Abstract
Intrinsically disordered regions of MED1 and BRD4 promote formation of phase-separated condensates.
Major finding: Intrinsically disordered regions of MED1 and BRD4 promote formation of phase-separated condensates.
Concept: Liquid-like condensates concentrate components of the transcriptional machinery at superenhancers.
Impact: Phase transitions mediated by intrinsically disordered regions may facilitate superenhancer activation.
Superenhancers are characterized by dense assembly of transcriptional machinery, leading to the hypothesis that phase separation may allow for the required high-density assembly. In this process, molecules in fluids separate into a dense phase and a dilute phase, allowing for phase-separated biomolecular condensates that compartmentalize and concentrate biochemical reactions within cells. Sabari, Dall’Agnese, and colleagues demonstrated that the transcriptional coactivators BRD4 and MED1, which are enriched at superenhancers, undergo phase separation to form condensates at superenhancers. Immunofluorescence imaging showed that BRD4 and MED1 formed nuclear puncta at superenhancers, and, after photobleaching, the rate of fluorescence recovery suggested a liquid-like state, consistent with a phase-separated condensate. These coactivator puncta were sensitive to 1,6-hexanediol, a compound known to disrupt liquid-like condensates, resulting in a reduction in the number of BRD4 and MED1 puncta, and loss of RNA polymerase II occupancy at superenhancers, suggesting a reduction in transcriptional activity. Both BRD4 and MED1 contain large intrinsically disordered regions (IDR), and these IDRs formed reversible phase-separated condensates (in contrast to irreversible aggregates) in vitro and in cells. MED1 contains an enrichment of serine residues that were required for phase separation, and MED1-mediated phase separation was abrogated when the serines were mutated to alanines. In vitro, the MED1–IDR condensate droplets compartmentalized MED1 and other proteins required for transcription into concentrated droplets. Taken together, these findings provide a mechanism by which transcriptional machinery can be compartmentalized at superenhancers to promote gene transcription via phase separation induced by the IDRs of coactivator proteins. These findings may have implications for the expression of superenhancer-driven oncogenes in cancer.
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