Nuclear transport receptor that mediates nuclear import of proteins, such as histones, proteasome and actin (PubMed:11493596, PubMed:11823430, PubMed:22323606). Serves as receptor for nuclear localization signals (NLS) in cargo substrates (PubMed:11493596, PubMed:11823430). 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 (PubMed:11493596, PubMed:11823430). 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 (PubMed:11493596, PubMed:11823430). 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:11493596, PubMed:11823430). Mediates the import of pre-assembled proteasomes into the nucleus; AKIRIN2 acts as a molecular bridge between IPO9 and the proteasome complex (By similarity). Mediates the nuclear import of histones H2A, H2B, H4 and H4 (PubMed:11493596, PubMed:11823430). In addition to nuclear import, also acts as a chaperone for histones by preventing inappropriate non-nucleosomal interactions (By similarity). Mediates the nuclear import of actin (PubMed:22323606). Interacts with histones H2A, H2B, H3 and H4 (PubMed:11493596, PubMed:11823430). The binding is coupled to RanGTP cycles (PubMed:11493596, PubMed:11823430). Interacts with AKIRIN2; promoting association with pre-assembled proteasomes (By similarity). Associates with pre-assembled proteasomes; interaction is indirect and mediated via interaction with AKIRIN2 (By similarity). Interacts with PPP2R1A and PPP2R1B (By similarity). Belongs to the importin beta family.