Importin are protein complexes in eukaryotes, which facilitate the transport of proteins from the cytoplasm of a cell to its nucleus. Importin recognize and bind the NLS sequence to proteins, and the resulting complex binds, in turn, to the nuclear pore complex (NPC), which is funneled from energy consumption in the cell nucleus.

Importinkomplexe consist of an α and a β subunit, of which there are at least seven and four partially cell type-specific isoforms. Some of these subunits are able to function autonomously and have even the name of importin. The α - subunit recognizes the NLS sequence and binds them. The dimeric complex now docked using the β - subunit of the nuclear pore. Now, the core protein is transported into the nucleus. In the nucleus, the complex dissociates. Bonded to the Ran protein β subunit is sluiced out of the core, the α - subunit alone is exported from the cell nucleus.