) is an intracellular second messenger used by many cells to release Ca 2ϩ from their internal stores (6). IP 3 acts on the IP 3 receptor, which is an important Ca 2ϩ release channel in the endoplasmic reticulum (7). There is, however, increasing evidence that IP 3 receptors are also localized in the Golgi apparatus (8 -12). The role of this latter organelle in the generation of cytosolic Ca 2ϩ signals is not well known, partly because the mechanism by which the Golgi complex takes up Ca 2ϩ is less well understood compared with the Ca 2ϩ transport into the endoplasmic reticulum mediated by the sarco(endo)plasmic reticulum Ca 2ϩ -ATPase (SERCA). The Golgi apparatus expresses a specific Ca 2ϩ pump belonging to the Pmr1 family of Ca 2ϩ -transport ATPases (13). Pmr1 is insensitive to the inhibitor thapsigargin (14), in contrast to the SERCA Ca 2ϩ pumps in the sarco-and endoplasmic reticulum (15).We expressed the Pmr1 Ca 2ϩ pump of Caenorhabditis elegans (12) in COS-1 cells and pretreated the cells with thapsigargin to prevent SERCA-mediated Ca 2ϩ uptake in the endoplasmic reticulum. We demonstrate that a Ca 2ϩ store filled by the Pmr1 Ca 2ϩ pump and probably belonging to the Golgi compartment can set up baseline Ca 2ϩ spiking.
EXPERIMENTAL PROCEDURESCell Culture and Transfection-COS-1 cells and HeLa cells were cultured as described previously (12, 16). The culture medium was replaced every 2 to 3 days. The cells were plated at a density of 2500 cells/cm 2 in Coverglass Chambers (Nunc Inc., Naperville, IL) or at a density of 10,000 cells/cm 2 in 12-well clusters (Costar, MA). Four days after plating, COS-1 cells were transiently transfected with Pmr1 from C. elegans or with the rabbit SERCA1a in pMT2 vector and investigated 3 days later (12). The full-length rabbit SERCA1a clone was kindly provided by Drs. J. P. Andersen and B. Vilsen (Department of Physiology, University of Aarhus, Denmark). Some cells were transfected with pig SERCA2a or SERCA2b in pSV57 (17).Immunofluorescence-Immunofluorescence microscopy was done as described previously (12). The primary antiserum Celpmrloop against Pmr1 was raised against a recombinant protein corresponding to the putative large cytoplasmic loop between transmembrane segments 4 and 5 of Pmr1 from C. elegans (12). The primary antiserum 809-27 against rabbit SERCA1 was kindly provided by Dr. J. Møller (Department of Biophysics, University of Aarhus, Denmark) (18). The antiserum against SERCA2a was described previously (19). The antibody against the 58K protein of the Golgi apparatus (clone 58K-9) was purchased from Abcams (Cambridge, UK).45 Ca 2ϩ Uptake-Fluxes were performed on saponin-permeabilized cells at 25°C as described previously (20). The cells were permeabilized by treating them for 10 min with 20 g/ml saponin at 25°C in a medium containing 120 mM KCl, 30 mM imidazole-HCl (pH 6.8), 2 mM MgCl 2 , 1 mM ATP and 1 mM EGTA. The non-mitochondrial Ca 2ϩ stores were loaded for 45 min in loading medium containing 120 mM KCl, 30 mM imidazole-HCl (pH 6.8), 5 mM MgCl 2 , 5 mM ATP, ...