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. Mediates the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5. In vitro, mediates nuclear import of H2A, H2B, H3 and H4 histones. Binds to CPEB3 and mediates its nuclear import following neuronal stimulation (PubMed:22730302). Interacts with RPS7 and RPL5 (By similarity). Directly interacts with RPL23A (via BIB domain) (By similarity). Interacts with H2A, H2B, H3 and H4 histones (PubMed:11493596). Interacts with CPEB3; this mediates CPEB3 nuclear import following neuronal stimulation which enhances the interaction in a RAN-regulated manner (By similarity). Interacts with AIFM2; this interaction likely mediates the translocation of AIFM2 into the nucleus upon oxidative stress (By similarity). Interacts with STX3 (isoform 3) (PubMed:26689472). Interacts with SRP19 (By similarity). Belongs to the importin beta family. Importin beta-3 subfamily. Extended N-terminus.