Loss of the major isoform of phosphoglucomutase results in altered calciumhomeostasis in Saccharomyces cerevisiae

Phosphoglucomutase (PGM) is a key enzyme in glucose metabolism, where it ca talyzes the interconversion of glucose 1-phosphate (Glc-1-P) and glucose 6- phosphate (Glc-6-P), In this study, we make the novel observation that PGM is also involved in the regulation of cellular Ca2+ homeostasis in Saccharo myces cerevisiae. When a strain lacking the major isoform of PGM (pgm2 Delt a) was grown on media containing galactose as sole carbon source, its rate of Ca2+ uptake was 5-fold higher than an isogenic wild-type strain. This in creased rate of Ca2+ uptake resulted in a 9-fold increase in the steady-sta te total cellular Ca2+ level. The fraction of cellular Ca2+ located in the exchangeable pool in the pgm2 Delta strain was found to be as large as the exchangeable fraction observed in wild-type cells, suggesting that the depl etion of Golgi Ca2+ stores is not responsible for the increased rate of Ca2 + uptake. We also found that growth of the pgm2 Delta strain on galactose m edia is inhibited by 10 mu M cyclosporin A, suggesting that activation of t he calmodulin/calcineurin signaling pathway is required to activate the Ca2 + transporters that sequester the increased cytosolic Ca2+ load caused by t his high rate of Ca2+ uptake. We propose that these Ca2+-related alteration s are attributable to a reduced metabolic flux between Glc-1-P and Glc-6-P due to a limitation of PGM enzymatic activity in the pgm2 Delta strain. Con sistent with this hypothesis, we found that this "metabolic bottleneck" res ulted in an 8-fold increase in the Glc-1-P level compared with the wild-typ e strain, while the Glc-6-P and ATP levels were normal. These results sugge st that Glc-1-P (or a related metabolite) may participate in the control of Ca2+ uptake from the environment.