Trans-SNARE interactions elicit Ca(2+) efflux from the yeast vacuole lumen

Ca(2+) transients trigger many SNARE-dependent membrane fusion events. The homotypic fusion of yeast vacuoles occurs after a release of lumenal Ca(2+). Here, we show that trans-SNARE interactions promote the release of Ca(2+) from the vacuole lumen. Ypt7p–GTP, the Sec1p/Munc18-protein Vps33p, and Rho GTPases, all of which function during docking, are required for Ca(2+) release. Inhibitors of SNARE function prevent Ca(2+) release. Recombinant Vam7p, a soluble Q-SNARE, stimulates Ca(2+) release. Vacuoles lacking either of two complementary SNAREs, Vam3p or Nyv1p, fail to release Ca(2+) upon tethering. Mixing these two vacuole populations together allows Vam3p and Nyv1p to interact in trans and rescues Ca(2+) release. Sec17/18p promote sustained Ca(2+) 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 Ca(2+) release from the vacuole lumen.