Department of Molecular Biology, Umeå University, 90187 Umeå, Sweden.
Department of Molecular Biology, Umeå University, 90187 Umeå, Sweden
G3 (Bethesda). 2019 Nov 5;9(11):3801-3812. doi: 10.1534/g3.119.400579.
Antagonistic functions of Polycomb and Trithorax proteins are essential for proper development of all metazoans. While the Polycomb proteins maintain the repressed state of many key developmental genes, the Trithorax proteins ensure that these genes stay active in cells where they have to be expressed. Ash1 is the Trithorax protein that was proposed to counteract Polycomb repression by methylating lysine 36 of histone H3. However, it was recently shown that genetic replacement of histone H3 with the variant that carried Arginine instead of Lysine at position 36 did not impair the ability of Ash1 to counteract Polycomb repression. This argues that Ash1 counteracts Polycomb repression by methylating yet unknown substrate(s) and that it is time to look beyond Ash1 methyltransferase SET domain, at other evolutionary conserved parts of the protein that received little attention. Here we used genetics to demonstrate that Ash1 requires each of the BAH, PHD and SET domains to counteract Polycomb repression, while AT hooks are dispensable. Our findings argue that, , Ash1 acts as a multimer. Thereby it can combine the input of the SET domain and PHD-BAH cassette residing in different peptides. Finally, using new loss of function alleles, we show that zygotic Ash1 is required to prevent erroneous repression of homeotic genes of the bithorax complex in the embryo.
多梳蛋白和三价蛋白的拮抗功能对所有后生动物的正常发育至关重要。多梳蛋白维持许多关键发育基因的沉默状态，而三价蛋白则确保这些基因在需要表达的细胞中保持活跃。ASH1 是三价蛋白，它被提议通过甲基化组蛋白 H3 的赖氨酸 36 来拮抗多梳蛋白的抑制作用。然而，最近的研究表明，用携带精氨酸而不是赖氨酸的变体替代组蛋白 H3，不会损害 Ash1 拮抗多梳蛋白抑制的能力。这表明 Ash1 通过甲基化未知的底物来拮抗多梳蛋白的抑制作用，现在是时候超越 Ash1 甲基转移酶 SET 结构域，关注该蛋白其他进化上保守但很少受到关注的部分了。在这里，我们利用遗传学证明，Ash1 需要 BAH、PHD 和 SET 结构域的每一个结构域来拮抗多梳蛋白的抑制作用，而 AT 钩是可有可无的。我们的研究结果表明，Ash1 作为一个多聚体发挥作用。这样，它可以结合来自不同肽段的 SET 结构域和 PHD-BAH 盒的输入。最后，利用新的功能丧失等位基因，我们证明合子型 Ash1 对于防止胚胎中同源盒基因的错误抑制是必需的。
