NDUFAF5 (formerly C20orf7) is a nuclear-encoded mitochondrial assembly factor for respiratory chain Complex I (NADH:ubiquinone oxidoreductase). It is imported into mitochondria and peripherally associated with the matrix face of the inner membrane. Although it adopts the S-adenosylmethionine (SAM)-dependent 7-beta-strand (class I Rossmann-fold) methyltransferase fold, its catalytic output is hydroxylation rather than methyl transfer: like the anthracycline enzyme RdmB, it uses SAM as a cofactor to introduce a hydroxyl group into a conserved arginine residue (Arg-73 of mature NDUFS7) of the Complex I core iron-sulfur subunit NDUFS7. This modification occurs at an early stage of Complex I biogenesis, before the peripheral and membrane arms are joined, and NDUFAF5 is not part of the mature holoenzyme. NDUFAF5 activity and stability are supported by its interactors NDUFAF8 and the SAM-methyltransferase chaperone PYURF. Biallelic loss-of-function variants cause mitochondrial complex I deficiency (MC1DN16), presenting as lethal neonatal mitochondrial disease and Leigh syndrome.
Definition: Catalysis of the S-adenosyl-L-methionine (SAM)-dependent hydroxylation of a peptidyl-arginine residue to peptidyl-hydroxyarginine, in which SAM acts as a cofactor rather than a methyl donor (as in the RdmB-type 7-beta-strand methyltransferase-fold hydroxylases). NDUFAF5 exemplifies this activity, hydroxylating Arg-73 of the Complex I subunit NDUFS7.
Justification: NDUFAF5 has a demonstrated protein-arginine hydroxylase activity (PMID:27226634) but no existing GO molecular-function term captures a SAM-dependent peptidyl-arginine hydroxylase. The available peptidyl-arginine 3-dioxygenase term (GO:0106157) is mechanistically inappropriate because its definition specifies a 2-oxoglutarate/O2-dependent dioxygenase reaction, which does not match NDUFAF5's SAM-cofactor mechanism. In the interim, the broad oxidoreductase activity term (GO:0016491) is used, consistent with UniProt's "Arginine-hydroxylase" RecName and EC 1.-.-.-.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred mitochondrial localization. Correct and consistent with all experimental evidence, but the more specific inner-membrane/matrix-face terms are the informative localizations for this protein. Reason: NDUFAF5 is experimentally localized to mitochondria (PMID:27226634, PMID:18940309); the IBA call is accurate, though less granular than the IDA matrix-side annotation. |
| GO:0032981 mitochondrial respiratory chain complex I assembly | IBA GO_REF:0000033 | ACCEPT | Summary: Core biological process. NDUFAF5 is a bona fide Complex I assembly factor; the IBA call is strongly corroborated by experimental IMP annotations. Reason: Directly supported experimentally (PMID:18940309, PMID:27226634): NDUFAF5 acts at an early stage of Complex I assembly. |
| GO:0005739 mitochondrion | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA electronic mitochondrial localization; correct but general. Retained as a non-specific but accurate localization. Reason: Consistent with experimental localization data; superseded in specificity by the inner-membrane and matrix-side annotations. |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: SubCell-keyword mapping to mitochondrial inner membrane. Accurate; NDUFAF5 is peripherally associated with the matrix face of the inner membrane. Reason: Matches UniProt subcellular location and experimental EXP/IDA annotations. |
| GO:0008757 S-adenosylmethionine-dependent methyltransferase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro-based prediction from the SAM-dependent 7-beta-strand methyltransferase fold. NDUFAF5 has the SAM-binding fold and binds SAM, but its demonstrated catalytic activity on NDUFS7 is arginine HYDROXYLATION, not methyl transfer; SAM acts only as a cofactor. This is a fold-based over-annotation of the specific catalytic activity. Reason: Direct substrate assays show NDUFAF5 hydroxylates Arg-73 of NDUFS7 and does not methylate it, analogous to RdmB where SAM is a cofactor for hydroxylation. The true molecular function is an oxidoreductase (hydroxylase) activity (GO:0016491), not SAM-dependent methyltransferase activity. Supporting Evidence: PMID:27226634 RdmB has no methyltransferase activity, and SAM acts as a cofactor in the process of hydroxylation PMID:27226634 SAM is a crucial feature of the hydroxylation reaction as S-adenosylhomocysteine cannot substitute for SAM |
| GO:0032981 mitochondrial respiratory chain complex I assembly | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA electronic annotation of the core Complex I assembly process. Correct and matches the experimental IMP annotations. Reason: Redundant with, and confirmed by, experimental IMP annotations (PMID:18940309, PMID:27226634). |
| GO:0005515 protein binding | IPI PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | MARK AS OVER ANNOTATED | Summary: IntAct-curated interaction with NDUFAF8 (A1L188). Experimentally real (NDUFAF8 stabilizes NDUFAF5), but bare "protein binding" is uninformative as a molecular function. Reason: Per curation policy, retain the experimental IPI but flag the non-informative MF term. The biology (NDUFAF8-mediated stabilization supporting Complex I assembly) is captured by the assembly BP and in core_functions; do not remove. |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | MARK AS OVER ANNOTATED | Summary: High-throughput BioPlex AP-MS interaction (mapped to NDUFAF8, A1L188). Bare "protein binding" is uninformative. Reason: Large-scale interactome screen; retained as experimental IPI but flagged as uninformative MF. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MARK AS OVER ANNOTATED | Summary: High-throughput BioPlex (cell-specific) AP-MS interaction (mapped to NDUFAF8, A1L188). Bare "protein binding" is uninformative. Reason: Large-scale interactome screen; retained as experimental IPI but flagged as uninformative MF. |
| GO:0005743 mitochondrial inner membrane | EXP PMID:18940309 Mutation of C20orf7 disrupts complex I assembly and causes l... | ACCEPT | Summary: Experimental localization to the mitochondrial inner membrane. NDUFAF5 is peripherally associated with the matrix face of the inner membrane. Reason: Directly demonstrated experimentally. Supporting Evidence: PMID:18940309 peripherally associated with the matrix face of the |
| GO:0005743 mitochondrial inner membrane | EXP PMID:27226634 NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembl... | ACCEPT | Summary: Experimental mitochondrial/inner-membrane localization corroborated in a second study using tagged NDUFAF5. Reason: Consistent with the primary localization data. Supporting Evidence: PMID:27226634 NDUFAF5 with a C-terminal FLAG tag was found uniquely in the mitochondria of human 143B cells |
| GO:0005743 mitochondrial inner membrane | EXP PMID:35614220 Defining mitochondrial protein functions through deep multio... | ACCEPT | Summary: Experimental inner-membrane localization corroborated by the deep multiomic profiling study. Reason: Consistent with UniProt subcellular location and other experimental data. |
| 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 all experimental localization evidence; less specific than inner-membrane terms. |
| GO:0099617 matrix side of mitochondrial inner membrane | IDA PMID:18940309 Mutation of C20orf7 disrupts complex I assembly and causes l... | ACCEPT | Summary: Most precise localization: NDUFAF5 is peripherally associated with the matrix (matrix-facing) side of the mitochondrial inner membrane, consistent with its role in modifying the incoming matrix-arm subunit NDUFS7. Reason: Directly demonstrated; captures both the membrane association and matrix-face topology. Supporting Evidence: PMID:18940309 peripherally associated with the matrix face of the |
| GO:0005515 protein binding | IPI PMID:35614220 Defining mitochondrial protein functions through deep multio... | MARK AS OVER ANNOTATED | Summary: IPI interaction with PYURF (Q96I23), a SAM-methyltransferase chaperone that directly binds (via its TRM112 domain) and stabilizes NDUFAF5. Experimentally real but bare "protein binding" is uninformative. Reason: Retain experimental IPI; flag non-informative MF. The stabilizing PYURF partnership supporting NDUFAF5 function is captured elsewhere; do not remove. Supporting Evidence: PMID:35614220 methyltransferase chaperone that supports both complex I assembly |
| GO:0005515 protein binding | IPI PMID:27226634 NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembl... | MARK AS OVER ANNOTATED | Summary: IPI interaction with NDUFS7 (O75251), the direct catalytic substrate of NDUFAF5. This is the mechanistically meaningful interaction (NDUFAF5 hydroxylates Arg-73 of NDUFS7), but the bare "protein binding" MF term does not convey the enzyme-substrate relationship. Reason: Retain the experimental IPI, but a bare "protein binding" term under-describes an enzyme-substrate relationship. The substrate interaction is captured in core_functions (hydroxylation of NDUFS7); do not remove. Supporting Evidence: PMID:27226634 the conserved arginine residue (residues 73 and 77 in the human and bovine complexes, respectively) is completely hydroxylated |
| GO:0005739 mitochondrion | IDA PMID:27226634 NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembl... | ACCEPT | Summary: Direct experimental mitochondrial localization of tagged NDUFAF5. Correct; less specific than the inner-membrane/matrix-side terms. Reason: Directly demonstrated. Supporting Evidence: PMID:27226634 NDUFAF5 with a C-terminal FLAG tag was found uniquely in the mitochondria of human 143B cells |
| GO:0032981 mitochondrial respiratory chain complex I assembly | IMP PMID:27226634 NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembl... | ACCEPT | Summary: Experimental IMP for the core biological process. siRNA suppression of NDUFAF5 impairs Complex I assembly at an early stage and reduces NDUFS7 Arg-73 hydroxylation, directly linking the enzymatic activity to Complex I biogenesis. Reason: Strongest evidence for the core function; this is the defining mechanistic study. Supporting Evidence: PMID:27226634 The suppression of NDUFAF5 affected the biogenesis of complex I at an early stage of assembly PMID:27226634 the participation of NDUFAF5 in the pathway of assembly of complex I affects both arms of the complex |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799179 | ACCEPT | Summary: Reactome traceable localization to the inner membrane within the Complex I biogenesis pathway. Correct. Reason: Consistent with experimental inner-membrane localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799191 | ACCEPT | Summary: Reactome traceable localization to the inner membrane (MT-ND1:NDUFAF5:NDUFAF6 assembly intermediate). Correct. Reason: Consistent with experimental data and with NDUFAF5's role in the membrane-anchored assembly intermediate. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799196 | ACCEPT | Summary: Reactome traceable inner-membrane localization within Complex I biogenesis. Correct. Reason: Consistent with experimental inner-membrane localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799197 | ACCEPT | Summary: Reactome traceable inner-membrane localization within Complex I biogenesis. Correct. Reason: Consistent with experimental inner-membrane localization. |
| GO:0005743 mitochondrial inner membrane | TAS Reactome:R-HSA-6799202 | ACCEPT | Summary: Reactome traceable inner-membrane localization within Complex I biogenesis. Correct. Reason: Consistent with experimental inner-membrane localization. |
| GO:0032981 mitochondrial respiratory chain complex I assembly | IMP PMID:18940309 Mutation of C20orf7 disrupts complex I assembly and causes l... | ACCEPT | Summary: Experimental IMP for the core biological process. RNAi silencing of C20orf7 (NDUFAF5) decreases Complex I activity, and patient fibroblasts show near-complete absence of Complex I holoenzyme with an early assembly defect. Reason: Establishes NDUFAF5 as essential for Complex I assembly. Supporting Evidence: PMID:18940309 crucial in the assembly of complex I and that mutations in C20orf7 cause |
| GO:0016491 oxidoreductase activity | IDA PMID:27226634 NDUFAF5 Hydroxylates NDUFS7 at an Early Stage in the Assembl... | NEW | Summary: Proposed correct catalytic molecular function. NDUFAF5 hydroxylates Arg-73 of NDUFS7 using SAM as a cofactor (an oxidoreductase/hydroxylase reaction), and UniProt classifies it as "Arginine-hydroxylase" with EC 1.-.-.-. This is not currently in GOA (GOA lists only the fold-based SAM-methyltransferase MF), so it is added here as a NEW annotation to replace that over-annotation. A dedicated SAM-dependent peptidyl-arginine hydroxylase term (see proposed_new_terms) would be preferable to this broad parent. Reason: Captures the demonstrated hydroxylase (oxidoreductase) activity on NDUFS7; GO:0106157 (peptidyl-arginine 3-dioxygenase) is inappropriate because it specifies a 2-oxoglutarate/O2 dioxygenase mechanism, whereas NDUFAF5 is SAM-dependent. Proposed replacements: oxidoreductase activity Supporting Evidence: PMID:27226634 RdmB has no methyltransferase activity, and SAM acts as a cofactor in the process of hydroxylation file:human/NDUFAF5/NDUFAF5-uniprot.txt FUNCTION: Arginine hydroxylase that mediates hydroxylation of 'Arg-111' |
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Download this section (compressed HTML)Q: What is the precise EC/reaction stoichiometry of NDUFAF5 (e.g. molecular oxygen incorporation, any additional cofactor/metal requirement beyond SAM), and can a dedicated SAM-dependent peptidyl-arginine hydroxylase GO molecular-function term be created to replace the fold-based SAM-methyltransferase annotation?
Q: Beyond NDUFS7 Arg-73, does NDUFAF5 hydroxylate (or otherwise modify) any additional substrates, given the residual proposed methyltransferase activity noted by UniProt?
Experiment: In vitro reconstitution of purified recombinant NDUFAF5 with a NDUFS7 substrate peptide and SAM, with mass spectrometry to quantify Arg-73 hydroxylation and to test for molecular-oxygen dependence and any metal/cofactor requirement, definitively establishing the EC subclass.
Experiment: Structure-guided mutagenesis of the SAM-binding pocket and predicted catalytic residues of NDUFAF5, assaying both NDUFS7 hydroxylation and rescue of Complex I assembly in NDUFAF5-null cells, to map catalysis to the assembly phenotype.
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