Mitochondria support multiple cell functions, but an accumulation of dysfunctional or excessive mitochondria is detrimental to cells. We previously demonstrated that a defect in the autophagic removal of mitochondria, termed mitophagy, leads to the acceleration of apoptosis induced by herpesvirus productive infection. However, the exact molecular mechanisms underlying activation of mitophagy and regulation of apoptosis remain poorly understood despite the identification of various mitophagy-associated proteins. Here, we report that the mitochondrial translation elongation factor Tu, a mitophagy-associated protein encoded by the TUFM gene, locates in part on the outer membrane of mitochondria (OMM) where it acts as an inhibitor of altered mitochondria-induced apoptosis through its autophagic function. Inducible depletion of TUFM potentiated caspase-8-mediated apoptosis in virus-infected cells with accumulation of altered mitochondria. In addition, TUFM depletion promoted caspase-8 activation induced by treatment with TNF-related apoptosis-inducing ligand in cancer cells, potentially via dysregulation of mitochondrial dynamics and mitophagy. Importantly, we revealed the existence of and structural requirements for autophagy-competent TUFM on the OMM; the GxxxG motif within the N-terminal mitochondrial targeting sequences of TUFM was required for self-dimerization and mitophagy. Furthermore, we found that autophagy-competent TUFM was subject to ubiquitin-proteasome-mediated degradation but stabilized upon mitophagy or autophagy activation. Moreover, overexpression of autophagy-competent TUFM could inhibit caspase-8 activation. These studies extend our knowledge of mitophagy regulation of apoptosis and could provide a novel strategic basis for targeted therapy of cancer and viral diseases.

线粒体支持多种细胞功能，但功能失调或过多线粒体的积累对细胞有害。我们之前证明，线粒体自噬去除的缺陷（称为线粒体自噬）导致疱疹病毒生产性感染诱导的细胞凋亡加速。然而，尽管鉴定了各种线粒体自噬相关蛋白，但对于线粒体自噬激活和细胞凋亡调节的确切分子机制仍知之甚少。在这里，我们报告了线粒体翻译延伸因子 Tu，一种由TUFM编码的线粒体自噬相关蛋白基因，部分位于线粒体外膜 (OMM)，在那里它通过其自噬功能充当改变的线粒体诱导的细胞凋亡的抑制剂。TUFM 的可诱导耗竭增强了病毒感染细胞中 caspase-8 介导的细胞凋亡，并伴有改变的线粒体积累。此外，TUFM 耗竭促进了癌细胞中 TNF 相关凋亡诱导配体处理诱导的 caspase-8 激活，可能是通过线粒体动力学和线粒体自噬的失调。重要的是，我们在 OMM 上揭示了具有自噬能力的 TUFM 的存在和结构要求；TUFM 的 N 末端线粒体靶向序列中的 GxxxG 基序是自二聚化和线粒体自噬所必需的。此外，我们发现具有自噬能力的 TUFM 受到泛素-蛋白酶体介导的降解，但在线粒体自噬或自噬激活后稳定下来。此外，具有自噬能力的 TUFM 的过表达可以抑制 caspase-8 的激活。这些研究扩展了我们对线粒体自噬调节细胞凋亡的认识，并可为癌症和病毒性疾病的靶向治疗提供新的战略基础。