The exchange of molecules between the nucleus and cytoplasm is mediated through nuclear pore complexes (NPCs) embedded in the nuclear envelope. The NPC is composed of highly conserved, distinct structural modules: the inner pore ring, the cytoplasmic and nuclear rings as well as the cytoplasmic filaments and the nuclear basket. Nuclear transport receptors (e.g. importins and exportins) mediate translocation of cargos through the NPC. Cargo proteins harbouring a nuclear export signal (NES) bind to exportins and Ran-GTP to form an export complex in the nucleoplasm. Nuclear export of bulk mRNA is provided by the non-karyopherin transport proteins TAP-p15 in a Ran-GTP-independent manner. Several EJC factors are known to function in mRNA export.
