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Department of Biochemistry, Dartmouth Medical School, Hanover, NH 03755

Address correspondence to William T. Wickner, Dept. of Biochemistry, 7200 Vail Bldg., Dartmouth Medical School, Hanover, NH 03755-3844. Tel.: (603) 650-1701. Fax: (603) 650-1353. email: william.wickner{at}dartmouth.edu

Ca²⁺ transients trigger many SNARE-dependent membrane fusion events. The homotypic fusion of yeast vacuoles occurs after a release of lumenal Ca²⁺. Here, we show that trans-SNARE interactions promote the release of Ca²⁺ from the vacuole lumen. Ypt7p–GTP, the Sec1p/Munc18-protein Vps33p, and Rho GTPases, all of which function during docking, are required for Ca²⁺ release. Inhibitors of SNARE function prevent Ca²⁺ release. Recombinant Vam7p, a soluble Q-SNARE, stimulates Ca²⁺ release. Vacuoles lacking either of two complementary SNAREs, Vam3p or Nyv1p, fail to release Ca²⁺ upon tethering. Mixing these two vacuole populations together allows Vam3p and Nyv1p to interact in trans and rescues Ca²⁺ release. Sec17/18p promote sustained Ca²⁺ release by recycling SNAREs (and perhaps other limiting factors), but are not required at the release step itself. We conclude that trans-SNARE assembly events during docking promote Ca²⁺ release from the vacuole lumen.

Key Words: membrane; docking; fusion; Rab GTPase; SM-protein

Abbreviations used in this paper: GDI, GDP dissociation inhibitor; RDI, Rho GDI; rVam7p, recombinant Vam7p; SM, Sec1p/Munc18.

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