Institute of Molecular Life Sciences, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland.
Development. 2013 Jun;140(11):2377-86. doi: 10.1242/dev.093591. Epub 2013 May 1.
Pygopus has been discovered as a fundamental Wnt signaling component in Drosophila. The mouse genome encodes two Pygopus homologs, Pygo1 and Pygo2. They serve as context-dependent β-catenin coactivators, with Pygo2 playing the more important role. All Pygo proteins share a highly conserved plant homology domain (PHD) that allows them to bind di- and trimethylated lysine 4 of histone H3 (H3K4me2/3). Despite the structural conservation of this domain, the relevance of histone binding for the role of Pygo2 as a Wnt signaling component and as a reader of chromatin modifications remains speculative. Here we generate a knock-in mouse line, homozygous for a Pygo2 mutant defective in chromatin binding. We show that even in the absence of the potentially redundant Pygo1, Pygo2 does not require the H3K4me2/3 binding activity to sustain its function during mouse development. Indeed, during tissue homeostasis, Wnt/β-catenin-dependent transcription is largely unaffected. However, the Pygo2-chromatin interaction is relevant in testes, where, importantly, Pygo2 binds in vivo to the chromatin in a PHD-dependent manner. Its presence on regulatory regions does not affect the transcription of nearby genes; rather, it is important for the recruitment of the histone acetyltransferase Gcn5 to chromatin, consistent with a testis-specific and Wnt-unrelated role for Pygo2 as a chromatin remodeler.
Pygopus 已被发现是果蝇中 Wnt 信号通路的基本组成部分。小鼠基因组编码两个 Pygopus 同源物，Pygo1 和 Pygo2。它们作为依赖于上下文的 β-连环蛋白共激活因子，Pygo2 发挥更重要的作用。所有的 Pygo 蛋白都共享一个高度保守的植物同源结构域（PHD），使它们能够结合二甲基化和三甲基化的组蛋白 H3 的赖氨酸 4（H3K4me2/3）。尽管这个结构域具有结构上的保守性，但组蛋白结合对于 Pygo2 作为 Wnt 信号通路成分和染色质修饰的阅读器的作用仍然是推测性的。在这里，我们生成了一个敲入小鼠系，Pygo2 突变体在染色质结合上是纯合的。我们表明，即使在缺乏潜在冗余的 Pygo1 的情况下，Pygo2 也不需要 H3K4me2/3 结合活性来维持其在小鼠发育过程中的功能。事实上，在组织稳态中，Wnt/β-连环蛋白依赖性转录受到的影响不大。然而，Pygo2-染色质相互作用在睾丸中是相关的，重要的是，Pygo2 以 PHD 依赖的方式在体内与染色质结合。它在调节区域的存在不会影响附近基因的转录；相反，它对于组蛋白乙酰转移酶 Gcn5 向染色质的募集是重要的，这与 Pygo2 作为染色质重塑因子在睾丸中的特异性和与 Wnt 无关的作用一致。
