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Interactions between actin filaments (F-actin) and myosin are critically important for a wide range of biological processes, including cell migration, cytokinesis, and morphogenesis. The motility assay with myosin motors fixed on a surface has been utilized for understanding various phenomena emerging from the interactions between F-actin and myosin. For example, F-actin in the motility assay exhibited distinct collective behaviors when actin concentration was above a critical threshold. Recent studies have performed the myosin motility assay on a lipid bilayer, meaning that myosin motors anchored on the fluidlike membrane have mobility. Interestingly, mobile motors led to very different collective behaviors of F-actin compared to those induced by stationary motors. However, the dynamics and mechanism of the unique collective behaviors have remained elusive. In this study, we employed our cutting-edge computational model to simulate the motility assay with mobile myosin motors. We reproduced the formation of actin clusters observed in experiments and showed that F-actin within clusters exhibits strong polar ordering and leads to phase separation between myosin motors and F-actin. The cluster formation was highly dependent on the average length and concentration of F-actin. Our study provides insights into understanding the collective behaviors of F-actins that could emerge under more physiological conditions. Published by the American Physical Society2025 

Abstract Living cells can adapt their shape in response to their environment, a process driven by the interaction between their flexible membrane and the activity of the underlying cytoskeleton. However, the precise physical mechanisms of this coupling remain unclear. Here we show how cytoskeletal forces acting on a biomimetic membrane affect its deformations. Using a minimal cell model that consists of an active network of microtubules and molecular motors encapsulated inside lipid vesicles, we observe large shape fluctuations and travelling membrane deformations. Quantitative analysis of membrane and microtubule dynamics demonstrates how active forces set the temporal scale of vesicle fluctuations, giving rise to fluctuation spectra that differ in both their spatial and temporal decays from their counterparts in thermal equilibrium. Using simulations, we extend the classical framework of membrane fluctuations to active cytoskeleton-driven vesicles, demonstrating how correlated activity governs membrane dynamics and the roles of confinement, membrane material properties and cytoskeletal forces. Our findings provide a quantitative foundation for understanding the shape-morphing abilities of living cells. 

Abstract Much like passive materials, active systems can be affected by the presence of imperfections in their microscopic order, called defects, that influence macroscopic properties. This suggests the possibility to steer collective patterns by introducing and controlling defects in an active system. Here we show that a self-assembled, passive nematic is ideally suited to control the pattern formation process of an active fluid. To this end, we force microtubules to glide inside a passive nematic material made from actin filaments. The actin nematic features self-assembled half-integer defects that steer the active microtubules and lead to the formation of macroscopic polar patterns. Moreover, by confining the nematic in circular geometries, chiral loops form. We find that the exact positioning of nematic defects in the passive material deterministically controls the formation and the polarity of the active flow, opening the possibility of efficiently shaping an active material using passive defects.