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First published online January 10, 2008
doi: 10.1242/10.1242/jcs.005777

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Shaping the endoplasmic reticulum into the nuclear envelope

Salk Institute for Biological Studies, Molecular and Cell Biology Laboratory, 10010 N. Torrey Pines Road, La Jolla, CA 92037, USA

View larger version (17K): [in this window] [in a new window]   Fig. 1. Metazoan cells undergo open mitosis. U2OS cells were transfected with a construct encoding the first 65 aa of Sec61 conjugated to (GFP-Sec61) and H2BtdTomato, and imaged in real time using spinning disk confocal microscopy throughout mitosis. Following chromatin condensation (prophase) the NE is torn apart and absorbed into the ER (pro-metaphase). When the chromosomes align on the metaphase plate there is little or no contact between chromosomes and the membranes of the ER (metaphase). The tubule tips of the ER first contact the chromatin during chromosome segregation (anaphase). The NE is reformed during cytokenisis (daughter cells). Scale bar, 10 µm.

View larger version (76K): [in this window] [in a new window]   Fig. 2. The morphology of the starting membranes dictates the biochemical requirements of cell-free nuclear assembly. Classically, nuclei were formed by mixing membrane fragments with sperm chromatin and cytosol (classical nuclear assembly). In this case, membrane fragments bind to the chromatin surface, fuse into a tubular network and expand to form the NE. Alternatively, when ER fragments are preformed into an ER network prior to chromatin addition, the network binds to chromatin and expands to enclose the chromatin. The addition of homotypic ER fusion inhibitors blocks nuclear assembly when fragmented membranes are mixed with chromatin (classical nuclear assembly). However, when these inhibitors are added after ER network assembly, there is no affect on NE formation.

View larger version (50K): [in this window] [in a new window]   Fig. 3. Hole closure is the final step for sealing of the nuclei. (A) After the ER network collapses onto the chromatin surface (left panel), membrane tubules flatten and merge into sheets (center panel). Connections between the INM and ONM in the form of small holes are the result of membrane flattening and merging (right panel). (B) There are three possible mechanisms for closure of the final connecting points between the INM and ONM. Protein-mediated constriction of the membrane hole may lead to fission of the two membrane sheets (`annular fusion'). This step may also be mediated by progressive tethering of the INM to chromatin by DNA-binding NETs (INM tethering). Alternatively, the final holes of NE formation may act as sites of NPC assembly (NPC assembly).