Regulaion of epigenome by the SUMO and ubiquitin modification systems

胸腺嘧啶DNA糖基化酶（TDG）是一种碱基切除修复（BER）酶，与小泛素相关修饰剂（SUMO）靶向泛素E3连接酶RNF 4相互作用，并在主动DNA去甲基化途径中发挥作用。在这里，我们发现，大多数的TDG是SUMO化的细胞进行有丝分裂，而SUMO化和未修饰的TDG被消除在S期停滞的细胞。在体外和体内结合分析表明，TDG与RNF 4在SUMO化独立的方式相互作用。在RNF 4耗尽的细胞中，两种形式的TDG都被有效降解，表明RNF 4不是控制TDG稳定性的主要E3连接酶。我们的研究结果不仅对理解TDG的翻译后调节很重要，而且还为细胞周期进程中BER驱动的主动DNA去甲基化中TDG-RNF 4相互作用提供了启示。

Thymine DNA glycosylase (TDG) is a base excision repair (BER) enzyme that interacts with the small ubiquitin-related modifier (SUMO)-targeted ubiquitin E3 ligase RNF4 and functions in the active DNA demethylation pathway. Here we found that the majority of TDG was SUMOylated in cells undergoing mitosis, while both SUMOylated and non-modified TDG were eliminated in cells arrested at S phase. In vitro and in vivo binding analyses revealed that TDG interacted with RNF4 in a SUMOylation-independent manner. In RNF4-depleted cells, both forms of TDG were efficiently degraded, suggesting that RNF4 was not the primary E3 ligase for controlling TDG stability. Our findings would not only be important for understanding posttranslational regulation of TDG but also provide implications for the TDG-RNF4 interaction in BER-driven active DNA demethylation during cell cycle progression.