Interaction of Clostridium botulinum C2-toxin with lipid bilayer membranes and vero cells:: inhibition of channel function by chloroquine and related compounds in vitro and intoxification in vivo

Several intracellularly acting bacterial protein toxins, which are known to enter cells by endocytosis, are shown to produce channels. This finding also holds true for the C2-II binding component of C2-toxin of Clostridium botulinum. There is evidence that toxin translocation across the target cell membrane and channel formation by the binding component are related phenomena. Here, we demonstrate that C2-II is fully reconstituted when it is added to only one side of the lipid bilayer membrane. Chloroquine and some related compounds, known as potent drugs against malaria infection of humans, efficiently block the C2-II-mediated channel in a dose-dependent way. The half-saturation constant for binding of chloroquine and some of the related compounds to C2-II is in the micromolar to millimolar range. Their binding to the C2-II channel is asymmetric with respect to its addition to one side of the membrane, and the half-saturation constant is smaller when both inhibitor and protein are added to the same side of the membrane, the cis-side, which corresponds to the external surface of target cells. In vivo experiments with Vero cells demonstrate that chloroquine and related compounds also efficiently block intoxication of the cells by C2-toxin in the same concentration range as they block in vitro the channels.