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Liquid-like protein condensates perform diverse physiological functions. Previous work showed that VASP, a processive actin polymerase, forms condensates that polymerize and bundle actin. To minimize their curvature, filaments accumulated at the inner condensate surface, ultimately deforming the condensate into a rod-like shape, filled with a bundle of parallel filaments. Here we show that this behavior does not require proteins with specific polymerase activity. Specifically, we found that condensates composed of Lamellipodin, a protein that binds actin but is not an actin polymerase, were also capable of polymerizing and bundling actin filaments. To probe the minimum requirements for condensate-mediated actin bundling, we developed an agent-based computational model. Guided by its predictions, we hypothesized that any condensate-forming protein that binds actin could bundle filaments through multivalent crosslinking. To test this idea, we added an actin-binding motif to Eps15, a condensate-forming protein that does not normally bind actin. The resulting chimera formed condensates that drove efficient actin polymerization and bundling. Collectively, these findings broaden the family of proteins that could organize cytoskeletal filaments to include any actin-binding protein that participates in protein condensation.
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Liquid-like condensates mediate competition between actin branching and bundling
Cellular remodeling of actin networks underlies cell motility during key morphological events, from embryogenesis to metastasis. In these transformations, there is an inherent competition between actin branching and bundling, because steric clashes among branches create a mechanical barrier to bundling. Recently, liquid-like condensates consisting purely of proteins involved in either branching or bundling of the cytoskeleton have been found to catalyze their respective functions. Yet in the cell, proteins that drive branching and bundling are present simultaneously. In this complex environment, which factors determine whether a condensate drives filaments to branch or become bundled? To answer this question, we added the branched actin nucleator, Arp2/3, to condensates composed of VASP, an actin bundling protein. At low actin to VASP ratios, branching activity, mediated by Arp2/3, robustly inhibited VASP-mediated bundling of filaments, in agreement with agent-based simulations. In contrast, as the actin to VASP ratio increased, addition of Arp2/3 led to formation of aster-shaped structures, in which bundled filaments emerged from a branched actin core, analogous to filopodia emerging from a branched lamellipodial network. These results demonstrate that multi-component, liquid-like condensates can modulate the inherent competition between bundled and branched actin morphologies, leading to organized, higher-order structures, similar to those found in motile cells.
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- Award ID(s):
- 2327244
- PAR ID:
- 10539643
- Publisher / Repository:
- Oxford
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 121
- Issue:
- 3
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2309152121
