Functions in nuclear protein import as nuclear transport receptor. Serves as receptor for nuclear localization signals (NLS) in cargo substrates. 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 (By similarity). Mediates the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5 (PubMed:9687515, PubMed:11682607). In vitro, mediates nuclear import of H2A, H2B, H3 and H4 histones. Binds to CPEB3 and mediates its nuclear import following neuronal stimulation (By similarity). In case of HIV-1 infection, binds and mediates the nuclear import of HIV-1 Rev. Interacts with RPS7 and RPL5 (PubMed:9687515). Interacts with RPL23A (via BIB domain) (PubMed:9687515, PubMed:11682607). Interacts with H2A, H2B, H3 and H4 histones (By similarity). Interacts with CPEB3; this mediates CPEB3 nuclear import following neuronal stimulation which enhances the interaction in a RAN-regulated manner (PubMed:22730302). Interacts with AIFM2; this interaction likely mediates the translocation of AIFM2 into the nucleus upon oxidative stress. Interacts with STX3 (isoform 3) (PubMed:29475951). Interacts with SRP19 (PubMed:11682607). (Microbial infection) Interacts with HIV-1 Rev. Nucleus; nuclear rim. Found particularly in the nuclear rim and nucleolus. Belongs to the importin beta family. Importin beta-3 subfamily.