A short carboxyl-terminal tail is required for single-stranded DNA binding, higher-order structural organization, and stability of the mitochondrial single-stranded annealing protein Mgm101

Rolling-Circle Replication in Mitochondrial DNA Inheritance: Scientific Evidence and Significance from Yeast to Human Cells

Studies of mitochondrial (mt)DNA replication, which forms the basis of mitochondrial inheritance, have demonstrated that a rolling-circle replication mode exists in yeasts and human cells. In yeast, rolling-circle mtDNA replication mediated by homologous recombination is the predominant pathway for replication of wild-type mtDNA. In human cells, reactive oxygen species (ROS) induce rolling-circle replication to produce concatemers, linear tandem multimers linked by head-to-tail unit-sized mtDNA that promote restoration of homoplasmy from heteroplasmy. The event occurs ahead of mtDNA replication mechanisms observed in mammalian cells, especially under higher ROS load, as newly synthesized mtDNA is concatemeric in hydrogen peroxide-treated human cells. Rolling-circle replication holds promise for treatment of mtDNA heteroplasmy-attributed diseases, which are regarded as incurable. This review highlights the potential therapeutic value of rolling-circle mtDNA replication.

ER-Mitochondria Microdomains in Cardiac Ischemia-Reperfusion Injury: A Fresh Perspective

The mitochondrial and endoplasmic reticulum (ER) homeostasis is pivotal to the maintenance of an array of physiological processes. The physical contact and association between ER and mitochondria, known as the ER–mitochondria microdomains or mitochondria-associated ER membrane (MAM), temporally and spatially regulates the mitochondria/ER structure and function. More evidence suggests a role for MAMs in energy production, cellular contraction and mobility, and normal extracellular signal transmission. In pathological states, such as cardiac ischemia–reperfusion (I/R injury), this ER–mitochondria microdomains may act to participate in the cellular redox imbalance, ER stress, mitochondrial injury, energy deletion, and programmed cell death. From a therapeutic perspective, a better understanding of the cellular and molecular mechanisms of the pathogenic ER–mitochondria contact should help to identify potential therapeutic target for cardiac I/R injury and other cardiovascular diseases and also pave the road to new treatment modalities pertinent for the treatment of reperfusion damage in clinical practice. This review will mainly focus on the possible signaling pathways involved in the regulation of the ER–mitochondria contact. In particular, we will summarize the downstream signaling modalities influenced by ER–mitochondria microdomains, for example, mitochondrial fission, mitophagy, calcium balance, oxidative stress, and programmed cell death in details.

The role of Lon-mediated proteolysis in the dynamics of mitochondrial nucleic acid-protein complexes

Mitochondrial nucleoids consist of several different groups of proteins, many of which are involved in essential cellular processes such as the replication, repair and transcription of the mitochondrial genome. The eukaryotic, ATP-dependent protease Lon is found within the central nucleoid region, though little is presently known about its role there. Aside from its association with mitochondrial nucleoids, human Lon also specifically interacts with RNA. Recently, Lon was shown to regulate TFAM, the most abundant mtDNA structural factor in human mitochondria. To determine whether Lon also regulates other mitochondrial nucleoid- or ribosome-associated proteins, we examined the in vitro digestion profiles of the Saccharomyces cerevisiae TFAM functional homologue Abf2, the yeast mtDNA maintenance protein Mgm101, and two human mitochondrial proteins, Twinkle helicase and the large ribosomal subunit protein MrpL32. Degradation of Mgm101 was also verified in vivo in yeast mitochondria. These experiments revealed that all four proteins are actively degraded by Lon, but that three of them are protected from it when bound to a nucleic acid; the Twinkle helicase is not. Such a regulatory mechanism might facilitate dynamic changes to the mitochondrial nucleoid, which are crucial for conducting mitochondrial functions and maintaining mitochondrial homeostasis.

The structure and DNA-binding properties of Mgm101 from a yeast with a linear mitochondrial genome

To study the mechanisms involved in the maintenance of a linear mitochondrial genome we investigated the biochemical properties of the recombination protein Mgm101 from Candida parapsilosis. We show that CpMgm101 complements defects associated with the Saccharomyces cerevisiae mgm101-1(ts) mutation and that it is present in both the nucleus and mitochondrial nucleoids of C. parapsilosis. Unlike its S. cerevisiae counterpart, CpMgm101 is associated with the entire nucleoid population and is able to bind to a broad range of DNA substrates in a non-sequence specific manner. CpMgm101 is also able to catalyze strand annealing and D-loop formation. CpMgm101 forms a roughly C-shaped trimer in solution according to SAXS. Electron microscopy of a complex of CpMgm101 with a model mitochondrial telomere revealed homogeneous, ring-shaped structures at the telomeric single-stranded overhangs. The DNA-binding properties of CpMgm101, together with its DNA recombination properties, suggest that it can play a number of possible roles in the replication of the mitochondrial genome and the maintenance of its telomeres.

Mechanism of homologous recombination and implications for aging-related deletions in mitochondrial DNA

Homologous recombination is a universal process, conserved from bacteriophage to human, which is important for the repair of double-strand DNA breaks. Recombination in mitochondrial DNA (mtDNA) was documented more than 4 decades ago, but the underlying molecular mechanism has remained elusive. Recent studies have revealed the presence of a Rad52-type recombination system of bacteriophage origin in mitochondria, which operates by a single-strand annealing mechanism independent of the canonical RecA/Rad51-type recombinases. Increasing evidence supports the notion that, like in bacteriophages, mtDNA inheritance is a coordinated interplay between recombination, repair, and replication. These findings could have profound implications for understanding the mechanism of mtDNA inheritance and the generation of mtDNA deletions in aging cells.