Steady‐state free Ca 2+ in the yeast endoplasmic reticulum reaches only 10 μM and is mainly controlled by the secretory pathway pump Pmr1

Over recent decades, diverse intracellular organelles have been recognized as key determinants of Ca 2+ signaling in eukaryotes. In yeast however, information on intra‐organellar Ca 2+ concentrations is scarce, despite the demonstrated importance of Ca 2+ signals for this microorganism. Here, we directly monitored free Ca 2+ in the lumen of the endoplasmic reticulum (ER) of yeast cells, using a specifically targeted version of the Ca 2+ ‐sensitive photoprotein aequorin. Ca 2+ uptake into the yeast ER displayed characteristics distinctly different from the mammalian ER. At steady‐state, the free Ca 2+ concentration in the ER lumen was limited to ∼10 μM, and ER Ca 2+ sequestration was insensitive to thapsigargin, an inhibitor specific for mammalian ER Ca 2+ pumps. In pmr1 null mutants, free Ca 2+ in the ER was reduced by 50%. Our findings identify the secretory pathway pump Pmr1, predominantly localized in the Golgi, as a major component of ER Ca 2+ uptake activity in yeast.