The Interaction Between Importin‐α and Nup153 Promotes Importin‐α/β‐Mediated Nuclear Import

Nuclear transport is mediated by transport factors, including the importin β family members. The directionality of nuclear transport is governed by the asymmetrical distribution of the small GTPase Ran. Of note, importin α/β‐mediated import of classical nuclear localization signal ( cNLS ) – containing cargo is more efficient than other Ran‐dependent import pathways that do not require importin α. In this study, we characterized the role of importin α in nuclear transport by examining import efficiencies of cNLS ‐cargo/importin α/β complexes. We first depleted digitonin‐permeabilized semi‐intact cells of endogenous importin α and used the cells to show that the interaction between importin α and Nup153 – a component of the nuclear pore complex ( NPC ) – is essential for efficient import of importin β‐binding domain containing substrates, but not other cargoes that directly bind to importin β. Moreover, we found that the binding of importin α to Nup153 facilitates cNLS ‐mediated import, and demonstrated that importin α in import complexes and cargo‐free importin α prebound to Nup153 promote efficient import of cNLS ‐containing proteins. This is the first in vitro study showing that in conjunction with Nup153, importin α contributes to directionally biased exit of cNLS ‐containing cargo to the nuclear side of NPCs .