Figure 6. Complexin-1 may hinder the final action of trans-SNARE complexes to bring membranes together.

(A–C) Close-up views of one of the SNARE complexes bridging a vesicle and a flat bilayer after simulation for 280 ns at 310 K (A) (shown in Figure 2D after 520 ns at 310 K and 454 ns at 325 K), of PC1 in the first MD simulation of primed complexes after 336 ns (B) (also shown in Figure 3E) and of PC1 in the second MD simulation of primed complexes after 310 ns (C) (also shown in Figure 4—figure supplement 2E). The complexes are illustrated by ribbon diagrams, with the SNAREs in salmon, Cpx1(27-72) in yellow and the Syt1 C₂AB fragment in cyan (C₂A domain) and violet (C₂B domain). The positions of the syntaxin-1 juxtamembrane linkers (sxl) are indicated. The comparison shows how the SNARE complex with the fully assembled four-helix bundle in (A) drew the two membranes closer than the two primed complexes of (B,C). (D) Close up view of the pose shown in (B) after moving the vesicle upwards so that it’s minimal distance from the flat bilayer is 3 nm.