Proteolytically activated Protective Antigen (PA) moiety of anthrax toxin self-associates to form a heptameric ring-shaped oligomer (the prepore). Acidic pH within the endosome converts the prepore to a pore that serves as a passageway for the toxin's enzymatic moieties to cross the endosomal membrane. Prepore is stable in solution under mildly basic conditions, and lowering the pH promotes a conformational transition to an insoluble pore-like state. N-tetradecylphosphocholine (FOS14) was the only detergent among 110 tested that prevented aggregation without dissociating the multimer into its constituent subunits. FOS14 maintained the heptamers as monodisperse, insertion-competent 440-kDa particles, which formed channels in planar phospholipid bilayers with the same unitary conductance and ability to translocate a model substrate protein as channels formed in the absence of detergent. Electron paramagnetic resonance analysis detected pore-like conformational changes within PA on solubilization with FOS14, and electron micrograph images of FOS14-solubilized pore showed an extended, mushroom-shaped structure. Circular dichroïsm measurements revealed an increase in α helix and a decrease in β structure in pore formation. Spectral changes caused by a deletion mutation support the hypothesis that the 2β2-2β3 loop transforms into the transmembrane segment of the β-barrel stem of the pore. Changes caused by selected point mutations indicate that the transition to α structure is dependent on residues of the luminal 2β11-2β12 loop that are known to affect pore formation. Stabilizing the PA pore in solution with FOS14 may facilitate further structural analysis and a more detailed understanding of the folding pathway by which the pore is formed.
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