Liquid-like VASP condensates drive actin polymerization and dynamic bundling.

Area of Science:

• Cell Biology
• Biophysics
• Biochemistry

Background:

• Actin filament organization into bundles is essential for cellular functions like motility and division.
• Actin-binding proteins regulate filament bundling, with some exhibiting liquid-liquid phase separation.
• The role of these liquid-like condensates in filament bundling remains unclear.

Purpose of the Study:

• To investigate how liquid-like condensates formed by the actin-binding protein VASP contribute to actin filament bundling.
• To elucidate the mechanism by which VASP droplets facilitate the formation of parallel actin filament bundles.

Main Methods:

• Formation and observation of VASP (vasodilator-stimulated phosphoprotein) droplets under physiological conditions.
• Studying actin polymerization dynamics within VASP droplets.
• Utilizing a continuum-scale computational model to predict droplet behavior and filament organization.

Main Results:

• VASP forms liquid-like droplets that promote actin polymerization.
• Actin filaments partition to droplet edges, forming a ring whose rigidity increases with polymerization.
• This increased rigidity eventually overcomes surface tension, deforming the droplet into a linear actin bundle.

Conclusions:

• The fluid nature of VASP droplets is critical for enabling actin filament rearrangement into bundles.
• This droplet-based mechanism drives the formation of long, parallel actin filament structures.
• The findings suggest a novel mechanism for assembling cellular architectures such as filopodia and stress fibers.