Skip to main page content

• HOME
• Current Issue
• Archive
• Alerts
• About ASM
• Contact us
• Tech Support
• Journals.ASM.org

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Fagan, R. P. Articles by Smith, S. G. J. Search for Related Content PubMed PubMed Citation Articles by Fagan, R. P. Articles by Smith, S. G. J.

 Previous Article  |  Next Article 

Infection and Immunity, March 2008, p. 1135-1142, Vol. 76, No. 3
0019-9567/08/$08.00+0     doi:10.1128/IAI.01327-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The Hek Outer Membrane Protein of Escherichia coli Strain RS218 Binds to Proteoglycan and Utilizes a Single Extracellular Loop for Adherence, Invasion, and Autoaggregation

Robert P. Fagan,^2, Matthew A. Lambert,^1,2 and Stephen G. J. Smith^1,2,3*

Department of Clinical Microbiology, Trinity College Dublin, St James's Hospital, Dublin 8, Ireland,¹ Department of Microbiology, Moyne Institute, Trinity College Dublin, Dublin 2, Ireland,² Institute for Molecular Medicine, Trinity Centre, Trinity College Dublin, St James's Hospital, Dublin 8, Ireland³

Received 3 October 2007/ Returned for modification 1 November 2007/ Accepted 17 December 2007

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Escherichia coli is the principal gram-negative causative agent of sepsis and meningitis in neonates. The pathogenesis of meningitis due to E. coli K1 involves mucosal colonization, transcytosis of epithelial cells, survival in the bloodstream, and eventually invasion of the meninges. The last two aspects have been well characterized at a molecular level. Less is known about the early stages of pathogenesis, i.e., adhesion to and invasion of epithelial cells. We have previously reported that the Hek protein causes autoaggregation and can mediate adherence to and invasion of epithelial cells. Here, we report that Hek-mediated adherence is dependent on binding to glycosoaminoglycan, in particular, heparin. The ability to hemagglutinate, autoaggregate, adhere, and invade is contingent on a putative 25-amino-acid loop that is exposed to the outside of the bacterial cells.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Escherichia coli is the principal gram-negative causative agent of sepsis and meningitis in neonates and is second only to the group B streptococci overall. The strains that cause these devastating infections are collectively known as neonatal meningitic E. coli (NMEC), and among NMEC strains, those expressing the K1 capsular antigen are predominant (13). E. coli K1 accounts for up to 40% of E. coli sepsis and 80% of E. coli meningitic isolates (29, 48) and generally results in a much poorer clinical outcome than other NMEC strains (36). The development of meningitis by E. coli K1 is a complex and multifactorial process. Initial colonization of the mucosae is followed by invasion and transcytosis of epithelial surfaces and eventual accession to intravascular space (50). The bacteria can then survive in the bloodstream, resulting in bacteremia, which is a prerequisite for subsequent invasion of the blood-brain barrier (BBB) (25). Inflammation accompanies the invasion of the central nervous system and results in neuronal damage, loss of integrity of the BBB, and pleocytosis (25).

Invasion of brain microvascular endothelial cells (BMECs), which comprise the BBB, is a complex process, though over the last decade much progress has been made in establishing the bacterial and host factors that are involved (24, 25). BMECs are efficiently invaded by NMEC, and this process is dependent on the bacterial OmpA outer membrane protein binding to a homolog of gp96 (43, 45, 46). In addition to its role as an adhesin, OmpA also protects NMEC from immune defenses, since these bacteria can multiply in macrophages and survive in serum (42, 52, 54, 55). The CNF1 toxin also promotes bacterial uptake (22). For both CNF1 and OmpA, bacterial uptake is mediated by the induction of host cell signaling pathways (21). A series of outer membrane proteins known as the Ibes and other proteins, such as TraJ, AslA, and YijP, contribute to invasion of the meninges (16-19). Aside from outer membrane proteins, fimbriae (type 1 fimbriae and S fimbriae) also have roles in attachment to BMECs (29, 44, 59). The K1 capsule is required for survival during traversal of the BBB (15, 23) and has a role in establishing high-level bacteremia (26).

Apart from endothelial cells, NMEC is known to interact with epithelial cells. NMEC has been shown to invade bladder epithelial cells, and this process requires cytoskeletal rearrangements (37). However, it is thought that the gastrointestinal tract is the more essential site for initial colonization leading to subsequent invasive disease (50). Transcytosis of gastrointestinal cells, such as T84 or Caco-2, by NMEC has been demonstrated and was maximal when cells were differentiated and polarized (3). In common with bladder cell invasion, transcytosis of gastrointestinal cells was also dependent on cytoskeletal rearrangements. The factors that contribute to gastrointestinal colonization, or more particularly invasion, have not been unequivocally identified. However, a number of novel factors, the so-called Dgcs, have been shown to be required for colonization (35).

We have previously shown that the Hek protein is expressed by NMEC strain RS218 and is localized to the outer membrane (9). This protein can cause agglutination of red blood cells and can mediate autoaggregation. In addition, E. coli bacteria expressing the protein can adhere to and invade epithelial cells. However, it is unknown what the protein binds to or what amino acid sequences within the protein are required for adhesion/invasion.

Glycosoaminoglycans (GAGs) are large linear molecules composed of disaccharide repeats of hexosamine and sulfated galactose or hexuronic acid (2). The GAG chains are usually covalently linked to a "core" transmembrane protein, thus forming a proteoglycan. GAGs can be grouped into different classes (heparin, heparan sulfate, dermatan sulfate, and chondroitin sulfate) based on the sugar composition and degree of modification (2). Proteoglycans are expressed by all nucleated cells and are often targets for binding by bacteria (49). Several pathogens, including Neisseria gonorrhoeae (56), Bordetella pertussis (38), Mycobacteria spp. (38), and Listeria monocytogenes (1), express surface proteins that recognize GAGs. Enterotoxigenic E. coli, a causative agent of traveler's diarrhea, also expresses a GAG-binding protein termed Tia (10, 11). Hek and Tia are 62% identical, though much of the similarity between the proteins is in the transmembrane regions (12). To our knowledge, Tia is not encoded by strains that have the hek gene.

Here, we show that the receptor for Hek is proteoglycan, in particular, heparinated proteoglycan. The ability to hemagglutinate, autoagglutinate, adhere, and invade is contingent on a putative 25-amino-acid loop that is exposed to the outside of bacterial cells. To our knowledge, this is the first factor from NMEC that utilizes this mechanism for adherence and invasion.

(Portions of this work were presented at the 155th and 152nd meetings of the Society for General Microbiology.)

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Bacterial strains, plasmids, and culture conditions. E. coli strain XL-1 Blue was used throughout this study. Bacterial plasmids are listed in Table 1. Bacteria were routinely cultured in Luria (L) broth or on L agar plates (39), and stocks were maintained in L broth supplemented with 8.7% (vol/vol) dimethyl sulfoxide at –70°C. Strains were also grown in Dulbecco's modified Eagle's medium (DMEM) or on Columbia agar plates containing 5% defribrinated horse blood when indicated in the text. Where appropriate, antibiotics were added to media at the following concentrations: carbenicillin, 50 µg/ml, and tetracycline, 30 µg/ml.

View this table: [in this window] [in a new window]

TABLE 1. E. coli plasmids used in this work

Eukaryotic cell lines and growth conditions. All cell lines used were obtained from ATCC (Manassas, VA). The mammalian cells lines used were CHO-K1 (ATCC CCL-61) and pgsA-745 (ATCC CRL-2242) cells. Stocks of cell lines were maintained in cell freezing medium-dimethyl sulfoxide (Sigma-Aldrich) under liquid nitrogen. All cell lines were grown in a 1:1 mixture of DMEM and Ham's F-12 medium supplemented with 10% (vol/vol) heat-inactivated fetal bovine serum (Life Technologies) at 37°C in 5% CO₂.

Recombinant DNA techniques. Plasmid DNA was isolated using the Genelute Plasmid Miniprep kit (Sigma-Aldrich) or the Qiagen Plasmid Midi kit, and total genomic DNA was routinely purified using the Puregene genomic DNA purification kit (Gentra Systems) according to the manufacturers' instructions. Restriction endonucleases were purchased from New England Biolabs and used according to the manufacturer's instructions. Standard methods were used for the ligation of DNA fragments and transformation of plasmid DNA (5, 7). The synthesis of oligonucleotides and all DNA sequencing were performed by MWG Biotech. Deletions within the hek gene of pHEK6 were made by inverse PCR with Phusion polymerase using the primers listed in Table 2. These primers contain the sequence CGC at their 5' ends; when the products of inverse PCR were religated, this resulted in a new BssHII site (CGCGCG). This sequence encodes arginine followed by alanine, which is a turn-promoting sequence. The products of inverse PCR were ligated and transformed into E. coli XL-1 Blue. The deletions were verified by DNA sequencing.

View this table: [in this window] [in a new window]

TABLE 2. Oligonucleotide primers used in this study

Preparation and analysis of membrane proteins. Samples enriched for outer membrane proteins were prepared as previously described (4). Briefly, 12 optical density at 600 nm (OD₆₀₀) units (corresponding to 4 ml of a culture with an OD₆₀₀ of 3) of bacteria were harvested by centrifugation at 6,000 x g for 10 min and resuspended in 600 µl of sonication buffer (10% sucrose, 50 mM Tris-Cl [pH 7.5], 100 mM NaCl, 1 mM EDTA) and lysed by sonication. Intact bacteria were removed by centrifugation at 9,300 x g for 5 min, and the supernatants were incubated with 0.5% sarcosyl for 30 min with continuous mixing to solubilize the inner membranes. The sarcosyl-insoluble fraction containing the outer membranes was harvested by centrifugation at 21,000 x g for 30 min and resuspended in 100 µl of Laemmli buffer. All protein samples were stored at –20°C and boiled for 5 min prior to use. Protein samples were separated on discontinuous denaturing polyacrylamide gels by the method of Laemmli (30) and visualized following staining with Coomassie brillliant blue R-250. Alternatively, proteins were transferred to Immobilon-P polyvinylidene difluoride membranes (Millipore) using a Biometra Fastblot semidry transfer apparatus at 5 mA/cm² for 20 min. Protein transfer was confirmed by staining the membrane with Ponceau S. The membranes were dried and incubated with a polyclonal primary anti-Hek antiserum raised in rabbits against a purified maltose-binding protein-Hek fusion protein, diluted 1:2,000 in blocking buffer (3% nonfat powdered milk in phosphate-buffered saline [PBS] containing 0.01% Tween 20) for 1 h at room temperature, followed by a secondary horseradish peroxidase-linked anti-rabbit antibody diluted 1:20,000 for 30 min. The blot was developed using the SuperSignal West Pico chemiluminescent horseradish peroxidase substrate (Pierce).

Amino-terminal sequencing of Hek. A sarcosyl-insoluble fraction containing the Hek protein was separated on a discontinuous denaturing gel and transferred to a Immobilon-P polyvinylidene difluoride membrane. The transferred protein was stained by Ponceau S, and the section of membrane containing Hek was excised. Sequencing was carried out by Alta Biosciences, Birmingham, United Kingdom.

Flow cytometry. Surface exposure of the Hek protein on bacterial cells was confirmed by flow cytometry using an anti-Hek polyclonal antiserum and a fluorescently labeled secondary antibody. Briefly, approximately 5 x 10⁷ bacteria from an overnight culture were harvested at 18,000 x g for 10 min, washed three times with PBS, and resuspended in PBS containing 2% (vol/vol) formaldehyde. Excess formaldehyde was removed by three successive washes with PBS, and the bacteria were then incubated overnight with a 1:10 dilution of absorbed anti-Hek antiserum at 4°C. The labeled bacteria were then washed three times with PBS and incubated with a 1:100 dilution of secondary fluorescein isothiocyanate-conjugated anti-rabbit antibody for 1 h at room temperature. The labeled samples were then washed again as before, and fluorescence was analyzed on a Beckman Coulter Epics XL flow cytometer according to the manufacturer's instructions.

Hemagglutination assays. The abilities of bacterial strains to agglutinate erythrocytes were determined using a 1% suspension of human blood containing 100 mM mannose. Overnight bacterial cultures were harvested by centrifugation at 15,800 x g for 1 min and resuspended in PBS to an OD₆₀₀ of 1.0. The bacteria were then serially twofold diluted with PBS in a final volume of 100 µl in a 96-well microtiter plate. To each well was added an equal volume of the 1% blood suspension, and the plate was incubated at room temperature for 2 h or at 4°C overnight to allow unagglutinated erythrocytes to settle out of suspension.

Autoaggregation assays. Overnight cultures were harvested by centrifugation, resuspended in PBS, and normalized to an OD₆₀₀ of approximately 4.0. Five milliliters of each culture was then transferred to a Kahn tube, and 50-µl samples were taken from the surfaces of the cultures at regular intervals to determine the OD₆₀₀. Assays were performed in duplicate, and the rate of autoaggregation was determined by the mean decrease in OD over time. Rates of autoaggregation were determined using Kaleidagraph software.

Quantitative cell association and invasion assays. Cell association and invasion assays were performed as previously described (6). CHO-K1 and pgsA-745 cells were seeded into 12-well trays at densities of 3.0 x 10⁴ and 6.0 x 10⁴ cells per well, respectively, in DMEM/Ham's F-12 medium supplemented with 10% (vol/vol) heat-inactivated fetal bovine serum at 37°C in 5% CO₂ and grown to confluence.

Bacterial strains were grown overnight, washed with PBS, and diluted 1:500 in warm tissue culture medium. All bacterial strains used in this study displayed similar growth profiles and reached similar ODs (as determined by spectrophotometry) following overnight growth. The exact inoculum sizes following dilution in tissue culture medium were determined by spreading dilutions on Columbia blood agar plates and were comparable for all bacterial strains used. Mammalian cell monolayers were washed once with warm PBS, and medium containing bacteria (1 ml) was added to each well. The infected cells were centrifuged at 600 x g for 5 min to initiate contact between the bacterial and mammalian cells and incubated at 37°C in 5% CO₂ for 1 h to allow adhesion to and invasion of the cultured cells. Infected monolayers were washed twice with warm PBS to remove any nonadherent bacteria. To determine the total number of cell-associated bacteria, the monolayer was disrupted by treatment with 0.1% Triton X-100 and the released bacteria were enumerated by spreading dilutions on Columbia blood agar plates. To determine the number of intracellular bacteria, a standard gentamicin protection assay was performed (20). Following the 1-h infection, cells were incubated with medium containing gentamicin (100 µg/ml) for 90 min at 37°C in 5% CO₂, washed with PBS, and disrupted with 0.1% Triton X-100, and the released bacteria were enumerated as before. The cell association or invasion efficiencies were expressed as the percentage of bacterial inocula recovered from triplicate wells. Experiments were performed at least three times.

In experiments in which exogenous GAGs were tested as potential inhibitors of invasion, the bacterial inocula were prepared as described previously in media containing the inhibitor to be tested and incubated for 30 min to 1 h at 37°C prior to infection of the cultured monolayers. The experiments were then carried out as previously described.

Enzymatic treatment of cells. To further establish the significance of GAGs in the Hek-mediated invasion of epithelial cells, eukaryotic cells were treated with heparinase prior to gentamicin protection assays. CHO-K1 cells were incubated at 37°C with heparinase III from Flavobacterium heparinum (75 mU/ml) for 3 h prior to the addition of bacteria. Heparinase III is a heparin-degrading lyase that recognizes heparin sulfate proteoglycan as its primary substrate, specifically hydrolyzing the glycosidic linkage present in heparan sulfate (33). This lyase has been shown to fully remove heparin moieties from proteoglycan core proteins (11).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Hek directs the invasion of epithelial cells via interactions with GAGs. Many pathogenic organisms have been shown to utilize interactions with the ubiquitous mammalian cell surface proteoglycans to both colonize and invade host tissues (12, 14, 49, 56). As stated above, the Tia invasin has identity with the Hek protein, particularly in the eight transmembrane regions. Tia mediates invasion by attaching to mammalian cell surface proteoglycans (10). Interactions with proteoglycans generally involve the GAG chains, for example, heparin and chondroitin sulfate, which are linked to the core protein of the proteoglycan. To examine if such interactions were important for Hek-promoted invasion of epithelial cells, the ability of Hek-expressing bacteria to invade epithelial cells deficient in GAG production or with reduced GAG expression was examined. In the first instance, CHO-K1 cells were treated with heparinase and the ability of Hek-expressing bacteria to invade these cells was ascertained. Cells that had been treated with heparinase were less well invaded (Fig. 1A). In fact, there was a greater than 60% decrease in the frequency of invasion of CHO-K1 cells that had been treated with heparinase. pgsA-745 cells are a CHO-K1 derivative that produces underglycosylated proteoglycans (8). Hek- or Tia-expressing E. coli K-12 bacteria are efficiently internalized by CHO-K1 cells, as described previously (9); however, invasion by these bacteria was greatly reduced in the xylotransferase mutant cell line pgsA-745 (Fig. 1B). The magnitude of invasion for Hek-expressing bacteria was reduced 23-fold in pgsA-745 cells.

View larger version (16K): [in this window] [in a new window]

FIG. 1. Characterization of the Hek GAG receptor. (A) Invasion of CHO-K1 cells by E. coli K-12 carrying pBSKII (white bar) or pHEK6 (Hek+). Cells were treated with heparinase (gray bar) or not treated with enzyme (black bar). **, P < 0.005 versus untreated CHO-K1 cells. (B) Removal of heparin reduces Hek-mediated invasion. Invasion of CHO-K1 (black bars) or pgsA-745 (gray bars) cells by E. coli K-12 carrying pHEK6 (Hek+), pBSKII (vector control), or pTia5 (Tia+). *, P < 0.05 for invasion of pgsa-745 versus invasion of CHO-K1 cells by Hek-expressing bacteria. The error bars indicate standard deviations.

In order to identify the specific GAG moiety involved in Hek-mediated interactions with epithelial cells, the abilities of different soluble GAG molecules to inhibit Hek-promoted invasion of CHO-K1 cells were investigated. Inhibition was tested with heparan sulfate, heparin (a more highly sulfated form), and chondroitin sulfate, the most common GAGs found on mammalian epithelial cell surfaces. The synthetic carbohydrate dextran sulfate (which is not a GAG) was included as a negative control to discount nonspecific charge-charge interactions, as it has a mass/charge ratio similar to that of heparin, the most highly sulfated of the GAGs tested (41). When cells were incubated with 10 µg/ml of GAG, only heparin was found to inhibit uptake of Hek-expressing E. coli K-12 at a statistically significant level (Fig. 2A). Heparin reduced the number of intracellular bacteria by 7.8-fold, whereas heparan sulfate, chondroitin sulfate, and dextran sulfate reduced uptake by only 1.6-, 1.4-, and 1.6-fold, respectively. Inhibition of Hek-promoted invasion by heparin was dose dependent and highly sensitive, with considerable inhibition (2.5-fold) apparent with as little as 100 ng/ml heparin (Fig. 2B). This suggests that Hek interacts specifically with heparin GAG moieties. Interestingly, heparan sulfate efficiently inhibited invasion at 100 µg/ml but had little or no effect at 10 µg/ml (not shown), suggesting that Hek is more specific for the more highly sulfated forms of heparin.

View larger version (23K): [in this window] [in a new window]

FIG. 2. Inhibition of Hek invasion by the addition of GAGs. (A) Inhibition of CHO-K1 invasion by E. coli K-12 harboring pHEK6 using exogenous GAGs. Shown is the invasion of CHO-K1 cells by E. coli K-12 carrying pHEK6 (black bar); E. coli K-12 carrying pHEK6 following incubation with 10 µg/ml of heparin (H), heparan sulfate (HS), chondroitin sulfate (CS), or dextran sulfate (DS) (gray bars); and E. coli K-12 harboring the vector control pBSKII. Bacteria were incubated for 30 min with each inhibitor prior to the assay. The non-GAG carbohydrate dextran sulfate was included as a negative control of nonspecific charge-charge interactions. Invasion efficiencies were expressed relative to the uninhibited control (normalized to 100%). *, P < 0.05 versus uninhibited control. (B) Inhibition of Hek-promoted invasion using exogenous heparin. Shown is invasion of CHO-K1 cells by E. coli K-12 carrying pHEK6 (black bar), E. coli K-12 carrying pHEK6 in the presence of various concentrations of heparin (gray bars), and E. coli K-12 harboring the vector control pBSKII (white bar). The error bars indicate standard deviations.

Topology of the Hek protein. Amino-terminal sequencing of the Hek protein, isolated from a sarcosyl-insoluble outer membrane fraction, yielded the sequence KEGK, corresponding to residues 23 to 26 of the predicted Hek sequence and consistent with the cleavage of 22 amino acids upon inner membrane translocation. The secondary structure of the 224-amino-acid mature Hek protein was modeled using several computational structural modeling tools, including PredictProtein (http://www.embl-heidelberg.de/predictprotein/predictprotein.html) and PORES (available upon request). The protein was predicted to contain eight β-strands separated by alternating short (3 to 6) or long (28 to 33) stretches of residues. A series of alternating hydrophobic amino acids occupying every second position along the strand can be found in each of the predicted β-strands of the Hek protein in a manner characteristic of β-barrel proteins (28). In support of the β-barrel prediction, several of the β-strands are also flanked by aromatic residues in a manner also characteristic of other β-barrel proteins (47, 51). Based on these observations, the Hek protein is likely to adopt a β-barrel conformation in the outer membrane consisting of eight antiparallel β-strands connected by four long external loops and three short periplasmic turns. The predicted external loops were also found to contain a very high number of charged residues compared to similar β-barrel proteins (27, 47, 58). Furthermore, the loops, or portions thereof, are predicted to be unstructured and disordered, as judged by using GlobPlot (32). Amino acids E31 to D48 (loop 1), Y80 to R93 (loop 2), W142 to N162 (loop 3), and S194 to K198 (loop 4) are predicted to be disordered, using Russell and Linding propensities for disorder. A topological model of the Hek protein is shown in Fig. 3, with the salient features highlighted, namely, the transmembrane β-strands and the surface-exposed charged residues.

View larger version (39K): [in this window] [in a new window]

FIG. 3. Two-dimensional model of the topology of the membrane-spanning mature Hek protein. Residues predicted to belong to transmembrane β-strands are indicated with diamonds, and residues in unstructured loops are circled. The extent of each loop deletion mutation and the position of the substituted alanine-arginine (AR) bridge used in each are indicated by hexagonal residues. Positively charged residues located in the surface-exposed loops are highlighted in black, and negatively charged residues are in gray.

The second surface-exposed loop of Hek is essential for all Hek-promoted phenotypes. In silico modeling of the Hek protein tertiary structure predicted the presence of four long external loops linking the eight strands of the protein barrel on the outer surface of the outer membrane. These four loops comprise the only predicted surface-exposed portion of the protein, and it can therefore be assumed that the biological function of the protein resides in one or more of these loops. In order to elucidate which of the loops play roles in the various Hek-promoted phenotypes, a series of hek loop deletion mutant genes were constructed, based on the pHEK6 plasmid, each lacking the coding sequence for a single loop. In each case, the coding sequence for each loop was replaced with two codons encoding arginine (CGC) and alanine (GCG), and the extents of the resulting deletions are shown in Fig. 3. Expression and surface exposure of each mutant protein was confirmed by Western immunoblot analysis and flow cytometry on intact bacterial cells using a polyclonal Hek-specific antibody (Fig. 4A). All four mutant proteins were expressed. For those proteins lacking loop 1, 2, or 4, the level of expression and surface localization was similar to that of wild-type Hek. The mutant lacking the third loop was less well expressed than the others and could be detected on the surfaces of recombinant bacteria at a level 10-fold lower than that of wild-type Hek. The ability of each to promote hemagglutination, autoaggregation, and invasion of CHO-K1 cells was assessed (Fig. 4B and Table 3). Deletion of the first loop had no discernible effect on hemagglutination, autoaggregation, or invasion. Deletion of the third loop abolished measurable hemagglutination and autoaggregation. The decreased levels of adhesion and hemagglutination displayed by Hek lacking loop 3 can be directly attributed to the decreased copy number of the protein in the cell membrane. Notwithstanding this decrease in the copy number of the loop 3 deletion mutant, E. coli K-12 expressing the mutant protein still efficiently invaded CHO-K1 cells at a level more than 50% of wild type. This may indicate that there is a saturable level of Hek required for invasion. Deletion of the fourth loop resulted in a protein that gave intermediate levels of hemagglutination and autoaggregation. Interestingly, deletion of this loop resulted in a protein that conferred enhanced invasiveness on the bacteria. Thus, loop 4 may have a modulatory effect on invasion. Hek lacking loop 2 could not promote heamagglutination and had a much reduced level of autoagglutination. The levels of invasion promoted by the protein were at vector control levels. Thus, loop 2 appears to be the most important region for all activities of the Hek protein.

View larger version (40K): [in this window] [in a new window]

FIG. 4. Identification of the functional loop of Hek. (A) Expression of the Hek loop deletion mutants (pLoop1 to -4) was confirmed by Western immunoblot analysis using a Hek-specific polyclonal antiserum. Densitometry values, normalized to the level of full-length Hek expression from pHEK6, are displayed below. The surface exposure of each was determined by flow cytometry on intact bacterial cells using the same anti-Hek antiserum and normalized to the fluorescence signal of bacteria expressing wild-type Hek. (B) Invasion of CHO-K1 cells by E. coli K-12 carrying pBSKII (white bar), pHEK6 (black bar), or each of the Hek loop deletion mutants (gray bars). The error bars indicate standard deviations.

View this table: [in this window] [in a new window]

TABLE 3. Hemagglutination and autoaggregation by Hek loop mutants

Deletion of putative transmembrane β-sheets 2 and 3. The Tia protein can bind to heparin using the sequence AVGYDFYQHYNVPVRTEVE from amino acids 51 to 69. In our model of Hek, the corresponding sequence is predicted to be the second and third membrane-spanning β-strands of Hek (Fig. 5). A deletion derivative of pHEK6, pTm23, lacking the DNA encoding the sequence AVGYDFYPQFSIPVRTELE, from amino acids 51 to 69, was constructed. Although a truncated derivative, Hek23, was expressed from this construct, it did not insert into the outer membrane as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sarcosyl-enriched fractions or by flow cytometry (data not shown).

View larger version (38K): [in this window] [in a new window]

FIG. 5. Comparison of the Hek and Tia proteins. The amino acid sequences of the mature forms of Hek and Tia were aligned by ClustalW. The positions of putative loops are indicated, and positively charged residues within loop 2 are marked by asterisks. The position of the peptide used to inhibit Tia adhesion (34) and binding of heparin (10) is underlined.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The pathogenesis of E. coli K1 neonatal meningitis is a complex and multifaceted process. A great deal is now known about the mechanism by which these organisms disseminate in the bloodstream, establishing septicemia, and breach the BBB. Considerably less is known about the initial stages of infection, in particular, colonization and invasion of the epithelia. We have previously described the first E. coli K1 factor to be implicated in both adhesion to and invasion of cultured human gastrointestinal epithelial cells (9). Hek is a 26-kDa protein that is predicted to form an eight-stranded β-barrel in the outer membrane of E. coli K1 strain RS218. Many pathogens have been shown to exploit host cell proteoglycans, in particular heparin-containing proteoglycans, in order to enable colonization and invasion of host tissues. Several proteins from pathogenic bacterial species implicated in this process bear a striking resemblance to Hek, including the Tia protein of enterotoxigenic E. coli and the broader family of Opa proteins of Neisseria spp. (10, 57). Although these similarities are mostly confined to amino acids predicted to be located in the membrane-spanning β-barrel, we undertook to determine if Hek could also utilize these ubiquitous host cell structures to promote adhesion to and invasion of cultured epithelial cells.

We have shown that expression of Hek in E. coli K-12 strains is sufficient to promote invasion of the human gastrointestinal epithelial cell line T84 and the model epithelial cell line CHO-K1 (9). Our results suggest that this process requires interactions with heparin-containing proteoglycans on the host cell surface. Pretreatment of CHO-K1 cells with heparinase significantly decreased the efficiency of Hek-dependent internalization. In keeping with this observation, E. coli K-12 expressing Hek or Tia was significantly less invasive in pgsA-745 cells, a derivative of CHO-K1 cells that produces underglycosylated proteoglycans. Inhibition of invasion with a number of soluble GAGs was also greatest with heparin, and this inhibition was dose dependent, suggesting a specific interaction with highly sulfated heparin proteoglycans. Heparin was also the only GAG found to inhibit the interaction of E. coli strain K1 with CHO-K1 cells (R. P. Fagan and S. G. J. Smith, unpublished observations). However, E. coli K-12 expressing Hek or Tia was still significantly more invasive than the negative control in pgsA-745 cells. This suggests that Hek and Tia may be capable of interacting with additional CHO-K1 cell surface components. It is possible that the initial interaction between the bacterial protein and a GAG moiety facilitates further interactions with an additional cell surface component, perhaps even with the core protein of the receptor proteoglycan, which would still be present on the surfaces of pgsA-745 cells.

Computational modeling of the Hek protein is consistent with an eight-stranded β-barrel with three short periplasmic turns and four longer surface-exposed loops. As these loops are the only predicted surface-exposed regions of the proteins, it is reasonable to assume that some or all of the loops are involved in mediating the various Hek-dependent phenotypes described to date (9). Although no secondary structure was predicted in the surface-exposed loops, the β-strands on either side of the third loop were predicted to extend a considerable distance beyond the membrane surface. Such an arrangement may serve to extend and support the loop farther from the membrane and to provide a support structure for the other loops in a manner similar to that observed with OpcA in Neisseria meningitidis and OmpX in E. coli (47, 51, 58). One striking feature of the surface-exposed loops of Hek is the extreme abundance of charged amino acids present, 20 basic and 26 acidic residues. The presence of surface-exposed basic residues has been shown to be involved in binding to proteoglycans (31), and in the case of OpcA in particular, the combination of extended β-strand structure and an abundance of basic residues resulted in the formation of a positively charged cleft that is theorized to be the site of binding to heparinated proteoglycans (47). Although the data gathered here do not allow any accurate prediction of the final structure of the surface-exposed loops of Hek, they may play a similar role in proteoglycan binding. Construction of a series of mutant Hek proteins, each lacking a single loop, has allowed a detailed analysis of the contribution of each to Hek-promoted hemagglutination, autoaggregation, and epithelial cell invasion. Deletion of a single 28-amino-acid loop, loop 2, was found to abrogate all three phenotypes. Individual deletions of the remaining three loops had little or no effect in the same assays. These data suggest that the second surface-exposed loop is essential for the interaction of Hek with its receptors, with only minor contributions from the remaining surface-exposed portions of the protein.

It has also been reported in the literature that the second loop of the related Tia protein is involved in invasion (34) and binding of heparin (10). The data are based on the abilities of two short peptides (GYDFYQHYNVPVRTEVEFY and AVGYDFYQHYNVPVRTEVEC) to inhibit invasion of HCT8 human epithelial cells by Tia-expressing bacteria. In addition, these peptides bind directly to heparin-albumin-biotin. The region corresponding to these peptides in Hek is predicted to be a transmembrane β-strand and is rich in hydrophobic residues. In support of this prediction, a deletion of this region in Hek yields a protein that is incapable of folding correctly and inserting into the outer membrane. If this sequence in Hek is also to engage in contacts with GAGs, it would suggest that the β-barrel may have a pore of sufficient size to permit access of the GAG chains. There is currently no evidence to support this hypothesis.

Hek is not restricted to NMEC isolates. In one study, 55% of urinary tract isolates were shown to have the hek gene (53). Given that the Hek homolog Hra1 can bind to cells under shear stress, it is tempting to speculate if Hek in these isolates can promote uroepithelial adherence. Furthermore, we have identified a protein in the recently completed sequence of the enteroaggregative E. coli strain 042 that is 90% identical to Hek. While aggregative adherence in enteroaggregative E. coli is mediated by fimbriae (40), considering the aggregative nature of Hek-mediated adherence, it may be that Hek contributes to this phenomenon. In addition, we have recently identified hek in 36% of a cohort of 221 bacteremic isolates of E. coli.

Hek can contribute to invasion of epithelial cells, and we have recently established that the OmpA and IbeB proteins of E. coli K1 also mediate entry into gastrointestinal epithelial cells (R. P. Fagan and S. G. Smith, unpublished data). For E. coli causing meningitis, invasion may thus be a multifactorial process involving several outer membrane adhesin/invasin proteins.

 
This work was supported by grants from The Health Research Board of Ireland (grant number HRB2001/RP85) and from the Irish Higher Education Authority to support the ITTAC project TCD and a start up fund from Trinity College Dublin.

We are grateful to Jim Fleckenstein and Seichi Yasuda for donations of strains and plasmids. Tadgh O'Cronin is thanked for assistance with flow cytometry, and we are grateful to Ronan O'Carroll for advice on cell culture.

 
* Corresponding author. Mailing address: Room 2.11, Department of Clinical Microbiology, Trinity College Dublin, The Stone Building, St James's Hospital, Dublin 8, Ireland. Phone: 353 1 896 8590. Fax: 353 1 896 8566. E-mail: sgsmith{at}tcd.ie

Published ahead of print on 26 December 2007.

Editor: B. A. McCormick

Present address: Centre for Molecular Microbiology and Infection, Imperial College London, London SW7 2AZ, United Kingdom.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

1
1. Alvarez-Dominguez, C., J. A. Vazquez-Boland, E. Carrasco-Marin, P. Lopez-Mato, and F. Leyva-Cobian. 1997. Host cell heparan sulfate proteoglycans mediate attachment and entry of Listeria monocytogenes, and the listerial surface protein ActA is involved in heparan sulfate receptor recognition. Infect. Immun. 65:78-88.[Abstract]
2
2. Bernfield, M., M. Gotte, P. W. Park, O. Reizes, M. L. Fitzgerald, J. Lincecum, and M. Zako. 1999. Functions of cell surface heparan sulfate proteoglycans. Annu. Rev. Biochem. 68:729-777.[CrossRef][Medline]
3
3. Burns, J. L., A. Griffith, J. J. Barry, M. Jonas, and E. Y. Chi. 2001. Transcytosis of gastrointestinal epithelial cells by Escherichia coli K1. Pediatr. Res. 49:30-37.[Medline]
4
4. Deighan, P., A. Free, and C. J. Dorman. 2000. A role for the Escherichia coli H-NS-like protein StpA in OmpF porin expression through modulation of micF RNA stability. Mol. Microbiol. 38:126-139.[CrossRef][Medline]
5
5. Dower, W. J., J. F. Miller, and C. W. Ragsdale. 1988. High efficiency transformation of E. coli by high voltage electroporation. Nucleic Acids Res. 16:6127-6145.[Abstract/Free Full Text]
6
6. Elsinghorst, E. A. 1994. Measurement of invasion by gentamicin resistance. Methods Enzymol. 236:405-420.[Medline]
7
7. Engler, M. J., and C. C. Richardson. 1982. DNA ligases, p. 3-30. In P. D. Boyer (ed.), The enzymes, vol. 15. Academic Press, San Diego, CA.
8
8. Esko, J. D., T. E. Stewart, and W. H. Taylor. 1985. Animal cell mutants defective in glycosaminoglycan biosynthesis. Proc. Natl. Acad. Sci. USA 82:3197-3201.[Abstract/Free Full Text]
9
9. Fagan, R. P., and S. G. Smith. 2007. The Hek outer membrane protein of Escherichia coli is an auto-aggregating adhesin and invasin. FEMS Microbiol. Lett. 269:248-255.[CrossRef][Medline]
10
10. Fleckenstein, J. M., J. T. Holland, and D. L. Hasty. 2002. Interaction of an outer membrane protein of enterotoxigenic Escherichia coli with cell surface heparan sulfate proteoglycans. Infect. Immun. 70:1530-1537.[Abstract/Free Full Text]
11
11. Fleckenstein, J. M., D. J. Kopecko, R. L. Warren, and E. A. Elsinghorst. 1996. Molecular characterization of the tia invasion locus from enterotoxigenic Escherichia coli. Infect. Immun. 64:2256-2265.[Abstract]
12
12. Gallay, P. 2004. Syndecans and HIV-1 pathogenesis. Microbes Infect. 6:617-622.[CrossRef][Medline]
13
13. Glode, M. P., A. Sutton, E. R. Moxon, and J. B. Robbins. 1977. Pathogenesis of neonatal Escherichia coli meningitis: induction of bacteremia and meningitis in infant rats fed E. coli K1. Infect. Immun. 16:75-80.[Abstract/Free Full Text]
14
14. Henry-Stanley, M. J., D. J. Hess, E. A. Erickson, R. M. Garni, and C. L. Wells. 2003. Role of heparan sulfate in interactions of Listeria monocytogenes with enterocytes. Med. Microbiol. Immunol. 192:107-115.[Medline]
15
15. Hoffman, J. A., C. Wass, M. F. Stins, and K. S. Kim. 1999. The capsule supports survival but not traversal of Escherichia coli K1 across the blood-brain barrier. Infect. Immun. 67:3566-3570.[Abstract/Free Full Text]
16
16. Huang, S. H., Y. H. Chen, Q. Fu, M. Stins, Y. Wang, C. Wass, and K. S. Kim. 1999. Identification and characterization of an Escherichia coli invasion gene locus, ibeB, required for penetration of brain microvascular endothelial cells. Infect. Immun. 67:2103-2109.[Abstract/Free Full Text]
17
17. Huang, S. H., and A. Y. Jong. 2001. Cellular mechanisms of microbial proteins contributing to invasion of the blood-brain barrier. Cell Microbiol. 3:277-287.[CrossRef][Medline]
18
18. Huang, S. H., Z. S. Wan, Y. H. Chen, A. Y. Jong, and K. S. Kim. 2001. Further characterization of Escherichia coli brain microvascular endothelial cell invasion gene ibeA by deletion, complementation, and protein expression. J. Infect. Dis. 183:1071-1078.[CrossRef][Medline]
19
19. Huang, S. H., C. Wass, Q. Fu, N. V. Prasadarao, M. Stins, and K. S. Kim. 1995. Escherichia coli invasion of brain microvascular endothelial cells in vitro and in vivo: molecular cloning and characterization of invasion gene ibe10. Infect. Immun. 63:4470-4475.[Abstract]
20
20. Isberg, R. R., and S. Falkow. 1985. A single genetic locus encoded by Yersinia pseudotuberculosis permits invasion of cultured animal cells by Escherichia coli K-12. Nature 317:262-264.[CrossRef][Medline]
21
21. Khan, N. A., S. Shin, J. W. Chung, K. J. Kim, S. Elliott, Y. Wang, and K. S. Kim. 2003. Outer membrane protein A and cytotoxic necrotizing factor-1 use diverse signaling mechanisms for Escherichia coli K1 invasion of human brain microvascular endothelial cells. Microb. Pathog. 35:35-42.[CrossRef][Medline]
22
22. Khan, N. A., Y. Wang, K. J. Kim, J. W. Chung, C. A. Wass, and K. S. Kim. 2002. Cytotoxic necrotizing factor-1 contributes to Escherichia coli K1 invasion of the central nervous system. J. Biol. Chem. 277:15607-15612.[Abstract/Free Full Text]
23
23. Kim, K. J., S. J. Elliott, F. Di Cello, M. F. Stins, and K. S. Kim. 2003. The K1 capsule modulates trafficking of E. coli-containing vacuoles and enhances intracellular bacterial survival in human brain microvascular endothelial cells. Cell Microbiol. 5:245-252.[CrossRef][Medline]
24
24. Kim, K. S. 2000. E. coli invasion of brain microvascular endothelial cells as a pathogenetic basis of meningitis. Subcell. Biochem. 33:47-59.[Medline]
25
25. Kim, K. S. 2003. Pathogenesis of bacterial meningitis: from bacteraemia to neuronal injury. Nat. Rev. Neurosci. 4:376-385.[CrossRef][Medline]
26
26. Kim, K. S., H. Itabashi, P. Gemski, J. Sadoff, R. L. Warren, and A. S. Cross. 1992. The K1 capsule is the critical determinant in the development of Escherichia coli meningitis in the rat. J. Clin. Investig. 90:897-905.[Medline]
27
27. Koebnik, R. 1999. Structural and functional roles of the surface-exposed loops of the beta-barrel membrane protein OmpA from Escherichia coli. J. Bacteriol. 181:3688-3694.[Abstract/Free Full Text]
28
28. Koebnik, R., K. P. Locher, and P. Van Gelder. 2000. Structure and function of bacterial outer membrane proteins: barrels in a nutshell. Mol. Microbiol. 37:239-253.[CrossRef][Medline]
29
29. Korhonen, T. K., M. V. Valtonen, J. Parkkinen, V. Vaisanen-Rhen, J. Finne, F. Orskov, I. Orskov, S. B. Svenson, and P. H. Makela. 1985. Serotypes, hemolysin production, and receptor recognition of Escherichia coli strains associated with neonatal sepsis and meningitis. Infect. Immun. 48:486-491.[Abstract/Free Full Text]
30
30. Laemmli, U. K. 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680-685.[CrossRef][Medline]
31
31. Libeu, C. P., S. Lund-Katz, M. C. Phillips, S. Wehrli, M. J. Hernaiz, I. Capila, R. J. Linhardt, R. L. Raffai, Y. M. Newhouse, F. Zhou, and K. H. Weisgraber. 2001. New insights into the heparan sulfate proteoglycan-binding activity of apolipoprotein E. J. Biol. Chem. 276:39138-39144.[Abstract/Free Full Text]
32
32. Linding, R., R. B. Russell, V. Neduva, and T. J. Gibson. 2003. GlobPlot: exploring protein sequences for globularity and disorder. Nucleic Acids Res. 31:3701-3708.[Abstract/Free Full Text]
33
33. Lohse, D. L., and R. J. Linhardt. 1992. Purification and characterization of heparin lyases from Flavobacterium heparinum. J. Biol. Chem. 267:24347-24355.[Abstract/Free Full Text]
34
34. Mammarappallil, J. G., and E. A. Elsinghorst. 2000. Epithelial cell adherence mediated by the enterotoxigenic Escherichia coli Tia protein. Infect. Immun. 68:6595-6601.[Abstract/Free Full Text]
35
35. Martindale, J., D. Stroud, E. R. Moxon, and C. M. Tang. 2000. Genetic analysis of Escherichia coli K1 gastrointestinal colonization. Mol. Microbiol. 37:1293-1305.[CrossRef][Medline]
36
36. McCracken, G. H., Jr., L. D. Sarff, M. P. Glode, S. G. Mize, M. S. Schiffer, J. B. Robbins, E. C. Gotschlich, I. Orskov, and F. Orskov. 1974. Relation between Escherichia coli K1 capsular polysaccharide antigen and clinical outcome in neonatal meningitis. Lancet ii:246-250.
37
37. Meier, C., T. A. Oelschlaeger, H. Merkert, T. K. Korhonen, and J. Hacker. 1996. Ability of Escherichia coli isolates that cause meningitis in newborns to invade epithelial and endothelial cells. Infect. Immun. 64:2391-2399.[Abstract]
38
38. Menozzi, F. D., K. Pethe, P. Bifani, F. Soncin, M. J. Brennan, and C. Locht. 2002. Enhanced bacterial virulence through exploitation of host glycosaminoglycans. Mol. Microbiol. 43:1379-1386.[CrossRef][Medline]
39
39. Miller, J. H. 1972. Experiments in molecular genetics. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY.
40
40. Nataro, J. P., T. Steiner, and R. L. Guerrant. 1998. Enteroaggregative Escherichia coli. Emerg. Infect. Dis. 4:251-261.[Medline]
41
41. Plotkowski, M. C., A. O. Costa, V. Morandi, H. S. Barbosa, H. B. Nader, S. de Bentzmann, and E. Puchelle. 2001. Role of heparan sulphate proteoglycans as potential receptors for non-piliated Pseudomonas aeruginosa adherence to non-polarised airway epithelial cells. J. Med. Microbiol. 50:183-190.[Abstract/Free Full Text]
42
42. Prasadarao, N. V., A. M. Blom, B. O. Villoutreix, and L. C. Linsangan. 2002. A novel interaction of outer membrane protein A with C4b binding protein mediates serum resistance of Escherichia coli K1. J. Immunol. 169:6352-6360.[Abstract/Free Full Text]
43
43. Prasadarao, N. V., P. K. Srivastava, R. S. Rudrabhatla, K. S. Kim, S. H. Huang, and S. K. Sukumaran. 2003. Cloning and expression of the Escherichia coli K1 outer membrane protein A receptor, a gp96 homologue. Infect. Immun. 71:1680-1688.[Abstract/Free Full Text]
44
44. Prasadarao, N. V., C. A. Wass, and K. S. Kim. 1997. Identification and characterization of S fimbria-binding sialoglycoproteins on brain microvascular endothelial cells. Infect. Immun. 65:2852-2860.[Abstract]
45
45. Prasadarao, N. V., C. A. Wass, M. F. Stins, H. Shimada, and K. S. Kim. 1999. Outer membrane protein A-promoted actin condensation of brain microvascular endothelial cells is required for Escherichia coli invasion. Infect. Immun. 67:5775-5783.[Abstract/Free Full Text]
46
46. Prasadarao, N. V., C. A. Wass, J. N. Weiser, M. F. Stins, S. H. Huang, and K. S. Kim. 1996. Outer membrane protein A of Escherichia coli contributes to invasion of brain microvascular endothelial cells. Infect. Immun. 64:146-153.[Abstract]
47
47. Prince, S. M., M. Achtman, and J. P. Derrick. 2002. Crystal structure of the OpcA integral membrane adhesin from Neisseria meningitidis. Proc. Natl. Acad. Sci. USA 99:3417-3421.[Abstract/Free Full Text]
48
48. Robbins, J. B., G. H. McCracken, Jr., E. C. Gotschlich, F. Orskov, I. Orskov, and L. A. Hanson. 1974. Escherichia coli K1 capsular polysaccharide associated with neonatal meningitis. N. Engl. J. Med. 290:1216-1220.[Medline]
49
49. Rostand, K. S., and J. D. Esko. 1997. Microbial adherence to and invasion through proteoglycans. Infect. Immun. 65:1-8.[Medline]
50
50. Sarff, L. D., G. H. McCracken, M. S. Schiffer, M. P. Glode, J. B. Robbins, I. Orskov, and F. Orskov. 1975. Epidemiology of Escherichia coli K1 in healthy and diseased newborns. Lancet i:1099-1104.
51
51. Schulz, G. E. 1992. Structure-function relationships in the membrane channel porin as based on the 1.8 Å resolution crystal structure, p. 403-412. In A. Pullman, J. Jortner, and B. Pullman (ed.), Membrane proteins: structures, interactions and models. Kluwer, Dordrecht, The Netherlands.
52
52. Smith, S. G., V. Mahon, M. A. Lambert, and R. P. Fagan. 2007. A molecular Swiss army knife: OmpA structure, function and expression. FEMS Microbiol. Lett. 273:1-11.[CrossRef][Medline]
53
53. Srinivasan, U., B. Foxman, and C. F. Marrs. 2003. Identification of a gene encoding heat-resistant agglutinin in Escherichia coli as a putative virulence factor in urinary tract infection. J. Clin. Microbiol. 41:285-289.[Abstract/Free Full Text]
54
54. Sukumaran, S. K., and N. V. Prasadarao. 2003. Escherichia coli K1 invasion increases human brain microvascular endothelial cell monolayer permeability by disassembling vascular-endothelial cadherins at tight junctions. J. Infect. Dis. 188:1295-1309.[CrossRef][Medline]
55
55. Sukumaran, S. K., S. K. Selvaraj, and N. V. Prasadarao. 2004. Inhibition of apoptosis by Escherichia coli K1 is accompanied by increased expression of BclXL and blockade of mitochondrial cytochrome c release in macrophages. Infect. Immun. 72:6012-6022.[Abstract/Free Full Text]
56
56. van Putten, J. P., and S. M. Paul. 1995. Binding of syndecan-like cell surface proteoglycan receptors is required for Neisseria gonorrhoeae entry into human mucosal cells. EMBO J. 14:2144-2154.[Medline]
57
57. Virji, M., K. Makepeace, I. R. Peak, D. J. Ferguson, M. P. Jennings, and E. R. Moxon. 1995. Opc- and pilus-dependent interactions of meningococci with human endothelial cells: molecular mechanisms and modulation by surface polysaccharides. Mol. Microbiol. 18:741-754.[CrossRef][Medline]
58
58. Vogt, J., and G. E. Schulz. 1999. The structure of the outer membrane protein OmpX from Escherichia coli reveals possible mechanisms of virulence. Struct. Fold Des. 7:1301-1309.[Medline]
59
59. Xie, Y., Y. Yao, V. Kolisnychenko, C. H. Teng, and K. S. Kim. 2006. HbiF regulates type 1 fimbriation independently of FimB and FimE. Infect. Immun. 74:4039-4047.[Abstract/Free Full Text]

---

Infection and Immunity, March 2008, p. 1135-1142, Vol. 76, No. 3
0019-9567/08/$08.00+0     doi:10.1128/IAI.01327-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Fagan, R. P. Articles by Smith, S. G. J. Search for Related Content PubMed PubMed Citation Articles by Fagan, R. P. Articles by Smith, S. G. J.