title: Expression of mutant dynamin protects cells against diphtheria toxin but not against ricin.
abstract: Diphtheria toxin is believed to enter sensitive mammalian cells via receptor-mediated endocytosis from clathrin-coated pits, while ricin can enter via both clathrin-dependent and clathrin-independent endocytosis. The present study has confirmed this by determining the toxin sensitivity of COS-7y cells which were transiently overexpressing a trans dominant negative mutant of dynamin, a GTPase required for the budding of clathrin-coated vesicles from the plasma membrane. Cells overexpressing wild-type dynamin showed normal receptor-mediated endocytosis of transferrin and remained sensitive to both diphtheria toxin and ricin. Cells overexpressing a mutant dynamin defective in GTP binding and hydrolysis were unable to endocytose transferrin and were protected against diphtheria toxin, but they remained completely sensitive to ricin intoxication. Treating non-transfected cells or cells overexpressing mutant dynamin with nystatin caused a redistribution of the caveolae membrane marker protein VIP21-caveolin from the cell surface to intracellular locations, but did not affect their sensitivity to ricin. The redistribution of caveolin seen after nystatin treatment may reflect the disappearance of caveolae. If this is the case, caveolae are not responsible for the endocytosis of ricin. An alternative clathrin-independent route may operate for ricin, since cellular uptake, intracellular transport, and translocation into the cytosol remain unaffected when clathrin-dependent endocytosis is effectively blocked.
authors.affiliation: Department of Biological Sciences, University of Warwick, Coventry, United Kingdom.
authors.forename: J C
authors.initials: JC
authors.lastname: Simpson
authors.forename: D C
authors.initials: DC
authors.lastname: Smith
authors.forename: L M
authors.initials: LM
authors.lastname: Roberts
authors.forename: J M
authors.initials: JM
authors.lastname: Lord
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D019556:COS Cells / Q000187:drug effects; D051242:Caveolin 1; D022461:Caveolins; D003034:Coated Pits, Cell-Membrane / Q000187:drug effects* / Q000502:physiology; D004167:Diphtheria Toxin / Q000378:metabolism / Q000633:toxicity*; D034281:Dynamins; D004705:Endocytosis / Q000187:drug effects*; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000494:pharmacology*; D006160:Guanosine Triphosphate / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D009761:Nystatin / Q000494:pharmacology; D012276:Ricin / Q000378:metabolism / Q000633:toxicity*; D014162:Transfection; D014168:Transferrin / Q000378:metabolism
chemical_list: D051242:Caveolin 1; D022461:Caveolins; D004167:Diphtheria Toxin; D008565:Membrane Proteins; D014168:Transferrin; D009761:Nystatin; D006160:Guanosine Triphosphate; D012276:Ricin; D020558:GTP Phosphohydrolases; D034281:Dynamins