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).