Functions in nuclear protein import as nuclear transport receptor. Serves as receptor for nuclear localization signals (NLS) in cargo substrates (PubMed:11493596). May 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 (By similarity). Involved in nuclear import of M9-containing proteins. In vitro, binds directly to the M9 region of the heterogeneous nuclear ribonucleoproteins (hnRNP), A1 and A2 and mediates their nuclear import. Involved in hnRNP A1/A2 nuclear export. Mediates the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5 (By similarity). In vitro, mediates nuclear import of SRP19 (By similarity). Mediates the import of histones H2A, H2B, H3 and H4 (PubMed:11493596). Mediates nuclear import of ADAR/ADAR1 in a RanGTP-dependent manner (By similarity). Identified in a complex that contains TNPO1, RAN and RANBP1 (By similarity). Binds HNRPA1, HNRPA2, HNRNPDL, RPS7, RPL5 and RAN. Interacts with H2A, H2B, H3 and H4 histones (PubMed:11493596). Interacts with isoform 1 and isoform 5 of ADAR/ADAR1 (via DRBM 3 domain). Interacts with SNAI1 (via zinc fingers); the interaction mediates SNAI1 nuclear import. Interacts with SNAI2 (via zinc fingers) (By similarity). Interacts with RPL23A (via BIB domain) AND SRP19; this interaction is involved in RPL23A and SRP19 import into the nucleus (By similarity). Belongs to the importin beta family. Importin beta-2 subfamily.