Functions in nuclear protein import as nuclear transport receptor. Serves as receptor for nuclear localization signals (NLS) in cargo substrates. 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. Required for nuclear import of Ho endonuclease and RFP1, and involved in rRNA-processing and assembly or export of 60S ribosomal subunits. Interacts with GTP-bound GSP1 and RFP1. Associates with the nuclear pore complex. Present with 6300 molecules/cell in log phase SD medium. Belongs to the importin beta family.