RBMP1 / BST4 (Cre06.g261750; UniProt A0A2K3DMS8) notes

Identity

• RBMP1 (Rubisco-binding membrane protein 1) and BST4 (bestrophin-like protein 4)
  are the same gene, Cre06.g261750. PMID:39240724
• The name RBMP1 was coined by Meyer et al. 2020. PMID:33177094
• UniProt A0A2K3DMS8 is an unreviewed TrEMBL entry ("Uncharacterized protein", 668 aa)
  with InterPro IPR044669 (YneE/VCCN1/2-like), Pfam PF25539 (Bestrophin_2) and PANTHER
  PTHR33281 (UPF0187 protein YneE). The UniProt "Cell membrane" location is an ARBA rule,
  not an observation. Disordered C-terminal regions 437-503 and 546-668 (MobiDB-lite).
• Naming hazard for the CCM project: projects/CO2_CONCENTRATING_MECHANISMS.md lists
  RBMP1 and BST4 as two separate genes, and modules/pyrenoid_ccm.yaml cites the BST4
  preprint (PMID:38014171) as describing a separate candidate tether. They are one gene.

Domain architecture

• N-terminal bestrophin channel domain (transmembrane) plus a long disordered C-terminus with
  two Rubisco-binding motifs (RBMs). PMID:39240724
• Diverged from BST1-3: phenylalanine at the first position of the putative selectivity pore,
  and a separate clade. PMID:39240724
• Meyer 2020 also called it a predicted bestrophin-family anion channel. PMID:33177094

Localization

• Pyrenoid tubules in Chlamydomonas (fluorescent tagging, two independent groups).
  PMID:33177094
  PMID:39240724
• Expansion microscopy places BST4 in a tubule subdomain at an intermediate radius, distinct
  from MITH1 and CAH3. PMID:42465278 (bioRxiv preprint)
• In Arabidopsis expression, BST4 sits in stroma lamellae thylakoids with the C-terminus facing
  the stroma (protease protection). PMID:39240724
• Tubule targeting depends on the RBM-bearing C-terminus. PMID:39240724

Rubisco binding

• Original evidence: AP-MS (cited by Meyer as ref 18, the spatial interactome; the cached text of
  PMID:28938113 does not name Cre06.g261750, so the gene-level hit is in supplementary tables),
  plus the RBM itself. PMID:33177094
• Y2H: the BST4 C-terminus interacts with RBCS1. WR-to-EE mutation of the RBMs abolishes the
  interaction. FRET shows BST4 and RBCS1 are close in vivo. PMID:39240724

Tether hypothesis: tested and not supported

• Meyer 2020 model: RBMs on RBMP1/RBMP2 recruit Rubisco to the tubules. PMID:33177094
• Adler et al. 2024 (peer-reviewed version of the 2023 preprint PMID:38014171):
• bst4 pyrenoids are normal, with tubules. PMID:39240724
• BST4 cannot pull thylakoids into an EPYC1-Rubisco condensate in Arabidopsis.
• Conclusion: "BST4 is a pentameric transmembrane channel found within the pyrenoid tubules but is not crucial for Rubisco matrix tethering."
• Possible redundancy with other RBM tubule proteins (RBMP2) is not excluded, but there is no
  positive evidence that RBMP1 tethers anything. So molecular adaptor activity (GO:0060090) is
  NOT justified. The Rubisco interaction looks like a targeting mechanism that holds the channel
  in the tubules.

Channel and physiology

• Pentameric assembly by Slimfield single-molecule counting, an AlphaFold pentamer, and a
  ~1 MDa BN-PAGE complex. PMID:39240724
• Xenopus oocytes gave no currents with KCl, NaHCO3, K-PEP or K-gluconate, including for
  C-terminal truncations. Permeant unknown. PMID:39240724
• bst4: no growth defect at air CO2 under continuous light, but a growth defect under
  fluctuating high light. NPQ (qE) is transiently higher during dark-to-light transitions,
  which suggests a lower lumenal pH. PMID:39240724
• Authors' model: BST4 is a tubule ion channel that contributes to lumen ion homeostasis/pH,
  possibly as an HCO3- channel. PMID:39240724

GOA assessment summary

• IBA voltage-gated chloride channel activity (from AtVCCN1): the family is right, but chloride
  and voltage gating are not shown, and no Cl- current was detected. Generalize to GO:0005216.
• IEA chloride transmembrane transport: comes from the IBA MF. Chloride is not shown, so mark
  it as over-annotated.
• IBA photosynthesis, light reaction: consistent with the bst4 NPQ and fluctuating-light
  phenotypes. Accept.
• IBA thylakoid membrane: consistent (the tubules are thylakoid-derived). Accept, and add pyrenoid
  tubule (GO:0160223) as NEW.
• IEA plasma membrane (ARBA): contradicted by every localization study. Remove.

Deep research

• deep_research_wrapper.py CHLRE RBMP1 falcon --fallback perplexity-lite was run once
  (2026-10-03). Falcon returned HTTP 402 Payment Required. The perplexity-lite fallback failed
  with "Provider 'perplexity' not available". It was not retried, and the review is based on
  the cached primary literature above.