Fig. S6.

Normalized force-displacement curves measured in the cytoplasm of NRK cells with F-actin depolymerized show a clear decrease of the characteristic viscoelastic timescale, τ, and a marked increase of the effective diffusivity, D, in the cytoplasm. (A) The cytoplasm of NRK cells with F-actin depolymerized behaves as a viscous fluid when the strain rate V/a is ∼0.2 s⁻¹, and the cytoplasm of normal NRK cells behaves as a fluid when V/a < 0.1 s⁻¹. This increase in the transition strain rate from a seemingly viscous fluid behavior to a viscoelastic material is attributed to the decrease of the characteristic viscoelastic timescale, τ. (B) The normalized force-displacement curves obtained with different bead sizes (0.5 µm and 1 µm) in the cytoplasm are close to each other when the same V/a is maintained as 2 s⁻¹; this proves that F-actin polymerized NRK cytoplasm behaves as a viscoelastic material in such a condition. (C and D) Under high loading speeds, the normalized force-displacement curves obtained with different bead sizes (0.5 µm and 1 µm) in the F-actin depolymerized cytoplasm are separated when the same Va is maintained (e.g., 5 µm²/s in C and 10 µm²/s in D), while force-displacement curves with the same speeds and sizes obtained in the cytoplasm of control NRK cells collapse well (Fig. 3B and Fig. S7A). This proves that the critical strain rate, V/a, at which we observe poroelasticity to start dominating the mechanical resistance increases because of an increase of the cytoplasmic diffusivity, D. (E and F) Bright-field and fluorescent imaging of NRK cells stained with phalloidine in the control cells (E) and cytochalasin D-treated cells (F) shows that F-actin structure is fully inhibited.