Functions in nuclear protein import as an adapter protein for importin beta nuclear receptors (PubMed:10913188). Binds specifically and directly to substrates containing either a simple or bipartite NLS motif (PubMed:10745017). Promotes docking of import substrates to the nuclear envelope (PubMed:8521485). Together with importin beta KAP95, mediates nuclear import of transcription factor GCN4 (PubMed:14648200). Together with tethering factor STS1, targets the proteasome to the nucleus (PubMed:10913188, PubMed:21075847). Forms a complex with an importin beta subunit. In the nucleus, interacts with NUP2 which accelerate release of NLSs, NUP2 is subsequently displaced by CSE1:RanGTP which mediates re-export and recycling. Interacts with HEH2, SHE2, and STS1. Mainly localized at the periphery of the nucleus. The NLS binding sites are mainly involved in recognition of simple or bipartite NLS motifs. Structurally located within in a helical surface groove they contain several conserved Trp and Asn residues of the corresponding third helices (H3) of ARM repeats which mainly contribute to binding. Binds to nucleoporin FxFG but not GLFG repeat regions. Ran-GTP can disrupt the karyopherin heterodimer by binding to the beta subunit and releases both subunits from the docking site. Present with 2790 molecules/cell in log phase SD medium. Belongs to the importin alpha family.