Chromatin insulators are responsible for orchestrating long-range interactions between enhancers and promoters throughout the genome and align with the boundaries of Topologically Associating Domains (TADs). Here, we demonstrate an association between gypsy insulator proteins and the phosphorylated histone variant H2Av (γH2Av), normally a marker of DNA double strand breaks. Gypsy insulator components colocalize with γH2Av throughout the genome, in polytene chromosomes and in diploid cells in which Chromatin IP data shows it is enriched at TAD boundaries. Mutation of insulator components su(Hw) and Cp190 results in a significant reduction in γH2Av levels in chromatin and phosphatase inhibition strengthens the association between insulator components and γH2Av and rescues γH2Av localization in insulator mutants. We also show that γH2Av, but not H2Av, is a component of insulator bodies, which are protein condensates that form during osmotic stress. Phosphatase activity is required for insulator body dissolution after stress recovery. Together, our results implicate the H2A variant with a novel mechanism of insulator function and boundary formation.

Mounting evidence implicates liquid-liquid phase separation (LLPS), the condensation of biomolecules into liquid-like droplets in the formation and dissolution of membraneless intracellular organelles (MLOs). Eukaryotic cells utilize MLOs or condensates for various biological processes, including emergency signaling, spatiotemporal control over steady-state biochemical reactions and heterochromatin formation. Insulator proteins function as architectural elements involved in establishing independent domains of transcriptional activity within eukaryotic genomes. In Drosophila, insulator proteins coalesce to form nuclear foci known as insulator bodies in response to osmotic stress and during apoptosis. However, the mechanism through which insulator proteins assemble into bodies and whether these bodies confer any genome function are yet to be fully investigated. Here, we identify signatures of liquid-liquid phase separation by insulator bodies, including high disorder tendency in insulator proteins, scaffold-client dependent assembly, extensive fusion behavior, sphericity, and sensitivity to 1,6-hexanediol. We also show that the cohesin subunit Rad21 is a component of insulator bodies adding to the known insulator proteins and the histone variant γH2Av constituents. Our data suggest a concerted role of cohesin and insulator proteins in insulator body formation and under physiological conditions. We propose a mechanism whereby these architectural proteins modulate 3D genome organization through LLPS.

Chromatin insulators are responsible for mediating long-range interactions between enhancers and promoters throughout the genome and align with the boundaries of topologically associating domains (TADs). Here, we demonstrate an interaction between proteins that associate with the gypsy insulator and the phosphorylated histone variant H2Av (γH2Av), a marker of DNA double strand breaks. Gypsy insulator components colocalize with γH2Av throughout the genome. Mutation of insulator components prevents stable H2Av phosphorylation in polytene chromatin. Phosphatase inhibition strengthens the association between insulator components and γH2Av and rescues γH2Av localization in insulator mutants. We also show that γH2Av is a component of insulator bodies, and that phosphatase activity is required for insulator body dissolution after recovery from osmotic stress. We further demonstrate a tight association between γH2Av and TAD boundaries. Together, our results indicate a novel mechanism linking insulator function with a histone H2A variant and with genome stability.

Mounting evidence implicates liquid–liquid phase separation (LLPS), the condensation of biomolecules into liquid-like droplets in the formation and dissolution of membraneless intracellular organelles (MLOs). Cells use MLOs or condensates for various biological processes, including emergency signaling and spatiotemporal control over steady-state biochemical reactions and heterochromatin formation. Insulator proteins are architectural elements involved in establishing independent domains of transcriptional activity within eukaryotic genomes. InDrosophila, insulator proteins form nuclear foci known as insulator bodies in response to osmotic stress. However, the mechanism through which insulator proteins assemble into bodies is yet to be investigated. Here, we identify signatures of LLPS by insulator bodies, including high disorder tendency in insulator proteins, scaffold–client–dependent assembly, extensive fusion behavior, sphericity, and sensitivity to 1,6-hexanediol. We also show that the cohesin subunit Rad21 is a component of insulator bodies, adding to the known insulator protein constituents and γH2Av. Our data suggest a concerted role of cohesin and insulator proteins in insulator body formation and under physiological conditions. We propose a mechanism whereby these architectural proteins modulate 3D genome organization through LLPS.