(A) C1C2BMUNC2C is represented by the structure of C1C2BMUN, with the C2B domain replaced by the structure of the isolated Ca2+-bound C2B domain (colored in salmon), and by a blue ellipse corresponding to the C2C domain. C1C2BMUNC2C is shown bridging the two membranes through interactions of the vesicle membrane with the C2C domain and the plasma membrane with the basic surface formed by the C1 and C2B domains (left panel) or with the DAG-binding region of the C1 domain and the Ca2+-binding region of the C2B domain (right panel) (see Liu et al., 2016). Arginine and lysine side chains are shown as blue spheres. The C1 domain ribbon is in cyan and Zn2+ ions are shown as yellow spheres. The two Ca2+ ions bound to the C2B domain are shown as green spheres on the right; on the left, they are shown as gray spheres to represent that the sites are not occupied. The NF sequence involved in opening syntaxin-1 (Yang et al., 2015) is represented by orange spheres. The closed syntaxin-1-Munc18–1 complex (PDB code 3C98) is shown to scale to allow comparison of its size with that of C1C2BMUN. The C-terminus of the syntaxin-1 SNARE motif and the cytoplasmic region of synaptobrevin are shown as dashed curved lines, and their transmembrane regions are represented by cylinders. B. C1C2BMUNC2C is represented as in A but, instead of being located between the two membranes, is bridging the two membranes from a peripheral location. The model must be visualized in three dimensions, such that the C2C domain is bound to the outer surface of the vesicle membrane rather than inserted into the vesicle lumen. On the right side, the C2C domain is in the front, whereas in the left side the C2C domain is in the back of the vesicle, which is represented by a semitransparent gray surface. In both panels, C1C2BMUNC2C is shown in the orientation proposed for the activated state induced during repetitive stimulation, where binding to the plasma membrane is mediated by the DAG-binding region of the C1 domain and the Ca2+-binding region of the C2B domain. The SNARE complex (PDB code 1N7S) (Ernst and Brunger, 2003), with the SNARE motifs in green for SNAP-25, in red for synaptobrevin and in yellow for syntaxin-1, is shown partially assembled at the top and fully assembled at the bottom. The arrows in the top panel are meant to illustrate that complete assembly of the C-terminus of the SNARE complex might pull the membranes radially outwards, which could create strong membrane tension to trigger membrane fusion. See text for further details.