Synonyms:IMP4B, RANBP4 Background:Functions in nuclear protein import as nuclear transport receptor. Serves as receptor for nuclear localization signals(NLS) in cargo substrates. 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. At the nucleoplasmic side of the NPC, Ran binds to the importin, the importin/substrate complex dissociates and importin is re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran. The directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP-and GDP-bound forms of Ran between the cytoplasm and nucleus(By similarity). Mediates the nuclear import of RPS3A. In vitro, mediates the nuclear import of human cytomegalovirus UL84 by recognizing a non-classical NLS.