Productive and non-productive binding of botulinum neurotoxin A to motor nerve endings are distinguished by its heavy chain.

Botulinum neurotoxin type A, a di-chain protein produced by Clostridium botulinum and responsible for botulism, blocks acetylcholine release from peripheral nerves by binding to the terminals, undergoing internalization and proteolyzing a protein essential for exocytosis. As butolinum neurotoxin is being used clinically for the treatment of dystonias and certain spasticities, deciphering the details of its specific targeting to cholinergic nerve endings has assumed great importance. Thus, interaction of butolinum neurotoxin type A with murine motor nerve terminals-a prime target in vivo-was investigated. Autoradiographic analysis revealed saturable, high-affinity interaction of radioiodinated toxin (0.4 nM) with two ecto-acceptor types, distinguished by an excess of the toxin's heavy chain which prevented only a fraction of this binding. Botulinum neurotoxin was also biotinylated through its free sulfhydryl groups, known not to be essential for neurotoxicity. Similar binding of this active derivative was, likewise, partially blocked by heavy chain, confirming the above results. This binding that is resistant to heavy chain equates to botulinum neurotoxin interacting with productive ecto-acceptors, leading to delivery to its cytosolic site of action, because heavy chain proved unable to antagonize toxin-induced neuromuscular paralysis. In contrast, it is deduced that botulinum neurotoxin bound to heavy chain-susceptible sites has a different fate, presumably due to trafficking via another route, and thus would be inefficient in causing neuroparalysis.