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Mammalian complex I can adopt catalytically active (A-) or deactive (D-) states. A defining feature of the reversible transition between these two defined states is thought to be exposure of the ND3 subunit Cys39 residue in the D-state and its occlusion in the A-state. As the catalytic A/D transition is important in health and disease, we set out to quantify it by measuring Cys39 exposure using isotopic labeling and mass spectrometry, in parallel with complex I NADH/CoQ oxidoreductase activity. To our surprise, we found significant Cys39 exposure during NADH/CoQ oxidoreductase activity. Furthermore, this activity was unaffected if Cys39 alkylation occurred during complex I-linked respiration. In contrast, alkylation of catalytically inactive complex I irreversibly blocked the reactivation of NADH/CoQ oxidoreductase activity by NADH. Thus, Cys39 of ND3 is exposed in complex I during mitochondrial respiration, with significant implications for our understanding of the A/D transition and the mechanism of complex I.

Post-transcriptional RNA modifications, the epitranscriptome, play important roles in modulating the functions of RNA species. Modifications of rRNA are key for ribosome production and function. Identification and characterization of enzymes involved in epitranscriptome shaping is instrumental for the elucidation of the functional roles of specific RNA modifications. Ten modified sites have been thus far identified in the mammalian mitochondrial rRNA. Enzymes responsible for two of these modifications have not been characterized. Here, we identify METTL15, show that it is the main N4-methylcytidine (m4C) methyltransferase in human cells and demonstrate that it is responsible for the methylation of position C839 in mitochondrial 12S rRNA. We show that the lack of METTL15 results in a reduction of the mitochondrial de novo protein synthesis and decreased steady-state levels of protein components of the oxidative phosphorylation system. Without functional METTL15, the assembly of the mitochondrial ribosome is decreased, with the late assembly components being unable to be incorporated efficiently into the small subunit. We speculate that m4C839 is involved in the stabilization of 12S rRNA folding, therefore facilitating the assembly of the mitochondrial small ribosomal subunits. Taken together our data show that METTL15 is a novel protein necessary for efficient translation in human mitochondria.

Significance The oxidation of energy rich compounds generates a proton motive force, a chemical potential difference for protons, across the inner membranes of the mitochondria. The proton motive force drives the turning of the rotor in the membrane domain of the ATP synthase. This rotation provides the energy to synthesize the ATP required to sustain life. The assembly in the inner organellar membrane of human ATP synthase from 27 nuclear encoded proteins and 2 mitochondrially encoded subunits involves the formation of intermediate modules representing the F1-catalytic domain, the peripheral stalk, and the c8-ring in the membrane part of the rotor. The assembly of the c8-ring requires the participation of two membrane-associated proteins, TMEM70 and, as we demonstrate, TMEM242., Human mitochondrial ATP synthase is a molecular machine with a rotary action bound in the inner organellar membranes. Turning of the rotor, driven by a proton motive force, provides energy to make ATP from ADP and phosphate. Among the 29 component proteins of 18 kinds, ATP6 and ATP8 are mitochondrial gene products, and the rest are nuclear gene products that are imported into the organelle. The ATP synthase is assembled from them via intermediate modules representing the main structural elements of the enzyme. One such module is the c8-ring, which provides the membrane sector of the enzyme’s rotor, and its assembly is influenced by another transmembrane (TMEM) protein, TMEM70. We have shown that subunit c interacts with TMEM70 and another hitherto unidentified mitochondrial transmembrane protein, TMEM242. Deletion of TMEM242, similar to deletion of TMEM70, affects but does not completely eliminate the assembly of ATP synthase, and to a lesser degree the assembly of respiratory enzyme complexes I, III, and IV. Deletion of TMEM70 and TMEM242 together prevents assembly of ATP synthase and the impact on complex I is enhanced. Removal of TMEM242, but not of TMEM70, also affects the introduction of subunits ATP6, ATP8, j, and k into the enzyme. TMEM70 and TMEM242 interact with the mitochondrial complex I assembly (the MCIA) complex that supports assembly of the membrane arm of complex I. The interactions of TMEM70 and TMEM242 with MCIA could be part of either the assembly of ATP synthase and complex I or the regulation of their levels.

The structural and functional organization of the mitochondrial respiratory chain (MRC) remains intensely debated. Here, we show the co-existence of two separate MRC organizations in human cells and postmitotic tissues, C-MRC and S-MRC, defined by the preferential expression of three COX7A subunit isoforms, COX7A1/2 and SCAFI (COX7A2L). COX7A isoforms promote the functional reorganization of distinct co-existing MRC structures to prevent metabolic exhaustion and MRC deficiency. Notably, prevalence of each MRC organization is reversibly regulated by the activation state of the pyruvate dehydrogenase complex (PDC). Under oxidative conditions, the C-MRC is bioenergetically more efficient, whereas the S-MRC preferentially maintains oxidative phosphorylation (OXPHOS) upon metabolic rewiring toward glycolysis. We show a link between the metabolic signatures converging at the PDC and the structural and functional organization of the MRC, challenging the widespread notion of the MRC as a single functional unit and concluding that its structural heterogeneity warrants optimal adaptation to metabolic function.

Funder: Akademie Věd České Republiky; Grant(s): RVO:67985823, The oxidative phosphorylation (OXPHOS) system localized in the inner mitochondrial membrane secures production of the majority of ATP in mammalian organisms. Individual OXPHOS complexes form supramolecular assemblies termed supercomplexes. The complexes are linked not only by their function but also by interdependency of individual complex biogenesis or maintenance. For instance, cytochrome c oxidase (cIV) or cytochrome bc1 complex (cIII) deficiencies affect the level of fully assembled NADH dehydrogenase (cI) in monomeric as well as supercomplex forms. It was hypothesized that cI is affected at the level of enzyme assembly as well as at the level of cI stability and maintenance. However, the true nature of interdependency between cI and cIV is not fully understood yet. We used a HEK293 cellular model where the COX4 subunit was completely knocked out, serving as an ideal system to study interdependency of cI and cIV, as early phases of cIV assembly process were disrupted. Total absence of cIV was accompanied by profound deficiency of cI, documented by decrease in the levels of cI subunits and significantly reduced amount of assembled cI. Supercomplexes assembled from cI, cIII, and cIV were missing in COX4I1 knock-out (KO) due to loss of cIV and decrease in cI amount. Pulse-chase metabolic labeling of mitochondrial DNA (mtDNA)-encoded proteins uncovered a decrease in the translation of cIV and cI subunits. Moreover, partial impairment of mitochondrial protein synthesis correlated with decreased content of mitochondrial ribosomal proteins. In addition, complexome profiling revealed accumulation of cI assembly intermediates, indicating that cI biogenesis, rather than stability, was affected. We propose that attenuation of mitochondrial protein synthesis caused by cIV deficiency represents one of the mechanisms, which may impair biogenesis of cI.

Many cellular processes involve the participation of large macromolecular assemblies. Understanding their function requires methods allowing to study their dynamic and mechanistic properties. Here we present a method for quantitative analysis of native protein or ribonucleoprotein complexes by mass spectrometry following their separation by density – qDGMS. Mass spectrometric quantitation is enabled through stable isotope labelling with amino acids in cell culture (SILAC). We provide a complete guide, from experimental design to preparation of publication-ready figures, using a purposely-developed R package – ComPrAn. As specific examples, we present the use of sucrose density gradients to inspect the assembly and dynamics of the human mitochondrial ribosome (mitoribosome), its interacting proteins, the small subunit of the cytoplasmic ribosome, cytoplasmic aminoacyl-tRNA synthetase complex and the mitochondrial PDH complex. ComPrAn provides tools for analysis of peptide-level data as well as normalization and clustering tools for protein-level data, dedicated visualization functions and graphical user interface. Although, it has been developed for the analysis of qDGMS samples, it can also be used for other proteomics experiments that involve 2-state labelled samples separated into fractions. We show that qDGMS and ComPrAn can be used to study macromolecular complexes in their native state, accounting for the dynamics inherent to biological systems and benefiting from its proteome-wide quantitative and qualitative capability., Graphical abstract Unlabelled Image, Highlights • qDGMS is a novel method to study macromolecular complex composition and assembly. • Complexes are separated in near-native form by density gradient ultracentrifugation. • SILAC enables simultaneous quantitative proteomic analysis of two biological samples. • R package ComPrAn allows analysis of SILAC complexome profiling and qDGMS data sets.

Summary Complex I (CI) is the largest enzyme of the mitochondrial respiratory chain, and its defects are the main cause of mitochondrial disease. To understand the mechanisms regulating the extremely intricate biogenesis of this fundamental bioenergetic machine, we analyze the structural and functional consequences of the ablation of NDUFS3, a non-catalytic core subunit. We show that, in diverse mammalian cell types, a small amount of functional CI can still be detected in the complete absence of NDUFS3. In addition, we determine the dynamics of CI disassembly when the amount of NDUFS3 is gradually decreased. The process of degradation of the complex occurs in a hierarchical and modular fashion in which the ND4 module remains stable and bound to TMEM126A. We, thus, uncover the function of TMEM126A, the product of a disease gene causing recessive optic atrophy as a factor necessary for the correct assembly and function of CI., Graphical abstract, Highlights • A fraction of functional complex I assembles without the core subunit NDUFS3 • Complex I disassembly differentially affects its submodules • With NDUFS3 absent, the ND4-module of the P-distal domain remains mostly stable • The TMEM126A/OPA7 factor interacts with the ND4-module and is necessary for CI maturation, D’Angelo et al. show that eliminating NDUFS3 does not completely abolish respiratory complex I maturation. Differential degradation of complex I subunits belonging to different structural/functional modules is triggered by NDUFS3 repression. The ND4 module remains stable and is bound to TMEM126A, which is, here, identified as a complex I assembly factor.

Acyl-CoAs are reactive metabolites that can non-enzymatically S-acylate and N-acylate protein cysteine and lysine residues, respectively. N-acylation is irreversible and enhanced if a nearby cysteine residue undergoes an initial reversible S-acylation, as proximity leads to rapid S → N-transfer of the acyl moiety. We reasoned that protein-bound acyl-CoA could also facilitate S → N-transfer of acyl groups to proximal lysine residues. Furthermore, as CoA contains an ADP backbone this may extend beyond CoA-binding sites and include abundant Rossmann-fold motifs that bind the ADP moiety of NADH, NADPH, FADH and ATP. Here, we show that excess nucleotides decrease protein lysine N-acetylation in vitro. Furthermore, by generating modelled structures of proteins N-acetylated in mouse liver, we show that proximity to a nucleotide-binding site increases the risk of N-acetylation and identify where nucleotide binding could enhance N-acylation in vivo. Finally, using glutamate dehydrogenase as a case study, we observe increased in vitro lysine N-malonylation by malonyl-CoA near nucleotide-binding sites which overlaps with in vivo N-acetylation and N-succinylation. Furthermore, excess NADPH, GTP and ADP greatly diminish N-malonylation near their nucleotide-binding sites, but not at distant lysine residues. Thus, lysine N-acylation by acyl-CoAs is enhanced by nucleotide-binding sites and may contribute to higher stoichiometry protein N-acylation in vivo.

Significance The production of ATP in mitochondria requires the oxidation of energy rich compounds to generate a proton motive force (pmf), a chemical potential difference for protons across the inner membrane. This pmf powers the ATP synthase, a molecular machine with a rotary action, to synthesize ATP. The assembly of human ATP synthase from 27 nuclear encoded proteins and two mitochondrially encoded subunits in the inner organellar membrane involves the formation of intermediate modules representing the F1-catalytic domain, the peripheral stalk, associated membrane subunits, and the c8 ring in the membrane part of the rotor. Here, we describe how components of the peripheral stalk and three associated membrane subunits are assembled and introduced into the enzyme complex., The adenosine triphosphate (ATP) synthase in human mitochondria is a membrane bound assembly of 29 proteins of 18 kinds organized into F1-catalytic, peripheral stalk (PS), and c8-rotor ring modules. All but two membrane components are encoded in nuclear genes, synthesized on cytoplasmic ribosomes, imported into the mitochondrial matrix, and assembled into the complex with the mitochondrial gene products ATP6 and ATP8. Intermediate vestigial ATPase complexes formed by disruption of nuclear genes for individual subunits provide a description of how the various domains are introduced into the enzyme. From this approach, it is evident that three alternative pathways operate to introduce the PS module (including associated membrane subunits e, f, and g). In one pathway, the PS is built up by addition to the core subunit b of membrane subunits e and g together, followed by membrane subunit f. Then this b-e-g-f complex is bound to the preformed F1-c8 module by subunits OSCP and F6. The final component of the PS, subunit d, is added subsequently to form a key intermediate that accepts the two mitochondrially encoded subunits. In another route to this key intermediate, first e and g together and then f are added to a preformed F1-c8-OSCP-F6-b-d complex. A third route involves the addition of the c8-ring module to the complete F1-PS complex. The key intermediate then accepts the two mitochondrially encoded subunits, stabilized by the addition of subunit j, leading to an ATP synthase complex that is coupled to the proton motive force and capable of making ATP.

Complex I (CI) is the largest enzyme of the mitochondrial respiratory chain and its defects are the main cause of mitochondrial disease. To understand the mechanisms regulating the extremely intricate biogenesis of this fundamental bioenergetic machine, we analyzed the structural and functional consequences of the ablation of NDUFS3, a non-catalytic core subunit. We prove that in diverse mammalian cell types a small amount of functional CI can still be detected in the complete absence of NDUFS3. In addition, we have determined the dynamics of CI disassembly when the amount of NDUFS3 is gradually decreased. The process of degradation of the complex occurs in a hierarchical and modular fashion where the ND4-module remains stable and bound to TMEM126A. We have thus, uncovered the function of TMEM126A, the product of a disease gene causing recessive optic atrophy, as a factor necessary for the correct assembly and function of CI.

Mitochondrial respiratory chain (MRC) enzymes associate in supercomplexes (SCs) that are structurally interdependent. This may explain why defects in a single component often produce combined enzyme deficiencies in patients. A case in point is the alleged destabilization of complex I in the absence of complex III. To clarify the structural and functional relationships between complexes, we have used comprehensive proteomic, functional, and biogenetical approaches to analyze a MT‐CYB‐deficient human cell line. We show that the absence of complex III blocks complex I biogenesis by preventing the incorporation of the NADH module rather than decreasing its stability. In addition, complex IV subunits appeared sequestered within complex III subassemblies, leading to defective complex IV assembly as well. Therefore, we propose that complex III is central for MRC maturation and SC formation. Our results challenge the notion that SC biogenesis requires the pre‐formation of fully assembled individual complexes. In contrast, they support a cooperative‐assembly model in which the main role of complex III in SCs is to provide a structural and functional platform for the completion of overall MRC biogenesis., Biogenesis of the human mitochondrial respiratory chain requires the cooperative and interdependent action of respiratory supercomplexes.

The F-ATPases (also called the F1 Fo -ATPases or ATP synthases) are multi-subunit membrane-bound molecular machines that produce ATP in bacteria and in eukaryotic mitochondria and chloroplasts. The structures and enzymic mechanisms of their F1 -catalytic domains are highly conserved in all species investigated hitherto. However, there is evidence that the F-ATPases from the group of protozoa known as Euglenozoa have novel features. Therefore, we have isolated pure and active F1 -ATPase from the euglenozoan parasite, Trypanosoma brucei, and characterized it. All of the usual eukaryotic subunits (α, β, γ, δ, and e) were present in the enzyme, and, in addition, two unique features were detected. First, each of the three α-subunits in the F1 -domain has been cleaved by proteolysis in vivo at two sites eight residues apart, producing two assembled fragments. Second, the T. brucei F1 -ATPase has an additional subunit, called p18, present in three copies per complex. Suppression of expression of p18 affected in vitro growth of both the insect and infectious mammalian forms of T. brucei. It also reduced the levels of monomeric and multimeric F-ATPase complexes and diminished the in vivo hydrolytic activity of the enzyme significantly. These observations imply that p18 plays a role in the assembly of the F1 domain. These unique features of the F1 -ATPase extend the list of special characteristics of the F-ATPase from T. brucei, and also, demonstrate that the architecture of the F1 -ATPase complex is not strictly conserved in eukaryotes.

The study of the mitochondrial respiratory chain (MRC) function in relation with its structural organization is of great interest due to the central role of this system in eukaryotic cell metabolism. The complexome profiling technique has provided invaluable information for our understanding of the composition and assembly of the individual MRC complexes, and also of their association into larger supercomplexes (SCs) and respirasomes. The formation of the SCs has been highly debated, and their assembly and regulation mechanisms are still unclear. Previous studies demonstrated a prominent role for COX7A2L (SCAFI) as a structural protein bridging the association of individual MRC complexes III and IV in the minor SC III2 + IV, although its relevance for respirasome formation and function remains controversial. In this work, we have used SILAC-based complexome profiling to dissect the structural organization of the human MRC in HEK293T cells depleted of SCAFI (SCAFIKO) by CRISPR-Cas9 genome editing. SCAFI ablation led to a preferential loss of SC III2 + IV and of a minor subset of respirasomes without affecting OXPHOS function. Our data suggest that the loss of SCAFI-dependent respirasomes in SCAFIKO cells is mainly due to alterations on early stages of CI assembly, without impacting the biogenesis of complexes III and IV. Contrary to the idea of SCAFI being the main player in respirasome formation, SILAC-complexome profiling showed that, in wild-type cells, the majority of respirasomes (ca. 70%) contained COX7A2 and that these species were present at roughly the same levels when SCAFI was knocked-out. We thus demonstrate the co-existence of structurally distinct respirasomes defined by the preferential binding of complex IV via COX7A2, rather than SCAFI, in human cultured cells.

The protein methylome in mammalian mitochondria has been little studied until recently. Here, we describe that lysine-368 of human citrate synthase is methylated and that the modifying enzyme, localized in the mitochondrial matrix, is methyltransferase-like protein 12 (METTL12), a member of the family of 7β-strand methyltransferases. Lysine-368 is near the active site of citrate synthase, but removal of methylation has no effect on its activity. In mitochondria, it is possible that some or all of the enzymes of the citric acid cycle, including citrate synthase, are organized in metabolons to facilitate the channelling of substrates between participating enzymes. Thus, possible roles for the methylation of Lys-368 are in controlling substrate channelling itself, or in influencing protein-protein interactions in the metabolon.

Summary Acetyl coenzyme A (AcCoA), a key intermediate in mitochondrial metabolism, N-acetylates lysine residues, disrupting and, in some cases, regulating protein function. The mitochondrial lysine deacetylase Sirtuin 3 (Sirt3) reverses this modification with benefits reported in diabetes, obesity, and aging. We show that non-enzymatic lysine N-acetylation by AcCoA is greatly enhanced by initial acetylation of a cysteine residue, followed by SN-transfer of the acetyl moiety to a nearby lysine on mitochondrial proteins and synthetic peptides. The frequent occurrence of an S-acetyl intermediate before lysine N-acetylation suggests that proximity to a thioester is a key determinant of lysine susceptibility to acetylation. The thioesterase glyoxalase II (Glo2) can limit protein S-acetylation, thereby preventing subsequent lysine N-acetylation. This suggests that the hitherto obscure role of Glo2 in mitochondria is to act upstream of Sirt3 in minimizing protein N-acetylation, thus limiting protein dysfunction when AcCoA accumulates., Graphical Abstract, Highlights • AcCoA and acetylglutathione reversibly acetylate protein cysteine residues • Non-enzymatic lysine acetylation proceeds via a proximal S-acetylated thiol intermediate • Glyoxalase II and glutathione limit lysine N-acetylation and N-succinylation • These findings have implications for N-acetylation of lysines in regulation and pathology, James et al. show that the non-enzymatic N-acetylation of lysine residues in mitochondrial proteins frequently occurs via a proximal S-acetylated thiol intermediate. Glutathione equilibrates with this intermediate, allowing the thioesterase glyoxalase II to limit protein lysine N-acetylation. These findings expand our understanding of how protein acetylation arises.

Complex I (NADH ubiquinone oxidoreductase) in mammalian mitochondria is an L-shaped assembly of 45 proteins. One arm lies in the inner membrane, and the other extends about 100 Å into the matrix of the organelle. The extrinsic arm contains binding sites for NADH, the primary electron acceptor FMN, and seven iron-sulfur clusters that form a pathway for electrons linking FMN to the terminal electron acceptor, ubiquinone, which is bound in a tunnel in the region of the junction between the arms. The membrane arm contains four antiporter-like domains, energetically coupled to the quinone site and involved in pumping protons from the matrix into the intermembrane space contributing to the proton motive force. Seven of the subunits, forming the core of the membrane arm, are translated from mitochondrial genes, and the remaining subunits, the products of nuclear genes, are imported from the cytosol. Their assembly is coordinated by at least thirteen extrinsic assembly factor proteins that are not part of the fully assembled complex. They assist in insertion of co-factors and in building up the complex from smaller sub-assemblies. One such factor, NDUFAF5, belongs to the family of seven-β-strand S-adenosylmethionine-dependent methyltransferases. However, similar to another family member, RdmB, it catalyzes the introduction of a hydroxyl group, in the case of NDUFAF5, into Arg-73 in the NDUFS7 subunit of human complex I. This modification occurs early in the pathway of assembly of complex I, before the formation of the juncture between peripheral and membrane arms.

The permeability transition in human mitochondria refers to the opening of a nonspecific channel, known as the permeability transition pore (PTP), in the inner membrane. Opening can be triggered by calcium ions, leading to swelling of the organelle, disruption of the inner membrane, and ATP synthesis, followed by cell death. Recent proposals suggest that the pore is associated with the ATP synthase complex and specifically with the ring of c-subunits that constitute the membrane domain of the enzyme's rotor. The c-subunit is produced from three nuclear genes, ATP5G1, ATP5G2, and ATP5G3, encoding identical copies of the mature protein with different mitochondrial-targeting sequences that are removed during their import into the organelle. To investigate the involvement of the c-subunit in the PTP, we generated a clonal cell, HAP1-A12, from near-haploid human cells, in which ATP5G1, ATP5G2, and ATP5G3 were disrupted. The HAP1-A12 cells are incapable of producing the c-subunit, but they preserve the characteristic properties of the PTP. Therefore, the c-subunit does not provide the PTP. The mitochondria in HAP1-A12 cells assemble a vestigial ATP synthase, with intact F1-catalytic and peripheral stalk domains and the supernumerary subunits e, f, and g, but lacking membrane subunits ATP6 and ATP8. The same vestigial complex plus associated c-subunits was characterized from human 143B ρ(0) cells, which cannot make the subunits ATP6 and ATP8, but retain the PTP. Therefore, none of the membrane subunits of the ATP synthase that are involved directly in transmembrane proton translocation is involved in forming the PTP.

Nitrate (NO3-) and nitrite (NO2-) are known to be cardioprotective and to alter energy metabolism in vivo NO3- action results from its conversion to NO2- by salivary bacteria, but the mechanism(s) by which NO2- affects metabolism remains obscure. NO2- may act by S-nitrosating protein thiols, thereby altering protein activity. But how this occurs, and the functional importance of S-nitrosation sites across the mammalian proteome, remain largely uncharacterized. Here we analyzed protein thiols within mouse hearts in vivo using quantitative proteomics to determine S-nitrosation site occupancy. We extended the thiol-redox proteomic technique, isotope-coded affinity tag labeling, to quantify the extent of NO2--dependent S-nitrosation of proteins thiols in vivo Using this approach, called SNOxICAT (S-nitrosothiol redox isotope-coded affinity tag), we found that exposure to NO2- under normoxic conditions or exposure to ischemia alone results in minimal S-nitrosation of protein thiols. However, exposure to NO2- in conjunction with ischemia led to extensive S-nitrosation of protein thiols across all cellular compartments. Several mitochondrial protein thiols exposed to the mitochondrial matrix were selectively S-nitrosated under these conditions, potentially contributing to the beneficial effects of NO2- on mitochondrial metabolism. The permeability of the mitochondrial inner membrane to HNO2, but not to NO2-, combined with the lack of S-nitrosation during anoxia alone or by NO2- during normoxia places constraints on how S-nitrosation occurs in vivo and on its mechanisms of cardioprotection and modulation of energy metabolism. Quantifying S-nitrosated protein thiols now allows determination of modified cysteines across the proteome and identification of those most likely responsible for the functional consequences of NO2- exposure.

The ATP synthase in human mitochondria is a membrane-bound assembly of 29 proteins of 18 kinds. All but two membrane components are encoded in nuclear genes, synthesized on cytoplasmic ribosomes, and imported into the matrix of the organelle, where they are assembled into the complex with ATP6 and ATP8, the products of overlapping genes in mitochondrial DNA. Disruption of individual human genes for the nuclear-encoded subunits in the membrane portion of the enzyme leads to the formation of intermediate vestigial ATPase complexes that provide a description of the pathway of assembly of the membrane domain. The key intermediate complex consists of the F1-c8 complex inhibited by the ATPase inhibitor protein IF1 and attached to the peripheral stalk, with subunits e, f, and g associated with the membrane domain of the peripheral stalk. This intermediate provides the template for insertion of ATP6 and ATP8, which are synthesized on mitochondrial ribosomes. Their association with the complex is stabilized by addition of the 6.8 proteolipid, and the complex is coupled to ATP synthesis at this point. A structure of the dimeric yeast Fo membrane domain is consistent with this model of assembly. The human 6.8 proteolipid (yeast j subunit) locks ATP6 and ATP8 into the membrane assembly, and the monomeric complexes then dimerize via interactions between ATP6 subunits and between 6.8 proteolipids (j subunits). The dimers are linked together back-to-face by DAPIT (diabetes-associated protein in insulin-sensitive tissue; yeast subunit k), forming long oligomers along the edges of the cristae.

Staphylococcus epidermidis has emerged as one of the major nosocomial pathogens associated with infections of implanted medical devices. The most important factor in the pathogenesis of these infections is the formation of bacterial biofilms. Bacteria grown in biofilms are more resistant to antibiotics and to the immune defence system than planktonic bacteria. In these infections, the antimicrobial therapy usually fails and the removal of the biofilm-coated implanted device is the only effective solution. In this study, three proteomic approaches were performed to investigate membrane proteins associated to biofilm formation: (i) sample fractionation by gel electrophoresis, followed by isotopic labelling and LC-MS/MS analysis, (ii) in-solution sample preparation, followed by isotopic labelling and LC-MS/MS analysis and (iii) in-solution sample preparation and label-free LC-MS/MS analysis. We found that the commensal strain S. epidermidis CECT 231 grown in biofilms expressed higher levels of five membrane and membrane-associated proteins involved in pathogenesis: accumulation-associated protein, staphylococcal secretory antigen, signal transduction protein TRAP, ribonuclease Y and phenol soluble modulin beta 1 when compared with bacteria grown under planktonic conditions. These results indicate that a commensal strain can acquire a pathogenic phenotype depending on the mode of growth.

The rotors of ATP synthases turn about 100 times every second. One essential component of the rotor is a ring of hydrophobic c-subunits in the membrane domain of the enzyme. The rotation of these c-rings is driven by a transmembrane proton-motive force, and they turn against a surface provided by another membrane protein, known as subunit a. Together, the rotating c-ring and the static subunit a provide a pathway for protons through the membrane in which the c-ring and subunit a are embedded. Vertebrate and invertebrate c-subunits are well conserved. In the structure of the bovine F1-ATPase-c-ring subcomplex, the 75 amino acid c-subunit is folded into two transmembrane α-helices linked by a short loop. Each bovine rotor-ring consists of eight c-subunits with the N- and C-terminal α-helices forming concentric inner and outer rings, with the loop regions exposed to the phospholipid head-group region on the matrix side of the inner membrane. Lysine-43 is in the loop region and its ε-amino group is completely trimethylated. The role of this modification is unknown. If the trimethylated lysine-43 plays some important role in the functioning, assembly or degradation of the c-ring, it would be expected to persist throughout vertebrates and possibly invertebrates also. Therefore, we have carried out a proteomic analysis of c-subunits across representative species from different classes of vertebrates and from invertebrate phyla. In the twenty-nine metazoan species that have been examined, the complete methylation of lysine-43 is conserved, and it is likely to be conserved throughout the more than two million extant metazoan species. In unicellular eukaryotes and prokaryotes, when the lysine is conserved it is unmethylated, and the stoichiometries of c-subunits vary from 9–15. One possible role for the trimethylated residue is to provide a site for the specific binding of cardiolipin, an essential component of ATP synthases in mitochondria.

The F-ATPases (also called the F

The opening of a non-specific channel, known as the permeability transition pore (PTP), in the inner membranes of mitochondria, can be triggered by calcium ions, leading to swelling of the organelle, disruption of the inner membrane and ATP synthesis, and cell death. Pore opening can be inhibited by cyclosporin A mediated via cyclophilin D. It has been proposed that the pore is associated with the dimeric ATP synthase, and that the OSCP (oligomycin sensitivity conferral protein), a component of the enzyme’s peripheral stalk, provides the site where cyclophilin D interacts. Subunit b contributes a central α-helical structure to the peripheral stalk, extending from near the top of the enzyme’s catalytic domain and crossing the membrane domain of the enzyme via two α-helices. We investigated the possible involvement of the subunit b and the OSCP in the PTP by generating clonal cells, HAP1-Δb and HAP1-ΔOSCP, lacking the membrane domain of subunit b or the OSCP, respectively, in which the correponding genes ATP5F1 and ATP5O had been disrupted. Both cell lines preserve the characteristic properties of the PTP. Therefore, the membrane domain of subunit b does not contribute to the PTP, and the OSCP does not provide the site of interaction with cyclophilin D. The membrane subunits ATP6, ATP8 and subunit c have been eliminated previously from possible participation in the PTP. Therefore, the only subunits of ATP synthase that could participate in pore formation are e, f, g, DAPIT (diabetes associated protein in insulin sensitive tissues) and the 6.8 kDa proteolipid.

UNLABELLED Impaired mitochondrial function is important in obesity and the development of insulin resistance and diabetes. The aim of this study was to identify human adipocyte derived mitochondrial proteins associated with obesity. Mitochondrial proteins from 20 abdominal omental adipose tissue biopsies (13 obese and 7 control subjects) were separated by anion exchange chromatography coupled to SDS PAGE. Protein contents were compared and identified by MALDI TOF TOF mass spectrometry. Proteins of interest were validated verified and quantified using immuno dot blot assays in a total of 76 mitochondrial preparations from both obese and non obese patients. Mass spectrometric comparison of 20 mitochondrial proteomes yielded 62 proteins that were differentially expressed in adipose tissue of obese subjects. The immunological quantification of 12 mitochondrial proteins from 76 omental adipose tissue biopsies revealed four proteins citrate synthase HADHA LETM1 and mitofilin inversely being associated with BMI and mitofilin being inversely correlated with gender. BIOLOGICAL SIGNIFICANCE The finding that obese human subjects have reduced levels of important mitochondrial proteins in adipocytes of omental adipose tissue as compared to non obese controls gives new insights in the impairment of mitochondrial function in this specialized compartment of human adipose tissue in obesity and may eventually lead to the definition of valuable obesity markers.

Nitrate (NO3−) and nitrite (NO2−) are known to be cardioprotective and to alter energy metabolism in vivo. NO3− action results from its conversion to NO2− by salivary bacteria, but the mechanism(s) by which NO2− affects metabolism remains obscure. NO2− may act by S-nitrosating protein thiols, thereby altering protein activity. But how this occurs, and the functional importance of S-nitrosation sites across the mammalian proteome, remain largely uncharacterized. Here we analyzed protein thiols within mouse hearts in vivo using quantitative proteomics to determine S-nitrosation site occupancy. We extended the thiol-redox proteomic technique, isotope-coded affinity tag labeling, to quantify the extent of NO2−-dependent S-nitrosation of proteins thiols in vivo. Using this approach, called SNOxICAT (S-nitrosothiol redox isotope-coded affinity tag), we found that exposure to NO2− under normoxic conditions or exposure to ischemia alone results in minimal S-nitrosation of protein thiols. However, exposure to NO2− in conjunction with ischemia led to extensive S-nitrosation of protein thiols across all cellular compartments. Several mitochondrial protein thiols exposed to the mitochondrial matrix were selectively S-nitrosated under these conditions, potentially contributing to the beneficial effects of NO2− on mitochondrial metabolism. The permeability of the mitochondrial inner membrane to HNO2, but not to NO2−, combined with the lack of S-nitrosation during anoxia alone or by NO2− during normoxia places constraints on how S-nitrosation occurs in vivo and on its mechanisms of cardioprotection and modulation of energy metabolism. Quantifying S-nitrosated protein thiols now allows determination of modified cysteines across the proteome and identification of those most likely responsible for the functional consequences of NO2− exposure.

The biogenesis of human cytochrome c oxidase (COX) is an intricate process in which three mitochondrial DNA (mtDNA)-encoded core subunits are assembled in a coordinated way with at least 11 nucleus-encoded subunits. Many chaperones shared between yeast and humans are involved in COX assembly. Here, we have used a MT-CO3 mutant cybrid cell line to define the composition of assembly intermediates and identify new human COX assembly factors. Quantitative mass spectrometry analysis led us to modify the assembly model from a sequential pathway to a module-based process. Each module contains one of the three core subunits, together with different ancillary components, including HIGD1A. By the same analysis, we identified the short isoform of the myofibrillogenesis regulator 1 (MR-1S) as a new COX assembly factor, which works with the highly conserved PET100 and PET117 chaperones to assist COX biogenesis in higher eukaryotes.

Loss-of-function mutations in TTC19 (tetra-tricopeptide repeat domain 19) have been associated with severe neurological phenotypes and mitochondrial respiratory chain complex III deficiency. We previously demonstrated the mitochondrial localization of TTC19 and its link with complex III biogenesis. Here we provide detailed insight into the mechanistic role of TTC19, by investigating a Ttc19?/? mouse model that shows progressive neurological and metabolic decline, decreased complex III activity, and increased production of reactive oxygen species. By using both the Ttc19?/? mouse model and a range of human cell lines, we demonstrate that TTC19 binds to the fully assembled complex III dimer, i.e., after the incorporation of the iron-sulfur Rieske protein (UQCRFS1). The in situ maturation of UQCRFS1 produces N-terminal polypeptides, which remain bound to holocomplex III. We show that, in normal conditions, these UQCRFS1 fragments are rapidly removed, but when TTC19 is absent they accumulate within complex III, causing its structural and functional impairment.

The glycation of protein and nucleic acids that occurs as a consequence of hyperglycemia disrupts cell function and contributes to many pathologies, including those associated with diabetes and aging. Intracellular glycation occurs after the generation of the reactive 1,2-dicarbonyls methylglyoxal and glyoxal, and disruption of mitochondrial function is associated with hyperglycemia. However, the contribution of these reactive dicarbonyls to mitochondrial damage in pathology is unclear owing to uncertainties about their levels within mitochondria in cells and in vivo. To address this we have developed a mitochondria-targeted reagent (MitoG) designed to assess the levels of mitochondrial dicarbonyls within cells. MitoG comprises a lipophilic triphenylphosphonium cationic function, which directs the molecules to mitochondria within cells, and an o-phenylenediamine moiety that reacts with dicarbonyls to give distinctive and stable products. The extent of accumulation of these diagnostic heterocyclic products can be readily and sensitively quantified by liquid chromatography–tandem mass spectrometry, enabling changes to be determined. Using the MitoG-based analysis we assessed the formation of methylglyoxal and glyoxal in response to hyperglycemia in cells in culture and in the Akita mouse model of diabetes in vivo. These findings indicated that the levels of methylglyoxal and glyoxal within mitochondria increase during hyperglycemia both in cells and in vivo, suggesting that they can contribute to the pathological mitochondrial dysfunction that occurs in diabetes and aging., Highlights • A mitochondria-targeted mass spectrometric probe, MitoG, has been developed to measure glyoxal and methylglyoxal. • Using MitoG we show that mitochondrial glyoxal and methylglyoxal can be measured in hyperglycemic cells. • MitoG can also be used in vivo to infer mitochondrial glyoxal and methylglyoxal production in a mouse model of type I diabetes. • These findings suggest that the accumulation of glyoxal and methylglyoxal within mitochondria may contribute to mitochondrial dysfunction in diabetes.

Mitochondrial respiratory complex I is a product of both the nuclear and mitochondrial genomes. The integration of seven subunits encoded in mitochondrial DNA into the inner membrane, their association with 14 nuclear-encoded membrane subunits, the construction of the extrinsic arm from 23 additional nuclear-encoded proteins, iron-sulfur clusters, and flavin mononucleotide cofactor require the participation of assembly factors. Some are intrinsic to the complex, whereas others participate transiently. The suppression of the expression of the NDUFA11 subunit of complex I disrupted the assembly of the complex, and subcomplexes with masses of 550 and 815 kDa accumulated. Eight of the known extrinsic assembly factors plus a hydrophobic protein, C3orf1, were associated with the subcomplexes. The characteristics of C3orf1, of another assembly factor, TMEM126B, and of NDUFA11 suggest that they all participate in constructing the membrane arm of complex I.

Summary The mitochondrial membrane potential (Δψm) is a major determinant and indicator of cell fate, but it is not possible to assess small changes in Δψm within cells or in vivo. To overcome this, we developed an approach that utilizes two mitochondria-targeted probes each containing a triphenylphosphonium (TPP) lipophilic cation that drives their accumulation in response to Δψm and the plasma membrane potential (Δψp). One probe contains an azido moiety and the other a cyclooctyne, which react together in a concentration-dependent manner by “click” chemistry to form MitoClick. As the mitochondrial accumulation of both probes depends exponentially on Δψm and Δψp, the rate of MitoClick formation is exquisitely sensitive to small changes in these potentials. MitoClick accumulation can then be quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). This approach enables assessment of subtle changes in membrane potentials within cells and in the mouse heart in vivo., Graphical Abstract, Highlights • Mass spectrometry and click chemistry can assess mitochondrial membrane potential • This approach can be applied to investigate membrane potential in cells and in vivo • Hypotheses dependent on small changes in membrane potential can be tested, The mitochondrial membrane potential is central to the organelle’s many functions. Combining mitochondria targeted probes, click chemistry, and mass spectrometry, Logan et al. develop a highly sensitive approach to assess small changes in membrane potential in cells and in vivo, and show its utility in proof-of-principle experiments.

Mitochondrial ribosomes and translation factors co-purify with mitochondrial nucleoids of human cells, based on affinity protein purification of tagged mitochondrial DNA binding proteins. Among the most frequently identified proteins were ATAD3 and prohibitin, which have been identified previously as nucleoid components, using a variety of methods. Both proteins are demonstrated to be required for mitochondrial protein synthesis in human cultured cells, and the major binding partner of ATAD3 is the mitochondrial ribosome. Altered ATAD3 expression also perturbs mtDNA maintenance and replication. These findings suggest an intimate association between nucleoids and the machinery of protein synthesis in mitochondria. ATAD3 and prohibitin are tightly associated with the mitochondrial membranes and so we propose that they support nucleic acid complexes at the inner membrane of the mitochondrion.

The bacterial homologue of C4orf14, YqeH, has been linked to assembly of the small ribosomal subunit. Here, recombinant C4orf14 isolated from human cells, co-purified with the small, 28S subunit of the mitochondrial ribosome and the endogenous protein co-fractionated with the 28S subunit in sucrose gradients. Gene silencing of C4orf14 specifically affected components of the small subunit, leading to decreased protein synthesis in the organelle. The GTPase of C4orf14 was critical to its interaction with the 28S subunit, as was GTP. Therefore, we propose that C4orf14, with bound GTP, binds to components of the 28S subunit facilitating its assembly, and GTP hydrolysis acts as the release mechanism. C4orf14 was also found to be associated with human mitochondrial nucleoids, and C4orf14 gene silencing caused mitochondrial DNA depletion. In vitro C4orf14 is capable of binding to DNA. The association of C4orf14 with mitochondrial translation factors and the mitochondrial nucleoid suggests that the 28S subunit is assembled at the mitochondrial nucleoid, enabling the direct transfer of messenger RNA from the nucleoid to the ribosome in the organelle.

The endoplasmic reticulum (ER)-localized Hsp70 chaperone BiP affects protein folding homeostasis and the response to ER stress. Reversible inactivating covalent modification of BiP is believed to contribute to the balance between chaperones and unfolded ER proteins, but the nature of this modification has so far been hinted at indirectly. We report that deletion of FICD, a gene encoding an ER-localized AMPylating enzyme, abolished detectable modification of endogenous BiP enhancing ER buffering of unfolded protein stress in mammalian cells, whilst deregulated FICD activity had the opposite effect. In vitro, FICD AMPylated BiP to completion on a single residue, Thr518. AMPylation increased, in a strictly FICD-dependent manner, as the flux of proteins entering the ER was attenuated in vivo. In vitro, Thr518 AMPylation enhanced peptide dissociation from BiP 6-fold and abolished stimulation of ATP hydrolysis by J-domain cofactor. These findings expose the molecular basis for covalent inactivation of BiP. DOI: http://dx.doi.org/10.7554/eLife.12621.001, eLife digest Newly made proteins are unstructured chains of amino acids that must fold into particular shapes to work effectively. Proteins that are destined to be exported from the cell undergo folding in a compartment of the cell called the endoplasmic reticulum. This folding is assisted by chaperones (which are themselves also proteins). Cells adjust the number of chaperones so that there are enough to cope with the burden of unfolded proteins. However, in the endoplasmic reticulum, the known mechanisms that regulate the production of chaperones are too slow to track the rapid fluctuations in the production of unfolded proteins. This suggests that other means exist to balance active chaperones and unfolded proteins that go beyond merely controlling chaperone or unfolded protein abundance. An important chaperone protein of the endoplasmic reticulum, called BiP, is chemically modified when the production of unfolded proteins declines, and loses the modification when more unfolded proteins are produced. This suggests that the modification might adjust BiP’s activity so that it can handle the unfolded proteins that are present. However, previous studies have failed to agree about the nature of the chemical modification and how it affects how BiP works. Preissler, Rato et al. compared the activity of BiP in normal mammalian cells and in cells engineered to lack an enzyme called FICD. This enzyme attaches a molecule of adenosine mono-phosphate (AMP) to proteins in a process known as AMPylation. The experiments revealed that AMPylation is the modification of BiP that tracks how many unfolded proteins are in the cell. Further studies showed that AMP attaches to a single amino acid of BiP, number 518, a threonine. Reconstructing the AMPylation of threonine 518 in a test tube caused the modified BiP to lose its ability to engage with unfolded proteins. Overall, Preissler, Rato et al.’s results indicate that cells inactivate BiP by AMPylating threonine 518 as the number of unfolded proteins decreases, and remove the modification to re-activate BiP in response to mounting levels of unfolded proteins. Further studies are now needed to determine how AMPylation inactivates BiP and to understand how the FICD enzyme is regulated so that it performs AMPylation at the right time. It also remains to be explored how important the regulation of BiP activity by AMPylation is for living cells. DOI: http://dx.doi.org/10.7554/eLife.12621.002

Mitochondrial DNA maintenance and segregation are dependent on the actin cytoskeleton in budding yeast. We found two cytoskeletal proteins among six proteins tightly associated with rat liver mitochondrial DNA: non-muscle myosin heavy chain IIA and β-actin. In human cells, transient gene silencing of MYH9 (encoding non-muscle myosin heavy chain IIA), or the closely related MYH10 gene (encoding non-muscle myosin heavy chain IIB), altered the topology and increased the copy number of mitochondrial DNA; and the latter effect was enhanced when both genes were targeted simultaneously. In contrast, genetic ablation of non-muscle myosin IIB was associated with a 60% decrease in mitochondrial DNA copy number in mouse embryonic fibroblasts, compared to control cells. Gene silencing of β-actin also affected mitochondrial DNA copy number and organization. Protease-protection experiments and iodixanol gradient analysis suggest some β-actin and non-muscle myosin heavy chain IIA reside within human mitochondria and confirm that they are associated with mitochondrial DNA. Collectively, these results strongly implicate the actomyosin cytoskeleton in mammalian mitochondrial DNA maintenance., Medical Research Council; the European Union; the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development and National Heart; Lung and Blood Institute; National Institutes of Health and grants [CMRPG360491-2, 380651, NSC 97-2321-B-182A-002-MY2] from the Chang Gung Memorial Hospital, Lin-Kou, Taiwan (to C.C.M.). Funding for open access charge: Medical Research Council.

Summary Hydrogen peroxide (H2O2) is central to mitochondrial oxidative damage and redox signaling, but its roles are poorly understood due to the difficulty of measuring mitochondrial H2O2 in vivo. Here we report a ratiometric mass spectrometry probe approach to assess mitochondrial matrix H2O2 levels in vivo. The probe, MitoB, comprises a triphenylphosphonium (TPP) cation driving its accumulation within mitochondria, conjugated to an arylboronic acid that reacts with H2O2 to form a phenol, MitoP. Quantifying the MitoP/MitoB ratio by liquid chromatography-tandem mass spectrometry enabled measurement of a weighted average of mitochondrial H2O2 that predominantly reports on thoracic muscle mitochondria within living flies. There was an increase in mitochondrial H2O2 with age in flies, which was not coordinately altered by interventions that modulated life span. Our findings provide approaches to investigate mitochondrial ROS in vivo and suggest that while an increase in overall mitochondrial H2O2 correlates with aging, it may not be causative., Highlights ► A mitochondria-targeted mass spectrometry probe measures mitochondrial H2O2 in vivo ► Overall mitochondrial H2O2 increases with age but can be independent of life span ► Increased physical activity leads to a decrease in mitochondrial H2O2 ► Hypotheses dependent on overall mitochondrial ROS can now be assessed in vivo

Protein cysteine residues are central to redox signaling and to protection against oxidative damage through their interactions with reactive oxygen and nitrogen species, and electrophiles. Although there is considerable evidence for a functional role for cysteine modifications, the identity and physiological significance of most protein thiol alterations are unknown. One way to identify candidate proteins involved in these processes is to utilize the proteomic methodologies that have been developed in recent years for the identification of proteins that undergo cysteine modification in response to redox signals or oxidative damage. These tools have proven effective in uncovering novel protein targets of redox modification and are important first steps that allow for a better understanding of how reactive molecules may contribute to signaling and damage. Here, we discuss a number of these approaches and their application to the identification of a variety of cysteine-centered redox modifications.

Modern α-proteobacteria are thought to be closely related to the ancient symbiont of eukaryotes, an ancestor of mitochondria. Respiratory complex I from α-proteobacteria and mitochondria is well conserved at the level of the 14 "core" subunits, consistent with that notion. Mitochondrial complex I contains the core subunits, present in all species, and up to 31 "supernumerary" subunits, generally thought to have originated only within eukaryotic lineages. However, the full protein composition of an α-proteobacterial complex I has not been established previously. Here, we report the first purification and characterization of complex I from the α-proteobacterium Paracoccus denitrificans. Single particle electron microscopy shows that the complex has a well defined L-shape. Unexpectedly, in addition to the 14 core subunits, the enzyme also contains homologues of three supernumerary mitochondrial subunits as follows: B17.2, AQDQ/18, and 13 kDa (bovine nomenclature). This finding suggests that evolution of complex I via addition of supernumerary or "accessory" subunits started before the original endosymbiotic event that led to the creation of the eukaryotic cell. It also provides further confirmation that α-proteobacteria are the closest extant relatives of mitochondria.

In the structure of bovine F1-ATPase inhibited with residues 1–60 of the bovine inhibitor protein IF1, the α-helical inhibitor interacts with five of the nine subunits of F1-ATPase. In order to understand the contributions of individual amino acid residues to this complex binding mode, N-terminal deletions and point mutations have been introduced, and the binding properties of each mutant inhibitor protein have been examined. The N-terminal region of IF1 destabilizes the interaction of the inhibitor with F1-ATPase and may assist in removing the inhibitor from its binding site when F1Fo-ATPase is making ATP. Binding energy is provided by hydrophobic interactions between residues in the long α-helix of IF1 and the C-terminal domains of the βDP-subunit and βTP-subunit and a salt bridge between residue E30 in the inhibitor and residue R408 in the C-terminal domain of the βDP-subunit. Several conserved charged amino acids in the long α-helix of IF1 are also required for establishing inhibitory activity, but in the final inhibited state, they are not in contact with F1-ATPase and occupy aqueous cavities in F1-ATPase. They probably participate in the pathway from the initial interaction of the inhibitor and the enzyme to the final inhibited complex observed in the structure, in which two molecules of ATP are hydrolysed and the rotor of the enzyme turns through two 120° steps. These findings contribute to the fundamental understanding of how the inhibitor functions and to the design of new inhibitors for the systematic analysis of the catalytic cycle of the enzyme., Graphical Abstract

Mitochondria sustain damage with aging, and the resulting mitochondrial dysfunction has been implicated in a number of diseases including Parkinson disease. We recently demonstrated that the E3 ubiquitin ligase Parkin, which is linked to recessive forms of parkinsonism, causes a dramatic increase in mitophagy and a change in mitochondrial distribution, following its translocation from the cytosol to mitochondria. Investigating how Parkin induces these changes may offer insight into the mechanisms that lead to the sequestration and elimination of damaged mitochondria. We report that following Parkin’s translocation from the cytosol to mitochondria, Parkin (but not a pathogenic mutant) promotes the K63-linked polyubiquitination of mitochondrial substrate(s) and recruits the ubiquitin- and LC3-binding protein, p62/SQSTM1, to mitochondria. After its recruitment, p62/SQSTM1 mediates the aggregation of dysfunctional mitochondria through polymerization via its PB1 domain, in a manner analogous to its aggregation of polyubiquitinated proteins. Surprisingly and in contrast to what has been recently reported for ubiquitin-induced pexophagy and xenophagy, p62 appears to be dispensable for mitophagy. Similarly, mitochondrial-anchored ubiquitin is sufficient to recruit p62 and promote mitochondrial clustering, but does not promote mitophagy. Although VDAC1 (but not VDAC2) is ubiquitinated following mitochondrial depolarization, we find VDAC1 cannot fully account for the mitochondrial K63-linked ubiquitin immunoreactivity observed following depolarization, as it is also observed in VDAC1/3-/- mouse embryonic fibroblasts. Additionally, we find VDAC1 and VDAC3 are dispensable for the recruitment of p62, mitochondrial clustering and mitophagy. These results demonstrate that mitochondria are aggregated by p62, following its recruitment by Parkin in a VDAC1-independent manner. They also suggest that proteins other than p62 are likely required for mitophagy downstream of Parkin substrates other than VDAC1.