Synchronized assembly of the oxidative phosphorylation system controls mitochondrial respiration in yeast

Control of protein stoichiometry is essential for cell function. Mitochondrial oxidative phosphorylation (OXPHOS) presents a complex stoichiometric challenge as the ratio of the electron transport chain (ETC) and ATP synthase must be tightly controlled, and assembly requires coordinated integration of proteins encoded in the nuclear and mitochondrial genome. How correct OXPHOS stoichiometry is achieved is unknown. We identify the Mitochondrial Regulatory hub for respiratory Assembly (MiRA) platform, which synchronizes ETC and ATP synthase biogenesis in yeast. Molecularly, this is achieved by a stop-and-go mechanism: the uncharacterized protein Mra1 stalls complex IV assembly. Two "Go" signals are required for assembly progression: binding of the complex IV assembly factor Rcf2 and Mra1 interaction with an Atp9-translating mitoribosome induce Mra1 degradation, allowing synchronized maturation of complex IV and the ATP synthase. Failure of the stop-and-go mechanism results in cell death. MiRA controls OXPHOS assembly, ensuring correct stoichiometry of protein machineries encoded by two different genomes.

An Early mtUPR: Redistribution of the Nuclear Transcription Factor Rox1 to Mitochondria Protects against Intramitochondrial Proteotoxic Aggregates

The mitochondrial proteome is built mainly by import of nuclear-encoded precursors, which are targeted mostly by cleavable presequences. Presequence processing upon import is essential for proteostasis and survival, but the consequences of dysfunctional protein maturation are unknown. We find that impaired presequence processing causes accumulation of precursors inside mitochondria that form aggregates, which escape degradation and unexpectedly do not cause cell death. Instead, cells survive via activation of a mitochondrial unfolded protein response (mtUPR)-like pathway that is triggered very early after precursor accumulation. In contrast to classical stress pathways, this immediate response maintains mitochondrial protein import, membrane potential, and translation through translocation of the nuclear HMG-box transcription factor Rox1 to mitochondria. Rox1 binds mtDNA and performs a TFAM-like function pivotal for transcription and translation. Induction of early mtUPR provides a reversible stress model to mechanistically dissect the initial steps in mtUPR pathways with the stressTFAM Rox1 as the first line of defense.

Pptc7 is an essential phosphatase for promoting mammalian mitochondrial metabolism and biogenesis

Mitochondrial proteins are replete with phosphorylation, yet its functional relevance remains largely unclear. The presence of multiple resident mitochondrial phosphatases, however, suggests that protein dephosphorylation may be broadly important for calibrating mitochondrial activities. To explore this, we deleted the poorly characterized matrix phosphatase Pptc7 from mice using CRISPR-Cas9 technology. Strikingly, Pptc7^−/− mice exhibit hypoketotic hypoglycemia, elevated acylcarnitines and serum lactate, and die soon after birth. Pptc7^−/− tissues have markedly diminished mitochondrial size and protein content despite normal transcript levels, and aberrantly elevated phosphorylation on select mitochondrial proteins. Among these, we identify the protein translocase complex subunit Timm50 as a putative Pptc7 substrate whose phosphorylation reduces import activity. We further find that phosphorylation within or near the mitochondrial targeting sequences of multiple proteins could disrupt their import rates and matrix processing. Overall, our data define Pptc7 as a protein phosphatase essential for proper mitochondrial function and biogenesis during the extrauterine transition.

The mitochondria houses several phosphatases, but their function is not well characterized. Here, the authors show that mitochondrial phosphatase Pptc7 is important during development for proper mitochondrial function and has a role regulating protein import with the translocase subunit Timm50.

Mutations in PMPCB Encoding the Catalytic Subunit of the Mitochondrial Presequence Protease Cause Neurodegeneration in Early Childhood

Mitochondrial disorders causing neurodegeneration in childhood are genetically heterogeneous, and the underlying genetic etiology remains unknown in many affected individuals. We identified biallelic variants in PMPCB in individuals of four families including one family with two affected siblings with neurodegeneration and cerebellar atrophy. PMPCB encodes the catalytic subunit of the essential mitochondrial processing protease (MPP), which is required for maturation of the majority of mitochondrial precursor proteins. Mitochondria isolated from two fibroblast cell lines and induced pluripotent stem cells derived from one affected individual and differentiated neuroepithelial stem cells showed reduced PMPCB levels and accumulation of the processing intermediate of frataxin, a sensitive substrate for MPP dysfunction. Introduction of the identified PMPCB variants into the homologous S. cerevisiae Mas1 protein resulted in a severe growth and MPP processing defect leading to the accumulation of mitochondrial precursor proteins and early impairment of the biogenesis of iron-sulfur clusters, which are indispensable for a broad range of crucial cellular functions. Analysis of biopsy materials of an affected individual revealed changes and decreased activity in iron-sulfur cluster-containing respiratory chain complexes and dysfunction of mitochondrial and cytosolic Fe-S cluster-dependent enzymes. We conclude that biallelic mutations in PMPCB cause defects in MPP proteolytic activity leading to dysregulation of iron-sulfur cluster biogenesis and triggering a complex neurological phenotype of neurodegeneration in early childhood.