Two dominant timescales of cytoskeletal crosslinking in the viscoelastic response of the cytoplasm

Cells precisely regulate their frequency-dependent viscoelastic properties in response to chemical and mechanical cues. We use optical trap-based active microrheology using intracellular probes measure the cytoplasmic of fibroblast macrophage cells over a broad frequency range ($\ensuremath{\sim}0.02--350$ Hz). Both cell types show similar behavior, suggesting that mechanisms control cell's are general many types. At frequencies above 1 Hz, behavior shows distribution relaxation timescales consistent with power-law mechanics. low ($<1$ Hz), exhibit fluidlike distinct timescales, observed reconstituted networks transiently crosslinked actin filaments. The across all can be captured by mathematical model combining term two crosslinker-unbinding terms. unbinding rates required describe low-frequency suggest cytoplasm is governed either multiple dominant crosslinkers or single crosslinker states.