Intracellular localization and degradation of diphtheria toxin

The internalization of surface‐bound diphtheria toxin (DT) in BS‐C‐1 cells correlated with its appearance in intracellular endosomal vesicles; essentially no toxin appeared within secondary lysosomal vesicles. In contrast, internalized epidermal growth factor (EGF) was localized within both endosomal and lysosomal vesicles. Upon prein‐cubation of cells with leupeptin, a lysosomal protease inhibitor, a threefold increase in the accumulation of EGF into lysosomes was observed. Under identical conditions, essentially all of the diphtheria toxin remained within endosomes (less than 2% of the intracellular diphtheria toxin accumulated in the lysosomaJ fraction), indicating that the inability to detect diphtheria toxin in lysosomes was not due to its rapid turnover within this vesicle. Following internalization of EGF or DT, up to 40% of the lgand appeared in the medium as TCA‐soluble radioactivity. EGF degradation was partially leupeptin‐sensitive and markedly NH 4 Cl‐sensitive, indicating lysosomal degradation. In contrast, DT A‐fragment degradation was resistant to these inhibitors, while B‐fragment showed only partial sensitivity. These data suggest that the bulk of endocytosed diphtheria toxin is localized within endosomes and degraded by a pathway essentially independent of lysosomes.