Calcium-induced restructuring of nuclear envelope and endoplasmic reticulum calcium stores.

The spatial organization of endoplasmic reticulum (ER) and nuclear envelope (NE) calcium stores is important for the regulation of localized calcium signals and sustained calcium gradients. Here, we have used a lumenal GFP fusion protein and shown that, in resting cells, large molecules can rapidly diffuse across the cell within the lumenal storage space defined by the ER and NE membranes. Increases in cytosolic calcium concentration reversibly fragmented ER tubules and prevented lumenal diffusion. However, the integrity of the NE was maintained, and a significant fraction of NE lumenal protein accumulated in an NE-associated vesicle. These dynamic properties of ER-NE calcium stores provide insights into the spatiotemporal control of calcium signaling.