The transport of proteins from the cytoplasm into the nucleus is a multistep process. The nuclear localization sequence (NLS) of a transport substrate associates with the heterodimeric NLS-receptor which binds to a subset of proteins of the nuclear pore complex (NPC). Translocation through the NPC is energy-dependent and requires the small GTPase Ran. Proteins that interact with Ran in either the GDP-bound or the GTP-bound state coordinate transfer through the NPC. Lastly, the NLS-receptor/substrate complex and Ran reach the nuclear side of the NPC where the complex disassembles.
Schlenstedt, G. (1996). Protein import into the nucleus. In FEBS Letters (Vol. 389, pp. 75–79). Elsevier B.V. https://doi.org/10.1016/0014-5793(96)00583-2