RICIN CYTOTOXICITY IS SENSITIVE TO RECYCLING BETWEEN THE ENDOPLASMIC-RETICULUM AND THE GOLGI-COMPLEX

Cytotoxic proteins that kill mammalian cells by catalytically inhibiti ng protein synthesis must enter the cytosol in order to reach their su bstrates. With the exception of diphtheria toxin, which enters the cyt osol from acidified endosomes, the intracellular site of translocation of other toxins including ricin, Escherichia coli Shiga-like toxin-1, and Pseudomonas exotoxin A is likely to involve early compartments of the secretory pathway. We have used a molecular approach to identify the site and mechanism of toxin delivery to the cytosol by transiently expressing mutant GTPases that inhibit the assembly of biochemical co mplexes mediating anterograde and retrograde transport in the exocytic and endocytic pathways, The results provide evidence to suggest that receptors actively recycling between the endoplasmic reticulum and ter minal Golgi compartments are essential for toxin translocation to the cytosol from the endoplasmic reticulum, The rapid kinetics of intoxica tion demonstrate a substantial level of bidirectional membrane flow an d sorting through the early secretory pathway.