The Golgi apparatus plays a significant role in the maintenance of Ca2+ homeostasis in the vps33 Delta vacuolar biogenesis mutant of 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, In this study, we examined how cellular Ca2+ homeostasis is maintained in a vps33 Delta vacuolar biogenesis mutant. We found that growth of the vp s33 Delta strain was sensitive to high or low extracellular Ca2+ This strai n could not properly regulate cytosolic Ca2+ levels and was able to retain only a small fraction of its total cellular Ca2+ in a nonexchangeable intra cellular pool. Surprisingly, the vps33 Delta strain contained more total ce llular Ca2+ than the wild type strain. Because most cellular Ca2+ is normal ly found within the vacuole, this suggested that other intracellular compar tments compensated for the reduced capacity to store Ca2+ within the vacuol e of this strain. To test this hypothesis, we examined the contribution of the Golgi-localized Ca2+ ATPase Pmr1p in the maintenance of cellular Ca2+ h omeostasis, We found that a vps33 Delta/pmr1 Delta strain was hypersensitiv e to high extracellular Ca2+. In addition, certain combinations of mutation s effecting both vacuolar and Golgi Ca2+ transport resulted in synthetic le thality. These results indicate that the Golgi apparatus plays a significan t role in maintaining Ca2+ homeostasis when vacuolar biogenesis is compromi sed.