Importin provides a link between nuclear protein import and U snRNA export

Area of Science:

• Molecular Biology
• Cell Biology
• RNA Transport

Background:

• Importin-alpha is crucial for nuclear protein import by binding nuclear localization signals and importin-beta.
• The nuclear cap-binding protein complex (CBC) is involved in nuclear export of capped U snRNAs and shuttles between the nucleus and cytoplasm.

Purpose of the Study:

• To investigate the interaction between importin-alpha and the cap-binding protein complex (CBC).
• To elucidate the mechanism by which CBC-mediated nuclear export of capped U snRNAs is regulated.

Main Methods:

• Co-immunoprecipitation assays to detect protein-protein interactions.
• RNA binding assays to assess the effect of importin-alpha and importin-beta on CBC-RNA complex formation.

Main Results:

• Approximately 30% of yeast importin-alpha (SRP1p) forms a nuclear complex with Saccharomyces cerevisiae CBC.
• Xenopus CBC is also significantly associated with importin-alpha in the nucleus.
• The CBC-importin-alpha complex specifically binds capped RNA.
• Importin-beta binding to the CBC-importin-alpha complex displaces the bound RNA.

Conclusions:

• The interaction between importin-alpha and CBC suggests a role for importin-alpha in regulating CBC's function in nuclear export.
• Importin-beta binding acts as a trigger for the release of export substrates (capped U snRNAs) from CBC into the cytoplasm.
• This mechanism ensures that importin-mediated RNA release occurs specifically in the cytoplasm, preventing premature release in the nucleus.