actI-ORF2 / KSβ / CLF (Q02062, SCO5088) — review notes

Part of the BGC exemplar curation project (projects/BGC.md). MIBiG BGC0000194
(Streptomyces coelicolor A3(2), actinorhodin type II PKS). GenBank CAC44201.1 →
UniProt Q02062 (ACTI2_STRCO), gene actI-ORF2 / SCO5088.

Function

ActI-ORF2 is the chain-length factor (CLF, KSβ) of the actinorhodin minimal
type II PKS. It is the non-catalytic partner of the ketosynthase (ActI-ORF1,
KSα): together they form the heterodimeric KS-CLF that polymerizes the actinorhodin
polyketide chain, with CLF determining the chain length.

• "Although CLF regulates chain length, it does not have an active site; KS must
  catalyze both chain initiation and elongation." PMID:15286722
• The polyketide is elongated "inside an amphipathic tunnel approximately 17 Å in
  length at the heterodimer interface"; the first cyclization occurs within the
  KS-CLF tunnel. PMID:15286722
• KSβ/CLF descends from a ketosynthase but lacks the active-site cysteine, so it is
  catalytically dead — directly analogous to PqsB in PqsBC (see genes/PSEAE/pqsB).

Annotation issues

• GO:0016746 acyltransferase activity (IEA) and GO:0016747 acyltransferase activity,
  transferring groups other than amino-acyl groups (IEA) → both
  MARK_AS_OVER_ANNOTATED: these attribute catalytic MF to a subunit with no
  active site. CLF contributes_to the KS-CLF activity / should be annotated to the
  complex, not as enables.

Add (NEW, via core_functions): GO:0034082 type II polyketide synthase complex
(CC) and GO:1901112 actinorhodin biosynthetic process (BP) — currently missing
from the GOA despite being well established.

Predicted-complex evidence (BGC project)

Moriwaki et al. (bioRxiv 2025.10.26.684697) predict the KSα–KSβ/CLF heterodimer
(BGC0000194; CAC44200.1/CAC44201.1) at ipTM 0.96, matching PDB 1TQY.

References

• PMID:15286722 — Keatinge-Clay et al. 2004, Nat Struct Mol Biol (act KS-CLF
  structure, PDB 1TQY). VERIFIED.