Karyopherin enrichment at the nuclear pore complex attenuates Ran permeability

Ran is a small GTPase whose nucleotide-bound forms cycle through nuclear pore complexes (NPCs) to direct nucleocytoplasmic transport (NCT). Generally, Ran guanosine triphosphate (GTP) binds cargo-carrying karyopherin receptors (Kaps) in the nucleus and liberates them in the cytoplasm following hydrolysis to Ran guanosine diphosphate (GDP). This generates a remarkably steep Ran gradient across the nuclear envelope that sustains compartment-specific cargo delivery and accumulation. However, because NPCs are permeable to small molecules of comparable size, it is unclear how an uncontrolled mixing of RanGTP and RanGDP is prevented. Here, we find that an NPC-enriched pool of Kapβ1 selectively mediates Ran diffusion across the pore, but not passive molecules of similar size (e.g. GFP). This is due to binding interactions with Kapβ1, which is stronger for RanGTP, but weaker for RanGDP. On this basis, the RanGDP importer, nuclear transport factor 2 (NTF2), facilitates the release of RanGDP from Kapβ1 following GTP hydrolysis. Accordingly, the enrichment of Kapβ1 at NPCs may function as a retention mechanism that preserves the sharp transition of RanGTP and RanGDP in the nucleus and cytoplasm, respectively.