Depletion-Driven Morphological Control of Bundled Actin Networks

, The actin cytoskeleton is a semiflexible biopolymer network whose morphology is controlled by a wide range of biochemical and physical factors. Actin is known to undergo a phase transition from a single-filament state to a bundled state by the addition of polyethylene glycol (PEG) molecules in sufficient concentration. While the depletion interaction experienced by these biopolymers is well-known, the effect of changing the molecular weight of the depletant is less well understood. Here, we experimentally identify a phase transition in solutions of actin from networks of filaments to networks of bundles by varying the molecular weight of PEG polymers, while holding the concentration of these PEG polymers constant. We examine the states straddling the phase transition in terms of micro and macroscale properties. We find that the mesh size, bundle diameter, persistence length, and intra-bundle spacing between filaments across the line of criticality do not show significant differences, while the relaxation time, storage modulus, and degree of bundling change between the two states do show significant differences. Our results demonstrate the ability to tune actin network morphology and mechanics by controlling depletant size, a property which could be exploited to develop actin-based materials with switchable rigidity. contributed equally in this work. Anne Crowell helped perform all rheological experiments and wrote much of the analysis and methods related to rheology. Justin Houser helped to prototype FRET experiments and provided direct assistance in all FRET data acquisition. Allison Green performed the DLS measurements, fitted the stretched exponential and relaxation time parameters, and provided insight and analysis with Delia Milliron. Kristin Graham helped with acceptor-donor labelling for actin-FRET measurements, along with experimental planning for all FRET data acquisition. Tom Truskett provided insights into the nature of the phase transition and limitations on bundle diameter. Adrianne Rosales was instrumental in conceptually designing rheological experiments and provided consistent feedback and insight as the experiments progressed. Jeanne Stachowiak was instrumental in planning FRET experiments. José Alvarado planned most of the inter-lab experimentation, assisted in the interpretation of all the results, and provided direct feedback on the structure and material of this paper. Lauren Melcher led development and analysis of the simulation and its results with Moumita Das.