ActI-ORF2 (SCO5088) is the chain-length factor (CLF, KSbeta) of the actinorhodin "minimal" type II polyketide synthase in Streptomyces coelicolor. It is the non-catalytic partner of the ketosynthase ActI-ORF1 (KSalpha): together they form the heterodimeric KS-CLF that polymerizes the poly-beta-keto chain of the aromatic polyketide antibiotic actinorhodin. CLF is descended from a ketosynthase but lacks an active site; it regulates the final polyketide chain length, while the KS subunit carries out chain initiation and elongation. Chain elongation and the first cyclization occur within an amphipathic tunnel at the KS-CLF heterodimer interface (PMID:15286722; PDB 1TQY). This makes ActI-ORF2 a structural/specificity subunit that is only functional as part of the assembled KS-CLF complex, directly analogous to PqsB in the P. aeruginosa PqsBC condensing enzyme.
Definition: The function of a catalytically inactive ketosynthase-like (KSbeta/CLF) subunit that, as part of a type II polyketide synthase (KS-CLF) complex, determines the number of chain-extension cycles and hence the final length of the nascent polyketide chain, without itself catalyzing condensation.
Justification: GO has no molecular function term capturing the chain-length-determining role of the type II PKS CLF/KSbeta subunit. The current IEA acyltransferase annotations mischaracterize this non-catalytic subunit as an enzyme; a dedicated term would let curators describe CLF accurately (cf. the non-catalytic PqsB partner of PqsBC).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0016746 acyltransferase activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro-based (IEA) acyltransferase activity from the ketosynthase (thiolase-like) signature. ActI-ORF2 (CLF/KSbeta) descends from a ketosynthase but lacks an active site and is catalytically dead; the condensation activity belongs to the KS subunit within the KS-CLF complex. Reason: Attributes a catalytic MF to a non-catalytic subunit. CLF contributes_to (is required for) the KS-CLF activity and sets chain length, but does not itself catalyze acyl transfer; the activity should be annotated to the type II PKS complex with a contributes_to qualifier. Supporting Evidence: PMID:15286722 Although CLF regulates chain length, it does not have an active site; KS must catalyze both chain initiation and elongation. |
| GO:0016747 acyltransferase activity, transferring groups other than amino-acyl groups | IEA GO_REF:0000117 | MARK AS OVER ANNOTATED | Summary: Orthology-based (IEA) more-specific acyltransferase activity. Same issue as GO:0016746: it assigns catalysis to the non-catalytic CLF subunit. Reason: CLF has no active site; this catalytic MF over-annotates the subunit. Better represented as contributes_to / annotation to the KS-CLF complex. Supporting Evidence: PMID:15286722 Although CLF regulates chain length, it does not have an active site; KS must catalyze both chain initiation and elongation. |
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Download this section (compressed HTML)Q: Should a contributes_to-qualified molecular function (or a dedicated "chain length factor" term) be the recommended annotation for type II PKS CLF subunits across the family, rather than enables acyltransferase activity?
Experiment: Reconstitute KS-CLF with CLF variants of differing length specificity to confirm that CLF, though non-catalytic, dictates chain length within the assembled complex.
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