Functions in nuclear protein import, either by acting as autonomous nuclear transport receptor or as an adapter-like protein in association with the importin-beta subunit KPNB1. Acting autonomously is thought to serve itself as receptor for nuclear localization signals (NLS) and to promote translocation of import substrates through the nuclear pore complex (NPC) by an energy requiring, Ran-dependent mechanism. At the nucleoplasmic side of the NPC, Ran binds to importin, the importin/substrate complex dissociates and importin is re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran. Mediates autonomously the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5 (By similarity). In association with KPNB1 mediates the nuclear import of H1 histone and the Ran-binding site of IPO7 is not required but synergizes with that of KPNB1 in importin/substrate complex dissociation (By similarity). In vitro, mediates nuclear import of H2A, H2B, H3 and H4 histones (PubMed:11493596). Forms a heterodimer with KPNB1 (By similarity). Interacts with histone H1 (By similarity). Interacts with H2A, H2B, H3 and H4 histones (PubMed:11493596). Interacts with SNUPN and XPO1 (By similarity). Interacts with RPS7 and RPL5 (By similarity). Interacts with RPL23A (via BIB domain) (By similarity). Binds directly to nuclear pore complexes (By similarity). Interacts with SMAD4 and NUP93; translocates SMAD4 to the nucleus through the NPC upon BMP7 stimulation resulting in activation of SMAD4 signaling (By similarity). Interacts with SRP19 (By similarity). Belongs to the importin beta family.