Mobility of Molecular Motors Regulates Contractile Behaviors of Actin Networks

Cells generate mechanical forces primarily from interactions between F-actin, cross-linking proteins, myosin motors, and other actin-binding proteins in the cytoskeleton. To understand how molecular interactions between the cytoskeletal elements generate forces, a number of in vitro experiments have been performed but are limited in their ability to accurately reproduce the diversity of motor mobility. In myosin motility assays, myosin heads are fixed on a surface and glide F-actin. By contrast, in reconstituted gels, the motion of myosin and F-actin are both unrestricted. Since only these two extreme conditions have been used, the importance of mobility of motors for network behaviors has remained unclear. In this study, to illuminate the impacts of motor mobility on the contractile behaviors of the actin cytoskeleton, we employed an agent-based computational model based on Brownian dynamics. We found that if motors can bind to only one F-actin like myosin I, networks are most contractile at intermediate mobility. On the contrary, if motors can bind to a pair of F-actins, a network can exhibit larger contraction with higher motor mobility. Results from this study imply that mobility of molecular motors may critically regulate contractile behaviors of actin networks in cells.