Dominant antigenic peptides located at the heavy chain terminal of botulinum neurotoxin B contain receptor-binding sites for synaptotagmin II

Botulism toxicity is caused by botulinum neurotoxins (BoNTs) that bind to the surface of the nerve terminals through the C-terminal portion of the heavy chain (Hc) and gain access to the neuron cells, thereby blocking the release of neurotransmitters into the synapse by the light chain. It has been recently found that synaptotagmin II (syt II) is the specific protein receptor of BoNTs; however, the molecular basis underlying the interaction between BoNTs and the receptor has been poorly understood. In this study, the recombinant fragment from the luminal domain of the human protein receptor syt II was prepared. Further, the Hc antigenic peptides of BoNTs type B (BoNTb) containing the possible epitopes were synthesized and analyzed for checking their ability for specific recognition and binding by using completely protective polyclonal antibody and were then evaluated for checking their ability to elicit protective immunity in mice against lethal toxin challenge. Finally, the interaction between the Hc-dominant antigenic peptides and syt II was analyzed. We found that two peptides located at the Hc terminal of the BoNTb showed the general character of the dominant antigenic epitopes in terms of protection against BoNTb intoxication. Further, they were found to contain the receptor-binding sites through the specific recognition of the receptor syt II. Both were derived from the uninterrupted segments of the amino acid residues H1241–1277 of BoNTb. Kinetic data of the antigen–receptor interaction were depicted in real-time and calculated as affinity constant of 2.99 × 10–7 M.