Coordinated glucose-induced Ca²⁺ and pH responses in yeast Saccharomyces cerevisiae

Ca2+ and pH homeostasis are closely intertwined and this interrelationship is crucial in the cells' ability to adapt to varying environmental conditions. To further understand this Ca2+-pH link, cytosolic Ca2+ was monitored using the aequorin-based bioluminescent assay in parallel with fluorescence reporter-based assays to monitor plasma membrane potentials and intracellular (cytosolic and vacuolar) pH in yeast Saccharomyces cerevisiae. At external pH 5, starved yeast cells displayed depolarized membrane potentials and responded to glucose readdition with small Ca2+ transients accompanied by cytosolic alkalinization and profound vacuolar acidification. In contrast, starved cells at external pH 7 were hyperpolarized and glucose re-addition induced large Ca2+ transients and vacuolar alkalinization. In external Ca2+-free medium, glucose-induced pH responses were not affected but Ca2+ transients were abolished, indicating that the intracellular [Ca2+] increase was not prerequisite for activation of the two primary proton pumps, being Pma1 at the plasma membrane and the vacuolar and Golgi localized V-ATPases. A reduction in Pma1 expression resulted in membrane depolarization and reduced Ca2+ transients, indicating that the membrane hyperpolarization generated by Pma1 activation governed the Ca2+ influx that is associated with glucose-induced Ca2+ transients. Loss of V-ATPase activity through concanamycin A inhibition did not alter glucose-induced cytosolic pH responses but affected vacuolar pH changes and Ca2+ transients, indicating that the V-ATPase established vacuolar proton gradient is substantial for organelle H+/Ca2+ exchange. Finally, a systematic analysis of yeast deletion strains allowed us to reveal an essential role for both the vacuolar H+/Ca2+ exchanger Vcx1 and the Golgi exchanger Gdt1 in the dissipation of intracellular Ca2+.