N-Ethylmaleimide-sensitive factor acts at a prefusion ATP-dependent step in Ca2+-activated exocytosis.

An ATP-dependent activity of NSF (N-ethylmaleimide-sensitive factor) that rearranges soluble NSF attachment protein (SNAP) receptor (SNARE) protein complexes was proposed to be the driving force for membrane fusion. The Ca2+-activated fusion of secretory vesicles with the plasma membrane in permeable PC12 cells requires ATP; however, the ATP requirement is for a priming step that precedes the Ca2+-triggered fusion reaction. While phosphoinositide phosphorylation is a key reaction required for priming, additional ATP-dependent reactions are also necessary. Here we report that the NSF-catalyzed rearrangement of SNARE protein complexes occurs during ATP-dependent priming. NSF with alpha-SNAP (soluble NSF attachment protein) were required for ATP-dependent priming but not Ca2+-triggered fusion, indicating that NSF acts at an ATP-dependent prefusion step rather than at fusion itself. NSF-catalyzed activation of SNARE proteins may reorganize membranes to generate a vesicle-plasma membrane prefusion intermediate that is poised for conversion to full fusion by Ca2+-dependent mechanisms.