Design of mutations to disrupt Munc18-1–SNARE interactions. (A) Domain diagram of syntaxin-1. NTS, N-terminal sequence; TM, transmembrane region. (B) Diagrams of the binary complex between Munc18-1 and closed syntaxin-1 (left) and the Munc18-1–SNARE complex assembly (right). Munc18-1 is purple, synaptobrevin is red, SNAP-25 is green, and syntaxin-1 is orange (H_abc domain) and yellow (SNARE motif and transmembrane region). The model of the binary complex is based on its crystal structure (Misura et al., 2000; Burkhardt et al., 2008). The model of the Munc18-1–SNARE complex assemblies is based on NMR data suggesting a multifaceted interaction and illustrates the overall notion that these assemblies are critical for membrane fusion (Dulubova et al., 2007; and for a concrete physical model of how these assemblies can induce membrane fusion, see Rizo et al. [2006]). (C) Models of potential interactions between Munc18-1 and open syntaxin-1 within syntaxin-1–SNAP-25 heterodimers. The model on the left is based on the finding that Munc18-1 can bind to the isolated syntaxin-1 N-terminal region (Khvotchev et al., 2007; Burkhardt et al., 2008), whereas the model on the right also incorporates interactions with the SNARE motifs (Weninger et al., 2008). (D) Ribbon diagram of the binary Munc18-1–syntaxin-1 complex with Munc18-1 colored in purple except for the N-terminal domain, which is in cyan, and syntaxin-1, which is in orange (H_abc domain) and yellow (linker and SNARE motif). The red asterisk indicates the position where cerulean was inserted for the rescue experiments. A close-up of the interface showing the mutated residues is shown on the right. The diagrams were prepared with Pymol (DeLano Scientific).