FIGURE 4. Simultaneous binding with membrane-anchored Syt-II and ganglioside imposes geometric restrictions on how BoNT/B binds to the membrane surface.

a, Proposed binding mode of BoNT/B on the membrane surface. The structure of a sialyllactose-bound BoNT/B (PDB code 1F31) was superimposed on the complex of H_CB–Syt-II by using the coordinates of the H_C fragment for the alignment. The light chain (LC), the N-terminal part of the heavy chain (H_N), and the C-terminal domain of the heavy chain (H_C) are shown in pink, green and gold, respectively. b, A close-up view of the proposed interface between BoNT/B and membrane. Four lysine residues that are conserved in Syt-I and Syt-II are coloured blue.