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Nuclear localization of NF-ATc by a calcineurin-dependent, cyclosporin-sensitive intramolecular interaction
C R Beals1, N A Clipstone, S N Ho
1Department of Developmental Biology, Stanford University, California 94305-5428, USA.
Abstract:
The NF-AT family of transcription factors participates in the regulation of early immune response genes such as IL-2, IL-4, CD40 ligand, and Fas ligand in response to Ca2+/calcineurin signals initiated at the antigen receptor. Calcineurin activation leads to the rapid translocation of NF-AT family members from cytoplasm to nucleus, an event that is blocked by the immunosuppressive drugs cyclosporin A and FK506. We show that translocation requires two redundant nuclear localization sequences and that one sequence is in an intramolecular association with phosphorserines in a conserved motif located at the amino terminus of each NF-AT protein. Mutation of serines in this motif in NF-ATc both disrupts this intramolecular interaction and leads to nuclear localization, suggesting a model of NF-AT nuclear import in which dephosphorylation by calcineurin causes exposure of two nuclear localization sequences.
Insights
Nuclear factor of activated T-cells (NF-AT) translocation to the nucleus is crucial for immune response gene regulation. Dephosphorylation by calcineurin exposes nuclear localization sequences, enabling NF-AT import.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- NF-AT transcription factors regulate key immune response genes like IL-2.
- Activation occurs via Ca2+/calcineurin signaling initiated at the antigen receptor.
- Calcineurin activation drives NF-AT nuclear translocation, a process inhibited by immunosuppressants.
Purpose of the Study:
- To elucidate the mechanism of NF-AT nuclear import.
- To identify the role of nuclear localization sequences (NLS) in NF-AT translocation.
- To investigate the interaction between calcineurin, phosphoserines, and NF-AT nuclear entry.
Main Methods:
- Site-directed mutagenesis of conserved motifs in NF-AT proteins.
- Analysis of NF-AT protein localization in response to calcineurin activation.
- Investigating the role of intramolecular interactions in regulating NLS accessibility.
Main Results:
- NF-AT nuclear translocation requires two redundant nuclear localization sequences.
- An intramolecular association between an NLS and phosphoserines in a conserved motif was identified.
- Mutation of motif serines disrupted this interaction, leading to constitutive nuclear localization.
Conclusions:
- Calcineurin-mediated dephosphorylation is essential for exposing NF-AT nuclear localization sequences.
- This dephosphorylation event facilitates the nuclear import of NF-AT proteins.
- A model for NF-AT nuclear import involves calcineurin-dependent unmasking of NLS.