Chromatin Mechanics Dictates Subdiffusion and Coarsening Dynamics of Embedded Condensates*

Liquid-liquid phase separation has emerged as a mechanism of biological organization, particularly within the cell nucleus. However, the impact of chromatin on the dynamics of phase separation is poorly understood. Here, we utilize a powerful optogenetic strategy to examine the interplay of droplet coarsening with the surrounding chromatin network. We demonstrate that droplet growth dynamics are inhibited by the viscoelastic environment, giving rise to a slow coarsening exponent, β≈0.12, contrasting with the classical prediction of β=1/3. Using scaling arguments and simulations, we show how this arrested growth can arise due to subdiffusion of individual condensates, predicting β=α/3, where α is the diffusive exponent. Tracking condensates within chromatin reveals α≈0.5, explaining the anomalous coarsening behavior. Our work has implications for the regulation of the size and shape of biomolecular condensates and suggests that condensate emulsions can be used to probe the viscoelastic environment within living cells.