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

Division of Molecular Genetics, Anderson College Complex, University of Glasgow, Glasgow, United Kingdom

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.

intracellular calcium; confocal microscopy; Malpighian tubule

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