Mtpbeta (CG4581; dHADHB) is the single Drosophila melanogaster ortholog of human HADHB and encodes the beta subunit of the mitochondrial trifunctional protein (MTP), an alpha2-beta2 heterotetramer formed with Mtpalpha (the fly HADHA ortholog). Within this complex the beta subunit provides the long-chain 3-ketoacyl-CoA thiolase (acetyl-CoA C-acyltransferase, EC 2.3.1.16) activity that catalyzes the final, thiolytic-cleavage step of each round of mitochondrial long-chain fatty acid beta-oxidation, releasing acetyl-CoA and a chain-shortened acyl-CoA. It is a member of the thiolase-like superfamily (thiolase family) with tandem N- and C-terminal thiolase domains and is imported into mitochondria. Loss of Mtpbeta in the fly impairs long-chain fatty acid beta-oxidation, causing accumulation of acylcarnitines and hydroxyacylcarnitines and organismal phenotypes including reduced lifespan, defective locomotion and reduced fecundity, mirroring human MTP deficiency.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003985 acetyl-CoA C-acetyltransferase activity | IBA GO_REF:0000033 | MODIFY | Summary: IBA (phylogenetic) annotation of thiolase/acetyltransferase activity to Mtpbeta. Mtpbeta is the fly ortholog of human HADHB and provides the thiolase activity of the MTP complex, so a thiolase-family molecular function is correct. GO:0003985 uses the C-acetyltransferase (EC 2.3.1.9) label, which is a closely related short-chain homolog activity; the specific long-chain MTP beta-subunit activity is better captured by GO:0003988 (acetyl-CoA C-acyltransferase, EC 2.3.1.16). Reason: The essence (thiolase/acyltransferase activity of the MTP beta subunit) is correct, but the more precise term for the long-chain trifunctional-protein thiolase is GO:0003988 (EC 2.3.1.16), which is separately annotated by ISS from human HADHB. Proposed replacements: acetyl-CoA C-acyltransferase activity |
| GO:0006635 fatty acid beta-oxidation | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation placing Mtpbeta in the fatty acid beta-oxidation pathway. This is the core biological process of the MTP complex and is directly supported in the fly by genetic loss-of-function evidence (PMID:22342726). Reason: Core biological process. Mtpbeta(KO) flies are impaired in long-chain fatty acid beta-oxidation (PMID:22342726), consistent with the conserved MTP function. Supporting Evidence: PMID:22342726 Our results indicated that both MtpΞ±(KO) and MtpΞ²(KO) flies were impaired in long-chain fatty acid Ξ²-oxidation. |
| GO:0016507 mitochondrial fatty acid beta-oxidation multienzyme complex | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation placing Mtpbeta as part of the mitochondrial fatty acid beta-oxidation multienzyme complex (the MTP / trifunctional protein). Mtpbeta is an obligate beta subunit of the alpha2-beta2 complex, so this is correct. Reason: Core cellular component / complex membership. Mtpbeta forms the MTP complex with Mtpalpha; the complex catalyzes long-chain fatty acid beta-oxidation (PMID:22342726). Supporting Evidence: PMID:22342726 Mitochondrial trifunctional protein (MTP), which consists of the MTPΞ± and MTPΞ² subunits, catalyzes long-chain fatty acid Ξ²-oxidation. |
| GO:0003985 acetyl-CoA C-acetyltransferase activity | IEA GO_REF:0000003 | MODIFY | Summary: IEA annotation derived from EC-number mapping (EC:2.3.1.9). This maps to the C-acetyltransferase activity, a related thiolase-family reaction, rather than the specific long-chain acyltransferase (EC 2.3.1.16) that defines the MTP beta subunit. Technically within the thiolase family but not the most precise term. Reason: The thiolase-family activity is correct, but GO:0003988 (EC 2.3.1.16) is the accurate MF for the long-chain MTP beta-subunit thiolase; the C-acetyltransferase (EC 2.3.1.9) mapping is a broader/related family activity. Proposed replacements: acetyl-CoA C-acyltransferase activity |
| GO:0003988 acetyl-CoA C-acyltransferase activity | IEA GO_REF:0000117 | ACCEPT | Summary: IEA annotation (ARBA machine-learning model) assigning acetyl-CoA C-acyltransferase activity (EC 2.3.1.16), which is the 3-ketoacyl-CoA thiolase activity that characterizes the MTP beta subunit. This is the correct, specific molecular function. Reason: Core molecular function. EC 2.3.1.16 (acetyl-CoA C-acyltransferase / long-chain 3-ketoacyl-CoA thiolase) is the reaction catalyzed by the MTP beta subunit; UniProt assigns this EC to Mtpbeta. Supporting Evidence: file:DROME/Mtpbeta/Mtpbeta-uniprot.txt RecName: Full=acetyl-CoA C-acyltransferase |
| GO:0005739 mitochondrion | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: IEA annotation based on UniProt subcellular-location mapping (mitochondrion). Consistent with the UniProt SUBCELLULAR LOCATION statement and with orthology to human HADHB, which resides in the mitochondrial inner membrane as part of the MTP complex. Reason: Correct compartment but non-specific. The functional pool is the MTP complex at the mitochondrial inner membrane; the complex-membership term GO:0016507 is more informative for the localization of the active enzyme. Supporting Evidence: file:DROME/Mtpbeta/Mtpbeta-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion |
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro-to-GO annotation capturing the thiolase-domain acyltransferase function. Correct but a broad parent of the specific acetyl-CoA C-acyltransferase activity. Reason: Technically correct parent term based on the thiolase domain, but uninformative. The specific child term GO:0003988 should be used for the core molecular function. |
| GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro-to-GO annotation from the thiolase domain signatures. Correct but a broad parent term. Reason: Correct thiolase-domain-derived parent term, but less informative than the specific acetyl-CoA C-acyltransferase activity (GO:0003988). |
| GO:0016507 mitochondrial fatty acid beta-oxidation multienzyme complex | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation transferring MTP complex membership from human HADHB (UniProtKB:P55084) by curator sequence-similarity judgment. Mtpbeta is the clear ortholog and an obligate beta subunit of the alpha2-beta2 MTP complex, so the transfer is appropriate. Reason: Core complex membership, well supported by orthology to human HADHB and by the fly MTP genetic model (PMID:22342726). Consistent with the IBA annotation of the same term. |
| GO:0003988 acetyl-CoA C-acyltransferase activity | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation transferring acetyl-CoA C-acyltransferase (3-ketoacyl-CoA thiolase, EC 2.3.1.16) activity from a mammalian HADHB ortholog (UniProtKB:Q99JY0, mouse) by curator sequence-similarity judgment. This is the correct, specific molecular function for the MTP beta subunit. Reason: Core molecular function. The long-chain 3-ketoacyl-CoA thiolase activity of the MTP beta subunit is well established in mammals and transferred by strong orthology to the single-copy fly gene. |
| GO:0005739 mitochondrion | HDA PMID:19317464 Mapping organelle proteins and protein complexes in Drosophi... | KEEP AS NON CORE | Summary: HDA annotation from a high-throughput LOPIT organelle-proteomics study of Drosophila embryos, which mapped proteins to subcellular compartments by mass spectrometry. Supports mitochondrial localization of Mtpbeta. Reason: Experimentally supported mitochondrial localization, but the general mitochondrion compartment is less informative than the complex-membership term GO:0016507. The specific inner-membrane MTP-complex pool is not resolved by this dataset. Supporting Evidence: PMID:19317464 we apply LOPIT, a mass-spectrometry based technique that simultaneously maps proteins to specific subcellular compartments, to Drosophila embryos. |
| GO:0005739 mitochondrion | ISM PMID:22758915 An inventory of peroxisomal proteins and pathways in Drosoph... | KEEP AS NON CORE | Summary: ISM annotation (sequence-model / targeting-signal prediction) of mitochondrial localization, arising from a Drosophila peroxisome-inventory bioinformatics survey that predicted subcellular localizations proteome-wide. Consistent with the experimental HDA mitochondrion annotation. Reason: Predicted mitochondrial localization, consistent with other evidence but non-specific and lower-confidence than the HDA annotation. The complex-membership term GO:0016507 is more informative. |
| GO:0006635 fatty acid beta-oxidation | IMP PMID:22342726 Impaired fatty acid oxidation in a Drosophila model of mitoc... | ACCEPT | Summary: IMP annotation from the Drosophila MTP-deficiency model. Mtpbeta(KO) flies accumulate acylcarnitines and hydroxyacylcarnitines and are impaired in long-chain fatty acid beta-oxidation, providing direct fly genetic evidence for Mtpbeta's role in this pathway. Reason: Core biological process with direct experimental (genetic) support in the fly. Supporting Evidence: PMID:22342726 both MtpΞ±(KO) and MtpΞ²(KO) flies accumulated acylcarnitine and hydroxyacylcarnitine, diagnostic markers of MTP deficiencies in humans. |
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Download this section (compressed HTML)Q: Does the FlyBase IMP "determination of adult lifespan" annotation (GO:0008340, based on the neuron-specific dHADHB knockdown study PMID:30953623) reflect a direct role of Mtpbeta in lifespan determination, or an indirect consequence of impaired neuronal energy metabolism? This annotation is present in UniProt GO cross-references but not in the current QuickGO export reviewed here.
Q: Is the substrate specificity of the fly MTP beta subunit restricted to long-chain 3-ketoacyl-CoAs (C10-C16) as in mammals, and does GO:0003985 (C-acetyltransferase, EC 2.3.1.9) reflect any genuine short-chain activity or only EC/phylogenetic mapping?
Experiment: Assay 3-ketoacyl-CoA thiolase activity of recombinant Mtpbeta (alone and reconstituted with Mtpalpha) across acyl-chain lengths.
Hypothesis: Mtpbeta supplies the sole thiolase activity of the fly MTP complex and prefers long-chain (C10-C16) 3-ketoacyl-CoA substrates, consistent with human HADHB.
Experiment: Test whether tissue-restricted (e.g. neuronal vs. fat body) rescue of Mtpbeta(KO) flies separates the beta-oxidation metabolic defect from the organismal lifespan/locomotor phenotypes.
Hypothesis: The lifespan and locomotor phenotypes of Mtpbeta loss arise from tissue-specific energy deficits downstream of impaired long-chain fatty acid beta-oxidation.
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