Mechanisms of SNARE proteins in membrane fusion

Since the discovery of SNARE proteins in the late 1980s, SNAREs have been recognized as key components of protein complexes that drive membrane fusion. Despite considerable sequence divergence among SNARE proteins, their mechanism seems to be conserved and is adaptable for fusion reactions as diverse as those involved in cell growth, membrane repair, cytokinesis and synaptic transmission. A fascinating picture of these robust nanomachines is emerging.

The two universally required components of the intracellular membrane fusion machinery, SNARE and SM (Sec1/Munc18-like) proteins, play complementary roles in fusion. Vesicular and target membrane-localized SNARE proteins zipper up into an alpha-helical bundle that pulls the two membranes tightly together to exert the force required for fusion. SM proteins, shaped like clasps, bind to trans-SNARE complexes to direct their fusogenic action. Individual fusion reactions are executed by distinct combinations of SNARE and SM proteins to ensure specificity, and are controlled by regulators that embed the SM-SNARE fusion machinery into a physiological context. This regulation is spectacularly apparent in the exquisite speed and precision of synaptic exocytosis, where synaptotagmin (the calcium-ion sensor for fusion) cooperates with complexin (the clamp activator) to control the precisely timed release of neurotransmitters that initiates synaptic transmission and underlies brain function.

The SNARE hypothesis holds that a transport vesicle chooses its target for fusion when a soluble NSF attachment protein (SNAP) receptor on the vesicle (v-SNARE) pairs with its cognate t-SNARE at the target membrane. Three synaptosomal membrane proteins have previously been identified: syntaxin, SNAP-25 (t-SNAREs), and vesicle-associated membrane protein (VAMP) (v-SNARE); all assemble with SNAPs and NSF into 20S fusion particles. We now report that in the absence of SNAP and NSF, these three SNAREs form a stable complex that can also bind synaptotagmin. Synaptotagmin is displaced by alpha-SNAP, suggesting that these two proteins share binding sites on the SNARE complex and implying that synaptotagmin operates as a "clamp" to prevent fusion from proceeding in the absence of a signal. The alpha-SNAP-SNARE complex can bind NSF, and NSF-dependent hydrolysis of ATP dissociates the complex, separating syntaxin, SNAP-25, and VAMP. ATP hydrolysis by NSF may provide motion to initiate bilayer fusion.