id: Q9VDT1
gene_symbol: Arc42
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:7227
  label: Drosophila melanogaster
description: >-
  Arc42 (CG4703; FlyBase FBgn0038742) is the Drosophila melanogaster ortholog of human ACADS,
  a mitochondrial short-chain specific acyl-CoA dehydrogenase (SCAD; butyryl-CoA dehydrogenase;
  EC 1.3.8.1). Despite its legacy gene name "Activator-recruited cofactor subunit 42", the protein
  is a member of the acyl-CoA dehydrogenase family and carries the three canonical ACAD domains
  (N-terminal, middle and C-terminal) together with a bound FAD cofactor. It is predicted to
  catalyze the first, FAD-dependent step of mitochondrial fatty acid beta-oxidation for short-chain
  substrates, the alpha,beta-dehydrogenation of saturated short-chain (C4-C6, optimum butyryl-CoA/C4)
  acyl-CoA thioesters to the corresponding trans-2-enoyl-CoA, transferring the abstracted electrons
  to electron-transfer flavoprotein (ETF). Drosophila has two predicted ACADS orthologs, Arc42 and
  CG4860; loss of Arc42, but not CG4860, produces the elevated C4 (butyrylcarnitine) signature
  characteristic of human and mouse SCAD deficiency, identifying Arc42 as the functional short-chain
  acyl-CoA dehydrogenase of the fly. The enzyme acts in the mitochondrial matrix and contributes to
  energy production from fats, particularly under nutrient stress such as starvation.
existing_annotations:
- term:
    id: GO:0016937
    label: short-chain fatty acyl-CoA dehydrogenase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: >-
      Phylogenetically inferred short-chain acyl-CoA dehydrogenase activity. This is the precise,
      correct molecular function for the ACADS ortholog Arc42, consistent with its SCAD-family
      domain architecture (CDD cd01158 SCAD_SBCAD), the UniProt/ARBA curated C4-C6 substrate
      specificity, and the in-vivo elevated-C4 (butyrylcarnitine) phenotype of Arc42 loss-of-function
      flies that mirrors human SCAD deficiency.
    action: ACCEPT
    reason: >-
      The IBA propagation gives the most informative MF term and is concordant with the ARBA/EC IEA
      evidence and the FlyBase IMP process evidence. This is the core molecular function of Arc42.
    supported_by:
      - reference_id: PMID:40519079
        supporting_text: "Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking functional SCAD"
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetically inferred mitochondrial localization (is_active_in). Arc42 is a mitochondrial
      beta-oxidation enzyme; this is correct but less specific than the mitochondrial matrix
      annotation also present.
    action: ACCEPT
    reason: >-
      Mitochondrial localization is expected for a SCAD-family fatty acid beta-oxidation enzyme and
      is transferred by ISS to the matrix compartment; the parent mitochondrion term is correct, with
      the more specific matrix term preferred as the core location.
- term:
    id: GO:0033539
    label: fatty acid beta-oxidation using acyl-CoA dehydrogenase
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Phylogenetically inferred involvement in the acyl-CoA dehydrogenase step of fatty acid
      beta-oxidation. This accurately captures the biological process in which Arc42 acts and is
      corroborated by the FlyBase IMP annotation from the same process (elevated C4 acylcarnitine in
      Arc42 knockouts) and by ISS from the pig SCAD ortholog.
    action: ACCEPT
    reason: >-
      Arc42 performs the ACAD-catalyzed first step of beta-oxidation for short-chain substrates. This
      is an appropriately specific and correct BP annotation and a core biological process.
    supported_by:
      - reference_id: PMID:40519079
        supporting_text: "C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected"
- term:
    id: GO:0046359
    label: butyrate catabolic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: >-
      Phylogenetically inferred butyrate (butanoate) catabolic process. The optimum short-chain
      substrate of SCAD-family enzymes is butyryl-CoA (butanoyl-CoA, C4), and Arc42 loss of function
      specifically elevates C4 (butyrylcarnitine), consistent with a role in catabolizing the C4
      short-chain acyl-CoA. This is a substrate-specific framing of the same short-chain
      beta-oxidation step Arc42 catalyzes.
    action: KEEP_AS_NON_CORE
    reason: >-
      Reasonable given butyryl-CoA is the optimal short-chain substrate and C4 accumulates upon Arc42
      loss, but it is a narrow substrate-specific restatement of the core short-chain fatty acid
      beta-oxidation role rather than an independent biological process. Retained as non-core.
    supported_by:
      - reference_id: PMID:40519079
        supporting_text: "C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected"
- term:
    id: GO:0003995
    label: acyl-CoA dehydrogenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      Electronic annotation (ARBA/InterPro) to the parent acyl-CoA dehydrogenase activity term.
      Correct but less specific than the short-chain term (GO:0016937); Arc42 is the fly short-chain
      family member (SCAD_SBCAD subfamily).
    action: KEEP_AS_NON_CORE
    reason: >-
      The broad parent term is accurate but subsumed by the more specific short-chain ACAD activity
      annotation, which better reflects Arc42 substrate specificity.
- term:
    id: GO:0016627
    label: oxidoreductase activity, acting on the CH-CH group of donors
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-derived electronic annotation to an intermediate oxidoreductase term. The acyl-CoA
      dehydrogenase reaction oxidizes the alpha,beta (C2-C3) CH-CH bond of the acyl-CoA, so this
      parent term is accurate.
    action: KEEP_AS_NON_CORE
    reason: >-
      Correct intermediate-level term between general oxidoreductase and the specific short-chain
      ACAD activity; subsumed by the more specific MF annotation.
- term:
    id: GO:0016937
    label: short-chain fatty acyl-CoA dehydrogenase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000003
  qualifier: enables
  review:
    summary: >-
      Electronic annotation from EC mapping (EC 1.3.8.1, butyryl-CoA dehydrogenase) to the short-chain
      ACAD activity term. Correct and concordant with the UniProt-curated catalytic activities
      (butanoyl-CoA/C4, pentanoyl-CoA/C5, hexanoyl-CoA/C6 with ETF) and with the IBA annotation.
    action: ACCEPT
    reason: >-
      Reflects the UniProt/ARBA-curated EC 1.3.8.1 and Rhea reactions for short-chain acyl-CoA
      dehydrogenation. This is the core molecular function.
- term:
    id: GO:0050660
    label: flavin adenine dinucleotide binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-derived FAD binding annotation. Arc42 is a flavoprotein of the acyl-CoA dehydrogenase
      family with a bound FAD cofactor (UniProt cofactor FAD, ChEBI:57692); FAD is essential for the
      dehydrogenase reaction. Well-supported and correct.
    action: ACCEPT
    reason: >-
      FAD binding is an intrinsic, functionally important molecular function of every acyl-CoA
      dehydrogenase and is recorded as the UniProt cofactor for Arc42.
- term:
    id: GO:0033539
    label: fatty acid beta-oxidation using acyl-CoA dehydrogenase
  evidence_type: IMP
  original_reference_id: PMID:40519079
  qualifier: involved_in
  review:
    summary: >-
      Experimental (IMP) annotation from Geronazzo et al. 2025. CRISPR-Cas9 frameshift knockout of
      Arc42 (8 bp deletion, exon 2) produces elevated C4 (butyrylcarnitine), the acylcarnitine
      signature of human and mouse SCAD/ACADS deficiency, and this elevation is exacerbated by
      starvation. This is direct in-vivo genetic evidence that Arc42 functions in the acyl-CoA
      dehydrogenase step of short-chain fatty acid beta-oxidation.
    action: ACCEPT
    reason: >-
      The strongest, gene-specific experimental support for Arc42 function: loss of function
      recapitulates the short-chain FAO defect of the human ortholog. This is the core biological
      process and identifies Arc42 (rather than the paralog CG4860) as the functional fly SCAD.
    supported_by:
      - reference_id: PMID:40519079
        supporting_text: "C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected"
      - reference_id: PMID:40519079
        supporting_text: "Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking functional SCAD"
- term:
    id: GO:0003995
    label: acyl-CoA dehydrogenase activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: >-
      ISS annotation transferring acyl-CoA dehydrogenase activity from the well-characterized pig
      SCAD ortholog (UniProtKB:P15651). Correct parent term; the short-chain child term (GO:0016937)
      better reflects Arc42 substrate specificity.
    action: KEEP_AS_NON_CORE
    reason: >-
      Accurate but broad; subsumed by the specific short-chain ACAD activity annotation. The pig SCAD
      source is an appropriate ortholog for sequence-similarity transfer.
- term:
    id: GO:0005759
    label: mitochondrial matrix
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: >-
      ISS annotation transferring mitochondrial matrix localization from the pig SCAD ortholog
      (UniProtKB:P15651). SCAD-family enzymes are soluble mitochondrial matrix beta-oxidation
      enzymes; this is the correct, specific compartment for Arc42.
    action: ACCEPT
    reason: >-
      Matrix localization is the documented compartment for SCAD orthologs and the appropriate core
      location for this soluble beta-oxidation flavoenzyme.
- term:
    id: GO:0033539
    label: fatty acid beta-oxidation using acyl-CoA dehydrogenase
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: >-
      ISS annotation transferring the acyl-CoA dehydrogenase step of fatty acid beta-oxidation from
      the pig SCAD ortholog. Duplicates the IBA and IMP process annotations; correct and
      appropriately specific for Arc42.
    action: ACCEPT
    reason: >-
      Consistent with the phylogenetic, orthology-based, and in-vivo experimental evidence that
      Arc42 catalyzes the ACAD step of short-chain fatty acid beta-oxidation. Core biological process.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: GO_REF:0000003
  title: Gene Ontology annotation based on Enzyme Commission mapping
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:40519079
  title: Characterizing fatty acid oxidation genes in Drosophila.
  findings:
    - statement: >-
        CRISPR-Cas9 loss-of-function of Arc42, the higher-scoring of two predicted Drosophila ACADS
        orthologs, produces elevated C4 (butyrylcarnitine) that is exacerbated by starvation,
        recapitulating the acylcarnitine signature of human and mouse SCAD deficiency; the paralog
        CG4860 does not, identifying Arc42 as the functional fly short-chain acyl-CoA dehydrogenase.
      supporting_text: >-
        Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking
        functional SCAD, while CG4860 knockout flies did not
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed/PMC-verified primary research (G3 Bethesda 2025, PMC12341901); full text available.
      Directly establishes Arc42 as the functional ACADS ortholog in Drosophila via CRISPR knockout
      and acylcarnitine profiling (elevated C4). Basis for the FlyBase IMP annotation on GO:0033539.
      Note the legacy fly name "Activator-recruited cofactor subunit 42" is a misnomer; the closest
      human symbol is ACADS.
core_functions:
- description: >-
    Arc42 catalyzes the first, FAD-dependent step of mitochondrial fatty acid beta-oxidation for
    short-chain substrates: the alpha,beta-dehydrogenation of saturated short-chain (C4-C6, optimum
    butyryl-CoA/C4) acyl-CoA thioesters to the corresponding trans-2-enoyl-CoA, transferring
    electrons to electron-transfer flavoprotein (ETF). It acts in the mitochondrial matrix as the
    functional Drosophila ortholog of human ACADS (SCAD); loss of Arc42 elevates C4 acylcarnitine,
    the biochemical hallmark of SCAD deficiency.
  molecular_function:
    id: GO:0016937
    label: short-chain fatty acyl-CoA dehydrogenase activity
  directly_involved_in:
  - id: GO:0033539
    label: fatty acid beta-oxidation using acyl-CoA dehydrogenase
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: PMID:40519079
    supporting_text: "Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking functional SCAD"
  - reference_id: PMID:40519079
    supporting_text: "C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected"
- description: >-
    Cofactor binding required for catalysis. As an acyl-CoA dehydrogenase family flavoprotein,
    Arc42 binds one non-covalently associated FAD per subunit, the redox cofactor that accepts the
    two electrons removed from the short-chain acyl-CoA substrate before passing them to
    electron-transfer flavoprotein (ETF); FAD is essential for the dehydrogenase reaction.
  molecular_function:
    id: GO:0050660
    label: flavin adenine dinucleotide binding
  directly_involved_in:
  - id: GO:0033539
    label: fatty acid beta-oxidation using acyl-CoA dehydrogenase
  locations:
  - id: GO:0005759
    label: mitochondrial matrix
  supported_by:
  - reference_id: file:DROME/Arc42/Arc42-uniprot.txt
    supporting_text: "Name=FAD; Xref=ChEBI:CHEBI:57692"
proposed_new_terms: []
suggested_questions:
- question: >-
    Does purified recombinant Arc42 protein exhibit short-chain acyl-CoA dehydrogenase activity in
    vitro with the expected C4-C6 substrate preference and butyryl-CoA optimum, confirming the
    in-vivo genetic inference?
- question: >-
    What is the division of labor between the two predicted fly ACADS orthologs Arc42 and CG4860,
    given that only Arc42 loss reproduces the SCAD-deficiency C4 signature while CG4860 loss lowers
    C4 and raises C2 acylcarnitine?
suggested_experiments:
- description: >-
    In-vitro enzyme kinetics of purified recombinant Arc42 across the C4-C8 acyl-CoA range (with ETF
    as electron acceptor) to directly demonstrate short-chain acyl-CoA dehydrogenase activity and
    delimit its substrate specificity relative to fly Mcad.
- description: >-
    Subcellular localization of tagged Arc42 (e.g. immunofluorescence or fractionation) to
    experimentally confirm mitochondrial matrix localization in Drosophila, which currently rests on
    ISS transfer from the pig SCAD ortholog.
- description: >-
    Comparative functional analysis of Arc42 and CG4860 (e.g. tissue-specific rescue, double
    knockouts, and metabolomics) to define whether CG4860 acts in short-chain acyl-CoA synthesis or
    ketone utilization as suggested by its distinct acylcarnitine phenotype.