Dispersal of PRC1 condensates disrupts polycomb chromatin domains and loops

Phase separation and inheritance of repressive chromatin domains

Polycomb-associated chromatin and pericentromeric heterochromatin form genomic domains important for the epigenetic regulation of gene expression. Both Polycomb complexes and heterochromatin factors rely on 'read and write' mechanisms, which, on their own, are not sufficient to explain the formation and the maintenance of these epigenetic domains. Microscopy has revealed that they form specific nuclear compartments separated from the rest of the genome. Recently, some subunits of these molecular machineries have been shown to undergo phase separation, both in vitro and in vivo, suggesting that phase separation might play important roles in the formation and the function of these two kinds of repressive chromatin. In this review, we will present the recent advances in the field of facultative and constitutive heterochromatin formation and maintenance through phase separation.

N-acetyl amino acid amide solubility in aqueous 1,6-hexanediol solutions: Insights into the protein droplet deformation mechanism

Proteinaceous liquid droplets, generated by liquid-liquid phase separation, function as membraneless compartments that are essential for diverse biological functions. Studies addressing droplet generation have used 1,6-hexanediol (1,6-HD) as a droplet-discerning agent owing to its capacity to induce droplet deformation. Despite the empirical utility of 1,6-HD, the mechanism underlying 1,6-HD-induced droplet deformation remains unknown. In this study, the solubilities of N-acetyl amino acid amides, which correspond to proteinogenic amino acid residues, were examined in the presence of 1,6-HD at 25 °C. Other solvents included ethanol, 1-propanol, and amides. Remarkably, 1,6-HD effectively solubilized hydrophobic species (particularly aromatic species) and exhibited reduced efficacy in solubilizing hydrophilic species and peptide bond moieties. These solubilizing effects are reflected in changes in protein solubility and structure. Specifically, 1,6-HD primarily targets the hydrophobic regions of a protein, increasing protein solubility without causing substantial structural changes. This solubilization mechanism is essential for elucidating the role of 1,6-HD as a droplet-discerning agent and recognizing its potential limitations.

Sparse CBX2 nucleates many Polycomb proteins to promote facultative heterochromatinization of Polycomb target genes

Facultative heterochromatinization of genomic regulators by Polycomb repressive complex (PRC) 1 and 2 is essential in development and differentiation; however, the underlying molecular mechanisms remain obscure. Using genetic engineering, molecular approaches, and live-cell single-molecule imaging, we quantify the number of proteins within condensates formed through liquid-liquid phase separation (LLPS) and find that in mouse embryonic stem cells (mESCs), approximately 3 CBX2 proteins nucleate many PRC1 and PRC2 subunits to form one non-stoichiometric condensate. We demonstrate that sparse CBX2 prevents Polycomb proteins from migrating to constitutive heterochromatin, demarcates the spatial boundaries of facultative heterochromatin, controls the deposition of H3K27me3, regulates transcription, and impacts cellular differentiation. Furthermore, we show that LLPS of CBX2 is required for the demarcation and deposition of H3K27me3 and is essential for cellular differentiation. Our findings uncover new functional roles of LLPS in the formation of facultative heterochromatin and unravel a new mechanism by which low-abundant proteins nucleate many other proteins to form compartments that enable them to execute their functions.