Liquid-liquid phase separation recapitulates the thermodynamics and kinetics of heterochromatin formation
The spatial segregation of pericentromeric heterochromatin (PCH) into distinct, membrane-less nuclear compartments involves the binding of Heterochromatin Protein 1 (HP1) to H3K9me2/3-rich genomic regions. While HP1 exhibits liquid-liquid phase separation properties in vitro, its mechanistic impact on the structure and dynamics of PCH condensate formation in vivo remains largely unresolved. Here, using biophysical modeling, we systematically investigate the mutual coupling between self-interacting HP1-like molecules and the chromatin polymer. We reveal that the specific affinity of HP1 for H3K9me2/3 loci facilitates coacervation in nucleo, and promotes the formation of stable PCH condensates at HP1 levels far below the concentration required to observe phase separation in purified protein assays in vitro. These heterotypic HP1-chromatin interactions give rise to a strong dependence of the nucleoplasmic HP1 density on HP1-H3K9me2/3 stoichiometry, consistent with the thermodynamics of multicomponent phase separation. The dynamical crosstalk between HP1 and the viscoelastic chromatin scaffold also leads to anomalously-slow equilibration kinetics, which strongly depend on the genomic distribution of H3K9me2/3 domains, and result in the coexistence of multiple long-lived, microphase-separated PCH compartments. The morphology of these complex coacervates is further found to be governed by the dynamic establishment of the underlying H3K9me2/3 landscape, which may drive their increasingly abnormal, aspherical shapes during cell development. These findings compare favorably to 4D microscopy measurements of HP1 condensates that we perform in live Drosophila embryos, and suggest a general quantitative model of PCH formation based on the interplay between HP1-based phase separation and chromatin polymer mechanics. SIGNIFICANCE STATEMENTThe compartmentalization of pericentromeric heterochromatin (PCH), the highly-repetitive part of the genome, into membrane-less organelles enriched in HP1 proteins, is critical to both genetic stability and cell fate determination. While HP1 can self-organize into liquid-like condensates in vitro, the roles of HP1 and the polymer chromatin in forming 3D PCH domains in vivo are still unclear. Using molecular simulations, we show that key kinetic and thermodynamic features of PCH condensates are consistent with a phase-separation mode of organization driven by the genomic distribution of methylated domains and HP1 self-attraction and affinity for heterochromatin. Our predictions are corroborated by live-microscopy performed during early fly embryogenesis, suggesting that a strong crosstalk between HP1-based phase separation and chromosome mechanics drive PCH condensate formation.