ANTHRAX PROTECTIVE ANTIGEN FORMS OLIGOMERS DURING INTOXICATION OF MAMMALIAN-CELLS

The protective antigen component (PA) of anthrax toxin binds to recept ors on target cells and conveys the toxin's edema factor (EF) and leth al factor (LF) components into the cytoplasm. PA (83 kDa) is processed by a cellular protease, yielding a 63-kDa fragment (PA(63)), which bi nds EF and/or LF. When exposed to acidic pH, PA(63) inserts into membr anes and forms ion-conductive channels. By electron microscopy, a sign ificant fraction of purified PA(63), was found to be in the form of a multisubunit ring-shaped oligomer (outer diameter, 10.4 nm). The rings are heptameric, as judged by inspection and by rotational power spect ra. Purified PA(63) showed a high M(r) band, apparently corresponding to the oligomer, on SDS-polyacrylamide gels, and oligomer of similar s ize was formed in cells in a time-dependent manner after addition of f ull-length PA. Inhibitors of internalization and endosome acidificatio n blocked conversion of cell-associated PA to a high molecular weight species, and medium at pH 5.0 induced oligomer formation in the presen ce or absence of the inhibitors. These results correlate PA(63) oligom erization with conditions required for translocation of EF and LF acro ss lipid bilayers, implying that the PA(63) oligomer may function in t ranslocation.