Outer membrane adhesion factor multivalent adhesion molecule 7 initiates host cell binding during infection by Gram-negative pathogens

Anne Marie Krachler, Hyeilin Ham, and Kim Orth
PNAS July 12, 2011 108 (28) 11614-11619; https://doi.org/10.1073/pnas.1102360108

  1. Edited by Harry L. T. Mobley, University of Michigan Medical School, Ann Arbor, Michigan, and accepted by the Editorial Board June 1, 2011 (received for review February 10, 2011)

Abstract

The initial binding of bacteria to host cells is crucial to the delivery of virulence factors and thus is a key determinant of the pathogen's success. We report a multivalent adhesion molecule (MAM) that enables a wide range of Gram-negative pathogens to establish high-affinity binding to host cells during the early stages of infection. MAM7 binds to the host by engaging in both protein–protein (with fibronectin) and protein–lipid (with phosphatidic acid) interactions with the host cell membrane. We find that MAM7 expression on the outer membrane of a Gram-negative pathogen is necessary for virulence in a nematode infection model and for efficient killing of cultured mammalian host cells. Expression of MAM7 on nonpathogenic strains produced a tool that can be used to impede infection by Gram-negative bacterial pathogens. Targeting or exploiting MAM7 might prove to be important in combating Gram-negative bacterial infections.

Bacterial pathogens have a large repertoire of virulence factors that target and manipulate the host cellular machinery to enable infection. Delivery of effector proteins to the host cytosol by type III, type IV, and type VI secretion systems, as well as delivery of extracellular toxins, is a common strategy used by bacterial pathogens to abrogate the host immune response and alter cellular pathways to the pathogen's advantage (1, 2). Because the secretion of effector and toxin proteins is contact-dependent, the bacteria need to establish tight binding to the host to successfully start an infection. We hypothesize that a common strategy exists across species enabling the pathogen to establish strong initial host binding that is complemented by other species-specific adhesion factors for efficient activation and secretion of virulence factors and toxins. During infection, a variety of adhesion factors are expressed by pathogens to facilitate host–pathogen interactions (3–5). Many of these adhesins are induced during infection and thus likely would not be involved in the initial adhesion of the bacterial pathogen with the host cell.

Using bioinformatics, we searched the genome of Vibrio parahaemolyticus, a Gram-negative bacterium that occurs in marine and estuarine environments and can cause shellfish-borne food poisoning, for a constitutively expressed protein that might be involved in the initial binding of bacteria to a host cell (6). We discovered a predicted outer membrane molecule, multivalent adhesion molecule (MAM), which includes a putative transmembrane motif followed by six (MAM6) or seven (MAM7) mammalian cell entry (mce) domains (Fig. 1A and Fig. S1A Unexpectedly, we found that MAM6 or MAM7 is encoded in a wide range of Gram-negative animal pathogens, but not in Gram-positive or plant pathogenic bacteria (Fig. 1A and Figs. S1B and S2). In contrast, proteins containing a single mce domain are widespread (Fig. 1A). In Mycobacterium spp. and some Gram-positive bacteria, including Rhodococcus spp. and Streptomyces spp., the mce domain occurs in conjunction with a second domain of unknown function (DUF3407) (7, 8). Proteins containing one mce domain and a C-terminal low-complexity region are thought to represent an accessory component of ABC transporters occurring in algae, higher plants, and bacteria (9). Here we tested whether MAMs, which constitute a distinct class of predicted outer membrane proteins from Gram-negative bacteria, are involved in cellular attachment. We initially used V. parahaemolyticus as the representative Gram-negative bacterium for analysis of MAM7s, and followed those studies with analyses on the role of MAMs in attachment for other Gram-negative pathogens (6).

Fig. 1.
Fig. 1.

MAM7 is an outer membrane protein involved in host cell adhesion. (A) Classification of mce-containing proteins using PFAM (26). (B–D) Western blot analyses of subcellular fractions of V. parahaemolyticus POR2 (B), E. coli BL21 expressing MAM7-myc (C), and E. coli BL21 expressing MAM7ΔN1–44-myc (D). (−), total lysate before induction; (+), total lysate after induction; sup, culture supernatant; OM, outer membrane; peri, periplasm; IM, inner membrane; cyto, cytoplasm. (E) Detection profile of MAM7 and MAM7ΔN1–44 after papain degradation. (F) Detection profile of N1–44-TEV-MAM7-myc after TEV treatment. P, cell pellet; S, supernatant. (G) Attachment of V. parahaemolyticus POR2, POR2ΔMAM7, and complemented POR2ΔMAM7 to HeLa, RAW264.7, Caco-2, and 3T3 cells. (H) Attachment of BL21-MAM7 or MAM7ΔN1–44.

Results

MAM7 Is an Outer Membrane Protein Mediating Host Cell Attachment.

We analyzed the intracellular localization of V. parahaemolyticus MAM7 (VP1611) by replacing the endogenous gene with a plasmid-borne, C-terminally myc-tagged version of MAM7 under the control of its endogenous promoter using a noncytotoxic strain of V. parahaemolyticus, POR2 (10) (Fig. S3). MAM7-myc was constitutively transcribed when the strain was grown in marine LB (Fig. S3) and, based on subcellular fractionation, localized exclusively to the outer membrane (Fig. 1B). Outer membrane localization was also observed after arabinose-induced heterologous expression of V. parahaemolyticus MAM7-myc in Escherichia coli strain BL21 (Fig. 1C), which does not contain a MAM7 ortholog. The first 44 N-terminal amino acids of MAM7 contain a stretch of hydrophobic residues (aa 21–40) predicted to form a transmembrane helix. Deletion of the 44 N-terminal amino acids (MAM7ΔN1–44-myc) led to cytoplasmic retention of the protein (Fig. 1D). To assess whether MAM7 was present on the surface of the bacteria, BL21-MAM7 and BL21-MAM7ΔN1–44-myc were tested for protease sensitivity. Treatment of these strains with increasing concentrations of papain led to a gradual loss of the epitope tag on cells expressing MAM7-myc but not on those expressing MAM7ΔN1–44-myc, further supporting localization of the C-terminal epitope to the extracellular space (Fig. 1E). To explore whether the N-terminal sequence is embedded in the outer membrane, we introduced a tobacco etch virus (TEV) protease-cleavable peptide between the hydrophobic N-terminal sequence (residues 1–44) and the mce domains (N1–44-TEV-MAM7-myc). The protein expressed from this construct was correctly localized and successfully cleaved by TEV protease incubated with intact bacteria (Fig. 1F), demonstrating that the N-terminal peptide contains the information necessary for outer membrane targeting and membrane anchoring of MAM7, whereas the rest of the protein is exposed extracellularly.

Having established that MAM7 is an outer membrane protein, we next tested whether MAM7 is important for early attachment of V. parahaemolyticus to host cells. Using V. parahaemolyticus POR2 (10) and the POR2ΔMAM7 derivative, we observed that in the absence of MAM7, attachment of V. parahaemolyticus was decreased from ∼80% to ∼35–40% for all tested host cell lines, including HeLa and Caco-2 epithelial cells, RAW264.7 macrophages, and 3T3 fibroblasts. The attachment of POR2ΔMAM7 was recovered by a plasmid expressing MAM7 with or without a TEV cleavage site or myc tag (Fig. 1G and Fig. S4). The nonadherent BL21 strain was converted to an adherent strain by inducing expression of V. parahaemolyticus MAM7, but not of the mutant MAM7ΔN1–44 (Fig. 1H). We conclude that MAM7 contributes to the attachment of V. parahaemolyticus to a broad range of mammalian cells and is sufficient to mediate efficient cellular attachment of a Gram-negative strain in the absence of other adhesion proteins.

MAM7-Mediated Attachment Augments Type III–Mediated Cell Death.

The translocation of effector proteins to manipulate host signaling pathways is a key step in the pathogenesis of many Gram-negative organisms. V. parahaemolyticus features two type III secretion systems (T3SS) that translocate at least eight different effector proteins into the host cytosol with the aim of altering the cellular response to infection to the pathogen's advantage (11, 12). We hypothesized that intimate association that can be mediated through a variety of adhesion mechanisms between pathogen and host is a prerequisite for successful T3SS effector translocation during infection. POR1, a V. parahaemolyticus strain containing both T3SSs but lacking the thermostable direct hemolysins tdhA and tdhS, causes T3SS-dependent cell lysis within 2–3 h after infection (10, 11). To test the contribution of MAM7 in POR1-mediated cell lysis, we created a POR1 strain deleted for MAM7 (POR1ΔMAM7) (Fig. S3) and a POR1ΔMAM7 strain complemented with MAM7 containing a TEV-cleavable sequence inserted between residues 44 and 45 and a C-terminal myc tag (N1–44-TEV-MAM7-myc). The N1–44-TEV-MAM7-myc protein was confirmed to localize to the outer membrane (Fig. S4B). In addition, treatment of the POR2ΔMAM7+N1–44-TEV-MAM7-myc strain with TEV-protease for 5 min resulted in a strain displaying a decrease in attachment comparable to that observed for POR2ΔMAM7 (Fig. S4A). We next used the POR1 strains to assess the contribution of MAM7 attachment to host cell cytotoxicity during V. parahaemolyticus infections.

Gentamycin protection assays were used to assess how long bacteria must remain attached to induce 100% lysis. V. parahaemolyticus strains were used to infect 3T3 cells, and gentamycin was added at various time points during the infection. After 4 h of infection, the cells were tested for cell lysis by a lactate dehydrogenase (LDH) release assay. When gentamycin was added at the start of the infection (0 min), minimal lysis was observed after 4 h, whereas without the addition of gentamycin, almost 100% lysis was observed after the same period. When infected cells were treated at 10, 30, 60, or 90 min after infection with gentamycin, minimal lysis was observed. These results support the hypothesis that V. parahaemolyticus must remain associated with 3T3 fibroblasts for at least 90 min to efficiently mediate T3SS-induced cell lysis (Fig. 2A). To test whether MAM7-mediated attachment contributes to host cell binding through this initial phase of infection, we performed infections using the POR1ΔMAM7+N1–44-TEV-MAM7 strain and treated cells with TEV protease at various time points during the infection. Cell lysis was measured at 4 h after infection. As predicted, cells infected with either the POR1 strain or the POR1ΔMAM7+N1–44-TEV-MAM7 strain, but not with the POR1ΔMAM7 strain, displayed 100% cell lysis. However, POR1ΔMAM7+N1–44-TEV-MAM7–infected cells treated with TEV protease immediately or at 10 or 30 min after the start of infection demonstrated a ∼40% decrease in cytotoxicity (Fig. 2B). This toxicity level is comparable to that seen with the POR1ΔMAM7 strain, supporting the hypothesis that MAM7 plays an important role in attachment during the early stages of infection.

Fig. 2.
Fig. 2.

Impact of MAM7-mediated adhesion on V. parahaemolyticus infection. (A and B) Cytotoxicity of 3T3 fibroblasts after infection with POR1 (black bar), POR1ΔMAM7 (white bar), or POR1ΔMAM7+pMAM7 (gray bar) and treatment with gentamycin (A) or TEV protease (B). (C–F) Lysis of 3T3 fibroblasts (C), RAW264.7 macrophages (D), Caco-2 epithelial cells (E), or HeLa epithelial cells (F) with POR1 (●), POR1ΔMAM7 (○), or POR1ΔMAM7+pMAM7 (6).

To further investigate the role of MAM7 in adhesion and cytotoxicity, we performed time course infection studies on a range of mammalian cell lines using POR1, POR1ΔMAM7, and POR1ΔMAM7 complemented strains. The POR1ΔMAM7 strain lysed 3T3 fibroblasts and RAW264.7 macrophages less efficiently than either POR1 or POR1ΔMAM7+ MAM7 (Fig. 2 C and D). In addition, the onset of lysis induced by the POR1ΔMAM7 strain was delayed by 30–40 min (Fig. 2 C and D). In contrast, no significant difference between POR1 and POR1ΔMAM7 was seen in either Caco-2 or HeLa epithelial cells (Fig. 2 E and F). Taken together, these results support the hypothesis that, with some cell lines, MAM7-mediated attachment plays an important role in mediating the initial attachment of bacteria to host cells during the early stages of infection. It also supports the hypothesis that other molecules, some of which might be host cell type dependent, play a role in attachment during later stages of infection with V. parahaemolyticus.

MAM7 Is Required for V. parahaemolyticus–Induced Pathogenicity in Caenorhabditis elegans.

The nematode C. elegans has been used as a model host for a variety of bacterial pathogens, including Vibrio cholerae and Vibrio vulnificus (13, 14). In the absence of a relevant animal model for V. parahaemolyticus, we tested whether MAM7-mediated adhesion plays a role during infection of nematodes. Synchronized germline-deficient L4 stage worms were fed with RIMD 2210633, POR1, POR1ΔMAM7, or POR1ΔMAM7+MAM7 strains. Worms fed either nonpathogenic E. coli HB101 or POR1ΔMAM7 exhibited a normal life expectancy profile (Fig. 3A). RIMD 2210633 and POR1-infected worms displayed severe phenotypic changes by day 2 of feeding, including growth retardation and increased frequency of intestinal tract distention leading to abdominal rupture, and died at a much faster rate than worms fed either HB101 or POR1ΔMAM7 (Fig. 3 A–C). Both RIMD 2210633 and POR1 killed the worms within 13 d, supporting the hypothesis that most of the lethality is mediated by T3SS effectors rather than by the thermostable hemolysins (10). Full virulence was reconstituted in the POR1ΔMAM7 by reconstituting the strain with plasmid-encoded MAM7 (Fig. 3A). Analysis of worms for abdominal rupture on day 7 revealed a significant increase in injury of the RIMD 2210633- and POR1-fed worms (Fig. 3D). Overall, the results support the hypothesis that MAM7 adhesion plays an important role in pathogenicity in the nemotode infection model.

Fig. 3.
Fig. 3.

MAM7 adhesin is an important factor for V. parahaemolyticus pathogenicity in the nematode C. elegans. (A) Lethality assays with C. elegans strain SS104 glp-4(bn2) fed on RIMD 2210633, POR1, POR1ΔMAM7, or a POR1ΔMAM7 complemented strain. (B and C) Morphology of worms fed on HB101 (B) or POR1 (C) for 48 h and pictured using Nomarski optics. (D) Quantitation of intestinal rupture on day 7.

Multiple mce Domains Are Required to Mediate Stable Attachment of MAM to Host Cells.

The number of tandem mce domains in pathogenic Gram-negative bacteria is strikingly constant (always six or seven domains). We thus hypothesized that six or seven mce domains are the minimum number of domains required for stable host cell attachment and thus explored the relationship between domain number and host cell affinity. We produced recombinant proteins containing one, two, six, or seven mce domains in tandem with a maltose-binding protein (MBP) tag and a single cysteine residue between mce domains and the MBP tag to allow labeling of the proteins with a single fluorophore. The amount of protein bound to host cells was measured using fluorescence spectroscopy, and the affinities of individual constructs were determined with saturation-binding experiments. The affinity increased nonlinearly with the number of mce domains, with equilibrium dissociation constants ranging from 15 ± 3 μM for one mce domain to 0.2 ± 0.1 μM for seven mce domains in tandem (Fig. 4A and Table S1). The affinities of MAM proteins for host cells also were determined indirectly, using unlabeled MAM proteins to block the host cell surface before measuring residual binding of E. coli BL21 expressing MAM7. Affinities determined by this method were consistent with those determined by fluorescence assays (Fig. 4B and Table S1).

Fig. 4.
Fig. 4.

Relationship between mce domain number, affinity, and competitiveness. (A) Saturation binding with HeLa cells and purified, Alexa Fluor–labeled MBP-MAM1, -MAM2, -MAM6 and -MAM7. KD values are listed in Table S1. (B) Indirect determination of binding affinities using MAM proteins to block the cell surface before attachment of BL21-MAM7. KD values are listed in Table S1. (C) HeLa cell attachment of BL21-MAM1, -MAM6, and -MAM7ΔN1–44 compared with BL21-MAM7 determined as CI. (D) Ability of BL21-MAM1, -MAM6, -MAM7, or -MAM7ΔN1–44 to block POR1 attachment to host cells and POR1-mediated cytotoxicity.

Having established the binding affinities for proteins containing varying numbers of mce domains, we analyzed whether strains expressing one mce domain were able to compete for host adhesion with strains expressing six or seven mce domains in tandem. All MAM constructs used were correctly localized to the outer membrane, as demonstrated by subcellular fractionation and attachment experiments (Fig. S5). E. coli BL21 strains expressing MAM1, MAM6, or MAM7ΔN1–44 were mixed with a strain expressing MAM7, and the ratio of attachment to host cells was determined as the competitive index (CI) between the strains. Whereas MAM6 and MAM7 were similarly effective in conferring adhesive properties on E. coli BL21 (CI of 0.8), MAM1 and MAM7ΔN1-44 were far less competitive (CI of 0.27 and 0.19, respectively) (Fig. 4C). We next investigated the ability of E. coli BL21 expressing MAM1, MAM6, or MAM7 to inhibit attachment of and cytolysis by V. parahaemolyticus POR1. When POR1 infections were performed in the presence of E. coli BL21 expressing MAM7 or MAM6 at an identical multiplicity of infection, the attachment of POR1, and thus pathogen-mediated cytotoxicity, were decreased significantly. The addition of BL21 expressing MAM1 or MAM7ΔN1–44, however, had little or no effect on the outcome of POR1 infection (Fig. 4D). These data demonstrate how the requirement for high-affinity attachment to host cells necessitates the presence of a large number of mce domains. Although we observed low-affinity binding with one or two mce domains, the expression of six or seven mce domains in tandem results in a steep increase in affinity, allowing the bacterial strain expressing these constructs to successfully compete for host binding. Because of problems with misfolding and insolubility, we could not study constructs containing between three and five mce repeats, and we hypothesize that such proteins might not occur in nature for similar reasons.

MAM7 Establishes Both Protein–Protein and Protein–Lipid Interactions with Host Cells.

The secondary structure of MAM7 is predicted to be rich in β-strands connected by flexible loop regions, a composition similar to that of fibronectin-binding proteins from Gram-positive bacteria (15). We thus tested whether MAM7 also could bind fibronectin. Immobilized GST-MAM7, but not GST alone, was able to pull down purified fibronectin from human plasma (Fig. 5 A and B). Titrations of fluorophore-labeled MBP-MAM7 against immobilized Fn showed that the interaction between hFn and MAM7 was of moderate affinity (KD of 15 ± 4 μM), whereas no measurable interaction between fibronectin and MAM1 was detected (Fig. 5C). To further explore the possibility that MAM7 binds fibronectin on cells, we treated the cells with trypsin to degrade extracellular proteins and then assessed whether MAM7 can bind to cells. Cells treated with trypsin reduced the number of MAM7 molecules on cells by >100-fold (Fig. 5D). Furthermore, the specificity of binding by MAM7 to fibronectin on cells was demonstrated by either blocking MAM7 binding to cells with an anti-fibronectin antibody or by competing with soluble fibronectin (Fig. 5E).

Fig. 5.
Fig. 5.

MAM7 attaches to host cells via fibronectin and phospholipid interactions. (A and B) Pull-down of hFn with GST-tagged MAM7 (A) or GST-tag (B). (C) Saturation binding of fluorescent- labeled MBP-MAM1 and MBP-MAM7 on immobilized Fn. MBP-MAM7 binds to Fn with an affinity of 15 ± 4 μM, whereas no binding was detected with MAM1. (D) Attachment of purified, labeled MBP-MAM7 or MBP to untreated (black) and trypsin-treated (white) 3T3 cells. (E) Attachment of BL21-MAM7 to 3T3 cells after preincubation with anti-fibronectin, anti-mouse IgG antibodies, or hFn. (F and G) Lipid overlay assays with MBP (F) and MBP-MAM7 (G). (H) Key for lipid strip. (I) Liposome association assays with MBP-tag, MBP-MAM1, and MBP-MAM7. Proteins were incubated with liposomes containing only PC (lane 1) or containing mixtures of PC and PA, with PA content in mol% as indicated (lanes 2–9). Supernatant (S) and pellet (P) fractions were analyzed by SDS/PAGE. (J) Effect of PLC treatment on BL21-MAM7 attachment to 3T3 cells. (K) Attachment to 3T3 cells after preincubation of bacteria with liposomes containing either PC or a mixture of 20 mol% PC and 80 mol% PA.

Arabidopsis Tgd2 is the substrate-binding component of a chloroplast lipid transporter that is involved in phosphatidic acid (PA) trafficking and is required for the biogenesis of thylakoid membrane lipids (9). Tgd2 contains a single mce domain that has been shown to display weak binding to PA (16). Using lipid overlay assays, we found that both MAM1 and MAM7 bound to PA, but detected no binding with the MBP tag alone (Fig. 5 F–H and Fig. S6). We compared the binding affinities of MAM1 and MAM7 to PA using liposome association assays (Fig. 5I). Whereas MAM7 demonstrated stoichiometric binding to PA when present in the liposomes at concentrations as low as 1 mol%, MAM1 bound only to liposomes containing at least 3 mol% PA. To assess whether MAM7 binding to PA occurs in vivo, we analyzed the binding of MAM7 to 3T3 cells incubated without and with previous phospholipase C (PLC) treatment. In the presence of PLC, PA is converted to diacylglycerol, and we noted compromised binding of BL21 expressing MAM7 to cells (Fig. 5J). To further assess whether MAM7 binds to PA on cells, we preincubated BL21-MAM7 with phospholipids and assessed the attachment of the bacteria to 3T3 cells. BL21-MAM7 could bind to cells after incubation with liposomes containing 1,2-Dioleoyl-sn-glycero-3-phosphocholine (PC), but not after incubation with liposomes containing 20 mol% PC and 80 mol% PA (Fig. 5K). These findings further support the model that MAM7 binds PA on the surface of cells.

Nonpathogenic E. coli Heterologously Expressing MAM7 Ameliorates Effects of Infection by Gram-Negative Pathogens.

Because we successfully used nonpathogenic E. coli BL21 expressing MAM7 to inhibit infection by V. parahaemolyticus POR1 (Figs. 4D and 6B; Fig. S7 G–I), we hypothesized that preincubation of host cells with BL21 expressing MAM7 would ameliorate infections caused by a broad range of pathogenic Gram-negative strains that are predicted to encode MAM7. We investigated the protective effect of E. coli BL21 + MAM7 on infection with the V. parahaemolyticus RIMD 2210633 strain that is equivalent to POR1 but features two thermostable direct hemolysins that are thought to contribute to cell lysis (17) (Fig. 6A and Fig. S7 D–F), V. cholerae El Tor N16961 (Fig. 6C and Fig. S7 J–L), Y. pseudotuberculosis YP126 (Fig. 6D and Fig. S7 M–O), and the enteropathogenic E. coli (EPEC) strain O127:H6 E2348/69 (Fig. 6E and Fig. S7 P–R). For all infections we performed, we observed a drastic decrease in pathogenicity as manifested either by decreased cytotoxicity (V. parahaemolyticus, V. cholerae, and Y. pseudotuberculosis) or decreased actin pedestal formation (EPEC). Preincubation with E. coli BL21-MAM7 alone did not induce any phenotypic changes in host cells (Fig. S7 A–C), and thus residual cytotoxicity was presumably due to the secretion of soluble toxins into the extracellular medium. Based on these results, we propose that MAM7 expressed on the nonpathogenic E. coli strain is masking sites needed by the pathogenic bacteria to initiate binding with the host cells.

Fig. 6.
Fig. 6.

Importance of MAM7 in V. parahaemolyticus, Y. pseudotuberculosis, V. cholerae and EPEC infection. (A–E) HeLa cells were infected with V. parahaemolyticus RIMD (A), V. parahaemolyticus POR1 (B), V. cholerae (C), Y. pseudotuberculosis (D), or EPEC (E). P, addition of pathogen; C, competition of pathogen with BL21-MAM7; Cm, competition of pathogen with BL21-MAM7ΔN1–44. (F) Attachment of BL21-MAM7 (black), V. parahaemolyticus MAM7ΔTM (gray), and MAM7 homologs from Y. pseudotuberculosis (Yp, red), V. cholerae (Vc, blue), or EPEC (green) to 3T3 fibroblasts. (G) Attachment of WT, ΔMAM7 (Δ), and complemented ΔMAM7 (C) strains of Yp, Vc, and EPEC to 3T3 fibroblasts. (H) Cytotoxicity (LDH release) of Yp WT (red), ΔMAM7 (pink), and complemented ΔMAM7 (purple) strains toward 3T3 fibroblasts over time. (I) Cytotoxicity of Vc WT (blue), ΔMAM7 (cyan), and complemented ΔMAM7 (dark-blue triangles) strains toward 3T3 cells. (J) Pedestal formation of EPEC WT (green), ΔMAM7 (green checkered), and complemented ΔMAM7 (green striped) strains on 3T3 cells over time.

We hypothesized that MAM7 from the various species are functionally redundant in that all mediate binding to host cells. To test this hypothesis, we cloned and expressed the MAM7 from Y. pseudotuberculosis, V. cholerae, or EPEC into nonadhesive E. coli BL21. Each of these MAM7 homologs enabled BL21 cells to attach to 3T3 cells at a level similar to that observed for BL21 expressing V. parahaemolyticus MAM7 (Fig. 6F). To assess whether MAM7 from Y. pseudotuberculosis, V. cholerae, or EPEC plays a role in host cell adhesion during infection, we created MAM7 deletion strains for each of these pathogens (YpΔMAM7, VcΔMAM7, and EPECΔMAM7) and then reconstituted the MAM7 deletion strains with a WT copy of MAM7 (YpΔMAM7+MAM7, VcΔMAM7+MAM7, and EPECΔMAM7+MAM7). After incubating these various strains with 3T3 cells, we found compromised attachment for the MAM7 deletion strains, but not for the WT or reconstituted deletion strains (Fig. 6G). We next tested whether the absence of MAM7 might attenuate cell culture cytotoxicity induced by Y. pseudotuberculosis and V. cholerae or reduce pedestal formation by EPEC. 3T3 cells infected with either YpΔMAM7 or VcΔMAM7 displayed reduced cytotoxicity over time compared with WT or reconstituted Y. pseudotuberculosis and V. cholerae strains (Fig. 6 H and I). When 3T3 cells were infected with EPECΔMAM7, fewer pedestals were observed over time compared with WT or reconstituted EPEC. Interestingly, the phenotype induced by the pathogen appeared to be attenuated only during the early time points in the infection (Fig. 6J). In agreement with previous findings from other groups, the attachment at the later times of infection was mediated by adherence molecules induced during infection, including Yersinia invasin, E. coli Tir, and type IV pili (4, 5, 18).

Discussion

Although many bacterial adhesins are known, most of these are species-specific, and many are induced during infection (19, 20). Here we have described the adherence factor, MAM7, used by a wide range of Gram-negative bacterial pathogens to mediate an initial, high-affinity interaction with host cells. Whereas the N-terminal hydrophobic sequence of MAM7 is necessary for outer membrane localization and anchoring of the protein, a dedicated system is not required for protein transport or membrane insertion, because MAM7 can be expressed heterologously and correctly localized by E. coli BL21. MAM7 is used for bacterial adhesion to a variety of mammalian cells and is a crucial factor contributing to host cell infection, as was observed in a tissue culture model for Gram-negative pathogens, including V. parahaemolyticus, Y. pseudotuberculosis, V. cholerae, and EPEC. Through multivalent interactions with the host cell surface, MAM7 contributes to the adherence of bacterial pathogens during the early stages of infection, thereby facilitating injection of T3SS effectors into the host cytoplasm. However, specificity of a pathogen for a certain cell type must be mediated by other, strain-specific adhesion molecules, which most likely are strengthened by the initial MAM7 interaction with host cells. The other adhesion factors are likely to dominate attachment during later phases of infection, when MAM7 binding becomes dispensable (Fig. 2).

The binding of the outer membrane adhesion factor MAM7 to host cells is mediated by a multivalent protein–protein interaction between the adhesin and the extracellular matrix component fibronectin. In contrast, the protein–lipid interaction between MAM7 and membrane-bound PA can be mediated by a single mce domain, albeit at a lower apparent affinity. Thus, even though mce domains have been integrated into a diverse range of proteins, the number of mce domains plays an important role in determining ligand-binding affinities and thus the functionality of mce containing proteins. Mce proteins involved in lipid transport require a low-affinity transient interaction with their ligand and contain only one mce domain (e.g., Tgd2). In contrast, constitutively expressed proteins with multiple mce domains appear to mediate an early, high-affinity interaction with the host cell surface. We show that this not only increases lipid-binding affinity, but also gives rise to a new functionality (fibronectin binding), which further strengthens the interaction between MAM7 and host cells. Although interactions with extracellular matrix components, such as fibronectin, are a common strategy used by both Gram-positive (FnBPs of Staphylococcus spp.) and Gram-negative (e.g., Yersinia invasin YadA, Salmonella MisL, or Campylobacter CadF) pathogens to achieve host cell adhesion, the direct binding of an adhesin to membrane phosphatidic acid is, to our knowledge, a previously unobserved mechanism for host–pathogen interaction (21–25). Although at 2–3 mol%, phosphatidic acid is only a minor component of eukaryotic membranes, we have shown that these concentrations are sufficient to mediate stable attachment of MAM7. The strategies for protein and lipid interactions are combined in MAM7-mediated adhesion to achieve efficient binding of Gram-negative pathogens to their host.

We have demonstrated that nonpathogenic BL21 expressing MAM7 can be used to prevent binding of a range of Gram-negative pathogens to host cells, thus offering protection against pathogen-mediated cytotoxicity. Based on bioinformatic analysis, it appears that a large number of Gram-negative pathogens contain a MAM7 (or MAM6) and are predicted to use this protein to mediate the initial, high-affinity attachment to host cells. Our studies indicate that MAM7 molecules from several Gram-negative pathogens, including V. parahaemolyticus, Y. pseudotuberculosis, V. cholerae, and EPEC, mediate early attachment of the pathogen to host cells. Based on these studies with the aforementioned pathogens, we propose that MAM7 could play an important role during the initial phase of infection for many Gram-negative pathogens that express this adhesion molecule.

The identification and initial characterization of this broadly expressed adhesion factor is important for understanding molecular interactions between Gram-negative pathogens and their target host cells. The adhesion factor MAM7 appears to be constitutively expressed, allowing the bacteria to be primed for immediate attachment when encountering a host cell. The MAM7-mediated attachment appears to be important during the initial phases of infection to allow for the production or presentation of other factors that might be involved in later stages of infection (Figs. 2 and 6). These initial studies demonstrate how bacteria expressing MAM7 have an advantage over other bacteria that have no MAM7 on their cell surface (Fig. 4). In addition, the multivalent interaction of the mce repeats in MAM7 allows for binding of substrates not recognized by a single repeat. Finally, future microbial and biochemical studies will address the possibility of countering Gram-negative pathogens with their own outer membrane adhesion factor, MAM7, to attenuate infection.

Materials and Methods

A detailed discussion of the materials and methods used in this study is provided in SI Materials and Methods.

Protease Protection Experiments.

Cells expressing myc-tagged MAM constructs were resuspended in PBS to give an OD600 of 1. Papain was added to final concentrations as indicated, and cells were incubated at 22 °C for 10 min. Reactions were stopped by adding Complete Protease Inhibitor Mixture (Roche) and 5× SDS loading buffer and then boiled for 10 min. Samples were separated by SDS/PAGE, and myc-tagged proteins were detected by Western blot analysis.

Acknowledgments

We thank A. Torres, the K.O. laboratory, E. L. Reddick, and L. Avery for their advice and discussion; N. Alto and V. Sperandio for providing EPEC and V. cholerae strains; and T. Silhavy and I. Collinson for providing antibodies against OmpA and SecE. K.O., H.H., and A.M.K. are supported by grants from the National Institute of Allergy and Infectious Diseases (R01-AI056404 and R01-AI087808) and the Welch Foundation (I-1561). K.O. is a Burroughs Wellcome Investigator in Pathogenesis of Infectious Disease and a W.W. Caruth Jr. Biomedical Scholar.

Footnotes

  • ↵1To whom correspondence should be addressed. E-mail: Kim.Orth{at}utsouthwestern.edu.

  • Author contributions: A.M.K., H.H., and K.O. designed research; A.M.K. and H.H. performed research; A.M.K. and H.H. contributed new reagents/analytic tools; A.M.K., H.H., and K.O. analyzed data; and A.M.K., H.H., and K.O. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. H.L.T.M. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1102360108/-/DCSupplemental.

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Outer membrane adhesion factor multivalent adhesion molecule 7 initiates host cell binding during infection by Gram-negative pathogens
Anne Marie Krachler, Hyeilin Ham, Kim Orth
Proceedings of the National Academy of Sciences Jul 2011, 108 (28) 11614-11619; DOI: 10.1073/pnas.1102360108
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