Actin crosslinker density tunes mesoscale mechanics in actin-microtubule composites*

The physical interactions between semiflexible actin filaments, rigid microtubules and the suit of smaller proteins that crosslink both filaments, allows the cytoskeleton to precisely tune its structure and mechanics to enable a vast array of cellular processes such as cytokinesis. Here we use optical tweezers microrheology to determine how steric entanglements versus chemical crosslinking impacts the nonlinear mesoscale mechanics of actin-microtubule composites. We create co-polymerized and co-entangled composites of actin and microtubules with varying concentrations of actin crosslinkers. We optically drive microspheres through the crosslinked composites at speeds much faster than the relaxation rates and distances larger than network mesh size. We simultaneously measure the force the filaments exert on the microspheres during and following the strain. We map network stiffness, relaxation profiles, and spatial heterogeneities to the concentration of actin crosslinkers in composites.