coq8a encodes the zebrafish ortholog of COQ8A/ADCK3, an atypical protein kinase-like (UbiB family) protein of the inner mitochondrial membrane, anchored by a single transmembrane segment with its catalytic domain facing the matrix. It is required for de novo biosynthesis of coenzyme Q (ubiquinone), the lipid electron carrier of the respiratory chain, where it binds early CoQ lipid intermediates and stabilizes the multi-subunit CoQ biosynthetic complex. Its kinase-like domain is autoinhibited by an N-terminal KxGQ motif that occludes the substrate pocket and by an A-rich loop that makes it bind ADP in preference to ATP; relief of this autoinhibition supports a membrane- and lipid-stimulated ATPase activity rather than transfer of phosphate to a protein substrate. Loss of the orthologous gene in yeast, mouse and human causes coenzyme Q deficiency, and in humans it underlies autosomal recessive cerebellar ataxia type 2.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006744 ubiquinone biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: Ubiquinone biosynthetic process, propagated from node PTN000059692, whose descendants include yeast Coq8p, human COQ8A and COQ8B, fly, worm and Arabidopsis members; loss of the gene lowers CoQ in every organism tested. Reason: The function assigned at this node is the one UbiB-family proteins were defined by, and it holds across the whole clade zebrafish coq8a sits in: chemical inhibition of endogenous yeast Coq8p depletes CoQ, and COQ8A knockout mice develop CoQ deficiency with ataxia. Coq8a does work in the pathway rather than merely being required for it, stabilizing the CoQ biosynthetic complex through its nucleotide-dependent activity. Supporting Evidence: PMID:29198567 acute chemical inhibition of its endogenous activity in yeast is sufficient to cause respiratory deficiency concomitant with CoQ depletion PMID:27499294 mice lacking COQ8A develop a slowly progressive cerebellar ataxia linked to Purkinje cell dysfunction and mild exercise intolerance PMID:27499294 COQ8A and yeast Coq8p specifically stabilize a CoQ biosynthesis complex through unorthodox PKL functions file:DANRE/coq8a/coq8a-uniprot.txt PATHWAY: Cofactor biosynthesis; ubiquinone biosynthesis. |
| GO:0031966 mitochondrial membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Mitochondrial membrane from the UniProt subcellular-location mapping; COQ8 proteins are anchored in the inner mitochondrial membrane with the kinase-like domain facing the matrix. Reason: Both ortholog studies place COQ8 at the matrix face of the inner mitochondrial membrane via its transmembrane segment, matching the single-pass membrane topology UniProt assigns to the zebrafish protein. Supporting Evidence: file:DANRE/coq8a/coq8a-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion membrane file:DANRE/coq8a/coq8a-uniprot.txt Single-pass membrane protein PMID:27499294 Coq8p and COQ8A localize to the matrix face of the inner mitochondrial membrane PMID:29198567 Endogenous COQ8 contains a TM domain that anchors it to the inner mitochondrial membrane (IMM) facing the matrix |
| GO:0006744 ubiquinone biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: Ubiquinone biosynthetic process from the UniPathway UPA00232 mapping, matching the UniProt PATHWAY line. Reason: The pathway assignment is the consensus function of the family and is supported experimentally in yeast, mouse and human orthologs. Supporting Evidence: file:DANRE/coq8a/coq8a-uniprot.txt PATHWAY: Cofactor biosynthesis; ubiquinone biosynthesis. file:DANRE/coq8a/coq8a-uniprot.txt Atypical kinase involved in the biosynthesis of coenzyme Q PMID:29198567 acute chemical inhibition of its endogenous activity in yeast is sufficient to cause respiratory deficiency concomitant with CoQ depletion |
| GO:0031966 mitochondrial membrane | ISS GO_REF:0000024 | ACCEPT | Summary: Mitochondrial membrane transferred from human COQ8A (UniProtKB:Q8NI60), which carries EXP evidence for this location. Reason: The donor annotation is experimentally grounded and the zebrafish protein has the same predicted transit peptide and single transmembrane anchor, so the transfer is sound. Consistent with the IEA row for the same term. Supporting Evidence: PMID:27499294 Coq8p and COQ8A localize to the matrix face of the inner mitochondrial membrane PMID:29198567 Endogenous COQ8 contains a TM domain that anchors it to the inner mitochondrial membrane (IMM) facing the matrix file:DANRE/coq8a/coq8a-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion membrane |
| GO:0004672 protein kinase activity | ISS GO_REF:0000024 | MODIFY | Summary: Protein kinase activity transferred from human COQ8B (UniProtKB:Q96D53). The paralog-derived positive claim is contradicted by direct assay of COQ8A itself, which lacks canonical protein kinase activity in trans; the in vitro autophosphorylation seen for truncated constructs requires mutating the A-rich loop or KxGQ motif and inhibits CoQ biosynthesis in vivo. Reason: The essence (an active kinase-like enzyme) is sound but the substrate class is wrong. The same GOA record also contains a NOT protein kinase activity row from the COQ8A donor, and the generic kinase activity term is what human curators assigned from the primary data; replacing this row with GO:0016301 removes the internal contradiction while keeping the supported claim. Propagation Review Root cause: SOURCE BAD Failure modes: WRONG ORTHOLOG OR PARALOG GRANULARITY MISMATCH Sources checked: UniProtKB:Q96D53 · COQ8B SOURCE BAD Human COQ8B paralog; its protein kinase activity annotation is not itself experimentally established, and direct assay of the COQ8A branch showed no canonical protein kinase activity in trans. UniProtKB:Q8NI60 · COQ8A SUPPORTS TRANSFER The orthologous donor carries NOT protein kinase activity (IDA, PMID:27499294) plus a positive generic kinase activity IDA, which is the level this row should be transferred at. Proposed replacements: kinase activity Supporting Evidence: PMID:27499294 we demonstrate that it lacks canonical protein kinase activity in trans PMID:27499294 These results suggest that complex Q proteins are not abundantly phosphorylated in vivo PMID:25498144 A single alanine-to-glycine mutation of this loop flips this coenzyme selectivity and enables autophosphorylation but inhibits coenzyme Q biosynthesis in vivo file:DANRE/coq8a/coq8a-uniprot.txt Its substrate specificity is still unclear |
| GO:0004672 protein kinase activity | ISS NOT GO_REF:0000024 | ACCEPT | Summary: NOT protein kinase activity: a negative assertion transferred from human COQ8A, where purified protein showed no canonical protein kinase activity in trans and complex Q proteins were not appreciably phosphorylated in vivo. Reason: The negation is directly supported in the donor by experiment and is the more reliable of the two conflicting protein kinase rows in this record; the positive row from the COQ8B paralog is proposed for modification to generic kinase activity instead. Accepting the negation records what the field actually established about this branch of the family. Supporting Evidence: PMID:27499294 we demonstrate that it lacks canonical protein kinase activity in trans PMID:27499294 These results suggest that complex Q proteins are not abundantly phosphorylated in vivo file:DANRE/coq8a/coq8a-uniprot.txt However, the small molecule kinase activity was not confirmed by another publication |
| GO:0006468 protein phosphorylation | ISS NOT GO_REF:0000024 | ACCEPT | Summary: NOT protein phosphorylation: the process counterpart of the negated protein kinase activity, transferred from human COQ8A. Reason: Phosphoproteomics of the CoQ complex found its subunits not abundantly phosphorylated in vivo, and the purified enzyme does not phosphorylate protein substrates in trans, so the gene product does not do the work of protein phosphorylation. Supporting Evidence: PMID:27499294 These results suggest that complex Q proteins are not abundantly phosphorylated in vivo PMID:27499294 we demonstrate that it lacks canonical protein kinase activity in trans |
| GO:0005739 mitochondrion | ISS GO_REF:0000024 | ACCEPT | Summary: Mitochondrion transferred from human COQ8A; the parent location of the mitochondrial membrane annotation. Reason: Correct but less informative than the membrane term: UbiB homologs are exclusively mitochondrial in eukaryotes and the zebrafish protein carries a mitochondrial transit peptide. Retained as the general location. Supporting Evidence: PMID:25498144 In eukaryotes, UbiB homologs are found exclusively in mitochondria file:DANRE/coq8a/coq8a-uniprot.txt SUBCELLULAR LOCATION: Mitochondrion membrane |
| GO:0006744 ubiquinone biosynthetic process | ISS GO_REF:0000024 | ACCEPT | Summary: Ubiquinone biosynthetic process transferred from human COQ8A, which has IMP evidence from two independent studies. Reason: This is the core process for the gene: the human and yeast orthologs are required for CoQ production, and the activity of COQ8 modulates the CoQ biosynthetic complex it belongs to. Consistent with the IBA and IEA rows for the same term. Supporting Evidence: PMID:27499294 COQ8A and yeast Coq8p specifically stabilize a CoQ biosynthesis complex through unorthodox PKL functions PMID:29198567 acute chemical inhibition of its endogenous activity in yeast is sufficient to cause respiratory deficiency concomitant with CoQ depletion file:DANRE/coq8a/coq8a-uniprot.txt PATHWAY: Cofactor biosynthesis; ubiquinone biosynthesis. |
| GO:0016301 kinase activity | ISS GO_REF:0000024 | ACCEPT | Summary: Kinase activity transferred from human COQ8A, which carries IDA evidence for this generic term from both structural studies; the protein is an atypical, autoinhibited member of the protein kinase-like superfamily whose physiological phosphoacceptor is unidentified. Reason: The generic kinase term is the right level: the fold and the nucleotide-dependent activity are demonstrated, and human curators chose exactly this term from the same experiments while negating protein kinase activity. UniProt records the unresolved substrate specificity, so nothing more specific can be asserted. Supporting Evidence: file:DANRE/coq8a/coq8a-uniprot.txt Atypical kinase involved in the biosynthesis of coenzyme Q file:DANRE/coq8a/coq8a-uniprot.txt Its substrate specificity is still unclear PMID:27499294 Instead, COQ8 has ATPase activity and interacts with lipid CoQ intermediates PMID:25498144 ADCK3 exhibits an unusual selectivity for binding ADP over ATP file:DANRE/coq8a/coq8a-deep-research-falcon.md the most defensible primary function is: *a mitochondrial UbiB-family ATPase/kinase-like protein that promotes CoQ biosynthesis by supporting the COQ metabolon and/or mobilizing lipidic CoQ intermediates in the IMM.* |
| GO:0016310 phosphorylation | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Phosphorylation (the general process) transferred from human COQ8A, which has IDA evidence for it; for COQ8 this rests on in vitro autophosphorylation of engineered constructs rather than on a physiological phosphotransfer to another molecule. Reason: Autophosphorylation is latent: it appears when the A-rich loop or the KxGQ motif is mutated, and that same mutation inhibits CoQ biosynthesis in vivo, so the phosphotransfer is not the productive output of the enzyme. The row is not wrong given the donor IDA, but it should not stand beside ubiquinone biosynthesis as a core process. Supporting Evidence: PMID:25498144 A single alanine-to-glycine mutation of this loop flips this coenzyme selectivity and enables autophosphorylation but inhibits coenzyme Q biosynthesis in vivo PMID:27499294 Mutating the A-rich loop or the invariant KxGQ motif enhances file:DANRE/coq8a/coq8a-uniprot.txt Autoinhibited by the N-terminal domain, containing the KxGQ motif, that completely occludes the typical substrate binding pocket. Nucleotide-binding relieves inhibition. |
| GO:0043531 ADP binding | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ADP binding transferred from human COQ8A, which has IDA evidence: thermal-shift assays showed the atypical A-rich loop gives an unusual selectivity for ADP over ATP. Reason: A real and unusual ligand-binding property that UniProt records for the zebrafish protein and that underlies the autoinhibition mechanism, but it is a nucleotide-binding feature rather than the gene product’s function; the core role is CoQ biosynthesis at the inner mitochondrial membrane. Supporting Evidence: PMID:25498144 ADCK3 exhibits an unusual selectivity for binding ADP over ATP file:DANRE/coq8a/coq8a-uniprot.txt Shows an unusual selectivity for binding ADP over ATP file:DANRE/coq8a/coq8a-uniprot.txt Autoinhibited by the N-terminal domain, containing the KxGQ motif, that completely occludes the typical substrate binding pocket. Nucleotide-binding relieves inhibition. |
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Download this section (compressed HTML)Q: Does zebrafish coq8a loss of function reproduce the coenzyme Q deficiency and neurological phenotype of COQ8A-null mice, and is CoQ content in zebrafish embryos dependent on it?
Q: Which molecular function term should capture COQ8 activity now that the conserved, lipid-stimulated ATPase activity is established but no physiological phosphoacceptor is known? Human COQ8A carries generic kinase activity (IDA) and no ATP hydrolysis activity annotation, so the family is currently described by a term that names an activity it may not perform on any substrate other than itself.
Experiment: Generate a coq8a null zebrafish line and quantify CoQ9/CoQ10 and CoQ pathway intermediates by LC-MS in embryos and adult tissues, testing whether the CoQ deficiency of mammalian orthologs is conserved.
Experiment: Assay recombinant zebrafish Coq8a for ATP hydrolysis in the presence and absence of cardiolipin-containing liposomes and CoQ-intermediate-like phenolic compounds, to test whether the lipid-stimulated ATPase activity described for yeast, human and bacterial COQ8 extends to the fish enzyme.
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