Mutational effects on the cooperativity of Ca2+ binding in calmodulin

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Science

Background:

• Calmodulin is a crucial calcium-binding protein involved in numerous cellular processes.
• EF-hands are characteristic motifs that bind calcium ions (Ca2+).
• The role of specific aspartate residues in Ca2+ coordination within calmodulin's EF-hands is not fully understood.

Purpose of the Study:

• To investigate the functional importance of aspartate ligands in Ca2+ binding sites II and III of calmodulin.
• To elucidate the impact of specific aspartate-to-asparagine mutations on Ca2+ binding affinity and cooperativity.

Main Methods:

• Site-directed mutagenesis was used to create engineered calmodulin proteins.
• Macroscopic Ca2+ binding constants were determined using Ca2+ titrations with 5,5'-Br2BAPTA.
• Analysis of intact calmodulin and tryptic fragments representing N- and C-terminal domains.

Main Results:

• Mutation in site II (Asp58Asn) increased Ca2+ binding energy and cooperativity in the N-terminal domain.
• Mutation in site III (Asp95Asn) decreased Ca2+ affinity and cooperativity in the C-terminal domain.
• Identical mutations yielded opposing effects on Ca2+ binding properties in different calmodulin domains.

Conclusions:

• Aspartate residues play critical roles beyond direct Ca2+ coordination in calmodulin's EF-hands.
• Interactions involving liganding side chains significantly influence Ca2+ binding affinity and cooperativity.
• The specific environment of each EF-hand dictates the functional outcome of mutations.