Blockage of Anthrax PA63 Pore by a Multicharged High-Affinity Toxin Inhibitor

Single channels of Bacillus anthracis protective antigen, PA63, were reconstituted into planar lipid membranes and their inhibition by cationic aminopropylthio-β-cyclodextrin, AmPrβCD, was studied. The design of the highly efficient inhibitor, the sevenfold symmetrical cyclodextrin molecule chemically modified to add seven positive charges, was guided by the symmetry and predominantly negative charge of the PA63 pore. The protective action of this compound has been demonstrated earlier at both single-molecule and whole-organism levels. In this study, using noise analysis, statistics of time-resolved single-channel closure events, and multichannel measurements, we find that AmPrβCD action is bimodal. The inhibitor, when added to the cis side of the membrane, blocks the channel reversibly. At high salt concentrations, the AmPrβCD blockage of the channel is well described as a two-state Markov process, in which both the on- and off-rates are functions of the salt concentration, whereas the applied voltage affects only the off-rate. At salt concentrations smaller than 1.5 M, the second mode of AmPrβCD action on the channel is discovered: addition of the inhibitor enhances voltage gating, making the closed states of the channel more favorable. The effect depends on the lipid composition of the membrane.