Targeting aequorin to the endoplasmic reticulum of living cells

The photoprotein aequorin has been engineered with an ER targeting sequence at the N-terminus, with and without KDEL at the C-terminus, so that it locates in the ER-secretory pathway. For the first time the free Ca2+ has been quantified inside the ER and shown to be 5–20 times that in the cytosol. In COS cells free Ca2+ in the ER ranged from 1–5mM at 37°C, decreasing 2–5-fold within 1 min of exposure to the Ca2+ ionophore ionomycin in the absence of external Ca2+.