cao-2 encodes CAO-2 (NCU11424), the second carotenoid cleavage oxygenase (CCO) of Neurospora crassa and the paralog of cao-1. In contrast to cao-1 (a stilbenoid/resveratrol cleaver), CAO-2 is a genuine carotenoid-cleaving enzyme: a torulene dioxygenase (EC 1.13.11.59) that catalyzes the committed cleavage step of the neurosporaxanthin biosynthetic pathway. It cleaves the C40 carotene torulene with molecular oxygen to yield the C35 apocarotenal 4'-apo-beta-carotenal plus 3-methyl-2-butenal; the apocarotenal is subsequently oxidized to the carboxylic xanthophyll neurosporaxanthin by the aldehyde dehydrogenase YLO-1. CAO-2 is cytosolic, is specific for torulene (it does not cleave gamma-carotene in vitro), and its expression is induced by light in a WC-1/WC-2 (White Collar) dependent manner, as expected for a structural gene of the carotenoid pathway. Disruption of cao-2 abolishes neurosporaxanthin production and causes torulene to accumulate.
Definition: Catalysis of the reaction: torulene + O2 = 4'-apo-beta-carotenal + 3-methyl-2-butenal. Oxidative cleavage of the C40 carotene torulene at the 4',5' double bond by a non-heme iron carotenoid cleavage oxygenase, producing a C35 apocarotenal.
Justification: CAO-2 (and its ortholog CarT in Fusarium fujikuroi) has a specific, experimentally-defined activity (EC 1.13.11.59) for which no dedicated GO MF term exists; only the parent GO:0010436 (carotenoid dioxygenase activity) is available. A specific term would let the neurosporaxanthin pathway step be annotated precisely.
Parent term: carotenoid dioxygenase activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0010436 carotenoid dioxygenase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation of carotenoid dioxygenase activity. For CAO-2 this is correct: the enzyme is a torulene dioxygenase (EC 1.13.11.59) that oxidatively cleaves the C40 carotene torulene, demonstrated directly with purified enzyme (PMID:17610084). No specific GO term for torulene dioxygenase activity exists, so this carotenoid-dioxygenase parent is the best available. Reason: Core molecular function, correctly propagated by IBA and independently supported by direct experimental evidence. This is the positive-control counterpart to cao-1, where the identical family IBA term is refuted; here it is right. (Currently only IBA in GOA despite experimental characterization - an IDA upgrade from PMID:17610084 is warranted.) Supporting Evidence: PMID:17610084 cleaved torulene to produce beta-apo-4'-carotenal, the corresponding aldehyde of neurosporaxanthin PMID:17610084 lack of gamma-carotene-cleaving activity in vitro |
| GO:0016121 carotene catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetic (IBA) annotation of carotene catabolic process. Accurate in that the carotene torulene is consumed/cleaved by CAO-2, but partial: the informative biological process is the biosynthesis of the apocarotenoid neurosporaxanthin, of which this torulene cleavage is the committed step (disruption abolishes neurosporaxanthin and accumulates torulene, PMID:17610084). Reason: Not wrong - torulene catabolism does occur - but it under-describes the role. The more informative process term is apocarotenoid biosynthetic process (GO:0043289), captured in core_functions; adding it (with IDA from PMID:17610084) is recommended. Supporting Evidence: PMID:17610084 CAO-2 is the enzyme responsible for the oxidative cleavage of torulene in the neurosporaxanthin biosynthetic pathway |
| GO:0005829 cytosol | IEA GO_REF:0000044 | ACCEPT | Summary: Automated (IEA) cytosol annotation from UniProt subcellular-location mapping, consistent with the curated cytoplasmic/cytosolic localization of CAO-2. Reason: Consistent core localization for this soluble cytosolic carotenoid oxygenase. |
| GO:0016702 oxidoreductase activity, acting on single donors with incorporation of molecular oxygen, incorporation of two atoms of oxygen | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based (IEA) dioxygenase MF term (incorporation of two oxygen atoms). Correct and consistent with the demonstrated torulene dioxygenase activity; a general parent of the carotenoid dioxygenase term. Reason: Accurate general dioxygenase MF, corroborating GO:0010436. The specific activity is captured by the carotenoid dioxygenase term. |
| GO:0043289 apocarotenoid biosynthetic process | IDA | NEW | Summary: Proposed NEW annotation (not currently in GOA). CAO-2's torulene cleavage is the committed step of neurosporaxanthin (a C35 carboxylic apocarotenoid) biosynthesis; disruption abolishes neurosporaxanthin and accumulates torulene (PMID:17610084). This apocarotenoid-biosynthesis process role is the informative complement to the existing (accurate but partial) carotene catabolic process IBA. Reason: Captures the experimentally-established biosynthetic pathway role directly, addressing the under-curation of this experimentally-characterized gene (currently only IBA/IEA in GOA). Supporting Evidence: PMID:17610084 CAO-2 is the enzyme responsible for the oxidative cleavage of torulene in the neurosporaxanthin biosynthetic pathway |
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Download this section (compressed HTML)Q: CAO-2 is experimentally characterized (disruption phenotype and purified-enzyme assay, PMID:17610084) yet carries only IBA/IEA GO annotations - should GOA add IDA annotations for torulene dioxygenase activity and apocarotenoid/neurosporaxanthin biosynthetic process?
Q: Since Neurospora lacks an identified retinal-forming enzyme and CAO-2's product beta-apo-4'-carotenal has been proposed as a candidate physiological chromophore of the NOP-1 rhodopsin, does CAO-2 (via this apocarotenal) contribute to NOP-1 photobiology in addition to its neurosporaxanthin role?
Experiment: Structural determination of CAO-2 (no experimental structure exists, unlike cao-1) to define the carotenoid-binding cleft and the basis of torulene versus gamma-carotene selectivity, enabling a direct structural comparison with the stilbenoid-adapted cleft of CAO-1.
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