Rice COQ5 (Q5JNC0; Os01g0976600 / LOC_Os01g74520) is the mitochondrial S-adenosyl-L-methionine (SAM)-dependent C-methyltransferase of the ubiquinone (coenzyme Q) biosynthetic pathway. It belongs to the class I-like SAM-binding methyltransferase superfamily, MenG/UbiE (UbiE/COQ5) family, and carries the conserved SAM-binding residues (UniProt BINDING sites at positions 100, 136 and 166-167). COQ5 catalyzes the single C-methylation step of the CoQ ring-modification stage (EC 2.1.1.201; Rhea:RHEA:28286): it transfers a methyl group from SAM to the C2 position of a 2-methoxy-6-(all-trans-polyprenyl)benzene-1,4-diol intermediate, converting 2-polyprenyl-6-methoxy-1,4-benzoquinol (DDMQH2) to 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinol (DMQH2). The other two methylation reactions of the pathway are O-methylations carried out by COQ3, so COQ5 is specifically the C-methyltransferase rather than an O-methyltransferase (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). The protein bears an N-terminal mitochondrial transit peptide (residues 1-10) and is a peripheral inner-membrane protein on the matrix side of the mitochondrial inner membrane, consistent with CoQ biosynthesis occurring at the inner mitochondrial membrane. COQ5 acts within a multi-subunit COQ enzyme complex (the "COQ metabolon" / "CoQ synthome") that channels hydrophobic prenylated intermediates between the late-pathway COQ3-COQ9 enzymes. Direct rice-specific biochemical or genetic characterization of Os01g0976600 is not available in the retrieved literature; the functional assignment rests on strong evolutionary conservation across eukaryotes plus plant-specific evidence that Arabidopsis AtCOQ5 functionally complements the corresponding Schizosaccharomyces pombe coq mutant, demonstrating that plant COQ5 proteins retain the conserved C-methyltransferase activity (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). CoQ is a redox-active, membrane-localized prenylquinone that functions as an electron carrier in mitochondrial respiration and as a lipophilic antioxidant.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0032259 methylation | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: SPKW (GO_REF:0000043) annotation derived from the UniProt keyword "Methyltransferase" (and the related "Transferase" keyword); snapshot-only, removed from the current GOA release. It maps the enzyme-class keyword to the bare biological-process term "methylation", which drops all pathway and substrate context for a CoQ-pathway C-methyltransferase. Reason: GOA's removal of this annotation was JUSTIFIED. This is a classic "enzyme-class keyword -> bare process" conflation: the UniProt "Methyltransferase" keyword is mapped to the very high-level process term "methylation" (GO:0032259), which says only that the protein methylates something and discards the specific pathway. COQ5's methylation is not a generic process - it is the single C-methylation step of coenzyme Q biosynthesis, in which a SAM-dependent C2 C-methyltransferase acts on a prenylated benzoquinol CoQ-pathway intermediate (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). The correct, specific process is "ubiquinone biosynthetic process" (GO:0006744), which is ALREADY present in the current GOA in two annotations (IBA from GO_REF:0000033 and IEA from GO_REF:0000120), and the specific molecular function "2-methoxy-6-polyprenyl-1,4-benzoquinol methyltransferase activity" (GO:0008425) is also already annotated (IBA and IEA). The bare "methylation" process term therefore adds no information beyond what the retained, more specific annotations already capture, and is redundant. Were a process term needed to replace it, the correct replacement is GO:0006744; but because that term is already present, the bare keyword-derived "methylation" is best treated as an over-annotation whose removal lost no biology. Proposed replacements: ubiquinone biosynthetic process Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md among the three methylation reactions in the canonical eukaryotic CoQ ring-modification stage (the other two methylations are O-methylations catalyzed by COQ3) file:ORYSJ/COQ5/COQ5-deep-research-falcon.md COQ5 acts on a prenylated benzoquinone/benzoquinol intermediate in the late pathway |
| GO:0006744 ubiquinone biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation propagated across the COQ5 phylogenetic group (PANTHER PTN001297884, including yeast SGD COQ5 and E. coli UbiE). Ubiquinone (coenzyme Q) biosynthesis is the core biological process of COQ5. Reason: This is the core biological process and is well supported. COQ5 is consistently assigned across eukaryotes and plants as the pathway C-methyltransferase of ubiquinone biosynthesis, performing the single C-methylation step of the CoQ ring-modification stage (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). The UniProt PATHWAY annotation ("Cofactor biosynthesis; ubiquinone biosynthesis") and UniPathway UPA00232 agree. Plant orthologs retain this role: Arabidopsis AtCOQ5 complements the corresponding S. pombe coq mutant. The IBA term is at the appropriate level of specificity. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md Across eukaryotes and plants, COQ5 is consistently assigned as the pathway C-methyltransferase rather than an O-methyltransferase. file:ORYSJ/COQ5/COQ5-deep-research-falcon.md Arabidopsis COQ genes including AtCOQ5 can complement the corresponding |
| GO:0008425 2-methoxy-6-polyprenyl-1,4-benzoquinol methyltransferase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA annotation for the specific COQ5 enzymatic activity, propagated across the COQ5/UbiE phylogenetic group. This is the precise, informative molecular function of the gene product. Reason: This is the core molecular function and is well supported. The UniProt entry records the catalytic activity (EC 2.1.1.201; Rhea:RHEA:28286): a 2-methoxy-6-(all-trans-polyprenyl)benzene-1,4-diol + SAM yields a 5-methoxy-2-methyl-3-(all-trans-polyprenyl)benzene-1,4-diol + S-adenosyl-L- homocysteine. The literature describes COQ5 as a mitochondrial SAM-dependent C2 C-methyltransferase acting on a prenylated benzoquinol CoQ-pathway intermediate (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). This is the exact, maximally informative MF term and should be retained as core. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md Reviews and primary studies describe this as the COQ5/UbiE-dependent C-methyltransferase reaction. file:ORYSJ/COQ5/COQ5-deep-research-falcon.md the single C-methylation step of the CoQ ring-modification phase, specifically methylation at the C2 position of the aromatic headgroup during CoQ biosynthesis |
| GO:0005743 mitochondrial inner membrane | IEA GO_REF:0000044 | ACCEPT | Summary: IEA annotation from the UniProt Subcellular Location vocabulary mapping (UniProtKB-SubCell:SL-0168). COQ5 is a peripheral inner-membrane protein on the matrix side of the mitochondrial inner membrane. Reason: Correct and consistent with the UniProt SUBCELLULAR LOCATION annotation ("Mitochondrion inner membrane; Peripheral membrane protein; Matrix side") and with the N-terminal mitochondrial transit peptide (residues 1-10). CoQ biosynthesis in eukaryotes occurs at the inner mitochondrial membrane, and late-pathway COQ enzymes are mitochondrial (matrix-localized and/or inner-membrane associated), so the mitochondrial inner-membrane localization is well supported (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). Direct rice microscopy was not retrieved, but the localization is consistent with strong pathway conservation. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md The best-supported localization is mitochondrial, with CoQ biosynthesis occurring at the inner mitochondrial membrane or matrix-facing environment file:ORYSJ/COQ5/COQ5-deep-research-falcon.md eukaryotic COQ proteins are described as matrix-localized and/or inner-membrane associated |
| GO:0006744 ubiquinone biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation (combined automated methods; ARBA/UniRule, UniPathway UPA00232) for ubiquinone biosynthesis. Duplicates the IBA annotation to the same term. Reason: Correct and consistent with the IBA annotation to the same term, with the UniProt PATHWAY statement ("Cofactor biosynthesis; ubiquinone biosynthesis"; UniPathway UPA00232) and with the conserved role of COQ5 as the CoQ-pathway C-methyltransferase (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). Duplicate annotations with different evidence codes are acceptable; the IEA provides additional computational support for the well-established core process. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md Across eukaryotes and plants, COQ5 is consistently assigned as the pathway C-methyltransferase rather than an O-methyltransferase. file:ORYSJ/COQ5/COQ5-deep-research-falcon.md COQ5 functions within a multisubunit CoQ biosynthetic assembly, often termed the COQ metabolon |
| GO:0008168 methyltransferase activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: IEA annotation from InterPro (IPR004033 UbiE/COQ5_MeTrFase; IPR023576 UbiE/COQ5_MeTrFase_CS) for the broad parent "methyltransferase activity". COQ5 is a methyltransferase, but a much more specific MF is already annotated. Reason: "Methyltransferase activity" is a broad parent term. It is not wrong - COQ5 is a SAM-dependent methyltransferase - but it is uninformative now that the specific MF "2-methoxy-6-polyprenyl-1,4-benzoquinol methyltransferase activity" (GO:0008425) is annotated (IBA and IEA). The InterPro signatures that produced this term (UbiE/COQ5 family) actually map to the specific UbiE/COQ5 activity, so the more precise GO:0008425 better represents the same evidence (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). Retaining a generic "methyltransferase activity" alongside the specific term adds no information, so the parent row is subsumed by the accepted GO:0008425 rows. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md COQ5 acts on a prenylated benzoquinone/benzoquinol intermediate in the late pathway file:ORYSJ/COQ5/COQ5-deep-research-falcon.md among the three methylation reactions in the canonical eukaryotic CoQ ring-modification stage (the other two methylations are O-methylations catalyzed by COQ3) |
| GO:0008425 2-methoxy-6-polyprenyl-1,4-benzoquinol methyltransferase activity | IEA GO_REF:0000120 | ACCEPT | Summary: IEA annotation (combined automated methods; RHEA:28286, EC:2.1.1.201, UniRule) for the specific COQ5 enzymatic activity. Duplicates the IBA annotation to the same term. Reason: Correct and consistent with the IBA annotation to the same term. This is the precise, maximally informative molecular function, directly supported by the UniProt CATALYTIC ACTIVITY (Rhea:RHEA:28286; EC 2.1.1.201) and by the literature description of COQ5 as a SAM-dependent C2 C-methyltransferase acting on a prenylated benzoquinol intermediate (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). Duplicate annotations with different evidence codes are acceptable. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md Primary and review sources describe Coq5-mediated C2 methylation yielding demethoxy-coenzyme Q (DMQ) file:ORYSJ/COQ5/COQ5-deep-research-falcon.md the single C-methylation step of the CoQ ring-modification phase, specifically methylation at the C2 position of the aromatic headgroup during CoQ biosynthesis |
| GO:0031314 extrinsic component of mitochondrial inner membrane | IEA GO_REF:0000104 | ACCEPT | Summary: IEA annotation (transferred from a related protein by UniRule) describing COQ5 as an extrinsic (peripheral) component of the mitochondrial inner membrane on the matrix side. Reason: Correct and more precise than the generic "mitochondrial inner membrane" term: the UniProt SUBCELLULAR LOCATION explicitly states "Peripheral membrane protein; Matrix side", i.e. an extrinsic/peripherally-attached inner-membrane component rather than an integral membrane protein. This matches the description of late-pathway COQ enzymes as matrix-localized and/or inner-membrane associated (file:ORYSJ/COQ5/COQ5-deep-research-falcon.md). The annotation is consistent with COQ5's role in the membrane-associated COQ biosynthetic complex. Supporting Evidence: file:ORYSJ/COQ5/COQ5-deep-research-falcon.md eukaryotic COQ proteins are described as matrix-localized and/or inner-membrane associated |
Loading supporting content…
Download this section (compressed HTML)Q: Does rice COQ5 (Os01g0976600) functionally complement a yeast coq5 (or S. pombe) null mutant, confirming that the rice protein retains the conserved CoQ-pathway C-methyltransferase activity as shown for Arabidopsis AtCOQ5?
Q: What is the loss-of-function phenotype of a rice coq5 mutant - is it seedling- or pollen-lethal (as expected for a CoQ-biosynthesis gene), and does it reduce total ubiquinone (UQ-9/UQ-10) content?
Experiment: Express rice COQ5 (mature, transit-peptide-removed) in a yeast coq5-null strain and assay rescue of respiratory growth on a non-fermentable carbon source plus restoration of CoQ6 levels by LC-MS, to confirm conserved C-methyltransferase activity.
Hypothesis: Rice COQ5 is a functional CoQ-pathway C2 C-methyltransferase and complements the yeast coq5 deletion, as Arabidopsis AtCOQ5 complements the S. pombe coq mutant.
Type: heterologous complementation
Experiment: Generate CRISPR/Cas9 knockout and knockdown lines of OsCOQ5 and quantify total ubiquinone (UQ-9) content by LC-MS/MS, mitochondrial respiratory capacity, and developmental/viability phenotypes relative to wild type.
Hypothesis: Loss of OsCOQ5 strongly reduces ubiquinone accumulation and impairs mitochondrial respiration, consistent with COQ5 being an essential late-pathway CoQ-biosynthesis enzyme.
Type: reverse genetics with metabolite profiling
Experiment: Express an OsCOQ5-GFP fusion in rice protoplasts or stable transformants and co-localize with a mitochondrial marker by confocal microscopy to verify mitochondrial (inner-membrane / matrix-side) localization directly in rice.
Hypothesis: OsCOQ5 localizes to mitochondria via its N-terminal transit peptide and associates peripherally with the matrix side of the inner membrane.
Type: subcellular localization
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)