Oligomerization of Clostridium perfringens ε-toxin is dependent upon membrane fluidity in liposomes

Clostridium perfringens epsilon-toxin binds to receptors on MDCK cells and forms a heptamer in membranes. The mechanism behind the oligomerization of epsilon-toxin was studied using carboxyfluorescein (CF)-loaded liposomes composed of various phosphatidylcholines (PCs). The toxin caused CF to leak from liposomes in a dose-dependent manner. The toxin-induced leakage of CF, binding of the toxin to liposomes, and formation of a functional oligomer increased as the phase-transition temperature (T-m) of the PC used in the liposomes decreased. Surface plasmon resonance analysis using an HPA sensorchip (BIAcore) also revealed that the binding of the toxin to liposomes increased with a decrease in the Tm of the PC used in liposomes. The oligomer that was formed in 3-(trifluoromethyl)-3-(m-[I-125]iodophenyl)diazirine ([I-125]TID)-treated liposomes was labeled, indicating that it inserts into a hydrophobic region. Furthermore, the rate of epsilon-toxin-induced CF leakage was enhanced by treatment with phosphatidylethanolamine or diacylglycerol, which is known to favor a lamellar-to-inverted hexagonal (L-H) phase transition. We show that membrane fluidity in the liposome plays an important role in the binding of the toxin to liposomes, insertion into the hydrophobic region in the bilayer of liposomes, and the assembly process in the bilayer.