In the yeast Saccharomyces cerevisiae, plasma membrane H+-ATPase plays key roles in the regulation of intracellular pH (by countering the pH-reducing effect of environmental factors) and in the transport of nutrients into the cell (by generating the necessary extracellular H+ gradient). The activation of the H+-ATPase pump by glucose is a well-known process that was first described by Serrano [1]. The underlying mechanism involves changes in kinetic parameters (e.g., increased Vmax and decreased KmATP) and increased expression of thepma1 gene at the transcriptional level. The decrease in intracellular pHinduced by organic acids [2] was shown to induce H+-ATPase activation via a Ca2+-dependent mechanism [3–5]. Lecchi and co-workers[6] demonstrated that this mechanism involves phosphorylation of the Ser 911 and Thr 912 residues of the ATPase and that such events are related to kinetic changes of the enzyme