Data from: HP1a dynamics in the early drosophila embryo

The spatial segregation of heterochromatin into distinct, membrane-less
nuclear compartments involves the binding of the heterochromatin protein 1
(HP1) to H3K9me2/3-rich genomic regions. While HP1 exhibits liquid-liquid
phase separation properties in vitro, its mechanistic role in vivo on the
structure and dynamics of heterochromatin 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
heterochromatin condensates at HP1 levels far below the concentration
required in vitro to observe phase separation in purified protein assays.
These heterotypic HP1-chromatin interactions give rise to a strong
dependence of the nucleoplasmic HP1 density on the HP1-H3K9me2/3
stoichiometry, consistent with the thermodynamics of multicomponent phase
separation. The dynamical crosstalk between HP1 and the visco-elastic
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
heterochromatin 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 heterochromatin formation based on the interplay between
HP1-based phase separation and chromatin mechanics.