Figure 1.

Active nuclear–cytoplasmic transport of proteins. The cargo proteins containing a nuclear localization signal (NLS) are recognized by importin βs alone or importin α/β1 heterodimer, whereas cargo proteins that possess a nuclear export signal (NES) bind to exportins in the presence of Ran-GTP. The transport complexes pass through nuclear pores by sequential and transient interaction of importin β with Nups within the NPC. Within the nucleus, Ran-GTP binding to importin β dissociates the import complex. Ran-GTP to Ran-GDP exchange facilitated by RanBP1 and RanGAP1 dissociates the nuclear export complex in the cytoplasm. Ran-GDP is converted to the GTP-bound form by RCC1 in the nucleus.