Dissociation and re-assembly of the endoplasmic reticulum in live cells

The endoplasmic reticulum (ER) of a typical interphase 3T3 fibroblast consists of a compact perinuclear arrangement of cisternae and lamellae which can be observed by immunofluorescence with anti-endoplasmin. During mitosis the reticulum dissociates into small fragments from which it appears to re-assemble in the daughter cells. When interphase 3T3 cells are exposed to calcium ionophores, but not other ionophores, there is a similar dissociation of the ER into small uniform fragments, which are dispersed throughout the cytoplasm. Electron microscopy shows that the fragments consist of small vesicular structures and that essentially all of the rough ER except the nuclear envelope is dissociated. The dissociation of the ER by calcium ionophore is a relatively specific process since other organelles and supramolecular assemblies remain unaffected. When cells with dissociated ER are returned to normal medium, there is a rapid reassembly of the fragments into the continuous reticulum. In a proportion of the cells it is possible to observe linear arrays of the fragments, which probably represent intermediates in the re-assembly process. These observations demonstrate that the ER in interphase 3T3 cells can be dissociated into, and re-assembled from, small fragments. Re-assembly of the ER from the fragments is dependent on the presence of millimolar levels of calcium in the external medium. In the presence of calcium, re-assembly is inhibited by the calcium channel blocker, verapamil. Thus calcium ions appear to play an important role in ER structure and assembly.