Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea.
Xaira Therapeutics, Foster City, CA 94404, USA.
Mol Cell. 2024 Aug 8;84(15):2935-2948.e7. doi: 10.1016/j.molcel.2024.06.023. Epub 2024 Jul 16.
Mitochondria are essential regulators of innate immunity. They generate long mitochondrial double-stranded RNAs (mt-dsRNAs) and release them into the cytosol to trigger an immune response under pathological stress conditions. Yet the regulation of these self-immunogenic RNAs remains largely unknown. Here, we employ CRISPR screening on mitochondrial RNA (mtRNA)-binding proteins and identify NOP2/Sun RNA methyltransferase 4 (NSUN4) as a key regulator of mt-dsRNA expression in human cells. We find that NSUN4 induces 5-methylcytosine (mC) modification on mtRNAs, especially on the termini of light-strand long noncoding RNAs. These mC-modified RNAs are recognized by complement C1q-binding protein (C1QBP), which recruits polyribonucleotide nucleotidyltransferase to facilitate RNA turnover. Suppression of NSUN4 or C1QBP results in increased mt-dsRNA expression, while C1QBP deficiency also leads to increased cytosolic mt-dsRNAs and subsequent immune activation. Collectively, our study unveils the mechanism underlying the selective degradation of light-strand mtRNAs and establishes a molecular mark for mtRNA decay and cytosolic release.
线粒体是先天免疫的重要调节因子。它们产生长的线粒体双链 RNA（mt-dsRNA），并将其释放到细胞质中，在病理应激条件下引发免疫反应。然而，这些自身免疫性 RNA 的调节在很大程度上仍然未知。在这里，我们通过 CRISPR 筛选线粒体 RNA（mtRNA）结合蛋白，并鉴定出 NOP2/Sun RNA 甲基转移酶 4（NSUN4）是人类细胞中 mt-dsRNA 表达的关键调节因子。我们发现 NSUN4 诱导 mtRNA 上的 5-甲基胞嘧啶（mC）修饰，特别是在轻链长非编码 RNA 的末端。这些 mC 修饰的 RNA 被补体 C1q 结合蛋白（C1QBP）识别，C1QBP 招募多核苷酸核苷酸转移酶以促进 RNA 周转。抑制 NSUN4 或 C1QBP 会导致 mt-dsRNA 表达增加，而 C1QBP 缺陷也会导致细胞质中 mt-dsRNA 增加和随后的免疫激活。总之，我们的研究揭示了轻链 mtRNA 选择性降解的机制，并为 mtRNA 降解和细胞质释放建立了分子标记。
