大陆学子Cell子刊同期发两篇文章

　　

生物通报道：本科毕业于福建师范大学陈志坚（Zhijian Chen，音译）现在已经是一名美国霍德华休斯医学院的研究员了，他在2005年曾获得Norman Hackerman Award生化奖，是一位前途无量的知名科学家。

2009年其研究小组接连在Nature，Cell杂志上发表文章，解析泛素及RNA聚合酶等方面的研究成果。近期在《Molecular Cell》杂志上，陈志坚研究组又同期发表两篇文章。

“A Ubiquitin Replacement Strategy in Human Cells Reveals Distinct Mechanisms of IKK Activation by TNFα and IL-1β
”这篇文章补充了IKK激酶信号传导过程中的一个关键机制。研究人员通过四环素诱导性RNAi实验发现IKK泛素依赖性激活机制有不同的途径，这为研究多种细胞途径中多泛素途径提供了重要信息。

“Key Role of Ubc5 and Lysine-63 Polyubiquitination in Viral Activation of IRF3”这篇文章发现内源性泛素K63R突变会导致IRF3病毒活性失火，从而证明K63多泛素作用在IRF活性中的重要作用。

泛素是一个由76个氨基酸组成的高度保守的多肽链，因其广泛分布于各类细胞而得名。泛素共价地结合于底物蛋白质的赖氨酸残基，被泛素标记的蛋白质将被特异性地识别并迅速降解，泛素的这种标记作用是非底物特异性的，在蛋白质降解过程中泛素的枢纽作用越来越得到研究者的重视。2004年，阿龙·切哈诺沃、阿夫拉姆·赫什科、欧文·罗斯因发现了泛素调节的蛋白质降解过程而获得了诺贝尔化学奖。

细胞中的蛋白质处于不断地降解与更新的过程中，保持细胞正常的蛋白质代谢对于生命的正常功能至关重要。控制蛋白质降解的机制尚未阐明，现在已清楚细胞蛋白的降解是一个复杂的、被严密调控的过程，此过程在细胞疾病和健康状态、生存和死亡的一系列基本过程中(细胞凋亡、DNA修复等）扮演重要角色， 蛋白质降解异常与许多疾病（恶性肿瘤,神经退行性疾患等）的发生密切相关。基因的功能是通过蛋白质的表达实现的，而泛素在蛋白质降解中的作用机制如能被阐明将对解释多种疾病的发生机制和遗传信息的调控表达有重要意义。

之前陈志坚研究组发表的Nature，Cell文章分别分析了天然免疫和获得性免疫中泛素的作用，以及通过体外重现TAK1激活，发现Lys63泛素链并没有与任何靶标蛋白连接，而是结合在TAB2上直接激活TAK1。给泛素研究带来了极大的进展。

（生物通：万纹）

原文链接：

A Ubiquitin Replacement Strategy in Human Cells Reveals Distinct Mechanisms of IKK Activation by TNFα and IL-1β

Lysine-63 (K63)-linked polyubiquitination has emerged as a mechanism regulating diverse cellular functions, including activation of the protein kinase IKK in the NF-κB pathways. However, genetic evidence for a key role of K63 polyubiquitination in IKK activation is lacking. Here, we devise a tetracycline-inducible RNAi strategy to replace endogenous ubiquitin with a K63R mutant in a human cell line. We demonstrate that K63 of ubiquitin and the catalytic activity of Ubc13, an E2 that catalyzes K63 polyubiquitination, are required for IKK activation by IL-1β, but surprisingly, not by TNFα. We further show that IKK activation by TNFα requires Ubc5, which functions with the E3 cIAP1 to catalyze polyubiquitination of RIP1 not restricted to K63 of ubiquitin. These results indicate that distinct ubiquitin-dependent mechanisms are employed for IKK activation by different pathways. The ubiquitin replacement methodology described here provides a means to investigate the function of polyubiquitin topology in various cellular processes.

Key Role of Ubc5 and Lysine-63 Polyubiquitination in Viral Activation of IRF3

The mitochondrial antiviral signaling protein (MAVS; also known as IPS-1, VISA, and CARDIF) is essential for innate immune response against RNA viruses. MAVS transduces signals from the cytosolic RIG-I-like receptors, which bind to viral RNAs. But how MAVS activates downstream transcription factors such as IRF3 to induce type-I interferons is not well understood. We have established a cell-free system in which mitochondria derived from virus-infected cells activate IRF3 in the cytosol. Fractionation of the cytosol led to the identification of Ubc5 as a ubiquitin-conjugating enzyme (E2) required for IRF3 activation. Using an inducible RNAi strategy, we demonstrate that catalytically active Ubc5 is required for IRF3 activation by viral infection. The activation of IRF3 also requires two ubiquitin-binding domains of NEMO. Furthermore, we show that replacement of endogenous ubiquitin with its K63R mutant abolishes viral activation of IRF3, demonstrating that K63 polyubiquitination plays a key role in IRF3 activation.