Abstract
Yeast vacuolar fusion is driven by Sec17, Sec18, SNAREs of four families (R [Nyv1], Qa [Vam3], Qb [Vti1], Qc [Vam7]) and HOPS, a catalyst of SNARE assembly. Qc, the only vacuolar SNARE that is not membrane-anchored, has a unique path of assembly with other fusion catalysts. Qc is the only SNARE that binds Sec17 with high affinity. Sec18 confers a high affinity for Qc (but not Qb) on HOPS-dependent fusion, but it has been unclear how Sec18 acts. The membrane complex of Sec17 and Sec18 binds Qc to form a membrane:Sec18:Sec17:Qc complex. Sec18 ATP hydrolysis, though dispensable for fusion, provides a measure of the physical and functional interactions between Qc, Sec17, Sec18, and membranes. Each binary interface in this quaternary complex regulates Sec18 ATPase and fusion. Qc is better than other SNAREs, alone or in combination, for stimulating ATP hydrolysis. We propose a working model in which membrane-bound Qc:Sec17:Sec18 associates with the trans complex of HOPS:R:QaQb, displacing HOPS while providing both Qc for complete SNARE zippering and localized Sec17 apolar loops, the twin driving forces for fusion.