Plasma membranes were purified from glucose-metabolizing cells by differential and sucrose gradient centrifugation and ATPase activity was assayed at pH 6.5 with 2 mM ATP [39]. Protein concentration was determined by the Bradford method [40] with the Bio-Rad Protein Assay reagent (Bio-Rad Laboratories, Hercules, CA) and bovine IgG as standard. Proton efflux from yeast cells was determined after starvation at 4 C and glucose addition at 30 C [41]. Simultaneous determination of potassium and rubidium content was performed as described [42]. In brief, cells were inoculated in AP medium containing 10 mM KCl and grown until absorbance at OD660 nm = 0.4. Then, the cells were washed twice with 20 mM MgCl2 and suspended in AP medium without potassium. 10 ml aliquots were taken, washed with 20 mM MgCl2 and suspended in 0.2 ml buffer containing 2% glucose, 50 mM succinic acid and 20 mM MgCl2. After 5 min, 5 mM RbCl was added and aliquots were taken, washed twice with 20 mM MgCl2, suspended in 0.5 ml MilliQ water, and boiled to extract the intracellular ions. After removal of cell debris by centrifugation, 1/50 dilutions of the supernatant were used for HPLC analysis in Waters equipment with a IC-PAK CM/D column and a Waters 432 conductivity detector. Elution was made in an isocratic flux, using as a mobile phase 0.1 mM EDTA and 3 mM HNO3. Sample analysis and preparation of rubidium and potassium standards was performed as described by the manufacturer.