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Calmodulin-Calcineurin Interaction beyond the Calmodulin-Binding Region Contributes to Calcineurin Activation.

Bin Sun1, Darin Vaughan1, Svetlana Tikunova2

  • 1Department of Chemistry , University of Kentucky , Lexington , Kentucky 40506 , United States.

Biochemistry
|September 5, 2019
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Summary

Calmodulin (CaM) binds to calcineurin (CaN) not only at the CaM recognition region but also at a distal helix. Mutations weakening this secondary interaction reduce CaN activity, revealing a new CaM regulatory mechanism.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • Calcineurin (CaN) is a calcium-dependent phosphatase crucial for cellular signaling.
  • Calmodulin (CaM) binding to CaN's regulatory domain (RD) activates its phosphatase activity.
  • Secondary CaM interactions with CaN's RD, beyond the canonical CaM recognition region (CaMBR), are proposed to be essential for full activation.

Purpose of the Study:

  • To investigate the hypothesis that CaM binds to CaN's C-terminal distal helix.
  • To identify potential CaM binding sites on the distal helix using computational methods.
  • To experimentally validate the role of identified CaM binding sites in CaN activation.

Main Methods:

  • Replica-exchange molecular dynamics simulations.
  • Protein-protein docking and computational mutagenesis.
  • In vitro phosphatase assays using mutated CaM variants.

Main Results:

  • A novel CaM binding site (site D) on the distal helix was identified computationally.
  • Mutations K30E and G40D in CaM weakened binding to the CaN distal helix.
  • These CaM variants exhibited reduced affinity for the CaN catalytic site substrate, suggesting impaired activation.

Conclusions:

  • CaM regulates calcineurin activity through secondary interactions with its distal helix, distinct from the CaMBR.
  • This distal helix interaction is critical for optimal CaN activation and substrate binding.
  • A novel mechanism of CaM-mediated regulation of CaN is proposed, potentially applicable to other CaM targets.