Large Scale Self-organization

Living cells need to impose inner order, transport organelles from site to site inside the cell, withstand external pressures and propagate themselves to different locations. All of these mechanical needs are supplied by the cells cytoskeleton. One of the components of the cytoskeleton is filamentous actin (F-actin). The two ends of F-actin are distinctly different, making it a polar rod with a ”plus end” and a ”minus end”. Actin defines and preserves the cells shape and allows for cell motion. Actin in the cell has many accessory proteins. One of the more interesting ones is Myosin II, a molecular motor which can convert chemical energy into mechanical work and progress along F-actin towards its plus end. These motors can aggregate and move along more than one filament simultaneously, causing their relative movement. In-vitro experiments have shown that mixtures of F-actin and myosin II motors can generate different patterns, including bundles, asters, and networks. The dynamics allowing this organization are not yet fully understood. In this work, we present a coarse-grained molecular model for systems consisting of myosin II motors and actin filaments. Within this model, the filaments are represented as flexible rods while the motors are modeled as ”mechanical units” that can bind to, exert forces, and move along the filaments. Thus, the approach balances the need for molecular details with computational simplicity in a manner that allows for simulations which mimic in-vitro experiments. We use the model to study, by means of molecular dynamics simulations, the self-organization behavior of the system. Our simulations reveal a very rich phase behavior with several steady-states (different types of bundles and asters both isolated and interconnected) and different global layouts of filaments and motors determined by the length of the filaments and the number of motors per filament, some of which have been observed experimentally. They also provide a visualization of the dynamics leading to these structures and shed light on the role played by the myosin II motors as active linkers which can bind to several filaments and (due to their non-processive nature) switch between them.