Nuclear transport factor (karyopherin) involved in protein transport between the cytoplasm and nucleoplasm. Required for the nuclear import of ribosomal proteins (RPL11, RPL16, RPL25, RPL31A), the poly(A)-binding protein PAB1, the HO endonuclease or the tRNA and snRNA chaperone LHP1. Indirectly involved in nuclear mRNA export through its PAB1 nuclear import activity. Associates with the nucleopore complex. Interacts with LHP1, PAB1, RPL11, RPL16, RPL25, RPL31A in order to import them into the nucleus. Interacts also with the nucleopore complex proteins (nucleoporins) NSP1, NUP1, NUP116 and NUP159. Present with 16300 molecules/cell in log phase SD medium. Belongs to the importin beta family.