Liquid-like condensates mediate competition between actin branching and bundling

Abstract

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 versus becoming 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.

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

“…11,39,69 In line with this reasoning, recent work in several labs has shown that many actin-interacting proteins participate in condensate networks or form condensates themselves and that many of these condensates facilitate actin polymerization. 11,13,23,[38][39][40][41][42] In this context, our results suggest that the ability to assemble and organize the actin cytoskeleton is an emergent property of liquid-like, multivalent protein networks, rather than the sole responsibility of specialized polymerase proteins.…”

“…Collectively, these results suggest that filament polymerization and bundling are emergent properties of liquid-like protein condensates that bind actin. Given that many actin-interacting proteins are now thought to participate in condensates, 11,13,23,[38][39][40][41][42] our results suggest a general principle of actin organization through multivalent interactions.…”

“…This observation illustrates that condensates of differing compositions can facilitate varying actin network morphologies. 23 Another VASP binding partner is Lamellipodin, which has been shown to interact with VASP in cytoskeletal protrusions such as lamellipodia and filopodia. 20,[24][25][26][27] Lamellipodin dimerizes via an N-terminal coiled-coil domain, 28 which is followed by a Ras-associating and Pleckstrin homology domain (RA-PH), which allows Lamellipodin to localize to the plasma membrane via lipid binding.…”