Elucidating the crystal structure of botulinum neurotoxin type A: a personal remembrance

Botulinum neurotoxins (BoNTs) are the bacterial proteins responsible for the flaccid paralysis and lethal effects of botulism. They act by inhibiting neurotransmitter release, primarily at peripheral cholinergic nerve terminals, and have traditionally been classified into seven serotypes, designated with letters A through G. Over a long history of investigation, scientists and physicians have learned to harness the selectivity and potency of these neurotoxins to advance therapeutic applications. These advances have come by leveraging the basic science understanding of how the BoNT's work. The BoNTs are typically produced as inactive single-chain proteins of 150 kDa that can be proteolytically activated to form a 50 kDa light chain (LC) and 100 kDa heavy chain (HC) that remain linked by a disulfide bond. The BoNTs act through a multi-step mechanism in which the HC mediates receptor binding and translocation of the LC into the neuronal cell cytosol. The LC is a zinc endopeptidase and each BoNT serotype cleaves a unique and specific bond within the three-protein soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. In the late 1990s, I had the opportunity to help visualize the atomic structure of this formidable molecule, a story I share to honor the legacy of Hans Bigalke, whose work set the stage for new careers and discoveries in BoNT research and medicine.