In eukaryotic cells, the GTPase Ran-GTP/GDP across nuclear pores is essential for the transport of many proteins and nucleic acids between the nuclear and cytoplasmic compartments (for review see ref. 1, 2).

All transport receptors probably exit the nucleus complexed with RanGTP, and so should deplete RanGTP continuously from the nucleus. The facilitated import is specific for cytoplasmic RanGDP and employs nuclear transport factor 2 (NTF2) as the actual carrier; NTF2 binds GDP-Ran selectively and this interaction is important for efficient nuclear protein import in vivo (ref. 3, 4).

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Ran has an a b -type structure in which the central b sheet comprises 7 b strand and 5 of which is parallel. There are 5 a helices. The loop regions that are involved in the activity of this protein are labeled G1-G5. G1 and G2 loops bind to Mg⁺⁺ and GTP or GDP. Loop G2 and G3 undergo large conformational changes when the molecule switched from the active GTP-bound form to inactive GDP-bound form (For more details, see Ran-GDP).

NTF2 is a dimer based on a distinctively bent b -sheet to which is angled a long a -helix. This arrangement of secondary structure produces a cone-shaped fold for the polypeptide chain which generates a distinctive hydrophobic cavity that is surrounded by negatively charged residues (see ref. 5 for more details on NTF2 dimer).

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