Polycomb group (PcG) proteins repress master regulators of development and differentiation through organization of chromatin structure. Mutation and dysregulation of PcG genes cause developmental defects and cancer. PcG proteins form condensates in the cell nucleus, and these condensates are the physical sites of PcG-targeted gene silencing via formation of facultative heterochromatin. However, the physiochemical principles underlying the formation of PcG condensates remain unknown, and their determination could shed light on how these condensates compact chromatin. Using fluorescence live-cell imaging, we observed that the Polycomb repressive complex 1 (PRC1) protein chromobox 2 (CBX2), a member of the CBX protein family, undergoes phase separation to form condensates and that the CBX2 condensates exhibit liquid-like properties. Using site-directed mutagenesis, we demonstrated that the conserved residues of CBX2 within the intrinsically disordered region (IDR), which is the region for compaction of chromatin in vitro, promote the condensate formation both in vitro and in vivo. We showed that the CBX2 condensates concentrate DNA and nucleosomes. Using genetic engineering, we report that trimethylation of Lys-27 at histone H3 (H3K27me3), a marker of heterochromatin formation produced by PRC2, had minimal effects on the CBX2 condensate formation. We further demonstrated that the CBX2 condensate formation does not require CBX2–PRC1 subunits; however, the condensate formation of CBX2–PRC1 subunits depends on CBX2, suggesting a mechanism underlying the assembly of CBX2–PRC1 condensates. In summary, our results reveal that PcG condensates assemble through liquid–liquid phase separation (LLPS) and suggest that phase-separated condensates can organize PcG-bound chromatin.

The Polycomb PRC1 plays essential roles in development and disease pathogenesis. Targeting of PRC1 to chromatin is thought to be mediated by the Cbx family proteins (Cbx2/4/6/7/8) binding to histone H3 with a K27me3 modification (H3K27me3). Despite this prevailing view, the molecular mechanisms of targeting remain poorly understood. Here, by combining live-cell single-molecule tracking (SMT) and genetic engineering, we reveal that H3K27me3 contributes significantly to the targeting of Cbx7 and Cbx8 to chromatin, but less to Cbx2, Cbx4, and Cbx6. Genetic disruption of the complex formation of PRC1 facilitates the targeting of Cbx7 to chromatin. Biochemical analyses uncover that the CD and AT-hook-like (ATL) motif of Cbx7 constitute a functional DNA-binding unit. Live-cell SMT of Cbx7 mutants demonstrates that Cbx7 is targeted to chromatin by co-recognizing of H3K27me3 and DNA. Our data suggest a novel hierarchical cooperation mechanism by which histone modifications and DNA coordinate to target chromatin regulatory complexes.DOI: http://dx.doi.org/10.7554/eLife.17667.001

Over 80% of diffuse intrinsic pontine gliomas (DIPGs) harbor a point mutation in histone H3.3 where lysine 27 is substituted with methionine (H3.3K27M); however, how the mutation affects kinetics and function of PcG proteins remains elusive. We demonstrate that H3.3K27M prolongs the residence time and search time of Ezh2, but has no effect on its fraction bound to chromatin. In contrast, H3.3K27M has no effect on the residence time of Cbx7, but prolongs its search time and decreases its fraction bound to chromatin. We show that increasing expression of Cbx7 inhibits the proliferation of DIPG cells and prolongs its residence time. Our results highlight that the residence time of PcG proteins directly correlates with their functions and the search time of PcG proteins is critical for regulating their genomic occupancy. Together, our data provide mechanisms in which the cancer-causing histone mutation alters the binding and search dynamics of epigenetic complexes.

Background: Polycomb proteins control transcription by regulating chromatin structure and dynamics. Results: By developing and applying a novel Sm-ChIPi technique, we identified that one PRC1 binds multiple nucleosomes within cells, although two PRC2s can bind a single nucleosome. Conclusion: PRC1 and PRC2 complexes employ distinct mechanisms to assemble on chromatin. Significance: The cellular assembly stoichiometry provides insight into repressive polycomb chromatin structure.

17Polycomb group (PcG) proteins are master regulators of development and differentiation. 18 Mutation and dysregulation of PcG genes cause developmental defects and cancer. PcG 19 proteins form condensates in the nucleus of cells and these condensates are the physical sites 20 of PcG-targeted gene silencing. However, the physiochemical principles underlying the PcG 21 condensate formation remain unknown. Here we show that Polycomb repressive complex 1 22 (PRC1) protein Cbx2, one member of the Cbx family proteins, contains a long stretch of 23 intrinsically disordered region (IDR). Cbx2 undergoes phase separation to form condensates. 24Cbx2 condensates exhibit liquid-like properties and can concentrate DNA and nucleosomes. We 25 demonstrate that the conserved residues within the IDR promote the condensate formation in 26 vitro and in vivo. We further indicate that H3K27me3 has minimal effects on the Cbx2 27 condensate formation while depletion of core PRC1 subunits facilitates the condensate 28 formation. Thus, our results reveal that PcG condensates assemble through liquid-liquid phase 29 separation (LLPS) and suggest that PcG-bound chromatin is in part organized through phase-30 separated condensates. 31 decrease of YFP-Cbx2 through diffusion. We did not detect YFP-Cbx2 condensates in the 131 lysate. We expected that formaldehyde crosslinking would preserve Cbx2 condensates. After 132 sonication and centrifugation, YFP-Cbx2 condensates would be within the pellets. To test this 133 speculation, prior to lysis, we cross-linked cells with formaldehyde. Lysates were prepared from 134 the cross-linked cells and subjected to sonication. Using fluorescence microscopy, we observed 135