Vacuole acidification is required for trans-SNARE pairing, LMA1 release, and homotypic fusion

Vacuole fusion occurs in three stages: priming, in which Sec18p mediates Sec17p release, LMA1 (low M-r activity 1) relocation, and cis-SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) complex disassembly; docking, mediated by Ypt7p and trans-SNARE association; and fusion of docked vacuoles. Ca2+ and calmodulin regulate late stages of the reaction. We now show that the vacuole proton gradient, generated by the vacuolar proton ATPase, is needed for trans-SNARE complex formation during docking and hence for the subsequent LMA1 release. Though neither the vacuolar Pmc1p Ca2+-ATPase nor the Vcx1p Ca2+/H+ exchanger are needed for the fusion reaction, they participate in Ca2+ and Delta mu(H)(+) homeostasis. Fusion itself does not require the maintenance of trans-SNARE pairs.