Stau is a new member of the family of the C3-like transferases but is also the prototype of a subfamily of RhoE/Rnd modifying transferases.Various bacterial protein toxins interfere with eukaryotic cell functions by catalyzing a posttranslational modification of essential cellular regulator proteins such as ADP-ribosylation of the Rho proteins by C3-like transferases. Clostridium botulinum exoenzyme C3 is the prototype of a family encompassing exoenzymes from Clostridium limosum and Bacillus cereus and from Staphylococcus aureus (EDIN) 1 (1-4). The members of this family are similar in structure and homologous to each other. They are single-chained ADP-ribosyltransferases with a molecular mass of ϳ25 kDa. The C3-like transferases are in fact mere exoenzymes devoid of the cell entry apparatus harbored by other toxins, and they are thought to enter cells by nonspecific pinocytosis. C3 catalyzes ADP-ribosylation of the RhoA, -B and -C subtypes but not of other members of Rho and Ras subfamilies (3-5). Only in the presence of the detergent sodium dodecyl sulfate, the Rac protein is a poor substrate (4). The ADP-ribose moiety is transferred from NAD ϩ to the acceptor amino acid Asn-41 and is linked N-glycosidically to the amide group of the carboxylate side chain of Asn-41 (6).The Rho proteins belong to the Ras superfamily of low molecular mass GTPases, which are the major regulators of the actin cytoskeleton, but they are also involved in cell cycle progression, transcriptional activity, and in cooperation with Ras in cell transformation (7-9). Because ADP-ribosylation of Rho in intact cells results in disaggregation of the actin cytoskeleton, ADP-ribosylation has been classified as an inactivating modification. The molecular basis of the inactivation is the decreased interaction of Rho with the exchange factors, which promote activation (10), and the sequestration by the negative regulator guanine nucleotide exchange factor.2 Although C3 induces the breakdown of the actin cytoskeleton, an effect that can be easily monitored, it is now clear that C3-catalyzed ADP-ribosylation inactivates all Rho functions (for reviews see .

Botulinum ADP-ribosyltransferase C3 modified 21-24 kDa proteins in a guanine nucleotide-dependent manner similar to that described for botulinum neurotoxin CI and D. Whereas GTP and GTP~S stimulated C3-catalyzed ADP-ribosylation in the absence of Mg a +, in the presence of added Mg 2 + ADP-ribosylation was impaired by GTPyS. C3 was about 1000-fold more potent than botulinum C1 neurotoxin in ADP-ribosylation of the 21-24 kDa protein(s) in human platelet membranes. Antibodies raised against C3 blocked ADP-ribosylation of the 21-24 kDa protein by C3 and neurotoxin C1 but neither cross reacted with neurotoxin C1 immunoblots nor neutralized the toxicity of neurotoxin CI in mice. The data indicate that the ADP-ribosylation of low molecular mass GTP-binding proteins in various eukaryotic cells is not caused by botulinum neurotoxins but is due to the action of botulinum ADP-ribosyltransferase C3. The weak enzymatic activities described for botulinum neurotoxins appear to be due to the contamination of CI and D preparations with ADP-ribosyltransferase C3.ADP-ribosylation; Botulinum ADP-ribosyltransferase C3; Botulinum-neurotoxin C1; Botulinum neurotoxin D; GTP-binding protein

Palytoxin (PTX) is a potent releaser of histamine from rat mast cells. The concentration response curve is bell-shaped with its range between 0.05 ng/ml an 5 g/ml and its maximum at 50 ng/ml. The release of histamine by PTX is specific because (a) heat-inactivated (50°C) mast cells are insensitive to PTX, and (b) extracellular calcium is essential. Removal of extracellular potassium as well as the presence of borate shifts the curve to the left by a factor of 10. At higher concentrations (5 µg/ml), PTX inhibits the release of histamine due to compound 48/80, MCD-peptide, Con-A, ionophore A-23187, and PTX itself.

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