A molecular basis underlying differences in the toxicity of botulinum serotypes A and E.

Botulinum neurotoxins (BoNTs) block neurotransmitter release through their specific proteolysis of the proteins responsible for vesicle exocytosis. Paradoxically, two serotypes of BoNTs, A and E, cleave the same molecule, synaptosome-associated protein with relative molecular mass 25K (SNAP-25), and yet they cause synaptic blockade with very different properties. Here we compared the action of BoNTs A and E on the plasma membrane fusion machinery composed of syntaxin and SNAP-25. We now show that the BoNT/A-cleaved SNAP-25 maintains its association with two syntaxin isoforms in vitro, which is mirrored by retention of SNAP-25 on the plasma membrane in vivo. In contrast, BoNT/E severely compromises the ability of SNAP-25 to bind the plasma membrane syntaxin isoforms, leading to dissociation of SNAP-25. The distinct properties of botulinum intoxication, therefore, can result from the ability of shortened SNAP-25 to maintain its association with syntaxins-in the case of BoNT/A poisoning resulting in unproductive syntaxin/SNAP-25 complexes that impede vesicle exocytosis.