The light chain of botulinum neurotoxin forms channels in a lipid membrane

The ability of botulinum neurotoxin and its isolated subunits, the heavy and light chains, to bind to a lipid membrane and to form channels in the membrane was examined. At pH 4.0, the neurotoxin caused aggregation of calcein-containing liposomes, providing evidence of binding of the neurotoxin to the surface of the outer lipid membrane. Aggregation was followed by the release of calcein, as a result of the formation of channels. The heavy chain evoked the same responses as those of the neurotoxin. The light chain did not cause aggregation of the liposomes but did evoke the release of calcein. The channel-forming ability of the light chain appeared to be higher than that of the neurotoxin or the heavy chain. This novel property of the light chain may help us to understand the mechanism of action of botulinum neurotoxin.