Probing the effects of calcium on gelsolin

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Gelsolin is a calcium-regulated protein crucial for actin dynamics.
• It possesses three actin-binding sites and interacts with actin monomers and filaments.
• Gelsolin's structure comprises six repeating sequence segments (G1-6).

Purpose of the Study:

• To investigate the impact of calcium ions on human gelsolin's structure and function.
• To analyze the effects of calcium on gelsolin's proteolysis and actin-binding properties.
• To explore calcium-induced structural changes in gelsolin and its subdomains.

Main Methods:

• Limited proteolysis of bacterially expressed human gelsolin using plasmin.
• Dynamic light scattering (DLS) to assess structural changes.
• Circular dichroism (CD) spectroscopy to analyze protein conformation.
• Actin binding assays at varying calcium concentrations.

Main Results:

• Cleavage of gelsolin between G1 and G2 in the absence of calcium did not dissociate the molecule.
• The noncovalently linked form of gelsolin exhibits three actin-binding sites in the presence of calcium.
• Calcium binding induces structural changes in gelsolin and its subdomains, detectable at nanomolar concentrations.
• Significant actin binding requires micromolar calcium concentrations, engaging both N-terminal and C-terminal domains.

Conclusions:

• Nanomolar calcium concentrations initiate structural changes that "unlatch" actin-binding sites.
• Micromolar calcium concentrations are necessary for substantial actin binding, indicating a multi-step activation process.
• Calcium ions play a critical role in controlling gelsolin's three-dimensional structure and actin-binding activity.