Clostridium botulinum neurotoxins (BoNT) are among the most toxic substances known to man. Seven immunologically distinct serotypes of neurotoxin, designated types A through G (BoNT/A, BoNT/B, BoNT/C, BoNT/D, BoNT/E, BoNT/F, BoNT/G), have been identified. These botulinum neurotoxins are naturally produced complexed with one or more nontoxic neurotoxin-associated proteins (NAPs). NAPs are important to toxicity by ingestion in that they protect the neurotoxin from proteases, acidity, heat and they may also play a role in translocation across the mucosal layer.
Synthesized as single 150 kDa polypeptide chains, the actual neurotoxin portion of the complex is subsequently activated by cleavage to produce two chains, a heavy chain and a light chain, which are linked by a single disulfide bond. For each toxin, the 50 kDa light chain is a zinc-dependent protease, which cleaves a single target protein essential for synaptic vesicle membrane fusion during neurotransmission. Neurotoxin types A, C, and E specifically bind to and selectively cleave the synaptosome-associated protein, SNAP-25 while types B, D, F and G cleave synaptobrevin-2, also known as VAMP-2. Cleavage of the target protein inhibits neurotransmitter release among neurons, which leads to muscular paralysis.
Each toxin is easily divided into three domains that reflect the three functions important for toxicity; binding, translocation, and cleavage of a specific substrate. The heavy chain is responsible for recognizing both a specific ganglioside and a specific protein receptor on the presynaptic membrane. The translocation domain portion of the heavy chain mediates the passage of light chain across the endosomal membrane and finally the target protein is cleaved by the enzymatic domain.
Botulinum neurotoxins are valuable research tools in studies aimed at elucidating the mechanisms involved in vesicle trafficking and the extreme toxicity, as well as in gaining an understanding of the underlying events of synaptic transmission. In addition to studies with the 150 kD holotoxin, recombinant nontoxic heavy chains can be used to evaluate the binding properties of the toxins and the recombinant nontoxic light chains can be used to screen for inhibition of the enzymatic activity of the protease domain.
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