NDUFA4

id: O00483
gene_symbol: NDUFA4
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  Cytochrome c oxidase subunit FA4 (COXFA4/NDUFA4), a small (~9.4 kDa, 81 aa) nuclear-encoded
  single-pass transmembrane protein that is a structural subunit of Complex IV (cytochrome
  c
  oxidase) in the mitochondrial inner membrane. Originally misclassified as a Complex
  I
  (NADH:ubiquinone oxidoreductase) accessory subunit, NDUFA4 was reclassified as the
  14th
  subunit of Complex IV by Balsa et al. (2012, PMID:22902835), based on proteomic,
  genetic,
  evolutionary, and biochemical analyses showing that NDUFA4 deletion does not perturb
  Complex I
  but instead reduces Complex IV levels and activity. This reclassification was structurally
  confirmed by Zong et al. (2018, PMID:30030519), who resolved the 3.3 Angstrom cryo-EM
  structure of intact 14-subunit human Complex IV from the supercomplex I1III2IV1,
  placing
  NDUFA4 at the dimeric interface of previously reported CIV crystal structures. NDUFA4
  is
  detergent-labile (dissociates in >1.5% DDM), explaining its absence from earlier
  CIV purified
  structures. As a Complex IV subunit, it contributes to the terminal step of the
  electron
  transport chain -- the reduction of molecular oxygen to water using electrons from
  cytochrome c,
  coupled with proton pumping across the inner mitochondrial membrane. Loss-of-function
  mutations
  in NDUFA4 cause mitochondrial Complex IV deficiency nuclear type 21 (MC4DN21), presenting
  as
  Leigh-like syndrome with reduced COX activity in patient fibroblasts. The protein
  is regulated
  post-transcriptionally and post-translationally by the C15ORF48/miR-147 axis in
  gut
  epithelium, linking CIV composition to immunometabolism.
existing_annotations:
- term:
    id: GO:0045277
    label: respiratory chain complex IV
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      NDUFA4 is the 14th subunit of Complex IV (cytochrome c oxidase), established
      by Balsa et al. (2012, PMID:22902835) and structurally confirmed by Zong et
      al.
      (2018, PMID:30030519). IBA annotation from phylogenetic inference correctly
      assigns NDUFA4 as part of Complex IV, consistent with the reclassification
      from Complex I.
    action: ACCEPT
    reason: >-
      Core annotation. NDUFA4 is a bona fide structural subunit of Complex IV. The
      IBA annotation reflects the phylogenetically supported reclassification. Balsa
      et al.
      demonstrated via proteomic, genetic, evolutionary, and biochemical analyses
      that
      NDUFA4 is a CIV subunit (PMID:22902835), and Zong et al. provided 3.3 Angstrom
      cryo-EM structural confirmation (PMID:30030519).
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV). Deletion of NDUFA4 does not
        perturb CI. Rather, proteomic, genetic, evolutionary, and biochemical analyses
        reveal that NDUFA4 plays a role in CIV function and biogenesis.
    - reference_id: PMID:30030519
      supporting_text: >-
        Combining previous structural and biochemical data shown by us and other groups,
        we propose that the intact
        complex-IV is a monomer containing 14 subunits.

    - reference_id: file:human/NDUFA4/NDUFA4-deep-research-falcon.md
      supporting_text: Belongs to the complex IV COXFA4 subunit family.
- term:
    id: GO:1902600
    label: proton transmembrane transport
  evidence_type: IEA
  original_reference_id: GO_REF:0000108
  review:
    summary: >-
      Complex IV pumps protons across the inner mitochondrial membrane coupled to
      electron transfer from cytochrome c to O2. NDUFA4 does not directly pump protons
      (catalytic core subunits MT-CO1/2/3 perform that), but as a structural subunit
      it is required for complex integrity and optimal activity. The IEA annotation
      is logically inferred from the cytochrome-c oxidase activity annotation.
    action: ACCEPT
    reason: >-
      Appropriate biological process annotation. Complex IV couples electron transfer
      to proton pumping. As a structural subunit essential for CIV maintenance,
      NDUFA4 is involved in this process. This parallels the accepted annotation
      pattern for COX5B (P10606).
    supported_by:
    - reference_id: PMID:30030519
      supporting_text: >-
        It accepts electrons from cytochrome c to reduce the oxygen to water and
        meanwhile pumps two protons from the matrix side to the intermembrane
        space (IMS)
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4 plays a role in CIV function and biogenesis

- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  review:
    summary: >-
      NDUFA4 is localized to the mitochondrial inner membrane as a single-pass
      transmembrane protein within Complex IV. UniProt and structural data confirm
      inner membrane localization.
    action: ACCEPT
    reason: >-
      Core localization. Cryo-EM structure (PMID:30030519) resolves NDUFA4 residues
      3-81 within CIV on the inner membrane. UniProt records NDUFA4 as a single-pass
      membrane protein in the inner mitochondrial membrane, with matrix-facing
      N-terminus (residues 1-14) and IMS-facing C-terminus (residues 38-81).
    supported_by:
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion inner membrane; Single-pass membrane protein
    - reference_id: PMID:30030519
      supporting_text: >-
        Here we obtained the 3.3 Å resolution structure of complex-IV derived
        from the human supercomplex I 1 III 2 IV 1 and assigned the NDUFA4 subunit
        into
        complex-IV

- term:
    id: GO:0022904
    label: respiratory electron transport chain
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: >-
      NDUFA4 participates in the respiratory electron transport chain as a subunit
      of Complex IV. This is a broad but correct annotation inferred from UniProt
      keyword mapping.
    action: KEEP_AS_NON_CORE
    reason: >-
      This term is too general. The more specific term GO:0006123 (mitochondrial
      electron transport, cytochrome c to oxygen) is already annotated and better
      captures the precise step of the ETC in which NDUFA4 participates. However,
      the annotation is not wrong -- NDUFA4 does function in the respiratory
      electron transport chain through CIV.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4 plays a role in CIV function and biogenesis
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        Component of the cytochrome c oxidase, the last enzyme in the mitochondrial
        electron transport chain which drives oxidative phosphorylation

- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32814053
  review:
    summary: >-
      This annotation derives from a large-scale neurodegenerative disease interactome
      study (Haenig et al. 2020) that mapped ~30,000 interactions among ~5,000 proteins.
      The with/from column indicates interaction with DPF1 (Q92782-2). This is a generic
      protein binding annotation from high-throughput yeast two-hybrid screening.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic protein binding from a high-throughput neurodegenerative disease
      interactome study. Per curation guidelines, protein binding is not informative
      about the actual molecular function. The interaction with DPF1 has no known
      functional significance for NDUFA4's role as a Complex IV subunit.
    supported_by:
    - reference_id: PMID:32814053
      supporting_text: >-
        Interactome maps are valuable resources to elucidate protein function and
        disease mechanisms.

- term:
    id: GO:0045271
    label: respiratory chain complex I
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  review:
    summary: >-
      This annotation incorrectly assigns NDUFA4 to Complex I. NDUFA4 was conclusively
      reclassified from Complex I to Complex IV by Balsa et al. (2012, PMID:22902835).
      Deletion of NDUFA4 does not perturb Complex I. The IEA annotation persists from
      ortholog transfer of an outdated annotation via Ensembl Compara, propagating
      the
      historical misclassification.
    action: REMOVE
    reason: >-
      Incorrect annotation based on historical misclassification. Balsa et al. (2012)
      definitively demonstrated that NDUFA4 is not a Complex I subunit: "Deletion
      of
      NDUFA4 does not perturb CI." Zong et al. (2018) structurally placed NDUFA4 in
      Complex IV. UniProt now explicitly states: "Was initially believed to be a subunit
      of the mitochondrial membrane respiratory chain NADH dehydrogenase (complex
      I)."
      The correct annotation is GO:0045277 (respiratory chain complex IV), which is
      already present.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV). Deletion of NDUFA4 does not
        perturb CI.
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        Was initially believed to be a subunit of the mitochondrial membrane respiratory
        chain NADH dehydrogenase (complex I).

- term:
    id: GO:0045277
    label: respiratory chain complex IV
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      IEA annotation from combined automated methods correctly placing NDUFA4 in
      Complex IV. Consistent with the IBA and IDA annotations for the same term.
    action: ACCEPT
    reason: >-
      Correct annotation. Same term as IBA (GO_REF:0000033) and IDA (PMID:22902835)
      annotations. Acceptable redundancy from automated pipeline that correctly
      reflects NDUFA4's reclassification as a CIV subunit.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV).

- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  review:
    summary: >-
      NDUFA4 localizes to mitochondria as confirmed by immunofluorescence data (HPA).
      This is a broad but correct localization.
    action: ACCEPT
    reason: >-
      Direct experimental evidence (IDA) from immunofluorescence confirms
      mitochondrial localization. While broad, this is valid. More specific inner
      membrane localization is captured by separate annotations.
    supported_by:
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion inner membrane; Single-pass membrane protein

- term:
    id: GO:0006123
    label: mitochondrial electron transport, cytochrome c to oxygen
  evidence_type: NAS
  original_reference_id: PMID:30030519
  review:
    summary: >-
      NDUFA4 is a structural subunit of Complex IV, which catalyzes the terminal step
      of
      the ETC -- transfer of electrons from cytochrome c to molecular oxygen. Zong
      et al.
      (2018, PMID:30030519) resolved the 14-subunit CIV structure including NDUFA4
      and
      confirmed its role in this process. As a CIV subunit, NDUFA4 is involved in
      this
      specific electron transport step.
    action: ACCEPT
    reason: >-
      Core biological process annotation. NDUFA4 is essential for this process as
      a
      structural subunit of Complex IV. Knockdown of NDUFA4 specifically reduces CIV
      activity (PMID:22902835), and loss-of-function mutations cause isolated CIV
      deficiency (MC4DN21) with reduced COX activity in patient fibroblasts.
    supported_by:
    - reference_id: PMID:30030519
      supporting_text: >-
        CIV is the terminal oxidase of the electron transport chain in mitochondria.
    - reference_id: PMID:22902835
      supporting_text: >-
        proteomic, genetic, evolutionary, and biochemical analyses reveal that NDUFA4
        plays a role in CIV function and biogenesis

- term:
    id: GO:0031966
    label: mitochondrial membrane
  evidence_type: IDA
  original_reference_id: PMID:30030519
  review:
    summary: >-
      NDUFA4 is specifically on the mitochondrial inner membrane. This term (mitochondrial
      membrane) is broader, encompassing both inner and outer membranes.
    action: MODIFY
    reason: >-
      While technically correct, this term is less precise than GO:0005743
      (mitochondrial inner membrane), which is already annotated with multiple lines
      of evidence. NDUFA4 is specifically a single-pass transmembrane protein of the
      inner mitochondrial membrane, as shown by the cryo-EM structure (PMID:30030519)
      and UniProt annotation. Following the same pattern as COX5B review.
    proposed_replacement_terms:
    - id: GO:0005743
      label: mitochondrial inner membrane
    supported_by:
    - reference_id: PMID:30030519
      supporting_text: >-
        Here we obtained the 3.3 Å resolution structure of complex-IV derived
        from the human supercomplex I 1 III 2 IV 1 and assigned the NDUFA4 subunit
        into
        complex-IV
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion inner membrane; Single-pass membrane protein

- term:
    id: GO:0045333
    label: cellular respiration
  evidence_type: NAS
  original_reference_id: PMID:30030519
  review:
    summary: >-
      Very broad biological process term. NDUFA4 participates in cellular respiration
      as a subunit of Complex IV, but more specific terms exist to capture its role.
    action: KEEP_AS_NON_CORE
    reason: >-
      Too general. More precise annotations for GO:0006123 (mitochondrial electron
      transport, cytochrome c to oxygen) and GO:1902600 (proton transmembrane transport)
      better describe NDUFA4's specific role. Keep as valid but non-core annotation,
      following the same pattern as COX5B review.
    supported_by:
    - reference_id: PMID:30030519
      supporting_text: >-
        CIV is the terminal oxidase of the electron transport chain in mitochondria.

- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  review:
    summary: >-
      NDUFA4 identified in quantitative high-confidence human mitochondrial proteome
      (Morgenstern et al. 2021). High-throughput proteomics data confirming
      mitochondrial localization.
    action: ACCEPT
    reason: >-
      High-confidence mitochondrial proteomics data from reputable study. HTP evidence
      code indicates high-throughput but the study is rigorous and well-validated.
      Confirms core localization.
    supported_by:
    - reference_id: PMID:34800366
      supporting_text: >-
        Quantitative high-confidence human mitochondrial proteome and its dynamics
        in cellular context

- term:
    id: GO:0045277
    label: respiratory chain complex IV
  evidence_type: IDA
  original_reference_id: PMID:22902835
  review:
    summary: >-
      Balsa et al. (2012) provided the foundational experimental evidence that NDUFA4
      is a subunit of Complex IV, not Complex I. This landmark paper used proteomic,
      genetic, evolutionary, and biochemical analyses to demonstrate CIV membership.
      IDA from the primary reclassification study.
    action: ACCEPT
    reason: >-
      Core annotation with strongest experimental evidence. This is the definitive
      study reclassifying NDUFA4 from CI to CIV. NDUFA4 knockdown specifically reduced
      CIV levels and activity without affecting CI. This is the most important single
      annotation for NDUFA4.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV). Deletion of NDUFA4 does not
        perturb CI. Rather, proteomic, genetic, evolutionary, and biochemical analyses
        reveal that NDUFA4 plays a role in CIV function and biogenesis.

- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:31536960
  review:
    summary: >-
      Moutaoufik et al. (2019) performed systematic mass spectrometry-based
      co-fractionation profiling of mitochondrial proteins. The with/from column
      indicates interaction with RAB5IF (Q9BUV8). UniProt confirms NDUFA4 interacts
      with RAB5IF. This is a high-throughput interactome study of mitochondrial proteins
      during neuronal reprogramming.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic protein binding from a high-throughput mitochondrial interactome study.
      While the interaction with RAB5IF was confirmed by UniProt, the protein binding
      term itself is uninformative about NDUFA4's actual molecular function as a CIV
      subunit. Per curation guidelines, protein binding does not tell us about actual
      function.
    supported_by:
    - reference_id: PMID:31536960
      supporting_text: >-
        Association of CI subunit (NDUFA4) or TFAM with the components of the respirasome
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        Interacts with RAB5IF

- term:
    id: GO:0044877
    label: protein-containing complex binding
  evidence_type: IDA
  original_reference_id: PMID:23209302
  review:
    summary: >-
      PMID:23209302 (Ahmed et al. 2012) is about KIF14 regulation of Rap1a-Radil
      signaling in breast cancer. This publication has no relevance to NDUFA4/COXFA4
      function. Searching the full text of the paper reveals no mention of NDUFA4,
      COXFA4, or O00483. This annotation appears to be a misattribution -- possibly
      transferred from a different protein erroneously.
    action: REMOVE
    reason: >-
      Likely erroneous annotation. The cited reference (PMID:23209302) is about KIF14
      and Rap1a-Radil signaling in breast cancer and has no connection to NDUFA4.
      The paper does not mention NDUFA4, COXFA4, or cytochrome c oxidase anywhere.
      This appears to be a curation error where the annotation was incorrectly
      assigned to NDUFA4. The term protein-containing complex binding is also vague
      and uninformative.
    supported_by:
    - reference_id: PMID:23209302
      supporting_text: >-
        KIF14 negatively regulates Rap1a-Radil signaling during breast cancer progression

- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19822128
  review:
    summary: >-
      Hayashi et al. (2009) reported that DJ-1 (PARK7, Q99497) directly binds to NDUFA4
      and ND1. However, this study was performed when NDUFA4 was still classified
      as a
      Complex I subunit. The finding that DJ-1 binds NDUFA4 is of some interest but
      the
      functional interpretation (DJ-1 maintaining CI activity through NDUFA4) is outdated
      given NDUFA4's reclassification to CIV. The protein binding annotation itself
      is
      generic.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic protein binding annotation. The DJ-1 interaction with NDUFA4 was reported
      in the context of Complex I biology, which is now known to be incorrect for
      NDUFA4.
      While the physical interaction may be real, the protein binding term is uninformative
      about NDUFA4's core function as a CIV subunit. The functional context of the
      interaction (CI activity maintenance) has been invalidated by NDUFA4 reclassification.
    supported_by:
    - reference_id: PMID:19822128
      supporting_text: >-
        DJ-1 directly bound to NDUFA4 and ND1, nuclear and mitochondrial DNA-encoding
        subunits of mitochondrial complex I, respectively
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV).

- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-163214
  review:
    summary: >-
      Reactome pathway for electron transfer from reduced cytochrome c to molecular
      oxygen, documenting Complex IV at the inner mitochondrial membrane.
    action: ACCEPT
    reason: >-
      Traceable author statement from Reactome curated pathway. NDUFA4 is part of
      Complex IV which localizes to and functions at the inner mitochondrial membrane.
      Appropriate and precise localization annotation.
    supported_by:
    - reference_id: Reactome:R-HSA-163214
      supporting_text: >-
        Electron transfer from reduced cytochrome c to molecular oxygen

- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9709406
  review:
    summary: >-
      Reactome reaction documenting CO binding to cytochrome c oxidase at the inner
      membrane.
    action: ACCEPT
    reason: >-
      Valid TAS evidence for NDUFA4 localization as subunit of Complex IV at the
      inner mitochondrial membrane. Reactome curated reaction.
    supported_by:
    - reference_id: Reactome:R-HSA-9709406
      supporting_text: >-
        CO binds to Cytochrome c oxidase

- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9865663
  review:
    summary: >-
      Reactome reaction for Complex IV assembly -- MT-CO3, COX6A, COX6B, COX7A
      and NDUFA4 bind to the holo-MT-CO1,2 complex at the inner membrane. This
      directly documents NDUFA4 incorporation into CIV.
    action: ACCEPT
    reason: >-
      Directly relevant Reactome reaction documenting NDUFA4 assembly into Complex
      IV
      at the inner mitochondrial membrane. Strong support for both localization and
      CIV membership.
    supported_by:
    - reference_id: Reactome:R-HSA-9865663
      supporting_text: >-
        MT-CO3, COX6A,B,7A and NDUFA4 bind to holo-MT-CO1,2 complex

- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: PMID:16729965
  review:
    summary: >-
      PMID:16729965 (Lamhonwah and Tein 2006) is about OCTN1 (organic cation/carnitine
      transporter) localization to mitochondria. While this study may have incidentally
      identified NDUFA4 in mitochondrial fractions, the paper is primarily about OCTN1.
      The mitochondrial localization of NDUFA4 is well-established from other sources.
    action: ACCEPT
    reason: >-
      Although the cited reference focuses on OCTN1, the mitochondrial localization
      of
      NDUFA4 is robustly supported by multiple independent lines of evidence including
      the cryo-EM structure (PMID:30030519), immunofluorescence (GO_REF:0000052),
      and
      proteomics (PMID:34800366). The broad mitochondrion term is valid.
    supported_by:
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        SUBCELLULAR LOCATION: Mitochondrion inner membrane; Single-pass membrane protein

    - reference_id: PMID:16729965
      supporting_text: Novel localization of OCTN1, an organic cation/carnitine transporter,
        to mammalian mitochondria.
- term:
    id: GO:0006120
    label: mitochondrial electron transport, NADH to ubiquinone
  evidence_type: NAS
  original_reference_id: PMID:9878551
  review:
    summary: >-
      This annotation reflects the outdated classification of NDUFA4 as a Complex
      I
      subunit. PMID:9878551 (Loeffen et al. 1998) reported cDNA sequences for nuclear
      encoded CI subunits including NDUFA4, based on the now-incorrect assumption
      that
      NDUFA4 was a Complex I component. Balsa et al. (2012) subsequently demonstrated
      that NDUFA4 is not involved in the NADH-to-ubiquinone electron transport step.
    action: REMOVE
    reason: >-
      Incorrect annotation based on historical misclassification. NDUFA4 is not a
      Complex I subunit and does not participate in mitochondrial electron transport
      from NADH to ubiquinone. Balsa et al. (2012) showed "Deletion of NDUFA4 does
      not perturb CI." The correct process annotation is GO:0006123 (mitochondrial
      electron transport, cytochrome c to oxygen), which is already annotated.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        NDUFA4, formerly considered a constituent of NADH Dehydrogenase (CI), is instead
        a component of the cytochrome c oxidase (CIV). Deletion of NDUFA4 does not
        perturb CI.
    - reference_id: PMID:9878551
      supporting_text: >-
        cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase:
        human
        complex I cDNA characterization completed [predates the reclassification]

- term:
    id: GO:0008137
    label: NADH dehydrogenase (ubiquinone) activity
  evidence_type: NAS
  original_reference_id: PMID:9878551
  review:
    summary: >-
      This annotation reflects the outdated classification of NDUFA4 as a Complex
      I
      subunit with NADH dehydrogenase activity. PMID:9878551 (Loeffen et al. 1998)
      characterized NDUFA4 cDNA as part of CI, which has since been disproved.
    action: REMOVE
    reason: >-
      Incorrect annotation based on historical misclassification. NDUFA4 does not
      have NADH dehydrogenase (ubiquinone) activity. It is a subunit of Complex IV
      (cytochrome c oxidase), not Complex I (NADH dehydrogenase). Balsa et al. (2012)
      definitively showed NDUFA4 deletion does not perturb CI. UniProt now explicitly
      cautions: "Was initially believed to be a subunit of the mitochondrial membrane
      respiratory chain NADH dehydrogenase (complex I)."
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        Deletion of NDUFA4 does not perturb CI. Rather, proteomic, genetic, evolutionary,
        and biochemical analyses reveal that NDUFA4 plays a role in CIV function and
        biogenesis.
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        Was initially believed to be a subunit of the mitochondrial membrane respiratory
        chain NADH dehydrogenase (complex I).

- term:
    id: GO:0008137
    label: NADH dehydrogenase (ubiquinone) activity
  evidence_type: TAS
  original_reference_id: PMID:9352085
  review:
    summary: >-
      PMID:9352085 (Kim et al. 1997) cloned the human NDUFA4 cDNA and described it
      as
      encoding the NADH:ubiquinone oxidoreductase MLRQ subunit. This was the initial
      characterization of NDUFA4, predating the reclassification by 15 years.
    action: REMOVE
    reason: >-
      Incorrect annotation based on original misclassification. Kim et al. (1997)
      characterized NDUFA4 under the assumption it was a Complex I subunit. Balsa
      et al.
      (2012) subsequently demonstrated this was incorrect. NDUFA4 does not contribute
      to NADH dehydrogenase activity; it is a cytochrome c oxidase subunit. The gene
      name NDUFA4 persists for historical reasons but is misleading.
    supported_by:
    - reference_id: PMID:9352085
      supporting_text: >-
        A cDNA clone encoding human NADH:ubiquinone oxidoreductase (complex I of
        mitochondrial respiratory chain) MLRQ subunit was isolated [predates reclassification]
    - reference_id: PMID:22902835
      supporting_text: >-
        The change in the attribution of the NDUFA4 protein requires renaming of the
        gene and reconsideration of the structure of CIV.

# NEW annotations suggested based on holistic review
- term:
    id: GO:0004129
    label: cytochrome-c oxidase activity
  evidence_type: IDA
  original_reference_id: PMID:22902835
  review:
    summary: >-
      As a structural subunit of Complex IV, NDUFA4 contributes to cytochrome-c oxidase
      activity. Balsa et al. (2012) showed that knockdown of NDUFA4 reduces CIV activity.
      Following GO annotation conventions for complex subunits (contributes_to qualifier),
      NDUFA4 should be annotated to the molecular function of the complex it enables.
      This annotation is present in UniProt GO cross-references but is missing from
      the
      GOA file.
    action: NEW
    reason: >-
      NDUFA4 is a subunit of Complex IV whose primary molecular function is cytochrome-c
      oxidase activity (GO:0004129). Following the pattern established for other CIV
      subunits like COX5B (P10606), which is annotated to GO:0004129, NDUFA4 should
      have this molecular function annotation. Balsa et al. showed NDUFA4 knockdown
      reduces CIV activity, and mutations cause MC4DN21 with reduced COX activity.
      Note this should use the contributes_to qualifier since NDUFA4 is a structural
      subunit, not the catalytic core.
    supported_by:
    - reference_id: PMID:22902835
      supporting_text: >-
        proteomic, genetic, evolutionary, and biochemical analyses reveal that NDUFA4
        plays a role in CIV function and biogenesis
    - reference_id: file:human/NDUFA4/NDUFA4-uniprot.txt
      supporting_text: >-
        Cytochrome c oxidase is the component of the respiratory chain that catalyzes
        the reduction of oxygen to water. COXFA4 is required for complex IV maintenance.

references:
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000043
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000108
  title: Automatic assignment of GO terms using logical inference, based on on inter-ontology
    links
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:16729965
  title: Novel localization of OCTN1, an organic cation/carnitine transporter, to
    mammalian mitochondria.
  findings: []
- id: PMID:19822128
  title: DJ-1 binds to mitochondrial complex I and maintains its activity.
  findings:
  - statement: DJ-1 directly binds to NDUFA4 and ND1; this study predates the reclassification
      of NDUFA4 to Complex IV.
    supporting_text: DJ-1 binds to mitochondrial complex I and maintains its activity.
- id: PMID:22902835
  title: NDUFA4 is a subunit of complex IV of the mammalian electron transport chain.
  findings:
  - statement: Definitive reclassification of NDUFA4 from Complex I to Complex IV
      based on proteomic, genetic, evolutionary, and biochemical analyses.
    supporting_text: NDUFA4 is a subunit of complex IV of the mammalian electron transport
      chain.
  - statement: NDUFA4 deletion does not perturb Complex I but reduces Complex IV levels
      and activity.
    supporting_text: NDUFA4 is a subunit of complex IV of the mammalian electron transport
      chain.
  - statement: NDUFA4 is required for CIV maintenance.
    supporting_text: NDUFA4 is a subunit of complex IV of the mammalian electron transport
      chain.
- id: PMID:23209302
  title: KIF14 negatively regulates Rap1a-Radil signaling during breast cancer progression.
  findings:
  - statement: This paper has no relevance to NDUFA4; likely a curation error in the
      GOA annotation.
    supporting_text: KIF14 negatively regulates Rap1a-Radil signaling during breast
      cancer progression.
- id: PMID:30030519
  title: Structure of the intact 14-subunit human cytochrome c oxidase.
  findings:
  - statement: 3.3 Angstrom cryo-EM structure of human Complex IV from supercomplex
      I1III2IV1.
    supporting_text: Structure of the intact 14-subunit human cytochrome c oxidase.
  - statement: NDUFA4 assigned as 14th subunit of CIV, located at the dimeric interface
      of previous crystal structures.
    supporting_text: Structure of the intact 14-subunit human cytochrome c oxidase.
  - statement: Intact CIV is proposed to be a monomer containing 14 subunits.
    supporting_text: Structure of the intact 14-subunit human cytochrome c oxidase.
- id: PMID:31536960
  title: Rewiring of the Human Mitochondrial Interactome during Neuronal Reprogramming
    Reveals Regulators of the Respirasome and Neurogenesis.
  findings:
  - statement: Mass spectrometry-based co-fractionation profiling shows NDUFA4 interacting
      with both CI and CIV subunits in differentiated neuronal-like cells.
    supporting_text: Rewiring of the Human Mitochondrial Interactome during Neuronal
      Reprogramming Reveals Regulators of the Respirasome and Neurogenesis.
  - statement: Interaction with RAB5IF identified.
    supporting_text: Rewiring of the Human Mitochondrial Interactome during Neuronal
      Reprogramming Reveals Regulators of the Respirasome and Neurogenesis.
  - statement: Paper still refers to NDUFA4 as CI subunit despite its reclassification.
    supporting_text: Rewiring of the Human Mitochondrial Interactome during Neuronal
      Reprogramming Reveals Regulators of the Respirasome and Neurogenesis.
- id: PMID:32814053
  title: Interactome Mapping Provides a Network of Neurodegenerative Disease Proteins
    and Uncovers Widespread Protein Aggregation in Affected Brains.
  findings:
  - statement: Large-scale neurodegenerative disease interactome mapping study.
    supporting_text: Interactome Mapping Provides a Network of Neurodegenerative Disease
      Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
  - statement: NDUFA4 interaction with DPF1 detected in yeast two-hybrid.
    supporting_text: Interactome Mapping Provides a Network of Neurodegenerative Disease
      Proteins and Uncovers Widespread Protein Aggregation in Affected Brains.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings:
  - statement: High-confidence identification of NDUFA4 in human mitochondrial proteome.
    supporting_text: Quantitative high-confidence human mitochondrial proteome and
      its dynamics in cellular context.
- id: PMID:9352085
  title: Cloning of the human cDNA sequence encoding the NADH:ubiquinone oxidoreductase
    MLRQ subunit.
  findings:
  - statement: Original cloning of NDUFA4 cDNA, predating the reclassification to
      Complex IV.
    supporting_text: Cloning of the human cDNA sequence encoding the NADH:ubiquinone
      oxidoreductase MLRQ subunit.
  - statement: Incorrectly classified NDUFA4 as a Complex I subunit.
    supporting_text: Cloning of the human cDNA sequence encoding the NADH:ubiquinone
      oxidoreductase MLRQ subunit.
- id: PMID:9878551
  title: 'cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase:
    human complex I cDNA characterization completed.'
  findings:
  - statement: Characterized NDUFA4 cDNA as part of Complex I characterization effort.
    supporting_text: 'cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase:
      human complex I cDNA characterization completed.'
  - statement: Predates the reclassification to Complex IV.
    supporting_text: 'cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase:
      human complex I cDNA characterization completed.'
- id: Reactome:R-HSA-163214
  title: Electron transfer from reduced cytochrome c to molecular oxygen
  findings: []
- id: Reactome:R-HSA-9709406
  title: CO binds to Cytochrome c oxidase
  findings: []
- id: Reactome:R-HSA-9865663
  title: MT-CO3, COX6A,B,7A and NDUFA4 bind to holo-MT-CO1,2 complex
  findings:
  - statement: Reactome reaction documenting NDUFA4 assembly into Complex IV during
      CIV biogenesis.
    supporting_text: MT-CO3, COX6A,B,7A and NDUFA4 bind to holo-MT-CO1,2 complex
- id: file:human/NDUFA4/NDUFA4-deep-research-falcon.md
  title: Deep research on NDUFA4/COXFA4 function
  findings:
  - statement: Comprehensive literature review confirming NDUFA4 as CIV subunit.
    supporting_text: Deep research on NDUFA4/COXFA4 function
  - statement: C15ORF48/miR-147 axis regulates NDUFA4 post-transcriptionally.
    supporting_text: Deep research on NDUFA4/COXFA4 function
  - statement: Biallelic NDUFA4 deletion causes Leigh-like syndrome (MC4DN21).
    supporting_text: Deep research on NDUFA4/COXFA4 function
  - statement: In situ cryo-EM confirms NDUFA4 in native CIV/respirasomes.
    supporting_text: Deep research on NDUFA4/COXFA4 function
aliases:
- COXFA4
- CI-MLRQ
- Complex I-MLRQ
- Cytochrome c oxidase subunit FA4
- Cytochrome c oxidase subunit NDUFA4
- NADH-ubiquinone oxidoreductase MLRQ subunit
core_functions:
- description: >-
    Structural subunit of mitochondrial respiratory chain Complex IV (cytochrome c
    oxidase) required for complex maintenance and optimal cytochrome-c oxidase activity.
    NDUFA4 is a single-pass transmembrane protein that occupies the position at the
    dimeric interface of CIV crystal structures, supporting the monomeric form of
    intact 14-subunit Complex IV within respirasomes. Knockdown reduces CIV levels
    and
    activity; loss-of-function mutations cause isolated CIV deficiency (MC4DN21).
  molecular_function:
    id: GO:0004129
    label: cytochrome-c oxidase activity
  directly_involved_in:
  - id: GO:0006123
    label: mitochondrial electron transport, cytochrome c to oxygen
  - id: GO:1902600
    label: proton transmembrane transport
  locations:
  - id: GO:0005743
    label: mitochondrial inner membrane
  in_complex:
    id: GO:0045277
    label: respiratory chain complex IV
  supported_by:
  - reference_id: PMID:22902835
    supporting_text: >-
      Rather, proteomic, genetic, evolutionary, and biochemical analyses reveal that
      NDUFA4 plays a role in CIV function and biogenesis
  - reference_id: PMID:30030519
    supporting_text: >-
      Intriguingly, NDUFA4 lies exactly at the dimeric interface observed in previously
      reported crystal structures of complex-IV homodimer which would preclude complex-IV
      dimerization
suggested_questions:
- question: >-
    Does NDUFA4 have a specific functional role within Complex IV beyond structural
    stabilization, such as modulating proton pumping efficiency or electron transfer
    kinetics?
- question: >-
    Is the DJ-1 (PARK7) interaction with NDUFA4 (PMID:19822128) still functionally
    relevant given the reclassification from CI to CIV?
suggested_experiments:
- description: >-
    Precise measurement of CIV kinetic parameters (kcat, Km for cytochrome c) in
    cells/mitochondria with and without NDUFA4, to determine whether NDUFA4 affects
    catalytic efficiency or only complex stability/assembly.
  hypothesis: >-
    NDUFA4 modulates CIV catalytic properties beyond simply maintaining complex
    stability, distinguishing it as a regulatory vs. purely structural subunit.