Nuclear transport receptor that mediates nuclear import of proteins, such as histones, RPS3A, TNP2 and VDR. 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 the histone H3-H4 dimer when in complex with ASF1 (ASF1A or ASF1B). Mediates the ligand-independent nuclear import of vitamin D receptor (VDR). Found in a cytosolic complex with ASF1 (ASF1A or ASF1B) and histones H3 and H4. Belongs to the importin beta family. Truncated N-terminus. Truncated N-terminus.