Figure 7.
Proposed life cycle of SNAREs. SNARE complexes residing in the same membrane (cis-SNARE complexes) are disrupted by αSNAP and NSF. The energy needed for this process is provided by NSF-catalyzed hydrolysis of ATP. SNAREpins (complexes between vesicles, also called trans-SNARE complexes), on the other hand, are resistant to the action of αSNAP and NSF. It is at this stage that the so-called flickering of the fusion pore (a rapid opening and closing of the fusion pore) might occur. Once the fusion pore is fully dilated, cis-SNARE complexes are formed that are, again, sensitive to αSNAP and NSF. The disruption of cis-SNARE complexes regenerates the SNAREs for a new round of docking and fusion. ATP hydrolysis by NSF supplies the necessary energy input at this point. The resistance of SNAREpins to the action of αSNAP and NSF makes the formation of these SNAREpins, and hence, docking of vesicles, an essentially irreversible step. This confers vectoriality to this process pushing the process to completion, i.e., full fusion.