PMR1, a P-type ATPase cloned from the yeast Saccharomyces cerevisiae, was previously localized to the Golgi, and shown to be required for normal secretory processes (Antebi, A., and Fink, G.R. (1992) Mol. Biol. Cell 3, 633-654). We provide biochemical evidence that PMR1 is a Ca 2؉ -transporting ATPase in the Golgi, a hitherto unusual location for a Ca 2؉ pump. As a starting point for structure-function analysis using a mutagenic approach, we used the strong and inducible heat shock promoter to direct high level expression of PMR1 from a multicopy plasmid. Yeast lysates were separated on sucrose density gradients, and fractions assayed for organellar markers. PMR1 is found in fractions containing the Golgi marker guanosine diphosphatase, and is associated with an ATP-dependent, protonophore-insensitive 45 Ca 2؉ uptake activity. This activity is virtually abolished in the absence of the expression plasmid. Furthermore, replacement of the active site aspartate within the phosphorylation domain had the expected effect of abolishing Ca 2؉ transport activity entirely. Interestingly, the mutant enzymes (Asp-371 3 Glu and Asp-371 3 Asn) demonstrated proper targeting to the Golgi, unlike analogous mutations in the related yeast H ؉ -ATPase. Detailed characterization of calcium transport by PMR1 showed that sensitivity to inhibitors (vanadate, thapsigargin, and cyclopiazonic acid) and affinity for substrates (MgATP and Ca 2؉ ) were different from the previously characterized sarco/endoplasmic reticulum and plasma membrane Ca 2؉ -ATPases. PMR1 therefore represents a new and distinct P-type Ca 2؉ -ATPase. Because close homologs of PMR1 have been cloned from rat and other organisms, we suggest that Ca 2؉ -ATPases in the Golgi will form a discrete subgroup that are important for functioning of the secretory pathway.In eukaryotic cells, the vast bulk of cellular calcium is sequestered within intracellular calcium stores, which maintain cytoplasmic calcium ion concentrations at submicromolar levels and release calcium in response to physiological signals. A major intracellular calcium pool is the endoplasmic reticulum, which is well known for its prominent role in inositol 1,4,5-trisphosphate-and caffeine-mediated calcium release (2). Filling of this store is accomplished by a thapsigargin-sensitive Ca 2ϩ -ATPase, a member of the ubiquitous family of P-type ion pumps, and best characterized by the isoform in skeletal muscle sarcoplasmic reticulum (SERCA1 1 ; Refs. 3 and 4). There are data supporting the existence of another ionomycin-sensitive pool of intracellular calcium that is unresponsive to both inositol 1,4,5-trisphosphate and caffeine, is non-mitochondrial, and appears to be loaded by a thapsigargin-insensitive Ca 2ϩ

We have analyzed Ca 2+ transport activity in defined subcellular fractions of an isogenic set of wild-type and mutant yeast. The results, together with measurements of polypeptide expression levels and promoter: :reporter gene activity, show that the Golgi CaZ+-ATPase, Pmrl, is the major Ca 2+ pump under normal growth conditions. In the absence of Pmrl, we show a massive, calcineurin-dependent compensatory induction of the vacuolar Ca2+-ATPase, Pmcl. In addition, H+/Ca 2+ exchange activity, that may be distinct from the vacuolar exchanger Vcxl, is also increased.