A PROTEOLYTIC PATHWAY THAT RECOGNIZES UBIQUITIN AS A DEGRADATION SIGNAL
A PROTEOLYTIC PATHWAY THAT RECOGNIZES UBIQUITIN AS A DEGRADATION SIGNAL
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DOI：
10.1074/jbc.270.29.17442
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发表时间：
1995-07-21
影响因子：
4.8
通讯作者：
VARSHAVSKY, A
中科院分区：
文献类型：
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作者：
JOHNSON, ES;MA, PCM;VARSHAVSKY, A
先前的工作表明，含有不可移除的N末端泛素(Uh)部分的融合蛋白在体内是短暂的，融合蛋白的Ub起着降解信号的作用。通过分析扰乱UFD途径的突变，在酿酒酵母中剖析了涉及的蛋白质分解系统，称为UFD途径(Ub融合降解)。这五个基因中的两个，UFD1和UFD5，在UFD途径的泛素化后步骤中发挥作用。UFD3在控制细胞内Ub的浓度方面起作用：Ufd3突变体显著降低了Ub的游离水平，并且这些突变体中Ub融合的降解可以通过过度表达Ub来恢复。UFD2和UFD4似乎影响与融合的Ub部分相连的多Ub链的形成和拓扑。UFD1、UFD2和UFD4分别编码40、110和170 kDa的蛋白。最后一个类似于Ufd4p 280个残基的序列与E6AP相似，E6AP是一种人类蛋白，既与致癌乳头状瘤病毒的E6蛋白结合，也与肿瘤抑制蛋白P53结合，其Ub依赖的降解涉及E6AP。UFD5与之前发现的SON1相同，SON1是作为sec63等位基因的基因外抑制因子分离出来的，后者会损害蛋白质向细胞核的运输。UFD5对UFD和N-端规则通路的活性都是必不可少的(后者系统降解带有某些N-末端残基的蛋白质)。我们还表明，在融合的Ub部分的29位或48位的Lys-->；Arg转换大大减少了Ub融合的液化和降解为β-半乳糖苷酶。相反，Ub(R29)抑制Ub融合成二氢叶酸还原酶的泛素化和降解，但Ub(R48)部分不抑制。Ufd4突变体不能在Lys(29)泛素化融合的Ub部分，而ufd2突变体在Lys(48)泛素化中受损。这些和相关的发现表明，底物连接的多Ub链中的Ub-Ub异肽键不仅涉及先前发现的Ub的Lys(48)，而且还涉及Ub的Lys(29)，并且结构不同的多Ub链在Ub依赖的蛋白质降解中具有不同的功能。
Previous work has shown that a fusion protein bearing a ''nonremovable'' N-terminal ubiquitin (Uh) moiety is short-lived in vivo, the fusion's Ub functioning as a degradation signal. The proteolytic system involved, termed the UFD pathway (Ub fusion degradation), was dissected in the yeast Saccharomyces cerevisiae by analyzing mutations that perturb the pathway. Two of the five genes thus identified, UFD1 and UFD5, function at post ubiquitination steps in the UFD pathway. UFD3 plays a role in controlling the concentration of Ub in a cell: ufd3 mutants have greatly reduced levels of free Ub, and the degradation of Ub fusions in these mutants can be restored by overexpressing Ub. UFD2 and UFD4 appear to influence the formation and topology of a multi-Ub chain linked to the fusion's Ub moiety. UFD1, UFD2, and UFD4 encode previously undescribed proteins of 40, 110, and 170 kDa, respectively. The sequence of the last similar to 280 residues of Ufd4p is similar to that of E6AP, a human protein that binds to both the E6 protein of oncogenic papilloma viruses and the tumor suppressor protein p53, whose Ub-dependent degradation involves E6AP. UFD5 is identical to the previously identified SON1, isolated as an extragenic suppressor of sec63 alleles that impair the transport of proteins into the nucleus. UFD5 is essential for activity of both the UFD and N-end rule pathways (the latter system degrades proteins that bear certain N-terminal residues). We also show that a Lys --> Arg conversion at either position 29 or position 48 in the fusion's Ub moiety greatly reduces ubliquitination and degradation of Ub fusions to beta-galactosidase. By contrast, the ubiquitination and degradation of Ub fusions to dihydrofolate reductase are inhibited by the Ub(R29) but not by the Ub(R48) moiety. ufd4 mutants are unable to ubiquitinate the fusion's Ub moiety at Lys(29), whereas ufd2 mutants are impaired in the ubiquitination at Lys(48). These and related findings suggest that Ub-Ub isopeptide bonds in substrate-linked multi-Ub chains involve not only the previously identified Lys(48) but also Lys(29) of Ub, and that structurally different multi-Ub chains have distinct functions in Ub-dependent protein degradation.