Introduction . SNARE-dependent membrane fusion is required for the secretion of most chemical signals1 , 2. As minimal fusion machinery, SNAREs couple their folding and assembly to membrane fusion3 , 4 , 5. The fully assembled SNARE complex is a four-helix bundle formed by characteristic SNARE motifs. While SNARE complexes can spontaneously form in vitro with low speed and specificity, their assembly in vivo is controlled by many proteins, such as Munc18s, Munc13s, syna...