Clostridium difficile
is the cause of antibiotics-associated diarrhea and pseudomembranous colitis. The pathogen produces three protein toxins:
C. difficile
toxins A (TcdA) and B (TcdB), and
C. ...difficile
transferase toxin (CDT). The single-chain toxins TcdA and TcdB are the main virulence factors. They bind to cell membrane receptors and are internalized. The N-terminal glucosyltransferase and autoprotease domains of the toxins translocate from low-pH endosomes into the cytosol. After activation by inositol hexakisphosphate (InsP6), the autoprotease cleaves and releases the glucosyltransferase domain into the cytosol, where GTP-binding proteins of the Rho Ras family are mono-
O
-glucosylated and, thereby, inactivated. Inactivation of Rho proteins disturbs the organization of the cytoskeleton and affects multiple Rho-dependent cellular processes, including loss of epithelial barrier functions, induction of apoptosis, and inflammation. CDT, the third
C. difficile
toxin, is a binary actin-ADP-ribosylating toxin that causes depolymerization of actin, thereby inducing formation of the microtubule-based protrusions. Recent progress in understanding of the toxins' actions include insights into the toxin structures, their interaction with host cells, and functional consequences of their actions.
Toxins A and B, which are the major virulence factors of antibiotic-associated diarrhea and pseudomembranous colitis caused by Clostridium difficile, are the prototypes of the family of clostridial ...glucosylating toxins. The toxins inactivate Rho and Ras proteins by glucosylation. Recent findings on the autocatalytic processing of the toxins and analysis of the crystal structures of their domains have made a revision of the current model of their actions on the eukaryotic target cells necessary.
Recently the crystal structure of the catalytic domain of Clostridium difficile toxin B was solved (Reinert, D. J., Jank, T., Aktories, K., and Schulz, G. E. (2005) J. Mol. Biol. 351, 973-981). On ...the basis of this structure, we studied the functional role of several amino acids located in the catalytic center of toxin B. Besides the 286DXD288 motif and Trp102, which were shown to be necessary for Mn2+ and UDP binding, respectively, we identified by alanine scanning Asp270, Arg273, Tyr284, Asn384, and Trp520 as being important for enzyme activity. The amino acids Arg455, Asp461, Lys463, and Glu472 and residues of helix α17 (e.g. Glu449) of toxin B are essential for enzyme-protein substrate recognition. Introduction of helix α17 of toxin B into Clostridium sordellii lethal toxin inhibited modification of Ras subfamily proteins but enabled glucosylation of RhoA, indicating that helix α17 is involved in RhoA recognition by toxin B. The data allow the design of a model of the interaction of the glucosyltransferase domain of toxin B with its protein substrate RhoA.
The action of Clostridium difficile toxins A and B depends on processing and translocation of the catalytic glucosyltransferase domain into the cytosol of target cells where Rho GTPases are modified. ...Here we studied the processing of the toxins. Dithiothreitol and β-mercaptoethanol induced auto-cleavage of purified native toxin A and toxin B into ∼250/210- and ∼63-kDa fragments. The 63-kDa fragment was identified by mass spectrometric analysis as the N-terminal glucosyltransferase domain. This cleavage was blocked by N-ethylmaleimide or iodoacetamide. Exchange of cysteine 698, histidine 653, or aspartate 587 of toxin B prevented cleavage of full-length recombinant toxin B and of an N-terminal fragment covering residues 1–955 and inhibited cytotoxicity of full-length toxin B. Dithiothreitol synergistically increased the effect of myo-inositol hexakisphosphate, which has been reported to facilitate auto-cleavage of toxin B (Reineke, J., Tenzer, S., Rupnik, M., Koschinski, A., Hasselmayer, O., Schrattenholz, A., Schild, H., and Von Eichel-Streiber, C. (2007) Nature 446, 415–419). N-Ethylmaleimide blocked auto-cleavage induced by the addition of myo-inositol hexakisphosphate, suggesting that cysteine residues are essential for the processing of clostridial glucosylating toxins. Our data indicate that clostridial glucosylating cytotoxins possess an inherent cysteine protease activity related to the cysteine protease of Vibrio cholerae RTX toxin, which is responsible for auto-cleavage of glucosylating toxins.
Background: TpeL is a member of the family of clostridial glucosylating toxins, produced by Clostridium perfringens.
Results: TpeL enters target cells by self-mediated entry and mono-glycosylates Ras ...proteins at Thr-35.
Conclusion: TpeL inhibits Ras signaling and induces apoptosis in target cells.
Significance: TpeL is a new glucosylating toxin produced by C. perfringens.
TpeL is a member of the family of clostridial glucosylating toxins produced by Clostridium perfringens type A, B, and C strains. In contrast to other members of this toxin family, it lacks a C-terminal polypeptide repeat domain, which is suggested to be involved in target cell binding. It was shown that the glucosyltransferase domain of TpeL modifies Ras in vitro by mono-O-glucosylation or mono-O-GlcNAcylation (Nagahama, M., Ohkubo, A., Oda, M., Kobayashi, K., Amimoto, K., Miyamoto, K., and Sakurai, J. (2011) Infect. Immun. 79, 905–910). Here we show that TpeL preferably utilizes UDP-N-acetylglucosamine (UDP-GlcNAc) as a sugar donor. Change of alanine 383 of TpeL to isoleucine turns the sugar donor preference from UDP-GlcNAc to UDP-glucose. In contrast to previous studies, we show that Rac is a poor substrate in vitro and in vivo and requires 1–2 magnitudes higher toxin concentrations for modification by TpeL. The toxin is autoproteolytically processed in the presence of inositol hexakisphosphate (InsP6) by an intrinsic cysteine protease domain, located next to the glucosyltransferase domain. A C-terminally extended TpeL full-length variant (TpeL1–1779) induces apoptosis in HeLa cells (most likely by mono-O-GlcNAcylation of Ras), and inhibits Ras signaling including Ras-Raf interaction and ERK activation. In addition, TpeL blocks Ras signaling in rat pheochromocytoma PC12 cells. TpeL is a glucosylating toxin, which modifies Ras and induces apoptosis in target cells without having a typical C-terminal polypeptide repeat domain.
Summary
Legionella pneumophila is a human pathogen causing severe pneumonia called Legionnaires' disease. Multiple Legionella effectors are type IV‐secreted into the host cell to establish a specific ...vesicular compartment for pathogen replication. Recently, it has been reported that the Legionella effector SetA shares sequence similarity with glycosyltransferases and interferes with vesicular trafficking of host cells. Here we show that SetA possesses glycohydrolase and mono‐O‐glucosyltransferase activity by using UDP‐glucose as a donor substrate. Whereas the catalytic activity is located at the N terminus of SetA, the C terminus (amino acids 401–644) is essential for guidance of SetA to vesicular compartments of host cells. EGFP‐SetA expressed in HeLa cells localizes to early endosomes by interacting with phosphatidylinositol 3‐phosphate. EGFP‐SetA, transiently expressed in RAW 264.7 macrophages, associates with early phagosomes after infection with Escherichia coli and L. pneumophila. Only the combined expression of the C‐ and N‐terminal domains induces growth defects in yeast similar to full‐length SetA. The data indicate that SetA is a multidomain protein with an N‐terminal glucosyltransferase domain and a C‐terminal phosphatidylinositol 3‐phosphate‐binding domain, which guides the Legionella effector to the surface of the Legionella‐containing vacuole. Both, the localization and the glucosyltransferase domains of SetA are crucial for cellular functions.
Binary enterotoxins Clostridium (C.) botulinum C2 toxin, C. perfringens iota toxin and C. difficile toxin CDT are composed of a transport (B) and a separate non-linked enzyme (A) component. Their ...B-components mediate endocytic uptake into mammalian cells and subsequently transport of the A-components from acidic endosomes into the cytosol, where the latter ADP-ribosylate G-actin resulting in cell rounding and cell death causing clinical symptoms. Protein folding enzymes, including Hsp90 and peptidyl-prolyl cis/trans isomerases facilitate transport of the A-components across endosomal membranes. Here, we identified Hsp70 as a novel host cell factor specifically interacting with A-components of C2, iota and CDT toxins to facilitate their transport into the cell cytosol. Pharmacological Hsp70-inhibition specifically prevented pH-dependent trans-membrane transport of A-components into the cytosol thereby protecting living cells and stem cell-derived human miniguts from intoxication. Thus, Hsp70-inhibition might lead to development of novel therapeutic strategies to treat diseases associated with bacterial ADP-ribosylating toxins.
Clostridium perfringens iota toxin is a binary toxin composed of the enzymatically active component Ia and receptor binding component Ib. Ia is an ADP-ribosyltransferase, which modifies Arg177 of ...actin. The previously determined crystal structure of the actin-Ia complex suggested involvement of Asp179 of actin in the ADP-ribosylation reaction. To gain more insights into the structural requirements of actin to serve as a substrate for toxin-catalyzed ADP-ribosylation, we engineered Saccharomyces cerevisiae strains, in which wild type actin was replaced by actin variants with substitutions in residues located on the Ia-actin interface. Expression of the actin mutant Arg177Lys resulted in complete resistance towards Ia. Actin mutation of Asp179 did not change Ia-induced ADP-ribosylation and growth inhibition of S. cerevisiae. By contrast, substitution of Glu270 of actin inhibited the toxic action of Ia and the ADP-ribosylation of actin. In vitro transcribed/translated human β-actin confirmed the crucial role of Glu270 in ADP-ribosylation of actin by Ia.
Toxin B is a member of the family of large clostridial cytotoxins which are of great medical importance. Its catalytic fragment was crystallized in the presence of UDP-glucose and Mn
2+. The ...structure was determined at 2.2
Å resolution, showing that toxin B belongs to the glycosyltransferase type A family. However, toxin B contains as many as 309 residues in addition to the common chainfold, which most likely contribute to the target specificity. A superposition with other glycosyltransferases shows the expected positions of the acceptor oxygen atom during glucosyl transfer and indicates further that the reaction proceeds probably along a single-displacement pathway. The C1″ donor carbon atom position is defined by the bound UDP and glucose. It assigns the surface area of toxin B that forms the interface to the target protein during the modifying reaction. A docking attempt brought the known acceptor atom, Thr37 O
γ1 of the switch I region of the RhoA:GDP target structure, near the expected position. The relative orientation of the two proteins was consistent with both being attached to a membrane. Sequence comparisons between toxin B variants revealed that the highest exchange rate occurs around the active center at the putative docking interface, presumably due to a continuous hit-and-evasion struggle between Clostridia and their eukaryotic hosts.
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