FIG. 2.

(color online) a) Depicts frequency sweeps of the viscoelastic moduli for K255E, QTAA and double-mutant Actn4 cross-linked networks without external prestress (data from [18]). (b) Shows the clear collapse of the viscoelastic response for a wide variety of network compositions onto the universal theory curve. The theoretical predictions for the elastic (solid) and viscous (dashed) response are obtained from the mean-field CGD model of [13] with t_eq → 0. It is important to note that the collapse is expected to fail at the highest frequencies, as born out by the data, both due to instrument inertial effects and viscous dynamics [34]. c) Depicts the three distinct regimes of WT network behavior, characterized as linear (yellow), elastically nonlinear (blue), and dynamically nonlinear (orange). The relaxation frequency ultimately approaches the zero stress K255E relaxation rate ω_R ≈ 0.03 Hz (represented by the blue line). d) Schematic illustration of the molecular origin of the three regimes of mechanical response [18]. At the lowest levels of prestress (purple arrow), the system is linear and the filaments exhibit thermal fluctuations as evidenced by the contortions of the polymer. In this regime, the linker’s unbinding rate (green arrow), which corresponds to ω_R, is also maximal. As the prestress increases, the conformational state of the linker changes and the ABS1 site is exposed. This induces a catch-bond-like behavior where cross-linking is stabilized by force, slowing down the relaxation dynamics and significantly decreasing ω_R. However, the applied stress is not yet strong enough to have pulled out the fluctuations of the filament; thus, the network stiffness remains unchanged. Finally, at the highest prestresses, the fluctuations of the filament are pulled out and the network depicts entropic stress-stiffening with a dramatic increase in the network’s plateau modulus.