Toxin A and B, the major virulence factors of Clostridium difficile, are the causative agents of antibiotic-associated pseudomembranous colitis. In cultured cell lines their potent cytotoxicity results from their ability to induce disaggregation of the microfilament cytoskeleton. Toxin B acts on the low-molecular-mass GTPase RhoA, which is involved in the regulation of the actin cytoskeleton. We report here that toxin B catalyses the incorporation of up to one mole of glucose per mole of RhoA at the amino acid threonine at position 37. The modification was identified and localized by tandem electrospray mass spectrometry. UDP-glucose selectively serves as cosubstrate for the monoglucosylation reaction catalysed by toxin B. Microinjection of RhoA previously glucosylated by toxin B into monolayer cells caused disaggregation of actin filaments, indicating a dominant-negative activity of glucosylated RhoA.

The actin cytoskeleton is regulated by GTP-hydrolysing proteins, the Rho GTPases, which act as molecular switches in diverse signal-transduction processes. Various bacterial toxins can inactivate Rho GTPases by ADP-ribosylation or glucosylation. Previous research has identified Rho proteins as putative targets for Escherichia coli cytotoxic necrotizing factors 1 and 2 (CNF1 and 2). These toxins induce actin assembly and multinucleation in culture cells. Here we show that treatment of RhoA with CNF1 inhibits the intrinsic GTPase activity of RhoA and completely blocks GTPase activity stimulated by the Rho-GTPase-activating protein (rhoGAP). Analysis by mass spectrometry and amino-acid sequencing of proteolytic peptides derived from CNF1-treated RhoA indicate that CNF1 induces deamidation of a glutamine residue at position 63 (Gln 63) to give constitutively active Rho protein.

The enterotoxin from Clostridium difficile (ToxA) is one of the causative agents of the antibiotic-associated pseudomembranous colitis. In cultured monolayer cells ToxA exhibits cytotoxic activity to induce disassembly of the actin cytoskeleton, which is accompanied by morphological changes. ToxA-induced depolymerization of actin filaments is correlated with a decrease in the ADP-ribosylation of the low molecular mass GTP-binding Rho proteins (Just, I., Selzer, J., von Eichel-Streiber, C., and Aktories, K. (1995) J. Clin. Invest. 95, 1026-1031). Here we report on the identification of the ToxA-induced modification of Rho. Applying electrospray mass spectrometry, the mass of the modification was determined as 162 Da, which is consistent with the incorporation of a hexose into Rho. From several hexoses tested UDP-glucose selectively served as cosubstrate for ToxA-catalyzed modification. The acceptor amino acid of glucosylation was identified from a Lys-C-generated peptide by tandem mass spectrometry as Thr-37. Mutation of Thr-37 to Ala completely abolished glucosylation. The members of the Rho family (RhoA, Rac1, and Cdc42Hs) were substrates for ToxA, whereas H-Ras, Rab5, and Arf1 were not glucosylated. ToxA-catalyzed glucosylation of lysates from ToxA-pretreated rat basophilic leukemia (RBL) cells resulted in a decreased incorporation of [14C]glucose, indicating previous glucosylation in the intact cell. Glucosylation of the Rho subtype proteins appears to be the molecular mechanism by which C. difficile ToxA mediates its cytotoxic effects on cells.

A fragment of the N-terminal 546 amino acid residues of Clostridium sordellii lethal toxin possesses full enzyme activity and glucosylates Rho and Ras GTPases in vitro. Here we identified several amino acid residues in C. sordellii lethal toxin that are essential for the enzyme activity of the active toxin fragment. Exchange of aspartic acid at position 286 or 288 with alanine or asparagine decreased glucosyltransferase activity by about 5000-fold and completely blocked glucohydrolase activity. No enzyme activity was detected with the double mutant D286A/D288A. Whereas the wild-type fragment of C. sordellii lethal toxin was labeled by azido-UDP-glucose after UV irradiation, mutation of the DXD motif prevented radiolabeling. At high concentrations (10 mM) of manganese ions, the transferase activities of the D286A and D288A mutants but not that of wild-type fragment were increased by about 20-fold. The exchange of Asp 270 and Arg 273 reduced glucosyltransferase activity by about 200-fold and blocked glucohydrolase activity. The data indicate that the DXD motif, which is highly conserved in all large clostridial cytotoxins and also in a large number of glycosyltransferases, is functionally essential for the enzyme activity of the toxins and may participate in coordination of the divalent cation and/or in the binding of UDP-glucose.

Recently, it has been reported that cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli induces formation of stress fibers by deamidation of glutamine 63 of RhoA (Schmidt, G., Sehr, P., Wilm, M., Selzer, J., Mann, M., and Aktories, K. (1997) Nature 387, 725-729); Flatau, G., Lemichez, E., Gauthier, M., Chardin, P., Paris, S., Fiorentini, C., and Boquet, P. (1997) Nature 387, 729 -733). By using mass spectrometric analysis, we show now that the toxin transfers ethylenediamine, putrescine, and dansylcadaverine specifically onto glutamine 63 of RhoA. RhoA was also a substrate for guinea pig liver transglutaminase, which modified not only glutamine 63, but also glutamine residues at positions 52 and 136. Treatment of the fully active N-terminal fragment of CNF1 (amino acid residues 709 -1014) with iodoacetamide inhibited both deamidation and transglutamination activities. Moreover, exchange of cysteine 866 with serine blocked the enzyme activity of the N-terminal CNF1 fragment. In addition, we identified histidine 881 to be essential for the enzyme activity of CNF1. The data indicate that CNF1 shares a catalytic dyad of cysteine and histidine residues with eukaryotic transglutaminases and cysteine proteases.Rho proteins are involved in the organization of the actin cytoskeleton and act as molecular switches in various signaling processes (1-4). These GTPases are targets for various bacterial toxins and exoenzymes. RhoA, RhoB, and RhoC are ADPribosylated by Clostridium botulinum C3 (5, 6) and related C3-like exoenzymes (7). All Rho subfamily proteins are glucosylated by large clostridial cytotoxins (e.g. Clostridium difficile toxins A and B) (8, 9). In addition, Rho is a target for cytotoxic necrotizing factors 1 and 2 of Escherichia coli (10, 11). These toxins induce actin polymerization and inhibit cytokinesis, which results in formation of multinucleated cells (10 -12). Recently, it was shown that CNF1 1 modifies RhoA by deamidation of glutamine 63, thereby forming a glutamic acid residue at this position (13,14). Because glutamine 63 is essential for the intrinsic and GTPase-activating protein (GAP)-stimulated GTPase activity of Rho, the GTPase is constitutively activated by CNF1 (13,15). A similar mechanism of Rho modification was recently reported for the CNF1-related dermonecrotic toxin (DNT) from Bordetella species (16).It is known that deamidation of glutamine residues of proteins is also observed with various transglutaminases (e.g. tissue transglutaminase or coagulation factor XIII). In general, transglutaminases are cross-linking enzymes that catalyze the exchange of the ␥-carboxamide group of glutamine residues for primary amines (e.g. peptide-bound lysine residues or polyamines) to form ⑀-(␥-glutamyl)lysine or (␥-glutamyl)polyamine bonds (17, 18). In the absence of an appropriate acceptor peptide, however, transglutaminases are capable of inducing the deamidation of glutamine residues. Therefore, we studied whether the Rho-deamidating CNF1 is somehow related to eukaryotic transglutaminases. Here...