The mechanics of cytoskeletal bundles

Semiflexible filament networks are ubiquitous in biology. One example is found in the cytoskeleton, a network of stiff protein filaments transiently cross linked and driven by molecular motors. These networks are well known to form large bundles (“stress fibers”) containing tens of nearly parallel filaments. We explore how the collective mechanical properties of these fibers emerge from the interactions of their constituent filaments with transient cross-linkers using large-scale Brownian dynamics simulations. Filaments making up the bundles are treated as flexible but nearly inextensible beams. They are cross linked by small molecule elastic elements that are in chemical equilibrium with a solution of such linkers. We observe that, under bending and torsional stresses applied at the boundary, the cross-linker distribution evolves, allowing for stress relaxation in the bundle. The time evolution of these internal degrees of freedom make the collective bundle mechanics viscoelastic. We comment on the implications for these measurements for in vitro experiments and cellular mechanics.