[The domain organization of calmodulin]

Area of Science:

• Biochemistry and biophysics
• Protein structure and dynamics
• Calcium-binding proteins

Context:

• Calmodulin (CaM) is a crucial calcium-binding protein involved in numerous cellular processes.
• Understanding CaM's domain organization is key to elucidating its regulatory mechanisms.
• Previous studies have investigated CaM's structure, but its cooperative domain stability requires further clarification.

Purpose:

• To investigate the domain organization and cooperative stability of calmodulin (CaM) using differential scanning microcalorimetry.
• To analyze the impact of calcium ion (Ca2+) concentration on CaM's structural integrity.
• To compare the stability of cooperative blocks in intact CaM versus its proteolytic fragments.

Summary:

• Differential scanning microcalorimetry demonstrated that CaM, at physiological Ca2+ levels, forms three cooperative blocks, with one block housing two Ca2+-binding domains.
• The stability of these cooperative blocks is highly dependent on Ca2+ concentration; domain III loses stability at room temperature with EDTA.
• Magnesium (Mg2+) or sodium (Na+) ions do not stabilize the cooperative structure of the Ca2+-binding domain III block in the absence of Ca2+.
• Thermodynamic melting parameters indicate that the structure of cooperative blocks in CaM fragments mirrors that of the intact molecule.

Impact:

• Provides novel insights into the cooperative domain organization of calmodulin.
• Highlights the critical role of calcium ions in maintaining calmodulin's structural stability.
• Suggests that proteolytic fragments retain key structural features of the intact protein, aiding in structure-function relationship studies.