MS4A1 (CD20) review notes

Setup / provenance

• just fetch-gene human MS4A1 run beforehand (UniProt P11836, GOA tsv, stub review).
• just fetch-gene-pmids human MS4A1 cached all 11 GOA PMIDs. Most are abstract-only
  (12920111, 18474602, 20458337, 2448768, 32792392, 3925015, 7684739); full text is
  available for 22664934, 23874341, 25416956, 32296183.
• Additional papers cached with ai-gene-review fetch-pmid: PMID:20038800 (human CD20
  deficiency), PMID:14688067 (mouse CD20 knockout), PMID:32079680 (CD20-rituximab
  cryo-EM), PMID:22615937 (CD20 palmitoylation; full text).
• Deep research: first just deep-research-falcon human MS4A1 attempt failed with
  Edison API 429 Too Many Requests; a retry was launched (see end of file for outcome).

Identity and structure

• B-lymphocyte antigen CD20, founding member of the MS4A (membrane-spanning 4-domains
  subfamily A) family; four transmembrane segments, N and C termini cytoplasmic
  PMID:7684739.
• Non-glycosylated four-pass protein with a large extracellular loop between TM3 and TM4;
  S-palmitoylated in B cells PMID:22615937.
• Cryo-EM: CD20 is a compact double-barrel dimer PMID:32079680.
  Earlier co-IP work proposed tetramers PMID:18474602. The dimer seen by cryo-EM may be the
  unit of higher-order assemblies; tetramer claim remains biochemical.

Expression

• B-cell restricted, from pre-B through mature B cells, lost on plasma cells
  PMID:3925015;
  mouse CD20 likewise B-cell restricted PMID:14688067.

Localization

• Plasma membrane, constitutively in membrane rafts PMID:12920111; GFP-CD20 at plasma membrane of BJAB cells PMID:23874341.
• Found in B-cell exosomes, co-IP with MHC II PMID:20458337.
• Tear-fluid proteomics (PMID:22664934) gives the "extracellular region" HDA; CD20 is
  an integral membrane protein with no signal sequence PMID:2448768, so presence in
  tears likely reflects shed cells/vesicles -> over-annotation.

Function: BCR-coupled calcium entry

• Ectopic CD20 generates a Ca2+ conductance PMID:7684739.
• CD20 is a component of a store-operated Ca entry pathway activated by the BCR; siRNA
  knockdown reduces BCR-stimulated influx PMID:12920111.
• Physically associates with IgM heavy/light chains of BCR; dissociates on BCR engagement,
  with recruitment of phosphoproteins and calmodulin-binding proteins
  PMID:18474602.
• Mouse knockout: reduced CD19-induced Ca2+ responses, otherwise largely normal B cells
  PMID:14688067.
• Whether CD20 is itself a channel pore or a regulator/organizer of an SOC channel
  (e.g., ORAI/STIM-based) is unresolved -> no channel MF asserted; knowledge gap.

Function: humoral immunity

• Human CD20 deficiency (CVID5): normal B-cell development, impaired T-independent antibody
  responses PMID:20038800.
• Mouse KO: T-dependent responses normal, B cell proliferation normal
  PMID:14688067.
  -> B cell proliferation / differentiation annotations (based on anti-CD20 mAb
  perturbation, PMID:3925015) are not core; mAb crosslinking effects are not
  loss-of-function evidence.

Interactions

• 30 IPI "protein binding" rows from HuRI/HI-II-14 Y2H (PMID:32296183, PMID:25416956);
  partners largely ER/membrane proteins (REEP2/4, TMX2, SLC10A1, HSD17B11...). These are
  uninformative for function; removed per project policy on GO:0005515.
• EGFR binding IEA (from rat Ensembl Compara) has no supporting evidence for B-cell CD20.

Therapeutics (context, not GO)

• Target of rituximab, ofatumumab, obinutuzumab; type I vs II mAb mechanisms
  PMID:32792392.

Deep research outcome

• Falcon attempt 1: Edison API 429 Too Many Requests.
• Falcon attempt 2: Provider falcon timed out after 600s / "All providers failed".
• No deep-research file was produced at review time (update: the falcon run in fact
  completed late and MS4A1-deep-research-falcon.md is now present; see reconciliation below); the review relies on the cached GOA publications,
  the UniProt record, and additional PubMed papers listed above (PMID:20038800,
  PMID:14688067, PMID:32079680, PMID:22615937).

Reconciliation with late falcon deep research (2026-10-05)

MS4A1-deep-research-falcon.md arrived after the review was drafted. Comparison:

• Consistent: B-cell-restricted four-pass plasma-membrane MS4A protein; cryo-EM dimer;
  CD20 deficiency (CVID5) with impaired T-independent antibody responses; no established
  channel/enzyme/ligand activity; calcium-channel pore status unresolved (matches the
  existing knowledge gap and the decision not to assert a channel MF).
• Material addition, verified and acted on: CD20 as a "gatekeeper" of the resting
  state. CRISPR ablation in Ramos B cells (with rescue by re-expression) and rituximab
  treatment of naive human B cells cause IgM-BCR/CD19 relocalization and transient
  activation PMID:33563755.
  PubMed-verified, full text cached via just fetch-pmid 33563755. Changes: one sentence
  added to description; PMID:33563755 added to references and to the GO:0050853 (B cell
  receptor signaling pathway) review support, with the reason noting the restraining role.
  Action unchanged (ACCEPT).
• Not acted on (unverifiable here): (i) that the Rouge 2020 structure (PMID:32079680)
  shows no plausible ion-permeation pathway; (ii) that CD20-deficient patient B-cell lines
  retain BCR-induced Ca2+ flux (Kuijpers 2010, PMID:20038800). Both cached papers are
  abstract-only and the abstracts do not state these points. If confirmed they would weaken
  the "required for SOC entry" framing of GO:0002115 / GO:1902656 (currently ACCEPT, based on
  siRNA and mouse KO data); flagged for future follow-up rather than changed.
• Noted only: CD20 on a subset (~3-5%) of human T cells (von Essen 2024); mouse
  olfactory CD20 (not transferable to human); therapeutic/clinical material.
• Deep-research file added to references as file:human/MS4A1/MS4A1-deep-research-falcon.md.