How crosslinking actin filaments influences the microscale viscoelastic properties of actin-microtubule composites*

The strength and mobility of cells is dependent upon the interactions between two protein filaments that comprise the cytoskeleton: actin and microtubules. These proteins form entangled networks that can also be chemically crosslinked to enable a wide range of mechanical properties. Here, we use optical tweezers microrheology to determine how varying concentrations of actin crosslinkers influences the viscoelastic properties of actin-microtubule composites. We create equimolar co-entangled networks of actin and microtubules with varying concentrations of actin crosslinkers. We use optical tweezers to apply both oscillatory and constant speed microscale strains over a range of rates and distances while simultaneously measuring the force the networks exert to resist these strains. We quantify the frequency-dependent complex viscosity, the nonlinear stress response, and the relaxation dynamics following strain. Surprisingly, we find that increasing the concentration of crosslinkers yields a decrease in network elasticity and stiffness.