Conformational analysis of isolated domains of Helicobacter pylori CagA

Pal power: Demonstration of the functional association of the Helicobacter pylori flagellar motor with peptidoglycan-associated lipoprotein (Pal) and its preliminary crystallographic analysis

The bacterial flagellar motor is a molecular nanomachine, the assembly and regulation of which requires many accessory proteins. Their identity, structure and function are often discovered through characterisation of mutants with impaired motility. Here, we demonstrate the functional association of the Helicobacter pylori peptidoglycan-associated lipoprotein (HpPal) with the flagellar motor by analysing the motility phenotype of the ∆pal mutant, and present the results of the preliminary X-ray crystallographic analysis of its globular C-terminal domain HpPal-C. Purified HpPal-C behaved as a dimer in solution. Crystals of HpPal-C were grown by the hanging drop vapour diffusion method using medium molecular weight polyethylene glycol (PEG) Smear as the precipitating agent. The crystals belong to the primitive orthorhombic space group P1 with unit cell parameters a = 50.7, b = 63.0, c = 75.1 Å. X-ray diffraction data were collected to 1.8 Å resolution on the Australian Synchrotron beamline MX2. Calculation of the Matthews coefficient (VM=2.24 Å3/Da) and molecular replacement showed that the asymmetric unit contains two protein subunits. This study is an important step towards elucidation of the non-canonical role of H. pylori Pal in the regulation, or function of, the flagellar motor.

Kinetics of CagA type IV secretion by Helicobacter pylori and the requirement for substrate unfolding

Type IV secretion of effector proteins is an important principle for interaction of human pathogens with their target cells. The corresponding secretion systems may transport a multitude of effector proteins that have to be deployed in the respective spatiotemporal context, or only a single translocated protein, as in the case of the CagA effector protein produced by the human gastric pathogen Helicobacter pylori. For a more detailed analysis of the kinetics and mode of action of CagA type IV secretion by H. pylori, we describe here, a novel, highly sensitive split luciferase-based translocation reporter which can be easily adapted to different end-point or real-time measurements. Using this reporter, we showed that H. pylori cells are able to rapidly inject a limited amount of their CagA supply into cultured gastric epithelial cells. We have further employed the reporter system to address the question whether CagA has to be unfolded prior to translocation by the type IV secretion system. We showed that protein domains co-translocated with CagA as protein fusions are more readily tolerated as substrates than in other secretion systems, but also provide evidence that unfolding of effector proteins is a prerequisite for their transport.

Bioinformatics analysis and biochemical characterisation of ABC transporter-associated periplasmic substrate-binding proteins ModA and MetQ from Helicobacter pylori strain SS1

The human gastric pathogen Helicobacter pylori relies on the uptake of host-provided nutrients for its proliferation and pathogenicity. ABC transporters that mediate import of small molecules into the cytoplasm of H. pylori employ their cognate periplasmic substrate-binding proteins (SBPs) for ligand capture in the periplasm. The genome of the mouse-adapted strain SS1 of H. pylori encodes eight ABC transporter-associated SBPs, but little is known about their specificity or structure. In this study, we demonstrated that the SBP annotated as ModA binds molybdate (MoO₄^2-, K_D = 3.8 nM) and tungstate (WO₄^2-, K_D = 7.8 nM). In addition, we showed that MetQ binds D-methionine (K_D = 9.5 μM), but not L-methionine, which suggests the existence of as yet unknown pathway for L-methionine uptake. Homology modelling has led to identification of the ligand-binding residues.

Purification, crystallization and preliminary X-ray crystallographic studies on the C-terminal domain of the flagellar protein FliL from Helicobacter pylori

FliL is an inner membrane protein, occupying a position between the rotor and the stator of the bacterial flagellar motor. Its proximity to, and interactions with, the MS (membrane and supramembranous) ring, the switch complex and the stator proteins MotA/B suggests a role in recruitment and/or stabilization of the stator around the rotor, although the precise role of FliL in the flagellum remains to be established. In this study, recombinant C-terminal domain of Helicobacter pylori FliL (amino-acid residues 81-183) has been expressed in Escherichia coli and purified to > 98% homogeneity. Purified recombinant protein behaved as a monomer in solution. Crystals were obtained by the hanging-drop vapour-diffusion method using ammonium phosphate monobasic as a precipitant. These crystals belong to space group P1, with unit-cell parameters a = 62.5, b = 82.6, c = 97.8 Å, α = 67.7, ꞵ = 83.4, γ = 72.8°. A complete data set has been collected to 2.8 Å resolution using synchrotron radiation. This is an important step towards elucidation of the function of FliL in the bacterial flagellar motor.

Structural Analysis of Variability and Interaction of the N-terminal of the Oncogenic Effector CagA of Helicobacter pylori with Phosphatidylserine

Helicobacter pylori cytotoxin-associated gene A protein (CagA) has been associated with the increase in virulence and risk of cancer. It has been demonstrated that CagA’s translocation is dependent on its interaction with phosphatidylserine. We evaluated the variability of the N-terminal CagA in 127 sequences reported in NCBI, by referring to molecular interaction forces with the phosphatidylserine and the docking of three mutations chosen from variations in specific positions. The major sites of conservation of the residues involved in CagA–Phosphatidylserine interaction were 617, 621 and 626 which had no amino acid variation. Position 636 had the lowest conservation score; mutations in this position were evaluated to observe the differences in intermolecular forces for the CagA–Phosphatidylserine complex. We evaluated the docking of three mutations: K636A, K636R and K636N. The crystal and mutation models presented a ΔG of −8.919907, −8.665261, −8.701923, −8.515097 Kcal/mol, respectively, while mutations K636A, K636R, K636N and the crystal structure presented 0, 3, 4 and 1 H-bonds, respectively. Likewise, the bulk effect of the ΔG and amount of H-bonds was estimated in all of the docking models. The type of mutation affected both the ΔG (χ2(1)=93.82, p-value <2.2×10−16) and the H-bonds (χ2(1)=91.93, p-value <2.2×10−16). Overall, 76.9% of the strains that exhibit the K636N mutation produced a severe pathology. The average H-bond count diminished when comparing the mutations with the crystal structure of all the docking models, which means that other molecular forces are involved in the CagA–Phosphatidylserine complex interaction.

The Middle Fragment of Helicobacter pylori CagA Induces Actin Rearrangement and Triggers Its Own Uptake into Gastric Epithelial Cells

Cytotoxin-associated gene product A (CagA) is a major virulence factor secreted by Helicobacter pylori. CagA activity in the gastric epithelium is associated with higher risk of gastric cancer development. Bacterial type IV secretion system (T4SS)-mediated translocation of CagA into the cytosol of human epithelial cells occurs via a poorly understood mechanism that requires CagA interaction with the host membrane lipid phosphatidylserine (PS) and host cell receptor integrin α₅β₁. Here we have characterized the isolated recombinant middle fragment of CagA (CagA-M) that contains the positively-charged PS-binding region (aa 613–636) and a putative β₁ integrin binding site, but lacks the EPIYA region, secretion signal peptide and the CagA multimerization motif. We show that CagA-M, when immobilized on latex beads, is capable of binding to, and triggering its own uptake into, gastric epithelial cells in the absence of infection with cagA-positive H. pylori. Using site-directed mutagenesis, fluorescent and electron microscopy, and highly-specific inhibitors, we demonstrate that the cell-binding and endocytosis-like internalization of CagA-M are dependent on (1) binding to PS; (2) β₁ integrin activity; and (3) actin dynamics. Interaction of CagA-M with the host cells is accompanied by the development of long filopodia-like protrusions (macrospikes). This novel morphology is different from the hummingbird phenotype induced by the translocation of full-length CagA. The determinants within CagA-M and within the host that are important for endocytosis-like internalization into host cells are very similar to those observed for T4SS-mediated internalization of full-length CagA, suggesting that the latter may involve an endocytic pathway.

Helicobacter pylori vacA s1m1 genotype but not cagA or babA2 increase the risk of ulcer and gastric cancer in patients from Southern Mexico

BACKGROUND

The vacA, cagA and babA2 genotypes of Helicobacter pylori are associated with gastric pathology. The objectives were to determine the frequency of infection and distribution of the vacA, cagA and babA2 genotypes of H. pylori in patients with gastric ulcer, chronic gastritis and gastric cancer, and to evaluate the association of virulent genotypes with diagnosis.

METHODS

We studied 921 patients with symptoms of dyspepsia or with presumptive diagnosis of gastric cancer. The DNA of H. pylori and the vacA, cagA and babA2 genes was detected by PCR in total DNA from gastric biopsies. The association of H. pylori and of its cagA, vacA and babA2 genotypes with diagnosis was determined by calculating the odds ratio (OR).

RESULTS

Chronic gastritis was confirmed in 767 patients, gastric ulcer in 115 and cancer in 39. The prevalence of H. pylori was 47.8, 49.6 and 61.5% in those groups, respectively. H. pylori was more frequent in the surrounding tissue (69.2%) than in the tumor (53.8%). The vacA s1m1 genotype predominated in the three groups (45.2, 61.4 and 83.3%, respectively). H. pylori was associated with cancer (OR_adjusted = 2.08; 95% CI 1.05-4.13; p = 0.035) but not with ulcer (OR_adjusted = 1.07; 95% CI 0.71-1.61; p = 0.728). The s1m1 genotype was associated with ulcer and cancer (OR_adjusted = 2.02; 95% CI 1.12-3.62; p = 0.019 and OR_adjusted = 6.58; 95% CI 2.15-20.08; p = 0.001, respectively). babA2 was associated with gastric cancer, and cagA was not associated with the diagnosis.

CONCLUSIONS

In population from Southern Mexico, H. pylori and the s1m1 genotype were associated with gastric cancer and the s1m1/cagA+/babA2+ strains predominated in tumor and adjacent tissue.

Sequence Polymorphism and Intrinsic Structural Disorder as Related to Pathobiological Performance of the Helicobacter pylori CagA Oncoprotein

CagA, an oncogenic virulence factor produced by Helicobacter pylori, is causally associated with the development of gastrointestinal diseases such as chronic gastritis, peptic ulcers, and gastric cancer. Upon delivery into gastric epithelial cells via bacterial type IV secretion, CagA interacts with a number of host proteins through the intrinsically disordered C-terminal tail, which contains two repeatable protein-binding motifs, the Glu-Pro-Ile-Tyr-Ala (EPIYA) motif and the CagA multimerization (CM) motif. The EPIYA motif, upon phosphorylation by host kinases, binds and deregulates Src homology 2 domain-containing protein tyrosine phosphatase 2 (SHP2), a bona fide oncoprotein, inducing pro-oncogenic mitogenic signaling and abnormal cell morphology. Through the CM motif, CagA inhibits the kinase activity of polarity regulator partitioning-defective 1b (PAR1b), causing junctional and polarity defects while inducing actin cytoskeletal rearrangements. The magnitude of the pathobiological action of individual CagA has been linked to the tandem repeat polymorphisms of these two binding motifs, yet the molecular mechanisms by which they affect disease outcome remain unclear. Recent studies using quantitative techniques have provided new insights into how the sequence polymorphisms in the structurally disordered C-terminal region determine the degree of pro-oncogenic action of CagA in the gastric epithelium.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori relies on multiple colonization and virulence factors to persist in the human stomach for life. In addition, these factors can be modulated and vary to suit the ever-changing environment within the host individual. This article outlines the novel developments in this field of research during the past year, highlighting the cag pathogenicity island, VacA, γ-glutamyl-transpeptidase as well as including recent advances in protein structure, bacteria-host interaction, and the role of stomach microbiota.

Proteomic characterization of Helicobacter pylori CagA antigen recognized by child serum antibodies and its epitope mapping by peptide array

Serum antibodies against pathogenic bacteria play immunologically protective roles, and can be utilized as diagnostic markers of infection. This study focused on Japanese child serum antibodies against Helicobacter pylori, a chronically-infected gastric bacterium which causes gastric cancer in adults. Serological diagnosis for H. pylori infection is well established for adults, but it needs to be improved for children. Serum samples from 24 children, 22 H. pylori (Hp)-positive and 2 Hp-negative children, were used to catalogue antigenic proteins of a Japanese strain CPY2052 by two-dimensional electrophoresis followed by immunoblot and LC-MS/MS analysis. In total, 24 proteins were identified as candidate antigen proteins. Among these, the major virulence factor, cytotoxin-associated gene A protein (CagA) was the most reactive antigen recognized by all the Hp-positive sera even from children under the age of 3 years. The major antigenic part of CagA was identified in the middle region, and two peptides containing CagA epitopes were identified using a newly developed peptide/protein-combined array chip method, modified from our previous protein chip method. Each of the epitopes was found to contain amino acid residue(s) unique to East Asian CagA. Epitope analysis of CagA indicated importance of the regional CagA antigens for serodiagnosis of H. pylori infection in children.

Phospholipid binding residues of eukaryotic membrane-remodelling F-BAR domain proteins are conserved in Helicobacter pylori CagA

Background

Cytotoxin associated gene product A (CagA) is an oncogenic protein secreted by the gastric bacterium Helicobacter pylori. Internalization of CagA by human epithelial cells occurs by an unknown mechanism that requires interaction with the host membrane lipid phosphatidylserine.

Findings

Local homology at the level of amino acid sequence and secondary structure has been identified between the membrane-tethering region of CagA and the lipid-binding Fes-CIP4 homology-Bin/Amphiphysin/Rvs (F-BAR) domains of eukaryotic proteins. The F-BAR proteins are major components of the endocytic machinery. In addition to the membrane-binding F-BAR domains, they contain other domains that interact with actin-regulatory networks and mediate interplay between membrane dynamics and cytoskeleton re-arrangements. Positively charged residues found on the lipid binding face of the F-BAR domains are conserved in CagA and represent residues involved in CagA binding to lipids.

Conclusions

The homologies with F-BAR proteins extend to lipid binding specificities and involvement in reorganization of the actin cytoskeleton. CagA and F-BAR domains share binding specificity for phosphatidylserine and phosphoinositides. Similar to the F-BAR proteins, CagA has a membrane-binding module and a module that shares structural homology with actin-binding proteins, and, like eukaryotic F-BAR domain proteins, CagA function is linked to actin dynamics. The uncovered similarities between the bacterial effector protein and eukaryotic F-BAR proteins suggest convergent evolution of CagA towards a similar function.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-0500-7-525) contains supplementary material, which is available to authorized users.