After their membrane assembly, Sec18 (NSF) and Sec17 (SNAP) promote membrane fusion

The energy that drives membrane fusion can come from either complete SNARE zippering, from Sec17 and Sec18, or both. Sec17 and Sec18 initially form a complex which binds membranes. Sec17, Sec18, and the apolarity of a loop on the N-domain of Sec17 are required for their interdependent membrane association. To determine whether Sec18 and the Sec17 loop apolarity are still required for fusion after their membrane arrival, a hydrophobic transmembrane (TM) anchor was affixed to the N-terminus of Sec17, forming TM-Sec17. Fusion without energy from complete SNARE zippering requires Sec18 as well as either Sec17 or TM-Sec17. Even without the need for membrane targeting, the TM-Sec17 apolar loop strongly stimulates Sec17/18-driven fusion. Thus, Sec18 and the Sec17 apolar loop are first required for membrane targeting, and once bound, drive rapid fusion. Each of these variables-the absence or presence of Sec17, its N-loop apolarity, addition or omission of Sec18, and unimpeded or diminished energy from SNARE zippering-has almost no effect on the amount of trans-SNARE complex, but instead regulates the capacity of docked membranes to fuse.