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. 2022 Feb 25;23(5):2578. doi: 10.3390/ijms23052578

Figure 2.

Figure 2

Highly schematic illustration of nuclear transport mechanisms. (A) Nuclear protein import whereby cargo proteins are bound by a karyopherin carrier (Kap) of the importin-β superfamily in the cytoplasm (often using an adapter protein such as importin-α or by “piggy-backing” on another protein that is being imported) that is able to circumvent the barrier generated by FG-nucleoporins (grey) in the nuclear pore transport channel and facilitate movement into the nucleus, where RanGTP binding releases the cargo. The karyopherin:RanGTP complex is then recycled to the cytoplasm where the GTPase activating protein RanGAP activates the RanGTPase, releasing RanGDP and freeing the karyopherin for a further import cycle. Energy, therefore, is used to rectify thermal motion rather than move material through the pores. (B) NTF2 then binds RanGDP and recycles it to the nucleus where the guanine nucleotide exchange factor RCC1 recharges it with GTP. To maintain the RanGTP gradient, RanGAP is retained in the cytoplasm by binding to Nup358, whereas RCC1 is retained in the nucleus bound to chromatin.