A major current issue in vesicle trafficking is whether NSF (N-ethylmaleimide-sensitive factor) and alpha-SNAP (alpha-soluble NSF attachment protein) are required prior to SNARE (SNAP receptor) complex formation to allow vesicle docking, or after docking at a step close to membrane fusion. Recent studies of yeast vacuolar fusion indicated that the requirement for ATP, NSF and alpha-SNAP could be completely satisfied prior to SNARE docking complex assembly; however, the universality of a predocking role for these factors remains to be established. The vacuolar fusion system has also been used to directly demonstrate a requirement for SNARE proteins on both fusing membranes, verifying a central postulate of current fusion models.