Chromatin Network Retards Droplet Coalescence

Nuclear bodies are membraneless condensates that may form via liquid-liquid phase separation. The viscoelastic chromatin network could impact their stability and may hold the key for understanding experimental observations that defy predictions of classical theories. However, quantitative studies on the role of the chromatin network in phase separation have remained challenging. Using a diploid human genome model parameterized with chromosome conformation capture (Hi-C) data, we studied the thermodynamics and kinetics of droplet formation inside the nucleus. Dynamical simulations predicted the formation of multiple droplets for protein particles that experience specific interactions with nucleolus-associated domains (NADs). Coarsening dynamics, surface tension, and coalescence kinetics of the simulated droplets are all in quantitative agreements with experimental measurements for nucleoli. Free energy calculations further supported that a two-droplet state, which is often observed for nucleoli seen in somatic cells, is metastable and separated from the single-droplet state with an entropic barrier. Our study suggests that protein-chromatin interactions facilitate the nucleation of droplets, but hinders their coarsening due to the correlated motion between droplets and the chromatin network: as droplets coalesce, the chromatin network becomes increasingly constrained. Therefore, protein-chromatin interactions arrest phase separation in multi-droplet states and may drive the variation of nuclear body numbers across cell types.