id: Q02059
gene_symbol: actI-ORF1
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:100226
  label: Streptomyces coelicolor (strain ATCC BAA-471 / A3(2) / M145)
description: >-
  ActI-ORF1 (SCO5087) is the ketosynthase subunit (KSalpha) of the actinorhodin
  "minimal" type II polyketide synthase in Streptomyces coelicolor. Together with
  ActI-ORF2, the chain-length factor (KSbeta/CLF), it forms the heterodimeric KS-CLF
  that catalyzes iterative decarboxylative (Claisen) condensation of malonyl units
  to polymerize the poly-beta-keto chain of the aromatic polyketide antibiotic
  actinorhodin. The KS subunit carries the catalytic activity and must catalyze both
  chain initiation and elongation, while CLF, which lacks an active site, governs the
  final chain length; chain elongation and the first cyclization occur within an
  amphipathic tunnel at the heterodimer interface (PMID:15286722; PDB 1TQY). Although
  the KS-CLF is evolutionarily related to fatty-acid beta-ketoacyl synthases, it is a
  polyketide synthase, not part of fatty-acid metabolism.
references:
- id: PMID:15286722
  title: An antibiotic factory caught in action.
  findings:
  - statement: >-
      In type II PKSs the polyketide chain is polymerized by the heterodimeric
      ketosynthase-chain length factor (KS-CLF); KS carries the catalytic activity.
    supporting_text: >-
      In type II polyketide synthases (PKSs), chains are polymerized by the
      heterodimeric ketosynthase-chain length factor (KS-CLF).
  - statement: >-
      The KS subunit catalyzes chain initiation and elongation; CLF regulates chain
      length but has no active site.
    supporting_text: >-
      Although CLF regulates chain length, it does not have an active site; KS must
      catalyze both chain initiation and elongation.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified; PDB 1TQY structure paper. Cached entry is abstract-only, but
      the abstract explicitly states the catalytic role of KS and the heterodimeric
      KS-CLF architecture.
- id: file:STRCO/actI-ORF1/actI-ORF1-notes.md
  title: actI-ORF1 review notes (BGC project) with predicted-complex evidence
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
- id: GO_REF:0000117
  title: Gene Ontology annotation based on Ensembl/EnsemblGenomes orthology
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple Source Sequence Analysis
existing_annotations:
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: >-
      Phylogenetic (IBA) inference of cytosolic localization. The actinorhodin type
      II PKS is a soluble cytosolic enzyme system, so this is appropriate.
    action: ACCEPT
    reason: Correct cellular component for a soluble type II PKS subunit.
- term:
    id: GO:0004315
    label: 3-oxoacyl-[acyl-carrier-protein] synthase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: >-
      InterPro/sequence-based (IEA) assignment from the beta-ketoacyl synthase (KAS)
      signature. The chemistry (decarboxylative condensation) is correct, but the
      specific term GO:0004315 denotes the fatty-acid-synthase (FAS) ketosynthase.
      ActI-ORF1 is the ketosynthase of a type II POLYKETIDE synthase, so the
      polyketide-specific term is more accurate.
    action: MODIFY
    reason: >-
      Wrong-specific: the enzyme is a polyketide ketosynthase, not a fatty-acid KAS.
      Replace with GO:0016218 (polyketide synthase activity).
    proposed_replacement_terms:
    - id: GO:0016218
      label: polyketide synthase activity
    supported_by:
    - reference_id: PMID:15286722
      supporting_text: >-
        Although CLF regulates chain length, it does not have an active site; KS must
        catalyze both chain initiation and elongation.
- term:
    id: GO:0006633
    label: fatty acid biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: >-
      InterPro-based (IEA) over-propagation from the KAS/FAS signature. ActI-ORF1 is
      part of actinorhodin (aromatic polyketide antibiotic) biosynthesis, not fatty
      acid biosynthesis.
    action: MODIFY
    reason: >-
      Wrong biological process. Replace with the specific, accurate term GO:1901112
      (actinorhodin biosynthetic process).
    proposed_replacement_terms:
    - id: GO:1901112
      label: actinorhodin biosynthetic process
    supported_by:
    - reference_id: PMID:15286722
      supporting_text: >-
        The synthesis of aromatic polyketides, such as actinorhodin, tetracycline and
        doxorubicin, begins with the formation of a polyketide chain.
- term:
    id: GO:0016746
    label: acyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based (IEA) general acyltransferase activity. This broad parent is a
      true (if non-specific) description of the ketosynthase condensation reaction.
    action: ACCEPT
    reason: >-
      Correct general molecular function (true parent of polyketide synthase
      activity); retained as accurate but non-specific.
    supported_by:
    - reference_id: PMID:15286722
      supporting_text: >-
        KS must catalyze both chain initiation and elongation
- term:
    id: GO:0030497
    label: fatty acid elongation
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: >-
      Orthology-based (IEA) fatty acid elongation. This is a fatty-acid-specific
      process term; ActI-ORF1 elongates a POLYKETIDE chain, not a fatty acid, and the
      polyketide chain-extension activity is already captured by the actinorhodin
      biosynthetic process and polyketide synthase activity assignments.
    action: REMOVE
    reason: >-
      Wrong-specific fatty-acid term that mischaracterizes polyketide chain extension;
      redundant once the actinorhodin/polyketide terms are applied.
    supported_by:
    - reference_id: PMID:15286722
      supporting_text: >-
        In type II polyketide synthases (PKSs), chains are polymerized by the
        heterodimeric ketosynthase-chain length factor (KS-CLF).
core_functions:
- description: >-
    Ketosynthase (KSalpha) subunit of the actinorhodin type II PKS; catalyzes the
    iterative decarboxylative condensation that builds the poly-beta-keto chain (KS
    must catalyze both chain initiation and elongation), thereby initiating actinorhodin
    biosynthesis. Acts as the KS subunit of the heterodimeric KS-CLF (type II PKS)
    complex with ActI-ORF2 (PMID:15286722; PDB 1TQY).
  molecular_function:
    id: GO:0016218
    label: polyketide synthase activity
  directly_involved_in:
    - id: GO:1901112
      label: actinorhodin biosynthetic process
  in_complex:
    id: GO:0034082
    label: type II polyketide synthase complex
  supported_by:
  - reference_id: PMID:15286722
    supporting_text: >-
      Although CLF regulates chain length, it does not have an active site; KS must
      catalyze both chain initiation and elongation.
suggested_questions:
- question: >-
    The minimal PKS additionally requires the holo-ACP (ActI-ORF3) for chain
    assembly; should the type II PKS complex annotation be extended to include the ACP
    subunit?
suggested_experiments:
- description: >-
    Active-site cysteine mutagenesis of ActI-ORF1 with in vitro reconstitution of the
    KS-CLF + ACP to confirm that catalysis (initiation and elongation) resides solely
    in the KS subunit.