Tuning of cytoplasmic effective 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, quantified by an effective temperature. We demonstrate that a jammed cytoplasm undergoes a transition from solid to liquid phase at a critical effective temperature. Intriguingly, the simulated complex shear modulus at the critical state exhibits a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>1</a:mn><a:mo>/</a:mo><a:mn>2</a:mn></a:mrow></a:math> power-law scaling with frequency and quantitatively matches the experimental data of live cells, given that the cytoplasmic volume fraction is slightly above the jamming transition. We further reveal the biological significance of the cytoplasm being slightly above jamming: It is a regime in which the effective viscosity, the magnitude of complex viscosity, is sensitive to changes in the effective temperature. Our results suggest that cells actively regulate their volume fraction into the sensitive regime, where they can efficiently tune cytoplasmic viscoelasticity through active processes.