Structure of the fusion pore...

Structure of the fusion pore. A: Helical wheel model for the transmembrane domain of syntaxin. The three residues that line the fusion pore are shown in red and marked with asterisks. B: Model for the opening of a fusion pore; six transmembrane domains of syntaxin are arranged in a triangle. Ca²⁺ triggers the rearrangement of these subunits to form an aqueous channel, potentially through the action of synaptotagmin (syt) on SNARE proteins. After the pore is fully opened, it either closes or expands. Expansion might involve the intercalation of lipids between SNARE subunits, and this step might also be controlled by the action of syt.

Adapted from Han, X., Wang, C.T., Bai, J., Chapman, E.R., and Jackson, M.B. 2004. Science 304:289–292.