Binds specifically and directly to substrates containing either a simple or bipartite NLS motif. Promotes docking of import substrates to the nuclear envelope. Seems to act as a cytosolic receptor for both simple and bipartite NLS motifs. Has an essential role in mitotic chromosome condensation. Involved in nuclear protein import. Required for efficient nuclear import of both an SV40 nuclear localization signal-containing reporter protein and the pap1 component of the stress response MAP kinase pathway. Required for proper mitotic progression. Interacts with pap1. Belongs to the importin alpha family.