Functions of Saccharomyces cerevisiae Ecm27p, a putative Na+/Ca2+ exchanger, in calcium homeostasis, carbohydrate storage and cell cycle reentry from the quiescent phase

The quiescent phase of the cell cycle is of fundamental importance for fungi, yet our understanding of this phase of the cycle is much less well understood than the mitotic cell cycle. We found that the ECM27 gene, which encodes a Na+/Ca2+ exchanger, is responsible for influx of calcium from the extracellular space and release from intracellular stores during membrane stress. Wild type cells increase total Ca2+ in quiescence but cells lacking ECM27 gene fail to do so and are defective in cell cycle reentry from the quiescent phase. ecm27Δ cells are also defective in maintaining trehalose levels throughout this phase. Addition of high levels of CaCl2 to the growth medium can increase total cellular calcium in ecm27Δ cells during quiescence and can also restore trehalose levels as well as partially restore ability of cells to reenter the mitotic cell cycle. ecm27Δ cells also have altered glycogen levels in exponentially growing cells. Our results show that Ecm27p and Ca2+ play roles in maintaining a high level of trehalose in quiescent cells, which in turn is important in the ability of cells to rapidly return to proliferation.