Novel subcellular locations and functions for secretory pathway Ca2+/Mn2+-ATPases

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+ and Mn2+ homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene (SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum ( ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+ handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian ( renal) tubule. Specifically, the ER isoform impacted on inositol ( 1,4,5)- trisphosphate-mediated Ca2+ signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+ spherites and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+ signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+ in transporting epithelia: to transport or store Ca2+ in bulk without compromising its use as a signal.