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  • Title: Receptor-mediated transport of the hybrid protein ricin-diphtheria toxin fragment A with subsequent ADP-ribosylation of intracellular elongation factor II.
    Author: Youle RJ, Neville DM.
    Journal: J Biol Chem; 1979 Nov 10; 254(21):11089-96. PubMed ID: 500625.
    Abstract:
    A hybrid protein of ricin and the enzymatically active fragment A of diphtheria toxin (toxin A) has been synthesized and purified. The diphtheria toxin A fragment of the hybrid protein is shown to enter the cytosol compartment of HeLa cells, its presence assayed by the fall of intracellular elongation factor II (EF-2) and the rise of ADP-ribosylated EF-2. Hybrid entrance to HeLa cells is blocked by lactose which blocks receptor-mediated entry of ricin but not by NH4Cl which blocks the transport of diphtheria toxin. It is concluded that the diphtheria toxin fragment A moiety of the hybrid enters the cell cytosol via the ricin receptor-mediated transport system. The kinetics of intracellular ADP-ribosylation of EF-2 by diphtheria toxin have also been studied. Ribosylation is preceded by a toxin dose-dependent lag period. The data suggest that the time constant responsible for the lag period is in the transport step. Models consistent with these data are discussed.
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