Functions in nuclear protein import via a substrate-importin alpha-beta transport complex that passes though the nuclear pore complexes (NPC). Binds specifically and directly to substrates containing either a simple or bipartite NLS motif (By similarity). Forms a complex with tnpo/importin subunit beta. The N-terminal hydrophilic region contains the importin beta binding domain (IBB domain), which is sufficient for binding importin beta and essential for nuclear protein import. Belongs to the importin alpha family.