Importin beta subunit that functions in nuclear protein import through association with the importin alpha subunit, which binds to the classical nuclear localization signal (cNLS) in cargo substrates (PubMed:7622450). Docking of the importin/substrate complex to the nuclear pore complex (NPC) is mediated by importin beta through binding to nucleoporin FxFG repeats and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism (PubMed:8521485). At the nucleoplasmic side of the NPC, GTP-Ran binds to importin beta and the three components separate, leading to release of the cargo (PubMed:15864302). Importin alpha and beta are re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran from importin beta. 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 (PubMed:11423015). Mediates the nuclear import of histones H2A and H2B (PubMed:11309407). Mediates the nuclear import of transcription factor GCN4 (PubMed:14648200).