Figure 2.

Cargo binding and release from the karyopherins is controlled by Ran. The asymmetric distribution of the GTPase-activating protein (RanGAP) and the guanine-exchange factor (RanGEF) generate a high RanGTP concentration in the nucleus and a high RanGDP concentration in the cytoplasm, respectively. Import cargoes bind to their receptors in the cytoplasm in the absence of RanGTP. In the nucleus, RanGTP binds to the import complex and triggers the release of the import cargoes. By contrast, export complexes form obligate trimeric complexes that contain receptor, cargo and RanGTP. Export complexes are dissociated in the cytoplasm where they encounter the RanGAP, and the RanGTP is hydrolysed to RanGDP.