Liquid-like condensates that bind actin drive filament polymerization and bundling

Area of Science:

• Cell Biology
• Biophysics
• Biochemistry

Background:

• Liquid-like protein condensates are crucial for cellular functions.
• Previously, VASP (a processive actin polymerase) condensates were shown to polymerize and bundle actin filaments, deforming into rod-like structures.
• Filament accumulation at the inner condensate surface minimized curvature, leading to parallel filament bundles.

Purpose of the Study:

• To investigate if actin polymerase activity is essential for condensate-mediated actin bundling.
• To determine the minimum requirements for protein condensates to bundle actin filaments.
• To broaden the understanding of proteins capable of organizing cytoskeletal filaments.

Main Methods:

• Experimental observation of actin filament behavior within condensates formed by Lamellipodin (an actin binder without polymerase activity).
• Development of an agent-based computational model to probe requirements for condensate-mediated actin bundling.
• Genetic engineering to create a chimeric protein (Eps15 with an actin-binding motif) to test hypotheses.

Main Results:

• Condensates of Lamellipodin, lacking polymerase activity, effectively polymerized and bundled actin filaments.
• The computational model suggested that multivalent crosslinking by any actin-binding condensate-forming protein could bundle filaments.
• The engineered Eps15 chimera formed condensates that efficiently drove actin polymerization and bundling, validating the hypothesis.

Conclusions:

• Actin polymerase activity is not a prerequisite for protein condensates to bundle actin filaments.
• Any protein capable of forming condensates and binding actin can organize cytoskeletal filaments via multivalent crosslinking.
• This expands the range of proteins, including non-polymerase actin binders, that can mediate cytoskeletal organization through phase separation.