Ca 2+ ‐ATPases of Saccharomyces cerevisiae : diversity and possible role in protein sorting

The PMR1 gene of Saccharomyces cerevisiae is thought to encode a putative Ca 2+ ‐ATPase [1]. Membranes isolated from wild‐type cells and from pmr1 null mutant of S. cerevisiae were fractionated on sucrose density gradients. In the pmr1 mutant we found a decrease in activity of the P‐type ATPase and of ATP‐dependent, protonophore‐insensitive Ca 2+ transport in light membranes, that comigrate with the Golgi marker GDPase. We conclude that the product of the PMR1 gene (Pmr1p) is indeed a Ca 2+ ‐ATPase of the Golgi and Golgi‐like membranes. Surprisingly, the pmr1 null mutation abolished Ca 2+ ‐ATPase activity in Golgi and/or Golgi‐like membranes only to 50% under conditions where they are separated from vacuolar membranes. This indicates that an additional Ca 2+ ‐ATPase is localized in Golgi and/or Golgi‐like membranes. Moreover, a third Ca 2+ ‐ATPase is found in the ER and ER‐like membranes. The data are consistent with the assumption that these Ca 2+ ‐ATPases are encoded by gene(s) different from PMR1 . Disruption of PMR1 Ca 2+ ‐ATPase causes significant redistribution of enzyme activities and of total protein in compartments of the secretory pathway. A decrease in activity is observed for three integral membrane proteins: NADPH cytochrome c reductase, dolichyl phosphate mannose synthase, and Ca 2+ ‐ATPase, and also for total protein in Golgi, Golgi‐like compartments and in vacuoles, whereas a corresponding increase of these activities is observed in endoplasmic reticulum and endoplasmic reticulum‐like membranes. We assume that Ca 2+ ‐ATPases and sufficient Ca 2+ gradients across the organellar membranes are important for the correct sorting of proteins to the various compartments of the secretory apparatus.