The effects of pH on the interaction of anthrax toxin lethal and edema factors with phospholipid vesicles

Bacillus anthracis secretes three distinct proteins which interact in binary combinations to produce two toxins. The two effector moieties, edema factor (EF) and lethal factor (LF), interact competitively with the cell receptor-binding moiety, protective antigen (PA), to produce biologically distinct effects. The passage of the toxins through an acidified endosomal compartment is an essential step in the intoxication process, and it has been shown that low pH triggers the insertion of the activated form of PA, PA63, into model lipid bilayers. In this study, we have examined the effects of pH on the interaction of LF and EF with a model membrane system. Protein labeling by radioactive phospholipid probes indicated that both LF and EF are able to insert into asolectin lipid bilayers in a pH-dependent manner. For LF, the extent of insertion into the bilayer was accompanied in parallel by the release of calcein from preloaded LUV (large unilamellar vesicles). The transition pH for protein insertion, however, was somewhat higher than that for membrane destabilization. The extent of protein radiolabeling and the release of calcein from LUV incubated with EF was similar to that seen with LF; however, the pH dependency was significantly less. Low pH-induced membrane insertion by both proteins was accompanied by only a minimal change in conformation. These results suggest that LF and EF may be actively involved in the process of toxin translocation.