The HCC-domain of botulinum neurotoxins A and B exhibits a singular ganglioside binding site displaying serotype specific carbohydrate interaction

Tetanus and botulinum neurotoxins selectively invade neurons following binding to complex gangliosides. Recent biochemical experiments demonstrate that two ganglioside binding sites within the tetanus neurotoxin H-C-fragment, originally identified in crystallographic studies to bind lactose or sialic acid, are required for productive binding to target cells. Here, we determine by mass spectroscopy studies that the H-C-fragment of botulinum neurotoxins A and B bind only one molecule of ganglioside GT1b. Mutations made in the presumed ganglioside binding site of botulinum neurotoxin A and B abolished the formation of these H-C-fragment/ganglioside complexes, and drastically diminished binding to neuronal membranes and isolated GT1b. Furthermore, correspondingly mutated full-length neurotoxins exhibit significantly reduced neurotoxicity, thus identifying a single ganglioside binding site within the carboxyl-terminal half of the H-C-fragment of botulinum neurotoxins A and B. These binding cavities are defined by the conserved peptide motif H...SXWY...G. The roles of tyrosine and histidine in botulinum neurotoxins A and B in ganglioside binding differ from those in the analogous tetanus neurotoxin lactose site. Hence, these findings provide valuable information for the rational design of potent botulinum neurotoxin binding inhibitors.