A novel target recognition revealed by calmodulin in complex with the basic helix--loop--helix transcription factor...

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Calmodulin (CaM) is a key calcium (Ca2+) sensor regulating cellular processes.
• CaM inhibits DNA binding of basic helix-loop-helix (bHLH) transcription factors via a novel interaction.
• Understanding this interaction is crucial for deciphering CaM-mediated gene regulation.

Purpose of the Study:

• To structurally characterize the novel interaction between CaM and a bHLH transcription factor peptide.
• To elucidate the binding stoichiometry and mode of the CaM:SEF2-1mp complex.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy was employed to study the complex.
• Resonance assignments and secondary structure determination for both CaM and SEF2-1mp peptide.
• Amide proton exchange rates and Nuclear Overhauser Effect (NOE) contacts were measured.

Main Results:

• The stoichiometry of the complex was determined as one dimeric SEF2-1mp peptide binding two CaM molecules.
• NMR data revealed a novel, open binding conformation of CaM, distinct from the classical wraparound mode.
• Hydrophobic surfaces of CaM's N- and C-terminal domains and charged residue interactions were identified as key binding sites.

Conclusions:

• Calmodulin binds the SEF2-1mp peptide in an unusual, open conformation, challenging the classical binding model.
• This interaction involves hydrophobic and charged residues, explaining sensitivity to ionic strength.
• The findings provide structural insights into CaM's regulation of bHLH transcription factors.