outcomes.

Temporal Control of the Helicobacter pylori Cag Type IV Secretion System in a Mongolian Gerbil Model of Gastric Carcinogenesis

The Helicobacter pylori Cag type IV secretion system (T4SS) translocates the effector protein CagA and nonprotein bacterial constituents into host cells. In this study, we infected Mongolian gerbils with an H. pylori strain in which expression of the cagUT operon (required for Cag T4SS activity) is controlled by a TetR/tetO system. Transcript levels of cagU were significantly higher in gastric tissue from H. pylori-infected animals receiving doxycycline-containing chow (to derepress Cag T4SS activity) than in tissue from infected control animals receiving drug-free chow. At 3 months postinfection, infected animals receiving doxycycline had significantly increased gastric inflammation compared to infected control animals. Dysplasia (a premalignant histologic lesion) and/or invasive gastric adenocarcinoma were detected only in infected gerbils receiving doxycycline, not in infected control animals. We then conducted experiments in which Cag T4SS activity was derepressed during defined stages of infection. Continuous Cag T4SS activity throughout a 3-month time period resulted in higher rates of dysplasia and/or gastric cancer than observed when Cag T4SS activity was limited to early or late stages of infection. Cag T4SS activity for the initial 6 weeks of infection was sufficient for the development of gastric inflammation at the 3-month time point, with gastric cancer detected in a small proportion of animals. These experimental results, together with previous studies of cag mutant strains, provide strong evidence that Cag T4SS activity contributes to gastric carcinogenesis and help to define the stages of H. pylori infection during which Cag T4SS activity causes gastric alterations relevant for cancer pathogenesis.IMPORTANCE The "hit-and-run model" of carcinogenesis proposes that an infectious agent triggers carcinogenesis during initial stages of infection and that the ongoing presence of the infectious agent is not required for development of cancer. H. pylori infection and actions of CagA (an effector protein designated a bacterial oncoprotein, secreted by the Cag T4SS) are proposed to constitute a paradigm for hit-and-run carcinogenesis. In this study, we report the development of methods for controlling H. pylori Cag T4SS activity in vivo and demonstrate that Cag T4SS activity contributes to gastric carcinogenesis. We also show that Cag T4SS activity during an early stage of infection is sufficient to initiate a cascade of cellular alterations leading to gastric inflammation and gastric cancer at later time points.

Lipoprotein Processing and Sorting in Helicobacter pylori

Our current understanding of lipoprotein synthesis and localization in Gram-negative bacteria is based primarily on studies of Escherichia coli Newly synthesized E. coli prolipoproteins undergo posttranslational modifications catalyzed by three essential enzymes (Lgt, LspA, and Lnt). The mature lipoproteins are then sorted to the inner or outer membrane via the Lol system (LolABCDE). Recent studies suggested that this paradigm may not be universally applicable among different classes of proteobacteria. In this study, we conducted a systematic analysis of lipoprotein processing and sorting in Helicobacter pylori, a member of the Epsilonproteobacteria that colonizes the human stomach. We show that H. pylorilgt, lspA, and lnt homologs can complement conditionally lethal E. coli mutant strains in which expression of these genes is conditionally regulated. Mutagenesis studies and analyses of conditionally lethal H. pylori mutant strains indicate that lgt and lspA are essential for H. pylori growth but lnt is dispensable. H. pylorilolA and the single lolC (or lolE) homolog are also essential genes. We then explored the role of lipoproteins in H. pylori Cag type IV secretion system (Cag T4SS) activity. Comparative analysis of the putative VirB7 homolog CagT in wild-type and lnt mutant H. pylori strains indicates that CagT undergoes amino-terminal modifications consistent with lipidation, and we show that CagT lipidation is essential for CagT stability and Cag T4SS function. This work demonstrates that lipoprotein synthesis and localization in H. pylori diverge from the canonical pathways and that lipidation of a T4SS component is necessary for H. pylori Cag T4SS activity.IMPORTANCE Bacterial lipoproteins have diverse roles in multiple aspects of bacterial physiology, antimicrobial resistance, and pathogenesis. Dedicated pathways direct the posttranslational lipidation and localization of lipoproteins, but there is considerable variation in these pathways among the proteobacteria. In this study, we characterized the proteins responsible for lipoprotein synthesis and localization in Helicobacter pylori, a member of the Epsilonproteobacteria that contributes to stomach cancer pathogenesis. We also provide evidence suggesting that lipidation of CagT, a component of the H. pylori Cag T4SS, is required for delivery of the H. pylori CagA oncoprotein into human gastric cells. Overall, these results constitute the first systematic analysis of H. pylori lipoprotein production and localization pathways and reveal how these processes in H. pylori differ from corresponding pathways in model proteobacteria.

Variations in cag pathogenicity island genes of Helicobacter pylori from Latin American groups may influence neoplastic progression to gastric cancer

Helicobacter pylori (HP) colonizes the human stomach and induces acute gastritis, peptic ulcer disease, atrophic gastritis, and gastric adenocarcinoma. Increased virulence in HP isolates derives from harboring the cag (cytotoxin-associated genes) pathogenicity island (cagPAI). We analyzed the microvariants in cagPAI genes with the hypothesis that they may play an important role in determining HP virulence. We tested DNAs from cagA positive patients HP isolates; a total of 74 patients with chronic gastritis (CG, N = 37), intestinal metaplasia (IM, N = 21) or gastric cancer (GC, N = 16) from Mexico and Colombia. We selected 520 non-synonymous variants with at least 7.5% frequency in the original sequence outputs or with a minimum of 5 isolates with minor allele. After adjustment for multiple comparisons, no variants were statistically significantly associated with IM or GC. However, 19 non-synonymous showed conventional P-values < 0.05 comparing the frequency of the alleles between the isolates from subjects with gastritis and isolates from subjects with IM or GC; 12 of these showed a significant correlation with the severity of the disease. The present study revealed that several cagPAI genes from Latin American Western HP strains contains a number of non-synonymous variants in relatively high frequencies which could influence on the clinical outcome. However, none of the associations remained statistically significant after adjustment for multiple comparison.

The Helicobacter pylori Cag Type IV Secretion System

Colonization of the human stomach with Helicobacter pylori strains containing the cag pathogenicity island is a risk factor for development of gastric cancer. The cag pathogenicity island contains genes encoding a secreted effector protein (CagA) and components of a type IV secretion system (Cag T4SS). The molecular architecture of the H. pylori Cag T4SS is substantially more complex than that of prototype T4SSs in other bacterial species. In this review, we discuss recent discoveries pertaining to the structure and function of the Cag T4SS and its role in gastric cancer pathogenesis.

Presence of cagPAI genes and characterization of vacA s, i and m regions in Helicobacter pylori isolated from Alaskans and their association with clinical pathologies

Introduction. Gastric cancer is a health disparity in the Alaska Native people. The incidence of Helicobacter pylori infection, a risk factor for non-cardia gastric adenocarcinoma, is also high. Gastric cancer is partially associated with the virulence of the infecting strain.Aim. To genotype the vacA s, m and i and cag pathogenicity island (cagPAI) genes in H. pylori from Alaskans and investigate associations with gastropathy.Methodology. We enrolled patients with gastritis, peptic ulcer disease (PUD) and intestinal metaplasia (IM) in 1998-2005 and patients with gastric cancer in 2011-2013. Gastric biopsies were collected and cultured and PCR was performed to detect the presence of the right and left ends of the cagPAI, the cagA, cagE, cagT and virD4 genes and to genotype the vacA s, m and i regions.Results. We recruited 263 people; 22 (8 %) had no/mild gastritis, 121 (46 %) had moderate gastritis, 40 (15%) had severe gastritis, 38 (14 %) had PUD, 30 (11 %) had IM and 12 (5 %) had gastric cancer. H. pylori isolates from 150 (57%) people had an intact cagPAI; those were associated with a more severe gastropathy (P≤0.02 for all comparisons). H. pylori isolates from 77 % of people had either the vacA s1/i1/m1 (40 %; 94/234) or s2/i2/m2 (37 %; 86/234) genotype. vacA s1/i1/m1 was associated with a more severe gastropathy (P≤0.03 for all comparisons).Conclusions. In this population with high rates of gastric cancer, we found that just over half of the H. pylori contained an intact cagPAI and 40 % had the vacA s1/i1/m1 genotype. Infection with these strains was associated with a more severe gastropathy.

Bacterial Energetic Requirements for Helicobacter pylori Cag Type IV Secretion System-Dependent Alterations in Gastric Epithelial Cells

Helicobacter pylori colonizes the stomach in about half of the world's population. H. pylori strains containing the cag pathogenicity island (cag PAI) are associated with a higher risk of gastric adenocarcinoma or peptic ulcer disease than cag PAI-negative strains. The cag PAI encodes a type IV secretion system (T4SS) that mediates delivery of the CagA effector protein as well as nonprotein bacterial constituents into gastric epithelial cells. H. pylori-induced nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) activation and interleukin-8 (IL-8) secretion are attributed to T4SS-dependent delivery of lipopolysaccharide metabolites and peptidoglycan into host cells, and Toll-like receptor 9 (TLR9) activation is attributed to delivery of bacterial DNA. In this study, we analyzed the bacterial energetic requirements associated with these cellular alterations. Mutant strains lacking Cagα, Cagβ, or CagE (putative ATPases corresponding to VirB11, VirD4, and VirB4 in prototypical T4SSs) were capable of T4SS core complex assembly but defective in CagA translocation into host cells. Thus, the three Cag ATPases are not functionally redundant. Cagα and CagE were required for H. pylori-induced NF-κB activation, IL-8 secretion, and TLR9 activation, but Cagβ was dispensable for these responses. We identified putative ATP-binding motifs (Walker-A and Walker-B) in each of the ATPases and generated mutant strains in which these motifs were altered. Each of the Walker box mutant strains exhibited properties identical to those of the corresponding deletion mutant strains. These data suggest that Cag T4SS-dependent delivery of nonprotein bacterial constituents into host cells occurs through mechanisms different from those used for recruitment and delivery of CagA into host cells.

In Vivo Structures of the Helicobacter pylori cag Type IV Secretion System

The type IV secretion system (T4SS) is a versatile nanomachine that translocates diverse effector molecules between microbes and into eukaryotic cells. Here, using electron cryotomography, we reveal the molecular architecture of the Helicobacter pylori cag T4SS. Although most components are unique to H. pylori, the cag T4SS exhibits remarkable architectural similarity to other T4SSs. Our images revealed that, when H. pylori encounters host cells, the bacterium elaborates membranous tubes perforated by lateral ports. Sub-tomogram averaging of the cag T4SS machinery revealed periplasmic densities associated with the outer membrane, a central stalk, and peripheral wing-like densities. Additionally, we resolved pilus-like rod structures extending from the cag T4SS into the inner membrane, as well as densities within the cytoplasmic apparatus corresponding to a short central barrel surrounded by four longer barrels. Collectively, these studies reveal the structure of a dynamic molecular machine that evolved to function in the human gastric niche.

Helicobacter pylori genotypes among Belarus patients with gastroduodenal disorders and their association with clinical outcome

The aim of this study was to evaluate the prevalence of Helicobacter pylori genotypes (vacA and cagPAI) directly in gastric biopsy specimens in patients with gastric diseases in Belarus. Gastric biopsies were collected from 461 patients with different gastrointestinal disorders: superficial gastritis (287 subjects), atrophy gastritis (59 subjects), erosive gastritis (47 subjects), duodenal ulcer disease (54 subjects), and stomach ulcer (14 subjects). PCR-based genotyping was used to detect s1a, s1b, s2, m1a, m1b, m2, cagM, cagA, and cagT genes. Overall prevalence of vacA s1a allele was 60.5% followed by m2 (47.1%) and m1a (37.5%). The analysis of data showed that genotype s1a/m1a was significantly more prevalent in patients with duodenal ulcer (21.4% vs. 45.1%, OR = 3.0, 95% CI = 1.5-6.1). The cagA gene was found with a high incidence in most patients with inflammatory diseases of stomach and duodenum. There was a significant increase in the frequency of cagT in patients with duodenal ulcer as compared to superficial gastritis. A high cagM prevalence was found in patients with atrophy gastritis and duodenal ulcer disease. All three island genes of pathogenicity of cagPAI are more often detected in patients with duodenal ulcer, which increases the risk of developing duodenal ulcer by 4.5 times.

Crystal structure of CagV, the Helicobacter pylori homologue of the T4SS protein VirB8

The VirB/D type IV secretion system (T4SS) plays an essential role in materials transport between host cells and pathogenic Helicobacter pylori and is considered the major pathogenic mediator of H. pylori-associated gastric disease. VirB8, an inner membrane protein that interacts with many other proteins, is a crucial component for secretory function. Here, we present a crystal structure of the periplasmic domain of CagV, the VirB8 counterpart in the H. pylori Cag-T4SS. The structure reveals a fold similar to that of other VirB8 members except for the absence of the α5 helix, a discontinuous β1 strand, a larger angle between the α2 and α3 helices, a more hydrophobic surface groove, but exhibits a different dimer interface. Whether the dimerization occurs in solution was proved by mutagenesis, size-exclusion chromatography and cross-linking assays. Unlike the classical dimerization mode, the interface of the CagV dimer is principally formed by several hydrogen bonds, which indicates instability of dimerization. The structure here demonstrates the difference in dimerization among VirB8 homologues and indicates the considerable compositional and functional diversity of them in T4SS. DATABASE: Coordinates and structure factors have been deposited in the Protein Data Bank under accession codes 6IQT.

Specific high affinity interaction of Helicobacter pylori CagL with integrin αV β6 promotes type IV secretion of CagA into human cells

CagL is an essential pilus surface component of the virulence-associated type IV secretion system (T4SS) employed by Helicobacter pylori to translocate the oncogenic effector protein CagA into human gastric epithelial cells. CagL contains an RGD motif and integrin α₅ β₁ is widely accepted as its host cell receptor. Here, we show that CagL binds integrin α_V β₆ with substantially higher affinity and that this interaction is functionally important. Cell surface expression of α_V β₆ on various cell lines correlated perfectly with cell adhesion to immobilized CagL and with binding of soluble CagL to cells. We found no such correlation for α₅ β₁ . The purified α_V β₆ ectodomain bound CagL with high affinity. This interaction was highly specific, as the affinity of CagL for other RGD-binding integrins was two to three orders of magnitude weaker. Mutation of either conserved leucine in the CagL RGDLXXL motif, a motif that generally confers specificity for integrin α_V β₆ and α_V β₈ , lowered the affinity of CagL for α_V β₆ . Stable expression of α_V β₆ in α_V β₆ -negative but α₅ β₁ -expressing human cells promoted two hallmarks of the functional H. pylori T4SS, namely translocation of CagA into host cells and induction of interleukin-8 secretion by host cells. These findings suggest that integrin α_V β₆ , although not essential for T4SS function, represents an important host cell receptor for CagL.

Antigen of 49.6-kDa subunitpili protein of Helicobacter pylori as a potential biomarker for early and rapid detection of the infection

Background and Aim:

Helicobacter pylori infection has been identified as a major cause of peptic ulcer diseases, including gastric and duodenal ulcers, gastritis, chronic and gastric carcinoma, and even gastric lymphoma. In vitro studies using Western blotting analysis, hemagglutination test, adherence inhibition assays, and immunocytochemical staining revealed that the 49.6-kDa subunit pili protein of H. pylori was considered an immunogenic protein. This study aimed to develop a serological diagnostic test using 49.6 kDa for detecting antibodies against H. pylori proteins in an early phase of the infection.

Materials and Methods:

An in-house immunochromatographic test (ICT) kit was developed and used to test a panel of sera sample obtained from a randomly selected symptomatic patient, in which 40 sera were H. pylori positive and 40 sera were H. pylori negative.

Results:

The results showed that ICT with 49.6 kDa as an antigen was highly sensitive and specific for detecting anti-H. pylori immunoglobulin G antibodies in human serum, with a high negative predictive value.

Conclusion:

The developed test could be used to exclude H. pylori infection in symptomatic patients.

In Situ Molecular Architecture of the Helicobacter pylori Cag Type IV Secretion System

Helicobacter pylori colonizes about half of humans worldwide, and its presence in the gastric mucosa is associated with an increased risk of gastric adenocarcinoma, gastric lymphoma, and peptic ulcer disease. H. pylori strains carrying the cag pathogenicity island (cagPAI) are associated with increased risk of disease progression. The cagPAI encodes the Cag type IV secretion system (Cag_T4SS), which delivers the CagA oncoprotein and other effector molecules into human gastric epithelial cells. We visualized structures of native and mutant Cag_T4SS machines on the H. pylori cell envelope by cryoelectron tomography. Individual H. pylori cells contain multiple Cag_T4SS nanomachines, each composed of a wheel-shaped outer membrane complex (OMC) with 14-fold symmetry and an inner membrane complex (IMC) with 6-fold symmetry. CagX, CagY, and CagM are required for assembly of the OMC, whereas strains lacking Cag3 and CagT produce outer membrane complexes lacking peripheral components. The IMC, which has never been visualized in detail, is configured as six tiers in cross-section view and three concentric rings surrounding a central channel in end-on view. The IMC contains three T4SS ATPases: (i) VirB4-like CagE, arranged as a hexamer of dimers at the channel entrance; (ii) a hexamer of VirB11-like Cagα, docked at the base of the CagE hexamer; and (iii) VirD4-like Cagβ and other unspecified Cag subunits, associated with the stacked CagE/Cagα complex and forming the outermost rings. The Cag_T4SS and recently solved Legionella pneumophila Dot/Icm system comprise new structural prototypes for the T4SS superfamily.IMPORTANCE Bacterial type IV secretion systems (T4SSs) have been phylogenetically grouped into two subfamilies. The T4ASSs, represented by the Agrobacterium tumefaciens VirB/VirD4_T4SS, include "minimized" machines assembled from 12 VirB- and VirD4-like subunits and compositionally larger systems such as the Helicobacter pylori Cag_T4SS T4BSSs encompass systems closely related in subunit composition to the Legionella pneumophila Dot/Icm_T4SS Here, we present structures of native and mutant H. pylori Cag machines determined by in situ cryoelectron tomography. We identify distinct outer and inner membrane complexes and, for the first time, visualize structural contributions of all three "signature" ATPases of T4SSs at the cytoplasmic entrance of the translocation channel. Despite their evolutionary divergence, the Cag_T4SS aligns structurally much more closely to the Dot/Icm_T4SS than an available VirB/VirD4 subcomplex. Our findings highlight the diversity of T4SSs and suggest a structural classification scheme in which T4SSs are grouped as minimized VirB/VirD4-like or larger Cag-like and Dot/Icm-like systems.

Biological Diversity and Evolution of Type IV Secretion Systems

The bacterial type IV secretion systems (T4SSs) are a highly functionally and structurally diverse superfamily of secretion systems found in many species of Gram-negative and -positive bacteria. Collectively, the T4SSs can translocate DNA and monomeric and multimeric protein substrates to a variety of bacterial and eukaryotic cell types. Detailed phylogenomics analyses have established that the T4SSs evolved from ancient conjugation machines whose original functions were to disseminate mobile DNA elements within and between bacterial species. How members of the T4SS superfamily evolved to recognize and translocate specific substrate repertoires to prokaryotic or eukaryotic target cells is a fascinating question from evolutionary, biological, and structural perspectives. In this chapter, we will summarize recent findings that have shaped our current view of the biological diversity of the T4SSs. We focus mainly on two subtypes, designated as the types IVA (T4ASS) and IVB (T4BSS) systems that respectively are represented by the paradigmatic Agrobacterium tumefaciens VirB/VirD4 and Legionella pneumophila Dot/Icm T4SSs. We present current information about the composition and architectures of these representative systems. We also describe how these and a few related T4ASS and T4BSS members evolved as specialized nanomachines through acquisition of novel domains or subunits, a process that ultimately generated extensive genetic and structural mosaicism among this secretion superfamily. Finally, we present new phylogenomics information establishing that the T4BSSs are much more broadly distributed than initially envisioned.

Inhibiting bacterial secretion systems in the fight against antibiotic resistance

Antimicrobial resistance is a mounting global health crisis that threatens a resurgence of life-threatening bacterial infections. Despite intensive drug discovery efforts, the rate of antimicrobial resistance outpaces the discovery of new antibiotic agents. One of the major mechanisms driving the rapid propagation of antibiotic resistance is bacterial conjugation mediated by the versatile type IV secretion system (T4SS). The search for therapeutic compounds that prevent the spread of antibiotic resistance via T4SS-dependent mechanisms has identified several promising molecular scaffolds that disrupt resistance determinant dissemination. In this brief review, we highlight the progress and potential of conjugation inhibitors and anti-virulence compounds that target diverse T4SS machineries. These studies provide a solid foundation for the future development of potent, dual-purpose molecular scaffolds that can be used as biochemical tools to probe type IV secretion mechanisms and target bacterial conjugation in clinical settings to prevent the dissemination of antibiotic resistance throughout microbial populations.

Structural and Molecular Biology of Type IV Secretion Systems

Type IV secretion systems (T4SSs) are nanomachines that Gram-negative, Gram-positive bacteria, and some archaea use to transport macromolecules across their membranes into bacterial or eukaryotic host targets or into the extracellular milieu. They are the most versatile secretion systems, being able to deliver both proteins and nucleoprotein complexes into targeted cells. By mediating conjugation and/or competence, T4SSs play important roles in determining bacterial genome plasticity and diversity; they also play a pivotal role in the spread of antibiotic resistance within bacterial populations. T4SSs are also used by human pathogens such as Legionella pneumophila, Bordetella pertussis, Brucella sp., or Helicobacter pylori to sustain infection. Since they are essential virulence factors for these important pathogens, T4SSs might represent attractive targets for vaccines and therapeutics. The best-characterized conjugative T4SSs of Gram-negative bacteria are composed of twelve components that are conserved across many T4SSs. In this chapter, we will review our current structural knowledge on the T4SSs by describing the structures of the individual components and how they assemble into large macromolecular assemblies. With the combined efforts of X-ray crystallography, nuclear magnetic resonance (NMR), and more recently electron microscopy, structural biology of the T4SS has made spectacular progress during the past fifteen years and has unraveled the properties of unique proteins and complexes that assemble dynamically in a highly sophisticated manner.

Biological and Structural Diversity of Type IV Secretion Systems

The bacterial type IV secretion systems (T4SSs) are a functionally diverse superfamily of secretion systems found in many species of bacteria. Collectively, the T4SSs translocate DNA and monomeric and multimeric protein substrates to bacterial and eukaryotic cell types. T4SSs are composed of two large subfamilies, the conjugation machines and the effector translocators that transmit their cargoes through establishment of direct donor-target cell contacts, and a third small subfamily capable of importing or exporting substrates from or to the milieu. This review summarizes recent mechanistic and structural findings that are shedding new light on how T4SSs have evolved such functional diversity. Translocation signals are now known to be located C terminally or embedded internally in structural folds; these signals in combination with substrate-associated adaptor proteins mediate the docking of specific substrate repertoires to cognate VirD4-like receptors. For the Legionella pneumophila Dot/Icm system, recent work has elucidated the structural basis for adaptor-dependent substrate loading onto the VirD4-like DotL receptor. Advances in definition of T4SS machine structures now allow for detailed comparisons of nanomachines closely related to the Agrobacterium tumefaciens VirB/VirD4 T4SS with those more distantly related, e.g., the Dot/Icm and Helicobacter pylori Cag T4SSs. Finally, it is increasingly evident that T4SSs have evolved a variety of mechanisms dependent on elaboration of conjugative pili, membrane tubes, or surface adhesins to establish productive contacts with target cells. T4SSs thus have evolved extreme functional diversity through a plethora of adaptations impacting substrate selection, machine architecture, and target cell binding.

Impact of Helicobacter pylori Virulence Factors on the Host Immune Response and Gastric Pathology

Helicobacter pylori chronically infects nearly half the world's population, yet most of those infected remain asymptomatic throughout their lifetime. The outcome of infection-peptic ulcer disease or gastric cancer versus asymptomatic colonization-is a product of host genetics, environmental influences, and differences in bacterial virulence factors. Here, we review the current understanding of the cag pathogenicity island (cagPAI), the vacuolating cytotoxin (VacA), and a large family of outer membrane proteins (OMPs), which are among the best understood H. pylori virulence determinants that contribute to disease. Each of these virulence factors is characterized by allelic and phenotypic diversity that is apparent within and across individuals, as well as over time, and modulates inflammation. From the bacterial perspective, inflammation is probably a necessary evil because it promotes nutrient acquisition, but at the cost of reduction in bacterial load and therefore decreases the chance of transmission to a new host. The general picture that emerges is one of a chronic bacterial infection that is dependent on both inducing and carefully regulating the host inflammatory response. A better understanding of these regulatory mechanisms may have implications for the control of chronic inflammatory diseases that are increasingly common causes of human morbidity and mortality.

Expression levels of A disintegrin and metalloproteases (ADAMs), and Th17-related cytokines and their association with Helicobacter pylori infection in patients with gastroduodenal diseases

Expression levels of A disintegrin and metalloproteases (ADAMs) (10 and 17) and Th17-related cytokines [interleukin (IL) 17A, IL-17F, IL-33, IL-23, IL-23R] were investigated by quantitative real time polymerase chain reaction in gastric biopsies of patients with different gastroduodenal pathologies in the presence and absence of Helicobacter pylori infection. Patients with gastric cancer (GC) (n = 70, intestinal-type 38 and diffuse type 32), peptic ulcer disease [n = 50, duodenal ulcer (DU) 16 and gastric ulcer (GU) 34] and functional dyspepsia (n = 120) were included in the study. Further, the expression levels of ADAMs and Th17 cytokines were correlated with H. pylori cytotoxin-associated genes pathogenicity island (cagPAI) status. Expression levels of ADAMs (10 and 17) and Th17-related cytokines (IL-17A, IL-23, IL-23R) were significantly higher in H. pylori-positive than in H. pylori-negative gastric biopsies. Significant increase in ADAM17 and Th17 cytokines (IL-17A and IL-23) expressions was observed in patients with GU and intestinal-type GC in the presence of H. pylori infection and in strains harbouring intact cagPAI. Expression levels of IL-17A, IL-23 and ADAM17 were strongly correlated with GU and intestinal-type GC and weakly with DU and diffuse-type GC in the presence of H. pylori infection. Higher expression levels of ADAM17 and Th17 cytokines (IL-17A and IL-23), and their strong correlation with GU and intestinal-type GC patients in the presence of H. pylori and its intact cagPAI status, suggest a possible role of strain specificity in the pathogenesis of these diseases.

Two pKM101-encoded proteins, the pilus-tip protein TraC and Pep, assemble on the Escherichia coli cell surface as adhesins required for efficient conjugative DNA transfer

Mobile genetic elements (MGEs) encode type IV secretion systems (T4SSs) known as conjugation machines for their transmission between bacterial cells. Conjugation machines are composed of an envelope-spanning translocation channel, and those functioning in Gram-negative species additionally elaborate an extracellular pilus to initiate donor-recipient cell contacts. We report that pKM101, a self-transmissible MGE functioning in the Enterobacteriaceae, has evolved a second target cell attachment mechanism. Two pKM101-encoded proteins, the pilus-tip adhesin TraC and a protein termed Pep, are exported to the cell surface where they interact and also form higher order complexes appearing as distinct foci or patches around the cell envelope. Surface-displayed TraC and Pep are required for an efficient conjugative transfer, 'extracellular complementation' potentially involving intercellular protein transfer, and activation of a Pseudomonas aeruginosa type VI secretion system. Both proteins are also required for bacteriophage PRD1 infection. TraC and Pep are exported across the outer membrane by a mechanism potentially involving the β-barrel assembly machinery. The pKM101 T4SS, thus, deploys alternative routing pathways for the delivery of TraC to the pilus tip or both TraC and Pep to the cell surface. We propose that T4SS-encoded, pilus-independent attachment mechanisms maximize the probability of MGE propagation and might be widespread among this translocation superfamily.

The type IV secretion system in Helicobacter pylori

Helicobacter pylori (H. pylori) has an essential role in the pathogenesis of gastritis, peptic ulcer disease, mucosa-associated lymphoid tissue lymphoma and gastric cancer. The severity of the host inflammatory responses against the bacteria have been straightly associated with a special bacterial virulence factor, the cag pathogenicity island, which is a type IV secretion system (T4SS) to deliver CagA into the host cells. Besides cag-T4SS, the chromosomes of H. pylori can encode another three T4SSs, including comB, tfs3 and tfs4. In this review, we systematically reviewed the four T4SSs of H. pylori and explored their roles in the pathogenesis of gastroduodenal diseases. The information summarized in this review might provide valuable insights into the pathogenic mechanism for H. pylori.

CagY-Dependent Regulation of Type IV Secretion in Helicobacter pylori Is Associated with Alterations in Integrin Binding

Strains of Helicobacter pylori that cause ulcer or gastric cancer typically express a type IV secretion system (T4SS) encoded by the cag pathogenicity island (cagPAI). CagY is an ortholog of VirB10 that, unlike other VirB10 orthologs, has a large middle repeat region (MRR) with extensive repetitive sequence motifs, which undergo CD4⁺ T cell-dependent recombination during infection of mice. Recombination in the CagY MRR reduces T4SS function, diminishes the host inflammatory response, and enables the bacteria to colonize at a higher density. Since CagY is known to bind human α₅β₁ integrin, we tested the hypothesis that recombination in the CagY MRR regulates T4SS function by modulating binding to α₅β₁ integrin. Using a cell-free microfluidic assay, we found that H. pylori binding to α₅β₁ integrin under shear flow is dependent on the CagY MRR, but independent of the presence of the T4SS pili, which are only formed when H. pylori is in contact with host cells. Similarly, expression of CagY in the absence of other T4SS genes was necessary and sufficient for whole bacterial cell binding to α₅β₁ integrin. Bacteria with variant cagY alleles that reduced T4SS function showed comparable reduction in binding to α₅β₁ integrin, although CagY was still expressed on the bacterial surface. We speculate that cagY-dependent modulation of H. pylori T4SS function is mediated by alterations in binding to α₅β₁ integrin, which in turn regulates the host inflammatory response so as to maximize persistent infection.IMPORTANCE Infection with H. pylori can cause peptic ulcers and is the most important risk factor for gastric cancer, the third most common cause of cancer death worldwide. The major H. pylori virulence factor that determines whether infection causes disease or asymptomatic colonization is the type IV secretion system (T4SS), a sort of molecular syringe that injects bacterial products into gastric epithelial cells and alters host cell physiology. We previously showed that recombination in CagY, an essential T4SS component, modulates the function of the T4SS. Here we found that these recombination events produce parallel changes in specific binding to α₅β₁ integrin, a host cell receptor that is essential for T4SS-dependent translocation of bacterial effectors. We propose that CagY-dependent binding to α₅β₁ integrin acts like a molecular rheostat that alters T4SS function and modulates the host immune response to promote persistent infection.

Biochemical characterization of the Helicobacter pylori Cag Type 4 Secretion System protein CagN and its interaction partner CagM

Highly virulent Helicobacter pylori strains contain the cag pathogenicity island (cagPAI). It codes for about 30 proteins forming a type IV secretion system (T4SS) which translocates the pro-inflammatory protein CagA into epithelial host cells. While CagA and various other Cag proteins have been extensively studied, several cagPAI proteins are poorly characterized or of unknown function. CagN (HP0538) is of unknown function but highly conserved in the cagPAI suggesting an important role. cagM (HP0537) is the first gene of the cagMN operon and its product is part of the CagT4SS core complex. Both proteins do not have detectable homologs in other type IV secretion systems. We have characterized the biochemical and structural properties of CagN and CagM and their interaction. We demonstrate by circular dichroism, Multi-Angle Light Scattering (MALS) and small angle X-ray scattering (SAXS) that CagN is a folded, predominantly monomeric protein with an elongated shape in solution. CagM is folded and forms predominantly dimers that are also elongated in solution. We found by various in vivo and in vitro methods that CagN and CagM directly interact with each other. CagM self-interacts stably with a low nanomolar K_D and can form stable multimers. Finally, in vivo experiments show that deletion of CagM reduces the amounts of CagN and other outer CagPAI proteins in H. pylori cells.

High-Salt Conditions Alter Transcription of Helicobacter pylori Genes Encoding Outer Membrane Proteins

Helicobacter pylori infection and high dietary salt intake are risk factors for the development of gastric adenocarcinoma. One possible mechanism by which a high-salt diet could influence gastric cancer risk is by modulating H. pylori gene expression. In this study, we utilized transcriptome sequencing (RNA-seq) methodology to compare the transcriptional profiles of H. pylori grown in media containing different concentrations of sodium chloride. We identified 118 differentially expressed genes (65 upregulated and 53 downregulated in response to high-salt conditions), including multiple members of 14 operons. Twenty-nine of the differentially expressed genes encode proteins previously shown to undergo salt-responsive changes in abundance, based on proteomic analyses. Real-time reverse transcription (RT)-PCR analyses validated differential expression of multiple genes encoding outer membrane proteins, including adhesins (SabA and HopQ) and proteins involved in iron acquisition (FecA2 and FecA3). Transcript levels of sabA, hopA, and hopQ are increased under high-salt conditions, whereas transcript levels of fecA2 and fecA3 are decreased under high-salt conditions. Transcription of sabA, hopA, hopQ, and fecA3 is derepressed in an arsS mutant strain, but salt-responsive transcription of these genes is not mediated by the ArsRS two-component system, and the CrdRS and FlgRS two-component systems do not have any detectable effects on transcription of these genes. In summary, these data provide a comprehensive view of H. pylori transcriptional alterations that occur in response to high-salt environmental conditions.

The Mosaic Type IV Secretion Systems

Escherichia coli and other Gram-negative and -positive bacteria employ type IV secretion systems (T4SSs) to translocate DNA and protein substrates, generally by contact-dependent mechanisms, to other cells. The T4SSs functionally encompass two major subfamilies, the conjugation systems and the effector translocators. The conjugation systems are responsible for interbacterial transfer of antibiotic resistance genes, virulence determinants, and genes encoding other traits of potential benefit to the bacterial host. The effector translocators are used by many Gram-negative pathogens for delivery of potentially hundreds of virulence proteins termed effectors to eukaryotic cells during infection. In E. coli and other species of Enterobacteriaceae, T4SSs identified to date function exclusively in conjugative DNA transfer. In these species, the plasmid-encoded systems can be classified as the P, F, and I types. The P-type systems are the simplest in terms of subunit composition and architecture, and members of this subfamily share features in common with the paradigmatic Agrobacterium tumefaciens VirB/VirD4 T4SS. This review will summarize our current knowledge of the E. coli systems and the A. tumefaciens P-type system, with emphasis on the structural diversity of the T4SSs. Ancestral P-, F-, and I-type systems were adapted throughout evolution to yield the extant effector translocators, and information about well-characterized effector translocators also is included to further illustrate the adaptive and mosaic nature of these highly versatile machines.

DNA Transfer and Toll-like Receptor Modulation by Helicobacter pylori

Helicobacter pylori is the most common bacterial infection worldwide, and virtually all infected persons develop co-existing gastritis. H. pylori is able to send and receive signals from the gastric mucosa, which enables both host and microbe to engage in a dynamic equilibrium. In order to persist within the human host, H. pylori has adopted dichotomous strategies to both induce inflammation as a means of liberating nutrients while simultaneously tempering the immune response to augment its survival. Toll-like receptors (TLRs) and Nod proteins are innate immune receptors that are present in epithelial cells and represent the first line of defense against pathogens. To ensure persistence, H. pylori manipulates TLR-mediated defenses using strategies that include rendering its LPS and flagellin to be non-stimulatory to TLR4 and TLR5, respectively; translocating peptidoglycan into host cells to induce NOD1-mediated anti-inflammatory responses; and translocating DNA into host cells to induce TLR9 activation.

Structural Insights into Helicobacter pylori Cag Protein Interactions with Host Cell Factors

The most virulent strains of Helicobacter pylori carry a genomic island (cagPAI) containing a set of 27-31 genes. The encoded proteins assemble a syringe-like apparatus to inject the cytotoxin-associated gene A (CagA) protein into gastric cells. This molecular device belongs to the type IV secretion system (T4SS) family albeit with unique characteristics. The cagPAI-encoded T4SS and its effector protein CagA have an intricate relationship with the host cell, with multiple interactions that only start to be deciphered from a structural point of view. On the one hand, the major roles of the interactions between CagL and CagA (and perhaps CagI and CagY) and host cell factors are to facilitate H. pylori adhesion and to mediate the injection of the CagA oncoprotein. On the other hand, CagA interactions with host cell partners interfere with cellular pathways to subvert cell defences and to promote H. pylori infection. Although a clear mechanism for CagA translocation is still lacking, the structural definition of CagA and CagL domains involved in interactions with signalling proteins are progressively coming to light. In this chapter, we will focus on the structural aspects of Cag protein interactions with host cell molecules, critical molecular events precluding H. pylori-mediated gastric cancer development.

Analyzing the role of CagV, a VirB8 homolog of the type IV secretion system of Helicobacter pylori

The type IV secretion system of Helicobacter pylori (Cag‐T4SS) is composed of ~ 27 components including a VirB8 homolog, CagV. We have characterized CagV and reported that it is an inner membrane protein and, like VirB8, forms a homodimer. Its stability is not dependent on the other Cag components and the absence of cagV affects the stability of only CagI, a protein involved in pilus formation. CagV is not required for the stability and localization of outer membrane subcomplex proteins, but interacts with them through CagX. It also interacts with the inner membrane‐associated components, CagF and CagZ, and is required for the surface localization of CagA. The results of this study might help in deciphering the mechanistic contributions of CagV in the Cag‐T4SS biogenesis and function.

Functional interaction and structural characteristics of unique components of Helicobacter pylori T4SS

The Helicobacter pylori infection of the human gastric mucosa causes chronic active gastritis and peptic ulcers and is associated with the development of gastric cancer. Epidemiological studies show that these gastric diseases are related to virulent H. pylori strains that harbor the cytotoxin-associated gene pathogenicity island (cag PAI). The cag PAI is a DNA insertion in the H. pylori chromosome that encodes ~ 27 proteins, including the oncoprotein CagA. Approximately 20 of these proteins have been designated as cag type IV secretion system (T4SS) components. However, only 11 of these proteins share function, structure, and/or sequence similarities with the prototypical VirB/VirD4 T4SS of Agrobacterium tumefaciens. The VirB/VirD4 orthologs of the cag T4SS of H. pylori are required for CagA translocation and stimulate the gastric epithelial cells to produce and secrete interleukin-8 (IL-8). The cag PAI encodes eight additional proteins, such as Cag3 (Cagδ/HP0522), CagM (Cag16/HP0537), CagU (Cag11/HP0531), CagI (Cag19/HP0540), and CagH (Cag20/HP0541), which are also required for the translocation of CagA and IL-8 secretion, meanwhile CagF (Cag22/HP0543), CagG (Cag21/HP0542), and CagZ (Cag6/HP0526) are just required for the translocation of CagA. However, relatively little is known about their functions and structural organization because they exhibit a nondetectable sequence similarity with T4SS components in the current databases. In this review, we conducted an exhaustive analysis of the literature to present the biochemistry, putative role, localization, and interactions of each of these eight additional cag T4SS components.

Fallacy of the Unique Genome: Sequence Diversity within Single Helicobacter pylori Strains

Many bacterial genomes are highly variable but nonetheless are typically published as a single assembled genome. Experiments tracking bacterial genome evolution have not looked at the variation present at a given point in time. Here, we analyzed the mouse-passaged Helicobacter pylori strain SS1 and its parent PMSS1 to assess intra- and intergenomic variability. Using high sequence coverage depth and experimental validation, we detected extensive genome plasticity within these H. pylori isolates, including movement of the transposable element IS607, large and small inversions, multiple single nucleotide polymorphisms, and variation in cagA copy number. The cagA gene was found as 1 to 4 tandem copies located off the cag island in both SS1 and PMSS1; this copy number variation correlated with protein expression. To gain insight into the changes that occurred during mouse adaptation, we also compared SS1 and PMSS1 and observed 46 differences that were distinct from the within-genome variation. The most substantial was an insertion in cagY, which encodes a protein required for a type IV secretion system function. We detected modifications in genes coding for two proteins known to affect mouse colonization, the HpaA neuraminyllactose-binding protein and the FutB α-1,3 lipopolysaccharide (LPS) fucosyltransferase, as well as genes predicted to modulate diverse properties. In sum, our work suggests that data from consensus genome assemblies from single colonies may be misleading by failing to represent the variability present. Furthermore, we show that high-depth genomic sequencing data of a population can be analyzed to gain insight into the normal variation within bacterial strains.IMPORTANCE Although it is well known that many bacterial genomes are highly variable, it is nonetheless traditional to refer to, analyze, and publish "the genome" of a bacterial strain. Variability is usually reduced ("only sequence from a single colony"), ignored ("just publish the consensus"), or placed in the "too-hard" basket ("analysis of raw read data is more robust"). Now that whole-genome sequences are regularly used to assess virulence and track outbreaks, a better understanding of the baseline genomic variation present within single strains is needed. Here, we describe the variability seen in typical working stocks and colonies of pathogen Helicobacter pylori model strains SS1 and PMSS1 as revealed by use of high-coverage mate pair next-generation sequencing (NGS) and confirmed by traditional laboratory techniques. This work demonstrates that reliance on a consensus assembly as "the genome" of a bacterial strain may be misleading.

The Helicobacter pylori Autotransporter ImaA Tempers the Bacterium's Interaction with α5β1 Integrin

The human pathogen Helicobacter pylori uses the host receptor α₅β₁ integrin to trigger inflammation in host cells via its cag pathogenicity island (cag PAI) type IV secretion system (T4SS). Here, we report that the H. pylori ImaA protein (HP0289) decreases the action of the cag PAI T4SS via tempering the bacterium's interaction with α₅β₁ integrin. Previously, imaA-null mutants were found to induce an elevated inflammatory response that was dependent on the cag PAI T4SS; here we extend those findings to show that the elevated response is independent of the CagA effector protein. To understand how ImaA could be affecting cag PAI T4SS activity at the host cell interface, we utilized the Phyre structural threading program and found that ImaA has a region with remote homology to bacterial integrin-binding proteins. This region was required for ImaA function. Unexpectedly, we observed that imaA mutants bound higher levels of α₅β₁ integrin than wild-type H. pylori, an outcome that required the predicted integrin-binding homology region of ImaA. Lastly, we report that ImaA directly affected the amount of host cell β1 integrin but not other cellular integrins. Our results thus suggest a model in which H. pylori employs ImaA to regulate interactions between integrin and the T4SS and thus alter the host inflammatory strength.

Systematic site-directed mutagenesis of the Helicobacter pylori CagL protein of the Cag type IV secretion system identifies novel functional domains

The Cag Type IV secretion system, which contributes to inflammation and cancerogenesis during chronic infection, is one of the major virulence factors of the bacterial gastric pathogen Helicobacter pylori. We have generated and characterized a series of non-marked site-directed chromosomal mutants in H. pylori to define domains of unknown function of the essential tip protein CagL of the Cag secretion system. Characterizing the CagL mutants, we determined that their function to activate cells and transport the effector CagA was reduced to different extents. We identified three novel regions of the CagL protein, involved in its structural integrity, its possible interaction with the CagPAI T4SS pilus protein CagI, and in its binding to integrins and other host cell ligands. In particular two novel variable CagL motifs were involved in integrin binding, TSPSA, and TASLI, which is located opposite of its integrin binding motif RGD. We thereby defined functionally important subdomains within the CagL structure, which can be used to clarify CagL contributions in the context of other CagPAI proteins or for inhibition of the CagT4SS. This structure-function correlation of CagL domains can also be instructive for the functional characterization of other potential VirB5 orthologs whose structure is not yet known.

Systematic analysis of phosphotyrosine antibodies recognizing single phosphorylated EPIYA-motifs in CagA of East Asian-type Helicobacter pylori strains

BACKGROUND

Highly virulent strains of the gastric pathogen Helicobacter pylori encode a type IV secretion system (T4SS) that delivers the effector protein CagA into gastric epithelial cells. Translocated CagA undergoes tyrosine phosphorylation by members of the oncogenic c-Src and c-Abl host kinases at EPIYA-sequence motifs A, B and D in East Asian-type strains. These phosphorylated EPIYA-motifs serve as recognition sites for various SH2-domains containing human proteins, mediating interactions of CagA with host signaling factors to manipulate signal transduction pathways. Recognition of phospho-CagA is mainly based on the use of commercial pan-phosphotyrosine antibodies that were originally designed to detect phosphotyrosines in mammalian proteins. Specific anti-phospho-EPIYA antibodies for each of the three sites in CagA are not forthcoming.

RESULTS

This study was designed to systematically analyze the detection preferences of each phosphorylated East Asian CagA EPIYA-motif by pan-phosphotyrosine antibodies and to determine a minimal recognition sequence. We synthesized phospho- and non-phosphopeptides derived from each predominant EPIYA-site, and determined the recognition patterns by seven different pan-phosphotyrosine antibodies using Western blotting, and also investigated representative East Asian H. pylori isolates during infection. The results indicate that a total of only 9-11 amino acids containing the phosphorylated East Asian EPIYA-types are required and sufficient to detect the phosphopeptides with high specificity. However, the sequence recognition by the different antibodies was found to bear high variability. From the seven antibodies used, only four recognized all three phosphorylated EPIYA-motifs A, B and D similarly well. Two of the phosphotyrosine antibodies preferentially bound primarily to the phosphorylated motif A and D, while the seventh antibody failed to react with any of the phosphorylated EPIYA-motifs. Control experiments confirmed that none of the antibodies reacted with non-phospho-CagA peptides and in accordance were able to recognize phosphotyrosine proteins in human cells.

CONCLUSIONS

The results of this study disclose the various binding preferences of commercial anti-phosphotyrosine antibodies for phospho-EPIYA-motifs, and are valuable in the application for further characterization of CagA phosphorylation events during infection with H. pylori and risk prediction for gastric disease development.

Mechanism and Function of Type IV Secretion During Infection of the Human Host

Bacterial pathogens employ type IV secretion systems (T4SSs) for various purposes to aid in survival and proliferation in eukaryotic hosts. One large T4SS subfamily, the conjugation systems, confers a selective advantage to the invading pathogen in clinical settings through dissemination of antibiotic resistance genes and virulence traits. Besides their intrinsic importance as principle contributors to the emergence of multiply drug-resistant "superbugs," detailed studies of these highly tractable systems have generated important new insights into the mode of action and architectures of paradigmatic T4SSs as a foundation for future efforts aimed at suppressing T4SS machine function. Over the past decade, extensive work on the second large T4SS subfamily, the effector translocators, has identified a myriad of mechanisms employed by pathogens to subvert, subdue, or bypass cellular processes and signaling pathways of the host cell. An overarching theme in the evolution of many effectors is that of molecular mimicry. These effectors carry domains similar to those of eukaryotic proteins and exert their effects through stealthy interdigitation of cellular pathways, often with the outcome not of inducing irreversible cell damage but rather of reversibly modulating cellular functions. This article summarizes the major developments for the actively studied pathogens with an emphasis on the structural and functional diversity of the T4SSs and the emerging common themes surrounding effector function in the human host.