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The HC fragment of BoNT/A can rotate around a linker connecting HN and HC. (A) Superposition of the free and complex forms of BoNT/A based on Cα atoms in LC and HN. HC of free BoNT/A is violet; LC and HN are omitted for clarity. Linkers in the free and complex forms of BoNT/A are gray and red, respectively. (B) Cartoon model showing two distinct conformations of BoNT/A that bring the receptor-binding site in HC (red star) close to the opposite tip of the long helical HN.
