ARF binds the C-terminal region of the Escherichia coli heat-labile toxin (LTA1) and competes for the binding of LTA2

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.

Reverse two-hybrid techniques in the yeast Saccharomyces cerevisiae

Use of the yeast two-hybrid system has provided definition to many previously uncharacterized pathways through the identification and characterization of novel protein-protein interactions. The two-hybrid system uses the bifunctional nature of transcription factors, such as the yeast enhancer Gal4, to allow protein-protein interactions to be monitored through changes in transcription of reporter genes. Once a positive interaction has been identified, either of the interacting proteins can be mutated by site-specific or randomly introduced changes, to produce proteins with a decreased ability to interact. Mutants generated using this strategy are very powerful reagents in tests of the biological significance of the interaction and in defining the residues involved in the interaction. Such techniques are termed reverse two-hybrid methods. We describe a reverse two-hybrid method that generates loss-of-interaction mutations of the catalytic subunit of the Escherichia coli heat-labile toxin (LTA1) with decreased binding to the active (GTP-bound) form of human ARF3, its protein cofactor. While newer methods are emerging for performing interaction screens in mammalian cells, instead of yeast, the use of reverse two-hybrid in yeast remains a robust and powerful means of identifying loss-of-interaction point mutants and compensating changes that remain among the most powerful tools of testing the biological significance of a protein-protein interaction.

ARF-Like (ARL) Proteins

The ARF-like (ARL) proteins, within the ARF family, are a collection of functionally diverse GTPases that share extensive (>40 %) identity with the ARFs and each other and are assumed to share basic mechanisms of regulation and a very incompletely documented degree of overlapping regulators. At least four ARLs were already present in the last eukaryotic common ancestor, along with one ARF, and these have been expanded to >20 members in mammals. We know little about the majority of these proteins so our review will focus on those about which the most is known, including ARL1, ARL2, ARL3, ARL4s, ARL6, ARL13s, and ARFRP1. From this fragmentary information we extract some generalizations and conclusions regarding the sources and extent of specificity and functions of the ARLs.

Characterization of a mutant Escherichia coli heat-labile toxin, LT(R192G/L211A), as a safe and effective oral adjuvant

Despite the fact that the adjuvant properties of the heat-labile enterotoxins of Escherichia coli (LT) and Vibrio cholerae (CT) have been known for more than 20 years, there are no available oral vaccines containing these molecules as adjuvants, primarily because they are both very potent enterotoxins. A number of attempts with various degrees of success have been made to reduce or eliminate the enterotoxicity of LT and CT so they can safely be used as oral adjuvants or immunogens. In this report we characterize the structural, enzymatic, enterotoxic, and adjuvant properties of a novel mutant of LT, designated LT(R192G/L211A), or dmLT. dmLT was not sensitive to trypsin activation, had reduced enzymatic activity for induction of cyclic AMP in Caco-2 cells, and exhibited no enterotoxicity in the patent mouse assay. Importantly, dmLT retained the ability to function as an oral adjuvant for a coadministered antigen (tetanus toxoid) and to elicit anti-LT antibodies. In vitro and in vivo data suggest that the reduced enterotoxicity of this molecule compared to native LT or the single mutant, LT(R192G), is a consequence of increased sensitivity to proteolysis and rapid intracellular degradation in mammalian cells. In conclusion, dmLT is a safe and powerful detoxified enterotoxin with the potential to function as a mucosal adjuvant for coadministered antigens and to elicit anti-LT antibodies without undesirable side effects.

Cholera toxin: A paradigm for multi-functional engagement of cellular mechanisms (Review)

Cholera toxin (Ctx) from Vibrio cholerae and its closely related homologue, heat-labile enterotoxin (Etx) from Escherichia coli have become superb tools for illuminating pathways of cellular trafficking and immune cell function. These bacterial protein toxins should be viewed as conglomerates of highly evolved, multi-functional elements equipped to engage the trafficking and signalling machineries of cells. Ctx and Etx are members of a larger family of A-B toxins of bacterial (and plant) origin that are comprised of structurally and functionally distinct enzymatically active A and receptor-binding B sub-units or domains. Intoxication of mammalian cells by Ctx and Etx involves B pentamer-mediated receptor binding and entry into a vesicular pathway, followed by translocation of the enzymatic A1 domain of the A sub-unit into the target cell cytosol, where covalent modification of intracellular targets leads to activation of adenylate cyclase and a sequence of events culminating in life-threatening diarrhoeal disease. Importantly, Ctx and Etx also have the capacity to induce a wide spectrum of remarkable immunological processes. With respect to the latter, it has been found that these toxins activate signalling pathways that modulate the immune system. This review explores the complexities of the cellular interactions that are engaged by these bacterial protein toxins, and highlights some of the new insights to have recently emerged.

Order-disorder-order transitions mediate the activation of cholera toxin

Cholera toxin (CT) holotoxin must be activated to intoxicate host cells. This process requires the intracellular dissociation of the enzymatic CTA1 domain from the holotoxin components CTA2 and B5, followed by subsequent interaction with the host factor ADP ribosylation factor 6 (ARF6)-GTP. We report the first NMR-based solution structural data for the CT enzymatic domain (CTA1). We show that this free enzymatic domain partially unfolds at the C-terminus and binds its protein partners at both the beginning and the end of this activation process. Deviations from random coil chemical shifts (Delta delta(coil)) indicate helix formation in the activation loop, which is essential to open the toxin's active site and occurs prior to its association with human protein ARF6. We performed NMR titrations of both free CTA1 and an active CTA1:ARF6-GTP complex with NAD(+), which revealed that the formation of the complex does not significantly enhance NAD(+) binding. Partial unfolding of CTA1 is further illustrated by using 4,4'-bis(1-anilinonaphthalene 8-sulfonate) fluorescence as an indicator of the exposed hydrophobic character of the free enzyme, which is substantially reduced when bound to ARF6-GTP. We propose that the primary role of ARF6's allostery is to induce refolding of the C-terminus of CTA1. Thus, as a folded globular toxin complex, CTA1 escapes the chaperone and proteasomal components of the endoplasmic reticulum associated degradation pathway in the cytosol and then proceeds to ADP ribosylate its target G(s)alpha, triggering the downstream events associated with the pathophysiology of cholera.

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.

Mucosal adjuvants

Induction of immune responses following oral immunization is frequently dependent upon the co-administration of appropriate adjuvants that can initiate and support the transition from innate to adaptive immunity. The three bacterial products with the greatest potential to function as mucosal adjuvants are the ADP-ribosylating enterotoxins (cholera toxin and the heat-labile enterotoxin of Escherichia coli), synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides (CpG ODN), and monophosphoryl lipid A (MPL). The mechanism of adjuvanticity of the ADP-ribosylating enterotoxins is the subject of considerable debate. Our own view is that adjuvanticity is an outcome and not an event. It is likely that these molecules exert their adjuvant function by interacting with a variety of cell types, including epithelial cells, dendritic cells, macrophages, and possibly B- and T-lymphocytes. The adjuvant activities of CpG and MPL are due to several different effects they have on innate and adaptive immune responses and both MPL and CpG act through MyD88-dependent and -independent pathways. This presentation will summarize the probable mechanisms of action of these diverse mucosal adjuvants and discuss potential synergy between these molecules for use in conjunction with plant-derived vaccines.

Cellular hijacking: a common strategy for microbial infection

Eukaryotic cells are under constant attack from microbial intruders seeking a selective advantage for survival, propagation and dissemination. Microbial infections can often result in disease and might even be lethal to the host if they are not combatted effectively. Studies of host-pathogen interactions have revealed that virulence often requires the usurpation of existing cell signaling pathways or membrane traffic machinery of the host. Such studies provide a rich source of cell biological data that will probably prove essential for future efforts designed to either thwart these attacks or learn from them.

The Escherichia coli heat labile toxin binds to Golgi membranes and alters Golgi and cell morphologies using ADP-ribosylation factor-dependent processes

The fate of the catalytic subunit of the Escherichia coli heat labile toxin (LTA(1)) was studied after expression in mammalian cells to assess the requirement for ADP-ribosylation factor (ARF) binding to localization and toxicity and ability to compete with endogenous ARF effectors. A progression in LTA(1) localization from cytosol to binding Golgi stacks to condensation of Golgi membranes was found to correlate with the time and level of LTA(1) expression. At the highest levels of LTA(1) expression the staining of LTA and both extrinsic and lumenal Golgi markers all became diffuse, in a fashion reminiscent of the actions of brefeldin A. Thus, LTA(1) binds to the Golgi and can alter its morphology in two distinct ways. However, point mutants of LTA(1) that are defective in the ability to bind activated ARF were also unable to bind Golgi membranes or modify Golgi morphology. Co-expression of mutants of ARF3 that regained binding to these same mutant LTA(1) proteins restored the localization and activities of the toxin. Thus, binding to ARF is required both for the localization of the toxin to the Golgi and for effects on Golgi membranes. A correlation was also seen between the ability of LTA mutants to bind ARF and the increase in cellular cAMP levels. These results demonstrate the importance of ARF binding to the toxicity and cellular effects of the ADP-ribosylating bacterial toxin and reveal that mutants defective in binding ARF retain basal ADP-ribosylation activity but are the least toxic LTA(1) mutants yet described, making them the best candidates for development as mucosal adjuvants.

ADP-ribosylation factors (ARFs) and ARF-like 1 (ARL1) have both specific and shared effectors: characterizing ARL1-binding proteins

Despite the 40-60% identity between ADP-ribosylation factors (ARFs) and ARF-like (ARL) proteins, distinct functional roles have been inferred from findings that ARLs lack the biochemical or genetic activities characteristic of ARFs. The potential for functional overlap between ARFs and ARLs was examined by comparing effects of expression on intact cells and the ability to bind effectors. Expression of [Q71L]ARL1 in mammalian cells led to altered Golgi structure similar to, but less dramatic than, that reported previously for [Q71L]ARF1. Two previously identified partners of ARFs, MKLP1 and Arfaptin2/POR1, also bind ARL1 but not ARL2 or ARL3. Two-hybrid screens of human cDNA libraries with dominant active mutants of human ARL1, ARL2, and ARL3 identified eight different but overlapping sets of binding partners. Specific interactions between ARL1 and two binding proteins, SCOCO and Golgin-245, are defined and characterized in more detail. Like ARFs and ARL1, the binding of SCOCO to Golgi membranes is rapidly reversed by brefeldin A, suggesting the presence of a brefeldin A-sensitive ARL1 exchange factor. These data reveal a complex network of interactions between GTPases in the ARF family and their effectors and reveal a potential for cross-talk not demonstrated previously.