Requirements for the translocation of diphtheria toxin fragment A across lipid membranes

The translocation of the enzymatic moiety of diphtheria toxin, fragment A, across the membranes of pure lipid vesicles was demonstrated. A new assay, which employed vesicles made to contain radiolabeled NAD and elongation factor-2, was used to measure the appearance of the enzymatic activity of the A fragment in the vesicles. When the vesicles were exposed to a low-pH medium in the presence of diphtheria toxin, small molecules, such as NAD, escaped into the extravesicular medium, whereas large molecules mostly remained inside the vesicles. The vesicle-entrapped elongation factor-2 became ADP-ribosylated, indicating the entry of fragment A into the vesicle. The translocation of the A fragment depended upon the pH of the medium, being negligible at pH greater than 7.0 and maximal at pH 4.5. The entire toxin molecule was needed for function; neither the A fragment nor the B fragment alone was able to translocate itself across and react with the sequestered substrates. After exposure of the toxin to low pH, the entry of the A fragment was rapid, being virtually complete within 2-3 min at pH 5.5, and within 1 min at pH 4.7. Translocation occurred in the absence of any protein in the vesicle membrane. These results are consistent with the notion that the diphtheria toxin molecule enters the cytoplasm of a cell by escaping from an acidic compartment such as an endocytic vesicle.