The role of transglutaminase in the mechanism of action of tetanus toxin.

Tetanus toxin is a zinc-dependent metalloendoprotease that cleaves synaptobrevin, a polypeptide found in the membranes of synaptic vesicles. This action is thought to account for toxin-induced blockade of transmitter release. However, Facchiano and Luini (Fachiano, F., and Luini, A. (1992) J. Biol Chem. 267, 13267-13271) have proposed that tetanus toxin can stimulate transglutaminase, and Facchiano et al. (Facchiano, F., Benfenati, F., Valtorta, F., and Luini, A. (1993) J. Biol Chem. 268, 4588-4591) have further proposed that the stimulated enzyme produces cross-linking of synapsin. These actions might also account for toxin-induced blockade of exocytosis. Therefore, a series of experiments were performed to evaluate the possibility that tetanus toxin exerts its effects via transglutaminase. The results indicated that clostridial neurotoxins were poor substrates for the cross-linking effects of transglutaminase, and transglutaminase was a poor substrate for the proteolytic actions of tetanus toxin. In addition, at concentrations relevant to blockade of exocytosis, clostridial neurotoxins did not act on intact cells to stimulate transglutaminase, nor did they act on the isolated enzyme to stimulate cross-linking of putrescine and dimethylcasein. When used as competitive inhibitors of endogenous transglutaminase substrates, glycine methyl ester and monodansylcadaverine did not block toxin action. Furthermore, concentrations of calcium that were too low to support transglutaminase activity did not prevent toxin action. The data suggest that stimulation of transglutaminase is not the principal mechanism by which tetanus toxin blocks exocytosis in nerve cells.