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Polycomb repressive complexes (PRCs) play a key role in gene repression and are indispensable for proper development. Canonical PRC1 forms condensates in vitro and in cells that are proposed to contribute to the maintenance of repression. However, how chromatin and the various subunits of PRC1 contribute to condensation is largely unexplored. Using a reconstitution approach and single-molecule imaging, we demonstrate that nucleosomal arrays and PRC1 act synergistically, reducing the critical concentration required for condensation by more than 20-fold. We find that the exact combination of PHC and CBX subunits determines condensate initiation, morphology, stability, and dynamics. Particularly, PHC2's polymerization activity influences condensate dynamics by promoting the formation of distinct domains that adhere to each other but do not coalesce. Live-cell imaging confirms CBX's role in condensate initiation and highlights PHC's importance for condensate stability. We propose that PRC1 composition can modulate condensate properties, providing crucial regulatory flexibility across developmental stages. [Display omitted] • Nucleosomal arrays and PRC1 act synergistically to form condensates • PRC1 pre-wetting of chromatin initiates switch-like transition to form condensates • Modularity of PRC1 complex composition fine-tunes condensate properties • PHC polymerization alters condensate properties forming distinct long-lived domains Through reconstitution and imaging, Niekamp et al. reveal that the Polycomb repressive complex 1 (PRC1) and chromatin act synergistically to form condensates. They show that specific combinations of PHC and CBX subunits determine condensate initiation, morphology, stability, and dynamics, which could provide important flexibility in regulatory functions across developmental stages. [ABSTRACT FROM AUTHOR]

The CBX family of proteins is central to proper mammalian development via key roles in Polycomb-mediated maintenance of repression. CBX proteins in differentiated lineages have chromatin compaction and phase separation activities that might contribute to maintaining repressed chromatin. The predominant CBX protein in pluripotent cells, CBX7, lacks the domain required for these activities. We inserted this functional domain into CBX7 in embryonic stem cells (ESCs) to test the hypothesis that it contributes a key epigenetic function. ESCs expressing this chimeric CBX7 were impaired in their ability to properly form embryoid bodies and neural progenitor cells and showed reduced activation of lineage-specific genes across differentiation. Neural progenitors exhibited a corresponding inappropriate maintenance of Polycomb binding at neural-specific loci over the course of differentiation. We propose that a switch in the ability to compact and phase separate is a central aspect of Polycomb group function during the transition from pluripotency to differentiated lineages. [Display omitted] • The CaPS domain confers chromatin compaction and phase separation activities • CBX7-CaPS chimeras compact chromatin and phase separate in vitro • CBX7-CaPS impairs efficient stem cell differentiation • Wild-type CBX7 allows for a poised chromatin environment in stem cells • Polycomb CaPS activity contributes to maintenance of a repressive chromatin state In this article, Jaensch et al. introduce a domain capable of chromatin compaction and phase separation (CaPS) into PRC1 in stem cells. They demonstrate that the addition of this domain leads to defects in differentiation-induced changes in gene expression, highlighting the importance of a changeable chromatin environment in pluripotent cells. [ABSTRACT FROM AUTHOR]