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 anthrac is edema toxin, protective antigen (PA63) and edema factor (EF), as we ll as the two EF mutants: CYA30 (containing the N-terminal PA63-bindin g 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 trans form infrared spectroscopy. Secondary structures were independent bf p H, whereas, in all cases, structural modifications were observed upon lipid binding. The ability of PA63 and EF to undergo hydrogen/deuteriu m exchange was evaluated. The binding of these proteins and the mutant s to the lipid membrane was also characterized and it was demonstrated that the association of PA63 to the lipid bilayer was pH-dependent, w hile the binding of EF to the Lipid membrane rook place at both neutra l and acidic pH. Interestingly, the two EF mutants are showing differe nt lipid binding properties in response to pH: CYA30 has a strong pH-d ependence 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 bindin g of PA63 to the membrane was found to be irreversible whereas that of CYA30 was reversible.