Intrinsically disordered caldesmon binds calmodulin via the "buttons on a string" mechanism

Area of Science:

• Biochemistry
• Protein Structure and Dynamics
• Molecular Interactions

Background:

• Chicken gizzard caldesmon (CaD) is a key protein involved in muscle contraction.
• The C-terminal domain of CaD (CaD136) is intrinsically disordered, posing challenges for understanding its interactions.
• Calmodulin (CaM) is a crucial calcium-binding protein involved in cellular signaling.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the interaction between chicken gizzard caldesmon C-terminal domain (CaD136) and calmodulin (CaM).
• To elucidate the role of specific tryptophan residues in the CaD136-CaM binding specificity.
• To propose a model for the CaD136-CaM interaction.

Main Methods:

• Computational and experimental analyses of wild-type CaD136 and its tryptophan mutants.
• Site-directed mutagenesis to create single and double tryptophan mutants (W674A, W707A, W737A, W674A/W707A).
• Analysis of CaD136-CaM binding affinity and specificity.

Main Results:

• CaD136 and CaM interaction is driven by electrostatic attraction but specified by tryptophan residue packing.
• A "buttons on a charged string" model is proposed, where tryptophan residues form "pliable buttons" that specificially pack at the interface.
• All three key tryptophan residues are important for CaD136-CaM binding, with mutations affecting interaction.

Conclusions:

• The specific packing of tryptophan residues in CaD136 is critical for the specificity of its interaction with CaM.
• The "buttons on a charged string" model effectively describes this unique binding mechanism.
• Understanding this interaction provides insights into the function of intrinsically disordered proteins in cellular processes.