Epe1 recruits the BET family bromodomain protein Bdf2 to establish heterochromatin boundaries. Schizosaccharomyces pombe

Heterochromatin spreading leads to the silencing of genes within its path, and boundary elements have evolved to constrain such spreading. In fission yeast, heterochromatin at centromeres I and III is flanked by inverted repeats termed IRCs, which are required for proper boundary functions. However, the mechanisms by which IRCs prevent heterochromatin spreading are unknown. Here, we identified Bdf2, homologous to the mammalian BET family of double bromodomain proteins involved in diverse types of cancers, as a factor required for proper boundary function at IRCs. Bdf2 is enriched at IRCs through its interaction with the boundary protein Epe1. The bromodomains of Bdf2 recognize acetylated histone H4 tails and antagonize Sir2-mediated deacetylation of histone H4K16 to prevent heterochromatin spreading. Our results thus illustrate a mechanism of establishing chromosome boundaries at specific sites through the recruitment of a factor that protects euchromatic histone modifications. They also reveal a previously unappreciated function of H4K16 acetylation, which cooperates with H3K9 methylation to regulate heterochromatin spreading. Overall design: Two samples, H4K16ac & Bdf2-Flag

异染色质扩散会导致其延伸路径内的基因发生沉默，而边界元件已进化出约束此类扩散的功能。在裂殖酵母中，第I号和第III号着丝粒处的异染色质侧翼带有被命名为IRCs的反向重复序列，该序列对维持正常的边界功能不可或缺。然而，IRCs阻止异染色质扩散的具体分子机制仍未明确。本研究鉴定出Bdf2——其与参与多种癌症发生的哺乳动物BET家族双溴结构域蛋白同源——是IRCs位点维持正常边界功能所必需的因子。Bdf2通过与边界蛋白Epe1相互作用，在IRCs位点发生富集。Bdf2的溴结构域可识别乙酰化组蛋白H4尾端，并拮抗Sir2介导的组蛋白H4K16去乙酰化，进而阻止异染色质扩散。综上，本研究结果阐明了一种通过招募保护常染色质组蛋白修饰的因子，在特定位点建立染色体边界的机制；同时还揭示了此前未被认知的H4K16乙酰化功能，该修饰可与H3K9甲基化协同调控异染色质扩散。实验整体设计：设置两组样本，分别为H4K16ac与带Flag标签的Bdf2（Bdf2-Flag）。