Single power-law rheology of crowded cytoplasm in living cells

Cytoplasmic viscoelasticity is crucial for various intracellular processes. However, the dynamic shear modulus, $G(\omega)$, has been reported to vary considerably, often without consistent patterns or rules, even within the same cell. Thus, uncovering the physical basis of cytoplasmic rheology, and whether any universal feature exists, remains a major challenge. Here, we employed microrheology with a 3D feedback technique to minimize artifacts such as laser phototoxicity and examined cytoplasmic viscoelasticity across varied mechanical environments, cell types, and cytoskeletal disruptions. Unlike previous studies, a single power-law rheology $G(\omega)\propto(-i\omega)^{0.5}$ was observed over a broad frequency range for all conditions except ATP depletion. While the vimentin cytoskeleton significantly contributed to steady shear viscosity measured by pulling a particle over large distances, cytoskeletal disruptions had only a minor effect on locally measured viscoelasticity. These findings demonstrate that molecular crowding governs the observed universality, providing a framework to systematically investigate cytoplasmic mechanics across diverse cellular contexts.