Mechanism of activation of cholera toxin by ADP-ribosylation factor (ARF): both low- and high-affinity interactions of ARF with guanine nucleotides promote toxin activation

Protein Toxins That Utilize Gangliosides as Host Receptors

Subsets of protein toxins utilize gangliosides as host receptors. Gangliosides are preferred receptors due to their extracellular localization on the eukaryotic cell and due to their essential nature in host physiology. Glycosphingolipids, including gangliosides, are mediators of signal transduction within and between eukaryotic cells. Protein toxins possess AB structure-function organization, where the A domain encodes a catalytic function for the posttranslational modification of a host macromolecule, including proteins and nucleic acids, and a B domain, which encodes host receptor recognition, including proteins and glycosphingolipids, alone or in combination. Protein toxins use similar strategies to bind glycans by pockets and loops, generally employing hydrogen bonding and aromatic stacking to stabilize interactions with sugars. In some cases, glycan binding facilitates uptake, while in other cases, cross-linking or a second receptor is necessary to stimulate entry. The affinity that protein toxins have for host glycans is necessary for tissue targeting, but not always sufficient to cause disease. In addition to affinity for binding the glycan, the lipid moiety also plays an important role in productive uptake and tissue tropism. Upon endocytosis, the protein toxin must escape to another intracellular compartment or into cytosol to modify a host substrate, modulating host signaling, often resulting in cytotoxic or apoptotic events in the cell, and a unique morbidity for the organism. The study of protein toxins that utilize gangliosides as host receptors has illuminated numerous eukaryotic cellular processes, identified the basis for developing interventions to prevent disease through vaccines and control bacterial diseases through therapies. In addition, subsets of these protein toxins have been utilized as therapeutic agents to treat numerous human inflictions.

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.

Novel bacterial ADP-ribosylating toxins: structure and function

Bacterial ADP-ribosyltransferase toxins (bARTTs) transfer ADP-ribose to eukaryotic proteins to promote bacterial pathogenesis. In this Review, we use prototype bARTTs, such as diphtheria toxin and pertussis toxin, as references for the characterization of several new bARTTs from human, insect and plant pathogens, which were recently identified by bioinformatic analyses. Several of these toxins, including cholix toxin (ChxA) from Vibrio cholerae, SpyA from Streptococcus pyogenes, HopU1 from Pseudomonas syringae and the Tcc toxins from Photorhabdus luminescens, ADP-ribosylate novel substrates and have unique organizations, which distinguish them from the reference toxins. The characterization of these toxins increases our appreciation of the range of structural and functional properties that are possessed by bARTTs and their roles in bacterial pathogenesis.

Bacillus cereus Certhrax ADP-ribosylates vinculin to disrupt focal adhesion complexes and cell adhesion

Bacillus cereus is often associated with mild to moderate gastroenteritis; however, some recent isolates cause inhalational anthrax-like diseases and death. These potential emerging human pathogens express multiple virulence factors. B. cereus strain G9241 expresses anthrax toxin, several polysaccharide capsules, and the novel ADP-ribosyltransferase, Certhrax. In this study, we show that Certhrax ADP-ribosylates Arg-433 of vinculin, a protein that coordinates actin cytoskeleton and extracellular matrix interactions. ADP-ribosylation of vinculin disrupted focal adhesion complexes and redistributed vinculin to the cytoplasm. Exogenous vinculin rescued these phenotypes. This provides a mechanism for strain G9241 to breach host barrier defenses and promote bacterial growth and spread. Certhrax is the first bacterial toxin to add a post-translational modification to vinculin to disrupt the actin cytoskeleton.

[Studies on the mode of action of bacterial AB5 toxins]

Bacterial AB5 toxins are proteins, produced by pathogenic bacteria including of Vibrio cholerae, Shigella dysenteriae, and enterohaemorrhagic Escherichia coli, which are usually released into the extracellular medium and cause disease by killing or altering the metabolism of target eukaryotic cells. The toxins are usually composed of one A subunit (a toxic domain) and five B subunits (a receptor-binding domain). This article overviews the characteristics and mode of actions of AB5 toxins including cholera toxin, Shiga-like toxin, and subtilase cytotoxin, and highlights current topics related to the roles of the effectors in promoting bacterial infection.

Characterization and expression of AmphiARF gene encoding a new member of ARF family from amphioxusBranchiostoma belcheri tsingtauense

The cDNA AmphiARF, encoding a novel member of ARF family was for the first time identified from the gut cDNA library of amphioxus Branchiostoma becheri tsingtauense. The deduced amino acid sequence includes the motifs involved in GTP binding (DVGG, NKQD and SAK) and hydrolysis (GXDXXGK), and a glycine at position 2 for N-terminal myristoylation, which are all characteristic of ARFs. However, it has a 46 amino acid C-terminal extension (positions 183-227), which was not found in any ARF family previously described including ARF-like proteins. This suggests that AmphiARF is a new member of ARF family, which was also supported by the phylogenetic analysis. It is highly likely that the extended C-terminal of AmphiARF is a novel addition occurring after cephalochordate/vertebrate split. Northern blotting analysis revealed that AmphiARF transcript was present in all tissues examined, consistent with the involvement of ARFs in the fundamental role like vesicular trafficking of proteins.

Calexcitin B is a new member of the sarcoplasmic calcium-binding protein family

Calexcitin (CE) is a calcium sensor protein that has been implicated in associative learning. The CE gene was previously cloned from the long-finned squid, Loligo pealei, and the gene product was shown to bind GTP and modulate K(+) channels and ryanodine receptors in a Ca(2+)-dependent manner. We cloned a new gene from L. pealei, which encodes a CE-like protein, here named calexcitin B (CE(B)). CE(B) has 95% amino acid identity to the original form. Our sequence analyses indicate that CEs are homologous to the sarcoplasmic calcium-binding protein subfamily of the EF-hand superfamily. Far and near UV circular dichroism and nuclear magnetic resonance studies demonstrate that CE(B) binds Ca(2+) and undergoes a conformational change. CE(B) is phosphorylated by protein kinase C, but not by casein kinase II. CE(B) does not bind GTP. Western blot experiments using polyclonal antibodies generated against CE(B) showed that CE(B) is expressed in the L. pealei optic lobe. Taken together, the neuronal protein CE represents the first example of a Ca(2+) sensor in the sarcoplasmic calcium-binding protein family.

Pseudomonas aeruginosa exoenzyme S, a bifunctional type-III secreted cytotoxin

Our recent studies have shown ExoS to be a bifunctional type-III secreted cytotoxin. Intracellular expression of the amino terminus of ExoS (C234) in eukaryotic cells stimulates actin reorganization without cytotoxicity, which involves small-molecular-weight GTPases of the Rho subfamily. Expression of the carboxyl terminus of ExoS comprises an ADP-ribosyltransferase domain, which is cytotoxic when expressed in cultured cells (Pederson and Barbieri, 1998). Rho and Ras are molecular switches, which control numerous cellular processes. Recent signaling studies suggest that there is crosstalk between Rho and Ras (Keely et al, 1997). Ras and Rho also contribute to wound healing processes and tissue regeneration. Recent studies have shown that microinjection of endothelial cells with activated Ras stimulated their motility, while microinjection of Ras-blocking antibodies inhibited cellular motility that is a component of the wound healing process (Fox et al., 1994). In addition, hepatocyte growth factor/scatter factor (HGF/ SF) and epidermal growth factor stimulate cellular motility through the Ras signal transduction pathway (Ridley et al., 1995). Rac and Rho are also involved in motility and tissue regeneration, since dominant negative Rac inhibits the cellular motility stimulated by HGF/SF (Santos et al., 1997) and inhibition of Rho by either C. difficile ToxA and ToxB or the C. botulinum C3 transferase inhibits wound healing (Santos et al., 1997). Inhibition of tissue regeneration and wound healing appear to play a role in the pathogenesis of C. difficile, since treatment of gastrointestinal mucosa with C. difficile ToxA and ToxB alone inhibits regeneration of the gastric mucosa. Thus, ExoS may contribute to the establishment of P. aeruginosa infections by inhibiting wound healing and tissue regeneration by two mechanisms. The amino terminus of ExoS could inhibit Rho function and wound healing in a manner similar to C. difficile. Alternatively, ExoS could inhibit the cellular motility and angiogenesis required for wound healing by ADP-ribosylating Ras. Through the inhibition of tissue regeneration and wound healing, ExoS may play a pivotal role in chronic disease by maintaining sites of colonization. Inhibition of Ras or Rho signaling may also interfere with both innate and acquired immunity. Small-molecular-weight GTP-binding proteins of the Ras superfamily are required for cellular processes, such as phagocytosis, as Rho proteins contribute to phagocytosis (Caron and Hall, 1998). Since Ras functions upstream of Rho in cellular signaling processes (Ridley et al., 1995), ADP-ribosylation of Ras by ExoS or the inhibition of Rho function by C234 may inhibit phagocytosis of P. aeruginosa by macrophages. Other studies indicate that Ras plays a role in T cell activation (Cantrell, 1994). Thus, ExoS may inhibit acquired immunity by inhibiting T-cell activation.

The delta subunit of rod specific cyclic GMP phosphodiesterase, PDE delta, interacts with the Arf-like protein Arl3 in a GTP specific manner

Recently, we have shown that the delta subunit of the cGMP phosphodiesterase (PDE delta) interacts with the retinitis pigmentosa guanine regulator (RPGR). Here, using the two-hybrid system, we identify a member of the Arf-like protein family of Ras-related GTP-binding proteins, Arl3, that interacts with PDE delta. The interaction was verified by fluorescence spectroscopy and co-immunoprecipitation. Arl3 features an unusually low affinity for guanine nucleotides, with a KD of 24 nM for GDP and 48 microM for GTP. Fluorescence spectroscopy shows that PDE delta binds and specifically stabilizes the GTP-bound form of Arl3 by strongly decreasing the dissociation rate of GTP. Thus, PDE delta is an effector of Arl3 and could provide a novel nucleotide exchange mechanism by which PDE delta stabilizes Arl3 in its active GTP-bound form.

Phospholipid- and GTP-dependent activation of cholera toxin and phospholipase D by human ADP-ribosylation factor-like protein 1 (HARL1)

ADP-ribosylation factors (ARFs), 20-kDa guanine nucleotide-binding proteins named for their ability to activate cholera toxin (CT) ADP-ribosyltransferase activity, have a critical role in vesicular transport and activate a phospholipase D (PLD) isoform. Although ARF-like (ARL) proteins are very similar in sequence to ARFs, they were initially believed not to activate CT or PLD. mRNA for human ARL1 (hARL1), which is 57% identical in amino acid sequence to hARF1, is present in all tissues, with the highest amounts in kidney and pancreas and barely detectable amounts in brain. Relative amounts of hARL1 protein were similar to mRNA levels. Purified hARL1 (rARL1) synthesized in Escherichia coli had less activity toward PLD than did rARF1, although PLD activation by both proteins was guanosine guanosine 5'-(gamma-thio)triphosphate (GTPgammaS)-dependent. ARL1 stimulation of CT-catalyzed ADP-ribosylation was considerably less than that by rARF1 and was phospholipid dependent. GTPgammaS-binding by rARL1 was also phospholipid- and detergent-dependent, and in assays containing phosphatidylserine, was greater than that by rARF1. In vitro, the activities of rARL1 and rARF1 are similar. Rather than being a member of a separate subfamily, hARL1, which activates PLD and CT in a phospholipiddependent manner, appears to be part of a continuum of ARF family proteins.

Characterization of an ADP-ribosylation factor-like 1 protein in Saccharomyces cerevisiae

ADP-ribosylation factors (ARFs) are highly conserved approximately 20-kDa guanine nucleotide-binding proteins that enhance the ADP-ribosyltransferase activity of cholera toxin and are believed to participate in vesicular transport in both exocytic and endocytic pathways. Several ARF-like proteins (ARLs) have been cloned from Drosophila, rat, and human; however, the biological functions of ARLs are unknown. We have identified a yeast gene (ARL1) encoding a protein that is structurally related (>60% identical) to human, rat, and Drosophila ARL1. Biochemical analyses of purified recombinant yeast ARL1 (yARL1) protein revealed properties similar to those ARF and ARL1 proteins, including the ability to bind and hydrolyze GTP. Like other ARLs, recombinant yARL1 protein did not stimulate cholera toxin-catalyzed auto-ADP-ribosylation. yARL1 was not recognized by antibodies against mammalian ARLs or yeast ARFs. Anti-yARL1 antibodies did not cross-react with yeast ARFs, but did react with human ARLs. On subcellular fractionation, yARL1, similar to yARF1, was localized to the soluble fraction. The amino terminus of yARL1, like that of ARF, was myristoylated. Unlike Drosophila Arl1, yeast ARL1 was not essential for cell viability. Like rat ARL1, yARL1 might be associated in part with the Golgi complex. However, yARL1 was not required for endoplasmic reticulum-to-Golgi protein transport, and it may offer an opportunity to define an ARL function in another kind of vesicular trafficking, such as the regulated secretory pathway.

Interspecies relationships among ADP-ribosylation factors (ARFs): evidence of evolutionary pressure to maintain individual identities

ADP-ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that are allosteric activators of the NAD:arginine ADP-ribosyltransferase activity of cholera toxin and appear to play a role in intracellular vesicular trafficking. Although the physiological roles of these proteins have not been defined, it has been presumed that each has a specific intracellular function. To obtain genetic evidence that each ARF is under evolutionary pressure to maintain its structure, and presumably function, rat ARF cDNA clones were isolated and their nucleotide and deduced amino acid sequences were compared to those of other mammalian ARFs. Deduced amino acid sequences for rat ARFs 1, 2, 3, 5 and 6 were identical to those of the known cognate human and bovine ARFs; rat ARF4 was 96% identical to human ARF4. Nucleotide sequences of both the untranslated as well as the coding regions were highly conserved. These results indicate that the ARF proteins are, as a family, extraordinarily well conserved across mammalian species. The unusually high degree of conservation of the untranslated regions is consistent with these regions having important regulatory roles and that individual ARFs contain structurally unique elements required for specific functions.

Functional domains of Pseudomonas aeruginosa exoenzyme S

Recombinant exoenzyme S (rHisExoS) of Pseudomonas aeruginosa was expressed in Escherichia coli as a soluble, cytosolic His fusion protein. rHisExoS was purified by Ni(2+)-affinity chromatography in the presence of protease inhibitors without detectable degradation. rHisExoS possessed a specific activity (within twofold) for the factor-activating exoenzyme S-dependent ADP-ribosylation of soybean trypsin inhibitor (SBTI) similar to that of native exoenzyme S. Analysis of several deletion peptides showed that delta N222, which encoded the carboxyl-terminal 222 amino acids of exoenzyme S, possessed factor-activating exoenzyme S-dependent ADP-ribosyltransferase activity. delta N222 catalyzed the ADP-ribosylation of SBTI at a rate sixfold greater than rHisExoS. Relative to rHisExoS, delta N222 had a similar affinity for NAD, a threefold greater affinity for SBTI, and a four- to eightfold greater kcat for the ADP-ribosylation of SBTI. Like native exoenzyme S, rHisExoS chromatographed as an aggregate with an apparent molecular mass of > 300 kDa. In contrast, delta N222 did not chromatograph as an aggregate, which showed that the amino-terminal 99 amino acids of exoenzyme S were responsible for the aggregation phenotype.

Isolation of an amino-terminal deleted recombinant ADP-ribosylation factor 1 in an activated nucleotide-free state

ADP-ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin ADP-ribosyltransferase in vitro and participate in intracellular vesicular membrane trafficking. ARFs are activated when bound GDP is replaced by GTP and inactivated by hydrolysis of bound GTP to yield ARF-GDP. Usually, ARFs are isolated in an inactive GDP-bound state and require addition of GTP along with detergent or phospholipid for activity. Purified mutant recombinant ARF1 lacking the first 13 amino acids (r delta 13ARF1-P) stimulated cholera toxin activity essentially equally with or without added GTP (and phospholipid or detergent), at least in part due to the presence of bound nucleotides, which later were identified as GTP and GDP. Nucleotide-free r delta 13ARF1 (r delta 13ARF1-F), prepared by dialysis against 7 M urea, was active without added GTP in the absence of SDS but inactive without added GTP in its presence. Renaturation of r delta 13ARF1-F in the presence of GTP, ITP, or GDP yielded, respectively, r delta 13ARF1-GTP and r delta 13ARF1-ITP, which were active, and r delta 13ARF1-GDP, which was inactive. Effects of phospholipids and detergents on nucleotide exchangeability evaluated as effects on activity of rARF1 and r delta 13ARF1-F differed. With r delta 13ARF1-F, 100 microM ITP and 100 microM GTP were essentially equally effective in the presence of cardiolipin or SDS. The finding that r delta 13ARF1 differs from rARF1 in the effects of phospholipids and detergents on nucleotide binding is consistent with the conclusion that the ARF amino terminus plays an important role in nucleotide binding and its specificity as well as the molecular conformation and associated activity.

Activation of rat brain phospholipase D by ADP-ribosylation factors 1,5, and 6: separation of ADP-ribosylation factor-dependent and oleate-dependent enzymes

Two major forms of phospholipase D (PLD) activity, solubilized from rat brain membranes with Triton X-100, were separated by HPLC on a heparin-5PW column with buffer containing octyl glucoside. One form was completely dependent on sodium oleate for activity. The other, which was dramatically activated by the addition of ADP-ribosylation factor (ARF) 1 and guanine 5' [gamma-thio]triphosphate, required the presence of phosphatidylinositol 4,5-bisphosphate in the phosphatidylcholine substrate for demonstration of activity, as described by others. Oleate-dependent activity was unaffected by guanine 5' [gamma-thio]triphosphate, or phosphatidylinositol 4,5-bisphosphate. Both sodium oleate-and ARF-dependent activities catalyzed transphosphatidylation, thus identifying them as PLDs. ARF-dependent PLD was activated by recombinant ARF5 (class II) and ARF6 (class III), as well as ARF1 (class I). Myristoylated recombinant ARFs were more effective than their nonmyristoylated counterparts. ARFs were originally identified as activators of cholera toxin ADP-ribosyltransferase activity. The effects of recombinant ARF proteins from the three classes on cholera toxin activity (assayed under conditions identical to those used to assay PLD activity) did not, however, correlate with those on PLD, consistent with the notion that different aspects of ARF structure are involved in the two functions.

ADP-ribosylation factors: a family of approximately 20-kDa guanine nucleotide-binding proteins that activate cholera toxin

ADP-ribosylation factors (ARFs) comprise a family of approximately 20 kDa guanine nucleotide-binding proteins that were discovered as one of several cofactors required in cholera toxin-catalyzed ADP-ribosylation of Gs alpha, the guanine nucleotide-binding protein responsible for stimulation of adenylyl cyclase, and was subsequently found to enhance all cholera toxin-catalyzed reactions and to directly interact with, and activate the toxin. ARF is dependent on GTP or its analogues for activity, binds GTP with high affinity in the presence of dimyristoylphosphatidylcholine/cholate and contains consensus sequences for GTP-binding and hydrolysis. Six mammalian family members have been identified which have been classified into three groups (Class I, II, and III) based on size, deduced amino acid sequence identity, phylogenetic analysis and gene structure. ARFs are ubiquitous among eukaryotes, with a deduced amino acid sequence that is highly conserved across diverse species. They have recently been shown to associate with phospholipid and Golgi membranes in a GTP-dependent manner and are involved in regulating vesicular transport.

Characterization of recombinant and endogenous ADP-ribosylation factors synthesized in Sf9 insect cells

ADP-ribosylation factors (ARFs) are a family of highly conserved, 20-kDa guanine nucleotide-binding proteins that participate in protein trafficking and enhance cholera toxin-catalyzed ADP-ribosylation. ARF 2 from bovine retinal cDNA was expressed in Sf9 insect cells using recombinant baculovirus and compared to the major insect cell ARF (Sf9 ARF) and to recombinant ARF 2 expressed in Escherichia coli (E. coli rARF 2). The 150000g supernatant and particulate fractions of freeze-thawed, recombinant ARF 2 baculovirus-infected cells contained immunoreactive proteins of 20 and 21 kDa at significantly higher levels than were found in uninfected cells. Infected Sf9 cells incorporated [3H]myristate only into the 20-kDa protein. Sf9 cell recombinant ARF 2 (Sf9 rARF 2) and Sf9 ARF were separated by isoelectric focusing or ion-exchange chromatography and identified by microsequencing of HPLC-purified tryptic peptides. Sf9 ARF displayed considerable sequence identity to mammalian class I ARFs. Both Sf9 ARF and Sf9 rARF 2 stimulated in a GTP-dependent manner cholera toxin-catalyzed ADP-ribosylation. The Ka for GTP of Sf9 ARF was, however, significantly lower than that of Sf9 rARF 2 or E. coli rARF 2. Myristoylation did not significantly affect the ability of ARF 2 to enhance cholera toxin-catalyzed ADP-ribosylation or the Ka for GTP. Despite the sequence identities and the fact that both were synthesized in insect cells, the endogenous Sf9 ARF was functionally different from Sf9 rARF 2.

Enhancement of Clostridium botulinum C3-catalysed ADP-ribosylation of recombinant rhoA by sodium dodecyl sulfate

The influence of sodium dodecyl sulfate (SDS) on ADP-ribosylation by Clostridium botulinum C3 exoenzyme (C3) was studied. SDS increased the ADP-ribosylation of recombinant rhoA and human platelet cytosolic proteins maximally at 0.01% whereas higher concentrations of the detergent (> 0.01%) inhibited the ADP-ribosylation. In contrast, ADP-ribosylation of human platelet membranes and of recombinant rhoB was inhibited by the detergent. The Km for NAD of the ADP-ribosylation of rhoA was decreased by SDS from about 10 to 0.6 microM. Whereas in the absence of SDS, the C3-induced ADP-ribosylation of recombinant rhoA is not affected by the amphiphilic wasp venom mastoparan, in the presence of SDS (0.01%) mastoparan (100 microM) inhibited the ADP-ribosylation. C3-associated NAD-glycohydrolase activity was maximally and half-maximally inhibited by 0.1 and 0.013% SDS, respectively. Inhibition of NAD-glycohydrolase activity was reversed by diluting out SDS indicating that C3 was not irreversibly denatured by SDS treatment. SDS (0.01%) completely inhibited the [3H]GTP binding of rhoA whereas the release of previously bound nucleotide was not affected. The data indicate that changes in the lipophilicity of rhoA protein largely affect its ability to serve as a substrate for C3-like ADP-ribosyltransferases.

Requirement for a GTPase-activating protein in vesicle budding from the endoplasmic reticulum

The binding and hydrolysis of guanosine triphosphate (GTP) by the small GTP-binding protein Sar1p is required to form transport vesicles from the endoplasmic reticulum (ER) in Saccharomyces cerevisiae. Experiments revealed that an interaction between Sar1p and the Sec23p subunit of an oligomeric protein is also required for vesicle budding. The isolated Sec23p subunit and the oligomeric complex stimulated guanosine triphosphatase (GTPase) activity of Sar1p 10- to 15-fold but did not activate two other small GTP-binding proteins involved in vesicle traffic (Ypt1p and ARF). Activation of GTPase was inhibited by an antibody to Sec23p but not by an antibody that inhibits the budding activity of the other subunit of the Sec23p complex. Also, activation was thermolabile in pure samples of Sec23p that were isolated from two independent sec23 mutant strains. It appears that Sec23p represents a new class of GTPase-activating protein because its sequence shows no similarity to any known member of this family.

Effects of temperature on ADP-ribosylation factor stimulation of cholera toxin activity

The effects of cholera toxin, a secretory product of Vibrio cholerae, result from ADP-ribosylation of the stimulatory guanine nucleotide-binding (Gs) protein of the adenylyl cyclase system. Cholera toxin A subunit (CTA) also uses agmatine, a simple guanidino compound, several proteins unrelated to Gs, and CTA itself as alternative ADP-ribose acceptors. The effects of toxin occur in the jejunum presumably at body core temperature. With agmatine as a model substrate, the optimal temperature for CTA-catalyzed ADP-ribosylation was 25-30 degrees C, and that for CTA-catalyzed auto-ADP-ribosylation was 20-25 degrees C. Both activities were significantly less at 37 degrees C, reflecting lower initial velocities, not heat-inactivation of the toxin. All the transferase activities of CTA are enhanced by ADP-ribosylation factors (ARFs), approximately 20-kDa guanine nucleotide-binding proteins that are ubiquitous in mammalian cells. Phospholipids and a soluble brain ARF, in a GTP-dependent manner, activated toxin NAD:agmatine ADP-ribosyltransferase activity; their simultaneous effect was maximal at physiological temperatures (approximately 37 degrees C). At lower temperatures, the stimulation by ARF was much less. There were similar effects on other toxin-catalyzed reactions, notably, the ADP-ribosylation of Gs alpha and the hydrolysis of NAD. Thus, host factors, such as ARF and phospholipid, synergistically increase cholera toxin activity at 37 degrees C and may be important in toxin action in the mammalian gut.

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.

Evidence for G proteins in rat parotid plasma membranes and secretory granule membranes

G proteins were identified in rat parotid plasma membrane-enriched fractions and in two populations of isolated secretory granule membrane fractions. Both [32P]ADP-ribosylation analysis with bacterial toxins and immunoblot analysis with crude and affinity-purified antisera specific for alpha subunits of G proteins were utilized. Pertussis toxin catalysed the ADP-ribosylation of a 41 kDa substrate in the plasma membrane fraction and both secretory granule membrane fractions. Cholera toxin catalysed the ADP-ribosylation of two substrates with molecular masses of 44 kDa and 48 kDa in the plasma membrane fraction but not in the secretory granule fractions. However, these substrates were detected in the secretory granule fractions when recombinant ADP-ribosylating factor was present in the assay medium. Immunoblot analysis of rat parotid membrane fractions using both affinity-purified and crude antisera revealed strong immunoreactivity of these membranes with anti-Gs alpha, -Gi alpha 1/alpha 2 and -Gi alpha 3 sera. In contrast Gs alpha was the major substrate found in both of the secretory granule fractions. Granule membrane fractions also reacted moderately with anti-Gi alpha 3 antiserum, and weakly with anti-Gi alpha 1/alpha 2 and -G(o) alpha sera. The results demonstrate that the parotid gland membranes express a number of G proteins. The presence of G proteins in secretory granule membranes suggests that they may play a direct role in regulating exocytosis in exocrine glands.

Activation of cholera toxin and Escherichia coli heat-labile enterotoxins by ADP-ribosylation factors, a family of 20 kDa guanine nucleotide-binding proteins

Cholera toxin and Escherichia coli heat-labile enterotoxins are responsible, in part, for the symptomatology of cholera and traveller's diarrhoea, respectively. Effects of the toxins result from ADP-ribosylation of regulatory guanine nucleotide-binding (G) proteins; the ADP-ribosylated G protein is stabilized in an activated state, resulting in prolonged effects on its target. Toxin-catalysed ADP-ribosylation is stimulated in vitro by a family of guanine nucleotide-binding proteins, c. 20 kDa, termed ADP-ribosylation factors or ARFs. In the presence of GTP, but not GDP or adenine analogues, ARFs serve as allosteric activators of the toxin. The effects are amplified by certain phospholipids and detergents which promote guanine nucleotide binding. Six different mammalian ARF genes have been identified. They encode highly conserved, ubiquitous proteins of 175 to 181 amino acids, containing consensus domains responsible for guanine nucleotide binding. Differences in amino acid sequences are localized near the amino terminus and in the carboxy half of the protein. Although the physiological functions of ARFs have not been precisely defined, their immunological localization to the Golgi is consistent with a role in the regulated orderly movement of newly synthesized proteins from the endoplasmic reticulum, through the Golgi system to their ultimate destination.

Activation of Escherichia coli heat-labile enterotoxins by native and recombinant adenosine diphosphate-ribosylation factors, 20-kD guanine nucleotide-binding proteins

Escherichia coli heat-labile enterotoxins (LT) are responsible in part for "traveler's diarrhea" and related diarrheal illnesses. The family of LTs comprises two serogroups termed LT-I and LT-II; each serogroup includes two or more antigenic variants. The effects of LTs result from ADP ribosylation of Gs alpha, a stimulatory component of adenylyl cyclase; the mechanism of action is identical to that of cholera toxin (CT). The ADP-ribosyltransferase activity of CT is enhanced by 20-kD guanine nucleotide-binding proteins, known as ADP-ribosylation factors or ARFs. These proteins directly activate the CTA1 catalytic unit and stimulate its ADP ribosylation of Gs alpha, other proteins, and simple guanidino compounds (e.g., agmatine). Because of the similarities between CT and LTs, we investigated the effects of purified bovine brain ARF and a recombinant form of bovine ARF synthesized in Escherichia coli on LT activity. ARF enhanced the LT-I-, LT-IIa-, and LT-IIb-catalyzed ADP ribosylation of agmatine, as well as the auto-ADP ribosylation of the toxin catalytic unit. Stimulation of ADP-ribosylagmatine formation by LTs and CT in the presence of ARF was GTP dependent and enhanced by sodium dodecyl sulfate. With agmatine as substrate, LT-IIa and LT-IIb exhibited less than 1% the activity of CT and LT-Ih. CT and LTs catalyzed ADP-ribosyl-Gs alpha formation in a reaction dependent on ARF, GTP, and dimyristoyl phosphatidylcholine/cholate. With Gs alpha as substrate, the ADP-ribosyltransferase activities of the toxins were similar, although CT and LT-Ih appeared to be slightly more active than LT-IIa and LT-IIb. Thus, LT-IIa and LT-IIb appear to differ somewhat from CT and LT-Ih in substrate specificity. Responsiveness to stimulation by ARF, GTP, and phospholipid/detergent as well as the specificity of ADP-ribosyltransferase activity are functions of LTs from serogroups LT-I and LT-II that are shared with CT.