FIGURE 1:

The cytoplasm is a crowded viscoelastic matrix of cross-linked polymers and proteins. (a) Schematic of the actin network being deformed by a microbead at frequencies (f) slower than the unbinding rate of the cross-linkers (k_off), before (opaque) and after deformation (transparent). Previously bound cross-linkers (blue heads) detach (magenta heads), and the filaments are free to slide past one another and rearrange (curved arrows), resulting in a more viscous, fluid-like network. (b) At frequencies faster than k_off, the cross-linkers stay bound and interlock the filaments, resulting in filament bending and a more solid-like, elastic response. (c) Schematic of the experimental setup showing the active oscillation of bead in the intracellular environment by the OT (x_trap). Not to scale. (d) Representative brightfield image of a live cell showing the optically trapped bead (arrow). Scale bar is 10 µm.