Biology · Cell Biology & Nuclear Transport

Regarding the nuclear pore complex (NPC) and nucleocytoplasmic transport, consider the following statements: I. The Ran-GTP gradient (high in nucleus, low in cytoplasm) is maintained by RCC1 (a Ran-GEF) concentrated in the nucleus and RanGAP (a Ran-GAP) anchored at the cytoplasmic face of the NPC. II. During classical NLS-mediated nuclear import, the importin-α/β heterodimer binds cargo in the cytoplasm; Ran-GTP in the nucleus binds importin-β and promotes release of the cargo–importin-α complex. III. CRM1 (exportin-1) mediates nuclear export of proteins bearing a leucine-rich nuclear export signal (NES) and requires Ran-GTP for cargo loading in the nucleus. IV. FG-Nups form a hydrophobic meshwork that acts as a selective barrier; transport factors (importins/exportins) move through by transiently interacting with FG repeats. Which of the above statements are correct?