FREE RICIN-A CHAIN REACHES AN EARLY COMPARTMENT OF THE SECRETORY PATHWAY BEFORE IT ENTERS THE CYTOSOL

During the intoxication of mammalian cells by ricin, the catalytically active A chain must cross the membrane of an intracellular compartmen t in order to reach its ribosomal substrates in the cytosol. The actua l site of ricin A chain translocation is unclear, and conflicting view s hold that it enters the cytosol from endosomes or from an early comp artment of the secretory pathway, possibly the lumen of the endoplasmi c reticulum. Here we show that treating cells with brefeldin A, or tra nsiently overexpressing mutant GTPases known to inhibit biochemical co mplexes mediating anterograde and retrograde transport between the end oplasmic reticulum and the Golgi complex, protected cells from intoxic ation by free ricin A chain. These data indicate that ricin A chain, e ither free or as part of intact ricin, reaches an early compartment of the secretory pathway before translocation into the cytosol occurs. ( C) 1996 Academic Press, Inc.