A 41 amino acid motif in importin‐alpha confers binding to importin‐beta and hence transit into the nucleus.

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