SS18 is a phase-separation protein relocating from pluripotent to somatic loci during mESC differentiation [RNA-Seq]

The transition from pluripotent to somatic states marks a critical checkpoint in mammalian development. How this is regulated remains largely unresolved. Here we report the identification of SS18 by whole genome CRISPR screen as a checkpoint regulator. SS18 regulates pluripotent to somatic transition or PST by relocating from pluripotent loci to somatic ones accompanied by corresponding chromatin closing-opening and remodeling of SS18-centric protein complexes. Mechanistically, SS18 forms phase separation granules (PSG) with an intrinsically disordered region (IDR) rich in tyrosine (Y), which, once mutated, no longer form PSG nor rescue SS18-/- defect in PST. These results reveal that rapid cell fate decision can be achieved by massive redistribution of chromatin remodeling37 complexes through phase separation. Overall design: We analyzed the transcriptome expression profiles (RNA-seq) in WT and Ss18-KO mESCs during the process of JUN triggered differentiation

多能状态向体细胞状态的转变是哺乳动物发育过程中的关键检查点，其调控机制目前仍未完全阐明。本研究通过全基因组CRISPR筛选鉴定出SS18作为该检查点的调控因子。SS18通过从多能基因座向体细胞基因座的动态移位，伴随相应的染色质开闭变化与以SS18为核心的蛋白质复合物的重塑，从而调控多能向体细胞转化（pluripotent to somatic transition, 简称PST）。机制层面，SS18可通过富含酪氨酸（tyrosine, Y）的内在无序区域（intrinsically disordered region, 简称IDR）形成相分离颗粒（phase separation granules, 简称PSG）；若该区域发生突变，则无法形成PSG，也无法挽救SS18敲除后PST过程中的缺陷。上述结果表明，细胞命运的快速决定可通过相分离介导的染色质重塑复合物（参考文献37）的大规模重分布来实现。整体实验设计：本研究分析了野生型（WT）与Ss18基因敲除的小鼠胚胎干细胞（mESCs）在JUN诱导分化过程中的转录组表达谱（RNA测序, RNA-seq）。