Jcb_201402003 1..9

The nuclear envelope (NE) in animal cells comprises three structures: the nuclear membrane, the nuclear pore complex (NPC), and the lamina. The nuclear membrane is divided into the inner nuclear membrane (INM) and outer nuclear membrane (ONM) based on protein content, but the membranes are contiguous with each other and with the ER. The nuclear membrane covers the chromatin and restricts nuclear–cytoplasmic trafficking to the NPCs. The NPCs extend through both the INM and ONM as well as the lamina (Schermelleh et al., 2008) and regulate the passage of macromolecules with molecular weights exceeding 40 kD between the nucleus and the cytoplasm (Wente and Rout, 2010). The nuclear lamina is a dense meshwork of lamin filaments attached to the INM. The two major types of lamin proteins are the B-type, lamins B1 and B2, and the A-type, lamins A and C, which are different isoforms of the same gene (Dechat et al., 2010). The lamin proteins interact with transmembrane INM proteins, like LBR and Lap2, and chromatin-binding proteins, like BAF, at the nuclear periphery to form a stable network that supports the membrane and links the INM to the chromatin (Ellenberg et al.,