Cancer and Vascular Biology Research Center, The Rappaport Faculty of Medicine and Research Institute, Technion-Israel Institute of Technology, Haifa 31096, Israel.
Biochem Biophys Res Commun. 2010 Sep 24;400(3):389-95. doi: 10.1016/j.bbrc.2010.08.082. Epub 2010 Aug 26.
The E3 ubiquitin ligase RING1B plays an important role in Polycomb-mediated gene silencing by monoubiquitinating histone H2A. Both the activity and stability of RING1B are controlled by ubiquitination in two distinct manners. Self ubiquitination of RING1B generates K6, K27 and K48-based mixed polyubiquitin chain, and is required for its activity as a ligase. On the other hand, its proteasomal degradation is mediated by another ligase; E6-AP catalyzes the formation of K48-based chains. Since these two modes of ubiquitination target the same lysine residues and are therefore mutually exclusive, an important mode of regulation of RING1B should be at the level of deubiquitination. Here we identify USP7 as a deubiquitinating enzyme that regulates the ubiquitination state of RING1B. RING1B interacts with USP7, which is mediated in part by its RING domain. In addition, USP7 was found in a complex with other Polycomb proteins, suggesting a broad role in regulating these complexes. Although, USP7 directly and specifically deubiquitinates RING1B in vitro and in vivo, it does not discriminate between the activating and proteolysis-targeting modes of ubiquitination, and therefore has a stabilizing effect on RING1B.
E3 泛素连接酶 RING1B 通过单泛素化组蛋白 H2A 在 Polycomb 介导的基因沉默中发挥重要作用。RING1B 的活性和稳定性通过两种不同的泛素化方式进行控制。RING1B 的自我泛素化产生基于 K6、K27 和 K48 的混合多泛素链，这是其作为连接酶发挥活性所必需的。另一方面，其蛋白酶体降解由另一种连接酶介导；E6-AP 催化基于 K48 的链的形成。由于这两种泛素化方式针对相同的赖氨酸残基，因此是相互排斥的，RING1B 的一种重要调节方式应该在去泛素化水平上。在这里，我们确定 USP7 是一种调节 RING1B 泛素化状态的去泛素化酶。RING1B 与 USP7 相互作用，其部分由其 RING 结构域介导。此外，USP7 被发现在与其他 Polycomb 蛋白的复合物中，表明其在调节这些复合物方面具有广泛的作用。尽管 USP7 可直接且特异性地体外和体内去泛素化 RING1B，但它不能区分激活和靶向蛋白水解的泛素化模式，因此对 RING1B 具有稳定作用。
