Functions in nuclear protein import. Binds very efficiently to importin subunit beta-1/KPNB1 via the IBB domain. This complex dissociates in the presence of RAN-GTP (By similarity). In MII-stage oocytes, localizes to the spindle. Expressed predominantly in ovary. Isoform 1 is the predominant form. Expressed at high levels in germinal vesicle-stage oocytes, as well as in zygotes and 2-cell embryos (at protein level). Drastically down-regulated after the 2-cell stage. Mutant mice exhibit abnormal preimplantation development. About half of the mutant embryos fail to develop into the blastocyst stage, or are delayed. Lethality is greater among female than among male embryos. Belongs to the importin alpha family.