Anthrax Protective Antigen: Prepore-to-Pore Conversion†

PA63, the active 63 kDa form of anthrax protective antigen, forms a heptameric ring-shaped oligomer that is believed to represent a precursor of the membrane pore formed by this protein. When maintained at pH ≥8.0, this “prepore” dissociated to monomeric subunits upon treatment with SDS at room temperature, but treatment at pH ≤7 (or with β-octylglucoside at pH 8.0) caused it to convert to an SDS-resistant pore-like form. Transition to this form involved major changes in the conformation of loop 2 of domain 2 (D2L2), as evidenced by (i) occlusion of a chymotrypsin site within D2L2 and (ii) excimer formation by pyrene groups linked to N306C within this loop. The pore-like form retained the capacity to bind anthrax toxin A moieties and cell surface receptors, but was unable to form pores in membranes or mediate translocation. Mutant PA63 in which D2L2 had been deleted was inactive in pore formation and translocation but, like the prepore, was capable of forming heptamers that converted to an SDS-resistant f...