cao-1 (CAO1_NEUCR, Q7S860 / NCU07008) — review notes

Summary of gene function

CAO-1 is one of two carotenoid cleavage oxygenase (CCO) family members encoded by
Neurospora crassa (the other is CAO-2). Despite the family name and the "carotenoid
cleavage oxygenase 1" designation it received when the genome was annotated, CAO-1 is
not a carotenoid-cleaving enzyme. It is a non-heme Fe(II) dioxygenase that cleaves the
interphenyl Cα–Cβ double bond of the stilbenoid resveratrol (and its dihydroxy derivative
piceatannol), yielding aromatic aldehyde products.

Molecular function: stilbene/resveratrol cleavage dioxygenase. Cleaves trans-resveratrol
→ 3,5-dihydroxybenzaldehyde + 4-hydroxybenzaldehyde; cleaves piceatannol →
3,5-dihydroxybenzaldehyde + 3,4-dihydroxybenzaldehyde. EC 1.13.11.- (incomplete). Uses a
mononuclear non-heme Fe(II) center coordinated by four His (four-His CCO motif).
Biological process: stilbene catabolism / degradation of the plant phytoalexin resveratrol.
Expression is strongly induced by resveratrol and is light-independent — the opposite of the
regulation of genuine carotenoid-pathway genes.
Localization: presumed cytoplasmic (external resveratrol is degraded only intracellularly;
no degradation halo around colonies, unlike secreted-laccase fungi). No direct GO CC annotation
exists in GOA.

Key experimental evidence

PMID:23893079 (Díaz-Sánchez et al. 2013, Eukaryot Cell) — functional characterization

CAO-1 was originally hypothesized to make retinal by cleaving β-carotene, but this was disproven:
PMID:23893079
PMID:23893079
PMID:23893079
Substrate specificity is narrow: PMID:23893079
Regulation opposite to carotenoid genes: PMID:23893079
Δcao-1 mutants are not more sensitive to resveratrol; instead cleavage products may be mildly toxic (faster growth of Δcao-1 under sorbose + resveratrol). Basis for the NOT carotenoid-metabolism annotation.

PMID:28493664 (Sui et al. 2017, Biochemistry) — structure & spectroscopy

Crystal structures (PDB 5U8X/5U8Y/5U8Z/5U90/5U97) of CAO1 with iron and stilbenoid ligands.
PMID:28493664
Iron center: PMID:28493664 and four-His coordination: PMID:28493664
Substrate (resveratrol/piceatannol) co-crystallized: PMID:28493664; activity confirmed PMID:28493664
UniProt binding residues from this structure: resveratrol/piceatannol contacts at His133, His164, His383; catalytic Fe ligands His197, His248, His313, His510.

PMID:29946976 (Sui et al. 2018) — metal-substituted CCOs; further cofactor characterization (referenced in UniProt COFACTOR, not in GOA).

Annotation review decisions

GO term	Evidence	Decision	Rationale
GO:0010436 carotenoid dioxygenase activity	IBA	REMOVE	Phylogenetic guess directly refuted by IDA: CAO-1 does not cleave carotenoids. Real MF captured by GO:0016702 IDA.
GO:0016121 carotene catabolic process	IBA	MODIFY → GO:0046272 stilbene catabolic process	Wrong substrate class; the real catabolic role is on stilbenes (resveratrol).
GO:0016702 oxidoreductase … two atoms of oxygen (IEA InterPro)	IEA	ACCEPT (KEEP_AS_NON_CORE not needed; core MF)	Correct dioxygenase parent term; consistent with IDA.
GO:0016116 carotenoid metabolic process — NOT	IDA	ACCEPT	Correct negative annotation; carotenoids explicitly tested and not converted.
GO:0016702 (IDA PMID:23893079)	IDA	ACCEPT	Best available MF term for the demonstrated dioxygenase activity.
GO:0016702 (IDA PMID:28493664)	IDA	ACCEPT	Structural + spectroscopic confirmation.
GO:0005506 iron ion binding	IDA	ACCEPT	Non-heme mononuclear Fe(II), four-His center, crystallographically defined.
GO:1905594 resveratrol binding	IDA	ACCEPT	Resveratrol co-crystallized in active-site cleft.

Notable point

This gene is a textbook example of a misnamed/over-propagated family annotation: the "carotenoid
cleavage oxygenase" family name and IBA propagation generated a carotenoid MF + process that
experimental work explicitly disproved. The curated GOA already carries the corrective NOT annotation,
and there is no dedicated GO MF term for stilbene/resveratrol dioxygenase activity (candidate new term).