bims-cytox1 Biomed News
on Cytochrome oxidase subunit 1
Issue of 2021‒08‒29
twelve papers selected by
Gavin McStay
Staffordshire University

  1. Biochim Biophys Acta Mol Cell Res. 2021 Aug 24. pii: S0167-4889(21)00187-7. [Epub ahead of print] 119133
      The respiratory chain, embedded in the inner mitochondrial membrane, is organized as a network of individual complexes, as well as large supercomplex structures. In the yeast S. cerevisiae, these supercomplexes consist of a dimeric cytochrome bc1-complex adjoined by one or two copies of cytochrome c oxidase. The formation of these complexes is a dynamic process and is regulated by various factors in order to adapt to environmental and metabolic changes. These adaptions occur at the level of enzyme regulation, complex assembly, as well as altered nuclear and mitochondrial transcription and translation. Members of the Rcf protein family (Rcf1, Rcf2 and Rcf3) are required for respiratory complex biogenesis and act mainly by regulating the assembly and enzyme activity of complex IV within supercomplexes. Rcf1 functions in the assembly process via the COX3 module, whereas Rcf2 and Rcf3 are responsible for enzymatic regulation. In this study, we have extended this knowledge to show that Rcf2 and Rcf3 can also associate with newly synthesized proteins, such as Cox3, and therefore contribute to complex IV assembly. Since the Rcf proteins have overlapping regions of sequence similarities, we engineered novel fusion proteins of Rcf1 and Rcf3, with parts of Rcf2, to probe which of the Rcf protein domains can be attributed to their functions. The fusion proteins could compensate for the individual phenotypes of the complex IV assembly defect and the lack of complex IV regulation. Finally, the role of Rcf proteins for defined species of respiratory chain complexes in a hypoxic model was investigated, uncovering a unique association of Rcf2 with the hypoxic III2IV supercomplex. We therefore suggest an involvement of Rcf2 for adaption of the respiratory chain to altering oxygen levels.
    Keywords:  OXPHOS; mitochondria; respiratory chain supercomplexes
  2. Biochem J. 2021 Aug 27. 478(16): 3125-3143
      Mitochondria import about 1000 proteins that are produced as precursors on cytosolic ribosomes. Defects in mitochondrial protein import result in the accumulation of non-imported precursor proteins and proteotoxic stress. The cell is equipped with different quality control mechanisms to monitor protein transport into mitochondria. First, molecular chaperones guide unfolded proteins to mitochondria and deliver non-imported proteins to proteasomal degradation. Second, quality control factors remove translocation stalled precursor proteins from protein translocases. Third, protein translocases monitor protein sorting to mitochondrial subcompartments. Fourth, AAA proteases of the mitochondrial subcompartments remove mislocalized or unassembled proteins. Finally, impaired efficiency of protein transport is an important sensor for mitochondrial dysfunction and causes the induction of cellular stress responses, which could eventually result in the removal of the defective mitochondria by mitophagy. In this review, we summarize our current understanding of quality control mechanisms that govern mitochondrial protein transport.
    Keywords:  TIM23 complex; TOM complex; mitochondria; protein sorting; protein transport
  3. Eur J Hum Genet. 2021 Aug 23.
      In a Dutch non-consanguineous patient having mitochondrial encephalomyopathy with complex I and complex IV deficiency, whole exome sequencing revealed two compound heterozygous variants in SLIRP. SLIRP gene encodes a stem-loop RNA-binding protein that regulates mitochondrial RNA expression and oxidative phosphorylation (OXPHOS). A frameshift and a deep-intronic splicing variant reduced the amount of functional wild-type SLIRP RNA to 5%. Consequently, in patient fibroblasts, MT-ND1, MT-ND6, and MT-CO1 expression was reduced. Lentiviral transduction of wild-type SLIRP cDNA in patient fibroblasts increased MT-ND1, MT-ND6, and MT-CO1 expression (2.5-7.2-fold), whereas mutant cDNAs did not. A fourfold decrease of citrate synthase versus total protein ratio in patient fibroblasts indicated that the resulting reduced mitochondrial mass caused the OXPHOS deficiency. Transduction with wild-type SLIRP cDNA led to a 2.4-fold increase of this ratio and partly restored OXPHOS activity. This confirmed causality of the SLIRP variants. In conclusion, we report SLIRP variants as a novel cause of mitochondrial encephalomyopathy with OXPHOS deficiency.
  4. Nucleic Acids Res. 2021 Aug 24. pii: gkab726. [Epub ahead of print]
      Diagnosing mitochondrial disorders remains challenging. This is partly because the clinical phenotypes of patients overlap with those of other sporadic and inherited disorders. Although the widespread availability of genetic testing has increased the rate of diagnosis, the combination of phenotypic and genetic heterogeneity still makes it difficult to reach a timely molecular diagnosis with confidence. An objective, systematic method for describing the phenotypic spectra for each variant provides a potential solution to this problem. We curated the clinical phenotypes of 6688 published individuals with 89 pathogenic mitochondrial DNA (mtDNA) mutations, collating 26 348 human phenotype ontology (HPO) terms to establish the MitoPhen database. This enabled a hypothesis-free definition of mtDNA clinical syndromes, an overview of heteroplasmy-phenotype relationships, the identification of under-recognized phenotypes, and provides a publicly available reference dataset for objective clinical comparison with new patients using the HPO. Studying 77 patients with independently confirmed positive mtDNA diagnoses and 1083 confirmed rare disease cases with a non-mitochondrial nuclear genetic diagnosis, we show that HPO-based phenotype similarity scores can distinguish these two classes of rare disease patients with a false discovery rate <10% at a sensitivity of 80%. Enriching the MitoPhen database with more patients will improve predictions for increasingly rare variants.
  5. Adv Mater. 2021 Aug 25. e2103307
      Leber's hereditary optic neuropathy (LHON) is a rare inherited blindness caused by mutations in the mitochondrial DNA (mtDNA). The disorder is untreatable and tricky, as the existing chemotherapeutic agent Idebenone alleviates symptoms rather than overcoming the underlying cause. Although some studies have made progress on allotopic expression for LHON, in situ mitochondrial gene therapy remains challenging, which may simplify delivery procedures to be a promising therapeutic for LHON. LHON becomes more difficult to manage in the changed mitochondrial microenvironment, including increasing reactive oxygen species (ROS) and decreasing mitochondrial membrane potential (MMP). Herein, a pathologically responsive mitochondrial gene delivery vector named [triphenylphosphine-terminated poly(sulfur-containing thioketal undecafluorohexylamine histamine) and Ide-terminated poly(sulfur-containing thioketal undecafluorohexylamine histamine)] (TISUH) is reported to facilitate commendable in situ mitochondrial gene therapy for LHON. TISUH directly targets diseased mitochondria via triphenylphosphine and fluorination addressing the decreasing MMP. In addition, TISUH can be disassembled by high mitochondrial ROS levels to release functional genes for enhancing gene transfection efficiency and fundamentally correcting genetic abnormalities. In both traditional and gene-mutation-induced LHON mouse models, TISUH-mediated gene therapy shows satisfactory curative effect through the sustained therapeutic protein expression in vivo. This work proposes a novel pathologically responsive in situ mitochondrial delivery platform and provides a promising approach for refractory LHON as well as other mtDNA mutated diseases treatments.
    Keywords:  Idebenone; Leber's hereditary optic neuropathy; fluorination; in situ mitochondrial gene therapy; pathologically responsive polymers
  6. Cell Chem Biol. 2021 Aug 24. pii: S2451-9456(21)00365-2. [Epub ahead of print]
      Mutations in mitochondrial DNA (mtDNA) cause mitochondrial diseases, characterized by abnormal mitochondrial function. Although eliminating mutated mtDNA has potential to cure mitochondrial diseases, no chemical-based drugs in clinical trials are capable of selective modulation of mtDNA mutations. Here, we construct a class of compounds encompassing pyrrole-imidazole polyamides (PIPs), mitochondria-penetrating peptide, and chlorambucil, an adenine-specific DNA-alkylating reagent. The sequence-selective DNA binding of PIPs allows chlorambucil to alkylate mutant adenine more efficiently than other sites in mtDNA. In vitro DNA alkylation assay shows that our compound 8950A-Chb(Cl/OH) targeting a nonpathogenic point mutation in HeLa S3 cells (m.8950G>A) can specifically alkylate the mutant adenine. Furthermore, the compound reduces the mtDNA possessing the target mutation in cultured HeLa S3 cells. The programmability of PIPs to target different sequences could allow this class of compounds to be developed as designer drugs targeting pathogenic mutations associated with mitochondrial diseases in future studies.
    Keywords:  DNA alkylation; DNA mutation; designer small molecule; heteroplasmy; mitochondria; mitochondrial DNA; mitochondrial disease; pyrrole-imidazole polyamide
  7. Crit Rev Biochem Mol Biol. 2021 Aug 25. 1-29
      Aerobic respiration is a key energy-producing pathway in many prokaryotes and virtually all eukaryotes. The final step of aerobic respiration is most commonly catalyzed by heme-copper oxidases embedded in the cytoplasmic or mitochondrial membrane. The majority of these terminal oxidases contain a prenylated heme (typically heme a or occasionally heme o) in the active site. In addition, many heme-copper oxidases, including mitochondrial cytochrome c oxidases, possess a second heme a cofactor. Despite the critical role of heme a in the electron transport chain, the details of the mechanism by which heme b, the prototypical cellular heme, is converted to heme o and then to heme a remain poorly understood. Recent structural investigations, however, have helped clarify some elements of heme a biosynthesis. In this review, we discuss the insight gained from these advances. In particular, we present a new structural model of heme o synthase (HOS) based on distance restraints from inferred coevolutionary relationships and refined by molecular dynamics simulations that are in good agreement with the experimentally determined structures of HOS homologs. We also analyze the two structures of heme a synthase (HAS) that have recently been solved by other groups. For both HOS and HAS, we discuss the proposed catalytic mechanisms and highlight how new insights into the heme-binding site locations shed light on previously obtained biochemical data. Finally, we explore the implications of the new structural data in the broader context of heme trafficking in the heme a biosynthetic pathway and heme-copper oxidase assembly.
    Keywords:  Cytochrome c oxidase; cytochrome c oxidase assembly; heme a synthase; heme o synthase; heme oxidation; heme trafficking; heme-copper oxidases; intramembrane aromatic prenyltransferase
  8. Mol Biol Cell. 2021 Aug 25. mbcE20070457
      The synthesis of Cox1, the conserved catalytic-core subunit of Complex IV, a multi-subunit machinery of the mitochondrial oxidative phosphorylation (OXPHOS) system under environmental stress is not sufficiently addressed. In this study, we show that the putative YihA superfamily GTPase, Mrx8 is a bonafide mitochondrial protein required for Cox1 translation initiation and elongation during suboptimal growth condition at 16°C. Mrx8 was found in a complex with mitochondrial ribosomes, consistent with a role in protein synthesis. Cells expressing mutant Mrx8 predicted to be defective in guanine nucleotide binding and hydrolysis were compromised for robust cellular respiration. We show that requirement of Pet309 and Mss51 for cellular respiration is not bypassed by overexpression of Mrx8 and vice versa. Consistently the ribosomal association of Mss51 is independent of Mrx8. Significantly, we find that GTPBP8, the human orthologue, complements the loss of cellular respiration in Δmrx8 cells and GTPBP8 localizes to the mitochondria in mammalian cells. This strongly suggest a universal role of MRX8 family of proteins in regulating mitochondrial function.
  9. Front Mol Biosci. 2021 ;8 711436
      Cellular respiration is a fundamental process required for energy production in many organisms. The terminal electron transfer complex in mitochondrial and many bacterial respiratory chains is cytochrome c oxidase (CcO). This converts the energy released in the cytochrome c/oxygen redox reaction into a transmembrane proton electrochemical gradient that is used subsequently to power ATP synthesis. Despite detailed knowledge of electron and proton transfer paths, a central question remains as to whether the coupling between electron and proton transfer in mammalian mitochondrial forms of CcO is mechanistically equivalent to its bacterial counterparts. Here, we focus on the conserved span between H376 and G384 of transmembrane helix (TMH) X of subunit I. This conformationally-dynamic section has been suggested to link the redox activity with the putative H pathway of proton transfer in mammalian CcO. The two helix X mutants, Val380Met (V380M) and Gly384Asp (G384D), generated in the genetically-tractable yeast CcO, resulted in a respiratory-deficient phenotype caused by the inhibition of intra-protein electron transfer and CcO turnover. Molecular aspects of these variants were studied by long timescale atomistic molecular dynamics simulations performed on wild-type and mutant bovine and yeast CcOs. We identified redox- and mutation-state dependent conformational changes in this span of TMH X of bovine and yeast CcOs which strongly suggests that this dynamic module plays a key role in optimizing intra-protein electron transfers.
    Keywords:  density functional theory; mitochondrial respiration; molecular dynamics simulations; proton pumping; yeast bioenergetics
  10. Curr Opin Chem Biol. 2021 Aug 23. pii: S1367-5931(21)00100-9. [Epub ahead of print]
      The ClpP protease is found across eukaryotic and prokaryotic organisms. It is well-characterized in bacteria where its function is important in maintaining protein homeostasis. Along with its ATPase partners, it has been shown to play critical roles in the regulation of enzymes involved in important cellular pathways. In eukaryotes, ClpP is found within cellular organelles. Proteomic studies have begun to characterize the role of this protease in the mitochondria through its interactions. Here, we discuss the proteomic techniques used to identify its interactors and present an atlas of mitochondrial ClpP substrates. The ClpP substrate pool is extensive and consists of proteins involved in essential mitochondrial processes such as the Krebs cycle, oxidative phosphorylation, translation, fatty acid metabolism, and amino acid metabolism. Discoveries of these associations have begun to illustrate the functional significance of ClpP in human health and disease.
    Keywords:  Cancer; ClpP protease; Mitochondria; Mitochondrial diseases; Parkinson's disease; Protein quality control; Proteolysis; Proteomics; Proteostasis
  11. Nat Med. 2021 Aug 23.
      Mitochondrial DNA (mtDNA) variants influence the risk of late-onset human diseases, but the reasons for this are poorly understood. Undertaking a hypothesis-free analysis of 5,689 blood-derived biomarkers with mtDNA variants in 16,220 healthy donors, here we show that variants defining mtDNA haplogroups Uk and H4 modulate the level of circulating N-formylmethionine (fMet), which initiates mitochondrial protein translation. In human cytoplasmic hybrid (cybrid) lines, fMet modulated both mitochondrial and cytosolic proteins on multiple levels, through transcription, post-translational modification and proteolysis by an N-degron pathway, abolishing known differences between mtDNA haplogroups. In a further 11,966 individuals, fMet levels contributed to all-cause mortality and the disease risk of several common cardiovascular disorders. Together, these findings indicate that fMet plays a key role in common age-related disease through pleiotropic effects on cell proteostasis.
  12. Microb Physiol. 2021 Aug 26. 1-20
      Fast adaptation to environmental changes ensures bacterial survival, and proteolysis represents a key cellular process in adaptation. The Clp protease system is a multi-component machinery responsible for protein homoeostasis, protein quality control, and targeted proteolysis of transcriptional regulators in prokaryotic cells and prokaryote-derived organelles of eukaryotic cells. A functional Clp protease complex consists of the tetradecameric proteolytic core ClpP and a hexameric ATP-consuming Clp-ATPase, several of which can associate with the same proteolytic core. Clp-ATPases confer substrate specificity by recognising specific degradation tags, and further selectivity is conferred by adaptor proteins, together allowing for a fine-tuned degradation process embedded in elaborate regulatory networks. This review focuses on the contribution of the Clp protease system to prokaryotic survival and summarises the current state of knowledge for exemplary bacteria in an increasing degree of interaction with eukaryotic cells. Starting from free-living bacteria as exemplified by a non-pathogenic and a pathogenic member of the Firmicutes, i.e., Bacillus subtilis and Staphylococcus aureus, respectively, we turn our attention to facultative and obligate intracellular bacterial pathogens, i.e., Mycobacterium tuberculosis, Listeria monocytogenes, and Chlamydia trachomatis, and conclude with mitochondria. Under stress conditions, the Clp protease system exerts its pivotal role in the degradation of damaged proteins and controls the timing and extent of the heat-shock response by regulatory proteolysis. Key regulators of developmental programmes like natural competence, motility, and sporulation are also under Clp proteolytic control. In many pathogenic species, the Clp system is required for the expression of virulence factors and essential for colonising the host. In accordance with its evolutionary origin, the human mitochondrial Clp protease strongly resembles its bacterial counterparts, taking a central role in protein quality control and homoeostasis, energy metabolism, and apoptosis in eukaryotic cells, and several cancer cell types depend on it for proliferation.
    Keywords:  Clp-ATPase; ClpP; Pathogenesis; Regulatory proteolysis; Stress response