@article{InterplayofStructure:3398,
      recid = {3398},
      author = {Scheff, Danielle Robin},
      title = {Interplay of Structure, Mechanics, and Dynamics in  Reconstituted Actin},
      publisher = {The University of Chicago},
      school = {Ph.D.},
      address = {2021-08},
      pages = {130},
      abstract = {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.},
      url = {http://knowledge.uchicago.edu/record/3398},
      doi = {https://doi.org/10.6082/uchicago.3398},
}