Liquid-like assembly of VASP catalyzes actin polymerization and bundling.

Abstract

Actin filament bundling underlies many critical cellular processes including morphogenesis, cell division, and motility. Actin filament bundling is regulated by a family of actin-binding proteins. Recent work has revealed that several proteins comprising this cytoskeletal network have been found to undergo liquid-liquid phase separation. How might liquid-like phases contribute to filament organization and bundling? Here, we show that the processive actin polymerase and bundling protein, VASP, forms liquid-like droplets under physiological conditions. These VASP droplets catalyze actin polymerization and bundling through a mechanism that depends on the relative material properties of the filaments and the droplets. Specifically, as actin polymerizes within VASP droplets, elongating filaments partition to the perimeter of the droplet to minimize filament curvature, forming an actin-rich ring within the droplet. The rigidity of this ring is balanced by the droplet's surface tension, as predicted by a continuum-scale computational model. However, as actin polymerizes and the ring grows thicker, its rigidity increases and eventually overcomes the surface tension of the droplet, deforming into a linear bundle. The resulting bundles contain long, parallel actin filaments that grow from their tips, reminiscent of filopodia. Once the parallel arrangement of filaments is created within a VASP droplet, it propagates through the addition of new actin monomers to achieve a length that is many times greater than the initial droplet. Significantly, the fluid nature of the droplets is critical for bundling, as more solid droplets resist deformation by preventing the rearrangement of filaments that is necessary to form bundles. These results reveal a novel mechanism of filament bundling that may be relevant to the assembly of cellular architectures such as filopodia, stress fibers, and focal adhesions.