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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016937 short-chain fatty acyl-CoA dehydrogenase activity | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:40519079 Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking functional SCAD |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | 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. 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. |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IBA GO_REF:0000033 | ACCEPT | 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. 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. Supporting Evidence: PMID:40519079 C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected |
| GO:0046359 butyrate catabolic process | IBA GO_REF:0000033 | KEEP AS NON CORE | 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. 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. Supporting Evidence: PMID:40519079 C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected |
| GO:0003995 acyl-CoA dehydrogenase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | 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). Reason: The broad parent term is accurate but subsumed by the more specific short-chain ACAD activity annotation, which better reflects Arc42 substrate specificity. |
| GO:0016627 oxidoreductase activity, acting on the CH-CH group of donors | IEA GO_REF:0000002 | KEEP AS NON CORE | 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. Reason: Correct intermediate-level term between general oxidoreductase and the specific short-chain ACAD activity; subsumed by the more specific MF annotation. |
| GO:0016937 short-chain fatty acyl-CoA dehydrogenase activity | IEA GO_REF:0000003 | ACCEPT | 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. 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. |
| GO:0050660 flavin adenine dinucleotide binding | IEA GO_REF:0000002 | ACCEPT | 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. Reason: FAD binding is an intrinsic, functionally important molecular function of every acyl-CoA dehydrogenase and is recorded as the UniProt cofactor for Arc42. |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IMP PMID:40519079 Characterizing fatty acid oxidation genes in Drosophila. | ACCEPT | 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. 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. Supporting Evidence: PMID:40519079 C4 was elevated in the homozygous Arc42 mutant flies, and this elevation was exacerbated in the starved condition, as expected PMID:40519079 Arc42 knockout flies exhibited the C4 elevation characteristic of humans and mice lacking functional SCAD |
| GO:0003995 acyl-CoA dehydrogenase activity | ISS GO_REF:0000024 | KEEP AS NON CORE | 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. 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. |
| GO:0005759 mitochondrial matrix | ISS GO_REF:0000024 | ACCEPT | 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. Reason: Matrix localization is the documented compartment for SCAD orthologs and the appropriate core location for this soluble beta-oxidation flavoenzyme. |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | ISS GO_REF:0000024 | ACCEPT | 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. 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. |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Experiment: 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.
Experiment: 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.
Experiment: 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.
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