BST1 (Chlamydomonas reinhardtii) review notes

UniProt A0A2K3CTN0 (TrEMBL, "Uncharacterized protein"); locus Cre16.g662600 / CHLRE_16g662600v5.
Paralogs (KEGG conv/uniprot mapping, 2026-10-03; UniProt REST returned 503 at the time):
BST2 = Cre16.g663400 -> A0A2K3CTQ2; BST3 (LCI11) = Cre16.g663450 -> A0A2K3CTP3.

Deep research

scripts/deep_research_wrapper.py CHLRE BST1 falcon --fallback perplexity-lite was run once
(2026-10-03): falcon returned HTTP 402 Payment Required, and the perplexity-lite fallback
failed because the perplexity provider is not available. No deep-research file exists; the
review is built from the primary literature below.

Identity and family

• Bestrophin / YneE / VCCN family (InterPro IPR044669, Pfam PF25539 Bestrophin_2,
  PANTHER PTHR33281:SF19), per UniProt record. Closest land-plant homolog is Arabidopsis
  VCCN1 (~30% identity) PMID:31391312
• Three paralogs clustered on chr 16 PMID:31391312.
• Predicted chloroplast transit peptide PMID:31391312.
• Homology model: pentameric bestrophin, anion-selective pore PMID:31391312.

Expression

• Low-CO2 induced, CIA5-dependent PMID:31391312; BST1 lowest of the three
  PMID:31391312.

Localization

• Thylakoid membranes, enriched at pyrenoid periphery (Venus fusion, PSAD promoter)
  PMID:31391312. The UniProt "Cell membrane" (ARBA) location and resulting GO plasma membrane
  IEA are wrong for this chloroplast paralog.
• Distinct from BST4/RBMP1, which sits in pyrenoid tubules PMID:39240724.

Interactions

• Interactome: BST1 interacts with LCIC and with BST3/LCI11 and BST2 PMID:28938113.

Phenotypes

• bst3 single knockout: weak phenotype -> redundancy PMID:31391312.
• Triple RNAi (bsti): poor growth at low CO2, higher K1/2(Ci), less 14Ci accumulation
  PMID:31391312. There is no BST1-specific mutant.
• pmf: knockdown gives a slight decrease, opposite to Arabidopsis vccn1 PMID:31391312.

Transport activity: status of evidence (key question for module)

• Bicarbonate delivery to lumenal CAH3 is a proposal PMID:31391312.
• I searched PubMed (2026-10-03; queries: bestrophin AND chlamydomonas; BST4 AND pyrenoid;
  bestrophin AND thylakoid; thylakoid AND bicarbonate AND channel/transporter; BST1/BST3 AND
  Chlamydomonas). I found no electrophysiology, reconstitution or heterologous transport
  assay for BST1, BST2 or BST3.
• The only Chlamydomonas thylakoid bestrophin tested electrophysiologically is BST4, which
  gave no currents in Xenopus oocytes. The authors state that HCO3- channel function is
  unsupported by data PMID:39240724.
• Diatom PtBST1 knockout nearly abolishes the inducible CCM PMID:38478576, which is
  consistent with the model but is also genetic evidence only.
• Conclusion: anion channel activity (GO:0005253) is a sound family-level inference. Bicarbonate
  transmembrane transporter / bicarbonate channel activity (GO:0015106 / GO:0160133) is a
  plausible but untested hypothesis. The IBA voltage-gated chloride channel (VCCN1-derived)
  over-specifies.

Annotation decisions (summary)

• GO:0005247 voltage-gated chloride channel activity (IBA): MODIFY -> GO:0005253
• GO:0005886 plasma membrane (IEA): REMOVE
• GO:0019684 photosynthesis, light reaction (IBA): MARK_AS_OVER_ANNOTATED
• GO:0042651 thylakoid membrane (IBA): ACCEPT
• GO:1902476 chloride transmembrane transport (IEA): MODIFY -> GO:0098656
• NEW GO:0009535 chloroplast thylakoid membrane (IDA, PMID:31391312)