Department of Biodiversity and Plant Systematics, Heidelberg Institute for Plant Science, University of Heidelberg, Im Neuenheimer Feld 345, Heidelberg, Germany.
Curr Biol. 2010 Oct 26;20(20):1853-9. doi: 10.1016/j.cub.2010.09.046. Epub 2010 Oct 7.
Polycomb group (PcG) proteins form conserved regulatory complexes that modify chromatin to repress the genes that are not required in a specific differentiation status [1]. In animals, the two best-characterized PcG complexes are PRC2 and PRC1, which respectively possess histone 3 lysine 27 (H3K27) trimethyltransferase [2-4] and histone 2A lysine 119 (H2AK119) E3 ubiquitin ligase activities [5-7]. In Arabidopsis, PRC2 activity is also required for the gene silencing mechanism [8]; however, the existence of PRC1 has been questioned, because plant genomes do not encode clear PRC1 components and H2A monoubiquitination has not been detected [6, 9]. Conversely, recent reports have unveiled the presence of homologs to PRC1 components that together with plant-specific proteins could be part of the long-sought PRC1-like complexes [10, 11]. Here we show that the PRC1 RING-finger homologs AtBMI1A and AtBMI1B are implicated in the repression of embryonic and stem cell regulators. Plants impaired in AtBMI1A and AtBMI1B show derepression of embryonic traits in somatic cells, displaying a phenotype similar to plants mutant in PRC2 components [12-14]. Our data demonstrate that the AtBMI1A/B proteins mediate H2A monoubiquitination in Arabidopsis and that this mark, together with PRC2-mediated H3K27 trimethylation, plays a key role in maintaining cell identity.
多梳蛋白（PcG）家族形成保守的调控复合物，修饰染色质以抑制特定分化状态下不需要的基因[1]。在动物中，两个研究最为深入的 PcG 复合物是 PRC2 和 PRC1，它们分别具有组蛋白 3 赖氨酸 27（H3K27）三甲基转移酶[2-4]和组蛋白 2A 赖氨酸 119（H2AK119）E3 泛素连接酶活性[5-7]。在拟南芥中，PRC2 活性也需要基因沉默机制[8]；然而，PRC1 的存在一直受到质疑，因为植物基因组不编码明确的 PRC1 成分，也没有检测到 H2A 单泛素化[6,9]。相反，最近的报道揭示了 PRC1 成分的同源物的存在，这些同源物与植物特异性蛋白一起可能是长期以来寻找的 PRC1 样复合物的一部分[10,11]。在这里，我们表明 PRC1 环指同源物 AtBMI1A 和 AtBMI1B 参与胚胎和干细胞调节因子的抑制。AtBMI1A 和 AtBMI1B 受损的植物在体细胞中表现出胚胎特征的去抑制，表现出与 PRC2 成分突变体相似的表型[12-14]。我们的数据表明，AtBMI1A/B 蛋白在拟南芥中介导 H2A 单泛素化，并且该标记与 PRC2 介导的 H3K27 三甲基化一起，在维持细胞身份方面发挥关键作用。
