Nuclear import of U snRNPs requires importin β

Isabel Palacios, Martin Hetzer, Stephen A. Adam, Lain W. Mattaj

Research output: Contribution to journalArticlepeer-review

118 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)6783-6792
Number of pages10
JournalEMBO Journal
Issue number22
StatePublished - 1997


  • Importin
  • Nuclear import
  • Nucleocytoplasmic transport
  • RNP transport
  • U snRNPs

ASJC Scopus subject areas

  • General Neuroscience
  • Molecular Biology
  • General Biochemistry, Genetics and Molecular Biology
  • General Immunology and Microbiology


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