Loss of endogenous estrogen alters mitochondrial metabolism and muscle clock-related protein Rbm20 in female mdx mice.

Female carriers of a Duchenne muscular dystrophy (DMD) gene mutation manifest exercise intolerance and metabolic anomalies that may be exacerbated following menopause due to the loss of estrogen, a known regulator of skeletal muscle function and metabolism. Here, we studied the impact of estrogen depletion (via ovariectomy) on exercise tolerance and muscle mitochondrial metabolism in female mdx mice and the potential of estrogen replacement therapy (using estradiol) to protect against functional and metabolic perturbations. We also investigated the effect of estrogen depletion, and replacement, on the skeletal muscle proteome through an untargeted proteomic approach with TMT-labelling. Our study confirms that loss of estrogen in female mdx mice reduces exercise capacity, tricarboxylic acid cycle intermediates, and citrate synthase activity but that these deficits are offset through estrogen replacement therapy. Furthermore, ovariectomy downregulated protein expression of RNA-binding motif factor 20 (Rbm20), a critical regulator of sarcomeric and muscle homeostasis gene splicing, which impacted pathways involving ribosomal and mitochondrial translation. Estrogen replacement modulated Rbm20 protein expression and promoted metabolic processes and the upregulation of proteins involved in mitochondrial dynamics and metabolism. Our data suggest that estrogen mitigates dystrophinopathic features in female mdx mice and that estrogen replacement may be a potential therapy for post-menopausal DMD carriers.

Human Tim8a, Tim8b and Tim13 are auxiliary assembly factors of mature Complex IV.

Human Tim8a and Tim8b are paralogous intermembrane space proteins of the small TIM chaperone family. Yeast small TIMs function in the trafficking of proteins to the outer and inner mitochondrial membranes. This putative import function for hTim8a and hTim8b has been challenged in human models, but their precise molecular function(s) remains undefined. Likewise, the necessity for human cells to encode two Tim8 proteins and whether any potential redundancy exists is unclear. We demonstrate that hTim8a and hTim8b function in the assembly of cytochrome c oxidase (Complex IV). Using affinity enrichment mass spectrometry, we define the interaction network of hTim8a, hTim8b and hTim13, identifying subunits and assembly factors of the Complex IV COX2 module. hTim8-deficient cells have a COX2 and COX3 module defect and exhibit an accumulation of the Complex IV S2 subcomplex. These data suggest that hTim8a and hTim8b function in assembly of Complex IV via interactions with intermediate-assembly subcomplexes. We propose that hTim8-hTim13 complexes are auxiliary assembly factors involved in the formation of the Complex IV S3 subcomplex during assembly of mature Complex IV.

Mitochondrial COA7 is a heme-binding protein with disulfide reductase activity, which acts in the early stages of complex IV assembly.

Cytochrome c oxidase (COX) assembly factor 7 (COA7) is a metazoan-specific assembly factor, critical for the biogenesis of mitochondrial complex IV (cytochrome c oxidase). Although mutations in COA7 have been linked to complex IV assembly defects and neurological conditions such as peripheral neuropathy, ataxia, and leukoencephalopathy, the precise role COA7 plays in the biogenesis of complex IV is not known. Here, we show that loss of COA7 blocks complex IV assembly after the initial step where the COX1 module is built, progression from which requires the incorporation of copper and addition of the COX2 and COX3 modules. The crystal structure of COA7, determined to 2.4 Å resolution, reveals a banana-shaped molecule composed of five helix-turn-helix (α/α) repeats, tethered by disulfide bonds. COA7 interacts transiently with the copper metallochaperones SCO1 and SCO2 and catalyzes the reduction of disulfide bonds within these proteins, which are crucial for copper relay to COX2. COA7 binds heme with micromolar affinity, through axial ligation to the central iron atom by histidine and methionine residues. We therefore propose that COA7 is a heme-binding disulfide reductase for regenerating the copper relay system that underpins complex IV assembly.

Sideroflexin 4 is a complex I assembly factor that interacts with the MCIA complex and is required for the assembly of the ND2 module.

SignificanceMitochondria are double-membraned eukaryotic organelles that house the proteins required for generation of ATP, the energy currency of cells. ATP generation within mitochondria is performed by five multisubunit complexes (complexes I to V), the assembly of which is an intricate process. Mutations in subunits of these complexes, or the suite of proteins that help them assemble, lead to a severe multisystem condition called mitochondrial disease. We show that SFXN4, a protein that causes mitochondrial disease when mutated, assists with the assembly of complex I. This finding explains why mutations in SFXN4 cause mitochondrial disease and is surprising because SFXN4 belongs to a family of amino acid transporter proteins, suggesting that it has undergone a dramatic shift in function through evolution.

Optic atrophy-associated TMEM126A is an assembly factor for the ND4-module of mitochondrial complex I.

Mitochondrial disease is a debilitating condition with a diverse genetic etiology. Here, we report that TMEM126A, a protein that is mutated in patients with autosomal-recessive optic atrophy, participates directly in the assembly of mitochondrial complex I. Using a combination of genome editing, interaction studies, and quantitative proteomics, we find that loss of TMEM126A results in an isolated complex I deficiency and that TMEM126A interacts with a number of complex I subunits and assembly factors. Pulse-labeling interaction studies reveal that TMEM126A associates with the newly synthesized mitochondrial DNA (mtDNA)-encoded ND4 subunit of complex I. Our findings indicate that TMEM126A is involved in the assembly of the ND4 distal membrane module of complex I. In addition, we find that the function of TMEM126A is distinct from its paralogue TMEM126B, which acts in assembly of the ND2-module of complex I.

Inwardly rectifying potassium channels mediate polymyxin-induced nephrotoxicity.

Polymyxin antibiotics are often used as a last-line defense to treat life-threatening Gram-negative pathogens. However, polymyxin-induced kidney toxicity is a dose-limiting factor of paramount importance and can lead to suboptimal treatment. To elucidate the mechanism and develop effective strategies to overcome polymyxin toxicity, we employed a whole-genome CRISPR screen in human kidney tubular HK-2 cells and identified 86 significant genes that upon knock-out rescued polymyxin-induced toxicity. Specifically, we discovered that knockout of the inwardly rectifying potassium channels Kir4.2 and Kir5.1 (encoded by KCNJ15 and KCNJ16, respectively) rescued polymyxin-induced toxicity in HK-2 cells. Furthermore, we found that polymyxins induced cell depolarization via Kir4.2 and Kir5.1 and a significant cellular uptake of polymyxins was evident. All-atom molecular dynamics simulations revealed that polymyxin B1 spontaneously bound to Kir4.2, thereby increasing opening of the channel, resulting in a potassium influx, and changes of the membrane potential. Consistent with these findings, small molecule inhibitors (BaCl2 and VU0134992) of Kir potassium channels reduced polymyxin-induced toxicity in cell culture and mouse explant kidney tissue. Our findings provide critical mechanistic information that will help attenuate polymyxin-induced nephrotoxicity in patients and facilitate the design of novel, safer polymyxins.

Biallelic pathogenic variants in COX11 are associated with an infantile-onset mitochondrial encephalopathy.

Primary mitochondrial diseases are a group of genetically and clinically heterogeneous disorders resulting from oxidative phosphorylation (OXPHOS) defects. COX11 encodes a copper chaperone that participates in the assembly of complex IV and has not been previously linked to human disease. In a previous study, we identified that COX11 knockdown decreased cellular adenosine triphosphate (ATP) derived from respiration, and that ATP levels could be restored with coenzyme Q10 (CoQ10 ) supplementation. This finding is surprising since COX11 has no known role in CoQ10 biosynthesis. Here, we report a novel gene-disease association by identifying biallelic pathogenic variants in COX11 associated with infantile-onset mitochondrial encephalopathies in two unrelated families using trio genome and exome sequencing. Functional studies showed that mutant COX11 fibroblasts had decreased ATP levels which could be rescued by CoQ10 . These results not only suggest that COX11 variants cause defects in energy production but reveal a potential metabolic therapeutic strategy for patients with COX11 variants.

Accessory subunits are integral for assembly and function of human mitochondrial complex I.

Complex I (NADH:ubiquinone oxidoreductase) is the first enzyme of the mitochondrial respiratory chain and is composed of 45 subunits in humans, making it one of the largest known multi-subunit membrane protein complexes. Complex I exists in supercomplex forms with respiratory chain complexes III and IV, which are together required for the generation of a transmembrane proton gradient used for the synthesis of ATP. Complex I is also a major source of damaging reactive oxygen species and its dysfunction is associated with mitochondrial disease, Parkinson's disease and ageing. Bacterial and human complex I share 14 core subunits that are essential for enzymatic function; however, the role and necessity of the remaining 31 human accessory subunits is unclear. The incorporation of accessory subunits into the complex increases the cellular energetic cost and has necessitated the involvement of numerous assembly factors for complex I biogenesis. Here we use gene editing to generate human knockout cell lines for each accessory subunit. We show that 25 subunits are strictly required for assembly of a functional complex and 1 subunit is essential for cell viability. Quantitative proteomic analysis of cell lines revealed that loss of each subunit affects the stability of other subunits residing in the same structural module. Analysis of proteomic changes after the loss of specific modules revealed that ATP5SL and DMAC1 are required for assembly of the distal portion of the complex I membrane arm. Our results demonstrate the broad importance of accessory subunits in the structure and function of human complex I. Coupling gene-editing technology with proteomics represents a powerful tool for dissecting large multi-subunit complexes and enables the study of complex dysfunction at a cellular level.

The Mitochondrial Acyl-carrier Protein Interaction Network Highlights Important Roles for LYRM Family Members in Complex I and Mitoribosome Assembly.

NDUFAB1 is the mitochondrial acyl carrier protein (ACP) essential for cell viability. Through its pantetheine-4'-phosphate post-translational modification, NDUFAB1 interacts with members of the leucine-tyrosine-arginine motif (LYRM) protein family. Although several LYRM proteins have been described to participate in a variety of defined processes, the functions of others remain either partially or entirely unknown. We profiled the interaction network of NDUFAB1 to reveal associations with 9 known LYRM proteins as well as more than 20 other proteins involved in mitochondrial respiratory chain complex and mitochondrial ribosome assembly. Subsequent knockout and interaction network studies in human cells revealed the LYRM member AltMiD51 to be important for optimal assembly of the large mitoribosome subunit, consistent with recent structural studies. In addition, we used proteomics coupled with topographical heat-mapping to reveal that knockout of LYRM2 impairs assembly of the NADH-dehydrogenase module of complex I, leading to defects in cellular respiration. Together, this work adds to the catalogue of functions executed by LYRM family of proteins in building mitochondrial complexes and emphasizes the common and essential role of NDUFAB1 as a protagonist in mitochondrial metabolism.

Abnormalities of mitochondrial dynamics and bioenergetics in neuronal cells from CDKL5 deficiency disorder.

CDKL5 deficiency disorder (CDD) is a rare neurodevelopmental disorder caused by pathogenic variants in the Cyclin-dependent kinase-like 5 (CDKL5) gene, resulting in dysfunctional CDKL5 protein. It predominantly affects females and causes seizures in the first few months of life, ultimately resulting in severe intellectual disability. In the absence of targeted therapies, treatment is currently only symptomatic. CDKL5 is a serine/threonine kinase that is highly expressed in the brain, with a critical role in neuronal development. Evidence of mitochondrial dysfunction in CDD is gathering, but has not been studied extensively. We used human patient-derived induced pluripotent stem cells with a pathogenic truncating mutation (p.Arg59*) and CRISPR/Cas9 gene-corrected isogenic controls, differentiated into neurons, to investigate the impact of CDKL5 mutation on cellular function. Quantitative proteomics indicated mitochondrial defects in CDKL5 p.Arg59* neurons, and mitochondrial bioenergetics analysis confirmed decreased activity of mitochondrial respiratory chain complexes. Additionally, mitochondrial trafficking velocity was significantly impaired, and there was a higher percentage of stationary mitochondria. We propose mitochondrial dysfunction is contributing to CDD pathology, and should be a focus for development of targeted treatments for CDD.

Building a complex complex: Assembly of mitochondrial respiratory chain complex I.

Mitochondrial complex I is the primary entry point for electrons into the electron transport chain, required for the bulk of cellular ATP production via oxidative phosphorylation. Complex I consists of 45 subunits, which are encoded by both nuclear and mitochondrial DNA. Currently, at least 15 assembly factors are known to be required for the complete maturation of complex I. Mutations in the genes encoding subunits and assembly factors lead to complex I deficiency, which can manifest as mitochondrial disease. The current model of complex I assembly suggests that the enzyme is built by the association of a set of smaller intermediate modules containing specific conserved core subunits and additional accessory subunits. Each module must converge in a spatially and temporally orchestrated fashion to allow assembly of the mature holoenzyme to occur. This review outlines the current understanding of complex I biogenesis, with an emphasis on the assembly factors that facilitate the building of this architectural giant.

A homozygous variant in NDUFA8 is associated with developmental delay, microcephaly, and epilepsy due to mitochondrial complex I deficiency.

Mitochondrial complex I deficiency is caused by pathogenic variants in mitochondrial and nuclear genes associated with complex I structure and assembly. We report the case of a patient with　NDUFA8-related mitochondrial disease. The patient presented with developmental delay, microcephaly, and epilepsy. His fibroblasts showed apparent biochemical defects in mitochondrial complex I. Whole-exome sequencing revealed that the patient carried a homozygous variant in NDUFA8. His fibroblasts showed a reduction in the protein expression level of not only NDUFA8, but also the other complex I subunits, consistent with assembly defects. The enzyme activity of complex I and oxygen consumption rate were restored by reintroducing wild-typeNDUFA8 cDNA into patient fibroblasts. The functional properties of the variant in NDUFA8 were also investigated using NDUFA8 knockout cells expressing wild-type or mutated NDUFA8 cDNA. These experiments further supported the pathogenicity of the variant in complex I assembly. This is the first report describing that the loss of NDUFA8, which has not previously been associated with mitochondrial disease, causes severe defect in the assembly of mitochondrial complex I, leading to progressive neurological and developmental abnormalities.

A patient with homozygous nonsense variants in two Leigh syndrome disease genes: Distinguishing a dual diagnosis from a hypomorphic protein-truncating variant.

Leigh syndrome is a mitochondrial disease caused by pathogenic variants in over 85 genes. Whole exome sequencing of a patient with Leigh-like syndrome identified homozygous protein-truncating variants in two genes associated with Leigh syndrome; a reported pathogenic variant in PDHX (NP_003468.2:p.(Arg446*)), and an uncharacterized variant in complex I (CI) assembly factor TIMMDC1 (NP_057673.2:p.(Arg225*)). The TIMMDC1 variant was predicted to truncate 61 amino acids at the C-terminus and functional studies demonstrated a hypomorphic impact of the variant on CI assembly. However, the mutant protein could still rescue CI assembly in TIMMDC1 knockout cells and the patient's clinical phenotype was not clearly distinct from that of other patients with the same PDHX defect. Our data suggest that the hypomorphic effect of the TIMMDC1 protein-truncating variant does not constitute a dual diagnosis in this individual. We recommend cautious assessment of variants in the C-terminus of TIMMDC1 and emphasize the need to consider the caveats detailed within the American College of Medical Genetics and Genomics (ACMG) criteria when assessing variants.

Biallelic Mutations in TMEM126B Cause Severe Complex I Deficiency with a Variable Clinical Phenotype.

Complex I deficiency is the most common biochemical phenotype observed in individuals with mitochondrial disease. With 44 structural subunits and over 10 assembly factors, it is unsurprising that complex I deficiency is associated with clinical and genetic heterogeneity. Massively parallel sequencing (MPS) technologies including custom, targeted gene panels or unbiased whole-exome sequencing (WES) are hugely powerful in identifying the underlying genetic defect in a clinical diagnostic setting, yet many individuals remain without a genetic diagnosis. These individuals might harbor mutations in poorly understood or uncharacterized genes, and their diagnosis relies upon characterization of these orphan genes. Complexome profiling recently identified TMEM126B as a component of the mitochondrial complex I assembly complex alongside proteins ACAD9, ECSIT, NDUFAF1, and TIMMDC1. Here, we describe the clinical, biochemical, and molecular findings in six cases of mitochondrial disease from four unrelated families affected by biallelic (c.635G>T [p.Gly212Val] and/or c.401delA [p.Asn134Ilefs(∗)2]) TMEM126B variants. We provide functional evidence to support the pathogenicity of these TMEM126B variants, including evidence of founder effects for both variants, and establish defects within this gene as a cause of complex I deficiency in association with either pure myopathy in adulthood or, in one individual, a severe multisystem presentation (chronic renal failure and cardiomyopathy) in infancy. Functional experimentation including viral rescue and complexome profiling of subject cell lines has confirmed TMEM126B as the tenth complex I assembly factor associated with human disease and validates the importance of both genome-wide sequencing and proteomic approaches in characterizing disease-associated genes whose physiological roles have been previously undetermined.

Preservation of skeletal muscle mitochondrial content in older adults: relationship between mitochondria, fibre type and high-intensity exercise training.

KEY POINTS: Ageing is associated with an upregulation of mitochondrial dynamics proteins mitofusin 2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) in human skeletal muscle with the increased abundance of Mfn2 being exclusive to type II muscle fibres. These changes occur despite a similar content of mitochondria, as measured by COXIV, NDUFA9 and complexes in their native states (Blue Native PAGE). Following 12 weeks of high-intensity training (HIT), older adults exhibit a robust increase in mitochondria content, while there is a decline in Mfn2 in type II fibres. We propose that the upregulation of Mfn2 and MiD49 with age may be a protective mechanism to protect against mitochondrial dysfunction, in particularly in type II skeletal muscle fibres, and that exercise may have a unique protective effect negating the need for an increased turnover of mitochondria. ABSTRACT: Mitochondrial dynamics proteins are critical for mitochondrial turnover and maintenance of mitochondrial health. High-intensity interval training (HIT) is a potent training modality shown to upregulate mitochondrial content in young adults but little is known about the effects of HIT on mitochondrial dynamics proteins in older adults. This study investigated the abundance of protein markers for mitochondrial dynamics and mitochondrial content in older adults compared to young adults. It also investigated the adaptability of mitochondria to 12 weeks of HIT in older adults. Both older and younger adults showed a higher abundance of mitochondrial respiratory chain subunits COXIV and NDUFA9 in type I compared with type II fibres, with no difference between the older adults and young groups. In whole muscle homogenates, older adults had higher mitofusin-2 (Mfn2) and mitochondrial dynamics protein 49 (MiD49) contents compared to the young group. Also, older adults had higher levels of Mfn2 in type II fibres compared with young adults. Following HIT in older adults, MiD49 and Mfn2 levels were not different in whole muscle and Mfn2 content decreased in type II fibres. Increases in citrate synthase activity (55%) and mitochondrial respiratory chain subunits COXIV (37%) and NDUFA9 (48%) and mitochondrial respiratory chain complexes (∼70-100%) were observed in homogenates and/or single fibres. These findings reveal (i) a similar amount of mitochondria in muscle from young and healthy older adults and (ii) a robust increase of mitochondrial content following 12 weeks of HIT exercise in older adults.

Characterization of mitochondrial FOXRED1 in the assembly of respiratory chain complex I.

Human mitochondrial complex I is the largest enzyme of the respiratory chain and is composed of 44 different subunits. Complex I subunits are encoded by both nuclear and mitochondrial (mt) DNA and their assembly requires a number of additional proteins. FAD-dependent oxidoreductase domain-containing protein 1 (FOXRED1) was recently identified as a putative assembly factor and FOXRED1 mutations in patients cause complex I deficiency; however, its role in assembly is unknown. Here, we demonstrate that FOXRED1 is involved in mid-late stages of complex I assembly. In a patient with FOXRED1 mutations, the levels of mature complex I were markedly decreased, and a smaller ∼475 kDa subcomplex was detected. In the absence of FOXRED1, mtDNA-encoded complex I subunits are still translated and transiently assembled into a late stage ∼815 kDa intermediate; but instead of transitioning further to the mature complex I, the intermediate breaks down to an ∼475 kDa complex. As the patient cells contained residual assembled complex I, we disrupted the FOXRED1 gene in HEK293T cells through TALEN-mediated gene editing. Cells lacking FOXRED1 had ∼10% complex I levels, reduced complex I activity, and were unable to grow on galactose media. Interestingly, overexpression of FOXRED1 containing the patient mutations was able to rescue complex I assembly. In addition, FOXRED1 was found to co-immunoprecipitate with a number of complex I subunits. Our studies reveal that FOXRED1 is a crucial component in the productive assembly of complex I and that mutations in FOXRED1 leading to partial loss of function cause defects in complex I biogenesis.

Gene knockout using transcription activator-like effector nucleases (TALENs) reveals that human NDUFA9 protein is essential for stabilizing the junction between membrane and matrix arms of complex I.

Transcription activator-like effector nucleases (TALENs) represent a promising approach for targeted knock-out of genes in cultured human cells. We used TALEN-technology to knock out the nuclear gene encoding NDUFA9, a subunit of mitochondrial respiratory chain complex I in HEK293T cells. Screening for the knock-out revealed a mixture of NDUFA9 cell clones that harbored partial deletions of the mitochondrial N-terminal targeting signal but were still capable of import. A cell line lacking functional copies of both NDUFA9 alleles resulted in a loss of NDUFA9 protein expression, impaired assembly of complex I, and cells incapable of growth in galactose medium. Cells lacking NDUFA9 contained a complex I subcomplex consisting of membrane arm subunits but not marker subunits of the matrix arm. Re-expression of NDUFA9 restored the defects in complex I assembly. We conclude that NDUFA9 is involved in stabilizing the junction between membrane and matrix arms of complex I, a late assembly step critical for complex I biogenesis and activity.

Phosphoproteomics-directed manipulation reveals SEC22B as a hepatocellular signaling node governing metabolic actions of glucagon.

The peptide hormone glucagon is a fundamental metabolic regulator that is also being considered as a pharmacotherapeutic option for obesity and type 2 diabetes. Despite this, we know very little regarding how glucagon exerts its pleiotropic metabolic actions. Given that the liver is a chief site of action, we performed in situ time-resolved liver phosphoproteomics to reveal glucagon signaling nodes. Through pathway analysis of the thousands of phosphopeptides identified, we reveal "membrane trafficking" as a dominant signature with the vesicle trafficking protein SEC22 Homolog B (SEC22B) S137 phosphorylation being a top hit. Hepatocyte-specific loss- and gain-of-function experiments reveal that SEC22B was a key regulator of glycogen, lipid and amino acid metabolism, with SEC22B-S137 phosphorylation playing a major role in glucagon action. Mechanistically, we identify several protein binding partners of SEC22B affected by glucagon, some of which were differentially enriched with SEC22B-S137 phosphorylation. In summary, we demonstrate that phosphorylation of SEC22B is a hepatocellular signaling node mediating the metabolic actions of glucagon and provide a rich resource for future investigations on the biology of glucagon action.

Two independent respiratory chains adapt OXPHOS performance to glycolytic switch.

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.

SILAC-based complexome profiling dissects the structural organization of the human respiratory supercomplexes in SCAFIKO cells.

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.