Ultrasensitive tuning of cytoplasmic viscosity via active noise

The mechanical properties of cytoplasm are crucial for cellular functions. While active processes significantly alter cytoplasmic viscoelasticity, the physical mechanisms remain elusive. Here, we model the cytoplasm as a colloidal suspension subject to passive and active noise, coarse-grained as an effective temperature. We show that a jammed cytoplasm transitions from a solid to a liquid phase at a critical effective temperature. Intriguingly, the simulated complex shear modulus at the critical state exhibits a 1/2 power-law scaling with frequency and quantitatively matches the experimental data of live cells without fitting parameters, given that the cytoplasmic volume fraction is slightly above the jamming transition. We further reveal the biological significance for the cytoplasm to be slightly above jamming: it is a regime in which the viscosity is ultrasensitive to changes in the effective temperature. Our results suggest that cells actively regulate their volume fractions in the sensitive regime in which they can tune cytoplasmic viscosity efficiently via active processes.