Membrane fusion as a team effort.

Abstract

Since the discovery that tetanus and botulinum toxins inhibit synaptic vesicle fusion by cleaving three proteins (synaptobrevin/VAMP, SNAP-25, and syntaxin/HPC) at the synapse, it has been known that these proteins, and, by extension, their many homologs [subsequently referred to as SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors)] are essential for fusion (1–5). The discovery that these three proteins bind to each other (6), and do so in a parallel fashion that forces the membranes in which they reside into close proximity (7, 8), prompted a facile model of fusion: namely, that SNAREs generally form trans complexes that link the two membranes destined to fuse with each other and that the full assembly of the SNARE complexes then forces the membranes into such a close proximity that their phospholipid surfaces disintegrate and reanneal to form a bilayer in which the SNAREs are now in cis (reviewed in ref. 9). Initial experiments using liposome containing reconstituted SNARE proteins supported this model and led to the hypothesis that SNAREs by themselves are sufficient to account for the membrane fusion process, i.e., constitute a minimal fusion machinery (10). However, two lines of evidence raised doubts about this hypothesis. First, members of two other protein families, the SM proteins (Sec1/Munc18-like proteins) and the Rab proteins, were found in many in vivo systems to be required for fusion (reviewed in ref. 11). At the synapse, for example, deleting the SM protein Munc18–1 causes a more severe block of fusion than deleting the SNARE proteins synaptobrevin/VAMP or SNAP-25 (12–14). Second, experiments with liposomes containing SNAREs revealed that unphysiologically high concentrations of SNAREs were required for fusion and that the fusion process observed was leaky, i.e., involved breakage of membranes, whereas physiologically, fusion is not leaky (15, 16). These findings demonstrated that SNAREs are not sufficient to account for biological membrane fusion and raised the issue of whether the SNAREs can, in fact, actually initiate fusion, or whether they just bring membranes together in preparation for the subsequent fusogenic activity of SM and/or Rab proteins. The feature article in this issue of PNAS by Starai et al. (17) addresses this issue by using yeast vacuole fusion as a model system. The article elegantly demonstrates that SNARE proteins acts as the motor of fusion as first shown by Weber et al. (10) but that control of this motor by a Rab and SM protein is required for fusion. This requirement is not simply regulatory and cannot be overcome by brute force (i.e., by increasing the concentration of SNARE proteins), but involves Rab and SM proteins as essential organizers of the fusion reaction. Thus, these experiments compare in a biologically relevant in vitro system “SNARE-only fusion” with fusion mediated by SNARE, Rab, and SM proteins (Fig. 1).