Ion-conducting channels produced by botulinum toxin in planar lipid membranes.

The interaction of botulinum neurotoxin (Botx) with planar lipid membranes was studied by measuring the ability of the toxin to form ion-conducting channels. Channel formation was pH dependent. At physiological pH, Botx formed no channels, whereas at pH 6.6, the toxin formed channels with a unit conductance of 12 pS in 0.1 M NaCl. The rate of channel formation increased with decreasing pH, reaching a maximum at pH 6.1, and then decreased at lower values of pH. The channels, once formed, were permanent entities in the membrane throughout the course of an experiment and fluctuated between an open and a closed state. The rate of channel formation depended upon the square of the toxin concentration, suggesting an aggregation step is involved in channel formation. The data were consistent with the hypothesis that Botx enters cells through endocytosis, followed by its release into the cytoplasm at low pH.