Blocking the Ca2+-induced conformational transitions in calmodulin with disulfide bonds

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Calcium-dependent regulation of cellular processes relies on Ca2+-binding proteins that undergo conformational changes.
• Calmodulin (CaM) and troponin C are key Ca2+-regulatory proteins utilizing the EF-hand motif.
• The Herzberg-Moult-James model proposed Ca2+-induced opening of helical interfaces in troponin C.

Purpose of the Study:

• To test if a similar Ca2+-induced conformational transition occurs in calmodulin.
• To investigate the role of helical interface opening in CaM's regulatory function.

Main Methods:

• Site-directed mutagenesis of human liver calmodulin (CaM) to introduce cysteine residues.
• Creation of CaM mutants (CaM41/75 and CaM85/112) designed to form intramolecular disulfide bonds.
• Assessing Ca2+ affinity and enzyme activation (phosphodiesterase, calcineurin) of wild-type and mutant CaM.
• Chemical modification of cysteine residues to probe the role of disulfide bonds.

Main Results:

• Intramolecular disulfide bonds formed readily in CaM mutants, decreasing Ca2+ affinity.
• Disulfide bond formation abolished or reduced CaM's ability to activate target enzymes.
• Reduction of disulfide bonds and subsequent modification restored CaM's regulatory activity.

Conclusions:

• The Ca2+-induced opening of interfaces between helical segments in both CaM domains is critical for its function.
• Findings support the Herzberg-Moult-James model's principles for Ca2+-binding protein regulation.
• This study elucidates a key mechanism in calmodulin's role in intracellular signaling.