A family of killer toxins. Exploring the mechanism of ADP-ribosylating toxins

The ADP-ribosylating toxins (ADPRTs) are a family of toxins that catalyse the hydrolysis of NAD and the transfer of the ADP-ribose moiety onto a target. This family includes many notorious killers, responsible for thousands of deaths annually including: cholera, enterotoxic Escherichia coli, whooping cough, diphtheria and a plethora of Clostridial binary toxins. Despite their notoriety as pathogens, the ADPRTs have been extensively used as cellular tools to study and elucidate the functions of the small GTPases that they target. There are four classes of ADPRTs and at least one structure representative of each of these classes has been determined. They all share a common fold and several motifs around the active site that collectively facilitate the binding and transfer of the ADP-ribose moiety of NAD to their protein targets. In this review, we present an overview of the physiology and cellular qualities of the bacterial ADPRTs and take an in-depth look at the structural motifs that differentiate the different classes of bacterial ADPRTs in relation to their function.     

Bacterial toxins modifying the actin cytoskeleton.

Numerous bacterial toxins recognize the actin cytoskeleton as a target. The clostridial binary toxins (Iota and C2 families) ADP-ribosylate the actin monomers causing the dissociation of the actin filaments. The large clostridial toxins from Clostridium difficile, Clostridium sordellii and Clostridium novyi inactivate, by glucosylation, proteins from the Rho family that regulate actin polymerization. In contrast, the cytotoxic necrotic factor from Escherichia coli activates Rho by deamidation and increases the formation of actin filaments. The enterotoxin of Bacteroides fragilis is a protease specific for E-cadherin and it promotes the reorganization of the actin cytoskeleton. The bacterial toxins that modify the actin cytoskeleton induce various cell disfunctions including changes in cell barrier permeability and disruption of intercellular junctions.     

Alkaloid toxins in endophyte-infected grasses.

Grasses infected with clavicipitaceous fungi have been associated with a variety of diseases including classical ergotism in humans and animals, fescue foot and summer syndrome in cattle, and rye-grass staggers in sheep. During the last decade it has been recognized that many of these fungal infections are endophytic; a fungal endophyte is a fungus that grows entirely within the host plant. Inspection of field collections and herbarium specimens has revealed that such infections are widespread in grasses. The chemistry associated with these grass-fungal interactions has proved to be interesting and complex, as each grass-fungal pair results in a unique "fingerprint" of various alkaloids, of which some are highly toxic to herbivores. In many cases the presence of an endophyte appears to benefit the plant by increasing drought resistance, or by increasing resistance to attack by insects, thus improving the overall survivability of the grass. This review will focus on alkaloids that have been reported in endophyte-infected grasses.     

Mollusc-specific toxins from the venom of Conus textile neovicarius.

Three peptide toxins exhibiting strong paralytic activity to molluscs, but with no paralytic effects on arthropods or vertebrates, were purified from the venom of the molluscivorous snail Conus textile neovicarius from the Red Sea. The amino acid sequences of these mollusc specific toxins are: TxIA, WCKQSGEMCNLLDQNCCDGYCI-VLVCT (identical to the so called 'King Kong peptide'); TxIB, WCKQSGEMCNVLDQNCCDGYCIVFVCT; TxIIA, WGGYSTYC gamma VDS gamma CCSDNCVRSYCT (gamma = gamma-carboxyglutamate). There is a similarity of the Cys framework of these toxins to that of the omega-conotoxins; however, their net negative charges, high content of hydrophobic residues and uneven number of Cys residues in TxIIA, are highly unusual for conotoxins. When assayed on isolated cultured Aplysia neurons, all three toxins induced membrane depolarization and spontaneous repetitive firing. The TxI toxins also induce a marked prolongation of the action potential duration, which is sodium dependent. These effects differ significantly from the blocking activities of piscivorous venom conotoxins. These mollusc specific conotoxins may therefore serve as new and selective probes for ion-channel functions in molluscan neuronal systems.     

ADP-ribosylation of gelsolin-actin complexes by clostridial toxins.

ADP-ribosylation of the 1:1 (G-A) and 1:2 (G-A-A) gelsolin-actin complexes by Clostridium perfringens iota toxin and Clostridium botulinum C2 toxin was studied. Iota toxin ADP-ribosylated actin in the G-A complex from human platelets as effectively as skeletal muscle actin. The Km for NAD (4 microM) was identical for both substrates. C2 toxin ADP-ribosylated actin in the G-A complex with lower efficacy than nonmuscle actin from platelet cytosol. In the G-A-A complex both actin molecules were ADP-ribosylated by iota toxin. The G-A complex bound ADP-ribosylated actin (Ar) to form the G-A-Ar complex in which the weakly bound actin is ADP-ribosylated. Vice versa, ADP-ribosylated 1:1 gelsolin-actin complex (G-Ar) was able to bind unmodified actin to yield the G-Ar-A complex. ADP-ribosylation did not change the nucleation activity of either the G-Ar complex or the G-Ar-A complex. When monomeric actin was added to the G-A-Ar complex, polymerization of actin was delayed by about 10 min. According to a quantitative kinetic analysis, the delay of polymerization corresponded to the rate of dissociation of ADP-ribosylated actin from the G-A-Ar complex. This suggests that the nucleation activity of the G-A-A complex is inhibited by ADP-ribosylation of the weakly bound actin and that the inhibition can be removed by dissociation of ADP-ribosylated actin from the G-A-Ar complex.     

Pregnancy interrupting effects of some bacterial toxins.

Embryotoxic properties of Shigella dysenteriae and Clostridium perfringens toxins, of E. coli endotoxin, V. cholerase and E. coli enterotoxins were compared in mice. E. coli endotoxin has embryotoxic effects at all stages of pregnancy. E. coli enterotoxin V. cholerae enterotoxin and Shigella dysenteriae toxin are most effective mainly at earlier stages of pregnancy. Clostridium perfringens toxin has no embryotoxic effect.     

Natural occurrence of Fusarium toxins in feedstuff.

The mycotoxins diacetoxyscirpenol, deoxynivalenol, and zearalenone, produced by Fusarium roseum, were found naturally occuring in mixed feed samples. In all cases analyzed, deoxynivalenol occurred together with zearalenone. The natural occurrence of zearalenone in sesame seed is reported for the first time. Strains of F. roseum isolated in various parts of the world form feed implicated in animal mycotoxicosis produced monoacetoxyscirpenol, diacetoxyscirpenol, deoxynivalenol, and zearalenone.     

Evaluation of synergism among Bacillus thuringiensis toxins.

A simple test for synergism among toxins is described and applied to previously reported data on independent and joint toxicities of insecticidal proteins from Bacillus thuringiensis. The analysis shows synergism between a 27-kDa (CytA) toxin and 130- or 65-kDa (CryIV) toxins from B. thuringiensis subsp. israelensis against Aedes aegypti larvae. No positive synergism between 130- and 65-kDa toxins or among three CryIA toxins tested against seven species of Lepidoptera occurred. Comparisons with the original interpretations of these data show one case in which synergism occurred but was reported previously as absent and two cases that were not synergistic but were reported previously as suggestive of synergism. These results show that lack of an appropriate test for synergism can produce misleading conclusions. The methods described here can be used to test for synergistic effects of any poisons.