pH-dependent bilayer destabilization and fusion of phospholipidic large unilamellar vesicles induced by diphtheria toxin and its fragments A and B.

The passage by the low endosomal pH is believed to be an essential step of the diphtheria toxin (DT) intoxication process in vivo. Several studies have suggested that this low pH triggers the insertion of DT into the membrane. We demonstrate here that its insertion into large unilamellar vesicles (LUV) is accompanied by a strong destabilization of the vesicles at low pH. The destabilization has been studied by following the release of a fluorescent dye (calcein) encapsulated in the liposomes. The influence of the lipid composition upon this process has been examined. At a given pH, the calcein release is always faster for a negatively charged (asolectin) than for a zwitterionic (egg PC) system. Moreover, the transition pH, which is the pH at which the toxin-induced release becomes significant, is shifted upward for the asolectin LUV as compared to the egg PC LUV. No calcein release is observed for rigid phospholipid vesicles (DPPC and DPPC/DPPA 9/1 mol/mol) below their transition temperature whereas DT induces an important release of the dye in the temperature range corresponding to the phase transition. The transition pH associated to the calcein release from egg PC vesicles is identical with that corresponding to the exposure of the DT hydrophobic domains, as revealed here by the binding of a hydrophobic probe (ANS) to the toxin. This suggests the involvement of these domains in the destabilization process. Both A and B fragments destabilize asolectin and PC vesicles in a pH-dependent manner but to a lesser extent than the entire toxin.(ABSTRACT TRUNCATED AT 250 WORDS)