ETFA is the alpha subunit of the electron-transfer flavoprotein (ETF), a soluble FAD-containing heterodimer of ETFA and ETFB that resides in the mitochondrial matrix (on the matrix face of the inner membrane). ETF is the common electron acceptor for at least a dozen mitochondrial FAD-dependent primary dehydrogenases, including the chain-length-specific acyl-CoA dehydrogenases of fatty-acid beta-oxidation (e.g. ACADM/MCAD, ACADVL) and dehydrogenases of amino-acid catabolism (IVD, glutaryl-CoA dehydrogenase GCDH) as well as sarcosine/dimethylglycine dehydrogenases of choline degradation. ETF accepts reducing equivalents from these dehydrogenases, storing them transiently on its single bound FAD, and passes the electrons to ETF-ubiquinone oxidoreductase (ETFDH / ETF-QO), which reduces the ubiquinone pool and thereby funnels the electrons into the mitochondrial respiratory chain. The heterodimer binds one FAD and one AMP per dimer; most of the FAD is coordinated by the C-terminal domain of the alpha subunit. ETF is thus the third major electron feeder to the respiratory chain after complexes I and II, and is required for normal mitochondrial fatty acid oxidation and amino acid metabolism. Biallelic loss-of-function of ETFA (or ETFB or ETFDH) causes multiple acyl-CoA dehydrogenase deficiency (MADD) / glutaric acidemia type II.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetic (IBA) localization of ETFA to the mitochondrion. Correct but general; ETF acts specifically in the mitochondrial matrix, which is separately and more precisely annotated (GO:0005759). Reason: The compartment is correct, but "mitochondrion" is the broad parent of the more specific and better-supported "mitochondrial matrix" location. Retained as non-core context; the matrix annotation is preferred as the core localization. Supporting Evidence: PMID:8617498 obligatory electron acceptor for several dehydrogenases and is located in the |
| GO:0009055 electron transfer activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment of electron transfer activity, the core molecular function of ETF: it accepts electrons from primary flavoprotein dehydrogenases and passes them to ETFDH. Well supported experimentally and by the phylogeny of the ETF alpha/FixB family. Reason: This is the central, defining molecular function of the ETF alpha subunit and is concordant with the direct experimental annotations (PMID:9334218, PMID:10423253) and the UniProt FUNCTION statement. Supporting Evidence: PMID:10423253 serves as an intermediate electron carrier |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment linking ETF to fatty-acid beta-oxidation. ETF is the obligate electron acceptor for the chain-length-specific acyl-CoA dehydrogenases of the beta-oxidation spiral, so its activity is required for flux through this pathway. Reason: Strongly supported: ETF accepts electrons from the beta-oxidation acyl-CoA dehydrogenases, and this term is also directly annotated (IDA, PMID:25416781). Loss of ETFA causes a functional block in fatty-acid beta-oxidation (MADD). Supporting Evidence: PMID:9334218 Among these dehydrogenases are the four chain PMID:25416781 Some of these dehydrogenases, e.g. medium chain acyl-CoA dehydrogenase (MCAD), are involved in Ξ²-oxidation of fatty acids |
| GO:0050660 flavin adenine dinucleotide binding | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) assignment of FAD binding. ETF carries a single FAD per heterodimer, most of which is coordinated by the alpha subunit; FAD is the redox cofactor that transiently stores the transferred electrons. Reason: Core cofactor-binding function of ETFA, corroborated by direct experimental annotations (IDA, PMID:9334218, PMID:10423253) and by the crystallographically defined FAD-binding residues on the alpha subunit in UniProt. Supporting Evidence: PMID:8962055 most of the FAD molecule residing in the |
| GO:0005739 mitochondrion | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: Electronic (ARBA) localization to the mitochondrion. Correct but general relative to the experimentally supported mitochondrial matrix location. Reason: Compartment is correct; broader than the specific matrix annotation. Retained as non-core. |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic annotation (UniProt Subcellular Location keyword mapping) placing ETFA in the mitochondrial matrix, the compartment where ETF operates. Reason: Consistent with the experimentally determined subcellular location (IDA, PMID:3760196; NAS, PMID:8504797) and with the biology of ETF as a matrix electron shuttle. Supporting Evidence: PMID:8617498 obligatory electron acceptor for several dehydrogenases and is located in the |
| GO:0009055 electron transfer activity | IEA GO_REF:0000002 | ACCEPT | Summary: Electronic (InterPro2GO, IPR001308 ETF alpha/FixB) assignment of electron transfer activity, the core molecular function of the ETF alpha subunit. Reason: The InterPro family (ETF alpha/FixB) maps correctly to electron transfer activity, in agreement with the direct experimental and phylogenetic annotations. Supporting Evidence: PMID:10423253 serves as an intermediate electron carrier |
| GO:0050660 flavin adenine dinucleotide binding | IEA GO_REF:0000002 | ACCEPT | Summary: Electronic (InterPro2GO, IPR001308) assignment of FAD binding, the redox cofactor carried by the alpha subunit. Reason: Correct InterPro-to-GO mapping for the ETF alpha/FixB family; concordant with direct experimental FAD-binding annotations and the crystallographic FAD-binding residues. Supporting Evidence: PMID:8962055 most of the FAD molecule residing in the |
| GO:0005515 protein binding | IPI PMID:24606901 Cochaperone binding to LYR motifs confers specificity of iro... | MARK AS OVER ANNOTATED | Summary: IntAct-curated binary interaction (with HSCB, Q8IWL3) captured as the uninformative term "protein binding". Derived from a study of the Fe-S cluster cochaperone HSC20/HSCB. Reason: "protein binding" (GO:0005515) is not an informative molecular function and does not document ETFA's role. The interaction is not further pursued in this HSC20-focused study; per curation policy this bare IPI is marked as over-annotated rather than removed. Supporting Evidence: PMID:24606901 LYR motifs |
| GO:0005515 protein binding | IPI PMID:26618866 βF508 CFTR interactome remodelling promotes rescue of cystic... | MARK AS OVER ANNOTATED | Summary: IntAct-curated binary interaction (with CFTR, P13569) from a large-scale CFTR-interactome remodelling study, captured as the uninformative term "protein binding". Reason: Bare "protein binding" from a high-throughput interactome dataset; not an informative molecular function for ETFA and no dedicated functional follow-up. Marked as over-annotated per policy. |
| GO:0005515 protein binding | IPI PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with ETFB, P38117) from the mitochondrial protein interaction map that identified LYRM5/ETFRF1 as a "deflavinase" directly regulating ETF. Captured only as the generic term "protein binding". Reason: The underlying interaction is biologically real and meaningful (ETF is directly regulated by LYRM5/ETFRF1, and ETFA obligately partners ETFB), but the GO term "protein binding" is uninformative. The functional relationships are better captured by the ETF-complex (GO:0045251) and electron-transfer annotations; the bare IPI is over-annotated. Supporting Evidence: PMID:27499296 We also establish LYRM5 as a βdeflavinaseβ that directly regulates the electron transferring flavoprotein (ETF) |
| GO:0005515 protein binding | IPI PMID:28380382 A Single Adaptable Cochaperone-Scaffold Complex Delivers Nas... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with HSCB, Q8IWL3) from a study of the HSC20 cochaperone- scaffold Fe-S delivery complex; captured as the generic term "protein binding". Reason: Bare "protein binding" from a study centred on respiratory-chain Fe-S cluster delivery, not on ETFA function. Uninformative molecular function; marked over-annotated per policy. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with ETFB, P38117) from a proteome-scale (BioPlex) interactome network, captured as the uninformative term "protein binding". Reason: Bare "protein binding" from a high-throughput interactome; the ETFA-ETFB partnership is informatively captured by the ETF complex annotation (GO:0045251). Marked over-annotated. |
| GO:0005515 protein binding | IPI PMID:35156780 CFTR interactome mapping using the mammalian membrane two-hy... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with CFTR, P13569) from a CFTR mammalian membrane two-hybrid screen, captured as the uninformative term "protein binding". Reason: Bare "protein binding" from a high-throughput CFTR screen; not an informative ETFA molecular function. Marked over-annotated per policy. |
| GO:0005515 protein binding | IPI PMID:36012204 Differential CFTR-Interactome Proximity Labeling Procedures ... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with CFTR, P13569) from a CFTR proximity-labeling interactome study, captured as the uninformative term "protein binding". Reason: Bare "protein binding" from a high-throughput proximity-labeling dataset; uninformative for ETFA function. Marked over-annotated per policy. |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with ETFB, P38117) from a multimodal cell-map / structural and functional genomics study, captured as the uninformative term "protein binding". Reason: Bare "protein binding" from a large-scale cell-mapping dataset; the ETFA-ETFB relationship is better captured by the ETF complex annotation. Marked over-annotated per policy. |
| GO:0005759 mitochondrial matrix | IDA PMID:3760196 Biosynthesis of electron transfer flavoprotein in a cell-fre... | ACCEPT | Summary: Direct experimental (IDA) evidence that alpha-ETF is synthesized in the cytosol as a larger precursor, imported into mitochondria, and processed to the mature form in the matrix. This is the core, experimentally defined localization of ETFA. Reason: Biosynthesis/import study demonstrating mitochondrial (matrix) localization of the processed alpha subunit; it is the basis for the UniProt Subcellular Location and matches the biology of ETF as a matrix electron shuttle. Supporting Evidence: PMID:3760196 translocated into the |
| GO:0009055 electron transfer activity | IDA PMID:9334218 Expression and characterization of two pathogenic mutations ... | ACCEPT | Summary: Direct experimental (IDA) demonstration of ETF electron transfer activity: recombinant wild-type and pathogenic-mutant ETF were assayed for electron transfer from primary dehydrogenases and onward to ETF-QO, with mutations impairing this activity. Reason: Direct biochemical characterization of the core electron-transfer function of ETF, including the effect of GA2A mutations on activity. This is a defining core function. Supporting Evidence: PMID:9334218 electron transfer from nine primary flavoprotein dehydrogenases to the main |
| GO:0005759 mitochondrial matrix | NAS PMID:8504797 cDNA cloning and mitochondrial import of the beta-subunit of... | ACCEPT | Summary: Non-traceable author statement (ComplexPortal) supporting mitochondrial matrix localization; the cited work shows the ETF beta subunit is imported to the mitochondrial matrix, consistent with the matrix location of the ETF heterodimer. Reason: Concordant with the direct experimental IDA matrix annotation (PMID:3760196) and the established matrix location of ETF. Retained as supporting the core localization. Supporting Evidence: PMID:8504797 reach the mitochondrial matrix |
| GO:0009063 amino acid catabolic process | IDA PMID:25416781 Human METTL20 is a mitochondrial lysine methyltransferase th... | ACCEPT | Summary: Direct experimental (IDA, ComplexPortal) evidence that ETF participates in amino-acid catabolism: the ETF alpha/beta heterodimer receives electrons from amino-acid-oxidation dehydrogenases such as glutaryl-CoA dehydrogenase (GCDH) and isovaleryl-CoA dehydrogenase. Reason: ETF is the obligate electron acceptor for amino-acid catabolic dehydrogenases; loss of ETFA impairs amino-acid metabolism (a component of the MADD phenotype). Supported by the cited study characterizing ETF electron transfer from GCDH. Supporting Evidence: PMID:25416781 others, including glutaryl-CoA dehydrogenase (GCDH) and isovaleryl-CoA dehydrogenase, are involved in the oxidation of amino acids |
| GO:0022904 respiratory electron transport chain | IDA PMID:25416781 Human METTL20 is a mitochondrial lysine methyltransferase th... | ACCEPT | Summary: Direct experimental (IDA, ComplexPortal) evidence linking ETF to the respiratory electron transport chain: ETF shuttles electrons from matrix dehydrogenases to ETF:quinone oxidoreductase and thence to the ubiquinone pool, acting as the third major electron provider to the respiratory chain after complexes I and II. Reason: ETF feeds electrons into the respiratory chain via ETFDH/ubiquinone; this is a core physiological role of ETFA, well documented in the cited study. Supporting Evidence: PMID:25416781 it is the third major provider of electrons to the ubiquinone |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IDA PMID:25416781 Human METTL20 is a mitochondrial lysine methyltransferase th... | ACCEPT | Summary: Direct experimental (IDA, ComplexPortal) evidence that ETF participates in acyl-CoA- dehydrogenase-dependent fatty-acid beta-oxidation: the heterodimer receives electrons from dehydrogenases such as medium-chain acyl-CoA dehydrogenase (MCAD) involved in beta-oxidation. Reason: Core physiological role: ETF is the obligate electron acceptor for the beta-oxidation acyl-CoA dehydrogenases. Duplicate of the IBA annotation to the same term; both accepted. Supporting Evidence: PMID:25416781 Some of these dehydrogenases, e.g. medium chain acyl-CoA dehydrogenase (MCAD), are involved in Ξ²-oxidation of fatty acids |
| GO:0045251 electron transfer flavoprotein complex | IPI PMID:8962055 Three-dimensional structure of human electron transfer flavo... | ACCEPT | Summary: ETFA is a subunit of the electron transfer flavoprotein complex, the FAD-containing alpha/beta heterodimer whose crystal structure was solved to 2.1 A. ETFA contributes two of the three structural domains and coordinates most of the FAD. Reason: Directly demonstrated by the X-ray structure of the human ETF heterodimer (also captured as ComplexPortal CPX-2731). This is the defining cellular complex for ETFA and a core annotation. Supporting Evidence: PMID:8962055 Mammalian electron transfer flavoproteins (ETF) are heterodimers containing a |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Immunofluorescence-based (HPA, IDA) localization of ETFA to the mitochondrion. Correct but general relative to the specific matrix location. Reason: Compartment is correct and independently corroborated (HPA imaging); broader than the specific mitochondrial matrix annotation. Retained as non-core. |
| GO:0005515 protein binding | IPI PMID:35362222 S1P defects cause a new entity of cataract, alopecia, oral m... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction (with MBTPS1/S1P, Q14703, and ETFB, P38117) demonstrated by Co-IP and GST pull-down; S1P forms a trimeric complex with ETFA/ETFB, stabilizing the heterodimer and promoting FAD incorporation. Captured only as the generic term "protein binding". Reason: The interaction is biologically meaningful and ETFA-specific (direct GST-ETFA pull-down, docking of specific ETFA residues), but "protein binding" is an uninformative molecular function. The functional consequence (heterodimer stabilization / FAD binding) is better captured by the FAD-binding and ETF-complex annotations; the bare IPI is over-annotated. Supporting Evidence: PMID:35362222 S1P forms a trimeric complex with ETFA and ETFB proteins |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: High-throughput (HTP) mitochondrial-proteome localization of ETFA to the mitochondrion, consistent with its established matrix location. Reason: Correct compartment from a high-confidence mitochondrial proteome; broader than the specific matrix annotation. Retained as non-core. |
| GO:0009055 electron transfer activity | IDA PMID:25416781 Human METTL20 is a mitochondrial lysine methyltransferase th... | ACCEPT | Summary: Direct experimental (IDA, ComplexPortal) evidence of ETF electron transfer activity: the ETF alpha/beta heterodimer mediates electron transfer from acyl-CoA dehydrogenases (MCAD, GCDH) to an electron acceptor, an activity modulated by ETFB methylation. Reason: Direct assay of the core electron-transfer function of the ETF heterodimer. Duplicate of other electron-transfer-activity annotations; accepted as core. Supporting Evidence: PMID:25416781 ETF acts as a mobile electron carrier that shuttles electrons between several FAD-containing dehydrogenases |
| GO:0005739 mitochondrion | HDA PMID:20833797 Phosphoproteome analysis of functional mitochondria isolated... | KEEP AS NON CORE | Summary: High-throughput direct assay (HDA) placing ETFA in the mitochondrion, from a phosphoproteome of mitochondria isolated from human muscle. Reason: Correct compartment from isolated-mitochondria mass spectrometry; broader than the specific matrix annotation. Retained as non-core. Supporting Evidence: PMID:20833797 mitochondria isolated from human muscle |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-169260 | ACCEPT | Summary: Traceable author statement (Reactome) placing ETF in the mitochondrial matrix, where it accepts reducing equivalents from fatty-acyl-CoA beta-oxidation. Reason: Concordant with the experimentally supported matrix localization and the reaction context (ETF residing on the matrix face of the inner membrane). |
| GO:0005759 mitochondrial matrix | TAS Reactome:R-HSA-169270 | ACCEPT | Summary: Traceable author statement (Reactome) placing ETF in the mitochondrial matrix in the reaction where ETFDH re-oxidizes reduced ETF and reduces CoQ. Reason: Concordant with the experimentally supported matrix localization and the ETF -> ETFDH -> ubiquinone electron-transfer reaction. |
| GO:0009055 electron transfer activity | IDA PMID:10423253 The intraflavin hydrogen bond in human electron transfer fla... | ACCEPT | Summary: Direct experimental (IDA) study of ETF as an intermediate electron carrier: substitution of the FAD analog 4'-deoxy-FAD altered redox potentials and abolished electron transfer to and from ETF-QO, dissecting the electron-transfer mechanism of ETF. Reason: Directly probes the core electron-transfer function of ETF and its dependence on the FAD cofactor. Duplicate of other electron-transfer-activity annotations; accepted as core. Supporting Evidence: PMID:10423253 serves as an intermediate electron carrier |
| GO:0050660 flavin adenine dinucleotide binding | IDA PMID:10423253 The intraflavin hydrogen bond in human electron transfer fla... | ACCEPT | Summary: Direct experimental (IDA) evidence of FAD binding: binding constants for FAD (and the 4'-deoxy-FAD analog) with the ETF apoprotein were measured, and the intra-cofactor 4'-hydroxyl-N1 hydrogen bond of the bound FAD was characterized. Reason: Direct biochemical characterization of the FAD-binding function of ETF, a core cofactor- binding function of the alpha subunit. Supporting Evidence: PMID:10423253 the 4'-hydroxyl of the ribityl side chain of FAD is hydrogen bonded to N(1) |
| GO:0050660 flavin adenine dinucleotide binding | IDA PMID:9334218 Expression and characterization of two pathogenic mutations ... | ACCEPT | Summary: Direct experimental (IDA) evidence of FAD binding: the flavin environment and kinetics of flavin release from oxidized and reduced ETF were characterized for wild-type and the pathogenic alphaT266M mutant, which perturbs the FAD-binding site. Reason: Direct biochemical characterization of ETFA FAD binding, including the effect of a GA2A mutation on the flavin environment. Core cofactor-binding function. Duplicate of the other FAD-binding annotations; accepted. Supporting Evidence: PMID:9334218 Among these dehydrogenases are the four chain |
| GO:0005739 mitochondrion | TAS PMID:3170610 Molecular cloning and nucleotide sequence of cDNAs encoding ... | KEEP AS NON CORE | Summary: Traceable author statement (PINC) placing alpha-ETF in the mitochondrion, from the molecular cloning of the alpha subunit precursor, whose identity was confirmed by mitochondrial processing of the translated protein. Reason: Correct compartment; broader than the specific matrix annotation. Retained as non-core. Supporting Evidence: PMID:3170610 mitochondrial processing of the |
| GO:0005759 mitochondrial matrix | TAS PMID:8617498 Assignment of Etfdh, Etfb, and Etfa to chromosomes 3, 7, and... | ACCEPT | Summary: Traceable author statement (PINC) placing ETF in the mitochondrial matrix, from the chromosomal-assignment study which states ETF is an obligatory electron acceptor located in the mitochondrial matrix. Reason: Concordant with the direct experimental matrix localization; supports the core location of ETFA. Supporting Evidence: PMID:8617498 obligatory electron acceptor for several dehydrogenases and is located in the |
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Download this section (compressed HTML)Q: Beyond serving all the acyl-CoA and amino-acid-catabolic dehydrogenases, how is electron flux through ETF regulated in vivo (e.g. by ETFRF1/LYRM5 deflavination or MBTPS1/S1P-mediated stabilization), and does this constitute a physiological control point for substrate selection?
Experiment: Quantitative flux analysis (e.g. stable-isotope tracing of fatty-acid and branched-chain amino-acid oxidation) in ETFA-null versus rescued cells to measure the contribution of ETF to each catabolic pathway and to respiratory-chain electron input.
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