Infect. Immun., Jun 1997, 2240-2242, Vol 65, No. 6
Copyright © 1997, American Society for Microbiology

Elevation of vacuolar pH inhibits the cytotoxic activity of furin- cleaved exotoxin A

MJ Corboy and RK Draper
Molecular and Cell Biology Program, University of Texas at Dallas, Richardson 75083-0688, USA.

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 fragment is apparently generated by the endoprotease furin after the toxin has been endocytosed. Cleavage of ETA by furin requires a low pH in vitro, and presumably also in vivo. Drugs that raise the pH of intracellular compartments are known to protect cells from ETA. The simplest hypothesis to explain this protection has been that the drugs interfere with furin cleavage. To test this idea, we measured the effect of pH- elevating drugs on the action of ETA that had been precleaved with recombinant furin before addition to cells. Surprisingly, we found that pH-elevating drugs protected cells from precleaved ETA as well as intact ETA. These results suggest that the process by which ETA intoxicates cells requires a low vacuolar pH for another event in addition to proteolysis by furin.

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