Large clostridial cytotoxins.

The large clostridial cytotoxins are a family of structurally and functionally related exotoxins from Clostridium difficile (toxins A and B), C. sordellii (lethal and hemorrhagic toxin) and C. novyi (alpha-toxin). The exotoxins are major pathogenicity factors which in addition to their in vivo effects are cytotoxic to cultured cell lines causing reorganization of the cytoskeleton accompanied by morphological changes. The exotoxins are single-chain protein toxins, which are constructed of three domains: receptor-binding, translocation and catalytic domain. These domains reflect the self-mediated cell entry via receptor-mediated endocytosis, translocation into the cytoplasm, and execution of their cytotoxic activity by an inherent enzyme activity. Enzymatically, the toxins catalyze the transfer of a glucosyl moiety from UDP-glucose to the intracellular target proteins which are the Rho and Ras GTPases. The covalent attachment of the glucose moiety to a conserved threonine within the effector region of the GTPases renders the Rho-GTPases functionally inactive. Whereas the molecular mode of cytotoxic effects is fully understood, the mechanisms leading to inflammatory processes in the context of disease (e.g., antibiotic-associated pseudomembranous colitis caused by Clostridium difficile) are less clear.