Energy requirements for diphtheria toxin translocation are coupled to the maintenance of a plasma membrane potential and a proton gradient.

Translocation of diphtheria toxin (DT) or ricin to the cytosol is the rate-limiting step responsible for (pseudo) first-order decline in protein synthesis observed in intoxicated cell populations. The requirements for energy utilization in the translocation of both toxins are examined by perturbing the intoxication during this period of protein synthesis decline. Translocation of either toxin is blocked at 4 degrees C and requires energy. Ricin translocation is tightly coupled to ATP hydrolysis with no involvement of membrane potential. Cell depolarization slows the rate of DT translocation but does not block completely. Elimination of transmembrane pH gradients alone does not affect DT translocation; however, in combination with depolarization, translocation is blocked virtually completely. Energy requirements for DT intoxication are mediated by establishing a plasma membrane potential and a pH gradient across some cellular membrane. It is proposed that a postendocytotic vesicle containing processed DT fuses with the plasma membrane. Either component of the proton motive force across the plasma membrane then drives DT translocation. Ricin apparently utilizes a different energy coupling mechanism at a different intracellular site, thus demonstrating toxin specificity in the translocation mechanism.