Cytosolic Ca Homeostasis Is a Constitutive Function of the V-ATPase in Saccharomyces cerevisiae*

The vacuole is the major site of intracellular Ca storage in yeast and functions to maintain cytosolic Ca levels within a narrow physiological range via a Ca pump (Pmc1p) and a H/Ca antiporter (Vcx1p) driven by the vacuolar H-ATPase (V-ATPase). We examined the function of the V-ATPase in cytosolic Ca homeostasis by comparing responses to a brief Ca challenge of a V-ATPase mutant (vma2D) and wild-type cells treated with the V-ATPase inhibitor concanamycin A. The kinetics of the Ca response were determined using transgenic aequorin as an in vivo cytosolic Ca reporter system. In wild-type cells, the V-ATPase-driven Vcx1p was chiefly responsible for restoring cytosolic Ca concentrations after a brief pulse. In cells lacking V-ATPase activity, brief exposure to elevated Ca compromised viability, even when there was little change in the final cytosolic Ca concentration. vma2D cells were more efficient at restoring cytosolic [Ca] after a pulse than concanamycin-treated wild-type cells, suggesting long term loss of V-ATPase triggers compensatory mechanisms. This compensation was dependent on calcineurin, and was mediated primarily by Pmc1p.