ELEVATION OF VACUOLAR PH INHIBITS THE CYTOTOXIC ACTIVITY OF FURIN-CLEAVED EXOTOXIN-A

Exotoxin A (ETA) inhibits protein synthesis in cells by a process that involves receptor-mediated endocytosis and the transport of a 37-kDa proteolytic fragment across a membrane into the cytoplasm, The fragmen t is apparently generated by the endoprotease furin after the toxin ha s been endocytosed, Cleavage of ETA by furin requires a low pH in vitr o, and presumably also in vivo. Drugs that raise the pH of intracellul ar compartments are known to protect cells from ETA, The simplest hypo thesis to explain this protection has been that the drugs interfere wi th furin cleavage, To test this idea, we measured the effect of DR-ele vating drugs on the action of ETA that had been precleaved with recomb inant furin before addition to cells, Surprisingly; we found that pH-e levating drugs protected cells from precleaved ETA as well as intact E TA. These results suggest that the process by which ETA intoxicates ce lls requires a low vacuolar pH for another event in addition to proteo lysis by furin.