Nuclear import of U snRNPs requires importin β

Macromolecules that are imported into the nucleus can be divided into classes according to their nuclear import signals. The best characterized class consists of proteins which carry a basic nuclear localization signal (NLS), whose transport requires the importin α/β heterodimer. U snRNP import depends on both the trimethylguanosine cap of the snRNA and a signal formed when the Sm core proteins bind the RNA. Here, factor requirements for U snRNP nuclear import are studied using an in vitro system. Depletion of importin α, the importin subunit that binds the NLS, is found to stimulate rather than inhibit U snRNP import. This stimulation is shown to be due to a common requirement for importin β in both U snRNP and NLS protein import. Saturation of importin β‐mediated transport with the importin β‐binding domain of importin α blocks U snRNP import both in vitro and in vivo . Immunodepletion of importin β inhibits both NLS‐mediated and U snRNP import. While the former requires re‐addition of both importin α and importin β, re‐addition of importin β alone to immunodepleted extracts was sufficient to restore efficient U snRNP import. Thus importin β is required for U snRNP import, and it functions in this process without the NLS‐specific importin α.