A balance between nucleating and elongating actin filaments controls deformation of protein condensates.

Protein condensates assemble actin filaments into diverse architectures, reminiscent of filopodia and stress fibers. During assembly, filament nucleation and elongation compete for a shared pool of actin monomers. Here we show that a balance between these processes is required to deform condensates of VASP, an actin polymerase, into high aspect ratio structures. Adding magnesium, which promotes filament nucleation, enhanced condensate deformation, while adding profilin, which favors elongation over nucleation, produced ring-like filament bundles that failed to deform condensates. Computational modeling predicted that a collection of short filaments would be more efficient at deforming condensates compared to a few long filaments. To test this prediction, filament capping protein was used to inhibit growth of long filaments. The resulting set of shorter filaments regained the ability to deform condensates, illustrating the importance of a balance between filament nucleation and elongation. More broadly, these findings illustrate how protein condensates balance actin nucleation, elongation, and bundling to direct the assembly of diverse cytoskeletal architectures.