MEMBRANE TRANSLOCATION OF DIPHTHERIA-TOXIN FRAGMENT - A EXPLOITS EARLY TO LATE ENDOSOME TRAFFICKING MACHINERY

After reaching early endosomes by receptor-mediated endocytosis, dipht heria toxin (DT) molecules have two possible fates. A large pool enter s the degradative pathway whereas a few molecules become cytotoxic by translocating their catalytic fragment A (DTA) into the cytosol. Impai rment of DT degradation by microtubule depolymerization does not block DT cytotoxicity. Therefore, DTA membrane translocation into the cytos ol occurs from an endocytic compartment located upstream of late endos omes. Comparisons between early endosomes and endocytic carrier vesicl es in a cell-free translocation assay have demonstrated that early end osomes are the earliest endocytic compartment from which DTA transloca tes. DTA translocation is ATP-dependent, requires early endosomal acid ification, and is increased by the addition of cytosol. Cytosol-depend ent DTA translocation is GTP gamma S-insensitive but is blocked by ant i-beta COP antibodies.