Nuclear transport receptor that mediates nuclear import of proteins (PubMed:33632744). Serves as receptor for nuclear localization signals (NLS) in cargo substrates (By similarity). Is thought to mediate docking of the importin/substrate complex to the nuclear pore complex (NPC) through binding to nucleoporin and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism (By similarity). Mediates the import of pre-assembled proteasomes into the nucleus during the late stages of sperm development (PubMed:33632744). Male and female sterility caused by disruption of chromosome segregation and condensation during meiosis (PubMed:33632744). Impaired nuclear localization of proteasome components (PubMed:33632744). Males form abnormally structured sperm and fail to properly exchange histones for protamines (PubMed:33632744). Belongs to the importin beta family.