Bridging the Gap between tRNA Modifications and the Respiratory Chain
Bridging the Gap between tRNA Modifications and the Respiratory Chain
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弥合 tRNA 修饰和呼吸链之间的差距
DOI：
10.1021/acs.biochem.8b00377
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发表时间：
2018
期刊：
影响因子：
2.9
通讯作者：
Ibba, Michael
中科院分区：
文献类型：
--
作者：
Steiner, Rebecca E.;Ibba, Michael
线粒体是细胞的动力室，通过氧化磷酸化（OXPHOS）产生细胞中使用的大部分ATP，这是一个由呼吸链催化的过程。由于其复杂性，OXPHOS是精心策划的，需要线粒体中编码的许多呼吸蛋白，使线粒体翻译成为一个必不可少的过程。细胞器是哺乳动物细胞中细胞质外唯一具有自身蛋白质合成机制的细胞器。除了在细胞质中合成并输入细胞器的1500多种蛋白质外，线粒体还编码13种基本肽，这些肽形成呼吸链的关键部分。1，2这些呼吸链蛋白，连同从细胞质输入的一个子集，在电子传递链中发挥作用，因此ATP的产生。重要的是，电子传递链的突变与许多人类疾病有关，特别是在使用大量能量的组织中，如大脑。1，2哺乳动物的翻译系统只有22个tRNA来解码遗传密码的64个密码子。1这些tRNA依赖于转录后修饰来准确解码剩余的42个密码子。3在线粒体内，tRNA可以在118个不同的位置上被15种不同的修饰。[1]对于编码氨基酸Leu、Glu、Gln、Lys和Trp的tRNA，反密码子U34的第一个位置通常在碱基的C-5位被牛磺酸修饰，形成5-牛磺酸甲基尿苷（tm 5 U）。牛磺酸的添加需要前体甲基化，如果没有这一点，牛磺酸的添加是有缺陷的，这与线粒体疾病有关。直到最近，这种甲基化的碳源还没有被确定。[4]在一项突破性的研究中，Morscher等人利用创新的生物化学技术确定了这种修饰的来源。最初，作者着手确定亚甲基-四氢叶酸（亚甲基-THF）在线粒体中的作用。一段时间以来，人们已经知道，亚甲基-THF是出口的核苷酸合成的胞质溶胶从duprons;然而，THF内的duprons的作用还没有得到阐明。亚甲基-THF由酶SHMT 2产生，其分解代谢丝氨酸以产生一个碳单元并将碳转移到THF。为了阐明亚甲基-THF在线粒体中的作用，Morscher等人开始删除HCT 116结肠癌细胞中的SHMT 2，然后通过增强葡萄糖摄取和乳酸盐分泌来促进糖酵解用于ATP生产。这与基础呼吸减少、呼吸能力低、NAD+/NADH比值降低和低水平的
Mitochondria, the powerhouses of the cell, generate the majority of the ATP used in cells by oxidative phosphorylation (OXPHOS), a process catalyzed by the respiratory chain. Because of its complexity, OXPHOS is carefully orchestrated and requires a number of respiratory proteins encoded in mitochondria, making mitochondrial translation an essential process. The mitochondrion is the only organelle in mammalian cells outside the cytoplasm that has its own protein synthesis machinery. In addition to more than 1500 proteins that are synthesized in the cytoplasm and imported into the organelle, mitochondria also encode 13 essential peptides that form critical parts of the respiratory chain. 1, 2 These respiratory chain proteins, together with a subset imported from the cytoplasm, play a role in the electron transport chain, and therefore ATP generation. Significantly, mutations in the electron transport chain are linked to many human diseases especially in tissues that utilize a lot of energy, such as the brain. 1, 2The mammalian mitochondrion translation system has only 22 tRNAs to decode the 64 codons of the genetic code. 1 These tRNAs rely on post-transcriptional modifications to accurately decode the 42 remaining codons. 3 Within the mitochondria, tRNAs can be modified with 15 distinct modifications at 118 different positions on tRNAs. 1 In the case of tRNAs encoding the amino acids Leu, Glu, Gln, Lys, and Trp, the first position of the anticodon, U34, is usually modified at the C-5 position of the base with taurine forming 5-taurinomethyluridine (tm5U). Taurine addition requires precursor methylation, and without this, the addition of taurine is defective, which has been correlated with mitochondrial disease. Until recently, the carbon source for this methylation had not been identified. 4 In a breakthrough study, Morscher et al. 5 identified the source of this modification using innovative biochemical techniques. Initially, the authors set out to determine the role of methylene-tetrahydrofolate (methylene-THF) in mitochondria. It has been known for some time that methylene-THF is exported from the mitochondrion to the cytosol for nucleotide synthesis; however, the role of THF within the mitochondrion had yet to be elucidated. Methylene-THF is generated by the enzyme SHMT2, which catabolizes serine to generate onecarbon units and transfers the carbon to THF. To elucidate the role of methylene-THF in the mitochondria, Morscher et al. began by deleting SHMT2 in HCT116 colon cancer cells, which then favors the use of glycolysis for ATP production through enhanced glucose uptake and lactate secretion. This was paired with reduced basal respiration, low respiratory capacity, decreased NAD+/NADH ratios, and low levels of