Viscoelasticity of F-actin and F-actin/gelsolin complexes

Area of Science:

• Biophysics
• Cell Biology
• Polymer Science

Background:

• Actin is a key protein in eukaryotic cytoplasm, influencing cell shape and motility.
• The mechanical properties of purified F-actin (viscoelastic fluid vs. elastic solid) remain debated.
• Polymer theory suggests long F-actin filaments appear immobile due to interpenetration.

Purpose of the Study:

• To investigate the rheological behavior of F-actin under varying shear conditions.
• To determine if F-actin exhibits viscoelastic fluid properties under specific experimental parameters.
• To elucidate the role of filament length and gelsolin in F-actin's mechanical characteristics.

Main Methods:

• Rheological measurements of F-actin under varying shear strain.
• Treatment of F-actin with gelsolin, an F-actin severing protein.
• Analysis of static and dynamic elastic moduli and viscosity.

Main Results:

• High static and dynamic elastic moduli were observed below critical shear strain, consistent with filament interpenetration.
• Increased shear strain or gelsolin treatment significantly reduced moduli, revealing F-actin's viscoelastic fluid behavior.
• F-actin exhibited shear rate independent viscosity below a critical shear rate after gelsolin treatment, indicating filament disruption.

Conclusions:

• Shortening F-actin filaments, via high strain or gelsolin, unmasks their inherent viscoelastic fluid properties.
• The unusual shear rate dependence of F-actin viscosity is attributed to filament disruption during measurement.
• Findings reconcile conflicting data by demonstrating F-actin's behavior is dependent on filament length and experimental conditions.