Bontoxilysins

This chapter describes the activity, specificity and structural chemistry of bontoxilysin. Botulinum neurotoxin (BoNT) type C cleaves syntaxin, a 30 kDa protein anchored on the cytosolic surface of the plasma membrane and several intracellular membranes. BoNT type C cleaves syntaxin inserted into liposomes, but is totally inactive on the recombinant molecules in solution. The BoNTs are released by autolysis of the bacteria as single 150 kDa polypeptide chains folded into three distinct domains. Selective proteolysis carried out with trypsin or other proteases cleaves an exposed loop between the first and the second domain and generates the active two-chain toxin with a single interchain disulfide bond. The crystallographic structures of BoNT types A and B have been recently determined and reveal three distinct 50 kDa domains. The N-terminal domain endowed with zinc-endopeptidase activity is disulfide-linked to a membrane translocation domain (HN) characterized by the presence of two 10 nm long a helices. HN mediates the transmembrane translocation of the L chain into the neuronal cytosol. The third domain is responsible for the neurospecific binding of the BoNTs, and consists of two unique subdomains similar to the legume lectins and Kunitz inhibitor.