Receptor-mediated entry of diphtheria toxin into monkey kidney (Vero) cells: electron microscopic evaluation.

To express toxicity in living cells, diphtheria toxin (DT) must cross a membrane barrier and reach its target in the cytosol. Here we examine the entry of DT into the toxin-sensitive monkey kidney (Vero) cells. Using electron microscopy we directly demonstrated for the first time that DT is internalized by receptor-mediated endocytosis, i.e., via clathrin-coated pits, and enters the endosomal system. Methylamine, which is known to protect cells from DT, stopped the movement of toxin to coated areas of the cell membrane. In the presence of amine, prebound biotinyl-DT was internalized, but toxicity was inhibited. Biochemical evidence revealed that methylamine maintained toxin molecules at a site accessible to neutralization by antitoxin. The data suggest that DT entering Vero cells in the presence of methylamine is sequestered within the cell and does not express toxicity.