Mtpalpha (CG4389) encodes the Drosophila alpha subunit of the mitochondrial trifunctional protein (MTP), the fly ortholog of human HADHA. With the beta subunit (Mtpbeta) it forms a membrane-associated alpha2-beta2 heterotetramer on the matrix face of the mitochondrial inner membrane that catalyzes the last three steps of long-chain fatty acid beta-oxidation. The alpha subunit carries TWO of these activities: long-chain enoyl-CoA hydratase (EC 4.2.1.17), which hydrates (2E)-enoyl-CoA to (3S)-3-hydroxyacyl-CoA, and long-chain 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.211), which oxidizes (3S)-3-hydroxyacyl-CoA to 3-oxoacyl-CoA using NAD+. The third step (thiolytic cleavage to acetyl-CoA) is carried by the beta subunit (Mtpbeta), not by the alpha subunit. The protein has the conserved two-domain architecture of its human ortholog: an N-terminal enoyl-CoA hydratase/crotonase domain and a central NAD-binding 3-hydroxyacyl-CoA dehydrogenase domain. Loss of Mtpalpha in the fly impairs long-chain fatty acid beta-oxidation, causing accumulation of acylcarnitine and hydroxyacylcarnitine, defective lipid mobilization on fasting, reduced adult lifespan and locomotor activity, modeling human MTP deficiency. Like human HADHA, the alpha subunit is additionally predicted to have a monolysocardiolipin acyltransferase activity implicated in cardiolipin remodeling, though this has not been experimentally demonstrated in Drosophila.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004300 enoyl-CoA hydratase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Enoyl-CoA hydratase activity (EC 4.2.1.17) assigned by curator sequence-similarity transfer from the rat MTP alpha subunit (UniProtKB:Q64428). This is one of the two core catalytic activities of the alpha subunit, hydrating (2E)-enoyl-CoA to (3S)-3-hydroxyacyl-CoA in step 2 of each long-chain beta-oxidation cycle. Mtpalpha has the conserved N-terminal crotonase/enoyl-CoA hydratase domain and the residues flagged as important for long-chain enoyl-CoA hydratase activity. Reason: Core molecular function of the alpha subunit, conserved from the human/rat ortholog and consistent with the domain architecture. Supporting Evidence: file:DROME/Mtpalpha/Mtpalpha-uniprot.txt Important for long-chain enoyl-CoA hydratase activity |
| GO:0004300 enoyl-CoA hydratase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) annotation of enoyl-CoA hydratase activity based on InterPro/RHEA/EC mapping. Consistent with the ISS annotation and the conserved crotonase domain. Reason: Redundant with the ISS annotation but correctly captures the core hydratase function of the alpha subunit. |
| GO:0016509 long-chain (3S)-3-hydroxyacyl-CoA dehydrogenase (NAD+) activity | ISS GO_REF:0000024 | ACCEPT | Summary: Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD, EC 1.1.1.211) assigned by curator sequence-similarity transfer from the rat MTP alpha subunit (UniProtKB:Q64428). This is the second core catalytic activity of the alpha subunit (step 3 of beta-oxidation, NAD+-dependent oxidation of (3S)-3-hydroxyacyl-CoA to 3-oxoacyl-CoA). The long-chain (rather than generic) term correctly reflects the substrate specificity of the MTP. Reason: Core molecular function of the alpha subunit, conserved from the ortholog and matching the central NAD-binding 3-hydroxyacyl-CoA dehydrogenase domain. Supporting Evidence: file:DROME/Mtpalpha/Mtpalpha-uniprot.txt 3-hydroxyacyl-CoA dehydrogenase NAD binding |
| GO:0016509 long-chain (3S)-3-hydroxyacyl-CoA dehydrogenase (NAD+) activity | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic (IEA) annotation of long-chain 3-hydroxyacyl-CoA dehydrogenase activity based on RHEA/EC mapping (RHEA:31159|RHEA:31167|RHEA:52656|EC:1.1.1.211). Consistent with the ISS annotation. Reason: Redundant with the ISS annotation but correctly captures the core dehydrogenase function with the appropriate long-chain specificity. |
| GO:0003857 (3S)-3-hydroxyacyl-CoA dehydrogenase (NAD+) activity | IEA GO_REF:0000120 | MODIFY | Summary: Electronic annotation of the generic (chain-length-unspecified) 3-hydroxyacyl-CoA dehydrogenase activity. Correct in essence, but the MTP alpha subunit is long-chain specific, so the more specific term GO:0016509 (already present) better captures the molecular function. Reason: Generalize/replace with the long-chain-specific term GO:0016509, which is the appropriate level of specificity for the MTP alpha subunit. Proposed replacements: long-chain (3S)-3-hydroxyacyl-CoA dehydrogenase (NAD+) activity |
| GO:0018812 3-hydroxyacyl-CoA dehydratase activity | IEA GO_REF:0000116 | MODIFY | Summary: RHEA-based electronic annotation. GO:0018812 describes the same chemical reaction as enoyl-CoA hydratase (3-hydroxyacyl-CoA <=> 2-enoyl-CoA + H2O) written in the dehydration (reverse) direction. This is the alpha subunit's hydratase activity captured under a reverse-direction label. Reason: The conventional molecular-function term for this reaction in the MTP alpha subunit is enoyl-CoA hydratase activity (GO:0004300, EC 4.2.1.17, physiological hydration direction); the dehydratase label is the reverse framing of the same reaction. Proposed replacements: enoyl-CoA hydratase activity |
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Root-level catalytic activity term from InterPro mapping. Correct but entirely uninformative. Reason: Too general; the specific hydratase and dehydrogenase terms capture the molecular function. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Broad oxidoreductase parent term from InterPro mapping, reflecting the dehydrogenase domain. Reason: Correct but too general; GO:0016509 is the appropriate specific term. |
| GO:0016616 oxidoreductase activity, acting on the CH-OH group of donors, NAD or NADP as acceptor | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Intermediate-specificity parent of the 3-hydroxyacyl-CoA dehydrogenase activity, from InterPro mapping. Reason: Correct parent term; the specific child GO:0016509 should be used for core function. |
| GO:0070403 NAD+ binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: NAD+ binding from InterPro mapping, reflecting the NAD-binding Rossmann domain of the dehydrogenase. A cofactor-binding capability that supports but does not by itself describe the catalytic function. Reason: Correct molecular characteristic (the dehydrogenase uses NAD+), but subsidiary to the catalytic dehydrogenase activity. |
| GO:0006635 fatty acid beta-oxidation | IMP PMID:22342726 Impaired fatty acid oxidation in a Drosophila model of mitoc... | ACCEPT | Summary: IMP from Drosophila Mtpalpha knockout flies. Kishita et al. showed that Mtpalpha(KO) flies accumulate acylcarnitine and hydroxyacylcarnitine (diagnostic markers of MTP deficiency) and are impaired in long-chain fatty acid beta-oxidation, directly establishing the fly gene's role in this pathway. Reason: Core biological process with direct loss-of-function (mutant phenotype) evidence in the fly. Supporting Evidence: PMID:22342726 both MtpΞ±(KO) and MtpΞ²(KO) flies were impaired in long-chain fatty acid Ξ²-oxidation. |
| GO:0006635 fatty acid beta-oxidation | ISS GO_REF:0000024 | ACCEPT | Summary: Sequence-similarity annotation of fatty acid beta-oxidation transferred from the rat MTP alpha subunit (UniProtKB:Q64428). Consistent with the direct fly IMP evidence. Reason: Redundant but correctly captures the core biological process. |
| GO:0006635 fatty acid beta-oxidation | IEA GO_REF:0000120 | ACCEPT | Summary: Electronic annotation of fatty acid beta-oxidation based on InterPro/UniPathway mapping, consistent with the ISS and IMP evidence. Reason: Redundant but correctly captures the core biological process. |
| GO:0006631 fatty acid metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Broad parent term of fatty acid beta-oxidation from InterPro mapping. Reason: Correct but too general; GO:0006635 (fatty acid beta-oxidation) is the appropriate specific term. |
| GO:0008340 determination of adult lifespan | IMP PMID:22342726 Impaired fatty acid oxidation in a Drosophila model of mitoc... | KEEP AS NON CORE | Summary: IMP based on the reduced lifespan of Mtpalpha(KO) flies. This is a whole-organism phenotypic consequence of impaired long-chain fatty acid beta-oxidation rather than a direct molecular role of the enzyme in lifespan determination. Reason: Downstream, pleiotropic phenotype of the metabolic deficiency; the core function is the beta-oxidation enzyme activity, not lifespan determination per se. Supporting Evidence: PMID:22342726 demonstrated reduced lifespan, defective locomotor activity, and reduced fecundity |
| GO:0042594 response to starvation | IMP PMID:22342726 Impaired fatty acid oxidation in a Drosophila model of mitoc... | KEEP AS NON CORE | Summary: IMP based on the fasting hypersensitivity of Mtpalpha(KO) flies, which retain lipid droplets and fail to mobilize lipid on fasting. This reflects the inability to catabolize fatty acids via beta-oxidation during nutrient deprivation. Reason: Physiological consequence of impaired beta-oxidation (defective lipid mobilization on fasting), not a distinct core molecular role. Supporting Evidence: PMID:22342726 MtpΞ±(KO) flies were hypersensitive to fasting, and retained lipid droplets in their fat body cells as in non-fasting conditions. |
| GO:0042060 wound healing | HMP PMID:19884309 Genetic screen in Drosophila melanogaster uncovers a novel s... | KEEP AS NON CORE | Summary: HMP annotation derived from a high-throughput transposon-insertion screen for embryonic epithelial (wound) repair, which recovered 30 lethal insertional mutants with repair defects. The abstract foregrounds karst/beta-Heavy-spectrin, DJUN and scab and does not name Mtpalpha; the FlyBase curator identified Mtpalpha as one of the screen hits. As a broad-phenotype screen hit for an energy-metabolism enzyme, this most likely reflects a pleiotropic/indirect requirement rather than a direct molecular role in wound healing. Reason: High-throughput screen hit for an essential metabolic gene; the wound-healing phenotype is most plausibly an indirect consequence of impaired energy metabolism, not a core function. Supporting Evidence: PMID:19884309 identification of 30 lethal insertional mutants with defects in embryonic epithelia repair |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation of mitochondrial inner membrane localization based on UniProt subcellular location. The MTP complex is membrane-associated on the matrix face of the inner membrane. Reason: Core cellular localization, consistent with the ortholog and the membrane-associated MTP complex. Supporting Evidence: file:DROME/Mtpalpha/Mtpalpha-uniprot.txt Mitochondrion inner membrane |
| GO:0016507 mitochondrial fatty acid beta-oxidation multienzyme complex | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic annotation of MTP complex membership. Mtpalpha is the alpha subunit of the alpha2-beta2 heterotetramer with Mtpbeta. Reason: Core cellular component; Mtpalpha is an obligate subunit of the mitochondrial trifunctional protein complex. Supporting Evidence: file:DROME/Mtpalpha/Mtpalpha-uniprot.txt Heterotetramer of 2 alpha/HADHA and 2 beta/HADHB subunits |
| GO:0005739 mitochondrion | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: General mitochondrial localization from InterPro mapping. Correct but less specific than the inner membrane annotation. Reason: Correct but subsumed by the more specific GO:0005743 / GO:0016507. |
| GO:0005739 mitochondrion | HDA PMID:19317464 Mapping organelle proteins and protein complexes in Drosophi... | KEEP AS NON CORE | Summary: HDA mitochondrial localization from a LOPIT organelle-mapping proteomics study of Drosophila embryos. Reason: Correct mitochondrial localization from proteomics; less specific than the inner membrane / complex terms. |
| GO:0005739 mitochondrion | ISM PMID:22758915 An inventory of peroxisomal proteins and pathways in Drosoph... | KEEP AS NON CORE | Summary: Sequence-model (ISM) prediction of mitochondrial localization associated with the Drosophila peroxisomal-proteome inventory study. Reason: Correct but predicted and less specific than the inner membrane / complex terms. |
| GO:0005739 mitochondrion | HDA PMID:16212416 Characterization of the Drosophila melanogaster mitochondria... | KEEP AS NON CORE | Summary: HDA mitochondrial localization from a 2D-gel/MALDI-TOF characterization of the Drosophila mitochondrial proteome. Reason: Correct mitochondrial localization from proteomics; less specific than the inner membrane / complex terms. |
| GO:0005777 peroxisome | IDA PMID:22758915 An inventory of peroxisomal proteins and pathways in Drosoph... | KEEP AS NON CORE | Summary: IDA peroxisomal localization from the Drosophila peroxisomal-proteome inventory. A peroxisomal pool is plausible for a fatty-acid beta-oxidation enzyme, but the canonical and functionally dominant location of the MTP alpha subunit is the mitochondrion. An independent OpenScientist analysis (held out from our review and run as a neutral function-assignment hypothesis) supports retaining this annotation: it finds a Drosophila-lineage-conserved C-terminal SKL peroxisomal targeting signal (PTS1), absent from vertebrate HADHA orthologs, and notes that targeting is isoform-dependent - the shorter Q8IPE8 isoform lacks the N-terminal mitochondrial presequence carried by the longer CG4389 isoform (Q9V397), consistent with genuine dual mitochondrial/peroxisomal localization rather than a spurious call. This dissociates Mtpalpha (peroxisome supported) from its sister gene Acat1, whose peroxisome prediction the same workflow refuted. Reason: Genuine but secondary localization; the core location is the mitochondrion (inner membrane) and the MTP complex, while a real peroxisomal pool is supported by a conserved C-terminal PTS1 and independent computational analysis. Supporting Evidence: PMID:22758915 The subcellular localization of five of these predicted peroxisomal proteins was confirmed. file:DROME/Mtpalpha/Mtpalpha-hypotheses/function-hypothesis-go-0005777/openscientist.md which lacks a mitochondrial targeting sequence (MTS) and was directly demonstrated to localize to peroxisomes by fluorescence microscopy |
| GO:0005777 peroxisome | ISS PMID:22758915 An inventory of peroxisomal proteins and pathways in Drosoph... | KEEP AS NON CORE | Summary: ISS peroxisomal localization transferred from an ortholog (UniProtKB:Q08426) in the peroxisomal-proteome study. Reason: Secondary localization; the core location is the mitochondrion. |
| GO:0005777 peroxisome | ISM PMID:22758915 An inventory of peroxisomal proteins and pathways in Drosoph... | KEEP AS NON CORE | Summary: ISM (sequence-model prediction) of peroxisomal localization from the peroxisomal-proteome study. Reason: Predicted secondary localization; the core location is the mitochondrion. |
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Download this section (compressed HTML)Q: Does Drosophila Mtpalpha possess the monolysocardiolipin acyltransferase / cardiolipin remodeling moonlighting activity described for its human ortholog HADHA, or is the UniProt MLCL-acyltransferase annotation purely an ARBA/orthology inference with no fly-specific experimental support?
Q: Is the peroxisomal pool of Mtpalpha functionally significant in Drosophila, or does the protein act essentially exclusively at the mitochondrial inner membrane? Given the OpenScientist finding of isoform-dependent targeting (the shorter Q8IPE8 isoform lacking the N-terminal mitochondrial presequence retained on the longer Q9V397 isoform, plus a conserved C-terminal SKL PTS1), do the two CG4389 isoforms partition between peroxisome and mitochondrion, and does the peroxisomal pool carry out beta-oxidation there?
Experiment: Express and purify recombinant Mtpalpha alone and as the alpha2-beta2 complex with Mtpbeta and assay enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities across an acyl-chain-length series (C4-C20).
Hypothesis: Fly Mtpalpha exhibits hydratase and dehydrogenase activities restricted to long-chain (>=C10) acyl-CoA substrates, as for the human ortholog.
Experiment: Test whether recombinant fly Mtpalpha can acylate monolysocardiolipin to cardiolipin in vitro, and whether Mtpalpha(KO) flies show altered cardiolipin acyl composition, to determine whether the cardiolipin-remodeling moonlighting activity is conserved in Drosophila.
Hypothesis: The MLCL acyltransferase / cardiolipin remodeling activity of the MTP alpha subunit is conserved in Drosophila and contributes to mitochondrial cardiolipin homeostasis.
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