The tumor suppressor protein p16INK4a and the human papillomavirus oncoprotein-58 E7 are naturally occurring lysine-less proteins that are degraded by the ubiquitin system -: Direct evidence for ubiquitination at the N-terminal residue
The tumor suppressor protein p16INK4a and the human papillomavirus oncoprotein-58 E7 are naturally occurring lysine-less proteins that are degraded by the ubiquitin system -: Direct evidence for ubiquitination at the N-terminal residue
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DOI：
10.1074/jbc.m407201200
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发表时间：
2004-10-01
影响因子：
4.8
通讯作者：
Ciechanover, A
中科院分区：
文献类型：
--
作者：
Ben-Saadon, R;Fajerman, I;Ciechanover, A
泛素与体内赖氨酸的结合是泛素系统大多数底物降解的第一步。对于几种底物，已经证明第一泛素部分连接到N-末端残基。然而，在所有这些底物中，内部赖氨酸也在调节它们的稳定性方面发挥了作用。为了更好地理解这种新的修饰模式的生理意义，重要的是识别其中的降解完全依赖于N-末端泛素化的蛋白质。此外，尽管N-末端泛素化的实验证据相当有力，但它仍然是间接的。在这里，我们证明了一组重要的蛋白质，通过N末端泛素化靶向是自然产生的无赖氨酸蛋白，如人乳头瘤病毒(HPV)-58E7癌蛋白和细胞周期抑制因子和肿瘤抑制因子p16(INK4a)。对于这些蛋白质，唯一可以靶向的残基是N-末端残基。有趣的是，p16(INK4a)以细胞密度依赖的方式降解。重要的是，我们首次提供了N末端泛素化的直接证据。对HPV-58E7泛素偶联物的胰酶消化分析表明，融合多肽由泛素的C-末端结构域和E7的N-末端结构域组成。随着天然赖氨酸蛋白的丰富，其中包括重要的病毒和细胞调节因子，这种新的蛋白质靶向模式对生理和病理生理过程都有意义。
Conjugation of ubiquitin to an internal lysine is the initial step in the degradation of the majority of the substrates of the ubiquitin system. For several substrates, it has been shown that the first ubiquitin moiety is conjugated to the N-terminal residue. In all these substrates, however, the internal lysines also played a role in modulating their stability. To better understand the physiological significance of this novel mode of modification, it was important to identify proteins in which degradation is completely dependent on N-terminal ubiquitination. Also, although the experimental evidence for N-terminal ubiquitination is rather strong, nevertheless, it has remained indirect. Here we demonstrate that an important group of proteins that are targeted via N-terminal ubiquitination are the naturally occurring lysine-less proteins such as the human papillomavirus (HPV)-58 E7 oncoprotein and the cell cycle inhibitor and tumor suppressor p16(INK4a). For these proteins, the only residue that can be targeted is the N-terminal residue. Interestingly, p16(INK4a) is degraded in a cell density-dependent manner. Importantly, we provide for the first time direct evidence for N-terminal ubiquitination. Analysis of tryptic digest of the ubiquitin conjugate of HPV-58 E7 revealed a fusion peptide that is composed of the C-terminal domain of ubiquitin and the N-terminal domain of E7. With the abundance of native lysine-less proteins, among which are important viral and cell regulators, this novel mode of protein targeting has implications for both physiological and pathophysiological processes.