Force transmission in an actomyosin cytoskeletal network revealed by particle fluctuation analysis

Actomyosin cytoskeleton, composed of actin filaments and myosin motor proteins, functions as nanoscale force generators within the cytoskeleton. While they generate contractile forces in the cytosol of living cells, the nature of non-thermal fluctuations driven by spontaneous generation of active force remains little understood. Here, by using in vitro reconstituted systems, we quantitatively analyzed the fluctuations of optically trapped microparticles within the bath of actomyosin cytoskeletons using optical tweezers. Statistical analysis of the trapped particles revealed active forces of approximately 0.16 pN under spontaneous advective flows with mean velocities of 0.25-0.29µm/s. Notably, these excess fluctuations emerged specifically under flow-generating conditions, and the associated energy dissipation was found to be on the order of thermal energy. Our results provide a quantitative estimate of force transmission in the actomyosin cytoskeleton and offer insights into how force-mediated interactions may contribute to intracellular self-organization.