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. SIGNIFICANCE STATEMENT Reorganization of actin filaments allows cells to migrate, which is required for embryonic development, wound healing, and cancer metastasis. During migration, the leading-edge of the cell consists of needle-like protrusions of bundled actin, which emanate from a sheet of branched actin. Given that the proteins responsible for both architectures are present simultaneously, what determines whether actin filaments will be branched or bundled? Here we show that liquid-like condensates, composed of both branching and bundling proteins, can mediate the inherent competition between these fundamentally different ways of organizing actin networks. This work demonstrates that by tuning the composition of condensates, we can recapitulate the transition from branched to bundled networks, a key step in cell migration.