Importin β-depending Nuclear Import Pathways: Role of the Adapter Proteins in the Docking and Releasing Steps
Nuclear imports of uridine-rich small nuclear ribonucleoprotein (U1 snRNP) and proteins with classical nuclear localization signal (cNLS-protein) are mediated by importin β. However, due to the presence of different import signals, the adapter protein of the imported molecules and importin β is different for each pathway. Although the adapter for cNLS-protein is importin α, the adapter for U1 snRNP is snurportin1 (SPN1). Herein, we show that the use of distinct adapters by importin β results in differences at the docking and releasing step for these two import pathways. Nuclear pore complex (NPC) docking of U1 snRNP but not of cNLS-protein was inhibited by an anti-CAN/Nup214 antibody. Thus, the initial NPC-binding site is different for each pathway. Pull-down assays between immobilized SPN1 and two truncated forms of importin β documented that SPN1 and importin α have different binding sites on importin β. Importin β fragment 1–618, which binds to SPN1 but not to importin α, was able to support the nuclear import of U1 snRNPs. After the translocation through the NPC, both import complexes associated with the nuclear side of the NPC. However, we found that the nature of the importin β-binding domain of the adapters influences the release of the cargo into the nucleoplasm.