A structural basis for clostridial neurotoxin target specificity

The process of extracellular secretion at chemical synapses is interrupted by the activity of clostridial neurotoxins (CNTs). Disruption of neuronal exocytosis inhibits quantal release of neurotransmitter into synapses. Clinical manifestations of CNT intoxication are the diseases botulism and tetanus. The essential components of exocytotic machinery selectively targeted by the CNTs are SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors). X-ray crystallography was employed to obtain a high-resolution structure of the protease component of botulism neurotoxin serotype A (BoNT/A) in complex with its target SNARE. The determinants of BoNT/A substrate specificity were identified as an array of exosites that provide a substrate-specific boost to the protease's catalytic efficiency. Additionally, the structure of the protease component of tetanus neurotoxin (TeNT) was determined to 2.3A resolution, but efforts to determine the structure of TeNT in complex with its target SNARE failed. A comparison of all available CNT protease structures reveals they share nearly identical folds, with differences primarily limited to surface features. An analysis of surface residue conservation reveals that the most variable residues follow the path of substrate binding observed in the BoNT/A complex. I propose that all CNT proteases utilize a similar arrangement of exosites to those observed in BoNT/A, with surface variations accommodating the differences in their respective SNARE targets. Further, I speculate that SNARE recognition exosites in CNTs may have eukaryotic origins. The detailed characterization of interactions between CNT proteases and their target SNAREs may be useful for development of novel and extremely specific inhibitors for these potent toxins.