AIGR Deep Research Report — cao-1 (NEUCR, Q7S860) OpenScientist openscientist-autonomous 4 citations 2 artifacts 2026-07-18T13:36:14.942187 citations file

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AIGR Deep Research Report — cao-1 (NEUCR, Q7S860)

Hypothesis under review: cao-1 has carotenoid dioxygenase activity (GO:0010436).
Focus type: function_assignment
Source: genes/NEUCR/cao-1/cao-1-ai-review.yaml → existing_annotations[1].function_hypothesis
Current annotation context: GO:0010436 carotenoid dioxygenase activity, evidence IBA, GO_REF:0000033 (phylogenetic inference).

Executive Judgment

Verdict: REFUTED (over-annotated).

The seed hypothesis that cao-1 directly possesses carotenoid dioxygenase activity (GO:0010436) is contradicted by direct primary experimental evidence. The one primary paper attached to the review context, Díaz-Sánchez et al. 2013 (PMID:23893079), explicitly tested purified CAO-1 against carotenoid substrates and found no conversion, while showing that CAO-1 cleaves the interphenyl Cα–Cβ double bond of the stilbenes resveratrol and piceatannol. The authors state CAO-1 "is not involved in carotenoid metabolism." UniProt (Q7S860, CAO1_NEUCR) reflects this: recommended activity is resveratrol/stilbene cleavage (EC 1.13.11.-), alt name "Resveratrol cleavage oxygenase cao-1," and an explicit note that it is not involved in carotenoid metabolism.

The GO:0010436 annotation is an IBA (phylogenetic) over-annotation: it was propagated across the carotenoid cleavage oxygenase (CCO) family (PANTHER PTHR10543, InterPro IPR004294, Pfam RPE65) via GO_REF:0000033 to a member that experimentally does not act on carotenoids. This is a textbook case of paralog/family carry-over that direct experimental data override.

Most important caveat: The gene product is a dioxygenase in the same structural family; the error is specifically the substrate class (carotenoid vs. stilbene). Notably (verified in Iteration 3 via QuickGO), the gene's own GO record already contains an experimental NOT annotation — GO:0016116 carotenoid metabolic process, NOT|involved_in, IDA, PMID:23893079 — which directly contradicts the two positive carotenoid IBA terms. The accurate catalytic MF is also already annotated experimentally as GO:0016702 (oxidoreductase acting on single donors with incorporation of two O atoms; i.e. dioxygenase), IDA from PMID:23893079 and PMID:28493664, and substrate specificity is captured by GO:1905594 resveratrol binding (IDA). Thus the recommended action is simply to remove the two carotenoid IBA terms; no new term is strictly required.

Evidence Matrix

Citation	Evidence type	Supports/Refutes/Qualifies	Claim tested	Key finding	Context	Confidence & limitations
PMID:23893079 (Díaz-Sánchez et al., 2013, Eukaryot Cell)	Direct enzyme assay + mutant + expression	Refutes GO:0010436	Does CAO-1 cleave carotenoids?	Carotenoid substrates "were, however, not converted"; CAO-1 instead cleaves resveratrol & piceatannol at the Cα–Cβ bond; resveratrol induces cao-1 mRNA, light does not; Δcao-1 not impaired by resveratrol	N. crassa CAO-1, heterologous expression, in vitro + in vivo	High. This is THE reference in the review context and is directly on-target.
UniProt Q7S860 / CAO1_NEUCR	Database (curated, cites PMID:23893079 & 28493664)	Refutes GO:0010436; supports stilbene activity	What activity does UniProt curate?	FUNCTION: "cleaves the interphenyl C-α-C-β double bond of resveratrol… Is not involved in carotenoid metabolism"; EC 1.13.11.-; catalytic activity trans-resveratrol + O2 → 3,5-dihydroxybenzaldehyde + 4-hydroxybenzaldehyde	Curated record	High (orientation-level, but faithfully reflects primary data). Note: UniProt still carries GO:0010436 IBA — the very annotation under review.
PMID:21073977 (Brefort et al., 2011)	Direct assay, orthologue	Qualifies/Supports refutation	Does the fungal CCO-family paralog cleave carotenoids?	U. maydis Rco1 shows "lack of activity on carotenoids"; cleaves resveratrol/piceatannol; homologs in A. fumigatus, C. globosum, Botrytis also cleave resveratrol	Fungal orthologue	High for family behavior; establishes a stilbene-cleaving (SCO) subclade lacking carotenoid activity.
PMID:30115012 (Loewen et al., 2018)	Structure + assay, orthologue	Qualifies	In vitro vs in vivo substrate range of SCO/LSD enzymes	SCOs are "one branch of the larger carotenoid cleavage oxygenases family"; preferential in vitro cleavage of resveratrol; only putative/in vivo activity toward lycopene	Pseudomonas brassicacearum	Medium. Shows carotenoid activity, where seen at all, is weak/in-vivo-only and not the primary function.
PMID:28493664 (Sui et al., 2017, Biochemistry)	Structural (X-ray) + spectroscopy	Qualifies/Supports refutation	Structural basis of CAO-1 substrate preference	Crystal structure of the fungal stilbenoid-cleaving CCO, CAO1: same four-His non-heme Fe(II) center as carotenoid CCOs but a "markedly different substrate-binding cleft"; 10 PDB entries map to Q7S860 (5U8X/8Y/8Z/5U90/5U97, 6B86, 7T8P/8Q, 8FU2/8FU5)	N. crassa CAO-1 recombinant protein	High. Structural evidence that the catalytic metal is conserved (source of family term) but the substrate pocket is stilbenoid-adapted.
InterPro IPR004294 / Pfam PF03055 (RPE65) / PANTHER PTHR10543	Computational (domain/family)	Explains the error	Basis for IBA propagation	Membership in the broad CCO/RPE65 superfamily is the source of the family-level carotenoid term; the family spans both carotenoid- and stilbene-cleaving activities	Sequence family	High as an explanation of provenance; family membership alone cannot assign substrate.

GO Curation Implications (leads — require curator verification)

GO decision table (current annotation set verified live via QuickGO, Iteration 3)

GO term	Aspect	Current annotation (evidence, qualifier)	Recommended action	Rationale
GO:0010436 carotenoid dioxygenase activity	MF	enables, IBA, GO_REF:0000033 ← the term under review	Remove / do not accept	Refuted by direct assay (PMID:23893079) and internally contradicted by the gene's own experimental NOT annotation on carotenoid metabolic process; over-annotation from CCO-family IBA.
GO:0016121 carotene catabolic process	BP	involved_in, IBA, GO_REF:0000033	Remove / do not accept	Same over-annotation; contradicts the NOT annotation below.
GO:0016116 carotenoid metabolic process	BP	NOT\|involved_in, IDA, PMID:23893079	Retain	Experimental negative annotation — CAO-1 is NOT in carotenoid metabolism; this is the direct counter-evidence to the two IBA terms above.
GO:0016702 oxidoreductase activity (single donors, 2 O atoms incorporated) — i.e. dioxygenase	MF	enables, IDA, PMID:23893079 & PMID:28493664 (already present)	Retain — this is the accurate MF	Experimentally supported; more specific than generic GO:0051213 and already captures the true catalytic activity. No new term strictly required.
GO:1905594 resveratrol binding	MF	enables, IDA, PMID:28493664 (already present)	Retain	Documents the true substrate specificity experimentally.
GO:0005506 iron ion binding	MF	enables, IDA, PMID:28493664 (already present)	Retain	Non-heme Fe cofactor confirmed structurally.
Stilbene/resveratrol α,β-dioxygenase activity	MF	none exists	Optional lead: request a new substrate-specific MF term (cf. EC 1.13.11.43 lignostilbene-α,β-dioxygenase)	Would make the MF maximally precise; combined with GO:1905594 the current set already conveys substrate + activity.

Do not default to "protein binding." The accurate catalytic MF (GO:0016702, IDA) is already annotated; the only required action is removal of the two carotenoid IBA terms (GO:0010436, GO:0016121). GO:0051213 is unnecessary because the more specific GO:0016702 is already present with experimental evidence.

Mechanistic Scope

Immediate molecular function (direct): O2-dependent oxidative cleavage of the interphenyl Cα–Cβ double bond of resveratrol (→ 3,5-dihydroxybenzaldehyde + 4-hydroxybenzaldehyde) and piceatannol; a non-heme Fe(II) (4-His) dioxygenase mechanism typical of the CCO/RPE65 fold. Not carotenoid cleavage.
Substrate specificity (direct): Requires unmodified hydroxyls on the stilbene; inactive on trans-stilbene, pinosylvin, trimethoxy-resveratrol, and a glycosylated stilbene.
Downstream/inferred (not the MF being tested): Regulatory induction by resveratrol; a subtle growth phenotype of Δcao-1 only under partial sorbose toxicity; presumed role in detoxifying/degrading the phytoalexin resveratrol. These are context/phenotype-level, not evidence for carotenoid activity.

Conflicts and Alternatives

Paralog/family confusion (the root cause): The sister gene CAO-2 (cao-2) is a genuine carotenoid (torulene) cleaving oxygenase in neurosporaxanthin biosynthesis. The carotenoid term likely bled onto cao-1 through family-level phylogenetic inference. cao-1 and cao-2 are distinct in substrate.
Original functional hypothesis was disproven: The 2013 study set out expecting CAO-1 to supply retinal (rhodopsin chromophore) by cleaving β-carotene — this a priori carotenoid hypothesis was explicitly tested and rejected.
Residual/in-vivo carotenoid activity in the family: PMID:30115012 reports putative in-vivo lycopene cleavage by a bacterial SCO. Even if some CCO-family SCOs have trace/in-vivo carotenoid side activity, for N. crassa CAO-1 specifically the direct in vitro test was negative, so GO:0010436 as a primary MF assignment is not warranted.

Knowledge Gaps

Gap	What was checked	Why it matters	What would resolve it
No dedicated GO MF term for stilbene/resveratrol dioxygenase	QuickGO search (stilbene/resveratrol/carotenoid): only generic GO:0051213 or catabolic-process BP terms exist	Prevents a precise MF replacement; forces use of a generic parent	Request a new GO MF term (align to EC 1.13.11.43 lignostilbene-α,β-dioxygenase).
Structure/active-site confirmation for CAO-1 itself	RESOLVED in Iteration 2: PMID:28493664 retrieved (crystal structure of stilbenoid-cleaving CAO1, distinct substrate cleft); 10 PDB entries confirmed for Q7S860	Confirms mechanism/substrate pocket structurally	Done — no longer a gap.
In vivo physiological role of resveratrol cleavage in N. crassa	Δcao-1 phenotype is subtle (only under sorbose stress)	Affects any BP annotation strength (stilbene catabolism)	Metabolite profiling of resveratrol turnover in Δcao-1 vs WT; broader stilbene panel.

Discriminating Tests

Definitive (already done): In vitro incubation of purified CAO-1 with β-carotene/torulene/lycopene vs resveratrol/piceatannol + LC-MS product ID — the discriminating experiment; result was negative for carotenoids, positive for stilbenes (PMID:23893079). No further test is needed to reject GO:0010436.
Confirmatory: Complement Δcao-1 and assay resveratrol→benzaldehyde flux; RNA induction by resveratrol vs light (both reported, both consistent with stilbene role).
Comparative bioinformatics: Phylogenetic placement of Q7S860 within PTHR10543 to show it groups with the SCO (Rco1/NOV1/LSD) subclade rather than the carotenoid-cleaving (CCD/CAO-2) subclade — supports removing the carotenoid IBA.

Curation Leads (require curator verification)

Action change (primary): Remove/reject IBA GO:0010436 (carotenoid dioxygenase activity) and IBA GO:0016121 (carotene catabolic process) for cao-1 as CCO-family over-annotations refuted by direct evidence and contradicted by the gene's own experimental NOT annotation.
Replacement MF — NOT needed: The accurate catalytic MF GO:0016702 (dioxygenase-type oxidoreductase) is already present with IDA evidence (PMID:23893079, PMID:28493664); GO:1905594 resveratrol binding (IDA) already captures the substrate. Do not add generic GO:0051213. Optionally file a request for a substrate-specific "resveratrol/stilbene dioxygenase activity" MF term (cf. EC 1.13.11.43).
Retain the experimental NOT annotation GO:0016116 (carotenoid metabolic process, NOT|involved_in, IDA, PMID:23893079) — the strongest single piece of counter-evidence.
Optional BP: Consider GO:0046272 stilbene catabolic process (evidence weak/subtle Δcao-1 phenotype — annotate cautiously).
Candidate references to verify:
PMID:23893079 — snippet to verify: "we tested CAO-1 activity with carotenoid substrates that were, however, not converted" and "CAO-1 cleaved the interphenyl Cα-Cβ double bond of resveratrol and its derivative piceatannol."
PMID:21073977 — orthologue support: "lack of activity on carotenoids."
PMID:28493664 — VERIFIED (Iteration 2): "The crystal structure of a fungal stilbenoid-cleaving CCO, CAO1, reveals strong similarity between its iron center and those of carotenoid-cleaving CCOs, but with a markedly different substrate-binding cleft." PDB: 5U8X/5U8Y/5U8Z/5U90/5U97/6B86/7T8P/7T8Q/8FU2/8FU5.
Suggested curator questions: (a) Is a new substrate-specific stilbene-dioxygenase MF term warranted, or is the existing IDA GO:0016702 + GO:1905594 sufficient? (b) Should the CAO-2 (torulene-cleaving) annotations be checked to ensure the family carotenoid term did not also mis-propagate elsewhere? (c) Given the experimental NOT annotation on carotenoid metabolic process, should the two carotenoid IBA terms be actively removed rather than merely down-weighted?

Provenance

UniProt REST fetch of Q7S860 (names, EC, GO xrefs, InterPro/Pfam/PANTHER, FUNCTION/CATALYTIC ACTIVITY) — executed via execute_code (output retained in iteration log).
QuickGO ontology search for stilbene/resveratrol/carotenoid MF terms — executed via execute_code.
PubMed retrieval of PMID:23893079, 21073977, 30115012.
NCBI E-utilities retrieval and verification of PMID:28493664 (Iteration 2).
RCSB PDB search API query mapping Q7S860 → 10 experimental structures (Iteration 2).
QuickGO annotation-search API query of the current authoritative GO annotation set for Q7S860 including qualifiers (Iteration 3): confirmed GO:0010436 & GO:0016121 are IBA/GO_REF:0000033; GO:0016116 is NOT|involved_in IDA/PMID:23893079; GO:0016702 & GO:1905594 & GO:0005506 are IDA.

All computed results above are from live API calls executed during this run.