Capture and release of partially zipped trans-SNARE complexes on intact organelles

Soluble N-ethyl-maleimide sensitive fusion protein attachment protein receptors (SNARES) are hypothesized to trigger membrane fusion by complexing in trans through their membrane-distal N termini and zippering toward their membrane-embedded C termini, which in turn drives the two membranes together. In this study, we use a set of truncated SNARES to trap kinetically stable, partially zipped traps-SNARE complexes on intact organelles in the absence of hemifusion and content mixing. We show that the C-terminal zippering of SNARE cytoplasmic domains controls the onset of lipid mixing but not the sub-sequent transition from hemifusion to full fusion. Moreover, we find that a partially zipped nonfusogenic traps-complex is rescued by Sec17, a universal SNARE cochaperone. Rescue occurs independently of the Sec17-binding partner Sec18, and it exhibits steep cooperativity, indicating that Sec17 engages multiple stalled traps-complexes to drive fusion. These experiments delineate distinct functions within the traps-complex, provide a straightforward method to trap and study prefusion complexes on native membranes, and reveal that Sec17 can rescue a stalled, partially zipped traps-complex.