(285c) To Bundle or Not. Morphological Transitions in Actin Bundles Are Determined By Their Initial Polarity, Myosin Activity, Crosslinking, and Filament Treadmilling

Bundled organization of actin filaments is crucial to ensure structural integrity of a cell. Along with various mechanosensitive components, bundles play an important role in phenomena such as locomotion and chemotaxis. We studied stability of actin bundles using MEDYAN, a publicly available software developed in our group. Furthemore, we developed a novel hierarchical clustering scheme that allowed classifying long time-scale morphologies of unipolar and apolar bundles. Using these computational techniques, we found that unipolar bundles, where all filaments point in the same direction, are significantly more stable than apolar bundles. Myosin walking within apolar bundles leads to mutual shearing and distortion of neighboring filaments. In addition, we also identified myosin concentration as the primary factor determining network morphology. For example, bundles transform into aster-like morphologies at high myosin concentrations. We also found that under certain conditions interesting architectures, such as sarcomere-like organization, spontaneously emerge. Finally, we suggest that the sensitivity of apolar bundles to environmental perturbations, which leads to their remarkable morphological responses, may be an important component of cytoplasm’s adaptation strategies.