FOXRED1

UniProt ID: Q96CU9
Organism: Homo sapiens
Review Status: INITIALIZED
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Gene Description

FOXRED1 (FAD-dependent oxidoreductase domain-containing protein 1) is a nuclear-encoded flavoprotein of the mitochondrial inner membrane that functions as a complex I-specific assembly factor (molecular chaperone) required for biogenesis of the respiratory chain NADH:ubiquinone oxidoreductase (complex I). It belongs to the FAD-dependent oxidoreductase (DAO/FAD-NAD(P)-binding) structural family, carries an FAD cofactor, and is annotated as an oxidoreductase (EC 1.-.-.-), but no in-vivo catalytic substrate or reaction has been defined and it is not a structural subunit of the mature 44-subunit holoenzyme. FOXRED1 acts at the mid-late stages of complex I assembly, transiently associating with assembly intermediates and complex I subunits; in its absence, a late (~815 kDa) intermediate fails to mature and instead degrades to a smaller (~475 kDa) subcomplex, and cells accumulate only residual complex I. Biallelic loss-of-function variants cause mitochondrial complex I deficiency, nuclear type 19 (MC1DN19), an autosomal-recessive infantile-onset encephalomyopathy.

Existing Annotations Review

GO Term Evidence Action Reason
GO:0005737 cytoplasm
IBA
GO_REF:0000033 		KEEP AS NON CORE
Summary: Phylogenetically inferred (PAN-GO/IBA) broad cytoplasmic localization. Not wrong (the mitochondrion is part of the cytoplasm) but uninformative for this protein, whose experimentally supported and functionally meaningful localization is the mitochondrial inner membrane.
Reason: Retained as a correct but general ancestor localization; the informative, experimentally supported location is mitochondrial inner membrane (GO:0005743, IDA/EXP).
GO:0032981 mitochondrial respiratory chain complex I assembly
IBA
GO_REF:0000033 		ACCEPT
Summary: Phylogenetically inferred core biological process. FOXRED1 is a bona fide complex I assembly factor; the IBA call is strongly corroborated by the experimental IMP annotation (PMID:25678554) and by disease genetics.
Reason: Directly supported experimentally; this is the core function of the gene.
GO:0005743 mitochondrial inner membrane
IEA
GO_REF:0000044 		ACCEPT
Summary: UniProt SubCell-keyword mapping to mitochondrial inner membrane. Accurate and matches the experimental IDA/EXP inner-membrane annotations and UniProt subcellular location.
Reason: Consistent with the experimental localization data (PMID:20858599, PMID:25678554).
Supporting Evidence:
file:human/FOXRED1/FOXRED1-uniprot.txt
SUBCELLULAR LOCATION: Mitochondrion inner membrane
GO:0005739 mitochondrion
IDA
GO_REF:0000052 		ACCEPT
Summary: Immunofluorescence-based (HPA) mitochondrial localization. Correct but general; less specific than the inner-membrane terms.
Reason: Consistent with all experimental localization evidence; superseded in specificity by the mitochondrial inner membrane annotations.
GO:0005739 mitochondrion
HTP
PMID:34800366
Quantitative high-confidence human mitochondrial proteome an... 		ACCEPT
Summary: High-throughput mitochondrial proteome localization. Correct but general.
Reason: Consistent with experimental localization; less specific than the inner-membrane terms.
Supporting Evidence:
PMID:34800366
Quantitative high-confidence human mitochondrial proteome and its dynamics
GO:0005743 mitochondrial inner membrane
EXP
PMID:20858599
FOXRED1, encoding an FAD-dependent oxidoreductase complex-I-... 		ACCEPT
Summary: Experimental localization of FOXRED1 to the mitochondrial inner membrane, from the original disease study. Consistent with UniProt subcellular location.
Reason: Directly demonstrated experimentally; the informative localization for this protein.
GO:0005739 mitochondrion
IDA
PMID:20858599
FOXRED1, encoding an FAD-dependent oxidoreductase complex-I-... 		ACCEPT
Summary: Direct experimental mitochondrial localization. Correct; less specific than the inner-membrane term from the same study.
Reason: Directly demonstrated; general mitochondrion is subsumed by the inner-membrane call.
GO:0005743 mitochondrial inner membrane
IDA
PMID:25678554
Characterization of mitochondrial FOXRED1 in the assembly of... 		ACCEPT
Summary: Direct experimental localization to the mitochondrial inner membrane in the definitive characterization study. Consistent with UniProt subcellular location and the primary localization data.
Reason: Directly demonstrated experimentally; the informative localization for this protein.
GO:0032981 mitochondrial respiratory chain complex I assembly
IMP
PMID:25678554
Characterization of mitochondrial FOXRED1 in the assembly of... 		ACCEPT
Summary: Experimental IMP for the core biological process. FOXRED1-null HEK293T cells retain only ~10% complex I, have reduced complex I activity, and cannot grow on galactose; a late ~815 kDa assembly intermediate forms but breaks down to ~475 kDa instead of maturing. FOXRED1 co-immunoprecipitates with complex I subunits. This establishes FOXRED1 as required for the mid-late stages of complex I assembly.
Reason: Strongest evidence for the core function; the defining mechanistic study.
Supporting Evidence:
PMID:25678554
Cells lacking FOXRED1 had ∼10% complex I levels,
PMID:25678554
that FOXRED1 is a crucial component in the productive assembly of complex I and
GO:0005739 mitochondrion
IDA
GO_REF:0000054 		ACCEPT
Summary: LIFEdb intracellular-localization (expressed fusion protein) mitochondrial annotation. Correct but general; consistent with all other localization evidence.
Reason: Consistent with experimental mitochondrial localization; less specific than the inner-membrane terms.
GO:0016491 oxidoreductase activity
IEA
GO_REF:0000043 		NEW
Summary: Generic oxidoreductase molecular function, grounded in the FAD-dependent oxidoreductase fold, the FAD cofactor, and the EC 1.-.-.- assignment. UniProt maps its Oxidoreductase keyword to this term (GO:0016491 F:oxidoreductase activity, IEA:UniProtKB-KW in the UniProt cross-references), though the row is not present in the current GOA snapshot, so it is recorded here as a proposed (NEW) annotation. This is the correct level of MF specificity: FOXRED1 has an oxidoreductase fold but no demonstrated in-vivo substrate/reaction, so a more specific catalytic term is not warranted, and NADH dehydrogenase activity (GO:0008137) is deliberately not assigned because FOXRED1 is not a catalytic subunit of the mature holoenzyme.
Reason: Supported by the UniProt oxidoreductase-fold/FAD/EC evidence; appropriately generic given no defined substrate reaction. Added as NEW because it is absent from the current GOA snapshot.
Proposed replacements: oxidoreductase activity
Supporting Evidence:
file:human/FOXRED1/FOXRED1-uniprot.txt
RecName: Full=FAD-dependent oxidoreductase domain-containing protein 1
GO:0050660 flavin adenine dinucleotide binding
IEA
GO_REF:0000043 		NEW
Summary: FAD-binding molecular function proposed as a NEW annotation. FOXRED1 is a flavoprotein with a FAD/NAD(P)-binding fold and carries an FAD cofactor (UniProt COFACTOR, by similarity; Flavoprotein/FAD keywords). This MF is not currently in GOA but is directly supported by the UniProt cofactor annotation and domain architecture.
Reason: Captures the flavoprotein cofactor-binding function evidenced by the UniProt FAD COFACTOR line and the DAO/FAD-binding domain; complements the generic oxidoreductase MF.
Supporting Evidence:
file:human/FOXRED1/FOXRED1-uniprot.txt
Name=FAD; Xref=ChEBI:CHEBI:57692; Evidence={ECO:0000250};

Core Functions

FAD-dependent oxidoreductase activity of an FAD-binding flavoprotein. FOXRED1 belongs to the DAO / FAD-NAD(P)-binding oxidoreductase structural family and carries an FAD cofactor, and is classified by UniProt as an oxidoreductase (EC 1.-.-.-). No physiological substrate or specific reaction has been demonstrated, so the activity is captured at the generic oxidoreductase level; FOXRED1 is not a catalytic subunit of the complex I holoenzyme, so NADH dehydrogenase activity is deliberately NOT assigned.

Molecular Function:
oxidoreductase activity
Directly Involved In:
mitochondrial respiratory chain complex I assembly
Cellular Locations:
mitochondrial inner membrane
Supporting Evidence:
  • file:human/FOXRED1/FOXRED1-uniprot.txt
    RecName: Full=FAD-dependent oxidoreductase domain-containing protein 1
  • file:human/FOXRED1/FOXRED1-uniprot.txt
    Name=FAD; Xref=ChEBI:CHEBI:57692; Evidence={ECO:0000250};

FAD (flavin adenine dinucleotide) binding. FOXRED1 is a flavoprotein whose FAD/NAD(P)-binding fold binds an FAD cofactor (by similarity), consistent with its Flavoprotein/FAD keywords. This molecular function is not currently in GOA; it is added here on the basis of the UniProt COFACTOR annotation and the DAO/FAD-binding domain architecture.

Molecular Function:
flavin adenine dinucleotide binding
Cellular Locations:
mitochondrial inner membrane
Supporting Evidence:
  • file:human/FOXRED1/FOXRED1-uniprot.txt
    Name=FAD; Xref=ChEBI:CHEBI:57692; Evidence={ECO:0000250};

Complex I-specific assembly factor (molecular chaperone). FOXRED1 is required for the mid-late stages of mitochondrial respiratory chain complex I (NADH:ubiquinone oxidoreductase) biogenesis, transiently associating with complex I subunits and assembly intermediates. In its absence a late (~815 kDa) intermediate fails to mature to the holoenzyme and degrades to a ~475 kDa subcomplex, leaving only residual assembled complex I. FOXRED1 is not a structural subunit of the mature complex.

Molecular Function:
oxidoreductase activity
Directly Involved In:
mitochondrial respiratory chain complex I assembly
Cellular Locations:
mitochondrial inner membrane
Supporting Evidence:
  • file:human/FOXRED1/FOXRED1-uniprot.txt
    Required for the assembly of the mitochondrial membrane
  • file:human/FOXRED1/FOXRED1-uniprot.txt
    Involved in mid-late stages of complex I assembly
  • PMID:25678554
    co-immunoprecipitate with a number of complex I subunits.
  • PMID:20858599
    human disease, is now shown to be a complex I-specific molecular chaperone.

References

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Suggested Questions for Experts

Q: What is FOXRED1's actual enzymatic reaction and physiological substrate, and does its FAD-dependent redox chemistry contribute to complex I assembly, or does the fold act as a non-catalytic FAD-stabilized scaffold/chaperone?

Q: At the mid-late (~815 kDa to mature) transition, which specific complex I subunit(s) or assembly intermediate does FOXRED1 act on, and how do the MC1DN19 hypomorphic variants (R352W, N430S) perturb this step?

Suggested Experiments

Experiment: In vitro reconstitution with purified recombinant FOXRED1 and FAD, screening candidate substrates (e.g. complex I subunits or intermediates) by mass spectrometry / redox assays to define the catalyzed reaction and EC subclass, and testing whether catalytically dead (FAD-binding-pocket) mutants still support complex I assembly in FOXRED1-null cells.

Experiment: Complexome profiling / cross-linking MS in FOXRED1-null versus rescued cells to map which assembly intermediate accumulates or is destabilized and to identify FOXRED1's direct binding partner within the mid-late complex I assembly pathway.

📚 Additional Documentation

Notes

(FOXRED1-notes.md)

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