, 1993 and Dulubova et al., 1999). When the closed conformation of syntaxin-1 “opens” in preparation to fusion and SNARE complexes form, Munc18-1 remains attached to syntaxin-1 in the assembling SNARE complex but switches its binding mode to an interaction with the SNARE complex ( Dulubova et al., 2007). Assembly of these SNARE/SM complexes mediates fusion, whereas disassembly of these complexes recycles SNARE and SM

proteins for further use ( Figure 2; reviewed in Südhof and Rothman, 2009). CCI-779 solubility dmso The continued association of Munc18-1 to SNARE complexes throughout their assembly/disassembly cycle is essential for fusion ( Khvotchev et al., 2007 and Zhou et al., 2013a). SNARE/SM protein complex assembly is maintained by chaperones whose dysfunction causes neurodegeneration (CSPs and synucleins; Burré et al., 2010 and Sharma et al., 2011), whereas disassembly is mediated by an evolutionarily conserved specialized ATPase (NSF) and its adaptors (SNAPs; Söllner et al., 1993b). The underlying principle of SNARE and SM protein function is simple (Figure 2): SNARE proteins embedded in the two fusing membranes form a trans-complex that involves a progressive zippering Protein Tyrosine Kinase inhibitor of the four-helical SNARE complex in an N- to C-terminal direction ( Hanson et al., 1997). Zippering of trans-SNARE complexes forces the fusing membranes into close proximity, destabilizing their hydrophilic surfaces. Assembly of the

full trans-SNARE complex together with the action of the SM protein opens the fusion pore. Fusion pore expansion transforms the initial “trans”-SNARE complexes into “cis”-SNARE complexes that are then dissociated by NSF (which binds to

SNARE complexes via SNAP adaptor proteins), completing the cycle. In a physiological context, SNARE complex assembly alone does not mediate fusion. We initially proposed that the SM protein Munc18-1 is an essential component of the core fusion machinery (Hata et al., 1993). This proposal was supported by pioneering work from the Novick laboratory demonstrating that the yeast SM protein Sec1p (the Munc18-1 homolog in yeast exocytosis) binds to assembled SNARE complexes (Carr et al., 1999). Moreover, nearly Novick and colleagues showed that Sec1p mutations completely block fusion but do not impair SNARE complex assembly (Grote et al., 2000). However, the proposal that Munc18-1 activates fusion met resistance because Munc18-1 binding to closed syntaxin-1 appeared to suggest that Munc18-1 is an inhibitor of fusion (Yang et al., 2000), despite the fact that Munc18-1 is absolutely required for synaptic fusion (Verhage et al., 2000). This issue was only resolved over the last decade when studies revealed that Munc18-1 remains associated during fusion with SNARE proteins throughout their assembly/disassembly cycle, thereby removing the notion of an inhibitor (Dulubova et al., 2007 and Shen et al., 2007).