Intracellular glucose 1-phosphate and glucose 6-phosphate levels modulate Ca2+ homeostasis in Saccharomyces cerevisiae

The enzyme phosphoglucomutase plays a key role in cellular metabolism by virtue of its ability to interconvert Glc-1-P and Glc-6-P. It was recently shown that a yeast strain lacking the major isoform of phosphoglucomutase (pgm2Delta) accumulates a high level of Glc-1-P and exhibits several phenotypes related to altered Ca2+ homeostasis when D-galactose is utilized as the carbon source (Fu, L., Miseta, A., Hunton, D., Marchase, R. B., and Bedwell, D. M. (2000) J. Biol. Chem 275, 5431-5440). These phenotypes include increased Ca2+ uptake and accumulation and sensitivity to high environmental Ca2+ levels. In the present study, we overproduced the enzyme UDP-Glc pyrophosphorylase to test whether the overproduction of a downstream metabolite produced from Glc-1-P can also mediate changes in Ca2+ homeostasis. We found that overproduction of UDP-Glc did not cause any alterations in Ca2+ uptake or accumulation. We also examined whether Glc-6-P can influence cellular Ca2+ homeostasis. A yeast strain lacking the beta-subunit of phosphofructokinase (ptk2Delta) accumulates a high level of Glc-6-P (Huang, D., Wilson, W. A., and Roach, P. J. (1997) J. BioL Chem. 272, 22495-22501). We found that this increase in Glc-6-P led to a 1.5-2-fold increase in total cellular Ca2+. We also found that the pgm2Delta/pfk2Delta strain, which accumulated high levels of both Glc-6-P and Glc-1-P, no longer exhibited the Ca2+-related phenotypes associated with high Glc-1-P levels in the pgm2Delta mutant. These results provide strong evidence that cellular Ca2+ homeostasis is coupled to the relative levels of Glc-6-P and Glc-1-P in yeast.