A 41 amino acid motif in importin-α confers binding to importin-β and hence transit into the nucleus

The complex of importin-α and -β is essential for nuclear protein import. It binds the import substrate in the cytosol, and the resulting trimeric complex moves through the nuclear pores, probably as a single entity. Importin-α provides the nuclear localization signal binding site, importin-β the site of initial docking to the pore. Here we show that the conserved, basic N-terminus of importin-α is sufficient for importin-β binding and essential for protein import. The fusion product of this 41 amino acid domain to a heterologous protein is transported into the nucleus in the same way as full-length importin-α itself. Transport is dependent on importin-β but competed by importin-α. As no additional part of importin-α is needed for translocation, the movement which drives the import substrate complex into the nucleus appears to be generated between importin-β and structures of the nuclear pore. The domain that binds to importin-β appears to confer import only, but not re-export out of the nucleus, suggesting that the return of importin-α into the cytoplasm is not a simple reversal of its entry.