CD320 (transcobalamin receptor, formerly 8D6 antigen) is a single-pass type I cell-surface glycoprotein that captures transcobalamin carrying vitamin B12 and mediates uptake of this complex by endocytosis. Its extracellular region contains two calcium-coordinating LDL receptor class A domains that contact transcobalamin, separated by an EGF-like region. Calcium stabilizes the receptor-ligand complex, and acidic pH reduces ligand affinity in vitro. CD320 has high selectivity for transcobalamin over the related circulating carrier haptocorrin. Its expression is associated with proliferation, with higher levels in actively dividing cells than in quiescent cells. It thereby supplies cells with cobalamin for subsequent intracellular metabolism. Human variants can impair this uptake and cause transcobalamin receptor deficiency with methylmalonic aciduria. CD320 also supports B-cell proliferation in follicular dendritic cell coculture experiments; this context-dependent activity is distinct from its broadly established nutrient uptake role.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | ACCEPT | Summary: Plasma-membrane localization is a core feature of CD320. Reason: The PAINT inference agrees with direct human receptor purification and cell-surface studies. The target in the donor list is expected when its own experimental evidence helped place the ancestral assertion; it is not circular support. The phylogenetic node has not been independently reconstructed here. |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: The subcellular-location mapping correctly places CD320 at the plasma membrane. Reason: The reviewed UniProt cell-membrane assignment and direct human receptor studies independently support this electronic mapping. |
| GO:0016020 membrane | IEA GO_REF:0000117 | MODIFY | Summary: The membrane prediction is correct but can be specified as plasma membrane. Reason: CD320 has a membrane-spanning segment and is directly demonstrated at the cell surface. This refinement uses target evidence rather than treating the ARBA rule as an independent experiment. Proposed replacements: plasma membrane |
| GO:0016192 vesicle-mediated transport | IEA GO_REF:0000117 | MODIFY | Summary: The supported transport mechanism is receptor-mediated endocytosis. Reason: CD320 captures extracellular transcobalamin-cobalamin and promotes its internalization; this supplies a specific mechanism for the broad electronic vesicle-transport annotation. It is a cargo receptor, not a vesicle motor or the enzyme that subsequently processes cobalamin. Proposed replacements: receptor-mediated endocytosis |
| GO:0005515 protein binding | IPI PMID:27411955 Structural basis of transcobalamin recognition by human CD32... | MODIFY | Summary: TCN2 binding is the ligand-recognition step of cargo receptor activity. Reason: The partner-resolved interaction and human ectodomain structure identify transcobalamin as the ligand. The receptor uptake experiments establish a functional refinement beyond generic protein binding. The structural experiment alone is not an endocytosis assay. Proposed replacements: cargo receptor activity |
| GO:0005515 protein binding | IPI PMID:35922511 A physical wiring diagram for the human immune system. | KEEP AS NON CORE | Summary: Retain the curated CD320-JAML extracellular association with a bounded interpretation. Reason: The GOA partner is JAML/Q86YT9, also recorded by the immutable UniProt interaction record. The source uses recombinant extracellular interaction screening and cellular phenotyping. Its target-specific supplementary interaction record was not independently inspected; defer to the interaction curators and do not turn this association into a new signaling or adhesion function. Soluble-CD320 effects in mixed leukocytes do not establish a JAML-specific physiological pathway. The quoted Fig.1a caption documents the assay format, not independent verification of the uninspected JAML pair. The exact positive partner attribution is retained from GOA and reviewed UniProt. Supporting Evidence: PMID:35922511 screening for protein binding interactions between recombinant extracellular domains file:human/CD320/CD320-uniprot.txt |
| GO:0015889 cobalamin transport | TAS Reactome:R-HSA-9758890 | ACCEPT | Summary: CD320 participates directly in cobalamin transport as the uptake receptor. Reason: The Reactome transport pathway includes delivery of circulating transcobalamin-cobalamin to cells. Receptor capture and internalization do part of this transport, unlike the later metabolic reactions that merely depend on vitamin supply. |
| GO:0005886 plasma membrane | EXP PMID:11418631 The distinct roles of T cell-derived cytokines and a novel f... | ACCEPT | Summary: Retain experimentally curated cell-surface localization in the FDC/8D6 study. Reason: The cached abstract identifies 8D6-expressing COS cells and antibody-sensitive effects on B cells. The full localization experiment is not in that abstract; the original cloning study and receptor purification independently corroborate the location, so retain the curator assignment without claiming to have reread the complete 2001 experiment. |
| GO:0038024 cargo receptor activity | EXP PMID:15652495 The binding properties of the human receptor for the cellula... | ACCEPT | Summary: Cargo receptor activity captures the principal biochemical role. Reason: The 2005 study characterizes a human placental TC-cobalamin receptor before its definitive gene identification; its abstract explicitly distinguishes the functional 58-kDa candidate from a competing receptor preparation. The later 2009 purification identifies that receptor as CD320 and connects binding with cellular uptake. |
| GO:0005509 calcium ion binding | IDA PMID:27411955 Structural basis of transcobalamin recognition by human CD32... | ACCEPT | Summary: CD320 directly coordinates calcium in both LDL receptor class A domains. Reason: The human ectodomain complex structure resolves calcium coordination by acidic residues and backbone carbonyls; chelation reduces complex stability. This is direct structural binding evidence supporting receptor architecture, not evidence for a calcium channel or calcium signaling activity. Supporting Evidence: PMID:27411955 central Ca2+ ions bound by four conserved acidic residues and two backbone carbonyls |
| GO:0005886 plasma membrane | IDA PMID:10727470 Identification of a human follicular dendritic cell molecule... | ACCEPT | Summary: The original 8D6 cloning study supports plasma-membrane localization. Reason: Antibody staining of human FDC/HK cells and expression cloning in COS cells identify the surface molecule. Intracellular staining in the same study does not negate its established cell-surface location. Supporting Evidence: PMID:10727470 expressed on the HK cell membrane surface |
| GO:0015889 cobalamin transport | IMP PMID:18779389 The protein and the gene encoding the receptor for the cellu... | ACCEPT | Summary: Human CD320 mediates uptake of transcobalamin-bound cobalamin. Reason: The 2009 study combines placental receptor purification and ligand binding with inhibition by receptor antiserum and increased uptake after CD320 transfection in HEK293 cells. Its heterozygous mouse ES-cell experiment is supporting evidence, not mislabeled as the human experiment. Supporting Evidence: |
| GO:0030890 positive regulation of B cell proliferation | IMP PMID:10727470 Identification of a human follicular dendritic cell molecule... | KEEP AS NON CORE | Summary: Retain experimentally supported enhancement of B-cell proliferation in the FDC coculture setting. Reason: Human FDC/HK cocultures and 8D6-transfected COS monolayers support B-cell growth under CD40 and cytokine stimulation; blocking antibody delays cell-cycle progression. This establishes a context-specific effect, not a universal growth-factor role or proof that nutrient uptake is its sole mechanism. Supporting Evidence: PMID:10727470 8D6 Ag cDNA stimulate B cell growth. |
| GO:0031296 B cell costimulation | IMP PMID:10727470 Identification of a human follicular dendritic cell molecule... | KEEP AS NON CORE | Summary: Retain the demonstrated B-cell costimulation phenotype. Reason: The original human B-cell coculture study tests costimulation alongside CD40 and cytokine signals with transfected cells and antibody controls. The receptor also has a well-established nutrient uptake function; neither role invalidates the other, and a direct downstream signaling mechanism was not established by these experiments. Supporting Evidence: PMID:10727470 blocks the costimulatory function completely. |
| GO:0015889 cobalamin transport | IMP PMID:20524213 Positive newborn screen for methylmalonic aciduria identifie... | ACCEPT | Summary: Human cellular loss and rescue experiments support cobalamin uptake. Reason: Patient fibroblasts show reduced holo-transcobalamin uptake, and restoring the deleted codon in the patient cDNA rescues uptake in transfected cells. The later purified ectodomain study found near-wild-type ligand affinity for the deletion variant; preserve the cellular uptake conclusion without asserting that complete loss of ligand binding explains it. Supporting Evidence: PMID:20524213 Inserting the codon by site-directed mutagenesis fully restored TCblR function. |
| GO:0016020 membrane | HDA PMID:19946888 Defining the membrane proteome of NK cells. | KEEP AS NON CORE | Summary: Retain the broad membrane proteomics observation without assigning more precision to that experiment. Reason: The cached abstract describes isolated membranes from the human NK-like YTS cell line and mass spectrometric identification. Its CD320-specific table is unavailable here. Membrane association is biologically credible and curator supported, but this fractionation result alone does not distinguish plasma membrane from other membranes; separate target experiments already establish the core location. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-3325546 | ACCEPT | Summary: The defective-uptake Reactome model places the receptor at the plasma membrane. Reason: This is the location of receptor-mediated capture before endosomal delivery in the disease reaction; it does not mean a defective receptor performs normal uptake. The normal receptor location is independently supported by human experiments. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-3000112 | ACCEPT | Summary: CD320 starts ligand uptake at the plasma membrane. Reason: The Reactome reaction describes internalization and subsequent receptor-ligand separation. Its recycling account supports pathway context, but the 2016 low-pH affinity experiment is an in vitro dissociation assay, not direct tracking of receptor recycling in cells. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-3000122 | ACCEPT | Summary: Extracellular transcobalamin-cobalamin is captured at the plasma membrane. Reason: The CD320 ligand-binding reaction explicitly represents the cell-surface capture step, consistent with the direct human receptor experiments. |
| GO:0005886 plasma membrane | IDA PMID:18779389 The protein and the gene encoding the receptor for the cellu... | ACCEPT | Summary: The human receptor identification study supports the cell-surface location. Reason: Purification from human placental membranes, antibody inhibition of binding and uptake in live cells, and transfection experiments fit the plasma-membrane receptor assignment. Recombinant soluble ectodomain binding does not imply that the normal intact protein is freely secreted. Supporting Evidence: |
| GO:0031419 cobalamin binding | IDA PMID:18779389 The protein and the gene encoding the receptor for the cellu... | MARK AS OVER ANNOTATED | Summary: The inspected experiments establish carrier recognition, not a separate cobalamin-binding molecular function. Reason: The original human receptor experiment tracks radiolabeled cobalamin already carried by transcobalamin. The human ectodomain structure independently resolves receptor contacts on the alpha domain of transcobalamin. GO:0031419 describes binding to cobalamin itself; mapping the carrier-complex uptake readout to this separate small-molecule-binding activity therefore exceeds what these inspected assays establish. The term does not require the vitamin to be free. This is an evidence-to-term scope judgment, not demonstrated inability to bind vitamin, a NOT annotation, or a wrong-gene claim. The structure paper's holo/apo-TC statement is conditional interpretation, not a direct apo-TC binding experiment; haptocorrin selectivity alone does not establish absence of receptor-vitamin contact. |
| GO:0005783 endoplasmic reticulum | IDA GO_REF:0000054 | UNDECIDED | Summary: The original target-specific GFP-fusion localization remains unverified. Reason: GO_REF:0000054 identifies the localization-curation method rather than the particular CD320 construct, image or controls. Those target-specific observations were not obtained. Established plasma-membrane function does not exclude a real ER pool, and neither secretory transit nor a fusion artifact has been demonstrated for this source record. Preserve the original experimental assertion while leaving the source-specific assessment unresolved; the method reference is not offered as independent biological support. The appended question concerns endogenous ER localization versus the reported fusion; the original construct, image and controls are needed for adjudication. ER residence is not promoted to a core function. |
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Q: Under what conditions, if any, does endogenous CD320 reside in the endoplasmic reticulum, and how does this compare with the reported GFP-fusion localization?
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