Cytosolic Ca2+ homeostasis is a constitutive function of the V-ATPase in Saccharomyces cerevisiae

The vacuole is the major site of intracellular Ca2+ storage in yeast and fu nctions to maintain cytosolic Ca2+ levels within a narrow physiological ran ge via a Ca2+ pump (Pmc1p) and a H+/Ca2+ antiporter (Vcx1p) driven by the v acuolar H+-ATPase (V-ATPase). We examined the function of the V-ATPase in c ytosolic Ca2+ homeostasis by comparing responses to a brief Ca2+ challenge of a V-ATPase mutant (vma2 Delta) and wild-type cells treated with the V-AT Pase inhibitor concanamycin A. The kinetics of the Ca2+ response were deter mined using transgenic aequorin as an in vivo cytosolic Ca2+ reporter syste m. In wild-type cells, the V-ATPase-driven Vcx1p was chiefly responsible fo r restoring cytosolic Ca2+ concentrations after a brief pulse. In cells lac king V-ATPase activity, brief exposure to elevated Ca2+ compromised viabili ty, even when there was little change in the final cytosolic Ca2+ concentra tion. vma2 Delta cells were more efficient at restoring cytosolic [Ca2+] af ter a pulse than concanamycin-treated wild-type cells, suggesting long term loss of V-ATPase triggers compensatory mechanisms. This compensation was d ependent on calcineurin, and was mediated primarily by Pmc1p.