Fig 8. Modeling positions HOPS as a dual regulator of guard cell vacuole fusion.

(A) HOPS promotes formation of the trans-SNARE fusion machinery, but then prevents fusion activity by hindering the access of Sec17. (B) A biological signal capable of reducing HOPS:trans-SNARE binding affinity (e.g., a signal resulting in the post-translational modification of a HOPS subunit, as indicated by the yellow star) could create the conditions required for Sec17 to displace HOPS and (C) thereby activate the otherwise stalled fusion complex, resulting in (D) membrane rearrangement and vacuole fusion.