Several actin-binding proteins undergo liquid-liquid phase-separation to form condensates that promote actin filament assembly and bundling, processes crucial for local actin network organization. Previous studies showed that condensates of actin-binding proteins, such as vasodilator-stimulated phosphoprotein (VASP) and lamellipodin, restrict the organization of actin filaments to structures such as rings, shells, discs, and rods through kinetic trapping. However, it remains unclear how crosslinker multivalency, actin growth, and condensate properties govern actin organization and droplet shape. Here, we combine agent-based simulations and in vitro experiments to investigate how the feedback between the mechanical and biochemical properties of protein condensates affects droplet shape dynamics. We utilize cytosim, an agent-based modeling framework that simulates the chemical dynamics and mechanical properties of filament networks, coupled with a simple deformable ellipsoid boundary to study how condensate deformability tunes actin filament structure. We find that deformable droplet interfaces extend our range of simulated actin networks from tightly bundled actin rings to also include weakly bundled actin discs. Our quantitative analysis reveals two key relationships between actin bundling and droplet deformation. First, crosslinked actin bundle thickness and droplet diameter follow a power law that is consistent with experimental observation. Second, droplet deformation exhibits a dynamic snapping behavior determined by droplet surface tension and multivalent VASP-actin binding kinetics. We further predicted that these two relationships were generalizable to dynamic multimers and to weak actin crosslinkers, which experimental results confirmed. Together, our results identify a mechanochemical feedback between droplet interfacial properties and crosslinker multivalency that tunes actin filament organization and controls the dynamics of droplet deformation by actin networks.
Mansour et al. (Sun,) studied this question.