Class C Vps protein complex regulates vacuolar SNARE pairing and is required for vesicle docking/fusion

In yeast, the Class C Vps protein complex (C-Vps complex), composed of Vps11, Vps16, Vps18, and Vps33, functions in Golgi-to-vacuole protein transport. In this study, we characterized and purified this complex and identified its interaction with the syntaxin homolog Vam3. Vam3 pairs with the SNAP-25 homolog Vam7 and VAMP homolog Vti1 to form SNARE complexes during vesicle docking/fusion with the vacuole. The C-Vps complex does not bind to Vam3-Vti1-Vam7 paired SNARE complexes but instead binds to unpaired Vam3. Antibodies to a component of this complex inhibited in vitro vacuole-to-vacuole fusion. Furthermore, temperature-conditional mutations in the Class C VPS genes destabilized Vam3-Vti1-Vam7 pairing. Therefore, we propose that the C-Vps complex associates with unpaired (activated) Vam3 to mediate the assembly of trans-SNARE complexes during both vesicle docking/fusion and vacuole-to-vacuole fusion.