Two low-affinity Ca(2+)-binding sites of gelsolin that regulate association with actin

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Dynamics

Background:

• Gelsolin is a calcium-dependent actin-binding protein crucial for regulating actin dynamics.
• Understanding the precise role of calcium in gelsolin-actin interactions is key to deciphering cellular processes like cell motility and cytokinesis.

Purpose of the Study:

• To quantitatively determine the calcium ion concentrations that regulate the binding kinetics of actin monomers to gelsolin.
• To elucidate the specific calcium-binding sites on gelsolin involved in sequential actin monomer association.

Main Methods:

• Fluorescence-based assay to monitor actin binding kinetics.
• Precise measurement of free calcium (Ca2+) concentrations using EGTA titration and Fura-2.
• Kinetic modeling incorporating calcium-binding equilibria to analyze gelsolin-actin interactions.

Main Results:

• Gelsolin's binding to the first actin monomer is regulated by a Ca2+ site with a dissociation constant (Kd1) of 25 microM.
• Binding of the second actin monomer is controlled by a Ca2+ site with a dissociation constant (Kd2) of 200 microM.
• Magnesium ions (Mg2+) do not interfere with Ca2+ binding to gelsolin within the tested concentration range.

Conclusions:

• Gelsolin exhibits distinct calcium sensitivities for the sequential binding of two actin monomers.
• Complex actin interactions, including nucleated polymerization, can occur at sub-micromolar Ca2+ concentrations, suggesting alternative regulatory mechanisms.