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  • The EMBO Journal (1999) 18, 4733 - 4743
  • doi:10.1093/emboj/18.17.4733

Steady-state free Ca2+ in the yeast endoplasmic reticulum reaches only 10 muM and is mainly controlled by the secretory pathway pump Pmr1

Jochen Strayle1, Tullio Pozzan2 and Hans K. Rudolph1

  1. Institut für Biochemie der Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
  2. Department of Biomedical Sciences and CNR Centre for the Study of Biomembranes, University of Padova, Via Colombo 3, 35121 Padova, Italy

Correspondence to:

Hans K. Rudolph, E-mail: rudolph@po.uni-stuttgart.de

Received 31 May 1999; Accepted 8 July 1999; Revised 8 July 1999

Over recent decades, diverse intracellular organelles have been recognized as key determinants of Ca2+ signaling in eukaryotes. In yeast however, information on intra-organellar Ca2+ concentrations is scarce, despite the demonstrated importance of Ca2+ signals for this microorganism. Here, we directly monitored free Ca2+ in the lumen of the endoplasmic reticulum (ER) of yeast cells, using a specifically targeted version of the Ca2+-sensitive photoprotein aequorin. Ca2+ uptake into the yeast ER displayed characteristics distinctly different from the mammalian ER. At steady-state, the free Ca2+ concentration in the ER lumen was limited to approx10 muM, and ER Ca2+ sequestration was insensitive to thapsigargin, an inhibitor specific for mammalian ER Ca2+ pumps. In pmr1 null mutants, free Ca2+ 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 Ca2+ uptake activity in yeast.

  • Keywords:

    • aequorin,
    • Ca2+ homeostasis,
    • Ca2+ stores,
    • endoplasmic reticulum,
    • Golgi apparatus