The Ca super(2+) Homeostasis Defects in a pgm2delta Strain of Saccharomyces cerevisiae Are Caused by Excessive Vacuolar Ca super(2+) Uptake Mediated by the Ca super(2+)-ATPase Pmc1p

Loss of the major isoform of phosphoglucomutase (PGM) causes an accumulation of glucose 1-phosphate when yeast cells are grown with galactose as the carbon and energy source. Remarkably, the pgm2delta strain also exhibits a severe imbalance in intracellular Ca super(2+) homeostasis when grown under these conditions. In the present study, we examined how the pgm2delta mutation alters yeast Ca super(2+) homeostasis in greater detail. We found that a shift from glucose to galactose as the carbon source resulted in a 2-fold increase in the rate of cellular Ca super(2+) uptake in wild-type cells, whereas Ca super(2+) uptake increased 8-fold in the pgm2delta mutant. Disruption of the PMC1 gene, which encodes the vacuolar Ca super(2+)-ATPase Pmc1p, suppressed the Ca super(2+)-related phenotypes observed in the pgm2delta strain. This suggests that excessive vacuolar Ca super(2+) uptake is tightly coupled to these defects in Ca super(2+) homeostasis. An in vitro assay designed to measure Ca super(2+) sequestration into intracellular compartments confirmed that the pgm2delta mutant contained a higher level of Pmc1p-dependent Ca super(2+) transport activity than the wild-type strain. We found that this increased rate of vacuolar Ca super(2+) uptake also coincided with a large induction of the unfolded protein response in the pgm2delta mutant, suggesting that Ca super(2+) uptake into the endoplasmic reticulum compartment was reduced. These results indicate that the excessive Ca super(2+) uptake and accumulation previously shown to be associated with the pgm2delta mutation are due to a severe imbalance in the distribution of cellular Ca super(2+) into different intracellular compartments.