CD320 antigen = transcobalamin receptor (TCblR), aka 8D6 antigen / FDC-signaling molecule 8D6.
Single-pass type I plasma-membrane glycoprotein (282 aa; signal 1-35, extracellular 36-229,
TM 230-250, cytoplasmic 251-282). Member of the LDLR family: two LDLR class A (LDLR-A) domains
(53-90, 131-168) separated by an EGF/complement-like cysteine-rich region. Non-enzymatic.
The original journal is preserved verbatim above. The reassessment below supersedes the earlier cobalamin-binding core claim, the unsupported ER-artifact explanation, and mechanistic interpretations linking the immune phenotypes or JAML association to a particular pathway. The current evidence and remaining uncertainties are recorded here.
This review reassesses the existing human CD320 review for the ClinGen Mendelian campaign. The frozen inventory identifies HGNC:16692, reviewed UniProt Q9NPF0 and the autosomal recessive, Definitive association with transcobalamin receptor defect (MONDO:0013341). It does not authorize a new clinical severity classification. The 282-residue protein and two recorded UniProt products are preserved. No isoform-specific activity is inferred for Q9NPF0-2.
The immutable GOA contains 23 rows representing 22 distinct assertions. The duplicate TCN2-binding assertion differs only in date and assigning source. Normal seed-goa --no-fetch-titles restored missing supporting-entity lists on six annotations in temporary storage, adding no annotations. All source IDs, evidence codes, qualifiers and ordered partners remain intact. Existing sources and the cobalamin module are protected; this review does not edit the module or generated source caches.
The normal deep-research recipe was attempted with falcon and the perplexity-lite fallback. Both stopped at dependency resolution because the pinned deep-research-client 0.2.7rc1 was unavailable in the offline environment. No provider report was produced or authored manually. The research here uses existing publications and selected original online text, with access limits below.
PMID:18779389(https://pubmed.ncbi.nlm.nih.gov/18779389/) identifies the human placental receptor by peptide sequencing and connects it to CD320. The locally cached abstract is supplemented by selected original Methods and Results: binding assays use radiolabeled cobalamin already carried by transcobalamin; receptor antibodies inhibit binding and uptake; human HEK293 transfection increases uptake. A mouse ES-cell allele experiment is separately supporting evidence. The entire online paper and figure pixels were not inspected; a later PMC request encountered a browser challenge.
PMID:15652495(https://pubmed.ncbi.nlm.nih.gov/15652495/) is the earlier receptor-characterization study. Its abstract is available, but the full 2005 methods were not accessed. It precedes definitive gene identification and distinguishes a functional 58-kDa receptor candidate from a competing preparation. The 2009 paper supplies the gene identification that makes the older functional evidence interpretable.
PMID:27411955(https://pubmed.ncbi.nlm.nih.gov/27411955/) provides the human TC-CD320 ectodomain structure, calcium coordination and ligand-affinity measurements. Selected cached Results, Discussion, expression/purification and binding methods were inspected. Human constructs were expressed in insect cells; these are not intact-cell transport measurements. Receptor contacts lie on transcobalamin, whereas the vitamin is carried within transcobalamin. Acidic pH lowers affinity in vitro; this is consistent with ligand release during uptake but does not directly demonstrate receptor recycling in cells.
The existing experimental cobalamin-binding assertion is retained as UNDECIDED. The inspected assays establish binding of the carrier-vitamin complex; direct receptor-vitamin contact and the intended ontology scope remain unresolved. Official QuickGO/OLS term API access failed. The local ontology cache confirms the term label but lacks its definition. No assertion of an experimental error or demonstrated absence of free-vitamin binding is made. A separate cobalamin-binding core entry would overstate what was established, so the synthesis has one cargo-receptor core function. Calcium coordination is a well-supported receptor structural feature, not a separate calcium-signaling function.
PMID:20524213(https://pubmed.ncbi.nlm.nih.gov/20524213/) reports reduced holo-transcobalamin uptake in patient fibroblasts and rescue after restoring the deleted codon in the patient cDNA. Selected original Methods and Results were read. The early patient series does not justify a broad prediction of lifelong clinical severity. The later purified deletion-mutant ectodomain retained high ligand affinity (PMID:27411955), so cellular uptake impairment cannot simply be equated with complete failure of carrier binding. Regulation, trafficking and assay context remain mechanistic questions.
PMID:10727470(https://pubmed.ncbi.nlm.nih.gov/10727470/) establishes the original 8D6/CD320 molecule using human FDC/HK cells, antibody staining and expression cloning. The cached abstract and selected Methods/Results support B-cell costimulation under CD40 and cytokine stimulation. Antibody controls and transfected COS cocultures strengthen the observation. These two existing process annotations are kept as non-core; neither an intrinsic growth-factor activity nor a resolved downstream mechanism is inferred. PMID:11418631(https://pubmed.ncbi.nlm.nih.gov/11418631/) provides complementary 8D6/B-cell differentiation evidence, but only its cached abstract was available.
PMID:35922511(https://pubmed.ncbi.nlm.nih.gov/35922511/) uses recombinant extracellular-protein screening and mixed-leukocyte phenotyping. The GOA row and immutable UniProt entry identify JAML as the CD320 partner. The target-specific supplementary pair record was not independently inspected; the curated association is retained with that limit. Soluble-CD320 phenotypes do not establish a JAML-specific signaling pathway. The official 2024 correction concerns a figure label, a concentration unit and missing single-cell dataset references, without retracting the interaction study.
PMID:19946888(https://pubmed.ncbi.nlm.nih.gov/19946888/) describes membrane proteomics in the human NK-like YTS line. Its abstract was read; its target-specific table was not. The broad experimental membrane assignment is retained as non-core without converting that fractionation into a more precise experiment. By contrast, the broad ARBA membrane prediction can be refined using independent target-specific evidence. This deliberate distinction accounts for the validator's inconsistent-action advisory.
The LIFEdb GO reference describes microscopic curation of GFP-fusion localization. The CD320-specific image and construct were not obtained. The ER assertion therefore remains UNDECIDED. Secretory transit or a fusion artifact must not be treated as demonstrated explanations without inspecting that record.
The four cached Reactome summaries were read. They support carrier capture, uptake and pathway location, without making CD320 an intracellular cobalamin-processing enzyme. The local GO-CAM index search found no CD320/Q9NPF0 entry; no absence-based new annotation is proposed.
All 22 source assertions were reassessed: 13 ACCEPT, three MODIFY, four KEEP_AS_NON_CORE and two UNDECIDED. There are no NEW assertions, 17 reference assessments, two preserved protein products and one core function. The three refinements specify plasma membrane, receptor-mediated endocytosis and cargo receptor activity. The generic JAML interaction remains non-core under the project's standing treatment of supported associations.
Temporary normal validation passes with two advisories: the generic protein-binding association and the deliberate evidence-specific action difference for membrane. DRAFT follows these advisory conditions; it is distinct from the two unresolved biological/source questions. Whole independent scientific review and canonical application remain subsequent steps.
The preceding journal remains an exact historical prefix. This focused follow-up changes only the cobalamin-binding action, adds short source-specific anchors, makes the unresolved ER record actionable, and clarifies the synthesis. All 22 machine assertions, source identifiers, partners, qualifiers and two products remain exact; no NEW annotation is added.
The official cobalamin-binding definition and parents name a vitamin-binding molecular function. They do not say the vitamin must be free. The original radiolabel assay measures carrier-bound vitamin, whereas the inspected human complex structure maps CD320 recognition to the alpha domain of transcobalamin. Assigning a separate small-molecule-binding activity from those results goes beyond their resolved molecular interaction. The original assertion is now MARK_AS_OVER_ANNOTATED on that source-to-term basis. This does not claim that free-vitamin binding was disproved, that the original paper is wrong, or that a NOT annotation is warranted. The conditional holo/apo-TC discussion is not a direct apo-TC assay, and the engineered TC-alpha construct is not assumed to reproduce every intact apo-TC property. The receptor remains the entity doing cargo recognition and uptake. PMID:18779389(https://pubmed.ncbi.nlm.nih.gov/18779389/); PMID:27411955(https://pubmed.ncbi.nlm.nih.gov/27411955/).
GO_REF:0000054 records the GFP-localization curation method. It is retained as the original source but removed from the ER review's biological support list, since it cannot supply the uninspected target image. UNDECIDED remains appropriate for that missing target evidence: a receptor's established cell-surface role does not by itself refute an ER observation. No artifact or secretory-transit explanation is asserted. Follow-up requires the original construct, image and controls; the structured question concerns endogenous ER localization versus the reported fusion.
The newly restored exact anchors cover the original 8D6 surface/growth/costimulation observations, the cellular codon-restoration rescue, structural calcium coordination and TC-domain recognition, and the extracellular screening format. The screening-method quotation does not validate the exact uninspected JAML pair; that positive attribution remains curator-supported. Newly authored YAML quotations remain within 25 words per source. The original historical notes already contain older quotations; their unchanged prefix is not represented as newly written quotation or as satisfying a new whole-file word limit.
The single cargo-receptor core now explicitly includes receptor-mediated endocytosis, already proposed as the replacement for the broad vesicle-transport row. This records the receptor's own ligand-capture work alongside its cargo-specific transport role. The description restores carrier selectivity and the proliferation-associated expression pattern. The ligand-capture Reactome assessment is limited to the actual reaction. The official 2024 correction is identified explicitly in the 2022 reference assessment; correction-notice verification is distinct from reading its target-pair supplement.