Evidence that diphtheria toxin and modeccin enter the cytosol from different vesicular compartments.

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Abstract
Inhibition of protein synthesis in Vero [African green monkey kidney] cells was measured at different times after treatment with diphtheria toxin and the related plant toxin modeccin. Diphtheria toxin acted much more rapidly than modeccin. Cells were protected against both toxins with antiserum and with agents like NH4Cl, procaine and the ionophores monensin, p-trifluoro-methoxyphenylhydrazone and carbonyl cyanide m-chlorophenylhydrazone, which increased the pH of intracellular vesicles. Antiserum, which is supposed to inactivate toxin only at the cell surface, protected only when it was added within a short time after modeccin. Compounds that increased the pH of intracellular vesicles protected even when added after 2 h, indicating that modeccin remains inside vesicles for a considerable time before it enters the cytosol. After addition of diphtheria toxin to the cells, compounds that increased the pH of intracellular vesicles protected only approximately to the same extent as antitoxin. Thus, after endocytosis diphtheria toxin rapidly enters the cytosol. At 20.degree. C, the cells were more strongly protected against modeccin than against diphtheria toxin. The residual toxic effect of diphtheria toxin at 20.degree. C was blocked with NH4Cl; this was not the case with modeccin. Thus, at 20.degree. C the uptake of diphtheria toxin occurs by the normal route; the uptake of modeccin occurs by a less efficient route than that dominating at 37.degree. C. After endocytosis diphtheria toxin rapidly enters the cytosol from early endosomes with low pH (receptosomes). Modeccin enters the cytosol much more slowly, possibly after fusion of the endocytic vesicles with another compartment.