Ca2+ flux across the mitochondrial inner membrane regulates bioenergetics, cytoplasmic Ca2+ signals and activation of cell death pathways1–11. Mitochondrial Ca2+ uptake occurs at regions of close apposition with intracellular Ca2+ release sites 12–14, driven by the inner membrane voltage generated by oxidative phosphorylation and mediated by a Ca2+ selective ion channel (MiCa15) called the uniporter16–18 whose complete molecular identity remains unknown. Mitochondrial calcium uniporter (MCU) was recently identified as the likely ion-conducting pore19, 20. In addition, MICU1 was identified as a mitochondrial regulator of uniporter-mediated Ca2+ uptake in HeLa cells 21. Here we identified CCDC90A, hereafter referred to as MCUR1 (Mitochondrial Calcium Uniporter Regulator 1), an integral membrane protein required for MCU-dependent mitochondrial Ca2+ uptake. MCUR1 binds to MCU and regulates ruthenium red-sensitive MCU-dependent Ca2+ uptake. MCUR1 knockdown does not alter MCU localization, but abrogates Ca2+ uptake by energized mitochondria in intact and permeabilized cells. Ablation of MCUR1 disrupts oxidative phosphorylation, lowers cellular ATP, and activates AMP kinase-dependent pro-survival autophagy. Thus, MCUR1 is a critical component of a mitochondrial uniporter channel complex required for mitochondrial Ca2+ uptake and maintenance of normal cellular bioenergetics.

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Defects in mitochondrial translation are among the most common causes of mitochondrial disease, but the mechanisms that regulate mitochondrial translation remain largely unknown. In the yeast Saccharomyces cerevisiae, all mitochondrial mRNAs require specific translational activators, which recognize sequences in 5' UTRs and mediate translation. As mammalian mitochondrial mRNAs do not have significant 5' UTRs, alternate mechanisms must exist to promote translation. We identified a specific defect in the synthesis of the mitochondrial DNA (mtDNA)-encoded COX I subunit in a pedigree segregating late-onset Leigh syndrome and cytochrome c oxidase (COX) deficiency. We mapped the defect to chromosome 17q by functional complementation and identified a homozygous single-base-pair insertion in CCDC44, encoding a member of a large family of hypothetical proteins containing a conserved DUF28 domain. CCDC44, renamed TACO1 for translational activator of COX I, shares a notable degree of structural similarity with bacterial homologs, and our findings suggest that it is one of a family of specific mammalian mitochondrial translational activators.

BackgroundMitochondrial DNA (mtDNA) deletions cause disease and accumulate during aging, yet our understanding of the molecular mechanisms underlying their formation remains rudimentary. Guanine-quadruplex (GQ) DNA structures are associated with nuclear DNA instability in cancer; recent evidence indicates they can also form in mitochondrial nucleic acids, suggesting that these non-B DNA structures could be associated with mtDNA deletions. Currently, the multiple types of GQ sequences and their association with human mtDNA stability are unknown.ResultsHere, we show an association between human mtDNA deletion breakpoint locations (sites where DNA ends rejoin after deletion of a section) and sequences with G-quadruplex forming potential (QFP), and establish the ability of selected sequences to form GQ in vitro. QFP contain four runs of either two or three consecutive guanines (2G and 3G, respectively), and we identified four types of QFP for subsequent analysis: intrastrand 2G, intrastrand 3G, duplex derived interstrand (ddi) 2G, and ddi 3G QFP sequences. We analyzed the position of each motif set relative to either 5' or 3' unique mtDNA deletion breakpoints, and found that intrastrand QFP sequences, but not ddi QFP sequences, showed significant association with mtDNA deletion breakpoint locations. Moreover, a large proportion of these QFP sequences occur at smaller distances to breakpoints relative to distribution-matched controls. The positive association of 2G QFP sequences persisted when breakpoints were divided into clinical subgroups. We tested in vitro GQ formation of representative mtDNA sequences containing these 2G QFP sequences and detected robust GQ structures by UV–VIS and CD spectroscopy. Notably, the most frequent deletion breakpoints, including those of the "common deletion", are bounded by 2G QFP sequence motifs.ConclusionsThe potential for GQ to influence mitochondrial genome stability supports a high-priority investigation of these structures and their regulation in normal and pathological mitochondrial biology. These findings emphasize the potential importance of helicases that subsequently resolve GQ to maintain the stability of the mitochondrial genome.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-677) contains supplementary material, which is available to authorized users.