Biological and biochemical characterization of variant A subunits of cholera toxin constructed by site-directed mutagenesis

cAMP-Independent Activation of the Unfolded Protein Response by Cholera Toxin

Cholera toxin (CT) is an AB₅ protein toxin that activates the stimulatory alpha subunit of the heterotrimeric G protein (Gsα) through ADP-ribosylation. Activation of Gsα produces a cytopathic effect by stimulating adenylate cyclase and the production of cAMP. To reach its cytosolic Gsα target, CT binds to the plasma membrane of a host cell and travels by vesicle carriers to the endoplasmic reticulum (ER). The catalytic CTA1 subunit then exploits the quality control mechanism of ER-associated degradation to move from the ER to the cytosol. ER-associated degradation is functionally linked to another quality control system, the unfolded protein response (UPR). However, the role of the UPR in cholera intoxication is unclear. We report here that CT triggers the UPR after 4 h of toxin exposure. A functional toxin was required to induce the UPR, but, surprisingly, activation of the adenylate cyclase signaling pathway was not sufficient to trigger the process. Toxin-induced activation of the UPR coincided with increased toxin accumulation in the cytosol. Chemical activation of the heterotrimeric G protein or the UPR also enhanced the onset of CTA1 delivery to the cytosol, thus producing a toxin-sensitive phenotype. These results indicate there is a cAMP-independent response to CT that activates the UPR and thereby enhances the efficiency of intoxication.

Heat-Labile Enterotoxins

Heat-labile enterotoxins (LTs) of Escherichia coli are closely related to cholera toxin (CT), which was originally discovered in 1959 in culture filtrates of the gram-negative bacterium Vibrio cholerae. Several other gram-negative bacteria also produce enterotoxins related to CT and LTs, and together these toxins form the V. cholerae-E. coli family of LTs. Strains of E. coli causing a cholera-like disease were designated enterotoxigenic E. coli (ETEC) strains. The majority of LTI genes (elt) are located on large, self-transmissible or mobilizable plasmids, although there are instances of LTI genes being located on chromosomes or carried by a lysogenic phage. The stoichiometry of A and B subunits in holotoxin requires the production of five B monomers for every A subunit. One proposed mechanism is a more efficient ribosome binding site for the B gene than for the A gene, increasing the rate of initiation of translation of the B gene independently from A gene translation. The three-dimensional crystal structures of representative members of the LT family (CT, LTpI, and LTIIb) have all been determined by X-ray crystallography and found to be highly similar. Site-directed mutagenesis has identified many residues in the CT and LT A subunits, including His44, Val53, Ser63, Val97, Glu110, and Glu112, that are critical for the structures and enzymatic activities of these enterotoxins. For the enzymatically active A1 fragment to reach its substrate, receptor-bound holotoxin must gain access to the cytosol of target cells.

A mutational analysis of residues in cholera toxin A1 necessary for interaction with its substrate, the stimulatory G protein Gsα

Pathogenesis of cholera diarrhea requires cholera toxin (CT)-mediated adenosine diphosphate (ADP)-ribosylation of stimulatory G protein (Gsα) in enterocytes. CT is an AB5 toxin with an inactive CTA1 domain linked via CTA2 to a pentameric receptor-binding B subunit. Allosterically activated CTA1 fragment in complex with NAD⁺ and GTP-bound ADP-ribosylation factor 6 (ARF6-GTP) differs conformationally from the CTA1 domain in holotoxin. A surface-exposed knob and a short α-helix (formed, respectively, by rearranging “active-site” and “activation” loops in inactive CTA1) and an ADP ribosylating turn-turn (ARTT) motif, all located near the CTA1 catalytic site, were evaluated for possible roles in recognizing Gsα. CT variants with one, two or three alanine substitutions at surface-exposed residues within these CTA1 motifs were tested for assembly into holotoxin and ADP-ribosylating activity against Gsα and diethylamino-(benzylidineamino)-guanidine (DEABAG), a small substrate predicted to fit into the CTA1 active site). Variants with single alanine substitutions at H55, R67, L71, S78, or D109 had nearly wild-type activity with DEABAG but significantly decreased activity with Gsα, suggesting that the corresponding residues in native CTA1 participate in recognizing Gsα. As several variants with multiple substitutions at these positions retained partial activity against Gsα, other residues in CTA1 likely also participate in recognizing Gsα.

ADP-ribosylation factor 6 acts as an allosteric activator for the folded but not disordered cholera toxin A1 polypeptide

The catalytic A1 subunit of cholera toxin (CTA1) has a disordered structure at 37°C. An interaction with host factors must therefore place CTA1 in a folded conformation for the modification of its Gsα target which resides in a lipid raft environment. Host ADP-ribosylation factors (ARFs) act as in vitro allosteric activators of CTA1, but the molecular events of this process are not fully characterized. Isotope-edited Fourier transform infrared spectroscopy monitored ARF6-induced structural changes to CTA1, which were correlated to changes in CTA1 activity. We found ARF6 prevents the thermal disordering of structured CTA1 and stimulates the activity of stabilized CTA1 over a range of temperatures. Yet ARF6 alone did not promote the refolding of disordered CTA1 to an active state. Instead, lipid rafts shifted disordered CTA1 to a folded conformation with a basal level of activity that could be further stimulated by ARF6. Thus, ARF alone is unable to activate disordered CTA1 at physiological temperature: additional host factors such as lipid rafts place CTA1 in the folded conformation required for its ARF-mediated activation. Interaction with ARF is required for in vivo toxin activity, as enzymatically active CTA1 mutants that cannot be further stimulated by ARF6 fail to intoxicate cultured cells.

Functional mapping of community-acquired respiratory distress syndrome (CARDS) toxin of Mycoplasma pneumoniae defines regions with ADP-ribosyltransferase, vacuolating and receptor-binding activities

Community-acquired respiratory distress syndrome (CARDS) toxin from Mycoplasma pneumoniae is a 591-amino-acid virulence factor with ADP-ribosyltransferase (ADPRT) and vacuolating activities. It is expressed at low levels during in vitro growth and at high levels during colonization of the lung. Exposure of experimental animals to purified recombinant CARDS toxin alone is sufficient to recapitulate the cytopathology and inflammatory responses associated with M. pneumoniae infection in humans and animals. Here, by molecular modelling, serial truncations and site-directed mutagenesis, we show that the N-terminal region is essential for ADP-ribosylating activity. Also, by systematic truncation and limited proteolysis experiments we identified a portion of the C-terminal region that mediates toxin binding to mammalian cell surfaces and subsequent internalization. In addition, the C-terminal region alone induces vacuolization in a manner similar to full-length toxin. Together, these data suggest that CARDS toxin has a unique architecture with functionally separable N-terminal and C-terminal domains.

The unfolded protein response element IRE1α senses bacterial proteins invading the ER to activate RIG-I and innate immune signaling

The plasma membrane and all membrane-bound organelles except for the Golgi and endoplasmic reticulum (ER) are equipped with pattern-recognition molecules to sense microbes or their products and induce innate immunity for host defense. Here, we report that inositol-requiring-1α (IRE1α), an ER protein that signals in the unfolded protein response (UPR), is activated to induce inflammation by binding a portion of cholera toxin as it co-opts the ER to cause disease. Other known UPR transducers, including the IRE1α-dependent transcription factor XBP1, are dispensable for this signaling. The inflammatory response depends instead on the RNase activity of IRE1α to degrade endogenous mRNA, a process termed regulated IRE1α-dependent decay (RIDD) of mRNA. The mRNA fragments produced engage retinoic-acid inducible gene 1 (RIG-I), a cytosolic sensor of RNA viruses, to activate NF-κB and interferon pathways. We propose IRE1α provides for a generalized mechanism of innate immune surveillance originating within the ER lumen.

Identification of host cell factors required for intoxication through use of modified cholera toxin

We describe a novel labeling strategy to site-specifically attach fluorophores, biotin, and proteins to the C terminus of the A1 subunit (CTA1) of cholera toxin (CTx) in an otherwise correctly assembled and active CTx complex. Using a biotinylated N-linked glycosylation reporter peptide attached to CTA1, we provide direct evidence that ~12% of the internalized CTA1 pool reaches the ER. We also explored the sortase labeling method to attach the catalytic subunit of diphtheria toxin as a toxic warhead to CTA1, thus converting CTx into a cytolethal toxin. This new toxin conjugate enabled us to conduct a genetic screen in human cells, which identified ST3GAL5, SLC35A2, B3GALT4, UGCG, and ELF4 as genes essential for CTx intoxication. The first four encode proteins involved in the synthesis of gangliosides, which are known receptors for CTx. Identification and isolation of the ST3GAL5 and SLC35A2 mutant clonal cells uncover a previously unappreciated differential contribution of gangliosides to intoxication by CTx.

Cholera toxin activates nonconventional adjuvant pathways that induce protective CD8 T-cell responses after epicutaneous vaccination

The ability to induce humoral and cellular immunity via antigen delivery through the unbroken skin (epicutaneous immunization, EPI) has immediate relevance for vaccine development. However, it is unclear which adjuvants induce protective memory CD8 T-cell responses by this route, and the molecular and cellular requirements for priming through intact skin are not defined. We report that cholera toxin (CT) is superior to other adjuvants in its ability to prime memory CD8 T cells that control bacterial and viral challenges. Epicutaneous immunization with CT does not require engagement of classic toll-like receptor (TLR) and inflammasome pathways and, surprisingly, is independent of skin langerin-expressing cells (including Langerhans cells). However, CT adjuvanticity required type-I IFN sensitivity, participation of a Batf3-dependent dendritic cell (DC) population and engagement of CT with suitable gangliosides. Chemoenzymatic generation of CT-antigen fusion proteins led to efficient priming of the CD8 T-cell responses, paving the way for development of this immunization strategy as a therapeutic option.

Structural and biochemical characterization of NarE, an iron-containing ADP-ribosyltransferase from Neisseria meningitidis

NarE is a 16 kDa protein identified from Neisseria meningitidis, one of the bacterial pathogens responsible for meningitis. NarE belongs to the family of ADP-ribosyltransferases (ADPRT) and catalyzes the transfer of ADP-ribose moieties to arginine residues in target protein acceptors. Many pathogenic bacteria utilize ADP-ribosylating toxins to modify and alter essential functions of eukaryotic cells. NarE is further the first ADPRT which could be shown to bind iron through a Fe-S center, which is crucial for the catalytic activity. Here we present the NMR solution structure of NarE, which shows structural homology to other ADPRTs. Using NMR titration experiments we could identify from Chemical Shift Perturbation data both the NAD binding site, which is in perfect agreement with a consensus sequence analysis between different ADPRTs, as well as the iron coordination site, which consists of 2 cysteines and 2 histidines. This atypical iron coordination is also capable to bind zinc. These results could be fortified by site-directed mutagenesis of the catalytic region, which identified two functionally crucial residues. We could further identify a main interaction region of NarE with antibodies using two complementary methods based on antibody immobilization, proteolytic digestion, and mass spectrometry. This study combines structural and functional features of NarE providing for the first time a characterization of an iron-dependent ADPRT.

Structural inferences for Cholera toxin mutations in Vibrio cholerae

Cholera is a global disease that has persisted for millennia. The cholera toxin (CT) from Vibrio cholerae is responsible for the clinical symptoms of cholera. This toxin is a hetero-hexamer (AB(5)) complex consisting of a subunit A (CTA) with a pentamer (B(5)) of subunit B (CTB). The importance of the AB(5) complex for pathogenesis is established for the wild type O1 serogroup using known structural and functional data. However, its role is not yet documented in other known serogroups harboring sequence level residue mutations. The sequences for the toxin from different serogroups are available in GenBank (release 177). Sequence analysis reveals mutations at several sequence positions in the toxin across serogroups. Therefore, it is of interest to locate the position of these mutations in the AB(5) structure to infer complex assembly for its functional role in different serogroups. We show that mutations in the CTA are at the solvent exposed regions of the AB(5) complex, whereas those in the CTB are at the CTB/CTB interface of the homo-pentamer complex. Thus, the role of mutations at the CTB/CTB interface for B(5) complex assembly is implied. It is observed that these mutations are often non-synonymous (e.g. polar to non-polar or vice versa). The formation of the AB(5) complex involves inter-subunit residue-residue interactions at the protein-protein interfaces. Hence, these mutations, at the structurally relevant positions, are of importance for the understanding of pathogenesis by several serogroups. This is also of significance in the improvement of recombinant CT protein complex analogs for vaccine design and their use against multiple serogroups.

Attenuated endocytosis and toxicity of a mutant cholera toxin with decreased ability to cluster ganglioside GM1 molecules

Cholera toxin (CT) moves from the plasma membrane (PM) of host cells to the endoplasmic reticulum (ER) by binding to the lipid raft ganglioside GM(1). The homopentomeric B-subunit of the toxin can bind up to five GM(1) molecules at once. Here, we examined the role of polyvalent binding of GM(1) in CT action by producing chimeric CTs that had B-subunits with only one or two normal binding pockets for GM(1). The chimeric toxins had attenuated affinity for binding to host cell PM, as expected. Nevertheless, like wild-type (wt) CT, the CT chimeras induced toxicity, fractionated with detergent-resistant membranes extracted from toxin-treated cells, displayed restricted diffusion in the plane of the PM in intact cells, and remained bound to GM(1) when they were immunoprecipitated. Thus, binding normally to two or perhaps only one GM(1) molecule is sufficient for association with lipid rafts in the PM and toxin action. The chimeric toxins, however, were much less potent than wt toxin, and they entered the cell by endocytosis more slowly, suggesting that clustering of GM(1) molecules by the B-subunit enhances the efficiency of toxin uptake and perhaps also trafficking to the ER.

The cholera toxin A1(3) subdomain is essential for interaction with ADP-ribosylation factor 6 and full toxic activity but is not required for translocation from the endoplasmic reticulum to the cytosol

Cholera toxin (CT) moves from the plasma membrane to the endoplasmic reticulum (ER) by retrograde vesicular traffic. In the ER, the catalytic CTA1 polypeptide dissociates from the rest of the toxin and enters the cytosol by a process that involves the quality control mechanism of ER-associated degradation (ERAD). The cytosolic CTA1 then ADP ribosylates Gsalpha, resulting in adenylate cyclase activation and intoxication of the target cell. It is hypothesized that the C-terminal A1(3) subdomain of CTA1 plays two crucial roles in the intoxication process: (i) it contains a hydrophobic domain that triggers the ERAD mechanism and (ii) it facilitates interaction with the cytosolic ADP-ribosylation factors (ARFs) that serve as allosteric activators of CTA1. In this study, we examined the role(s) of the CTA1(3) subdomain in CT intoxication. Full-length CTA1 constructs and truncated CTA1 constructs lacking the A1(3) subdomain were generated and used to conduct two-hybrid studies of interactions with ARF6, in vitro enzyme assays, in vivo toxicity assays, and in vivo processing/degradation assays. Direct, plasmid-mediated expression of CTA1 constructs in the ER or cytosol of transfected CHO cells was used to perform the in vivo assays. With these methods, we found that the A1(3) subdomain of CTA1 is important both for interaction with ARF6 and for full expression of enzyme activity in vivo. Surprisingly, however, the A1(3) subdomain was not required for ERAD-mediated passage of CTA1 from the ER to the cytosol. A possible alternative trigger for CTA1 to activate the ERAD mechanism is discussed.

Structural basis for the activation of cholera toxin by human ARF6-GTP

The Vibrio cholerae bacterium causes devastating diarrhea when it infects the human intestine. The key event is adenosine diphosphate (ADP)-ribosylation of the human signaling protein GSalpha, catalyzed by the cholera toxin A1 subunit (CTA1). This reaction is allosterically activated by human ADP-ribosylation factors (ARFs), a family of essential and ubiquitous G proteins. Crystal structures of a CTA1:ARF6-GTP (guanosine triphosphate) complex reveal that binding of the human activator elicits dramatic changes in CTA1 loop regions that allow nicotinamide adenine dinucleotide (NAD+) to bind to the active site. The extensive toxin:ARF-GTP interface surface mimics ARF-GTP recognition of normal cellular protein partners, which suggests that the toxin has evolved to exploit promiscuous binding properties of ARFs.

Construction of tumor-specific toxins using ubiquitin fusion technique

The use of cytotoxic agents to eliminate cancer cells is limited because of their nonselective toxicity and unwanted side effects. One of the strategies to overcome these limitations is to use latent prodrugs that become toxic in situ after being enzymatically activated in target cells. In this work we describe a method for producing tumor-specific toxins by using a ubiquitin fusion technique. The method is illustrated by the production of recombinant toxins by in-frame fusion of ubiquitin to saporin, a toxin from the plant Saponaria officinalis. Ubiquitin-fused toxins were rapidly degraded via the ubiquitin-proteasome system, significantly reducing their nonspecific toxicity. The insertion of the protease-cleavage sequence between ubiquitin and saporin led to the removal of ubiquitin by the protease and resulted in protease-dependent stabilization of the toxin. We engineered toxins that can be stabilized by specific proteases such as deubiquitinating enzymes and prostate-specific antigen (PSA). Both constructs were activated in vitro and in cultured cells by the appropriate enzyme. Processing by the protease resulted in a greater than 10-fold increase in the toxicity of these constructs. Importantly, the PSA-cleavable toxin was able to kill specifically the PSA-producing prostate cancer cells. The ubiquitin fusion technique is thus a versatile and reliable method for obtaining selective cytotoxic agents and can easily be adapted for different kinds of toxins and activating proteases.

Use of translational fusion of the MrpH fimbrial adhesin-binding domain with the cholera toxin A2 domain, coexpressed with the cholera toxin B subunit, as an intranasal vaccine to prevent experimental urinary tract infection by Proteus mirabilis

This is a follow-up to our previous study using an intranasal vaccine composed of MrpH, the tip adhesin of the MR/P fimbria, and cholera toxin to prevent urinary tract infection by Proteus mirabilis (X. Li, C. V. Lockatell, D. E. Johnson, M. C. Lane, J. W. Warren, and H. L. Mobley, Infect. Immun. 72:66-75, 2004). Here, we have expressed a cholera toxin-like chimera in which the MrpH adhesin-binding domain (residues 23 to 157) replaces the cholera toxin A1 ADP-ribosyltransferase domain. This chimera, when administered intranasally without additional adjuvant, is sufficient to induce protective immunity in mice.

Vesicular transport is not required for the cytoplasmic pool of cholera toxin to interact with the stimulatory alpha subunit of the heterotrimeric g protein

Cholera toxin (CT) moves from the cell surface to the endoplasmic reticulum (ER) by retrograde vesicular transport. The catalytic A1 polypeptide of CT (CTA1) then crosses the ER membrane, enters the cytosol, ADP-ribosylates the stimulatory alpha subunit of the heterotrimeric G protein (Gsalpha) at the cytoplasmic face of the plasma membrane, and activates adenylate cyclase. The cytosolic pool of CTA1 may reach the plasma membrane and its Gsalpha target by traveling on anterograde-directed transport vesicles. We examined this possibility with the use of a plasmid-based transfection system that directed newly synthesized CTA1 to either the ER lumen or the cytosol of CHO cells. Such a system allowed us to bypass the CT retrograde trafficking itinerary from the cell surface to the ER. Previous work has shown that the ER-localized pool of CTA1 is rapidly exported from the ER to the cytosol. Expression of CTA1 in either the ER or the cytosol led to the activation of Gsalpha, and Gsalpha activation was not inhibited in transfected cells exposed to drugs that inhibit vesicular traffic. Thus, anterograde transport from the ER to the plasma membrane is not required for the cytotoxic action of CTA1.

Crystal structures of an intrinsically active cholera toxin mutant yield insight into the toxin activation mechanism

Cholera toxin (CT) is a heterohexameric bacterial protein toxin belonging to a larger family of A/B ADP-ribosylating toxins. Each of these toxins undergoes limited proteolysis and/or disulfide bond reduction to form the enzymatically active toxic fragment. Nicking and reduction render both CT and the closely related heat-labile enterotoxin from Escherichia coli (LT) unstable in solution, thus far preventing a full structural understanding of the conformational changes resulting from toxin activation. We present the first structural glimpse of an active CT in structures from three crystal forms of a single-site A-subunit CT variant, Y30S, which requires no activational modifications for full activity. We also redetermined the structure of the wild-type, proenzyme CT from two crystal forms, both of which exhibit (i) better geometry and (ii) a different A2 "tail" conformation than the previously determined structure [Zhang et al. (1995) J. Mol. Biol. 251, 563-573]. Differences between wild-type CT and active CTY30S are observed in A-subunit loop regions that had been previously implicated in activation by analysis of the structure of an LT A-subunit R7K variant [van den Akker et al. (1995) Biochemistry 34, 10996-11004]. The 25-36 activation loop is disordered in CTY30S, while the 47-56 active site loop displays varying degrees of order in the three CTY30S structures, suggesting that disorder in the activation loop predisposes the active site loop to a greater degree of flexibility than that found in unactivated wild-type CT. On the basis of these six new views of the CT holotoxin, we propose a model for how the activational modifications experienced by wild-type CT are communicated to the active site.

Cholera toxin toxicity does not require functional Arf6- and dynamin-dependent endocytic pathways

Cholera toxin (CT) and related AB(5) toxins bind to glycolipids at the plasma membrane and are then transported in a retrograde manner, first to the Golgi and then to the endoplasmic reticulum (ER). In the ER, the catalytic subunit of CT is translocated into the cytosol, resulting in toxicity. Using fluorescence microscopy, we found that CT is internalized by multiple endocytic pathways. Inhibition of the clathrin-, caveolin-, or Arf6-dependent pathways by overexpression of appropriate dominant mutants had no effect on retrograde traffic of CT to the Golgi and ER, and it did not affect CT toxicity. Unexpectedly, when we blocked all three endocytic pathways at once, although fluorescent CT in the Golgi and ER became undetectable, CT-induced toxicity was largely unaffected. These results are consistent with the existence of an additional retrograde pathway used by CT to reach the ER.

NarE: a novel ADP-ribosyltransferase from Neisseria meningitidis

Mono ADP-ribosyltransferases (ADPRTs) are a class of functionally conserved enzymes present in prokaryotic and eukaryotic organisms. In bacteria, these enzymes often act as potent toxins and play an important role in pathogenesis. Here we report a profile-based computational approach that, assisted by secondary structure predictions, has allowed the identification of a previously undiscovered ADP-ribosyltransferase in Neisseria meningitidis (NarE). NarE shows structural homologies with E. coli heat-labile enterotoxin (LT) and cholera toxin (CT) and possesses ADP-ribosylating and NAD-glycohydrolase activities. As in the case of LT and CT, NarE catalyses the transfer of the ADP-ribose moiety to arginine residues. Despite the absence of a signal peptide, the protein is efficiently exported into the periplasm of Neisseria. The narE gene is present in 25 out of 43 strains analysed, is always present in ET-5 and Lineage 3 but absent in ET-37 and Cluster A4 hypervirulent lineages. When present, the gene is 100% conserved in sequence and is inserted upstream of and co-transcribed with the lipoamide dehydrogenase E3 gene. Possible roles in the pathogenesis of N. meningitidis are discussed.

Cholera holotoxin assembly requires a hydrophobic domain at the A-B5 interface: mutational analysis and development of an in vitro assembly system

Cholera toxin (CT) and related Escherichia coli enterotoxins LTI and LTIIb have a conserved hydrophobic region at the AB(5) interface postulated to be important for toxin assembly. Hydrophobic residue F223 in the A subunit of CT (CTA) as well as residues 174, L77, and T78 in the B subunit of CT (CTB) were replaced individually with aspartic acid, and the resulting CTA and CTB variants were analyzed for their ability to assemble into holotoxin in vivo. CTA-F223D holotoxin exhibited decreased stability and toxicity and increased susceptibility to proteolysis by trypsin. CTB-L77D was unable to form functional pentamers. CTB-I74D and CTB-T78D formed pentamers that bound to GM(1) and D-galactose but failed to assemble with CTA to form holotoxin. In contrast, CTB-T78D and CTA-F223H interacted with each other to form a significant amount of holotoxin in vivo. Our findings support the importance of hydrophobic interactions between CTA and CTB in holotoxin assembly. We also developed an efficient method for assembly of CT in vitro, and we showed that CT assembled in vitro was comparable to wild-type CT in toxicity and antigenicity. CTB-I74D and CTB-T78D did not form pentamers or holotoxin in vitro, and CTA-F223D did not form holotoxin in vitro. The efficient system for in vitro assembly of CT described here should be useful for future studies on the development of drugs to inhibit CT assembly as well as the development of chimeric CT-like molecules as potential vaccine candidates.

A class of mutant CHO cells resistant to cholera toxin rapidly degrades the catalytic polypeptide of cholera toxin and exhibits increased endoplasmic reticulum-associated degradation

After binding to the eukaryotic cell surface, cholera toxin undergoes retrograde transport to the endoplasmic reticulum. The catalytic A1 polypeptide of cholera toxin (CTA1) then crosses the endoplasmic reticulum membrane and enters the cytosol in a process that may involve the quality control mechanism known as endoplasmic reticulum-associated degradation. Other toxins such as Pseudomonas exotoxin A and ricin are also thought to exploit endoplasmic reticulum-associated degradation for entry into the cytosol. To test this model, we mutagenized Chinese hamster ovary cells and selected clones that survived a prolonged coincubation with Pseudomonas exotoxin A and ricin. These lethal endoplasmic reticulum-translocating toxins bind different surface receptors and target different cytosolic substrates, so resistance to both would likely result from disruption of a shared trafficking or translocation event. Here we characterize two Pseudomonas exotoxin A/ricin-resistant clones that exhibited increased endoplasmic reticulum-associated degradation. Both clones acquired the following unselected traits: (i) resistance to cholera toxin; (ii) increased degradation of an endoplasmic reticulum-localized CTA1 construct; (iii) increased degradation of an established endoplasmic reticulum-associated degradation substrate, the Z variant of alpha1-antitrypsin (alpha1AT-Z); and (iv) reduced secretion of both alpha1AT-Z and the transport-competent protein alpha1AT-M. Proteosome inhibition partially rescued the alpha1AT-M secretion deficiencies. However, the mutant clones did not exhibit increased proteosomal activity against cytosolic proteins, including a second CTA1 construct that was expressed in the cytosol rather than in the endoplasmic reticulum. These results suggested that accelerated endoplasmic reticulum-associated degradation in the mutant clones produced a cholera toxin/Pseudomonas exotoxin A/ricin-resistant phenotype by increasing the coupling efficiency between toxin translocation and toxin degradation.

Inhibition of endoplasmic reticulum-associated degradation in CHO cells resistant to cholera toxin, Pseudomonas aeruginosa exotoxin A, and ricin

Many plant and bacterial toxins act upon cytosolic targets and must therefore penetrate a membrane barrier to function. One such class of toxins enters the cytosol after delivery to the endoplasmic reticulum (ER). These proteins, which include cholera toxin (CT), Pseudomonas aeruginosa exotoxin A (ETA), and ricin, move from the plasma membrane to the endosomes, pass through the Golgi apparatus, and travel to the ER. Translocation from the ER to the cytosol is hypothesized to involve the ER-associated degradation (ERAD) pathway. We developed a genetic strategy to assess the role of mammalian ERAD in toxin translocation. Populations of CHO cells were mutagenized and grown in the presence of two lethal toxins, ETA and ricin. Since these toxins bind to different surface receptors and attack distinct cytoplasmic targets, simultaneous acquisition of resistance to both would likely result from the disruption of a shared trafficking or translocation mechanism. Ten ETA- and ricin-resistant cell lines that displayed unselected resistance to CT and continued sensitivity to diphtheria toxin, which enters the cytosol directly from acidified endosomes, were screened for abnormalities in the processing of a known ERAD substrate, the Z form of alpha1-antitrypsin (alpha1AT-Z). Compared to the parental CHO cells, the rate of alpha1AT-Z degradation was decreased in two independent mutant cell lines. Both of these cell lines also exhibited, in comparison to the parental cells, decreased translocation and degradation of a recombinant CTA1 polypeptide. These findings demonstrated that decreased ERAD function was associated with increased cellular resistance to ER-translocating protein toxins in two independently derived mutant CHO cell lines.

Transfer of the cholera toxin A1 polypeptide from the endoplasmic reticulum to the cytosol is a rapid process facilitated by the endoplasmic reticulum-associated degradation pathway

The active pool of internalized cholera toxin (CT) moves from the endosomes to the Golgi apparatus en route to the endoplasmic reticulum (ER). The catalytic CTA1 polypeptide is then translocated from the ER to the cytosol, possibly through the action of the ER-associated degradation (ERAD) pathway. Translocation was previously measured indirectly through the downstream effects of CT action. We have developed a direct biochemical assay for CTA1 translocation that is independent of toxin activity. Our assay is based upon the farnesylation of a CVIM motif-tagged CTA1 polypeptide (CTA1-CVIM) after it enters the cytosol. When expressed from a eukaryotic vector in transfected CHO cells, CTA1-CVIM was targeted to the ER, but was not secreted. Instead, it was translocated into the cytosol and degraded in a proteosome-dependent manner. Translocation occurred rapidly and was monitored by the appearance of farnesylated CTA1-CVIM in the detergent phase of cell extracts generated with Triton X-114. Detergent-phase partitioning of CTA1-CVIM resulted from the cytoplasmic addition of a 15-carbon fatty acid farnesyl moiety to the cysteine residue of the CVIM motif. Our use of the CTA1-CVIM translocation assay provided supporting evidence for the ERAD model of toxin translocation and generated new information on the timing of CTA1 translocation.

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.