All S. cerevisiae strains used were derivatives of BY4742 (ResGen; Invitrogen). The complementation studies33 and measurement of vacuolar pH33,38,61 were done as follows. Yeast strains, media and growth conditions: Derivatives of BY4742 S. cerevisiae strains were grown in synthetic complete medium (SC) to saturation, washed three times in water and used to seed 200 µl of APG medium (Arginine phosphate glucose, a synthetic minimal medium containing 10 mM arginine, 8 mM phosphoric acid, 2% (w/v) glucose, 2 mM MgSO4, 1 mM KCl and 0.2 mM CaCl2 and trace minerals and vitamins) to a starting attenuance of 0.05 D600 units/ml. Phosphoric acid was used to adjust the pH to 4.0 or 2.7., NaCl, KCl or hygromycin was added as indicated and growth was monitored by measuring OD600 after culturing for 24 h at 30° C.Measurement of vacuolar pH: Cells were grown for 18 h at 30 °C in APG growth medium, absorbance readings were taken at 600 nm to measure growth, and cultures were then incubated with 50µM BCECF [2´,7´-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein]-acetoxymethyl ester at 30 °C for 20 min, washed and suspended in
