To study the role of calreticulin in Ca 2؉ homeostasis and apoptosis, we generated cells inducible for fulllength or truncated calreticulin and measured Ca 2؉ signals within the cytosol, the endoplasmic reticulum (ER), and mitochondria with "cameleon" indicators. Induction of calreticulin increased the free Ca 2؉ concentration within the ER lumen, [Ca 2؉ ] ER , from 306 ؎ 31 to 595 ؎ 53 M, and doubled the rate of ER refilling. [Ca 2؉ ] ER remained elevated in the presence of thapsigargin, an

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... ibitor of SERCA-type Ca 2؉ ATPases. Under these conditions, store-operated Ca 2؉ influx appeared inhibited but could be reactivated by decreasing [Ca 2؉ ] ER with the low affinity Ca 2؉ chelator N,N,N,Ntetrakis(2-pyridylmethyl)ethylenediamine. In contrast, [Ca 2؉ ] ER decreased much faster during stimulation with carbachol. The larger ER release was associated with a larger cytosolic Ca 2؉ response and, surprisingly, with a shorter mitochondrial Ca 2؉ response. The reduced mitochondrial signal was not associated with visible morphological alterations of mitochondria or with disruption of the contacts between mitochondria and the ER but correlated with a reduced mitochondrial membrane potential. Altered ER and mitochondrial Ca 2؉ responses were also observed in cells expressing an N-truncated calreticulin but not in cells overexpressing calnexin, a P-domain containing chaperone, indicating that the effects were mediated by the unique C-domain of calreticulin. In conclusion, calreticulin overexpression increases Ca 2؉ fluxes across the ER but decreases mitochondrial Ca 2؉ and membrane potential. The increased Ca 2؉ turnover between the two organelles might damage mitochondria, accounting for the increased susceptibility of cells expressing high levels of calreticulin to apoptotic stimuli.