Toxin B is an intracellularly acting cytotoxin and enters the cell via a receptor-mediated endocytosis pathway to reach the endosomes, from which the toxin is translocated to the cytoplasm (Florin and Thelestam, 1986; Henriques etal., 1987). Because of this specific mode of entry, the toxin concentration needed for intoxication of cells is low (lOOng/ml for about 4h). In contrast, Clostridium botulinum exoen-zyme C3 (23.5 kDa), which ADP-ribosylates the Rho subtype proteins RhoA, B and C only, enters the cells by a non-specific uptake process, possibly by pinocytosis. Therefore, C3 has to be applied in high concentrations (about 30ng/ml) for 24 h or longer.
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