Structure and interaction of PA63 and EF (edema toxin) of Bacillus anthracis with lipid membrane

The secondary structures of the two components of the Bacillus anthracis edema toxin, protective antigen (PA63) and edema factor (EF), as well as the two EF mutants: CYA30 (containing the N-terminal PA63-binding domain) and CYA62 (containing the C-terminal catalytic domain) were investigated as a function of pH in the absence and in the presence of phospholipid vesicles using attenuated total reflection Fourier transform infrared spectroscopy. Secondary structures were independent of pH, whereas, in all cases, structural modifications were observed upon lipid binding. The ability of PA63 and EF to undergo hydrogen/deuterium exchange was evaluated. The binding of these proteins and the mutants to the lipid membrane was also characterized and it was demonstrated that the association of PA63 to the lipid bilayer was pH-dependent, while the binding of EF to the lipid membrane took place at both neutral and acidic pH. Interestingly, the two EF mutants are showing different lipid binding properties in response to pH: CYA30 has a strong pH-dependence whereas CYA62, as EF, binds to the lipid vesicles at all pHs. For the two proteins characterized by a pH-dependent lipid binding, the reversibility of binding upon neutralization was tested and binding of PA63 to the membrane was found to be irreversible whereas that of CYA30 was reversible.