BRD4, a major tandem-bromodomain-containing transcription regulator, has two isoforms. The long isoform (BRD4L) has an extended C terminus that binds transcription cofactors, while the short isoform (BRD4S) lacks this C-terminal extension. Unlike BRD4L, the role of BRD4S in gene transcription remains unclear. Here, we report that, in human cancer cells, BRD4S forms nuclear puncta that possess liquid-like properties and that colocalize with BRD4L, MED1 and sites of histone H3 lysine 27 acetylation. BRD4 puncta are correlated with BRD4S but not BRD4L expression levels. BRD4S knockdown reduces BRD4S condensation, and ectopic expression promotes puncta formation and target gene transcription. BRD4S nuclear condensation is mediated by its intrinsically disordered regions and binding of its bromodomains to DNA and acetylated chromatin, respectively, and BRD4S phosphorylation diminishes BRD4 condensation. Our study illuminates a previously unappreciated role of BRD4S in organizing chromatin and transcription factors through phase separation to sustain gene transcription in chromatin for cancer cell proliferation.

BRD4是主要的串联-溴结构域包含的转录调节剂，具有两个同工型。长同工型（BRD4L）具有结合转录辅因子的扩展C末端，而短同工型（BRD4S）缺少此C端延伸。与BRD4L不同，BRD4S在基因转录中的作用尚不清楚。在这里，我们报道，在人类癌细胞中，BRD4S形成核突点，具有液体状特性，并与BRD4L，MED1和组蛋白H3赖氨酸27乙酰化位点共定位。BRD4点与BRD4S相关，但与BRD4L表达水平无关。BRD4S敲低可减少BRD4S凝聚，而异位表达则可促进点状形成和靶基因转录。BRD4S核凝聚是由其固有的无序区域以及其溴结构域分别与DNA和乙酰化染色质结合而介导的，BRD4S磷酸化可减少BRD4缩合。我们的研究阐明了BRD4S在通过相分离来组织染色质和转录因子以维持染色质中的基因转录以促进癌细胞增殖方面的前所未有的作用。