The Mechanism of Elimination of the Mitochondrial DNA Replisome

摘要 人类线粒体DNA（mtDNA）复制过程中的缺陷导致 突变引起广泛的退行性疾病，涉及多个器官和系统， 人体的在这个项目中，我们建议调查一个假定的机制，防止形成 线粒体DNA中的大规模缺失，这是线粒体基因组中最常见的（从头）缺陷。 缺失形成的机制尚不清楚，但迄今报道的研究表明，它们起源于 线粒体DNA复制机制（复制体）停滞，导致DNA断裂 股。值得注意的是，突出的mtDNA复制停滞位点对应于主要的线粒体DNA的结合位点。 线粒体蛋白酶，Lon，这表明它可能参与停止复制体的消除。在 此外，已发现人线粒体Hsp 70/40分子伴侣系统的组分共沉淀 与线粒体复制聚合酶的催化亚基，这表明这些可能涉及 在线粒体复制体的组装或拆卸中。事实上，在模式生物中，Hsp 70/40系统具有 已发现通过解折叠和递送蛋白质底物来辅助Lon蛋白酶。Hsp70/40 还发现系统与另一种蛋白酶ClpXP协作，其缺陷导致 线粒体基因组不稳定。 该提议旨在评估停滞的mtDNA复制体被两个 参与Lon或ClpXP蛋白酶的替代机制。此外，Hsp 70/40分子伴侣系统 可用于分解复制体并将其组分递送至客户蛋白酶。确认和 表征细胞去除缺陷线粒体的能力之间的直接关系 复制体和线粒体基因组中损伤的积累，将带来一个新的和令人兴奋的 mtDNA缺失相关人类疾病的发展前景，可能确定目标， 前瞻性治疗策略。为此，我们将采取综合办法， 用于分子亲和力和动力学参数分析的生物膜干涉边缘技术 系统的生化分析，需要专门的酶测定，并测试推定的相关性 在细胞水平的Lon，Hsp 70/40和ClpX与体内mtDNA缺失的发展之间，使用 酿酒酵母作为模式生物。到目前为止，我们已经证明了Lon蛋白酶水解酶 线粒体复制聚合酶的催化亚基，但只有在缺乏其余的 聚合酶亚单位。这一发现证实了我们最初的假设，并保证调查的作用 Hsp 70/40和/或ClpX在复制体复合物的分解中的作用，以使催化核心能够降解 Lon蛋白酶。

Abstract Defects in the human mitochondrial DNA (mtDNA) replication process result in the accumulation of mutations that give rise to a broad spectrum of degenerative disorders involving multiple organs and systems of the human body. In this project, we propose to investigate a putative mechanism that prevents the formation of large-scale deletions in mtDNA, which are the most common (de novo) defects of the mitochondrial genome. The mechanism of deletion formation is unknown, but studies reported to date indicate that they originate from stalling of the mitochondrial DNA replication machinery (the replisome), which leads to the breaking of DNA strands. Notably, prominent mtDNA replication stalling sites correspond with binding sites of the major mitochondrial protease, Lon, which suggests that it might be involved in the elimination of halted replisomes. In addition, the components of human mitochondrial Hsp70/40 chaperone system have been found to co-precipitate with the catalytic subunit of the mitochondrial replicative polymerase, which suggests that these may be involved in the assembly or disassembly of mitochondrial replisomes. In fact, in model organisms, Hsp70/40 systems have been found to assist Lon protease by unfolding and delivering protein substrates. On the other hand, Hsp70/40 systems have also been found to cooperate with another protease, ClpXP, which deficiency results in mitochondrial genome destabilization. This proposal aims to evaluate the hypothesis that stalled mtDNA replisomes are eliminated by two alternative mechanisms that engage either Lon or ClpXP protease. In addition, the Hsp70/40 chaperone system may serve to disassemble the replisome and deliver its components to the client protease. Confirmation and characterization of a direct relationship between the capacity of a cell to remove defective mitochondrial replisomes and the accumulation of damage in the mitochondrial genome, would bring a novel and exciting perspective on the development of mtDNA deletions-linked human disorders, potentially identifying targets for prospective therapeutic strategies. To this end, we will apply a comprehensive approach combining the cutting- edge technique of biolayer interferometry for the analysis of molecular affinities and kinetic parameters, a methodical biochemical analysis that entails specialized enzymatic assays, and testing the putative correlation between cellular levels of Lon, Hsp70/40 and ClpX and the development of mtDNA deletions in vivo, using Saccharomyces cerevisiae as the model organism. To date, we have demonstrated that Lon protease hydrolases the catalytic subunit of the mitochondrial replicative polymerase, but only in the absence of the remaining polymerase subunits. This finding confirms our initial hypothesis and warrants the investigation of the role of Hsp70/40 and/or ClpX in the disassembly of the replisome complex to enable degradation of the catalytic core by Lon protease.