The actin cytoskeleton plays critical roles in cellular morphogenesis, motility, trafficking, and force generation. In many of these processes, actin filaments must form bundles of parallel filaments to perform their mechanical function. The polymerization and bundling of actin filaments is regulated through a dense network of actin binding proteins. How does this network support actin assembly? Here we show that dynamic assembly of multivalent, actin-binding proteins can drive actin polymerization and filament bundling. In particular, we have found that VASP, a tetrameric actin polymerase and cross-linking protein, self-assembles into liquid-like, micrometer-scale condensates. These assemblies concentrate actin monomers, catalyzing actin polymerization (Fig. A, B). Filaments are then bundled by a unique mechanism driven by the comparative mechanical properties of both the filaments and the condensate. Specifically, we have found that short filaments can be contained within the condensate with minimal deformation. However, filaments longer than the diameter of the condensate must bend in order to be contained within the condensate (Fig. C). As more filaments polymerize and bend, they concentrate around the edges of the condensate and begin to form a bundle (Fig. D, E). With increasing bundle size, the rigidity of the bundle increases enough to deform the initially spherical condensate into a rod (Fig. F, G). Here the liquid-like deformability of the condensate uniquely enables both local concentration of actin monomers and dynamic rearrangement to support filament growth and bundling. Based on these results, we hypothesize that liquid-like assembly of actin regulators provides optimum support for dynamic actin bundling, which underlies multiple cellular processes, including formation of filopodia.

Liquid-like assembly of cytoskeletal proteins drives actin polymerization and bundling

ACS Spring Meeting 2021

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