COQ8A (also known as ADCK3, CABC1, COQ8) is a mitochondrial member of the ancient UbiB family of atypical protein-kinase-like (PKL) enzymes, and is one of two human co-orthologs (with COQ8B/ADCK4) of yeast Coq8p. It is required for the biosynthesis of coenzyme Q (ubiquinone/CoQ10), a lipid-soluble electron carrier of the respiratory chain and a membrane antioxidant. The 647-residue precursor is imported into mitochondria (an N-terminal transit peptide of ~162 residues is removed) and anchors to the inner mitochondrial membrane as a single-pass membrane protein, with its catalytic domain facing the matrix, where CoQ is produced. Although COQ8A adopts a protein-kinase-like fold, several UbiB-specific features suppress canonical protein-kinase activity: an N-terminal KxGQ domain occludes the substrate-binding cleft, and an alanine-rich (AAAS) loop replaces the canonical glycine-rich nucleotide-binding loop, conferring an unusual selectivity for binding ADP over ATP. Biochemically, the protein binds and hydrolyzes ATP (ATPase activity that is enhanced by the KxGQ motif and by coenzyme-Q head-group intermediates and cardiolipin), and it interacts with lipid CoQ intermediates, rather than exhibiting canonical protein-kinase phosphotransfer in trans. Functionally, COQ8A stabilizes and helps assemble the multi-subunit CoQ biosynthetic complex (the "COQ synthome"/complex Q; COQ3-COQ7, COQ9), thereby streamlining CoQ production. Biallelic loss-of-function variants cause primary coenzyme Q10 deficiency-4 (COQ10D4), presenting as autosomal-recessive cerebellar ataxia (ARCA2/SCAR9) with cerebellar atrophy, exercise intolerance, and variable seizures.
Definition: A protein-containing complex, located at the matrix face of the mitochondrial inner membrane, that carries out head-group modification steps of ubiquinone (coenzyme Q) biosynthesis. In humans it comprises COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9, and is stabilized by the atypical UbiB-family kinase COQ8A (and its paralog COQ8B).
Justification: COQ8A's core cellular role is membership in and stabilization of this complex ("complex Q"), but there is no specific protein-complex GO term to record it; only the generic GO:0032991 protein-containing complex is currently available for the in_complex slot.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006744 ubiquinone biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (PAN-GO) inference of the core, well-established role of COQ8A/Coq8p orthologs in ubiquinone (coenzyme Q) biosynthesis. This is consistent with abundant experimental evidence and is the central function of the gene. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt Atypical kinase involved in the biosynthesis of coenzyme Q |
| GO:0031966 mitochondrial membrane | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic mapping from the UniProt Subcellular Location keyword (mitochondrion membrane). Correct and corroborated by experimental localization; mitochondrial (inner) membrane association is well established. Kept as non-core relative to the more specific matrix/inner membrane localization. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt Mitochondrion membrane |
| GO:0005515 protein binding | IPI PMID:16189514 Towards a proteome-scale map of the human protein-protein in... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" (GO:0005515) from a large-scale human protein-protein interaction network study (IntAct partner Q9UI14/RABAC1). The generic term conveys no specific molecular function and the partner is not part of the CoQ biosynthetic machinery. Over-annotation of a high-throughput binary interaction; not a core function. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from a proteome-scale human interactome map (Rolland et al.). Multiple binary partners captured (e.g. AGTRAP, TMEM239, LDAF1, REEP6, TFIP11, RABAC1); none are CoQ-biosynthesis proteins and the term is uninformative. High-throughput over-annotation; not a core function. |
| GO:0005515 protein binding | IPI PMID:25910212 Widespread macromolecular interaction perturbations in human... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from a study of macromolecular interaction perturbations in genetic disorders (Sahni et al.). Binary partners (DTX2, SH3GLB1, REEP6, TFIP11, etc.) are not CoQ pathway components; the generic term is uninformative. High-throughput over-annotation. |
| GO:0005515 protein binding | IPI PMID:27499296 Mitochondrial Protein Interaction Mapping Identifies Regulat... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from mitochondrial protein interaction mapping (Floyd et al.); the IntAct partner here is COQ9 (O75208), a genuine member of the CoQ biosynthetic complex ("complex Q"). The interaction is biologically real and central to COQ8A function, but the generic GO:0005515 term does not capture it. Marked as over-annotated at the term level; the underlying complex membership is represented via in_complex in core_functions. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9 |
| GO:0005515 protein binding | IPI PMID:31515488 Extensive disruption of protein interactions by genetic vari... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from a systematic study of interaction disruption by genetic variants (Fragoza et al.). Partners (AGTRAP, REEP6, TFIP11) are not CoQ-pathway proteins; term is uninformative. High-throughput over-annotation; not a core function. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from the HuRI human binary interactome reference map (Luck et al.). Many low-specificity Y2H partners captured; none are CoQ-biosynthesis components and the generic term is uninformative. High-throughput over-annotation; not a core function. |
| GO:0005515 protein binding | IPI PMID:32814053 Interactome Mapping Provides a Network of Neurodegenerative ... | MARK AS OVER ANNOTATED | Summary: Bare "protein binding" from an interactome map of neurodegenerative-disease proteins. Partners (MAOB, LITAF, SPRY4, ATG10, etc.) are not CoQ-pathway components; the generic term is uninformative. High-throughput over-annotation; not a core function. |
| GO:0006744 ubiquinone biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: Electronic annotation from UniPathway vocabulary mapping (ubiquinone biosynthesis, UPA00232). Correct and consistent with the core function, though redundant with the experimental IMP/IDA annotations to the same term. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt Cofactor biosynthesis; ubiquinone biosynthesis. |
| GO:0005739 mitochondrion | IDA GO_REF:0000052 | KEEP AS NON CORE | Summary: Mitochondrial localization from HPA immunofluorescence curation. Correct; COQ8A is a well-established mitochondrial protein. Kept as non-core relative to the more specific inner-membrane/matrix-facing localization. |
| GO:0031966 mitochondrial membrane | EXP PMID:11888884 Isolation of a novel gene, CABC1, encoding a mitochondrial p... | ACCEPT | Summary: Experimental mitochondrial membrane localization from the original CABC1/COQ8A isolation study (Iiizumi et al.), which identified CABC1 as a mitochondrial, p53-inducible protein. Consistent with the well-established inner-mitochondrial-membrane localization. Accepted as the core localization; the mature protein is a single-pass inner-membrane protein with a matrix-facing catalytic domain. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt Single-pass membrane protein |
| GO:0031966 mitochondrial membrane | EXP PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | ACCEPT | Summary: Experimental mitochondrial-membrane localization from the structural/biochemical study (Stefely et al. 2015), which mapped the mature form and confirmed matrix-facing residence at the inner membrane. Accepted; corroborates the core localization. Supporting Evidence: PMID:25498144 ADCK3 is known to reside in the mitochondrial matrix |
| GO:0031966 mitochondrial membrane | EXP PMID:33988507 A subcellular map of the human kinome. | KEEP AS NON CORE | Summary: Mitochondrial-membrane localization from the subcellular kinome map (Zhang et al. 2021), a systematic assignment of kinome members to compartments. Consistent with the established mitochondrial localization; corroborating, non-core. |
| GO:0004672 protein kinase activity | ISS GO_REF:0000024 | MODIFY | Summary: "Protein kinase activity" transferred by sequence similarity from COQ8B (Q96D53). Although COQ8A adopts a protein-kinase-like fold, direct biochemical study demonstrated that it lacks canonical protein kinase activity in trans (the same PMID:27499294 result underpins an explicit NOT protein-kinase-activity annotation on this gene). The demonstrated activity is instead ATP binding/hydrolysis (ATPase). This ISS annotation therefore over-asserts a canonical protein-kinase function; recommend replacing with ATP binding and ATP hydrolysis (ATPase) activity, which the evidence supports. Proposed replacements: ATP binding ATP hydrolysis activity Supporting Evidence: PMID:27499294 interacts with lipid CoQ intermediates |
| GO:0006744 ubiquinone biosynthetic process | IDA PMID:38425362 In vitro construction of the COQ metabolon unveils the molec... | ACCEPT | Summary: Direct evidence that COQ8 promotes coenzyme Q biosynthesis, from in-vitro reconstruction of the animal COQ metabolon (Nicoll et al. 2024), which shows COQ8 increases and streamlines CoQ production. (The experimentally characterized paralog in that study was ancestral COQ8B, since recombinant COQ8A gave very low yields, but COQ8A and COQ8B are chordate co-orthologs of the same COQ8 clade and the CoQ-biosynthesis role is shared.) Accepts the gene's core biological process. Supporting Evidence: PMID:38425362 increases and streamlines coenzyme Q production |
| GO:0005739 mitochondrion | HTP PMID:34800366 Quantitative high-confidence human mitochondrial proteome an... | KEEP AS NON CORE | Summary: Mitochondrion localization from a high-throughput high-confidence mitochondrial proteome study. Consistent with the established localization; corroborating, non-core. |
| GO:0005739 mitochondrion | IDA PMID:33988507 A subcellular map of the human kinome. | KEEP AS NON CORE | Summary: Mitochondrion localization (IDA) from the subcellular kinome map (Zhang et al. 2021). Correct and consistent with the well-established mitochondrial localization; corroborating. |
| GO:0004672 protein kinase activity | IDA NOT PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | ACCEPT | Summary: NOT protein kinase activity. Stefely et al. 2016 directly demonstrated that COQ8, despite its protein-kinase-like fold, lacks canonical protein kinase activity in trans. This negated annotation is correct and important — it explicitly counters the naive "protein kinase" assignment implied by the family name (ADCK, aarF domain-containing kinase). Accept as-is. Supporting Evidence: PMID:27499294 Instead, COQ8 has ATPase activity |
| GO:0006468 protein phosphorylation | IDA NOT PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | ACCEPT | Summary: NOT involved_in protein phosphorylation. Consistent with the finding that COQ8 does not perform canonical protein-kinase phosphotransfer in trans; the protein instead functions via ATPase / small-molecule (possibly lipid) activity and complex-Q stabilization. Correct negated annotation; accept as-is. Supporting Evidence: PMID:27499294 interacts with lipid CoQ intermediates |
| GO:0006744 ubiquinone biosynthetic process | IMP PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | ACCEPT | Summary: IMP evidence that COQ8A is required for ubiquinone biosynthesis: Coq8a-knockout mice develop cerebellar ataxia recapitulating ARCA2 with disruption of complex Q and CoQ deficiency, and active-site mutations abolish CoQ production. This is the core biological process of the gene. Accept. Supporting Evidence: PMID:27499294 the K134H mutation eliminates CoQ production in vivo |
| GO:0016301 kinase activity | IDA PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | MODIFY | Summary: Generic "kinase activity" IDA. The activity actually measured in this paper is ATP hydrolysis (ATPase), which the authors describe as small-molecule "water kinase" activity and which is enhanced by the UbiB-specific KxGQ motif. The broad "kinase activity" term is imprecise and, given the explicit NOT protein kinase annotation, risks implying phosphotransfer to a macromolecular substrate that was not demonstrated. Recommend the more precise ATP hydrolysis (ATPase) activity term. Proposed replacements: ATP hydrolysis activity Supporting Evidence: PMID:27499294 Coq8p ATPase activity is enhanced by the UbiB-specific KxGQ motif |
| GO:0016310 phosphorylation | IDA PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | MARK AS OVER ANNOTATED | Summary: Generic BP "phosphorylation" paired with the "kinase activity" MF. The only phosphotransfer demonstrated for COQ8 is autophosphorylation, and that is seen only when the native A-rich loop is mutated (A-to-G); native COQ8 does not carry out canonical protein phosphorylation (NOT-annotated). The measured wild-type activity is ATP hydrolysis. "Phosphorylation" as a biological process is an over-annotation here and is not the gene's endogenous role (ubiquinone biosynthesis). |
| GO:0005739 mitochondrion | IDA PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | KEEP AS NON CORE | Summary: Mitochondrion localization (IDA) from Stefely et al. 2015, which characterized endogenous ADCK3/COQ8A as residing in the mitochondrial matrix at the inner membrane. Correct; corroborating the core localization. Supporting Evidence: PMID:25498144 ADCK3 is known to reside in the mitochondrial matrix |
| GO:0006744 ubiquinone biosynthetic process | IMP PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | ACCEPT | Summary: IMP evidence for the ubiquinone-biosynthesis role: structure-guided mutations of conserved nucleotide-pocket, A-rich loop and QKE-triad residues in Coq8p/ADCK3 eliminate respiratory growth and reduce CoQ abundance in vivo. Core biological process; accept. Supporting Evidence: PMID:25498144 A single alanine-to-glycine mutation of this loop flips |
| GO:0016301 kinase activity | IDA PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | MODIFY | Summary: Generic "kinase activity" IDA. The demonstrable ATP-utilizing activity in this study is autophosphorylation enabled only by the engineered A339G mutation of the A-rich loop; wild-type ADCK3 shows an unusual ADP-over-ATP selectivity and does not display canonical kinase activity. The umbrella "kinase activity" term over-asserts the native biochemistry; the better-supported activities are ATP binding and (per PMID:27499294) ATP hydrolysis. Proposed replacements: ATP hydrolysis activity Supporting Evidence: PMID:25498144 unusual selectivity for binding ADP over ATP |
| GO:0016310 phosphorylation | IDA PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | MARK AS OVER ANNOTATED | Summary: Generic BP "phosphorylation", paired with the "kinase activity" MF above. Based on autophosphorylation that is only unmasked by the artificial A339G mutation and that inhibits CoQ biosynthesis in vivo — i.e. not the gene's native process. Over-annotation; the endogenous process is ubiquinone biosynthesis. |
| GO:0043531 ADP binding | IDA PMID:25498144 Mitochondrial ADCK3 employs an atypical protein kinase-like ... | ACCEPT | Summary: ADP binding (IDA). A distinctive and well-supported molecular feature of COQ8A: thermal-shift assays show adenine nucleotides stabilize the protein, with an unusual selectivity for ADP over ATP driven by the A-rich (AAAS) loop. This nucleotide-binding activity is central to the atypical UbiB mechanism. Accept. Supporting Evidence: PMID:25498144 unusual selectivity for binding ADP over ATP |
| GO:0005759 mitochondrial matrix | IDA PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | NEW | Summary: Proposed refinement of the existing mitochondrion / mitochondrial-membrane localization: COQ8A (and yeast Coq8p) localize to the matrix face of the inner mitochondrial membrane, the site of CoQ production. Added to make the core-function localization explicit. Supporting Evidence: PMID:27499294 localize to the matrix face of the inner mitochondrial membrane |
| GO:0032991 protein-containing complex | IPI PMID:27499294 Cerebellar Ataxia and Coenzyme Q Deficiency through Loss of ... | NEW | Summary: Proposed annotation capturing COQ8A's membership in the multi-subunit CoQ biosynthetic complex (COQ synthome / complex Q; COQ3-COQ7, COQ9), which it stabilizes. Recorded via the generic protein-containing complex term as no specific COQ synthome term currently exists. Supporting Evidence: file:human/COQ8A/COQ8A-uniprot.txt composed of at least COQ3, COQ4, COQ5, COQ6, COQ7 and COQ9 PMID:27499294 Coq8p and COQ8A specifically maintain complex Q |
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Download this section (compressed HTML)Q: What is the physiological small-molecule substrate of COQ8A's ATPase / atypical kinase activity in vivo - is it truly a "water kinase" (ATPase), a lipid/prenyl-quinone kinase, or does ATP hydrolysis primarily power conformational cycling that assembles complex Q?
Q: Does COQ8A phosphorylate any endogenous target (protein or lipid) under native conditions, given that canonical protein-kinase activity in trans is absent and autophosphorylation is only seen with engineered A-rich-loop mutants?
Experiment: Reconstitute human complex Q with recombinant COQ8A (not only ancestral/COQ8B) and measure CoQ-intermediate-stimulated ATPase activity and its dependence on the KxGQ motif and cardiolipin.
Experiment: Use activity-based or metabolomic profiling in COQ8A-null cells rescued with catalytically dead (D507N) vs KxGQ-mutant vs wild-type COQ8A to distinguish the roles of nucleotide binding, ATP hydrolysis, and complex-Q stabilization in CoQ production.
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