NDUFA2 (Complex I-B8/CI-B8) is a small (99 aa) nuclear-encoded accessory (supernumerary) subunit of mitochondrial NADH:ubiquinone oxidoreductase (respiratory chain Complex I). It resides in the peripheral (matrix) arm of the holoenzyme as a peripheral membrane protein on the matrix side of the mitochondrial inner membrane. The isolated subunit adopts a thioredoxin-like fold and carries a redox-active Cys24-Cys58 disulfide, but it is a non-catalytic structural subunit and is not itself an electron-transfer/redox catalyst; the 14 conserved core subunits perform Complex I catalysis. NDUFA2 is required for proper assembly and stability of Complex I, and loss-of-function variants cause autosomal-recessive mitochondrial complex I deficiency (Leigh disease/leukoencephalopathy, MC1DN13).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0045271 respiratory chain complex I | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) placement of NDUFA2 in respiratory chain complex I. This matches the extensive experimental evidence that NDUFA2/B8 is a stable structural subunit of the peripheral arm of Complex I, and is consistent with UniProt. Correct and core. Reason: The PAINT node placing Complex I membership on this family is confirmed for the human protein itself by immunopurification/MS (PMID:12611891), assembly-intermediate tracing (PMID:17209039), knockout complexome profiling (PMID:27626371) and cryo-EM subunit assignment (PMID:28844695). No human-specific loss or divergence argues against the node placement, so the inherited assertion is accepted rather than merely tolerated. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt Accessory subunit of the mitochondrial membrane respiratory |
| GO:0005739 mitochondrion | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Electronic (ARBA) mitochondrial localization. Correct but non-informative relative to the more specific inner-membrane / Complex I localizations that are experimentally supported. Reason: The ARBA model's call is correct but GO:0005739 is an ancestor of the inner-membrane term that NDUFA2 already carries with experimental support, and UniProt localizes it more precisely to the matrix face of the inner membrane. Kept as consistent background. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt Mitochondrion inner membrane |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic inner-membrane localization, consistent with UniProt (peripheral membrane protein on the matrix side of the inner membrane) and with experimental IDA annotations. Correct and core localization. Reason: This reproduces an experimentally evidenced UniProt location rather than a by-similarity one, and matches the compartment resolved by cryo-EM. Accepted as the core localization alongside the IDA row to the same term. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt Mitochondrion inner membrane |
| GO:1902600 proton transmembrane transport | IEA GO_REF:0000108 | MARK AS OVER ANNOTATED | Summary: Electronic term inferred logically from GO:0008137 (NADH dehydrogenase activity). Proton translocation is a property of the Complex I holoenzyme, driven by the membrane-arm antiporter-like core subunits; NDUFA2 is a non-catalytic peripheral-arm accessory subunit and does not itself transport protons. This is a complex/pathway-level process, not the molecular activity of NDUFA2, so it is an over-annotation for this subunit. Reason: A purely logical inter-ontology inference whose premise is the GO:0008137 NAS row that this review marks as an over-annotation; an inference is no stronger than its premise. Independently, the entity that performs proton translocation is the antiporter-like membrane-arm core (ND2/ND4/ND5), whereas NDUFA2 sits in the peripheral matrix arm and is stated by UniProt not to be involved in catalysis, so it does none of the work of this process. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt that is believed not to be |
| GO:0045271 respiratory chain complex I | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) part_of respiratory chain complex I, duplicating the experimentally supported component annotations. Correct and core. Reason: Correct, and the same term is asserted by four independent experimental annotations. Since the claim is not in doubt and the term is the core structural fact about NDUFA2, the electronic call is accepted rather than demoted. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt Complex I is composed of 45 different subunits. |
| GO:0005743 mitochondrial inner membrane | IDA PMID:28844695 Architecture of Human Mitochondrial Respiratory Megacomplex ... | ACCEPT | Summary: IDA inner-membrane localization from cryo-EM of the human respiratory megacomplex (ComplexPortal). NDUFA2 is resolved within Complex I embedded in the inner membrane. Correct and core localization. Reason: Cryo-EM of the intact human megacomplex assigned each Complex I subunit to a position in the structure, which localizes NDUFA2 to the inner membrane by direct observation rather than by inference, and agrees with UniProt's peripheral inner-membrane, matrix-side assignment. Supporting Evidence: PMID:28844695 The structure not only reveals the precise assignment of individual subunits of file:human/NDUFA2/NDUFA2-uniprot.txt Matrix side |
| GO:0009060 aerobic respiration | NAS PMID:30030361 Assembly of mammalian oxidative phosphorylation complexes I-... | KEEP AS NON CORE | Summary: High-level process annotation (ComplexPortal, NAS). NDUFA2 contributes to aerobic respiration only indirectly, as a structural subunit of Complex I. Retained as a non-core, high-level pathway context rather than a specific NDUFA2 function; the more precise NDUFA2 processes are GO:0006120 and Complex I assembly. Reason: The cited source is a review stating that supernumerary subunits serve assembly, regulation and stability roles while the core proteins perform the catalytic activities; it demonstrates no step of aerobic respiration carried out by NDUFA2. The annotation is true of the OXPHOS system NDUFA2 belongs to, so it is kept as context and excluded from the core functions. Supporting Evidence: PMID:30030361 supernumerary' subunits that play |
| GO:0042776 proton motive force-driven mitochondrial ATP synthesis | NAS PMID:30030361 Assembly of mammalian oxidative phosphorylation complexes I-... | MARK AS OVER ANNOTATED | Summary: ATP synthesis is performed by Complex V, and the proton-motive force is generated by the coupled respiratory chain complexes as a whole. A non-catalytic accessory subunit of Complex I does not participate in ATP synthesis in any specific way; this is a distal pathway-level term and is an over-annotation for NDUFA2. Reason: Proton-motive-force-driven ATP synthesis is performed by Complex V. Complex I contributes only upstream, by pumping protons, and NDUFA2 contributes to Complex I only structurally - so the claim is two steps removed from anything NDUFA2 does. The cited reference is a general OXPHOS assembly review that asserts nothing specific to NDUFA2 here. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt that is believed not to be PMID:30030361 The assembly of the five oxidative phosphorylation system (OXPHOS) complexes in |
| GO:0045271 respiratory chain complex I | IPI PMID:28844695 Architecture of Human Mitochondrial Respiratory Megacomplex ... | ACCEPT | Summary: IPI part_of respiratory chain complex I from the megacomplex cryo-EM structure (ComplexPortal), where individual Complex I subunits are precisely assigned. Direct physical evidence that NDUFA2 is a subunit of Complex I. Correct and core. Reason: The megacomplex structure resolves 140 subunits and assigns them individually, which is direct physical evidence that NDUFA2 is a constituent of the assembled complex rather than a transient interactor. Accepted as core. Supporting Evidence: PMID:28844695 reveals the precise assignment of individual subunits of |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput mitochondrial-proteome localization. Correct but non-informative relative to the specific inner-membrane / Complex I localizations. Non-core. Reason: MitoCoP is a quantitatively filtered high-confidence mitochondrial proteome, so inclusion is trustworthy evidence of mitochondrial residence; but an organelle-level proteomic inventory cannot resolve submitochondrial compartment, so it supports only the parent term. Kept as coarser corroboration of the IDA inner-membrane annotation. Supporting Evidence: PMID:34800366 mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP) |
| GO:0045271 respiratory chain complex I | IDA PMID:12611891 The subunit composition of the human NADH dehydrogenase obta... | ACCEPT | Summary: IDA part_of respiratory chain complex I from one-step immunopurification and mass-spectrometric identification of the human NADH dehydrogenase subunit composition. Direct experimental evidence that NDUFA2 co-purifies as a Complex I subunit. Correct and core. Reason: One-step immunocapture of the intact complex followed by MALDI-TOF peptide mass fingerprinting and LC-MS/MS resolved 42 polypeptides, NDUFA2 among them. Co-purification with the immunoisolated holoenzyme is direct evidence of subunit status, and the authors note concordance with independently purified bovine Complex I. Supporting Evidence: PMID:12611891 homologues of 42 polypeptides detected so far in the more extensively studied |
| GO:0045271 respiratory chain complex I | IDA PMID:17209039 Identification of mitochondrial complex I assembly intermedi... | ACCEPT | Summary: IDA part_of respiratory chain complex I from tracking Complex I assembly intermediates by blue-native analysis. Supports NDUFA2 as a bona fide Complex I subunit incorporated during assembly. Correct and core. Reason: An inducible NDUFS3-GFP system resolved six distinct assembly subcomplexes on blue-native blots, distinguished from breakdown products by differential solubilization and heat treatment; composition analysis of those subcomplexes is what places NDUFA2 in the complex. Co-assembly with a traced core subunit is direct evidence of membership, not merely of co-migration. Supporting Evidence: PMID:17209039 stepwise assembly of CI |
| GO:0045271 respiratory chain complex I | IDA PMID:27626371 Accessory subunits are integral for assembly and function of... | ACCEPT | Summary: IDA part_of respiratory chain complex I from systematic accessory-subunit knockout and quantitative proteomics. This is the study underpinning the UniProt FUNCTION statement: accessory subunits (including NDUFA2) are required for assembly of a functional complex and stabilize other subunits in the same module. Correct and core. Reason: Gene-edited knockout lines were made for every accessory subunit and profiled by quantitative proteomics, which both confirms NDUFA2 as a constituent and assigns it to a structural module through the destabilization pattern of its neighbours. This is the study UniProt cites (ECO:0000269) for the FUNCTION statement. Supporting Evidence: PMID:27626371 Accessory subunits are integral for assembly and function of human |
| GO:0045271 respiratory chain complex I | NAS PMID:9878551 cDNA of eight nuclear encoded subunits of NADH:ubiquinone ox... | ACCEPT | Summary: NAS part_of respiratory chain complex I from the completion of human Complex I cDNA characterization. Consistent with all experimental evidence that NDUFA2 is a Complex I subunit. Correct and core (though redundant with stronger IDA/IPI annotations). Reason: The paper completed the cDNA inventory of the 41 then-known human Complex I subunits, NDUFA2/B8 among them, so its assertion of subunit identity is well grounded even though the evidence code is NAS. It agrees with four experimental annotations to the same term. Supporting Evidence: PMID:9878551 subunits of NADH:ubiquinone oxidoreductase |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-163217 | ACCEPT | Summary: TAS inner-membrane localization from the Reactome reaction for mature Complex I oxidising NADH. Consistent with the experimentally supported inner-membrane localization. Correct and core. Reason: This is the reaction for the mature holoenzyme, so it states the compartment of assembled Complex I and hence of each of its subunits. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-163217 the HP arm lying within the inner mitochondrial membrane |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799178 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from a Reactome Complex I biogenesis reaction. Consistent with NDUFA2 localization; a redundant assembly-pathway restatement. Non-core. Reason: An assembly step rather than an NDUFA2-specific observation, so it restates the compartment without adding evidence. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799178 anchored to the inner mitochondrial membrane by Intermediate |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799179 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from the Reactome reaction in which peripheral-arm subunits bind the 815 kDa complex during Complex I assembly. Consistent with NDUFA2 localization; assembly-pathway restatement. Non-core. Reason: This is the peripheral-arm step, the module NDUFA2 belongs to, but it is a pathway restatement rather than independent evidence. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799179 Subunits NDUFA12, NDUFS1, 4, 6, NDUFV1, 2 and 3 with the assembly factor |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799191 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from a Reactome Complex I assembly reaction. Consistent with NDUFA2 localization; assembly-pathway restatement. Non-core. Reason: An assembly step in which the IP subcomplex is anchored to the membrane by MT-ND1; a pathway restatement of the compartment. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799191 to the inner mitochondrial membrane by NADH-ubiquinone oxidoreductase chain 1 |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799196 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from the Reactome reaction in which assembly factors dissociate to yield mature Complex I. Consistent with NDUFA2 localization; assembly-pathway restatement. Non-core. Reason: The maturation step at which the transient assembly factors leave; it confirms that NDUFA2, which stays, is a true subunit, but adds no localization evidence. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799196 The MCIA complex, NDUFAF2-7 all dissociate from the 980kDa complex |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799197 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from a Reactome Complex I membrane-arm assembly reaction. Consistent with NDUFA2 localization; assembly-pathway restatement. Non-core. Reason: A membrane-arm step that does not involve NDUFA2 directly; a pathway restatement of the compartment. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799197 Distal components of the membrane arm MT-ND4 and 5 associate with the 550kDa |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799202 | KEEP AS NON CORE | Summary: TAS inner-membrane localization from a Reactome Complex I subcomplex assembly reaction. Consistent with NDUFA2 localization; assembly-pathway restatement. Non-core. Reason: An intermediate assembly step; a pathway restatement of the compartment. The inner-membrane compartment is correct and agrees with the experimental IDA annotation, which remains the annotation of record. Reactome entries carry no primary-literature snippet, so the cached reaction record is cited instead. Supporting Evidence: Reactome:R-HSA-6799202 The 315kDa and 370kDa subcomplexes associate to form a 550kDa complex |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9837978 | KEEP AS NON CORE | Summary: TAS inner-membrane localization derived from a Reactome LONP1 protein- degradation reaction (LONP1 binds mitochondrial inner-membrane proteins). The localization call is correct, but it comes from a model of NDUFA2 as a degradation substrate, not from an NDUFA2-function assay. Non-core. Reason: LONP1 is the protease; NDUFA2 is one of the inner-membrane proteins it binds. The localization the annotation records is therefore a property NDUFA2 must have for the reaction to occur, and is correct - but it is inferred from a substrate relationship rather than observed in an NDUFA2-function assay, and the same compartment is already carried by an IDA. Kept as non-core. Supporting Evidence: Reactome:R-HSA-9837978 LONP1 binds, unfolds, and degrades several mitochondrial |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-9838004 | KEEP AS NON CORE | Summary: TAS inner-membrane localization derived from a Reactome LONP1 degradation reaction (LONP1 degrades mitochondrial inner-membrane proteins). Localization is correct but originates from a substrate-of-degradation model. Non-core. Reason: The companion degradation step to the LONP1 binding reaction above, and the same reasoning applies: the compartment is right, but it follows from NDUFA2 being a degradation substrate, not from anything NDUFA2 does. Kept as non-core. That NDUFA2 turnover is LONP1-dependent is itself interesting but is a fact about its regulation, not about its function. Supporting Evidence: Reactome:R-HSA-9838004 LONP1 binds and degrades several mitochondrial inner membrane proteins |
| GO:0006120 mitochondrial electron transport, NADH to ubiquinone | NAS PMID:9878551 cDNA of eight nuclear encoded subunits of NADH:ubiquinone ox... | ACCEPT | Summary: NAS involvement in mitochondrial electron transport (NADH to ubiquinone), the defining process of Complex I. NDUFA2 is a subunit of Complex I and thus participates in this process as part of the holoenzyme. Appropriate core biological process for a Complex I subunit. Reason: NDUFA2 catalyses no step of electron transfer - the flavin and Fe-S chain belong to the NDUFV/NDUFS core subunits. It nevertheless does part of the work of the process in the sense GO requires: it is a structural constituent of the peripheral arm that the complex needs in order to assemble and hold together, and patient loss-of-function abolishes functional Complex I. Retained as a core process, with the catalytic activity itself represented as contributes_to rather than enables. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt Complex I functions in the transfer of electrons |
| GO:0008137 NADH dehydrogenase (ubiquinone) activity | NAS PMID:9878551 cDNA of eight nuclear encoded subunits of NADH:ubiquinone ox... | MARK AS OVER ANNOTATED | Summary: This annotates NDUFA2 as directly enabling NADH dehydrogenase (ubiquinone) activity. NADH:ubiquinone oxidoreductase catalysis is carried out by the 14 conserved core subunits (FMN/Fe-S-bearing subunits of the holoenzyme). UniProt explicitly states NDUFA2 is an accessory subunit "believed not to be involved in catalysis." Assigning the catalytic MF directly to a non-catalytic accessory subunit is an over-annotation. The honest relationship is captured in core_functions via contributes_to_molecular_function (GO:0008137), with the subunit's own MF being structural molecule activity (GO:0005198). Reason: PMID:9878551 reports cDNA sequences for eight previously uncharacterized nuclear-encoded subunits; it assays no enzymatic activity, so the MF assignment reflects the name of the parent enzyme rather than a measurement. NDUFA2 does have a thioredoxin-like fold and a redox-active Cys24-Cys58 disulfide, but no thioredoxin, oxidoreductase or disulfide-isomerase activity has ever been demonstrated for it, and UniProt states it is believed not to be involved in catalysis - so the fold is not a licence for a catalytic MF either. The right representation is contributes_to GO:0008137 with a structural molecule activity MF. Supporting Evidence: file:human/NDUFA2/NDUFA2-uniprot.txt that is believed not to be PMID:9878551 Here we report the cDNA sequences of the hitherto file:human/NDUFA2/NDUFA2-uniprot.txt Redox-active |
| GO:0031966 mitochondrial membrane | IDA PMID:17209039 Identification of mitochondrial complex I assembly intermedi... | KEEP AS NON CORE | Summary: IDA mitochondrial-membrane localization from the Complex I assembly- intermediate study. Correct but less specific than the mitochondrial inner membrane / respiratory chain complex I annotations. Non-core. Reason: Correct but strictly less informative: GO:0031966 is an ancestor of GO:0005743, which NDUFA2 already carries with experimental support, and UniProt specifies the matrix face of the inner membrane. Retained as consistent, non-core. Supporting Evidence: PMID:17209039 Biogenesis of human mitochondrial complex I (CI) requires the coordinated |
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Download this section (compressed HTML)Q: NDUFA2 adopts a thioredoxin fold and carries a redox-active Cys24-Cys58 disulfide, yet UniProt records no catalytic function and GOA has no redox MF for it. Is the disulfide functional in the assembled complex - for example as a redox or oxidative-stress sensor that modulates Complex I - or is it a vestige of the fold with no activity?
Q: Does the oxidation state of the Cys24-Cys58 disulfide change with the redox or oxidative-stress status of the matrix, and does that state affect NDUFA2's incorporation into, or the stability of, Complex I?
Q: Reactome models NDUFA2 as a LONP1 degradation substrate. Is LONP1-mediated turnover of unassembled NDUFA2 a physiological quality-control route, and does it contribute to the Complex I deficiency seen in MC1DN13 patients?
Q: The isoform O43678-2 replaces C-terminal residues 71-99 with the hexapeptide SRVQNS, truncating the thioredoxin-like core just downstream of Cys58. Is this isoform translated, and does it retain the disulfide or any capacity to be incorporated into Complex I?
Q: Which structural module of Complex I does NDUFA2 belong to, and which other subunits are destabilized when it is lost? The knockout proteomics framework of PMID:27626371 answers this for every accessory subunit; making the NDUFA2-specific answer explicit would sharpen the assembly annotation beyond the generic accessory-subunit claim.
Suggested experts: Stroud DA, Ryan MT
Experiment: Determine the in-cell oxidation state of Cys24 and Cys58 by differential thiol alkylation and quantitative mass spectrometry under basal, reducing and oxidative-stress conditions, then re-express Cys-to-Ser mutants in NDUFA2-null cells and assay Complex I assembly by blue-native PAGE and complexome profiling. If the disulfide is structural only, the mutants should still assemble; if it is regulatory, assembly or activity should track the redox condition.
Hypothesis: The Cys24-Cys58 disulfide of NDUFA2 is redox-responsive in situ and its reduction destabilizes the peripheral arm of Complex I.
Type: Redox proteomics with mutant complementation
Experiment: Generate NDUFA2 knockouts in a human cell line and perform complexome profiling alongside quantitative whole-mitochondrial proteomics, identifying which subunits fall in abundance and which assembly intermediate accumulates. Compare the destabilization signature against those catalogued for the other accessory subunits.
Hypothesis: NDUFA2 is required for assembly of a specific structural module of Complex I, and its loss selectively destabilizes the subunits of that module.
Type: Knockout complexome profiling
Experiment: Inhibit or deplete LONP1 in wild-type cells and in cells expressing an assembly-incompetent NDUFA2 variant, and measure free versus complex-incorporated NDUFA2 by blue-native PAGE and pulse-chase labelling. A selective rise in the free pool would confirm the Reactome substrate relationship as a physiological quality-control route.
Hypothesis: Unassembled NDUFA2 is cleared by LONP1, so LONP1 inhibition should stabilize the free subunit pool without restoring assembled Complex I.
Type: Protease inhibition with pulse-chase turnover analysis
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