Novel chimeras of botulinum and tetanus neurotoxins yield insights into their distinct sites of neuroparalysis.

Botulinum neurotoxin (BoNT) A or E and tetanus toxin (TeTx) bind to motor-nerve endings and undergo distinct trafficking; their light-chain (LC) proteases cleave soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) peripherally or centrally and cause flaccid or spastic paralysis, respectively. To seek protein domains responsible for local blockade of transmitter release (BoNTs) rather than retroaxonal transport to spinal neurons (TeTx), their acceptor-binding moieties (H(C))--or in one case, heavy chain (HC)--were exchanged by gene recombination. Each chimera, expressed and purified from Escherichia coli, entered rat cerebellar neurons to cleave their substrates, blocked in vitro nerve-induced muscle contractions, and produced only flaccid paralysis in mice. Thus, the local cytosolic delivery of BoNT/A or BoNT/E proteases and the contrasting retrograde transport of TeTx are not specified solely by their HC or H(C); BoNT/A LC translocated locally irrespective of being targeted by either of the latter TeTx domains. In contrast, BoNT/E protease fused to a TeTx enzymatically inactive mutant (TeTIM) caused spastic paralysis and cleaved SNAP-25 in spinal cord but not the injected muscle. Apparently, TeTIM precludes cytosolic release of BoNT/E protease at motor nerve endings. It is deduced that the LCs of the toxins, acting in conjunction with HC domains, dictate their local or distant destinations.