Intermediates in translocation of diphtheria toxin across the plasma membrane

Active diphtheria toxin consists of two parts, fragments A and B. Fragment A has enzymatic activity and inhibits protein synthesis. Fragment B binds to cellular receptors, and upon exposure to low pH it inserts into the membrane and facilitates translocation of the A fragment into the cytosol, concomitantly with formation of cation-selective channels. Reduction of the interfragment disulfide bridge is required for release of fragment A and intoxication. In cells treated with N-ethylmaleimide (NEM), which inhibits reduction of the disulfide bridge, fragment A was translocated to the cytosol but not released from fragment B. In the presence of NEM a peptide larger than fragment A was protected against extracellularly added Pronase. This peptide (M(r) approximately 24,000) was released to the supernatant fraction of saponin-treated cells. This indicates that fragment A, which is 21 kDa, is covalently attached via a disulfide bond to an N-terminal (M(r) approximately 3,000) piece of fragment B. The 24-kDa fragment disappeared upon reduction, and the 21-kDa fragment A appeared instead. NEM did not prevent channel activity by fragment B in the context of full-length toxin, demonstrating that channel formation occurs in spite of inhibited reduction of the disulfide bond. Thus, channel formation is not dependent on release of fragment A from the toxin-receptor complex.