Cells dynamically control their material properties through remodeling of the actin cytoskeleton, an assembly of cross-linked networks and bundles formed from the biopolymer actin. Actin thus serves as an ideal model system to study mechanical adaptation of the cytoskeleton towards understanding both the functioning of cells and inform the creation of novel materials. In this work, I reconstitute networks in vitro to investigate the interplay of three aspects of actin: structure, mechanical properties, and dynamics.First, I investigate the influence of filament dopants on the internal structure and material properties of protein liquids. I find that the short, biopolymer filaments of actin spontaneously partition into phase separated FUS to form composite liquid droplets. The droplet shape is tunable and ranges from spherical to tactoid as the filament length or concentration is increased. I find that the tactoids are well described by a model of a quasi bipolar liquid crystal droplet, where nematic order from the anisotropic actin filaments competes with isotropic interfacial energy from the FUS, controlling droplet shape in a size-dependent manner. These results demonstrate a versatile approach to construct tunable, anisotropic macromolecular liquids. Next, I explore how actin networks adapt to external stimuli through structural changes. It was recently found that cross-linked networks of actin filaments can exhibit adaptive behavior. In these networks, training, in the form of applied shear stress, can induce asymmetry in the nonlinear elasticity. Here, I explore control over this response, called mechanical hysteresis, by tuning the concentration and mechanical properties of cross-linking proteins in both experimental and simulated networks. I find that this effect depends on two conditions: the initial network must exhibit nonlinear strain stiffening, and filaments in the network must be able to reorient during training. Hysteresis depends strongly and non-monotonically on cross-linker concentration. At low concentrations, where the network does not strain stiffen, or at high concentrations, where filaments are unable to rearrange, there is little response to training. Remarkably plotting hysteresis against alignment after training yields a single curve regardless of the physical properties or concentration of the cross-linkers. Finally, I investigate the ability of cross-linkers and myosin activity to control turnover, a dynamic process in which actin continuously polymerizes on one end while depolymerizing on the other. Using fluorescence recovery after photobleaching, I measure actin severing and turnover in these networks. I find that when actin is bundled by the cross-linker α-actinin, cofilin mediated severing and turnover vanishes. Additionally, I find that myosin mediated severing is sufficient to increase the rate of actin turnover, even in systems without cofilin. This increase depends on actin buckling and severing. When buckling is reduced by decreasing filament length, turnover is similarly reduced. Remarkably, α-actinin does not impact myosin mediated severing, and myosin is able to increase turnover even in bundled networks. These results not only suggest that myosin can regulate turnover of actin filaments, but also that different methods of disassembly might be needed to remodel actin depending on its local structure.




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