Abstract：
Apoptosis-inducing factor (AIF) is a mitochondrion-localized flavoprotein with NADH oxidase activity. AIF normally acts as an oxidoreductase to catalyze the transfer of electrons between molecules, but it can also kill cells when exposed to certain stimuli. For example, intact AIF is cleaved upon exposure to DNA-damaging agents such as etoposide, and truncated AIF (tAIF) is released from the mitochondria to the cytoplasm and translocated to the nucleus where it induces apoptosis. Although the serial events during tAIF-mediated apoptosis and the transition of AIF function have been widely studied from various perspectives, their underlying regulatory mechanisms and the factors involved are not fully understood. Here, we demonstrated that tAIF is a target of the covalent conjugation of the ubiquitin-like moiety ISG15 (referred to as ISGylation), which is mediated by the ISG15 E3 ligase HERC5. In addition, ISGylation increases the stability of tAIF protein as well as its K6-linked polyubiquitination. Moreover, we found that ISGylation increases the nuclear translocation of tAIF upon cytotoxic etoposide treatment, subsequently causing apoptotic cell death in human lung A549 carcinoma cells. Collectively, these results suggest that HERC5-mediated ISG15 conjugation is a key factor in the positive regulation of tAIF-mediated apoptosis, highlighting a novel role of posttranslational ISG15 modification as a switch that allows cells to live or die under the stress that triggers tAIF release.
摘要：
凋亡诱导因子（AIF）是一种线粒体定位的具有NADH氧化酶活性的黄素蛋白。AIF通常作为氧化还原酶催化分子之间的电子转移，但当暴露于某些刺激时，它也可以杀死细胞。例如,完整的AIF在暴露于DNA损伤剂如依托泊苷时被切割，截短的AIF（tAIF）从线粒体释放到细胞质，并转移到细胞核，在那里它诱导细胞凋亡。虽然tAIF介导的细胞凋亡和AIF功能转变过程中的一系列事件已经从不同的角度进行了广泛的研究，其潜在的监管机制和所涉及的因素尚未完全了解。这里,我们证明了tAIF是泛素样部分ISG15共价缀合的靶标（称为ISGylation），由ISG15E3连接酶HERC5介导。此外,ISGylation增加了tAIF蛋白及其K6连接的聚泛素化的稳定性。此外，我们发现，在细胞毒性依托泊苷治疗后，ISGylation增加了tAIF的核易位，随后在人肺A549癌细胞中引起凋亡细胞死亡。总的来说，这些结果表明，HERC5介导的ISG15结合是tAIF介导的细胞凋亡正调节的关键因素，强调翻译后ISG15修饰作为开关的新作用，允许细胞在触发tAIF释放的压力下存活或死亡。
