The nuclear envelope
The nuclear envelope is an essential structure of the eukaryotic cell that separates the cytoplasmic from the nucleoplasmic compartment. It is composed of three distinct membrane domains, the outer and inner nuclear membranes, and the wall of nuclear pore complexes. The inner nuclear membrane is structurally and functionally distinct from the other two membrane domains and contains specific integral membrane proteins (IMPs). IMPs have at least one membrane spanning domain and the N-terminal domain protrudes into the nucleoplasm where it binds during interphase to the underlying lamina and/or the peripheral chromatin. IMPs are synthesised at the Endoplasmic Reticulum, transported by lateral diffusion in the lipid bilayer to the outer nuclear membrane, and via the membrane of the pore wall they reach the inner nuclear membrane where they are anchored to components of the lamina or to chromatinThe lamina is a proteinaceous layer bordering the nucleoplasmic surface of the inner nuclear membrane. The major constituents of this karyoskeletal structure are the lamins, type V intermediate filament proteins. They contain an alpha-helical rod domain centered between a non helical N-terminal head domain and a globular C-terminal tail domain. The C-terminal tail of all lamins contains a nuclear localisation sequence and most lamins possess in addition at their C-terminus a motif that facilitates their targeting to the inner nuclear membrane.