Fig 4. Final schematic offering the hypothesis that HOPS displacement is required to activate the trans-SNARE complex and drive fusion.

The literature on yeast vacuole fusion offers a candidate signal with precisely that function – i.e., activating assembled, but inactive, trans-SNARE complexes. Furthermore, the Arabidopsis analogue of that yeast protein is expressed in guard cells. In our schematic, this protein, Sec17, is a cytoplasmic species recruited to the membrane (event 6), where it can associate with and activate trans-SNARE complexes (event 9). We posit that Sec17 cannot displace HOPS from HOPS:trans-SNARE super-complexes until the system receives some upstream signal that triggers stoma opening (event 8). After HOPS displacement, the fusion-competent complex drives fusion events (event 10). We allow the abstraction in event 10 to be inclusive of post-fusion phenomena such as disassembly of the cis-SNARE complex, freeing individual SNARE proteins to participate in further rounds of fusion. Event 7 represents turnover of Sec17 from the membrane. Events 1 through 5 are retained from earlier schematics (Figure 3).