Mechanism of toxin secretion by Vibrio cholerae investigated in strains harboring plasmids that encode heat-labile enterotoxins of Escherichia coli

Using Vibrio natriegens for High-Yield Production of Challenging Expression Targets and for Protein Perdeuteration

Production of soluble proteins is essential for structure/function studies; however, this usually requires milligram amounts of protein, which can be difficult to obtain with traditional expression systems. Recently, the Gram-negative bacterium Vibrio natriegens emerged as a novel and alternative host platform for production of proteins in high yields. Here, we used a commercial strain derived from V. natriegens (Vmax X2) to produce soluble bacterial and fungal proteins in milligram scale, which we struggled to achieve in Escherichia coli. These proteins include the cholera toxin (CT) and N-acetyl glucosamine-binding protein A (GbpA) from Vibrio cholerae, the heat-labile enterotoxin (LT) from E. coli and the fungal nematotoxin CCTX2 from Coprinopsis cinerea. CT, GbpA, and LT are secreted by the Type II secretion system in their natural hosts. When these three proteins were produced in Vmax, they were also secreted and could be recovered from the growth media. This simplified the downstream purification procedure and resulted in considerably higher protein yields compared to production in E. coli (6- to 26-fold increase). We also tested Vmax for protein perdeuteration using deuterated minimal media with deuterium oxide as solvent and achieved a 3-fold increase in yield compared to the equivalent protocol in E. coli. This is good news, since isotopic labeling is expensive and often ineffective but represents a necessary prerequisite for some structural biology techniques. Thus, Vmax represents a promising host for production of challenging expression targets and for protein perdeuteration in amounts suitable for structural biology studies.

Unraveling the mechanistic insights of sophorolipid-capped gold nanoparticle-induced cell death in Vibrio cholerae

IMPORTANCE

Vibrio cholerae, a Gram-negative bacterium, is the causative agent of a fatal disease, "cholera." Prevention of cholera outbreak is possible by eliminating the bacteria from the environment. However, antimicrobial resistance developed in microorganisms has posed a threat and challenges to its treatment. Application of nanoparticles is a useful and effective option for the elimination of such microorganisms. Metal-based nanopaticles exhibit microbial toxicity through non-specific mechanisms. To prevent resistance development and increase antibacterial efficiency, rational designing of nanoparticles is required. Thus, knowledge on the exact mechanism of action of nanoparticles is highly essential. In this study, we explore the possible mechanisms of antibacterial activity of AuNPs-SL against V. cholerae. We show that the interaction of AuNPs-SL with V. cholerae enhances ROS production and membrane depolarization, change in permeability, and leakage of intracellular content. This action leads to the depletion of cellular ATP level, DNA damage, and subsequent cell death.

Using Vibrio natriegens for high-yield production of challenging expression targets and for protein deuteration

Production of soluble proteins is essential for structure/function studies, however, this usually requires milligram amounts of protein, which can be difficult to obtain with traditional expression systems. Recently, the Gram-negative bacterium Vibrio natriegens appeared as a novel and alternative host platform for production of proteins in high yields. Here, we used a commercial strain derived from V. natriegens (Vmax™ X2) to produce soluble bacterial and fungal proteins in milligram scale, which we struggled to achieve in Escherichia coli. These proteins include the cholera toxin (CT) and N-acetyl glucosamine binding protein A (GbpA) from Vibrio cholerae, the heat-labile enterotoxin (LT) from E. coli and the fungal nematotoxin CCTX2 from Coprinopsis cinerea. CT, GbpA and LT are secreted by the Type II secretion system in their natural hosts. When these three proteins were produced in Vmax, they were also secreted, and could be recovered from the growth media. This simplified the downstream purification procedure and resulted in considerably higher protein yields compared to production in E. coli (6- to 26-fold increase). We also tested Vmax for protein deuteration using deuterated minimal media with deuterium oxide as solvent, and achieved a 3-fold increase in yield compared to the equivalent protocol in E. coli. This is good news since isotopic labeling is expensive and often ineffective, but represents a necessary prerequisite for some structural techniques. Thus, Vmax represents a promising host for production of challenging expression targets and for protein deuteration in amounts suitable for structural biology studies.

In-Depth Characterization of a Re-Engineered Cholera Toxin Manufacturing Process Using Growth-Decoupled Production in Escherichia coli

Non-toxic derivatives of the cholera toxin are extensively used in neuroscience, as neuronal tracers to reveal the location of cells in the central nervous system. They are, also, being developed as vaccine components and drug-delivery vehicles. Production of cholera-toxin derivatives is often non-reproducible; the quality and quantity require extensive fine-tuning to produce them in lab-scale settings. In our studies, we seek a resolution to this problem, by expanding the molecular toolbox of the Escherichia coli expression system with suitable production, purification, and offline analytics, to critically assess the quality of a probe or drug delivery, based on a non-toxic derivative of the cholera toxin. We present a re-engineered Cholera Toxin Complex (rCTC), wherein its toxic A1 domain was replaced with Maltose Binding Protein (MBP), as a model for an rCTC-based targeted-delivery vehicle. Here, we were able to improve the rCTC production by 11-fold (168 mg/L vs. 15 mg/L), in comparison to a host/vector combination that has been previously used (BL21(DE3) pTRBAB5-G1S). This 11-fold increase in the rCTC production capability was achieved by (1) substantial vector backbone modifications, (2) using Escherichia coli strains capable of growth-decoupling (V strains), (3) implementing a well-tuned fed-batch production protocol at a 1 L scale, and (4) testing the stability of the purified product. By an in-depth characterization of the production process, we revealed that secretion of rCTC across the E. coli Outer Membrane (OM) is processed by the Type II secretion-system general secretory pathway (gsp-operon) and that cholera toxin B-pentamerization is, likely, the rate-limiting step in complex formation. Upon successful manufacturing, we have validated the biological activity of rCTC, by measuring its binding affinity to its carbohydrate receptor GM1 oligosaccharide (K_d = 40 nM), or binding to Jurkat cells (93 pM) and delivering the cargo (MBP) in a retrograde fashion to the cell.

Deciphering the possible role of ctxB7 allele on higher production of cholera toxin by Haitian variant Vibrio cholerae O1

Cholera continues to be an important public health concern in developing countries where proper hygiene and sanitation are compromised. This severe diarrheal disease is caused by the Gram-negative pathogen Vibrio cholerae belonging to serogroups O1 and O139. Cholera toxin (CT) is the prime virulence factor and is directly responsible for the disease manifestation. The ctxB gene encodes cholera toxin B subunit (CTB) whereas the A subunit (CTA) is the product of ctxA gene. Enzymatic action of CT depends on binding of B pentamers to the lipid-based receptor ganglioside GM1. In recent years, emergence of V. cholerae Haitian variant strains with ctxB7 allele and their rapid spread throughout the globe has been linked to various cholera outbreaks in Africa and Asia. These strains produce classical type (WT) CTB except for an additional mutation in the signal sequence region where an asparagine (N) residue replaces a histidine (H) at the 20th amino acid position (H20N) of CTB precursor (pre-CTB). Here we report that Haitian variant V. cholerae O1 strains isolated in Kolkata produced higher amount of CT compared to contemporary O1 El Tor variant strains under in vitro virulence inducing conditions. We observed that the ctxB7 allele, itself plays a pivotal role in higher CT production. Based on our in silico analysis, we hypothesized that higher accumulation of toxin subunits from ctxB7 allele might be attributed to the structural alteration at the CTB signal peptide region of pre-H20N CTB. Overall, this study provides plausible explanation regarding the hypertoxigenic phenotype of the Haitian variant strains which have spread globally, possibly through positive selection for increased pathogenic traits.

Diversification of the type IV filament superfamily into machines for adhesion, protein secretion, DNA uptake, and motility

Processes of molecular innovation require tinkering and shifting in the function of existing genes. How this occurs in terms of molecular evolution at long evolutionary scales remains poorly understood. Here, we analyse the natural history of a vast group of membrane-associated molecular systems in Bacteria and Archaea-the type IV filament (TFF) superfamily-that diversified in systems involved in flagellar or twitching motility, adhesion, protein secretion, and DNA uptake. The phylogeny of the thousands of detected systems suggests they may have been present in the last universal common ancestor. From there, two lineages-a bacterial and an archaeal-diversified by multiple gene duplications, gene fissions and deletions, and accretion of novel components. Surprisingly, we find that the 'tight adherence' (Tad) systems originated from the interkingdom transfer from Archaea to Bacteria of a system resembling the 'EppA-dependent' (Epd) pilus and were associated with the acquisition of a secretin. The phylogeny and content of ancestral systems suggest that initial bacterial pili were engaged in cell motility and/or DNA uptake. In contrast, specialised protein secretion systems arose several times independently and much later in natural history. The functional diversification of the TFF superfamily was accompanied by genetic rearrangements with implications for genetic regulation and horizontal gene transfer: systems encoded in fewer loci were more frequently exchanged between taxa. This may have contributed to their rapid evolution and spread across Bacteria and Archaea. Hence, the evolutionary history of the superfamily reveals an impressive catalogue of molecular evolution mechanisms that resulted in remarkable functional innovation and specialisation from a relatively small set of components.

Cholera Toxin Encapsulated within Several Vibrio cholerae O1 Serotype Inaba Outer Membrane Vesicles Lacks a Functional B-Subunit

Cholera toxin (CT), the major virulence factor of Vibrio cholerae, is an AB5 toxin secreted through the type II secretion system (T2SS). Upon secretion, the toxin initiates endocytosis through the interaction of the B pentamer with the GM1 ganglioside receptor on small intestinal cells. In addition to the release of CT in the free form, the bacteria secrete CT in association with outer membrane vesicles (OMVs). Previously, we demonstrated that strain 569B releases OMVs that encapsulate CT and which interact with host cells in a GM1-independent mechanism. Here, we have demonstrated that OMV-encapsulated CT, while biologically active, does not exist in an AB5 form; rather, the OMVs encapsulate two enzymatic A-subunit (CTA) polypeptides. We further investigated the assembly and secretion of the periplasmic CT and found that a major fraction of periplasmic CTA does not participate in the CT assembly process and instead is continuously encapsulated within the OMVs. Additionally, we found that the encapsulation of CTA fragments in OMVs is conserved among several Inaba O1 strains. We further found that under conditions in which the amount of extracellularly secreted CT increases, the concentration of OMV-encapsulated likewise CTA increases. These results point to a secondary mechanism for the secretion of biologically active CT that does not depend on the CTB-GM1 interaction for endocytosis.

Enhanced soluble production of cholera toxin B subunit in Escherichia coli by co-expression of SKP chaperones

The cholera toxin B subunit (CTB) is a nontoxic portion of the cholera toxin that retains mucosal adjuvant properties. Expression of CTB in Escherichia coli is difficult as CTB aggregates and accumulates as insoluble inclusion bodies. To remedy this problem, the periplasmic chaperone, SKP, was investigated as possible co-expression partner to increase the solubility of recombinant CTB (rCTB) in E. coli. The result showed co-expression of SKP enhanced the soluble expression of rCTB in E. coli. Moreover, soluble rCTB was successfully expressed and secreted into the periplasmic space through the direction of the LTB leader signal. rCTB in periplasm was purified using an immobilized d-galactose resin; GM1-ELISA experiments showed that rCTB retains strong GM1 ganglioside-binding activity. Intranasal administration of ovalbumin (OVA) with rCTB significantly induced both mucosal and humoral immune responses specific to OVA. These data indicate that co-expression of the molecular chaperone SKP with CTB increased the solubility of rCTB while maintaining its function.

Animal Enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is the most common cause of E. coli diarrhea in farm animals. ETEC are characterized by the ability to produce two types of virulence factors: adhesins that promote binding to specific enterocyte receptors for intestinal colonization and enterotoxins responsible for fluid secretion. The best-characterized adhesins are expressed in the context of fimbriae, such as the F4 (also designated K88), F5 (K99), F6 (987P), F17, and F18 fimbriae. Once established in the animal small intestine, ETEC produce enterotoxin(s) that lead to diarrhea. The enterotoxins belong to two major classes: heat-labile toxins that consist of one active and five binding subunits (LT), and heat-stable toxins that are small polypeptides (STa, STb, and EAST1). This review describes the disease and pathogenesis of animal ETEC, the corresponding virulence genes and protein products of these bacteria, their regulation and targets in animal hosts, as well as mechanisms of action. Furthermore, vaccines, inhibitors, probiotics, and the identification of potential new targets by genomics are presented in the context of animal ETEC.

Cholera Toxin Subunit B as Adjuvant--An Accelerator in Protective Immunity and a Break in Autoimmunity

Cholera toxin subunit B (CTB) is the nontoxic portion of cholera toxin. Its affinity to the monosialotetrahexosylganglioside (GM1) that is broadly distributed in a variety of cell types including epithelial cells of the gut and antigen presenting cells, macrophages, dendritic cells, and B cells, allows its optimal access to the immune system. CTB can easily be expressed on its own in a variety of organisms, and several approaches can be used to couple it to antigens, either by genetic fusion or by chemical manipulation, leading to strongly enhanced immune responses to the antigens. In autoimmune diseases, CTB has the capacity to evoke regulatory responses and to thereby dampen autoimmune responses, in several but not all animal models. It remains to be seen whether the latter approach translates to success in the clinic, however, the versatility of CTB to manipulate immune responses in either direction makes this protein a promising adjuvant for vaccine development.

Crystal structure of the full-length ATPase GspE from the Vibrio vulnificus type II secretion system in complex with the cytoplasmic domain of GspL

The type II secretion system (T2SS) is present in many Gram-negative bacteria and is responsible for secreting a large number of folded proteins, including major virulence factors, across the outer membrane. The T2SS consists of 11-15 different proteins most of which are present in multiple copies in the assembled secretion machinery. The ATPase GspE, essential for the functioning of the T2SS, contains three domains (N1E, N2E and CTE) of which the N1E domain is associated with the cytoplasmic domain of the inner membrane protein GspL. Here we describe and analyze the structure of the GspE•cyto-GspL complex from Vibrio vulnificus in the presence of an ATP analog, AMPPNP. There are three such ∼83 kDa complexes per asymmetric unit with essentially the same structure. The N2E and CTE domains of a single V. vulnificus GspE subunit adopt a mutual orientation that has not been seen before in any of the previous GspE structures, neither in structures of related ATPases from other secretion systems. This underlines the tremendous conformational flexibility of the T2SS secretion ATPase. Cyto-GspL interacts not only with the N1E domain, but also with the CTE domain and is even in contact with AMPPNP. Moreover, the cyto-GspL domains engage in two types of mutual interactions, resulting in two essentially identical, but crystallographically independent, "cyto-GspL rods" that run throughout the crystal. Very similar rods are present in previous crystals of cyto-GspL and of the N1E•cyto-GspL complex. This arrangement, now seen four times in three entirely different crystal forms, involves contacts between highly conserved residues suggesting a role in the biogenesis or the secretion mechanism or both of the T2SS.

Preparation of biocompatible heat-labile enterotoxin subunit B-bovine serum albumin nanoparticles for improving tumor-targeted drug delivery via heat-labile enterotoxin subunit B mediation

Heat-labile enterotoxin subunit B (LTB) is a non-catalytic protein from a pentameric subunit of Escherichia coli. Based on its function of binding specifically to ganglioside GM1 on the surface of cells, a novel nanoparticle (NP) composed of a mixture of bovine serum albumin (BSA) and LTB was designed for targeted delivery of 5-fluorouracil to tumor cells. BSA-LTB NPs were characterized by determination of their particle size, polydispersity, morphology, drug encapsulation efficiency, and drug release behavior in vitro. The internalization of fluorescein isothiocyanate-labeled BSA-LTB NPs into cells was observed using fluorescent imaging. Results showed that BSA-LTB NPs presented a narrow size distribution with an average hydrodynamic diameter of approximately 254±19 nm and a mean zeta potential of approximately −19.95±0.94 mV. In addition, approximately 80.1% of drug was encapsulated in NPs and released in the biphasic pattern. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that BSA-LTB NPs exhibited higher cytotoxic activity than non-targeted NPs (BSA NPs) in SMMC-7721 cells. Fluorescent imaging results proved that, compared with BSA NPs, BSA-LTB NPs could greatly enhance cellular uptake. Hence, the results indicate that BSA-LTB NPs could be a potential nanocarrier to improve targeted delivery of 5-fluorouracil to tumor cells via mediation of LTB.

Hexamers of the type II secretion ATPase GspE from Vibrio cholerae with increased ATPase activity

The type II secretion system (T2SS), a multiprotein machinery spanning two membranes in Gram-negative bacteria, is responsible for the secretion of folded proteins from the periplasm across the outer membrane. The critical multidomain T2SS assembly ATPase GspE(EpsE) had not been structurally characterized as a hexamer. Here, four hexamers of Vibrio cholerae GspE(EpsE) are obtained when fused to Hcp1 as an assistant hexamer, as shown with native mass spectrometry. The enzymatic activity of the GspE(EpsE)-Hcp1 fusions is ∼20 times higher than that of a GspE(EpsE) monomer, indicating that increasing the local concentration of GspE(EpsE) by the fusion strategy was successful. Crystal structures of GspE(EpsE)-Hcp1 fusions with different linker lengths reveal regular and elongated hexamers of GspE(EpsE) with major differences in domain orientation within subunits, and in subunit assembly. SAXS studies on GspE(EpsE)-Hcp1 fusions suggest that even further variability in GspE(EpsE) hexamer architecture is likely.

A dodecameric ring-like structure of the N0 domain of the type II secretin from enterotoxigenic Escherichia coli

In many bacteria, secretins from the type II secretion system (T2SS) function as outer membrane gated channels that enable passage of folded proteins from the periplasm into the extracellular milieu. Cryo-electron microscopy of the T2SS secretin GspD revealed previously the dodecameric cylindrical architecture of secretins, and crystal structures of periplasmic secretin domains showed a modular domain organization. However, no high-resolution experimental data has as yet been provided about how the entire T2SS secretin or its domains are organized in a cylindrical fashion. Here we present a crystal structure of the N0 domain of the T2SS secretin GspD from enterotoxigenic Escherichia coli containing a helix with 12 subunits per turn. The helix has an outer diameter of ∼125Å and a pitch of only 24Å which suggests a model of a cylindrical dodecameric N0 ring whose dimensions correspond with the cryo-electron microscopy map of Vibrio cholerae GspD. The N0 domain is known to interact with the HR domain of the inner membrane T2SS protein GspC. When the new N0 ring model is combined with the known N0·HR crystal structure, a dodecameric double-ring of twelve N0-HR heterodimers is obtained. In contrast, the previously observed compact N0-N1 GspD module is not compatible with the N0 ring. Interestingly, a N0-N1 T3SS homolog is compatible with forming a N0-N1 dodecameric ring, due to a different N0-vs-N1 orientation. This suggests that the dodecameric N0 ring is an important feature of T2SS secretins with periplasmic domains undergoing considerable motions during exoprotein translocation.

Nanoparticles functionalized with ampicillin destroy multiple-antibiotic-resistant isolates of Pseudomonas aeruginosa and Enterobacter aerogenes and methicillin-resistant Staphylococcus aureus

We show here that silver nanoparticles (AgNP) were intrinsically antibacterial, whereas gold nanoparticles (AuNP) were antimicrobial only when ampicillin was bound to their surfaces. Both AuNP and AgNP functionalized with ampicillin were effective broad-spectrum bactericides against Gram-negative and Gram-positive bacteria. Most importantly, when AuNP and AgNP were functionalized with ampicillin they became potent bactericidal agents with unique properties that subverted antibiotic resistance mechanisms of multiple-drug-resistant bacteria.

Instability of toxin A subunit of AB(5) toxins in the bacterial periplasm caused by deficiency of their cognate B subunits

Shiga toxin (STx) belongs to the AB(5) toxin family and is transiently localized in the periplasm before secretion into the extracellular milieu. While producing outer membrane vesicles (OMVs) containing only A subunit of the toxin (STxA), we created specific STx1B- and STx2B-deficient mutants of E. coli O157:H7. Surprisingly, STxA subunit was absent in the OMVs and periplasm of the STxB-deficient mutants. In parallel, the A subunit of heat-labile toxin (LT) of enterotoxigenic E. coli (ETEC) was absent in the periplasm of the LT-B-deficient mutant, suggesting that instability of toxin A subunit in the absence of the B subunit is a common phenomenon in the AB(5) bacterial toxins. Moreover, STx2A was barely detectable in the periplasm of E. coli JM109 when stx2A was overexpressed alone, while it was stably present when stxB was co-expressed. Compared with STx2 holotoxin, purified STx2A was degraded rapidly by periplasmic proteases when assessed for in vitro proteolytic susceptibility, suggesting that the B subunit contributes to stability of the toxin A subunit in the periplasm. We propose a novel role for toxin B subunits of AB(5) toxins in protection of the A subunit from proteolysis during holotoxin assembly in the periplasm.

Context-dependent activation kinetics elicited by soluble versus outer membrane vesicle-associated heat-labile enterotoxin

Enterotoxigenic Escherichia coli (ETEC) is the leading cause of traveler's diarrhea and children's diarrhea worldwide. Among its virulence factors, ETEC produces heat-labile enterotoxin (LT). Most secreted LT is associated with outer membrane vesicles that are rich in lipopolysaccharide. The majority of prior studies have focused on soluble LT purified from ETEC periplasm. We investigated the hypothesis that the extracellular vesicle context of toxin presentation might be important in eliciting immune responses. We compared the polarized epithelial cell responses to apically applied soluble LT and LT-containing vesicles (LT(+) vesicles) as well as controls using a catalytically inactive mutant of LT and vesicles lacking LT. Although vesicle treatments with no or catalytically inactive LT induced a modest amount of interleukin-6 (IL-6), samples containing catalytically active LT elicited higher levels. A combination of soluble LT and LT-deficient vesicles induced significantly higher IL-6 levels than either LT or LT(+) vesicles alone. The responses to LT(+) vesicles were found to be independent of the canonical LT pathway, because the inhibition of cyclic AMP response element (CRE)-binding protein (CREB) phosphorylation did not lead to a decrease in cytokine gene expression levels. Furthermore, soluble LT caused earlier phosphorylation of CREB and activation of CRE compared with LT(+) vesicles. Soluble LT also led to the activation of activator protein 1, whereas LT(+) vesicle IL-6 responses appeared to be mediated by NF-κB. In summary, the results demonstrate that soluble LT and vesicle-bound LT elicit ultimately similar cytokine responses through distinct different activation pathways.

The binding of cholera toxin to the periplasmic vestibule of the type II secretion channel

The type II secretion system (T2SS) is a large macromolecular complex spanning the inner and outer membranes of many gram-negative bacteria. The T2SS is responsible for the secretion of virulence factors such as cholera toxin (CT) and heat-labile enterotoxin (LT) from Vibrio cholerae and enterotoxigenic Escherichia coli, respectively. CT and LT are closely related AB5 heterohexamers, composed of one A subunit and a B-pentamer. Both CT and LT are translocated, as folded protein complexes, from the periplasm across the outer membrane through the type II secretion channel, the secretin GspD. We recently published the 19 Å structure of the V. cholerae secretin (VcGspD) in its closed state and showed by SPR measurements that the periplasmic domain of GspD interacts with the B-pentamer complex. Here we extend these studies by characterizing the binding of the cholera toxin B-pentamer to VcGspD using electron microscopy of negatively stained preparations. Our studies indicate that the pentamer is captured within the large periplasmic vestibule of VcGspD. These new results agree well with our previously published studies and are in accord with a piston-driven type II secretion mechanism.

In vivo cross-linking of EpsG to EpsL suggests a role for EpsL as an ATPase-pseudopilin coupling protein in the Type II secretion system of Vibrio cholerae

The type II secretion system is a multi-protein complex that spans the cell envelope of Gram-negative bacteria and promotes the secretion of proteins, including several virulence factors. This system is homologous to the type IV pilus biogenesis machinery and contains five proteins, EpsG-K, termed the pseudopilins that are structurally homologous to the type IV pilins. The major pseudopilin EpsG has been proposed to form a pilus-like structure in an energy-dependent process that requires the ATPase, EpsE. A key remaining question is how the membrane-bound EpsG interacts with the cytoplasmic ATPase, and if this is a direct or indirect interaction. Previous studies have established an interaction between the bitopic inner membrane protein EpsL and EpsE; therefore, in this study we used in vivo cross-linking to test the hypothesis that EpsG interacts with EpsL. Our findings suggest that EpsL may function as a scaffold to link EpsG and EpsE and thereby transduce the energy generated by ATP hydrolysis to support secretion. The recent discovery of structural homology between EpsL and a protein in the type IV pilus system implies that this interaction may be conserved and represent an important functional interaction for both the type II secretion and type IV pilus systems.

Cholera toxin - a foe & a friend

After De΄s pivotal demonstration in 1959 of a diarrhoeogenic exo-enterotoxin in cell-free culture filtrates from Vibrio cholerae (of classical biotype), much insight has been gained about cholera toxin (CT), which is arguably now the best known of all microbial toxins. The subunit structure and function of CT, its receptor (the GM1 ganglioside), and its effects on the cyclic AMP system and on intestinal secretion were defined in the 1970s, and the essential aspects of the genetic organization in the 1980s. Recent findings have generated additional perspectives. The 3D-crystal structure of CT has been established, the CT-encoding operon has been shown to be carried by a non-lytic bacteriophage, and in depth knowledge has been gained on how the bacterium controls CT gene expression in response to cell density and various environmental signals. The mode of entry into target cells and the intracellular transport of CT are becoming clearer. CT has become the prototype enterotoxin and a widely used tool for elucidating important aspects of cell biology and physiology, e.g., cell membrane receptors, the cyclic AMP system, G proteins, as well as normal and pathological ion transport mechanisms. In immunology, CT has emerged as a potent, widely used experimental adjuvant, and the strong oral-mucosal immunogenicity of the non-toxic B-subunit (CTB) has led to the use of CTB as a protective antigen together with killed vibrios in a widely licensed oral cholera vaccine. CTB has also been shown to promote immunological tolerance against certain types of mucosally co-administered antigens, preferably tissue antigens linked to the CTB molecule; this has stimulated research and development to use CTB in this context for treatment of autoimmune and allergic diseases. In summary, in the 50 years after De΄s discovery of CT, this molecule has emerged from being the cholera patient΄s "foe" to also becoming a highly useful scientist΄s "friend".

Cholera toxin B subunits assemble into pentamers--proposition of a fly-casting mechanism

The cholera toxin B pentamer (CtxB₅), which belongs to the AB₅ toxin family, is used as a model study for protein assembly. The effect of the pH on the reassembly of the toxin was investigated using immunochemical, electrophoretic and spectroscopic methods. Three pH-dependent steps were identified during the toxin reassembly: (i) acquisition of a fully assembly-competent fold by the CtxB monomer, (ii) association of CtxB monomer into oligomers, (iii) acquisition of the native fold by the CtxB pentamer. The results show that CtxB₅ and the related heat labile enterotoxin LTB₅ have distinct mechanisms of assembly despite sharing high sequence identity (84%) and almost identical atomic structures. The difference can be pinpointed to four histidines which are spread along the protein sequence and may act together. Thus, most of the toxin B amino acids appear negligible for the assembly, raising the possibility that assembly is driven by a small network of amino acids instead of involving all of them.

Structure of the cholera toxin secretion channel in its closed state

The type II secretion system (T2SS) is a macromolecular complex spanning the inner and outer membranes of Gram-negative bacteria. Remarkably, the T2SS secretes folded proteins, including multimeric assemblies such as cholera toxin and heat-labile enterotoxin from Vibrio cholerae and enterotoxigenic Escherichia coli, respectively. The major outer membrane T2SS protein is the 'secretin' GspD. Cryo-EM reconstruction of the V. cholerae secretin at 19-Å resolution reveals a dodecameric structure reminiscent of a barrel, with a large channel at its center that contains a closed periplasmic gate. The GspD periplasmic domain forms a vestibule with a conserved constriction, and it binds to a pentameric exoprotein and to the trimeric tip of the T2SS pseudopilus. By combining our results with structures of the cholera toxin and T2SS pseudopilus tip, we provide a structural basis for a possible secretion mechanism of the T2SS.

Type II Secretion in Escherichia coli

The type II secretion system (T2SS) is used by Escherichia coli and other gram-negative bacteria to translocate many proteins, including toxins and proteases, across the outer membrane of the cell and into the extracellular space. Depending on the bacterial species, between 12 and 15 genes have been identified that make up a T2SS operon. T2SSs are widespread among gram-negative bacteria, and most E. coli appear to possess one or two complete T2SS operons. Once expressed, the multiple protein components that form the T2S system are localized in both the inner and outer membranes, where they assemble into an apparatus that spans the cell envelope. This apparatus supports the secretion of numerous virulence factors; and therefore secretion via this pathway is regarded in many organisms as a major virulence mechanism. Here, we review several of the known E. coli T2S substrates that have proven to be critical for the survival and pathogenicity of these bacteria. Recent structural and biochemical information is also reviewed that has improved our current understanding of how the T2S apparatus functions; also reviewed is the role that individual proteins play in this complex system.

AB toxins: a paradigm switch from deadly to desirable

To ensure their survival, a number of bacterial and plant species have evolved a common strategy to capture energy from other biological systems. Being imperfect pathogens, organisms synthesizing multi-subunit AB toxins are responsible for the mortality of millions of people and animals annually. Vaccination against these organisms and their toxins has proved rather ineffective in providing long-term protection from disease. In response to the debilitating effects of AB toxins on epithelial cells of the digestive mucosa, mechanisms underlying toxin immunomodulation of immune responses have become the focus of increasing experimentation. The results of these studies reveal that AB toxins may have a beneficial application as adjuvants for the enhancement of immune protection against infection and autoimmunity. Here, we examine similarities and differences in the structure and function of bacterial and plant AB toxins that underlie their toxicity and their exceptional properties as immunomodulators for stimulating immune responses against infectious disease and for immune suppression of organ-specific autoimmunity.

Heat-labile enterotoxin: beyond G(m1) binding

Enterotoxigenic Escherichia coli (ETEC) is a significant source of morbidity and mortality worldwide. One major virulence factor released by ETEC is the heat-labile enterotoxin LT, which is structurally and functionally similar to cholera toxin. LT consists of five B subunits carrying a single catalytically active A subunit. LTB binds the monosialoganglioside G_M1, the toxin’s host receptor, but interactions with A-type blood sugars and E. coli lipopolysaccharide have also been identified within the past decade. Here, we review the regulation, assembly, and binding properties of the LT B-subunit pentamer and discuss the possible roles of its numerous molecular interactions.

Testing the efficacy and toxicity of adenylyl cyclase inhibitors against enteric pathogens using in vitro and in vivo models of infection

Enterotoxigenic Escherichia coli (ETEC) produces the ADP-ribosyltransferase toxin known as heat-labile enterotoxin (LT). In addition to the toxic effect of LT resulting in increases of cyclic AMP (cAMP) and disturbance of cellular metabolic processes, this toxin promotes bacterial adherence to intestinal epithelial cells (A. M. Johnson, R. S. Kaushik, D. H. Francis, J. M. Fleckenstein, and P. R. Hardwidge, J. Bacteriol. 191:178-186, 2009). Therefore, we hypothesized that the identification of a compound that inhibits the activity of the toxin would have a suppressive effect on the ETEC colonization capabilities. Using in vivo and in vitro approaches, we present evidence demonstrating that a fluorenone-based compound, DC5, which inhibits the accumulation of cAMP in intoxicated cultured cells, significantly decreases the colonization abilities of adenylyl cyclase toxin-producing bacteria, such as ETEC. These findings established that DC5 is a potent inhibitor both of toxin-induced cAMP accumulation and of ETEC adherence to epithelial cells. Thus, DC5 may be a promising compound for treatment of diarrhea caused by ETEC and other adenylyl cyclase toxin-producing bacteria.

Specificity of the type II secretion systems of enterotoxigenic Escherichia coli and Vibrio cholerae for heat-labile enterotoxin and cholera toxin

The Gram-negative type II secretion (T2S) system is a multiprotein complex mediating the release of virulence factors from a number of pathogens. While an understanding of the function of T2S components is emerging, little is known about what identifies substrates for export. To investigate T2S substrate recognition, we compared mutations affecting the secretion of two highly homologous substrates: heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) from Vibrio cholerae. Each toxin consists of one enzymatic A subunit and a ring of five B subunits mediating the toxin's secretion. Here, we report two mutations in LT's B subunit (LTB) that reduce its secretion from ETEC without global effects on the toxin. The Q3K mutation reduced levels of secreted LT by half, and as with CT (T. D. Connell, D. J. Metzger, M. Wang, M. G. Jobling, and R. K. Holmes, Infect. Immun. 63:4091-4098, 1995), the E11K mutation impaired LT secretion. Results in vitro and in vivo show that these mutants are not degraded more readily than wild-type LT. The Q3K mutation did not significantly affect CT B subunit (CTB) secretion from V. cholerae, and the E11A mutation altered LT and CTB secretion to various extents, indicating that these toxins are identified as secretion substrates in different ways. The levels of mutant LTB expressed in V. cholerae were low or undetectable, but each CTB mutant expressed and secreted at wild-type levels in ETEC. Therefore, ETEC's T2S system seems to accommodate mutations in CTB that impair the secretion of LTB. Our results highlight the exquisitely fine-tuned relationship between T2S substrates and their coordinate secretion machineries in different bacterial species.

Shiga toxins--from cell biology to biomedical applications

Shiga toxin-producing Escherichia coli is an emergent pathogen that can induce haemolytic uraemic syndrome. The toxin has received considerable attention not only from microbiologists but also in the field of cell biology, where it has become a powerful tool to study intracellular trafficking. In this Review, we summarize the Shiga toxin family members and their structures, receptors, trafficking pathways and cellular targets. We discuss how Shiga toxin affects cells not only by inhibiting protein biosynthesis but also through the induction of signalling cascades that lead to apoptosis. Finally, we discuss how Shiga toxins might be exploited in cancer therapy and immunotherapy.

Overexpression, purification, crystallization and preliminary X-ray studies of Vibrio cholerae EpsG

EpsG is the major pseudopilin protein of the Vibrio cholerae type II secretion system. An expression plasmid that encodes an N-terminally truncated form of EpsG with a C-terminal noncleavable His tag was constructed. Recombinant EpsG was expressed in Escherichia coli; the truncated protein was purified and crystallized by hanging-drop vapor diffusion against a reservoir containing 6 mM zinc sulfate, 60 mM MES pH 6.5, 15% PEG MME 550. The crystals diffracted X-rays to a resolution of 2.26 A and belonged to space group P2(1), with unit-cell parameters a = 88.61, b = 70.02, c = 131.54 A.

Nanotransportation system for cholera toxin in Vibrio cholerae 01

Vibrio cholerae (V. cholerae) cholera toxin (CT), which causes a severe watery diarrheal illness, is secreted via the type II secretion machinery; it remains unclear, however, how this toxin is transported toward the machinery. In this study, we determined that the pH-dependent intrabacterial transport system correlates with the priming of CT secretion by V. cholerae. The secretion and production of V. cholerae treated at different pHs were examined by enzyme immunoassay. The localization of the CT was analyzed by immunoelectron microscopy. The CT secretion level rapidly increases in the alkaline-pH-treated V. cholerae but does so more slowly in neutral- and acidic-pH-treated V. cholerae. The CT was found to be densely localized near the membrane in the alkaline-pH-treated bacterial cytoplasm, suggesting that the CT shifts from the center to the peripheral portion of the cytoplasm following an extracellular rise in pH. The shift was observed in V. cholerae treated at alkaline pH for more than 10 min. The pH treatment did not enhance CT production at the same stage at which secretion and intrabacterial transport of the CT were enhanced. We propose that V. cholerae possesses a pH-dependent intrabacterial nanotransportation system that probably accelerates priming for CT secretion.

Docking and assembly of the type II secretion complex of Vibrio cholerae

Secretion of cholera toxin and other virulence factors from Vibrio cholerae is mediated by the type II secretion (T2S) apparatus, a multiprotein complex composed of both inner and outer membrane proteins. To better understand the mechanism by which the T2S complex coordinates translocation of its substrates, we are examining the protein-protein interactions of its components, encoded by the extracellular protein secretion (eps) genes. In this study, we took a cell biological approach, observing the dynamics of fluorescently tagged EpsC and EpsM proteins in vivo. We report that the level and context of fluorescent protein fusion expression can have a bold effect on subcellular location and that chromosomal, intraoperon expression conditions are optimal for determining the intracellular locations of fusion proteins. Fluorescently tagged, chromosomally expressed EpsC and EpsM form discrete foci along the lengths of the cells, different from the polar localization for green fluorescent protein (GFP)-EpsM previously described, as the fusions are balanced with all their interacting partner proteins within the T2S complex. Additionally, we observed that fluorescent foci in both chromosomal GFP-EpsC- and GFP-EpsM-expressing strains disperse upon deletion of epsD, suggesting that EpsD is critical to the localization of EpsC and EpsM and perhaps their assembly into the T2S complex.

Crystal structure of the N-terminal domain of the secretin GspD from ETEC determined with the assistance of a nanobody

Secretins are among the largest bacterial outer membrane proteins known. Here we report the crystal structure of the periplasmic N-terminal domain of GspD (peri-GspD) from the type 2 secretion system (T2SS) secretin in complex with a nanobody, the VHH domain of a heavy-chain camelid antibody. Two different crystal forms contained the same compact peri-GspD:nanobody heterotetramer. The nanobody contacts peri-GspD mainly via CDR3 and framework residues. The peri-GspD structure reveals three subdomains, with the second and third subdomains exhibiting the KH fold which also occurs in ring-forming proteins of the type 3 secretion system. The first subdomain of GspD is related to domains in phage tail proteins and outer membrane TonB-dependent receptors. A dodecameric peri-GspD model is proposed in which a solvent-accessible beta strand of the first subdomain interacts with secreted proteins and/or T2SS partner proteins by beta strand complementation.

Vaccine development for the control of cholera and related toxin-induced diarrhoeal diseases

The toxin-induced diarrhoeal diseases in greatest need of effective vaccines for use in control programmes are cholera and diarrhoea due to enterotoxigenic Escherichia coli. Such vaccines, whether consisting of inactivated immunogens or live attenuated organisms, should be administered by the oral route to stimulate the gut mucosal immune system to a maximal extent. For optimal efficacy they should probably contain or produce immunogens evoking both antibacterial and antitoxic immunity that can interfere in a synergistically cooperative manner with colonization as well as toxin action (binding) events in the pathogenesis. The actual or predicted advantages and limitations of oral vaccines based on protective antigen cocktails and different approaches to live, attenuated organisms are discussed. A conclusion is that effective vaccines could play an important role in the control of diarrhoeal disease by reducing mortality and morbidity, and ideally also the transmission of disease.

Cholera toxin up-regulates endoplasmic reticulum proteins that correlate with sensitivity to the toxin

Cholera toxin (CT) contains one A chain and five B chains. The A chain is an enzyme that covalently modifies a trimeric G protein in the cytoplasm, resulting in the overproduction of cAMP. The B chain binds the glycosphingolipid G(M1), the cell surface receptor for CT, which initiates receptor-mediated endocytosis of the toxin. After endocytosis, CT enters the endoplasmic reticulum (ER) via retrograde vesicular traffic where the A chain retro-translocates through the ER membrane to reach the cytoplasm. The retro-translocation mechanism is poorly understood, but may involve proteins of the ER stress response, including the ER associated degradation (ERAD) pathway. We report here that treating cells with CT or CTB quickly up-regulates the levels of BiP, Derlin-1, and Derlin-2, known participants in the ER stress response and ERAD. CT did not induce calnexin, another known responder to ER stress, indicating that the CT-mediated induction of ER proteins is selective in this time frame. These data suggest that CT may promote retro-translocation of the A chain to the cytoplasm by rapidly up-regulating a set of ER proteins involved in the retro-translocation process. In support of this idea, a variety of conditions that induced BiP, Derlin-1, and Derlin-2 sensitized cells to CT and conditions that inhibited their induction de-sensitized cells to CT. Moreover, specifically suppressing Derlin-1 with siRNA protected cells from CT. In addition, Derlin-1 co-immunoprecipitated with CTA or CTB from CT-treated cells using anti-CTA or anti-CTB antibodies. Altogether, the results are consistent with the hypothesis that the B chain of CT up-regulates ER proteins that may assist in the retro-translocation of the A chain across the ER membrane.

Deletion mutations in N-terminal alpha1 helix render heat labile enterotoxin B subunit susceptible to degradation

Heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli is a heterohexameric protein consisting of an enzymatically active A subunit, LTA, and a carrier pentameric B subunit, LTB. It is clear from the crystal structure of LTB that the N-terminal alpha1 helix lies outside the core structure. However, the function of the N-terminal alpha1 helix of LTB is unknown. The present work was carried out to investigate the effect of site-directed mutagenesis of the alpha1 helix on LTB synthesis. Six amino acids (PQSITE) located at positions 2-7 from the N terminus, including 4 aa from the alpha1 helix, were deleted by site-directed mutagenesis. The deletion resulted in complete inhibition of LTB expression in E. coli when expressed along with its signal sequence. A single amino acid deletion within the alpha1 helix also resulted in loss of expression. However, a single amino acid deletion outside the alpha1 helix did not affect LTB synthesis. Mutant proteins, whose synthesis was not detected in vivo, could be successfully translated in vitro by using the coupled transcription-translation system. Immunoblot analysis, Northern blot analysis, and in vitro transcription-translation data collectively indicate that the lack of synthesis of the mutant proteins is caused by the immediate degradation of the expressed product by cellular proteases rather than by faulty translation of mutant LTB mRNA. Coexpression of the LTA could not rescue the degradation of LTB mutants.

Mapping critical interactive sites within the periplasmic domain of the Vibrio cholerae type II secretion protein EpsM

The type II secretion (T2S) system is present in many gram-negative species, both pathogenic and nonpathogenic, where it supports the delivery of a variety of toxins, proteases, and lipases into the extracellular environment. In Vibrio cholerae, the T2S apparatus is composed of 12 Eps proteins that assemble into a multiprotein complex that spans the entire cell envelope. Two of these proteins, EpsM and EpsL, are key components of the secretion machinery present in the inner membrane. In addition to likely forming homodimers, EpsL and EpsM have been shown to form a stable complex in the inner membrane and to protect each other from proteolytic degradation. To identify and map the specific regions of EpsM involved in protein-protein interactions with both another molecule of EpsM and EpsL, we tested the interactions of deletion constructs of EpsM with full-length EpsM and EpsL by functional characterization and copurification as well as coimmunoprecipitation. Analysis of the truncated EpsM mutants revealed that the region of EpsM from amino acids 100 to 135 is necessary for EpsM to form homo-oligomers, while residues 84 to 99 appear to be critical for a stable interaction with EpsL.

Compromised outer membrane integrity in Vibrio cholerae Type II secretion mutants

The type II secretion (T2S) system of Vibrio cholerae is a multiprotein complex that spans the cell envelope and secretes proteins important for pathogenesis as well as survival in different environments. Here we report that, in addition to the loss of extracellular secretion, removal or inhibition of expression of the T2S genes, epsC-N, results in growth defects and a broad range of alterations in the outer membrane that interfere with its barrier function. Specifically, the sensitivity to membrane-perturbing agents such as bile salts and the antimicrobial peptide polymyxin B is increased, and periplasmic constituents leak out into the culture medium. As a consequence, the sigma(E) stress response is induced. Furthermore, due to the defects caused by inactivation of the T2S system, the Deltaeps deletion mutant of V. cholerae strain N16961 is incapable of surviving the passage through the infant mouse gastrointestinal tract. The growth defect and leaky outer membrane phenotypes are suppressed when the culture medium is supplemented with 5% glucose or sucrose, although the eps mutants remain sensitive to membrane-damaging agents. This suggests that the sugars do not restore the integrity of the outer membrane in the eps mutant strains per se but may provide osmoprotective functions.

The serine 31 residue of the B subunit of Shiga toxin 2 is essential for secretion in enterohemorrhagic Escherichia coli

Shiga toxins produced by enterohemorrhagic Escherichia coli (EHEC) include Shiga toxin 1 (Stx1) as well as Shiga toxin 2 (Stx2). Stx1 is cell associated, whereas Stx2 is localized to the culture supernatant. We have analyzed the secretion of Stx2 by generating histidine-tagged StxB (StxB-H). Although neither StxB1-H nor StxB2-H was secreted in StxB-H-overexpressed EHEC, StxB2-H-overexpressed EHEC showed inhibited Stx2 secretion. On the other hand, StxB1-H-overexpressed EHEC showed no alteration of Stx2 secretion. B-subunit chimeras of Stx1 and Stx2 were used to identify the specific residue of StxB2 that the Stx2 secretory system recognizes. Alteration of the serine 31 residue to an asparagine residue (S31N) in StxB2-H enabled the recovery of Stx2 secretion. On the other hand, alteration of the asparagine 32 residue to a serine residue (N32S) in StxB1-H caused the partial secretion of a point-mutated histidine-tagged B subunit in EHEC. Based on the evidence, it appeared possible that this residue might contain secretion-related information for Stx2 secretion. To investigate this hypothesis, we constructed an isogenic mutant EHEC (Stx1B subunit, N32S) strain and an isogenic mutant EHEC (Stx2B subunit, S31N) strain. Although the mutant Stx2 was cell associated in isogenic mutant EHEC, mutant Stx1 was not extracellular. However, when we used plasmids for the expression of the mutant holotoxins, the overexpressed mutant Stx1 was found in the supernatant fraction, and the overexpressed mutant Stx2 was found in the cell-associated fraction in mutant holotoxin gene-transformed EHEC. These results indicate that the serine 31 residue of the B subunit of Stx2 contains secretion-related information.

Directed delivery of heat-labile enterotoxin by enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC), leading causes of diarrhoeal morbidity and mortality in developing countries, are heterogenous pathogens that elaborate heat-labile (LT) and/or heat-stable (ST) enterotoxins which elicit watery, cholera-like diarrhoea. The molecular events permitting efficient delivery of LT remain undefined. Here, we characterize the role of host-pathogen interaction as it relates to the delivery of LT by ETEC. Separation of bacteria from target intestinal epithelial monolayers by semipermeable filters prevented activation of adenylate cyclase suggesting that pathogen-host cell contact is required for efficient toxin delivery. Likewise, a non-motile strain bearing a mutation in the flagellar fliD gene was deficient in delivery of LT relative to the ETEC (H10407) prototype. Although LT secretion via the type II secretion system (T2SS) was responsive to a variety of environmental factors, neither toxin release nor delivery depended on transcriptional activation of genes encoding LT or the T2SS. Fusions of green fluorescent protein to GspM (a component of the T2SS system for LT) and to LT demonstrated that both T2SS and toxin are distributed at one pole of the ETEC bacterium. Optimal LT delivery may occur in a polarized fashion with transfer of preformed toxin upon close interaction with host cells, preventing neutralization of LT.

The structure of the cytoplasmic domain of EpsL, an inner membrane component of the type II secretion system of Vibrio cholerae: an unusual member of the actin-like ATPase superfamily

The type II secretion system (T2SS) is used by several Gram-negative bacteria for the secretion of hydrolytic enzymes and virulence factors across the outer membrane. In these secretion systems, a complex of 12-15 so-called "Gsp proteins" spans from a regulatory ATPase in the cytoplasm, via several signal or energy transducing proteins in the inner membrane and the pseudopilins in the periplasm, to the actual pore in the outer membrane. The human pathogen Vibrio cholerae employs such an assembly, called the Eps system, for the export of its major virulence factor, cholera toxin, from its periplasm into the lumen of the gastro-intestinal tract of the host. Here, we report the atomic structure of the major cytoplasmic domain of the inner membrane-spanning EpsL protein from V. cholerae. EpsL is the binding partner of the regulatory ATPase EpsE as well as of EpsM and pseudopilins, and is therefore a critical link between the cytoplasmic and the periplasmic part of the Eps-system. The 2.7A resolution structure was determined by a combination of Se-Met multiple anomalous dispersion (MAD) and multiple isomorphous replacement with anomalous scattering (MIRAS) phasing methods. The 28kDa cytoplasmic domain of EpsL (cyto-EpsL) consists of three beta-sheet-rich domains. With domains I and III similar to the RNaseH-fold, cyto-EpsL unexpectedly shows structural homology with the superfamily of actin-like ATPases. cyto-EpsL, however, is an unusual member of this superfamily as it misses the canonical actin domains 1B and 2B, which are common yet variable in this superfamily. Moreover, cyto-EpsL has an additional domain II, which has the topology of an SHS2-fold module. Within the superfamily this fold module has been observed only for domain 1C of the cell division protein FtsA, in which it mediates protein-protein interactions. This domain II displays great flexibility and contributes to a pronounced negatively charged canyon on the surface of cyto-EpsL. Functional data as well as structural homology and sequence conservation suggest that domain II interacts with EpsE, the major cytoplasmic binding partner of EpsL.

Lipopolysaccharide 3-deoxy-D-manno-octulosonic acid (Kdo) core determines bacterial association of secreted toxins

In contrast to cholera toxin (CT), which is secreted solubly by Vibrio cholerae across the outer membrane, heat-labile enterotoxin (LT) is retained on the surface of enterotoxigenic Escherichia coli (ETEC) via an interaction with lipopolysaccharide (LPS). We examined the nature of the association between LT and LPS. Soluble LT binds to the surface of LPS deep-rough biosynthesis mutants but not to lipid A, indicating that only the Kdo (3-deoxy-d-manno-octulosonic acid) core is required for binding. Although capable of binding truncated LPS and Kdo, LT has a higher affinity for longer, more complete LPS species. A putative LPS binding pocket is proposed based on the crystal structure of the toxin. The ability to bind LPS and remain associated with the bacterial surface is not unique to LT, as CT also binds to E. coli LPS. However, neither LT nor CT is capable of binding to the surface of Vibrio. The core structures of Vibrio and E. coli LPS differ in that Vibrio contains a phosphorylated single Kdo-lipid A, and E. coli LPS contains unphosphorylated Kdo2-lipid A. We determined that the phosphate group on the Kdo core of Vibrio LPS prevents CT from binding, resulting in the secretion of soluble toxin. Because LT binds E. coli LPS, it remains associated with the extracellular bacterial surface and is released in association with outer membrane vesicles. We propose that difference in the extracellular fates of LT and CT contribute to the differences in disease caused by ETEC and Vibrio cholerae.

The intestinal zonula occludens toxin (ZOT) receptor recognises non-native ZOT conformers and localises to the intercellular contacts

A preliminary structural analysis of Vibrio cholerae zonula occludens toxin (ZOT) was made by equilibrium denaturation and circular dichroism. ZOT is a structurally unstable protein in aqueous solution (DeltaG((H2O)) 3.82 kcal/mol), the putative intra- and extracellular domains unfold co-operatively, with complete denaturation via observed conformational intermediates. Refolding of denatured ZOT is not dependent on disulphide bridge formation. Partial refolding of a maltose binding protein-ZOT fusion did not prevent its specific binding to the ZOT receptor on Caco-2 cells. Immuno-gold labelling showed that the ZOT receptor localises to the intercellular contacts between cells in a confluent monolayer.