Human cytochrome c oxidase assembly factor 4 homolog, mitochondrial.
Synonyms: CHCHD8, E2IG2. HGNC:24604. 87 aa. Chromosome 11.
COA4 was one of the ~100 "orphan" mitochondrial proteins in MitoCarta3.0. UniProt still
carries only FUNCTION: Putative COX assembly factor. {ECO:0000250} — i.e. by similarity,
with no experimental function statement. The 2026 MitoMatch paper
(doi:10.1038/s41467-026-77112-z) supplies the
missing direct physical interaction (COA4–COX11) and the first human loss-of-function
characterization, converting a long-standing genetic inference into experimentally grounded
human data. See projects/MITO_INTERACTOME.md.
ECO:0000305|PubMed:23676665).copper chaperone activity (GO:0016531) or copper binding.Discovery (2009, as Cmc3). Systematic survey of the 14 yeast twin Cx9C proteins; most
deletions impair respiratory chain assembly or stability [PMID:19703468 — abstract only in cache].
Renaming and first characterization (2010). Isolated as an allele-specific suppressor of a
Shy1 Leigh-syndrome mutant; renamed Coa4. Key phenotypes PMID:20624914:
- "Cells lacking Coa4 are depressed in CcO activity but show no impairment in Cox1 maturation or
formation of the Shy1-stabilized Cox1 assembly intermediate" — so Coa4 acts downstream of
Cox1 synthesis and hemylation.
- "Cells lacking Coa4 resemble shy1 Δ cells in exhibiting a reduced mitochondrial copper content."
- "Coa4 may likewise be part of the Cu(I) routing pathway."
- Respiratory function restored by CYC1 (cytochrome c) overexpression.
ROS confound (2013). coa4Δ respiratory growth is rescued by DTT/GSH/ascorbate without
restoring CcO assembly — the growth defect is largely H₂O₂ from partially assembled CcO
intermediates, not the assembly defect itself PMID:23198688. Important caveat when reading respiratory-growth rescue as evidence of restored assembly.
Genetic placement in the copper pathway (2022). Targeted suppressor screen PMID:35666203:
- COX11 overexpression restores Cox1 abundance, CcO assembly and respiration in coa4Δ.
- The rescue requires the copper-coordinating cysteines of Cox11 — so it is restored copper
delivery, not ROS suppression: "cysteine mutants of Cox11 that are incapable of binding copper,
failed to rescue the respiratory defect".
- Coa4 and Cox11 abundance are reciprocally regulated in mitochondria.
- coa4Δ cells have reduced cellular copper; copper supplementation partially rescues.
- Human COA4 complements yeast coa4Δ — "we demonstrate that human COA4 can replace the
function of yeast Coa4 indicating its evolutionarily conserved role". This is what licenses
transferring the yeast mechanism to the human protein.
- Directionality: "only Cox11 can rescue coa4Δ and not the other way around ... suggests that
Coa4 acts upstream of Cox11 in the copper delivery pathway".
- Explicit negative result: "Our initial attempts to detect protein:protein interaction
between Cox11 and Coa4 via coimmunoprecipitation/mass spectrometry experiments were not
successful (data not shown)." The 2026 paper closes exactly this gap.
Swaminathan et al., The predicted interactome of the human mitochondrial proteome,
Nat Commun 2026 (in press), doi:10.1038/s41467-026-77112-z.
No PMID assigned yet, so it is cited and cached by DOI as
publications/DOI_10.1038_s41467-026-77112-z.md (full text retrieved via OpenAlex under
CC-BY), which means its supporting quotes are machine-verifiable like any PMID reference.
Prediction: AlphaFold-Multimer predicts an evolutionarily conserved COA4–COX11 interaction, with
IMS-localized COA4 contacting the IMS-facing domain of COX11 — topologically coherent.
Experimental validation and human phenotyping:
| Experiment | Result |
|---|---|
| Yeast Coa4-V5 co-IP/MS, DSSO-crosslinked mitochondria (n=3) | Cox11 recovered; also IMS phosphatase Ptc5 |
| COX11-FLAG IP from 293T mitochondria (n=3) | Recovers COA4-V5, plus COX1 and COX2 |
| Reciprocal COA4-V5 IP (n=3) | Recovers COX11-FLAG; not COX1 or COX2 |
| CRISPR COA4-KO, MCH58 fibroblasts, 2 clones | COA4 absent |
| COX11 immunoblot in KO (n=3) | Striking reduction in COX11 abundance |
| ICP-MS of KO mitochondria (n=3) | Reduced Cu; Fe, Zn, Mn unaffected |
| BCS copper-chelator titration (n=3) | COX1 loss more pronounced in KO than WT |
| BN-PAGE/western (n=2) | Drastic, specific loss of complex IV–containing supercomplexes |
| Seahorse OCR (n=3) | Reduced respiration |
The crosslinking step is plausibly why 2026 succeeded where 2022 failed — consistent with a
transient or low-affinity contact.
UniProt Q9NYJ1 records INTERACTION: Q9NYJ1; Q9Y6N1: COX11; NbExp=2; IntAct=EBI-22303661,
EBI-2963275, sourced from BioPlex PMID:33961781. This is also the origin of the GOA
IPI GO:0005515 protein binding row with WITH/FROM UniProtKB:Q9Y6N1. So the human COA4–COX11
interaction had affinity-purification support before MitoMatch; the 2026 work adds reciprocal
targeted co-IP, cross-species conservation, and functional consequence.
Well supported and core
- GO:0033617 mitochondrial respiratory chain complex IV assembly — previously IBA/IEA only.
Now backed by human KO data (complex IV supercomplex loss, reduced OCR, COX1 sensitized to
copper limitation) plus two decades of yeast genetics. This is the core BP.
- GO:0005758 mitochondrial intermembrane space — twin CX9C/MIA substrate, yeast localization
data, InterPro IPR039870. Core CC.
Uninformative but not wrong
- GO:0005515 protein binding (IPI, PMID:33961781, with COX11). Per repository guidelines this
term is uninformative regardless of evidence quality. The real content — a specific,
conserved, functionally consequential contact with COX11 — is carried by the complex IV
assembly annotation and by core_functions. Mark over-annotated, keep the partner recorded in
supporting_entities.
Redundant generalization
- GO:0005739 mitochondrion (×3: IEA, HTP, IDA). Correct but subsumed by the IMS annotation.
Candidate not currently annotated
- GO:0006878 intracellular copper ion homeostasis. Reduced mitochondrial/cellular copper is
reproducible across yeast coa4Δ [PMID:20624914, PMID:35666203] and human COA4-KO (2026, with
Fe/Zn/Mn controls). But it may be a downstream consequence of failed CcO metallation
rather than a separate role — the 2022 authors themselves frame it as one of "2 distinct but
closely related roles", and comparable copper deficiency is seen in SCO1/SCO2/COX10/COX15
patient fibroblasts, where it is attributed to CTR1 turnover/mislocalization. Left out of
core_functions; raised in suggested_questions instead.
Molecular function: still unknown. No catalytic activity is established, and metallochaperone
activity is positively excluded. The best current statement is an accessory/regulatory role
supporting COX11-mediated CuB metallation of COX1 — which the ontology does not currently express
well for this case. Asserting a specific MF term here would over-reach; core_functions therefore
carries the BP + CC with a mechanistic description rather than an invented MF.
INDUCTION, PMID:11085516] — biological significance unknown.