Inactivation of the Escherichia coli heat-labile enterotoxin by in vitro mutagenesis of the A-subunit gene

Review of Newly Identified Functions Associated With the Heat-Labile Toxin of Enterotoxigenic Escherichia coli

Heat-labile toxin (LT) is a well-characterized powerful enterotoxin produced by enterotoxigenic Escherichia coli (ETEC). This toxin is known to contribute to diarrhea in young children in developing countries, international travelers, as well as many different species of young animals. Interestingly, it has also been revealed that LT is involved in other activities in addition to its role in enterotoxicity. Recent studies have indicated that LT toxin enhances enteric pathogen adherence and subsequent intestinal colonization. LT has also been shown to act as a powerful adjuvant capable of upregulating vaccine antigenicity; it also serves as a protein or antigenic peptide display platform for new vaccine development, and can be used as a naturally derived cell targeting and protein delivery tool. This review summarizes the epidemiology, secretion, delivery, and mechanisms of action of LT, while also highlighting new functions revealed by recent studies.

Significance of Enterotoxigenic Escherichia coli (ETEC) Heat-Labile Toxin (LT) Enzymatic Subunit Epitopes in LT Enterotoxicity and Immunogenicity

Enterotoxigenic Escherichia coli (ETEC) strains producing heat-labile toxin (LT) and/or heat-stable toxin (STa) are a top cause of children's diarrhea and travelers' diarrhea. Holotoxin-structured GM1-binding LT is a strong immunogen and an effective adjuvant, and can serve a carrier or a platform for multivalent vaccine development. However, the significance of peptide domains or epitopes of LT particularly enzymatic LTA subunit in association with LT enterotoxicity and immunogenicity has not been characterized. In this study, we identified B-cell epitopes in silico from LTA subunit and examined epitopes for immunogenicity and association with LT enterotoxicity. Epitopes identified from LTA subunit were individually fused to a modified chicken ovalbumin carrier protein, and each epitope-ovalbumin fusion was used to immunize mice. Data showed all 11 LTA epitopes were immunogenic; epitope 7 (105SPHPYEQEVSA115) induced greater titers of anti-LT antibodies which neutralized LT enterotoxicity more effectively. To examine these epitopes for the significance in LT enterotoxicity, we constructed LT mutants by substituting each of 10 epitopes at the toxic A1 domain of LTA subunit with a foreign epitope and examined LT mutants for enterotoxicity and GM1-binding activity. Data showed that LT mutants exhibited no enterotoxicity but retained GM1-binding activity. The results from this study indicated that while not all immunodominant LTA epitopes were neutralizing, LT mutants with an individual epitope substituted lost enterotoxicity but retained GM1-binding activity. These results provided additional information to understand LT immunogenicity and enterotoxicity and suggested the potential application of LT platform for multivalent vaccines against ETEC diarrhea and other diseases.IMPORTANCE No vaccine is licensed for enterotoxigenic Escherichia coli (ETEC) strains, which remain a leading cause of diarrhea in children from developing countries and international travelers. GM1-binding heat-labile toxin (LT) which is a key virulence factor of ETEC diarrhea is a strong vaccine antigen and a self-adjuvant. LT can also serve a backbone or platform for MEFA (multiepitope fusion antigen), a newly developed structural vaccinology technology, to present heterogeneous epitopes (by replacing LT epitopes) and to mimic epitope antigenicity for development of broadly protective vaccines. Data from this study identified neutralizing LT epitopes and demonstrated that substitution of LT epitopes eliminated LT enterotoxicity without altering GM1-binding activity, suggesting LT is potentially a versatile MEFA platform to present heterogeneous epitopes for multivalent vaccines against ETEC and other pathogens.

Salmonella enterotoxin (Stn) regulates membrane composition and integrity

The mechanism of action of Salmonella enterotoxin (Stn) as a virulence factor in disease is controversial. Studies of Stn have indicated both positive and negative effects on Salmonella virulence. In this study, we attempted to evaluate Stn function and its effects on Salmonella virulence. To investigate Stn function, we first performed in vitro and in vivo analysis using mammalian cells and a murine ileal loop model. In these systems, we did not observe differences in virulence phenotypes between wild-type Salmonella and an stn gene-deleted mutant. We next characterized the phenotypes and molecular properties of the mutant strain under various in vitro conditions. The proteomic profiles of the total cell membrane protein fraction differed between wild type and mutant in that there was an absence of a protein in the mutant strain, which was identified as OmpA. By far-western blotting, OmpA was found to interact directly with Stn. To verify this result, the morphology of Salmonella was examined by transmission electron microscopy, with OmpA localization being analyzed by immunogold labeling. Compared with wild-type Salmonella, the mutant strain had a different pole structure and a thin periplasmic space; OmpA was not seen in the mutant. These results indicate that Stn, via regulation of OmpA membrane localization, functions in the maintenance of membrane composition and integrity.

Detoxification of cholera toxin without removal of its immunoadjuvanticity by the addition of (STa-related) peptides to the catalytic subunit. A potential new strategy to generate immunostimulants for vaccination

Peptides related to the heat-stable enterotoxin STa were fused to the N terminus of the A-subunit of cholera toxin (CTA) to explore whether peptide additions could help generate detoxified cholera toxin (CT) derivatives. Proteins carrying APRPGP (6-CTA), ASRCAELCCNPACPAP (16-CTA), or ANSSNYCCELCCNPACTGCYPGP (23-CTA) were genetically constructed. Using a two-plasmid system these derivatives were co-expressed in Vibrio cholerae with cholera toxin B-subunit (CTB) to allow formation and secretion of holotoxin-like molecules (engineered CT, eCTs). Purified eCTs maintained all normal CT properties yet they were more than 10-fold (eCT-6), 100-fold (eCT-16), or 1000-fold (eCT-23) less enterotoxic than wild-type CT. The inverse correlation between enterotoxicity and peptide length indicated sterical interference with the ADP-ribosylating active site in CTA. This interpretation agreed with greater than 1000-fold reductions in cAMP induction, with reductions, albeit not proportional, in in vitro agmatine ADP-ribosylation, and was supported by molecular simulations. Intranasal immunization of mice demonstrated that eCTs retained their inherent immunogenicity and ability to potentiate immune responses to a co-administered heterologous protein antigen, although in variable degrees. Therefore, the addition of STa-related peptides to CTA reduced the toxicity of CT while partly preserving its natural immunoadjuvanticity. These results suggest peptide extensions to CTA are a useful alternative to site-directed mutagenesis to detoxify CT. The simplicity of the procedure, combined with efficient expression and assembly of derivatives, suggests this approach could allow for large scale production of detoxified, yet immunologically active CT molecules.

Interplay of cytokines and adjuvants in the regulation of mucosal and systemic HIV-specific CTL

We examined the interplay between cytokines and adjuvants to optimize the induction of CTL by a mucosal HIV peptide vaccine. We show synergy between IL-12 and GM-CSF when administered together with the HIV peptide PCLUS3-18IIIB and cholera toxin (CT) in the induction of CTL activity and protection against mucosal viral transmission. Further, we examine the efficacy of mutant Escherichia coli labile toxin, LT(R192G), as a less toxic adjuvant than CT. LT(R192G) was as effective as or more effective than CT at inducing a mucosal CTL response. Moreover, LT(R192G) was as effective without IL-12 as CT was when combined with IL-12, and the response elicited by LT(R192G) with the vaccine was not further enhanced by the addition of IL-12. GM-CSF synergized with LT(R192G) without exogenous IL-12. Therefore, LT(R192G) may induce a more favorable cytokine response by not inhibiting IL-12 production. In particular, less IL-4 is made after LT(R192G) than CT immunization, and the response is less susceptible to anti-IL-12 inhibition. Thus, the choice of mucosal adjuvant affects the cytokine environment, and the mucosal response and protection can be enhanced by manipulating the cytokine environment with synergistic cytokine combinations incorporated in the vaccine.

Functional comparison of the NAD binding cleft of ADP-ribosylating toxins

Although a common core structure forms the active site of ADP-ribosylating (ADPRT) toxins, the limited-sequence homology within this region suggests that different mechanisms are being used by toxins to perform their shared function. To explain differences in their mechanisms of NAD binding and hydrolysis, the functional interrelationship of residues predicted to perform similar functions in the beta3-strand of the NAD binding cleft of different ADPRT toxins was compared. Replacing Tyr54 in the A-subunit of diphtheria toxin (DTA) with a serine, its functional homologue in cholera toxin (CT), resulted in the loss of catalytic function but not NAD binding. The catalytic role of the aromatic portion of Tyr54 in the ADPRT reaction was confirmed by the ability of a Tyr54-to-phenylalanine DTA mutant to retain ADPRT activity. In reciprocal studies, positioning a tyrosine in the beta3-strand of the A1-subunit of CT (CTA1) caused both loss of function and altered structure. The restricted flexibility of the CTA1 active site relative to function became evident upon the loss of ADPRT activity when a conservative Val60-to-leucine mutation was performed. We conclude from our studies that DT and CT maintain a similar mechanism of NAD binding but differ in their mechanisms of NAD hydrolysis. The aromatic moiety at position 54 in DT is integral to NAD hydrolysis, while NAD hydrolysis in CT appears highly dependent on the precise positioning of specific residues within the beta3-strand of the active-site cleft.

New approaches to mucosal immunization

Every year more than 17 million deaths worldwide are caused by infectious diseases. The great majority of these deaths occur in underdeveloped countries and are attributed to diseases preventable by existing vaccines, or diseases that could potentially be prevented with new vaccines. The fact that most human and veterinary pathogens establish infection in the host by initiating contact at a mucosal surface, provide the rationale for the development of mucosal vaccines. An increasing number of strategies have been proposed to facilitate mucosal immunization. Among the most widely investigated strategies are the use of attenuated microorganisms; the inclusion of immunizing antigens in lipid-based carriers, the genetic creation of transgenic plants and the use of mucosal adjuvants derived from bacterial toxins. This review provides a brief summary of the most recent advances in the field of mucosal immunization with an special emphasis on a promising genetically detoxified mucosal adjuvant, LT(R192G), derived from the heat-labile toxin of enterotoxigenic E. coli. We present evidence regarding the safety, immunogenicity, and efficacy of LT(R192G) for the development of a new generation of mucosal vaccines.

The role of cAMP in mucosal adjuvanticity of Escherichia coli heat-labile enterotoxin (LT)

Heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) produced by Vibrio cholerae have been shown to function as potent mucosal adjuvants. A number of studies have examined the effects of different mutations at either the active site or the protease site of LT and CT and the influence of those mutations on toxicity and adjuvanticity. However, different observations reported by various groups using a variety of animal models with different antigens or different routes of immunization have provided contradictory findings and evoked many questions regarding the underlying mechanisms of mucosal adjuvanticity of LT and CT. In this study, the role of cAMP in mucosal adjuvanticity was examined by comparing three LT active site mutants (S61F, A69G, E112K), a protease site mutant (R192G) and recombinant LT-B for toxicity, cAMP activity and mucosal adjuvanticity using tetanus toxoid (TT) as a model antigen. While all mutants examined showed reduced toxicity, the effects of each mutation on its ability to function as an adjuvant varied. Following intranasal immunization, native LT as well as protease and active site mutants of LT induced serum anti-TT IgG and their responses were virtually indistinguishable from one another. In addition, LT-B was also able to enhance production of serum anti-TT IgG, though at a level significantly lower than that achieved by native LT and mutants. Following oral immunization, the best serum anti-TT IgG responses were obtained with native LT and mutants that retained the ability to induce accumulation of cAMP. Despite the nearly identical serum anti-TT IgG responses following intranasal immunization, there was a strong correlation between the ability to induce accumulation of cAMP in cultured Caco-2 cells and the ability to elicit production of antigen-specific Th1 or Th2 cytokines.

Bacterial toxins as mucosal adjuvants

The use of mucosally administered killed bacteria or viruses as vaccines has a number of attractive features over the use of viable attenuated organisms, including safety, cost, storage and ease of delivery. Unfortunately, mucosally administered killed organisms are not usually effective as vaccines. The use of LT(R192G), a genetically detoxified derivative of LT, as a mucosal adjuvant enables the use of killed bacteria or viruses as vaccines by enhancing the overall humoral and cellular host immune response to these organisms, especially the Th1 arm of the immune response. With this adjuvant, protective responses equivalent to those elicited by live attenuated organisms can be achieved with killed organisms without the potential side effects. These findings have significant implications for vaccine development and further support the potential of LT(R192G) to function as a safe, effective adjuvant for mucosally administered vaccines. There are a number of unresolved issues regarding the use of LT and CT mutants as mucosal adjuvants. Both active-site and protease-site mutants of LT and CT have been constructed and adjuvanticity reported for these molecules in various animal models and with different antigens. There needs to be a side-by-side comparison of CT, LT, active-site mutants, protease-site mutants and recombinant B subunits regarding the ability to induce specific, targeted immunological outcomes as a function of route of immunization and nature of the co-administered antigen. Those side-by-side comparisons have not been carried out and there is a substantial body of evidence indicating that the outcomes may very well be different. With that information, vaccine strategies could be designed employing the optimum adjuvant/antigen formulation and route of administration for a variety of bacterial and viral pathogens. Also lacking is an understanding of the underlying cellular and intracellular signaling pathways activated by these different molecules and an understanding of the mechanisms of adjuvanticity at the cellular level. These are important issues because they take us beyond the phenomenological observations of "enhanced immunity" to a more clear understanding of the mechanisms of adjuvant activity.

Mutants of Escherichia coli heat-labile enterotoxin as an adjuvant for nasal influenza vaccine

The effectiveness and safety of known mutants of Escherichia coli heat-labile enterotoxin (LT) as an adjuvant for nasal influenza vaccine were examined. Six mutants, called LT7K (Arg to Lys), LT61F (Ser to Phe), LT112K (Glu to Lys), LT118E (Gly to Glu), LT146E (Arg to Glu) and LT192G (Arg to Gly) were constructed by the replacement of one amino acid at one position of the A1 subunit to another using site-directed mutagenesis. All mutants were confirmed to be less toxic than wild-type LT when analyzed using Y-1 adrenal cells in vitro. When influenza vaccine was administered intranasally with LT7K and LT192G, BALB/c mice developed high levels of serum and local antibodies to the HA molecules. The adjuvant activity of these mutant LTs corresponded to that of wild-type LT when 1 microgram of these mutant LTs (or wild-type LT) was coadministered with the vaccine. From the point of view of safety, LT7K was considered to be the most potent mucosal adjuvant and was examined in more detail. The adjuvant activity of the mutant was lowered more rapidly with a decrease in dose than was that of wild-type LT. The low level of adjuvant of a relatively small amount of LT7K was heightened by adding LTB to the mutant LT. These results suggest that LT7K supplemented with LTB can be used as a less toxic, effective adjuvant for nasal influenza vaccine.

Crystal structure of a non-toxic mutant of heat-labile enterotoxin, which is a potent mucosal adjuvant

Two closely related bacterial toxins, heat-labile enterotoxin (LT-I) and cholera toxin (CT), not only invoke a toxic activity that affects many victims worldwide but also contain a beneficial mucosal adjuvant activity that significantly enhances the potency of vaccines in general. For the purpose of vaccine design it is most interesting that the undesirable toxic activity of these toxins can be eliminated by the single-site mutation Ser63Lys in the A subunit while the mucosal adjuvant activity is still present. The crystal structure of the Ser63Lys mutant of LT-I is determined at 2.0 A resolution. Its structure appears to be essentially the same as the wild-type LT-I structure. The substitution Ser63Lys was designed, based on the wild-type LT-I crystal structure, to decrease toxicity by interfering with NAD binding and/or catalysis. In the mutant crystal structure, the newly introduced lysine side chain is indeed positioned such that it could potentially obstruct the productive binding mode of the substrate NAD while at the same time its positive charge could possibly interfere with the critical function of nearby charged groups in the active site of LT-I. The fact that the Ser63Lys mutant of LT-I does not disrupt the wild-type LT-I structure makes the non-toxic mutant potentially suitable, from a structural point of view, to be used as a vaccine to prevent enterotoxigenic E. coli infections. The structural similarity of mutant and wild-type toxin might also be the reason why the inactive Ser63Lys variant retains its adjuvant activity.

Loss of biological activity due to Glu-->Arg mutation at residue 11 of the B subunit of cholera toxin

Since it has been reported that a single amino acid mutation of Gly-->Arg in the CAGYC region of the beta chain of human thyroid stimulating hormone (hTSH) was responsible for congenital isolated TSH deficiency, and that the same amino acid substitution in this site of hTSH and human chorionic gonadotropin (hCG) introduced by site-directed mutagenesis resulted in loss of activity, the authors studied the role of glutamic acid at position 11 (Glu-11) from the N-terminus of the B subunit of cholera toxin (CT), which corresponds to the glycine in the CAGYC region of the beta chain of hTSH and hCG. A mutant CT constructed by site-directed mutagenesis in which Glu-11 was replaced by Arg (CT-E11R) did not induce either morphological changes or accumulation of cytosolic cyclic AMP in Chinese hamster ovary cells, although it formed the holotoxin AB5, retained the ability to bind to GM1-ganglioside and showed ADP-ribosyltransferase activity. Weak assembly of the B subunits in mutant CT-E11R demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-heating conditions might explain the loss of biological activity.

Histidine-44 of the A subunit of Escherichia coli enterotoxin is involved in its enzymatic and biological activities

We examined the role in toxicity of histidine-44 of the A subunit of Escherichia coli enterotoxin, which is located in the active site cavity close to glutamic acid-112. Although amino acid substitution of histidine-44 usually renders a mutant toxin unstable to trypsin, one mutant, alanine-44 (His44Ala) was found to be stable. His44Ala did not show any agmatine:ADP-ribosyltransferase activity in the presence or absence of recombinant ADP-ribosylation factor. It showed no diarrheal or rabbit skin permeability activity and was a competitor in enterotoxin-ADP-ribosyltransferase assays containing recombinant ADP-ribosylation factor. These results suggest that like glutamic acid-112, histidine-44 plays an essential role in toxicity. A tentative model, which explains NAD+ catalysis and the transfer of the ADP-ribosyl moiety to a target amino acid, is proposed for histidine-44 and glutamic acid-112.

Mutants in the ADP-ribosyltransferase cleft of cholera toxin lack diarrheagenicity but retain adjuvanticity

Cholera toxin (CT), the most commonly used mucosal adjuvant in experimental animals, is unsuitable for humans because of potent diarrhea-inducing properties. We have constructed two CT-A subunit mutants, e.g., serine-->phenylalanine at position 61 (S61F), and glutamic acid-->lysine at 112 (E112K) by site-directed mutagenesis. Neither mutant CT (mCT), in contrast to native CT (nCT), induced adenosine diphosphate-ribosylation, cyclic adenosine monophosphate formation, or fluid accumulation in ligated mouse ileal loops. Both mCTs retained adjuvant properties, since mice given ovalbumin (OVA) subcutaneously with mCTs or nCT, but not OVA alone developed high-titered serum anti-OVA immunoglobulin G (IgG) antibodies (Abs) which were largely of IgG1 and IgG2b subclasses. Although nCT induced brisk IgE Ab responses, both mCTs elicited lower anti-OVA IgE Abs. OVA-specific CD4+ T cells were induced by nCT and by mCTs, and quantitative analysis of secreted cytokines and mRNA revealed a T helper cell 2 (Th2)-type response. These results now show that the toxic properties of CT can be separated from adjuvanticity, and the mCTs induce Ab responses via a Th2 cell pathway.

Crystal structure of a new heat-labile enterotoxin, LT-IIb

BACKGROUND

Cholera toxin from Vibrio cholerae and the type I heat-labile enterotoxins (LT-Is) from Escherichia coli are oligomeric proteins with AB5 structures. The type II heat-labile enterotoxins (LT-IIs) from E. coli are structurally similar to, but antigenically distinct from, the type I enterotoxins. The A subunits of type I and type II enterotoxins are homologous and activate adenylate cyclase by ADP-ribosylation of a G protein subunit, G8 alpha. However, the B subunits of type I and type II enterotoxins differ dramatically in amino acid sequence and ganglioside-binding specificity. The structure of LT-IIb was determined both as a prototype for other LT-IIs and to provide additional insights into structure/function relationships among members of the heat-labile enterotoxin family and the superfamily of ADP-ribosylating protein toxins.

RESULTS

The 2.25 A crystal structure of the LT-IIb holotoxin has been determined. The structure reveals striking similarities with LT-I in both the catalytic A subunit and the ganglioside-binding B subunits. The latter form a pentamer which has a central pore with a diameter of 10-18 A. Despite their similarities, the relative orientation between the A polypeptide and the B pentamer differs by 24 degrees in LT-I and LT-IIb. A common hydrophobic ring was observed at the A-B5 interface which may be important in the cholera toxin family for assembly of the AB5 heterohexamer. A cluster of arginine residues at the surface of the A subunit of LT-I and cholera toxin, possibly involved in assembly, is also present in LT-IIb. The ganglioside receptor binding sites are localized, as suggested by mutagenesis, and are in a position roughly similar to the sites where LT-I binds its receptor.

CONCLUSIONS

The structure of LT-IIb provides insight into the sequence diversity and structural similarity of the AB5 toxin family. New knowledge has been gained regarding the assembly of AB5 toxins and their active-site architecture.

Protein engineering studies of A-chain loop 47-56 of Escherichia coli heat-labile enterotoxin point to a prominent role of this loop for cytotoxicity

Heat-labile enterotoxin (LT), produced by enterotoxigenic Escherichia coli, is a close relative of cholera toxin (CT). These two toxins share approximately 80% sequence identity, and consists of one 240-residue A chain and five 103-residue B subunits. The B pentamer is responsible for GM1 receptor recognition, whereas the A subunit carries out an ADP-ribosylation of an arginine residue in the G protein, Gs alpha, in the epithelial target cell. This paper explores the importance of specific amino acids in loop 47-56 of the A subunit. This loop was observed to be highly mobile in the inactive R7K mutant of the A subunit. The position of the loop in wild-type protein is such that it might require considerable reorganization during substrate binding and is likely to have a crucial role in substrate binding. Five single-site substitutions have been made in the LT-A subunit 47-56 loop to investigate its possible role in the enzymatic activity and toxicity of LT and CT. The wild-type residues Thr-50 and Val-53 were replaced either by a glycine or by a proline. The glycine substitutions were intended to increase the mobility of this active-site loop, and the proline substitutions were intended to decrease the mobility of this same loop by restricting the accessible conformational space. Under the hypothesis that mobility of the loop is important for catalysis, the glycine-substitution mutants T50G and V53G would be expected to exhibit activity equal to or greater than that of the wild-type A subunit, while the proline substitution mutants T50P and T53P would be less active. Cytotoxicity assays showed, however, that all four of these mutants were considerably less active than wild-type LT. These results lend support for assignment of a prominent role to loop 47-56 in catalysis by LT and CT.

Site-directed mutagenic alteration of potential active-site residues of the A subunit of Escherichia coli heat-labile enterotoxin. Evidence for a catalytic role for glutamic acid 112

Escherichia coli heat-labile enterotoxin (LT) and the related cholera toxin exert their effects on eukaryotic cells through the ADP-ribosylation of guanine nucleotide-binding proteins of the adenylate cyclase complex. The availability of the crystal structure for LT has permitted the tentative identification of residues that lie within or are vicinal to a presumptive NAD(+)-binding site and thus may play a role in substrate binding or catalysis. Using a plasmid clone encoding the A subunit of LT, we have introduced substitutions at such potential active-site residues and analyzed the enzymatic properties of the resultant mutant analogs. Enzymatic analyses, employing both transducin and agmatine as acceptor substrates, revealed that substitutions at serine 61, glutamic acid 110, and glutamic acid 112 resulted in reduction of enzyme activity to < 10% of wild-type levels. Kinetic analyses indicated that alteration of these sites affected the catalytic rate of the enzyme and had little or no effect on the binding of either NAD+ or agmatine. Of the mutant analogs analyzed, only glutamic acid 112 appeared to represent an essential catalytic residue as judged by the relative effects on kcat and kcat/Km. The results provide formal evidence that glutamic acid 112 of the A subunit of LT represents a functional homolog or analog of catalytic glutamic acid residues that have been identified in several other bacterial ADP-ribosylating toxins and that it may play an essential role in rendering NAD+ susceptible to nucleophilic attack by an incoming acceptor substrate.

The Arg7Lys mutant of heat-labile enterotoxin exhibits great flexibility of active site loop 47-56 of the A subunit

The heat-labile enterotoxin from Escherichia coli (LT) is a member of the cholera toxin family. These and other members of the larger class of AB5 bacterial toxins act through catalyzing the ADP-ribosylation of various intracellular targets including Gs alpha. The A subunit is responsible for this covalent modification, while the B pentamer is involved in receptor recognition. We report here the crystal structure of an inactive single-site mutant of LT in which arginine 7 of the A subunit has been replaced by a lysine residue. The final model contains 103 residues for each of the five B subunits, 175 residues for the A1 subunit, and 41 residues for the A2 subunit. In this Arg7Lys structure the active site cleft within the A subunit is wider by approximately 1 A than is seen in the wild-type LT. Furthermore, a loop near the active site consisting of residues 47-56 is disordered in the Arg7Lys structure, even though the new lysine residue at position 7 assumes a position which virtually coincides with that of Arg7 in the wild-type structure. The displacement of residues 47-56 as seen in the mutant structure is proposed to be necessary for allowing NAD access to the active site of the wild-type LT. On the basis of the differences observed between the wild-type and Arg7Lys structures, we propose a model for a coordinated sequence of conformational changes required for full activation of LT upon reduction of disulfide bridge 187-199 and cleavage of the peptide loop between the two cysteines in the A subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Construction of nontoxic derivatives of cholera toxin and characterization of the immunological response against the A subunit

Using computer modelling, we have identified some of the residues of the A subunit of cholera toxin (CT) and heat-labile toxin that are involved in NAD binding, catalysis, and toxicity. Here we describe the site-directed mutagenesis of the CT gene and the construction of CT mutants. Nine mutations of the A subunit gene were generated. Six of them encoded proteins that were fully assembled in the AB5 structure and were nontoxic; these proteins were CT-D53 (Val-53-->Asp), CT-K63 (Ser-63-->Lys), CT-K97 (Val-97-->Lys), CT-K104 (Tyr-104-->Lys), CT-S106 (Pro-106-->Ser), and the double mutant CT-D53/K63 (Val-53-->Asp, Ser-63-->Lys). Two of the mutations encoded proteins that were assembled into the AB5 structure but were still toxic; these proteins were CT-H54 (Arg-54-->His) and CT-N107 (His-107-->Asn). Finally, one of the mutant proteins, CT-E114 (Ser-114-->Glu), was unable to assemble the A and the B subunits and produced only the B oligomer. The six nontoxic mutants were purified from the culture supernatants of recombinant Vibrio cholerae strains and further characterized. The CT-K63 mutant, which was the most efficient in assembly of the AB5 structure, was used to immunize rabbits and was shown to be able to induce neutralizing antibodies against both the A and B subunits. This molecule may be useful for the construction of improved vaccines against cholera.

Dissociation of Escherichia coli heat-labile enterotoxin adjuvanticity from ADP-ribosyltransferase activity

The heat-labile enterotoxin (LT) of Escherichia coli is immunologically and physiochemically related to cholera enterotoxin. A number of studies have been performed to determine the relationship of the ADP-ribosylating enzymatic activity of these enterotoxins to toxicity and adjuvanticity. These studies have generally examined the effect of abolishing the ADP-ribosyltransferase activity of A1 by a variety of chemical or genetic manipulations. In every case, loss of enzymatic activity was associated with loss of biological activity and also with the ability of the molecules to function as oral adjuvants. Consequently, we explored an alternate approach to detoxification of LT without altering its adjuvanticity. Specifically, we generated a novel mutant form of LT by genetic modification of the proteolytically sensitive residues that join the A1 and A2 components of the A subunit. This mutant contains a single amino acid substitution within the disulfide subtended region joining A1 and A2. This mutant toxin, designated LT(R192G), is not sensitive to proteolytic activation, has negligible activity on mouse Y-1 adrenal tumor cells, and is devoid of ADP-ribosyltransferase activity. Nonetheless, LT(R192G) retains the ability to function as a mucosal adjuvant, increasing the serum immunoglobulin G (IgG) and mucosal IgA responses to coadministered antigen (OVA) beyond that achieved with administration of that antigen alone. Further, LT(R192G) prevented the induction of tolerance to coadministered antigen and did not induce tolerance against itself, as demonstrated by the presence of significant serum anti-LT IgG and mucosal anti-LT IgA antibodies in immunized mice.

Probing the structure-activity relationship of Escherichia coli LT-A by site-directed mutagenesis

Computer analysis of the crystallographic structure of the A subunit of Escherichia coli heat-labile toxin (LT) was used to predict residues involved in NAD binding, catalysis and toxicity. Following site-directed mutagenesis, the mutants obtained could be divided into three groups. The first group contained fully assembled, non-toxic new molecules containing mutations of single amino acids such as Val-53-->Glu or Asp, Ser-63-->Lys, Val-97-->Lys, Tyr-104-->Lys or Asp, and Ser-114-->Lys or Glu. This group also included mutations in amino acids such as Arg-7, Glu-110 and Glu-112 that were already known to be important for enzymatic activity. The second group was formed by mutations that caused the collapse or prevented the assembly of the A subunit: Leu-41-->Phe, Ala-45-->Tyr or Glu, Val-53-->Tyr, Val-60-->Gly, Ser-68-->Pro, His-70-->Pro, Val-97-->Tyr and Ser-114-->Tyr. The third group contained those molecules that maintained a wild-type level of toxicity in spite of the mutations introduced: Arg-54-->Lys or Ala, Tyr-59-->Met, Ser-68-->Lys, Ala-72-->Arg, His or Asp and Arg-192-->Asn. The results provide a further understanding of the structure-function of the active site and new, non-toxic mutants that may be useful for the development of vaccines against diarrhoeal diseases.

Common structure of the catalytic sites of mammalian and bacterial toxin ADP-ribosyltransferases

The amino acid sequences of several bacterial toxin ADP-ribosyltransferases, rabbit skeletal muscle transferases, and RT6.2, a rat T-cell NAD glycohydrolase, contain three separate regions of similarity, which can be aligned. Region I contains a critical histidine or arginine residue, region II, a group of closely spaced aromatic amino acids, and region III, an active-site glutamate which is at times seen as part of an acidic amino acid-rich sequence. In some of the bacterial ADP-ribosyltransferases, the nicotinamide moiety of NAD has been photo-crosslinked to this glutamate, consistent with its position in the active site. The similarities within these three regions, despite an absence of overall sequence similarity among the several transferases, are consistent with a common structure involved in NAD binding and ADP-ribose transfer.

Structure and function of cholera toxin and the related Escherichia coli heat-labile enterotoxin

Cholera and the related Escherichia coli-associated diarrheal disease are important problems confronting Third World nations and any area where water supplies can become contaminated. The disease is extremely debilitating and may be fatal in the absence of treatment. Symptoms are caused by the action of cholera toxin, secreted by the bacterium Vibrio cholerae, or by a closely related heat-labile enterotoxin, produced by Escherichia coli, that causes a milder, more common traveler's diarrhea. Both toxins bind receptors in intestinal epithelial cells and insert an enzymatic subunit that modifies a G protein associated with the adenylate cyclase complex. The consequent stimulated production of cyclic AMP, or other factors such as increased synthesis of prostaglandins by intoxicated cells, initiates a metabolic cascade that results in the excessive secretion of fluid and electrolytes characteristic of the disease. The toxins have a very high degree of structural and functional homology and may be evolutionarily related. Several effective new vaccine formulations have been developed and tested, and a growing family of endogenous cofactors is being discovered in eukaryotic cells. The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins. This information may improve our understanding of the disease process itself, as well as illuminate the role of the toxin in studies of signal transduction and G-protein function.

The adjuvant effect of Vibrio cholerae and Escherichia coli heat-labile enterotoxins is linked to their ADP-ribosyltransferase activity

This study addressed the question of whether the mucosal adjuvant property of cholera toxin (CT) and the structurally closely related Escherichia coli heat-labile toxin (LT) requires the enterotoxic and adenylate cyclase/cAMP activating property of these molecules. Therefore, we investigated the cytotoxic and adjuvant abilities of the enterotoxins and compared the results with those obtained with the non-toxic CT and LT derivatives; recombinant CTB (rCTB) and a mutated LT (mLT), which had a single amino acid substitution in position 112 (Glu----Lys) of the A subunit. Detailed functional studies revealed that, in contrast to the enterotoxins, both rCTB and mLT lacked ADP-ribosylating and cAMP-stimulating abilities. However, similar membrane ganglioside GM1-receptor binding ability of all the putative adjuvants was demonstrated. When the probe antigen, keyhole limpet hemocyanin (KLH), was given perorally together with CT or LT strong gut mucosal anti-KLH immune responses were stimulated, whereas no or very low anti-KLH responses were seen in the groups which received antigen admixed with rCTB or the mLT. Moreover, the specific serum antibody responses to the various immunization protocols closely paralleled the local anti-KLH response in the gut. From these results it appears that the adjuvant mechanism of LT, and probably also of CT, is linked to the ability to ADP-ribosylate and to stimulate cAMP formation. However, this study does not unequivocally rule out other possibilities such as interactions by the A1 fragment of CT or LT with other G-proteins than Gs alpha or events that parallel or precede the effects on the adenylate cyclase/cAMP system. Thus, the levels of ADP-ribosylation and cAMP-induction that are required and the key event or target cell that is responsible for the adjuvant effect of CT and LT remain to be elucidated. Studies are underway to address these issues.

Glutamic acid-112 of the A subunit of heat-labile enterotoxin from enterotoxigenic Escherichia coli is important for ADP-ribosyltransferase activity

A mutant strain of enterotoxigenic Escherichia coli (E. coli pTUH 6A) produced an abnormal heat-labile enterotoxin (LT), the A subunit of which has a single amino acid substitution at position 112 (Glu-112 to Lys-112). As already reported, this mutant LT had no ileal loop and vascular permeability activities [(1990) J. Biol. Chem. 265, 22520-22525]. In this paper we report that the mutant LT showed no CHO cell elongation activity and did not activate adenylate cyclase of target cells. Moreover, no ADP-ribosyltransferase activity was detected in the mutant LT. It is concluded that the amino acid substitution at position 112 abolished the ADP-ribosyltransferase activity of the A subunit and this leads to the loss of toxic activities of LT.

Effect of site-directed mutagenic alterations on ADP-ribosyltransferase activity of the A subunit of Escherichia coli heat-labile enterotoxin

Previous studies of the S1 subunit of pertussis toxin, an NAD(+)-dependent ADP-ribosyltransferase, suggested that a small amino-terminal region of amino acid sequence similarity to the active fragments of both cholera toxin and Escherichia coli heat-labile enterotoxin represents a region containing critical active-site residues that might be involved in the binding of the substrate NAD+. Other studies of two other bacterial toxins possessing ADP-ribosyltransferase activity, diphtheria toxin and Pseudomonas exotoxin A, have revealed the presence of essential glutamic acid residues vicinal to the active site. To help determine the relevance of these observations to activities of the enterotoxins, the A-subunit gene of the E. coli heat-labile enterotoxin was subjected to site-specific mutagenesis in the region encoding the amino-terminal region of similarity to the S1 subunit of pertussis toxin delineated by residues 6 through 17 and at two glutamic acid residues, 110 and 112, that are conserved in the active domains of all of the heat-labile enterotoxin variants and in cholera toxin. Mutant proteins in which arginine 7 was either deleted or replaced with lysine exhibited undetectable levels of ADP-ribosyltransferase activity. However, limited trypsinolysis of the arginine 7 mutants yielded fragmentation kinetics that were different from that yielded by the wild-type recombinant subunit or the authentic A subunit. In contrast, mutant proteins in which glutamic acid residues at either position 110 or 112 were replaced with aspartic acid responded like the wild-type subunit upon limited trypsinolysis, while exhibiting severely depressed, but detectable, ADP-ribosyltransferase activity. The latter results may indicate that either glutamic acid 110 or glutamic acid 112 of the A subunit of heat-labile enterotoxin is analogous to those active-site glutamic acids identified in several other ADP-ribosylating toxins.

Crystal structure of a cholera toxin-related heat-labile enterotoxin from E. coli

Examination of the structure of Escherichia coli heat-labile enterotoxin in the AB5 complex at a resolution of 2.3A reveals that the doughnut-shaped B pentamer binds the enzymatic A subunit using a hairpin of the A2 fragment, through a highly charged central pore. Putative ganglioside GM1-binding sites on the B subunits are more than 20A removed from the membrane-crossing A1 subunit. This ADP-ribosylating (A1) fragment of the toxin has structural homology with the catalytic region of exotoxin A and hence also to diphtheria toxin.