SCO2

id: O43819
gene_symbol: SCO2
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  SCO2 (Synthesis of Cytochrome c Oxidase 2) is a copper metallochaperone essential for
  the assembly of cytochrome c oxidase (Complex IV). It is an inner mitochondrial membrane
  protein with a thioredoxin-like fold and a conserved CXXXC copper-binding motif. SCO2
  functions within a redox-sensitive copper relay module together with SCO1 and COA6 to
  deliver copper to the CuA site of the mtDNA-encoded COX2 subunit. SCO2 also acts as a
  thiol-disulfide oxidoreductase that regulates the redox state of cysteines in SCO1 during
  COX2 maturation. Importantly, SCO2 is an assembly factor for Complex IV, NOT a structural
  subunit of the complex. Mutations cause fatal infantile cardioencephalomyopathy with
  Complex IV deficiency (MC4DN2), and variants are also associated with autosomal-dominant
  high-grade myopia (MYP6). SCO2 is a transcriptional target of TP53 (p53), linking
  mitochondrial respiration to tumor suppression.
existing_annotations:
# ===== ANNOTATION 1: GO:0033617 (BP) - IBA =====
- term:
    id: GO:0033617
    label: mitochondrial respiratory chain complex IV assembly
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  review:
    summary: >-
      SCO2 is a well-established assembly factor for mitochondrial Complex IV
      (cytochrome c oxidase). The IBA phylogenetic inference is strongly supported
      by the conserved function of SCO proteins from yeast to human. SCO2 participates
      in the CuA-site assembly of COX2, a core step of CIV biogenesis (PMID:15229189,
      PMID:19336478, PMID:25959673). The founding disease paper (PMID:10545952)
      identified SCO2 as a COX assembly gene through mutations causing COX deficiency.
    action: ACCEPT
    reason: >-
      Mitochondrial respiratory chain complex IV assembly is the core biological process
      of SCO2. This IBA annotation is well-supported by extensive experimental evidence
      from multiple organisms and is at the correct level of specificity. SCO2 functions
      specifically in the CuA-site assembly step of COX2 maturation within the broader
      CIV assembly pathway.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        we have identified mutations in the human homologue, SCO2, in three unrelated
        infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency
    - reference_id: PMID:15229189
      supporting_text: >-
        We propose a model in which COX17 delivers copper to SCO2, which in turn
        transfers it directly to the CuA site at an early stage of COX assembly in
        a reaction that is facilitated by SCO1.
    - reference_id: PMID:19336478
      supporting_text: >-
        Based on these data we present a model in which each SCO protein fulfills
        distinct, stage-specific functions during CO II synthesis and CuA site maturation.

# ===== ANNOTATION 2: GO:0005507 (MF) - IEA =====
- term:
    id: GO:0005507
    label: copper ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      SCO2 binds copper ions via its conserved CXXXC motif (Cys133 and Cys137) and
      an additional His224 ligand, as demonstrated by the NMR structure (PMID:17850752,
      PDB:2RLI). The InterPro-based IEA annotation is correct and well-supported by
      structural evidence. Copper ion binding is a core molecular function of SCO2,
      essential for its role as a copper chaperone in CuA-site assembly.
    action: ACCEPT
    reason: >-
      Copper ion binding is a core molecular function of SCO2. The structural data
      from PMID:17850752 directly demonstrates copper coordination by the CXXXC motif
      (C133 and C137) and H224. The IEA annotation from InterPro is consistent with
      experimental evidence and is at the correct level of specificity.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        Human SCO1 and SCO2 are paralogous genes that code for metallochaperone
        proteins with essential, but poorly understood, roles in copper delivery to
        cytochrome c oxidase (COX).
    - reference_id: file:human/SCO2/SCO2-deep-research-falcon.md
      supporting_text: >-
        SCO2 is a member of the conserved SCO1/2 family with a thioredoxin-like fold
        and the hallmark Cys-x-x-x-Cys motif central to redox and copper-binding
        chemistry in CuA assembly

# ===== ANNOTATION 3: GO:0005743 (CC) - IEA =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  review:
    summary: >-
      SCO2 is an integral protein of the mitochondrial inner membrane with a single
      transmembrane helix (residues 61-78) and its functional thioredoxin domain
      exposed to the intermembrane space (PMID:15229189). The IEA annotation from
      combined automated methods is correct and supported by direct experimental
      evidence (IDA from PMID:15229189).
    action: ACCEPT
    reason: >-
      Mitochondrial inner membrane localization is well-established for SCO2 through
      both topology studies and fractionation experiments. This IEA is consistent
      with the experimentally verified IDA annotation from PMID:15229189.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        To investigate the molecular function of the SCO proteins, we characterized
        the mitochondrial copper delivery pathway in SCO1 and SCO2 patient backgrounds.

# ===== ANNOTATION 4: GO:0006878 (BP) - IEA =====
- term:
    id: GO:0006878
    label: intracellular copper ion homeostasis
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      SCO2 has a primary role as a copper chaperone delivering copper to the CuA site of
      COX2 during Complex IV assembly. PMID:17189203 (Leary et al. 2007) demonstrated
      that SCO proteins have regulatory roles in the maintenance of cellular copper
      homeostasis, providing some support for this annotation. However, SCO2's primary
      function is copper delivery to COX2 for CIV assembly, not broad cellular copper
      homeostasis. The copper homeostasis role is secondary and context-dependent.
    action: KEEP_AS_NON_CORE
    reason: >-
      While SCO2 does participate in copper handling, its primary role is copper delivery
      to COX2 in the context of CIV assembly, not broad intracellular copper homeostasis.
      The copper homeostasis function described in PMID:17189203 represents a regulatory
      or secondary effect rather than the core evolved function of SCO2. Keeping as
      non-core rather than removing because there is published evidence for a regulatory
      role in copper homeostasis.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        Our results demonstrate that the human SCO proteins have non-overlapping,
        cooperative functions in mitochondrial copper delivery.

# ===== ANNOTATION 5: GO:0008535 (BP) - IEA =====
- term:
    id: GO:0008535
    label: respiratory chain complex IV assembly
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      GO:0008535 (respiratory chain complex IV assembly) is the broader parent term
      of GO:0033617 (mitochondrial respiratory chain complex IV assembly). Since SCO2
      specifically functions in mitochondrial CIV assembly, the more specific child
      term GO:0033617 is more appropriate. However, the broader term is not incorrect
      and is acceptable as an IEA annotation.
    action: ACCEPT
    reason: >-
      This is the parent term of GO:0033617 which is already annotated with stronger
      evidence. While slightly less specific than the mitochondrial-specific child term,
      it is not incorrect for an IEA annotation and is subsumed by the more specific
      annotation.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in
        SCO2, a COX assembly gene.

# ===== ANNOTATION 6: GO:0016531 (MF) - IEA =====
- term:
    id: GO:0016531
    label: copper chaperone activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  review:
    summary: >-
      Copper chaperone activity (directly binding to and delivering copper ions to a
      target protein) is a core molecular function of SCO2. SCO2 delivers copper to the
      CuA site of COX2 in cooperation with SCO1. Multiple studies confirm this role
      (PMID:15229189, PMID:19336478, PMID:25959673). The InterPro-based IEA annotation
      correctly captures this core function.
    action: ACCEPT
    reason: >-
      Copper chaperone activity is the primary molecular function of SCO2. It directly
      binds copper and delivers it to the CuA site of COX2. This is strongly supported
      by structural (PMID:17850752), genetic (PMID:10545952, PMID:15229189), and
      biochemical (PMID:19336478, PMID:25959673) evidence.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        Human SCO1 and SCO2 are paralogous genes that code for metallochaperone
        proteins with essential, but poorly understood, roles in copper delivery to
        cytochrome c oxidase (COX).
    - reference_id: PMID:25959673
      supporting_text: >-
        Our analyses define COA6 as a constituent of the mitochondrial copper relay
        system, linking defects in COX2 metallation to cardiac cytochrome c oxidase
        deficiency.

# ===== ANNOTATION 7: GO:0017004 (BP) - IEA =====
- term:
    id: GO:0017004
    label: cytochrome complex assembly
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: >-
      GO:0017004 (cytochrome complex assembly) is a broader parent of GO:0033617
      (mitochondrial respiratory chain complex IV assembly). SCO2 is specifically
      involved in cytochrome c oxidase (Complex IV) assembly, not general cytochrome
      complex assembly. The UniProtKB-KW keyword mapping generated this broader term.
      While not incorrect, the more specific term GO:0033617 is preferred.
    action: ACCEPT
    reason: >-
      This is a correct but overly broad IEA annotation. SCO2 does participate in
      cytochrome complex assembly (specifically cytochrome c oxidase), so the term is
      not wrong. It is subsumed by the more specific annotation GO:0033617. Acceptable
      as an IEA annotation.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        Fatal infantile cardioencephalomyopathy with COX deficiency and mutations in
        SCO2, a COX assembly gene.

# ===== ANNOTATION 8: GO:0033617 (BP) - IEA =====
- term:
    id: GO:0033617
    label: mitochondrial respiratory chain complex IV assembly
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  review:
    summary: >-
      This ARBA machine-learning IEA annotation for mitochondrial respiratory chain
      complex IV assembly is correct and consistent with the IBA and experimental
      annotations for the same term. It is a duplicate in terms of the GO term but
      from a different evidence source.
    action: ACCEPT
    reason: >-
      Correct IEA annotation consistent with the core function of SCO2 in CIV assembly.
      The ARBA model correctly identified this function.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        we have identified mutations in the human homologue, SCO2, in three unrelated
        infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency

# ===== ANNOTATION 9: GO:0046872 (MF) - IEA =====
- term:
    id: GO:0046872
    label: metal ion binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000043
  review:
    summary: >-
      GO:0046872 (metal ion binding) is a very broad parent term. SCO2 specifically
      binds copper ions (GO:0005507), which is already annotated. The keyword-mapped
      IEA is not incorrect but is too general to be informative.
    action: ACCEPT
    reason: >-
      While this is a very generic term, it is not incorrect for an IEA annotation
      derived from keyword mapping. It is subsumed by the more specific copper ion
      binding annotation (GO:0005507). Acceptable to keep as IEA.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        Human SCO1 and SCO2 are paralogous genes that code for metallochaperone
        proteins with essential, but poorly understood, roles in copper delivery to
        cytochrome c oxidase (COX).

# ===== ANNOTATION 10: GO:0005515 (MF) - IPI (CIDEB) =====
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  review:
    summary: >-
      This IPI annotation is based on a high-throughput binary interactome screen
      (HI-Union, PMID:32296183) that detected an interaction between SCO2 and CIDEB
      (Q9UHD4). CIDEB is a cell death-inducing DFFA-like effector involved in lipid
      metabolism and apoptosis. This interaction lacks biological plausibility in the
      context of SCO2's known function as a mitochondrial copper chaperone for CIV
      assembly. The interaction with CIDEB is likely a false positive from the
      high-throughput screen.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic protein binding annotation based on a high-throughput binary interactome
      screen. The detected interactor (CIDEB) has no known role in mitochondrial
      copper delivery or CIV assembly. SCO2 has well-characterized, functionally
      relevant interactions with COX20, SCO1, COA6, COX16, and TMEM177 that are
      already captured by other IPI annotations. This annotation does not provide
      informative functional information.
    supported_by:
    - reference_id: PMID:32296183
      supporting_text: >-
        A reference map of the human binary protein interactome.

# ===== ANNOTATION 11: GO:0005739 (CC) - IEA =====
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  review:
    summary: >-
      SCO2 is unambiguously a mitochondrial protein. This IEA annotation via
      Ensembl Compara ortholog transfer is correct and consistent with the IDA and
      HTP annotations for the same term. UniProt annotates SCO2 with a mitochondrial
      transit peptide (residues 1-41).
    action: ACCEPT
    reason: >-
      Correct IEA annotation. Mitochondrial localization is universally supported.
      While the more specific mitochondrial inner membrane annotation (GO:0005743)
      is also present, having the broader mitochondrion annotation is acceptable.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        To investigate the molecular function of the SCO proteins, we characterized
        the mitochondrial copper delivery pathway in SCO1 and SCO2 patient backgrounds.

# ===== ANNOTATION 12: GO:0005739 (CC) - IDA =====
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  review:
    summary: >-
      This IDA annotation from Human Protein Atlas immunofluorescence data (HPA)
      confirms mitochondrial localization of SCO2 by direct assay. This is consistent
      with SCO2's known function as a mitochondrial inner membrane protein.
    action: ACCEPT
    reason: >-
      Direct experimental evidence (immunofluorescence) for mitochondrial localization,
      consistent with all other evidence sources. The broader mitochondrion term is
      acceptable from IDA/immunofluorescence which may not resolve to the inner membrane.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        To investigate the molecular function of the SCO proteins, we characterized
        the mitochondrial copper delivery pathway in SCO1 and SCO2 patient backgrounds.

# ===== ANNOTATION 13: GO:0005739 (CC) - HTP =====
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: HTP
  original_reference_id: PMID:34800366
  review:
    summary: >-
      This HTP annotation comes from the quantitative high-confidence human
      mitochondrial proteome study (Morgenstern et al. 2021). SCO2 was identified
      as a bona fide mitochondrial protein by mass spectrometry-based proteomics.
      This is consistent with its known function and localization.
    action: ACCEPT
    reason: >-
      High-throughput proteomics confirmation of mitochondrial localization.
      Consistent with all other localization evidence for SCO2.
    supported_by:
    - reference_id: PMID:34800366
      supporting_text: >-
        Quantitative high-confidence human mitochondrial proteome and its dynamics
        in cellular context.

# ===== ANNOTATION 14: GO:0005743 (CC) - TAS (TP53 Reactome) =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5632766
  review:
    summary: >-
      This TAS annotation derives from the Reactome pathway entry for TP53-stimulated
      SCO2 transcription. The Reactome entry describes SCO2 as a "copper-binding
      assembly protein" that localizes to the mitochondrial inner membrane. This is
      consistent with experimental evidence (PMID:15229189).
    action: ACCEPT
    reason: >-
      Correct localization annotation supported by Reactome curation. SCO2 is
      experimentally verified as a mitochondrial inner membrane protein with a
      single transmembrane helix.
    supported_by:
    - reference_id: Reactome:R-HSA-5632766
      supporting_text: >-
        SCO2, synthesis of cytochrome c oxidase 2, is a copper-binding assembly
        protein for the mitochondrial COX (cytochrome C oxidase) complex

# ===== ANNOTATION 15: GO:0005743 (CC) - TAS (Cu into MT-CO1) =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9865449
  review:
    summary: >-
      This TAS annotation derives from the Reactome pathway entry describing the
      metallochaperone complex that inserts Cu2+ into MT-CO1. The Reactome entry
      describes SCO2 as part of a nine-subunit metallochaperone complex on the inner
      mitochondrial membrane. Note that SCO2's primary role is copper delivery to
      COX2 (CuA site), not COX1 (CuB site). The Reactome entry groups SCO2 into
      a broader metallochaperone complex that may be somewhat oversimplified.
    action: ACCEPT
    reason: >-
      The localization to mitochondrial inner membrane is correct regardless of
      whether the broader metallochaperone complex model in Reactome is precisely
      accurate. SCO2 is an inner membrane protein.
    supported_by:
    - reference_id: Reactome:R-HSA-9865449
      supporting_text: >-
        A metallochaperone complex consisting of nine subunits (COA3, COA5, COX10,
        dimeric COX11, COX15, COX16, COX19, SCO1, SCO2)

# ===== ANNOTATION 16: GO:0005743 (CC) - TAS (heme moieties) =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9865579
  review:
    summary: >-
      This TAS annotation derives from the Reactome pathway entry describing the
      association of MT-CO1 and MT-CO2 complexes with heme moiety installation.
      The Reactome entry describes SCO2 as part of the COX2-specific copper
      chaperone module (with COA6 and SCO1). The localization to the mitochondrial
      inner membrane is correct.
    action: ACCEPT
    reason: >-
      Correct localization annotation. SCO2 functions at the mitochondrial inner
      membrane during CIV assembly.
    supported_by:
    - reference_id: Reactome:R-HSA-9865579
      supporting_text: >-
        A dynamic metallochaperone complex involving the heme A biosynthetic enzymes
        COX10 and COX15, together with COX2-specific copper chaperones COA6, SCO1,
        and SCO2

# ===== ANNOTATION 17: GO:0005743 (CC) - TAS (Cu into MT-CO2) =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-9865630
  review:
    summary: >-
      This TAS annotation derives from the Reactome pathway entry describing
      copper insertion into MT-CO2 (the CuA site). This is the most directly
      relevant Reactome entry for SCO2's function, as it describes the specific
      step SCO2 participates in. The localization to the mitochondrial inner
      membrane is correct.
    action: ACCEPT
    reason: >-
      Correct localization annotation from the most functionally relevant Reactome
      pathway entry for SCO2. The CuA insertion into MT-CO2 is the core function
      of SCO2.
    supported_by:
    - reference_id: Reactome:R-HSA-9865630
      supporting_text: >-
        A metallochaperone complex consisting of nine subunits (COA3, COA5, COX10,
        dimeric COX11, COX15, COX16, COX19, SCO1, SCO2) carries two copper cations
        ...to the pre-assembled MT-CO2 subunit. These copper ions are frequently
        called CuA

# ===== ANNOTATION 18: GO:0033617 (BP) - TAS =====
- term:
    id: GO:0033617
    label: mitochondrial respiratory chain complex IV assembly
  evidence_type: TAS
  original_reference_id: PMID:10545952
  review:
    summary: >-
      This TAS annotation from the founding SCO2 disease paper (Papadopoulou et al.
      1999) is well-supported. The paper identified SCO2 as a COX assembly gene
      through the discovery that mutations cause fatal infantile
      cardioencephalomyopathy with COX deficiency.
    action: ACCEPT
    reason: >-
      Core biological process annotation supported by the founding publication
      for SCO2 disease. The paper established that SCO2 mutations cause COX
      deficiency, demonstrating its role in CIV assembly.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        we have identified mutations in the human homologue, SCO2, in three unrelated
        infants with a newly recognized fatal cardioencephalomyopathy and COX
        deficiency...Immunohistochemical studies implied that the enzymatic deficiency,
        which was most severe in cardiac and skeletal muscle, was due to the loss of
        mtDNA-encoded COX subunits.

# ===== ANNOTATION 19: GO:0005515 (MF) - IPI (COX16) =====
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:29381136
  review:
    summary: >-
      This IPI annotation records the interaction between SCO2 and COX16 (Q9P0S2)
      demonstrated in PMID:29381136. COX16 was shown to interact with newly
      synthesized COX2 and its copper center-forming metallochaperones SCO1, SCO2,
      and COA6. This is a functionally relevant interaction in the context of CIV
      assembly, but the generic "protein binding" term is uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      While the interaction with COX16 is genuine and functionally relevant (COX16
      promotes COX2 metallation), the generic "protein binding" term does not capture
      the biological significance of this interaction. The interaction is better
      represented by SCO2's role in CIV assembly (GO:0033617). Protein binding is
      uninformative per GO curation guidelines.
    supported_by:
    - reference_id: PMID:29381136
      supporting_text: >-
        COX16, a protein required for cytochrome c oxidase assembly, interacts
        specifically with newly synthesized COX2 and its copper center-forming
        metallochaperones SCO1, SCO2, and COA6.

# ===== ANNOTATION 20: GO:0033617 (BP) - IMP =====
- term:
    id: GO:0033617
    label: mitochondrial respiratory chain complex IV assembly
  evidence_type: IMP
  original_reference_id: PMID:15229189
  review:
    summary: >-
      This IMP annotation from Leary et al. (2004) is strongly supported. The study
      characterized the mitochondrial copper delivery pathway in SCO2 patient
      backgrounds and demonstrated that SCO2 patient cells have a defect in COX
      assembly with the appearance of a common assembly intermediate. This is direct
      mutant phenotype evidence for SCO2's role in CIV assembly.
    action: ACCEPT
    reason: >-
      Strong experimental evidence (IMP) from a key functional study. Leary et al.
      showed that SCO2 mutations result in defective COX assembly and reduced levels
      of the mutant protein, with a specific assembly intermediate accumulating.
      This directly demonstrates SCO2's role in CIV assembly.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        Immunoblot analysis of patient cell lines showed reduced levels of the mutant
        proteins, resulting in a defect in COX assembly, and the appearance of a common
        assembly intermediate.

# ===== ANNOTATION 21: GO:0005743 (CC) - IDA =====
- term:
    id: GO:0005743
    label: mitochondrial inner membrane
  evidence_type: IDA
  original_reference_id: PMID:15229189
  review:
    summary: >-
      This IDA annotation from Leary et al. (2004) directly demonstrates SCO2
      localization to the mitochondrial inner membrane. The study characterized
      the topology of SCO2 as a single-pass membrane protein with the N-terminus
      in the matrix and the C-terminal thioredoxin domain in the intermembrane space.
    action: ACCEPT
    reason: >-
      Direct experimental evidence for mitochondrial inner membrane localization
      with topology determination. This is the primary experimental evidence for
      SCO2 localization and is at the correct level of specificity.
    supported_by:
    - reference_id: PMID:15229189
      supporting_text: >-
        To investigate the molecular function of the SCO proteins, we characterized
        the mitochondrial copper delivery pathway in SCO1 and SCO2 patient backgrounds.
        ...Size exclusion chromatography suggests that both the proteins function as
        homodimers.

# ===== ANNOTATION 22: GO:0015035 (MF) - IDA =====
- term:
    id: GO:0015035
    label: protein-disulfide reductase activity
  evidence_type: IDA
  original_reference_id: PMID:19336478
  review:
    summary: >-
      This IDA annotation from Leary et al. (2009) reports that SCO2 acts as a
      thiol-disulfide oxidoreductase for SCO1. Specifically, the paper demonstrates
      that SCO2 regulates the redox state of the copper-coordinating cysteines in
      SCO1, and that overexpression or knockdown of SCO2 alters the ratio of oxidized
      to reduced cysteines in SCO1. However, the paper specifically states that SCO2
      acts to OXIDIZE the copper-coordinating cysteines in SCO1 (thiol-disulfide
      oxidoreductase), whereas GO:0015035 (protein-disulfide reductase activity) is
      defined as reduction of disulfide bonds. The directionality of the redox
      reaction may be more nuanced than the GO term captures. More recent reviews
      (Swaminathan & Gohil, 2022) describe a complex redox relay where COA6 reduces
      SCO proteins.
    action: MODIFY
    reason: >-
      The annotation captures an important secondary molecular function of SCO2, but
      the specific GO term may not precisely match the described activity. PMID:19336478
      states SCO2 acts as a "thiol-disulphide oxidoreductase to oxidize the
      copper-coordinating cysteines in SCO1." The GO term GO:0015035 describes
      reductase activity (reduction of disulfide bonds), while the paper describes
      oxidoreductase activity in the oxidizing direction. A more appropriate term
      would be the broader GO:0015036 (disulfide oxidoreductase activity) which
      encompasses both directions.
    proposed_replacement_terms:
    - id: GO:0015036
      label: disulfide oxidoreductase activity
    supported_by:
    - reference_id: PMID:19336478
      supporting_text: >-
        SCO2 acts as a thiol-disulphide oxidoreductase to oxidize the
        copper-coordinating cysteines in SCO1 during CO II maturation.
    - reference_id: file:human/SCO2/SCO2-deep-research-falcon.md
      supporting_text: >-
        SCO2 functions within a redox-sensitive copper-relay module...acting primarily
        through thiol-disulfide redox chemistry on cysteine pairs required to
        coordinate Cu in COX2 and SCO proteins

# ===== ANNOTATION 23: GO:0005515 (MF) - IPI (COX20, PMID:24403053) =====
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:24403053
  review:
    summary: >-
      This IPI annotation records the interaction between SCO2 and COX20 (Q5RI15,
      also known as FAM36A) demonstrated in PMID:24403053. COX20 acts as a chaperone
      that stabilizes newly synthesized COX2 and presents it to the SCO1/SCO2
      metallochaperone module. The interaction is functionally significant for CIV
      assembly but the generic "protein binding" term is uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      The interaction with COX20 is genuine and functionally important (COX20
      presents COX2 to the SCO metallochaperone module), but "protein binding" is
      uninformative per GO curation guidelines. The functional significance is
      better captured by the CIV assembly annotation (GO:0033617).
    supported_by:
    - reference_id: PMID:24403053
      supporting_text: >-
        We propose that COX20 acts as a chaperone in the early steps of COX2
        maturation, stabilizing the newly synthesized protein and presenting COX2
        to its metallochaperone module, which in turn facilitates the incorporation
        of mature COX2 into the CIV assembly line.

# ===== ANNOTATION 24: GO:0005515 (MF) - IPI (COX20, PMID:28330871) =====
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:28330871
  review:
    summary: >-
      This IPI annotation records the interaction between SCO2 and COX20 (Q5RI15)
      from Bourens & Barrientos (2017). The paper demonstrates that after COX18
      promotes translocation of the COX2 C-tail, the SCO1-SCO2-COA6 copper
      metallation module binds to COX2-COX20 to finalize COX2 biogenesis. This is
      a duplicate interactor (COX20) from a different publication.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Same issue as the other protein binding annotations -- the generic "protein
      binding" term is uninformative. The interaction with COX20 is genuine and
      important for CIV assembly but is better captured by the assembly process
      annotation. This is a second independent publication confirming the
      SCO2-COX20 interaction, but "protein binding" remains uninformative.
    supported_by:
    - reference_id: PMID:28330871
      supporting_text: >-
        The release of COX18 from this complex coincides with the binding of the
        SCO1-SCO2-COA6 copper metallation module to COX2-COX20 to finalize COX2
        biogenesis.

# ===== ANNOTATION 25: GO:0005515 (MF) - IPI (TMEM177/COX20, PMID:29154948) =====
# Note: GOA has two rows for PMID:29154948 -- one with Q53S58 (TMEM177) and one with Q5RI15 (COX20)
# The ai-review stub collapsed these into one entry. This entry covers the TMEM177 interaction.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:29154948
  review:
    summary: >-
      This IPI annotation records interactions between SCO2 and both TMEM177 (Q53S58)
      and COX20 (Q5RI15) from Lorenzi et al. (2018). The paper shows that TMEM177
      associates with newly synthesized COX2 and SCO2 in a COX20-dependent manner,
      and that SCO2 was found in a complex with TMEM177, COX20, COA6, MT-CO2, COX18,
      and SCO1. The GOA has two separate rows for this PMID. Both interactions are
      functionally relevant to the COX2 assembly pathway.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic "protein binding" is uninformative per GO curation guidelines. The
      interactions with TMEM177 and COX20 are genuine and functionally relevant
      to CIV assembly, but are better captured by the assembly process annotation
      (GO:0033617).
    supported_by:
    - reference_id: PMID:29154948
      supporting_text: >-
        TMEM177 associates with newly synthesized COX2 and SCO2 in a COX20-dependent
        manner

# ===== ANNOTATION 26: GO:0005515 (MF) - IPI (COA6) =====
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25959673
  review:
    summary: >-
      This IPI annotation records the interaction between SCO2 and COA6 (Q5JTJ3)
      demonstrated in Pacheu-Grau et al. (2015). COA6 was shown to interact
      with SCO2 and pathogenic mutations in each protein affect complex formation.
      COA6 is a constituent of the mitochondrial copper relay system and cooperates
      with SCO2 in COX2 maturation. This is a highly significant functional
      interaction, but "protein binding" is still uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      Generic "protein binding" is uninformative. The interaction with COA6 is
      one of the most functionally significant interactions for SCO2 -- COA6
      cooperates with SCO2 in COX2 metallation and acts as a disulfide reductase
      in the copper relay. But the term "protein binding" does not capture this.
      The functional significance is better represented by the assembly annotation.
    supported_by:
    - reference_id: PMID:25959673
      supporting_text: >-
        We show that COA6 and SCO2 interact and that corresponding pathogenic
        mutations in each protein affect complex formation

# ===== ANNOTATION 27: GO:0001654 (BP) - IMP =====
- term:
    id: GO:0001654
    label: eye development
  evidence_type: IMP
  original_reference_id: PMID:23643385
  review:
    summary: >-
      This IMP annotation is based on Tran-Viet et al. (2013) who identified SCO2
      mutations in families with autosomal-dominant high-grade myopia (MYP6). The
      paper showed that SCO2 mRNA levels were significantly downregulated in myopic
      mouse retinae and that SCO2 protein localizes to retina, retinal pigment
      epithelium, and sclera. However, the connection between SCO2's core function
      (mitochondrial copper chaperone for CIV) and eye development is indirect.
      The myopia phenotype likely reflects a tissue-specific consequence of
      mitochondrial dysfunction rather than a direct role in eye development.
    action: KEEP_AS_NON_CORE
    reason: >-
      SCO2 mutations are associated with high-grade myopia, and SCO2 protein is
      expressed in the eye. However, this represents a pleiotropic effect of
      mitochondrial dysfunction on eye tissue rather than a direct role in eye
      development. SCO2 is not an eye development gene per se; it is a
      mitochondrial assembly factor whose loss has tissue-specific consequences
      including in the eye. Keeping as non-core to document the clinical
      association while accurately representing that this is not the core function.
    supported_by:
    - reference_id: PMID:23643385
      supporting_text: >-
        Messenger RNA levels of SCO2 were significantly downregulated in myopic
        mouse retinae. Immunohistochemistry in mouse eyes confirmed SCO2 protein
        localization in retina, retinal pigment epithelium, and sclera.

# ===== ANNOTATION 28: GO:0030016 (CC) - IDA =====
- term:
    id: GO:0030016
    label: myofibril
  evidence_type: IDA
  original_reference_id: PMID:20864674
  review:
    summary: >-
      This IDA annotation from Brosel et al. (2010) is based on
      immunohistochemistry of mouse tissues showing SCO2 expression patterns.
      The paper focused primarily on the differential tissue expression of SCO1
      vs SCO2, showing that SCO2 is highly expressed in muscle (the tissue most
      affected in SCO2 patients) while SCO1 predominates in liver and vasculature.
      The myofibril localization is surprising for a mitochondrial inner membrane
      protein and likely reflects mitochondrial localization within muscle fibers
      rather than true myofibril localization. This annotation was made by MGI
      (mouse annotation transferred to human).
    action: MARK_AS_OVER_ANNOTATED
    reason: >-
      SCO2 is an integral mitochondrial inner membrane protein. Its detection in
      myofibrils by immunohistochemistry more likely reflects the abundance of
      mitochondria in muscle tissue (where SCO2 is highly expressed) rather than
      true localization to the myofibril contractile apparatus. The paper was
      primarily about tissue expression patterns, not subcellular localization
      beyond mitochondria. The localization to the myofibril compartment is
      likely an artifact of the resolution of immunohistochemistry in muscle tissue.
    supported_by:
    - reference_id: PMID:20864674
      supporting_text: >-
        the expression of SCO1, but not of SCO2, is very high in liver (the tissue
        most affected in SCO1-mutant patients), whereas the reverse holds true in
        muscle (the tissue most affected in SCO2-mutant patients).

# ===== ANNOTATION 29: GO:0005507 (MF) - NAS =====
- term:
    id: GO:0005507
    label: copper ion binding
  evidence_type: NAS
  original_reference_id: PMID:10545952
  review:
    summary: >-
      This NAS annotation for copper ion binding is based on the founding SCO2 disease
      paper (PMID:10545952). The paper identified SCO2 as a homolog of yeast Sco1/2
      which are known copper-binding proteins. While the paper did not directly assay
      copper binding by human SCO2, the inference from yeast homologs was correct
      and subsequently confirmed by NMR structure (PMID:17850752). The NAS evidence
      is appropriate for the time of annotation.
    action: ACCEPT
    reason: >-
      Copper ion binding is a core molecular function of SCO2, confirmed by
      structural studies. This NAS annotation correctly inferred the function
      from the yeast homolog. The annotation is a duplicate of the IEA annotation
      from InterPro but from a different evidence source.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        SCO2, a COX assembly gene...In yeast, two related COX assembly genes, SCO1
        and SCO2 (for synthesis of cytochrome c oxidase), enable subunits I and II
        to be incorporated into the holoprotein.

# ===== ANNOTATION 30: GO:0005739 (CC) - TAS =====
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: TAS
  original_reference_id: PMID:10545952
  review:
    summary: >-
      This TAS annotation for mitochondrial localization is from the founding SCO2
      disease paper (PMID:10545952). The paper identified SCO2 as a mitochondrial
      gene (yeast Sco1/Sco2 are mitochondrial) and showed that mutations cause
      COX deficiency with loss of mtDNA-encoded COX subunits, implying a
      mitochondrial function. Subsequent studies directly confirmed mitochondrial
      localization (PMID:15229189).
    action: ACCEPT
    reason: >-
      Correct localization annotation. Mitochondrial localization was implicit
      in the founding paper and subsequently confirmed experimentally. TAS
      evidence is appropriate.
    supported_by:
    - reference_id: PMID:10545952
      supporting_text: >-
        In yeast, two related COX assembly genes, SCO1 and SCO2 (for synthesis of
        cytochrome c oxidase), enable subunits I and II to be incorporated into the
        holoprotein.

core_functions:
- molecular_function:
    id: GO:0016531
    label: copper chaperone activity
  directly_involved_in:
  - id: GO:0033617
    label: mitochondrial respiratory chain complex IV assembly
  - id: GO:0008535
    label: respiratory chain complex IV assembly
  locations:
  - id: GO:0005743
    label: mitochondrial inner membrane
  description: >-
    SCO2 is a mitochondrial copper metallochaperone and assembly factor that
    enables copper relay into the CuA maturation pathway of COX2 during Complex
    IV biogenesis. Its core role is copper chaperone activity coupled to
    respiratory chain Complex IV assembly at the mitochondrial inner membrane.
  supported_by:
  - reference_id: PMID:15229189
    supporting_text: >-
      We propose a model in which COX17 delivers copper to SCO2, which in turn
      transfers it directly to the CuA site at an early stage of COX assembly in
      a reaction that is facilitated by SCO1.
  - reference_id: PMID:10545952
    supporting_text: >-
      we have identified mutations in the human homologue, SCO2, in three unrelated
      infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency
  - reference_id: file:human/SCO2/SCO2-deep-research-falcon.md
    supporting_text: >-
      SCO2 is a member of the conserved SCO1/2 family with a thioredoxin-like fold
      and the hallmark Cys-x-x-x-Cys motif central to redox and copper-binding
      chemistry in CuA assembly

references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings:
  - statement: InterPro2GO mapping infers GO terms from conserved InterPro domain signatures, providing IEA support for SCO2 copper ion binding and copper chaperone activity from the SCO family signature.
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings:
  - statement: PAINT/PANTHER phylogenetic annotation transfers experimentally validated SCO family functions across orthologs, supporting SCO2 annotation to mitochondrial respiratory chain complex IV assembly.
- id: GO_REF:0000043
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword mapping
  findings:
  - statement: UniProtKB keyword-to-GO mappings (e.g., Metal-binding, Mitochondrion, Copper) produce broad IEA annotations such as metal ion binding and cytochrome complex assembly for SCO2.
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings:
  - statement: Curation of Human Protein Atlas immunofluorescence images supports SCO2 mitochondrial localization at IDA evidence level.
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings:
  - statement: Ensembl Compara orthology-based transfer propagates experimentally verified mitochondrion localization to human SCO2 from validated orthologs.
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings:
  - statement: ARBA machine learning rules predict GO:0033617 mitochondrial respiratory chain complex IV assembly for SCO2 from sequence and family features.
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings:
  - statement: A combined automated pipeline integrating multiple IEA methods produces a high-confidence IEA annotation of SCO2 to the mitochondrial inner membrane.
- id: PMID:10545952
  title: Fatal infantile cardioencephalomyopathy with COX deficiency and mutations
    in SCO2, a COX assembly gene.
  findings:
  - statement: SCO2 was identified as a nuclear COX assembly gene whose recessive mutations cause fatal infantile cardioencephalomyopathy with COX deficiency.
    supporting_text: we have identified mutations in the human homologue, SCO2, in three unrelated infants with a newly recognized fatal cardioencephalomyopathy and COX deficiency
  - statement: The enzymatic deficiency caused by SCO2 mutations is most severe in cardiac and skeletal muscle and results from loss of mtDNA-encoded COX subunits, implicating SCO2 in COX holoenzyme assembly rather than catalysis.
    supporting_text: Immunohistochemical studies implied that the enzymatic deficiency, which was most severe in cardiac and skeletal muscle, was due to the loss of mtDNA-encoded COX subunits.
- id: PMID:15229189
  title: Human SCO1 and SCO2 have independent, cooperative functions in copper delivery
    to cytochrome c oxidase.
  findings:
  - statement: SCO1 and SCO2 are paralogous metallochaperones that have independent but cooperative functions in delivering copper to the CuA site of cytochrome c oxidase.
    supporting_text: Human SCO1 and SCO2 are paralogous genes that code for metallochaperone proteins with essential, but poorly understood, roles in copper delivery to cytochrome c oxidase (COX).
  - statement: SCO2 patient cells show a defect in COX assembly with accumulation of a common assembly intermediate, demonstrating the role of SCO2 in CIV assembly by mutant phenotype.
    supporting_text: Immunoblot analysis of patient cell lines showed reduced levels of the mutant proteins, resulting in a defect in COX assembly, and the appearance of a common assembly intermediate.
  - statement: COX17 delivers copper to SCO2, which transfers it to the CuA site of COX2 in a reaction facilitated by SCO1, defining the SCO2-mediated step of the mitochondrial copper relay.
    supporting_text: We propose a model in which COX17 delivers copper to SCO2, which in turn transfers it directly to the CuA site at an early stage of COX assembly in a reaction that is facilitated by SCO1.
- id: PMID:16728594
  title: p53 regulates mitochondrial respiration.
  findings:
  - statement: SCO2 is a transcriptional target of p53, and its expression couples p53 status to mitochondrial respiration; loss of SCO2 in p53 wild-type cancer cells reproduces the glycolytic (Warburg) phenotype of p53-deficient cells.
    supporting_text: Disruption of the SCO2 gene in human cancer cells with wild-type p53 recapitulated the metabolic switch toward glycolysis that is exhibited by p53-deficient cells.
- id: PMID:19336478
  title: Human SCO2 is required for the synthesis of CO II and as a thiol-disulphide
    oxidoreductase for SCO1.
  findings:
  - statement: SCO2 acts upstream of SCO1 and is indispensable for synthesis of COX2 (CO II); subsequent COX2 maturation requires both SCO proteins with intact CXXXC copper-coordinating motifs.
    supporting_text: These results indicate that SCO2 acts upstream of SCO1, and that it is indispensable for CO II synthesis. The subsequent maturation of CO II is contingent upon the formation of a complex that includes both SCO proteins, each with a functional CxxxC copper-coordinating motif.
  - statement: SCO2 functions as a thiol-disulphide oxidoreductase that oxidizes the copper-coordinating cysteines of SCO1 during CuA site maturation.
    supporting_text: SCO2 acts as a thiol-disulphide oxidoreductase to oxidize the copper-coordinating cysteines in SCO1 during CO II maturation.
- id: PMID:20864674
  title: 'Unexpected vascular enrichment of SCO1 over SCO2 in mammalian tissues: implications
    for human mitochondrial disease.'
  findings:
  - statement: SCO1 and SCO2 show divergent tissue expression patterns in mammals (SCO1 enriched in liver and vasculature; SCO2 enriched in muscle), partially explaining the tissue specificity of SCO1- and SCO2-related mitochondrial disease phenotypes.
    supporting_text: the expression of SCO1, but not of SCO2, is very high in liver (the tissue most affected in SCO1-mutant patients), whereas the reverse holds true in muscle (the tissue most affected in SCO2-mutant patients).
- id: PMID:23643385
  title: Mutations in SCO2 are associated with autosomal-dominant high-grade myopia.
  findings:
  - statement: SCO2 mutations segregate with autosomal-dominant high-grade myopia (MYP6); SCO2 mRNA is down-regulated in myopic mouse retina and SCO2 protein is detected in retina, retinal pigment epithelium and sclera.
    supporting_text: Messenger RNA levels of SCO2 were significantly downregulated in myopic mouse retinae. Immunohistochemistry in mouse eyes confirmed SCO2 protein localization in retina, retinal pigment epithelium, and sclera.
- id: PMID:24403053
  title: Human COX20 cooperates with SCO1 and SCO2 to mature COX2 and promote the
    assembly of cytochrome c oxidase.
  findings:
  - statement: COX20 acts as a chaperone for newly synthesized COX2, stabilizing it and presenting it to the SCO1/SCO2 metallochaperone module for CuA-site maturation and CIV assembly.
    supporting_text: We propose that COX20 acts as a chaperone in the early steps of COX2 maturation, stabilizing the newly synthesized protein and presenting COX2 to its metallochaperone module, which in turn facilitates the incorporation of mature COX2 into the CIV assembly line.
- id: PMID:25959673
  title: Cooperation between COA6 and SCO2 in COX2 maturation during cytochrome c
    oxidase assembly links two mitochondrial cardiomyopathies.
  findings:
  - statement: COA6 is a constituent of the mitochondrial copper relay that physically interacts with SCO2; pathogenic mutations in either protein disrupt complex formation, mechanistically linking COA6- and SCO2-associated cardiomyopathies.
    supporting_text: We show that COA6 and SCO2 interact and that corresponding pathogenic mutations in each protein affect complex formation
  - statement: COA6 functions in the metallation of COX2 during cytochrome c oxidase assembly and its loss converges on the same CuA-maturation pathway as SCO2 loss.
    supporting_text: Our analyses define COA6 as a constituent of the mitochondrial copper relay system, linking defects in COX2 metallation to cardiac cytochrome c oxidase deficiency.
- id: PMID:28330871
  title: Human mitochondrial cytochrome c oxidase assembly factor COX18 acts transiently
    as a membrane insertase within the subunit 2 maturation module.
  findings:
  - statement: COX18 acts as a transient membrane insertase for the COX2 C-tail; on its release, the SCO1-SCO2-COA6 copper-metallation module binds the COX2-COX20 complex to finalize COX2 biogenesis.
    supporting_text: The release of COX18 from this complex coincides with the binding of the SCO1-SCO2-COA6 copper metallation module to COX2-COX20 to finalize COX2 biogenesis.
- id: PMID:29154948
  title: The mitochondrial TMEM177 associates with COX20 during COX2 biogenesis.
  findings:
  - statement: TMEM177 associates with newly synthesized COX2 and SCO2 in a COX20-dependent manner, expanding the COX2 maturation interactome to include TMEM177 alongside the SCO1/SCO2/COA6 module.
    supporting_text: TMEM177 associates with newly synthesized COX2 and SCO2 in a COX20-dependent manner
- id: PMID:29381136
  title: COX16 promotes COX2 metallation and assembly during respiratory complex IV
    biogenesis.
  findings:
  - statement: COX16 interacts with newly synthesized COX2 and with its copper-center-forming metallochaperones SCO1, SCO2, and COA6, promoting COX2 metallation during CIV biogenesis.
    supporting_text: COX16, a protein required for cytochrome c oxidase assembly, interacts specifically with newly synthesized COX2 and its copper center-forming metallochaperones SCO1, SCO2, and COA6.
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings:
  - statement: Reports a high-throughput binary protein interactome (HI-Union) in which SCO2 was scored as interacting with CIDEB; in the absence of orthogonal validation or functional rationale this binding is treated as likely false-positive for SCO2 curation.
    supporting_text: A reference map of the human binary protein interactome.
- id: PMID:34800366
  title: Quantitative high-confidence human mitochondrial proteome and its dynamics
    in cellular context.
  findings:
  - statement: Quantitative mass-spectrometry-based mitochondrial proteomics confirms SCO2 as a bona fide mitochondrial protein and provides high-throughput evidence for its mitochondrion localization.
    supporting_text: Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context.
- id: Reactome:R-HSA-5632766
  title: TP53 stimulates SCO2 gene transcription
  findings:
  - statement: Reactome curates SCO2 as a transcriptional target of TP53 in the pathway "TP53 stimulates SCO2 gene transcription", supporting the p53/SCO2/COX/respiration axis at TAS evidence level.
- id: Reactome:R-HSA-9865449
  title: Metallochaperone inserts Cu2+ into MT-CO1
  findings:
  - statement: Reactome describes a nine-subunit metallochaperone complex (COA3, COA5, COX10, dimeric COX11, COX15, COX16, COX19, SCO1, SCO2) that inserts Cu2+ into MT-CO1; SCO2 is placed in this complex on the mitochondrial inner membrane.
    supporting_text: A metallochaperone complex consisting of nine subunits (COA3, COA5, COX10, dimeric COX11, COX15, COX16, COX19, SCO1, SCO2)
- id: Reactome:R-HSA-9865579
  title: MT-CO1 and MT-CO2 complexes associate, installing heme moieties
  findings:
  - statement: Reactome places SCO2 with COA6, SCO1, COX10 and COX15 in the dynamic metallochaperone complex that mediates association of MT-CO1 and MT-CO2 modules and heme A installation during CIV assembly.
    supporting_text: A dynamic metallochaperone complex involving the heme A biosynthetic enzymes COX10 and COX15, together with COX2-specific copper chaperones COA6, SCO1, and SCO2
- id: Reactome:R-HSA-9865630
  title: Metallochaperone inserts 2Cu2+ into MT-CO2
  findings:
  - statement: Reactome describes insertion of two Cu2+ ions (the CuA site) into MT-CO2 by the SCO2-containing metallochaperone complex, which is the most functionally relevant Reactome entry for SCO2's core role.
    supporting_text: A metallochaperone complex consisting of nine subunits (COA3, COA5, COX10, dimeric COX11, COX15, COX16, COX19, SCO1, SCO2) carries two copper cations ...to the pre-assembled MT-CO2 subunit. These copper ions are frequently called CuA

suggested_questions:
- question: Does SCO2 act primarily as a copper donor to SCO1, as a thiol-disulfide oxidoreductase that licenses SCO1 for copper transfer, or both, and is one activity dispensable in vivo?
  experts:
  - Leary SC
  - Shoubridge EA
- question: How does the SCO1/SCO2/COA6 disulfide-relay module sense and respond to changes in mitochondrial redox state, and is it coupled to the cytosolic glutathione or thioredoxin systems?
  experts:
  - Leary SC
  - Rehling P
- question: To what extent does SCO2 contribute to maintenance of cellular copper homeostasis beyond its role in COX2 metallation, and is this a regulated function or a passive consequence of copper handling?
  experts:
  - Leary SC
- question: Is the reported SCO2-CIDEB interaction from high-throughput interactome screens reproducible by orthogonal assays, and if so, does it have any role in lipid droplet or apoptosis biology?
  experts:
  - Calvo SE

suggested_experiments:
- hypothesis: Separation-of-function mutations in the SCO2 CXXXC motif that abolish copper binding but preserve oxidoreductase activity (or vice versa) will reveal which activity is rate-limiting for COX2 CuA-site maturation in human cells.
  description: Generate CRISPR knock-in human cells expressing SCO2 CXXXC variants that selectively disrupt Cu coordination vs disulfide-bond catalysis. Assay COX2 synthesis (mitochondrial pulse-labeling), CIV assembly (BN-PAGE), and SCO1 cysteine redox state (alkylation/mass spectrometry).
  experiment_type: separation-of-function mutagenesis with redox proteomics
- hypothesis: SCO2 contributes to mitochondrial copper homeostasis independently of CIV assembly via copper sequestration or transfer to other targets.
  description: Quantify mitochondrial labile and total copper pools (ICP-MS, copper-responsive fluorescent probes) in SCO2-null, SCO1-null, and double-null human cells, with rescue by separation-of-function variants. Combine with proximity labeling (BioID/TurboID) of SCO2 to identify non-COX copper recipients.
  experiment_type: trace metal proteomics and proximity labeling
- hypothesis: The myopia phenotype caused by SCO2 mutations reflects retinal pigment epithelium and photoreceptor mitochondrial dysfunction rather than a non-mitochondrial developmental role.
  description: Generate retinal organoids or RPE cultures from SCO2-MYP6 patient iPSCs and isogenic controls. Measure CIV activity, mitochondrial bioenergetics, and photoreceptor maturation markers; rescue with WT SCO2 vs CXXXC-mutant SCO2.
  experiment_type: iPSC-derived retinal model with mitochondrial functional assays
- hypothesis: The composition of the COX2 maturation interactome (SCO1, SCO2, COA6, COX16, COX18, COX20, TMEM177) is dynamic across discrete assembly intermediates and is altered by pathogenic SCO2 variants.
  description: Apply native MS or crosslinking-MS to immunopurified SCO2 complexes in WT and SCO2-mutant cells, combined with siRNA depletion of each candidate partner; correlate complex composition with COX2 maturation intermediates resolved by BN-PAGE.
  experiment_type: native/crosslinking mass spectrometry of assembly intermediates