A Properly Configured Ring Structure Is Critical for the Function of the Mitochondrial DNA Recombination Protein,Mgm101

Mgm101 is a Rad52-type recombination protein of bacteriophage origin required for the repair and maintenance of mitochondrial DNA (mtDNA). It forms large oligomeric rings of ∼14-fold symmetry that catalyze the annealing of single-stranded DNAs in vitro. In this study, we investigated the structural elements that contribute to this distinctive higher order structural organization and examined its functional implications. A pair of vicinal cysteines, Cys-216 and Cys-217, was found to be essential for mtDNA maintenance. Mutations to the polar serine, the negatively charged aspartic and glutamic acids, and the hydrophobic amino acid alanine all destabilize mtDNA in vivo. The alanine mutants have an increased propensity of forming macroscopic filaments. In contrast, mutations to aspartic acid drastically destabilize the protein and result in unstructured aggregates with severely reduced DNA binding activity. Interestingly, the serine mutants partially disassemble the Mgm101 rings into smaller oligomers. In the case of the C216S mutant, a moderate increase in DNA binding activity was observed. By using small angle x-ray scattering analysis, we found that Mgm101 forms rings of ∼200 Å diameter in solution, consistent with the structure previously established by transmission electron microscopy. We also found that the C216A/C217A double mutant tends to form broken rings, which likely provide free ends for seeding the growth of the super-stable but functionally defective filaments. Taken together, our data underscore the importance of a delicately maintained ring structure critical for Mgm101 activity. We discuss a potential role of Cys-216 and Cys-217 in regulating Mgm101 function and the repair of damaged mtDNA under stress conditions.     

Mgm101: A double-duty Rad52-like protein

Mgm101 has well-characterized activity for the repair and replication of the mitochondrial genome. Recent work has demonstrated a further role for Mgm101 in nuclear DNA metabolism, contributing to an S-phase specific DNA interstrand cross-link repair pathway that acts redundantly with a pathway controlled by Pso2 exonuclease. Due to involvement of FANCM, FANCJ and FANCP homologues (Mph1, Chl1 and Slx4), this pathway has been described as a Fanconi anemia-like pathway. In this pathway, Mgm101 physically interacts with the DNA helicase Mph1 and the MutSα (Msh2/Msh6) heterodimer, but its precise role is yet to be elucidated. Data presented here suggests that Mgm101 functionally overlaps with Rad52, supporting previous suggestions that, based on protein structure and biochemical properties, Mgm101 and Rad52 belong to a family of proteins with similar function. In addition, our data shows that this overlap extends to the function of both proteins at telomeres, where Mgm101 is required for telomere elongation during chromosome replication in rad52 defective cells. We hypothesize that Mgm101 could, in Rad52-like manner, preferentially bind single-stranded DNAs (such as at stalled replication forks, broken chromosomes and natural chromosome ends), stabilize them and mediate single-strand annealing-like homologous recombination event to prevent them from converting into toxic structures.     

Mgm101p is a novel component of the mitochondrial nucleoid that binds DNA and is required for the repair of oxidatively damaged mitochondrial DNA

Maintenance of mitochondrial DNA (mtDNA) during cell division is required for progeny to be respiratory competent. Maintenance involves the replication, repair, assembly, segregation, and partitioning of the mitochondrial nucleoid. MGM101 has been identified as a gene essential for mtDNA maintenance in S. cerevisiae, but its role is unknown. Using liquid chromatography coupled with tandem mass spectrometry, we identified Mgm101p as a component of highly enriched nucleoids, suggesting that it plays a nucleoid-specific role in maintenance. Subcellular fractionation, indirect immunofluorescence and GFP tagging show that Mgm101p is exclusively associated with the mitochondrial nucleoid structure in cells. Furthermore, DNA affinity chromatography of nucleoid extracts indicates that Mgm101p binds to DNA, suggesting that its nucleoid localization is in part due to this activity. Phenotypic analysis of cells containing a temperature sensitive mgm101 allele suggests that Mgm101p is not involved in mtDNA packaging, segregation, partitioning or required for ongoing mtDNA replication. We examined Mgm101p's role in mtDNA repair. As compared with wild-type cells, mgm101 cells were more sensitive to mtDNA damage induced by UV irradiation and were hypersensitive to mtDNA damage induced by gamma rays and H2O2 treatment. Thus, we propose that Mgm101p performs an essential function in the repair of oxidatively damaged mtDNA that is required for the maintenance of the mitochondrial genome.     

Replication Intermediates of the Linear Mitochondrial DNA of Candida parapsilosis Suggest a Common Recombination Based Mechanism for Yeast Mitochondria

Variation in the topology of mitochondrial DNA (mtDNA) in eukaryotes evokes the question if differently structured DNAs are replicated by a common mechanism. RNA-primed DNA synthesis has been established as a mechanism for replicating the circular animal/mammalian mtDNA. In yeasts, circular mtDNA molecules were assumed to be templates for rolling circle DNA-replication. We recently showed that in Candida albicans, which has circular mapping mtDNA, recombination driven replication is a major mechanism for replicating a complex branched mtDNA network. Careful analyses of C. albicans-mtDNA did not reveal detectable amounts of circular DNA molecules. In the present study we addressed the question of how the unit sized linear mtDNA of Candida parapsilosis terminating at both ends with arrays of tandem repeats (mitochondrial telomeres) is replicated. Originally, we expected to find replication intermediates diagnostic of canonical bi-directional replication initiation at the centrally located bi-directional promoter region. However, we found that the linear mtDNA of Candida parapsilosis also employs recombination for replication initiation. The most striking findings were that the mitochondrial telomeres appear to be hot spots for recombination driven replication, and that stable RNA:DNA hybrids, with a potential role in mtDNA replication, are also present in the mtDNA preparations.     

Mne1 is a novel component of the mitochondrial splicing apparatus responsible for processing of a COX1 group I intron in yeast

Saccharomyces cerevisiae cells lacking Mne1 are deficient in intron splicing in the gene encoding the Cox1 subunit of cytochrome oxidase but contain wild-type levels of the bc(1) complex. Thus, Mne1 has no role in splicing of COB introns or expression of the COB gene. Northern experiments suggest that splicing of the COX1 aI5β intron is dependent on Mne1 in addition to the previously known Mrs1, Mss116, Pet54, and Suv3 factors. Processing of the aI5β intron is similarly impaired in mne1Δ and mrs1Δ cells and overexpression of Mrs1 partially restores the respiratory function of mne1Δ cells. Mrs1 is known to function in the initial transesterification reaction of splicing. Mne1 is a mitochondrial matrix protein loosely associated with the inner membrane and is found in a high mass ribonucleoprotein complex specifically associated with the COX1 mRNA even within an intronless strain. Mne1 does not appear to have a secondary function in COX1 processing or translation, because disruption of MNE1 in cells containing intronless mtDNA does not lead to a respiratory growth defect. Thus, the primary defect in mne1Δ cells is splicing of the aI5β intron in COX1.     

Voltage-dependent anion channel involved in the mitochondrial calcium cycle of cell lines carrying the mitochondrial DNA A4263G mutation

In this study, we investigated the effects of the voltage-dependent anion channel (VDAC) on the mitochondrial calcium cycle in cell lines carrying the mitochondrial DNA A4263G mutation. We established lymphoblastoid cell lines from three symptomatic individuals and one asymptomatic individual from the large Chinese Han family carrying the A4263G mutation; these were compared with three control cell lines. The mitochondrial Ca²⁺ concentration and membrane potential were detected by loading cells with Rhod-2 and JC-1, respectively. Confocal imagines showed the average Rhod-2 and JC-1 fluorescence levels of individuals carrying the tRNA^Ile A4263G mutation were lower than those of the control group (P < 0.05). The baseline Rhod-2 fluorescence in the control group increased after exposure to atractyloside (an opener of the adenine nucleotide translocator, P < 0.05), but no significant change was detected in the cell line harboring the A4263G mutation (P > 0.05). The baseline JC-1 fluorescence in both the mutated and control cell lines decreased after subsequent exposure to atractyloside (P < 0.05), whereas this effect of atractyloside was inhibited by Cyclosporin A (CsA, a VDAC blocker). We conclude that the mitochondrial VDAC is involved in both the increase of mitochondrial permeability to Ca²⁺ and the decrease of mitochondrial membrane potential in cell lines carrying the mtDNA A4263G mutation.     

Rtt107 is required for recruitment of the SMC5/6 complex to DNA double strand breaks

Genome integrity is maintained by a network of DNA damage response pathways, including checkpoints and DNA repair processes. In Saccharomyces cerevisiae, the BRCT domain-containing protein Rtt107/Esc4 is required for the restart of DNA replication after successful repair of DNA damage and for cellular resistance to DNA-damaging agents. In addition to its well characterized interaction with the endonuclease Slx4, Rtt107 interacts with a number of other DNA repair and recombination proteins. These include the evolutionarily conserved SMC5/6 complex, which is involved in numerous chromosome maintenance activities, such as DNA repair, chromosome segregation, and telomere function. The interaction between Rtt107 and the SMC5/6 complex was mediated through the N-terminal BRCT domains of Rtt107 and the Nse6 subunit of SMC5/6 and was independent of methyl methane sulfonate-induced damage and Slx4. Supporting a shared function in the DNA damage response, Rtt107 was required for recruitment of SMC5/6 to DNA double strand breaks. However, this functional relationship did not extend to other types of DNA lesions such as protein-bound nicks. Interestingly, Rtt107 was phosphorylated when SMC5/6 function was compromised in the absence of DNA-damaging agents, indicating a connection beyond the DNA damage response. Genetic analyses revealed that, although a subset of Rtt107 and SMC5/6 functions was shared, these proteins also contributed independently to maintenance of genome integrity.     

Vital roles of the second DNA-binding site of Rad52 protein in yeast homologous recombination

RecA/Rad51 proteins are essential in homologous DNA recombination and catalyze the ATP-dependent formation of D-loops from a single-stranded DNA and an internal homologous sequence in a double-stranded DNA. RecA and Rad51 require a "recombination mediator" to overcome the interference imposed by the prior binding of single-stranded binding protein/replication protein A to the single-stranded DNA. Rad52 is the prototype of recombination mediators, and the human Rad52 protein has two distinct DNA-binding sites: the first site binds to single-stranded DNA, and the second site binds to either double- or single-stranded DNA. We previously showed that yeast Rad52 extensively stimulates Rad51-catalyzed D-loop formation even in the absence of replication protein A, by forming a 2:1 stoichiometric complex with Rad51. However, the precise roles of Rad52 and Rad51 within the complex are unknown. In the present study, we constructed yeast Rad52 mutants in which the amino acid residues corresponding to the second DNA-binding site of the human Rad52 protein were replaced with either alanine or aspartic acid. We found that the second DNA-binding site is important for the yeast Rad52 function in vivo. Rad51-Rad52 complexes consisting of these Rad52 mutants were defective in promoting the formation of D-loops, and the ability of the complex to associate with double-stranded DNA was specifically impaired. Our studies suggest that Rad52 within the complex associates with double-stranded DNA to assist Rad51-mediated homologous pairing.     

In vivo conformation of mitochondrial DNA revealed by pulsed-field gel electrophoresis in the true slime mold, Physarum polycephalum.

Pulsed-field gel electrophoresis (PFGE) was used to examine the in vivo and in vitro conformations of Physarum polycephalum mitochondrial DNA (mtDNA). We used plugs containing isolated mitochondria, isolated mitochondrial nucleoids (mt-nuclei), and isolated mtDNA, in addition to whole cells. The mtDNA contained in the myxamoebae, plasmodia, isolated mitochondria, and isolated mt-nuclei was circular, but most of the isolated mtDNA had been site-specifically fragmented and linearized during DNA preparation and storage under low ionic strength conditions. Restriction mapping of Physarum mtDNA by the direct digestion of the isolated mt-nuclei from two different strains, DP89 x AI16 and KM88 x AI16, resulted in the circular form. A linear mitochondrial plasmid, mF, is known to promote mitochondrial fusion and integration of itself into the mtDNA in Physarum. Linearization of mtDNA by the integration of the mF plasmid was demonstrated when we used PFGE to analyze isolated mitochondria from the plasmodial strain DP89 x NG7 carrying the mF plasmid (mF+). The PFGE system can be used not only to determine whether the form of mtDNA is linear or circular but also to analyze the dynamic conformational changes of mtDNA.     

Human mitochondrial DNA diseases and Drosophila models

Mutations that disrupt the mitochondrial genome cause a number of human diseases whose phenotypic presentation varies widely among tissues and individuals. This variability owes in part to the unconventional genetics of mitochondrial DNA(mtDNA), which includes polyploidy, maternal inheritance and dependence on nuclear-encoded factors. The recent development of genetic tools for manipulating mitochondrial genome in Drosophila melanogaster renders this powerful model organism an attractive alternative to mammalian systems for understanding mtDNA-related diseases. In this review, we summarize mtDNA genetics and human mtDNA-related diseases. We highlight existing Drosophila models of mtDNA mutations and discuss their potential use in advancing our knowledge of mitochondrial biology and in modeling human mitochondrial disorders. We also discuss the potential and present challenges of gene therapy for the future treatment of mtDNA diseases.     

The investigation and diagnosis of pathogenic mitochondrial DNA mutations in human urothelial cells

Patients with mitochondrial DNA disease are amongst the most challenging to diagnose and manage given the striking phenotypic and genetic heterogeneity, which characterise these conditions. Recently, we and others have demonstrated the m.3243A>G mutation, one of the most common mitochondrial DNA pathogenic mutations, is present at clinically relevant levels in urinary epithelium, thus providing a practical, non-invasive test for diagnosis and mutation screening. In this study we further evaluate the use of these cells in detecting the m.3243A>G mutation, other mtDNA tRNA gene point mutations including the m.8344A>G mutation and single large-scale mtDNA deletions. We observe a robust relationship between m.3243A>G levels in urothelial cells and clinically affected tissues that does not change with time. Conversely, single large-scale mtDNA deletions can be detected in urothelial cells, with higher levels present in younger patients with more severe disease, but generally mtDNA deletion levels are not representative of those seen in a clinically affected tissue. Our results have implications for the diagnosis, management and counselling of families with mtDNA disease.     

Uncoupling the roles of the SUV3 helicase in maintenance of mitochondrial genome stability and RNA degradation

Yeast SUV3 is a nuclear encoded mitochondrial RNA helicase that complexes with an exoribonuclease, DSS1, to function as an RNA degradosome. Inactivation of SUV3 leads to mitochondrial dysfunctions, such as respiratory deficiency; accumulation of aberrant RNA species, including excised group I introns; and loss of mitochondrial DNA (mtDNA). Although intron toxicity has long been speculated to be the major reason for the observed phenotypes, direct evidence to support or refute this theory is lacking. Moreover, it remains unknown whether SUV3 plays a direct role in mtDNA maintenance independently of its degradosome activity. In this paper, we address these questions by employing an inducible knockdown system in Saccharomyces cerevisiae with either normal or intronless mtDNA background. Expressing mutants defective in ATPase (K245A) or RNA binding activities (V272L or ΔCC, which carries an 8-amino acid deletion at the C-terminal conserved region) resulted in not only respiratory deficiencies but also loss of mtDNA under normal mtDNA background. Surprisingly, V272L, but not other mutants, can rescue the said deficiencies under intronless background. These results provide genetic evidence supporting the notion that the functional requirements of SUV3 for degradosome activity and maintenance of mtDNA stability are separable. Furthermore, V272L mutants and wild-type SUV3 associated with an active mtDNA replication origin and facilitated mtDNA replication, whereas K245A and ΔCC failed to support mtDNA replication. These results indicate a direct role of SUV3 in maintaining mitochondrial genome stability that is independent of intron turnover but requires the intact ATPase activity and the CC conserved region.     

植物细胞质雄性不育分子机理研究进展

本文从线粒体基因组、线粒体基因、线粒体转录RNA、线粒体蛋白、转基因植物以及花粉败育机理六个方面详细介绍了植物细胞质雄性不育分子生物学研究的技术和方法。综述了植物细胞质雄性不育分子机理研究的进展 ,并对植物细胞质雄性不育分子机理的前景作了展望。     

Mapping Topoisomerase Sites in Mitochondrial DNA with a Poisonous Mitochondrial Topoisomerase I (Top1mt)

Mitochondrial topoisomerase I (Top1mt) is a type IB topoisomerase present in vertebrates and exclusively targeted to mitochondria. Top1mt relaxes mitochondrial DNA (mtDNA) supercoiling by introducing transient cleavage complexes wherein the broken DNA strand swivels around the intact strand. Top1mt cleavage complexes (Top1mtcc) can be stabilized in vitro by camptothecin (CPT). However, CPT does not trap Top1mtcc efficiently in cells and is highly cytotoxic due to nuclear Top1 targeting. To map Top1mtcc on mtDNA in vivo and to overcome the limitations of CPT, we designed two substitutions (T546A and N550H) in Top1mt to stabilize Top1mtcc. We refer to the double-mutant enzyme as Top1mt*. Using retroviral transduction and ChIP-on-chip assays with Top1mt* in Top1mt knock-out murine embryonic fibroblasts, we demonstrate that Top1mt* forms high levels of cleavage complexes preferentially in the noncoding regulatory region of mtDNA, accumulating especially at the heavy strand replication origin O_H, in the ribosomal genes (12S and 16S) and at the light strand replication origin O_L. Expression of Top1mt* also caused rapid mtDNA depletion without affecting mitochondria mass, suggesting the existence of specific mitochondrial pathways for the removal of damaged mtDNA.     

Rad51 replication fork recruitment is required for DNA damage tolerance

Homologous recombination (HR) is essential for genome integrity. Recombination proteins participate in tolerating DNA lesions that interfere with DNA replication, but can also generate toxic recombination intermediates and genetic instability when they are not properly regulated. Here, we have studied the role of the recombination proteins Rad51 and Rad52 at replication forks and replicative DNA lesions. We show that Rad52 loads Rad51 onto unperturbed replication forks, where they facilitate replication of alkylated DNA by non‐repair functions. The recruitment of Rad52 and Rad51 to chromatin during DNA replication is a prerequisite for the repair of the non‐DSB DNA lesions, presumably single‐stranded DNA gaps, which are generated during the replication of alkylated DNA. We also show that the repair of these lesions requires CDK1 and is not coupled to the fork but rather restricted to G2/M by the replicative checkpoint. We propose a new scenario for HR where Rad52 and Rad51 are recruited to the fork to promote DNA damage tolerance by distinct and cell cycle‐regulated replicative and repair functions.     

Mitochondrial DNA nucleoids determine mitochondrial genetics and dysfunction

Mitochondrial DNA plays a crucial role in cellular homeostasis; however, the molecular mechanisms underlying mitochondrial DNA inheritance and propagation are only beginning to be understood. To ensure the distribution and propagation of the mitochondrial genome, mitochondrial DNA is packaged into macromolecular assemblies called nucleoids, composed of one or more copies of mitochondrial DNA and associated proteins. We review current research on the mitochondrial nucleoid, including nucleoid-associated proteins, nucleoid dynamics within the cell, potential mechanisms to ensure proper distribution of nucleoids, and the impact of nucleoid organization on mitochondrial dysfunction. The nucleoid is the molecular organizing unit of mitochondrial genetics, and is the site of interactions that ultimately determine the bioenergetic state of the cell as a whole. Current and future research will provide essential insights into the molecular and cellular interactions that cause bioenergetic crisis, and yield clues for therapeutic rescue of mitochondrial dysfunction.     

Discovery of a Novel Function for Human Rad51: MAINTENANCE OF THE MITOCHONDRIAL GENOME*

Homologous recombination (HR) plays a critical role in facilitating replication fork progression when the polymerase complex encounters a blocking DNA lesion, and it also serves as the primary mechanism for error-free repair of DNA double strand breaks. Rad51 is the central catalyst of HR in all eukaryotes, and to this point studies of human Rad51 have focused exclusively on events occurring within the nucleus. However, substantial amounts of HR proteins exist in the cytoplasm, yet the function of these protein pools has not been addressed. Here, we provide the first demonstration that Rad51 and the related HR proteins Rad51C and Xrcc3 exist in human mitochondria. We show stress-induced increases in both the mitochondrial levels of each protein and, importantly, the physical interaction between Rad51 and mitochondrial DNA (mtDNA). Depletion of Rad51, Rad51C, or Xrcc3 results in a dramatic decrease in mtDNA copy number as well as the complete suppression of a characteristic oxidative stress-induced copy number increase. Our results identify human mtDNA as a novel Rad51 substrate and reveal an important role for HR proteins in the maintenance of the human mitochondrial genome.     

The human mitochondrial replication fork in health and disease

Mitochondria are organelles whose main function is to generate power by oxidative phosphorylation. Some of the essential genes required for this energy production are encoded by the mitochondrial genome, a small circular double stranded DNA molecule. Human mtDNA is replicated by a specialized machinery distinct from the nuclear replisome. Defects in the mitochondrial replication machinery can lead to loss of genetic information by deletion and/or depletion of the mtDNA, which subsequently may cause disturbed oxidative phosphorylation and neuromuscular symptoms in patients. We discuss here the different components of the mitochondrial replication machinery and their role in disease. We also review the mode of mammalian mtDNA replication.