TNPO1, also named as KPNB2, MIP1, TRN and MIP, belongs to the importin beta family. TNPO1 functions in nuclear protein import as nuclear transport receptor. It serves as receptor for nuclear localization signals (NLS) in cargo substrates. TNPO1 is thought to mediate docking of the importin/substrate complex to the nuclear pore complex (NPC) through binding to nucleoporin and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism. At the nucleoplasmic side of the NPC, Ran binds to the importin, the importin/substrate complex dissociates and importin is re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran. The directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP- and GDP-bound forms of Ran between the cytoplasm and nucleus. TNPO1 is involved in nuclear import of M9-containing proteins. In vitro, it binds directly to the M9 region of the heterogeneous nuclear ribonucleoproteins (hnRNP), A1 and A2 and mediates their nuclear import. It is involved in hnRNP A1/A2 nuclear export. TNPO1 mediates the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5. It binds to a beta-like import receptor binding (BIB) domain of RPL23A. In vitro, it mediates nuclear import of H2A, H2B, H3 and H4 histones, and SRP19. In case of HIV-1 infection, binds and mediates the nuclear import of HIV-1 Rev.