CYB5D2 (Neuferricin) is a secreted heme-binding protein belonging to the membrane-associated progesterone receptor (MAPR) family and the cytochrome b5 superfamily. The protein contains an N-terminal signal peptide and a conserved cytochrome b5-like heme-binding domain, with Asp86 being critical for heme coordination. CYB5D2 promotes neuronal differentiation (but not astrocyte differentiation) through heme-dependent mechanisms involving MAPK/ERK and PI3K/AKT signaling pathways. It localizes primarily to the endoplasmic reticulum where it co-localizes with cytochrome P450 reductase (CYPOR) and modulates CYP enzyme activities, including CYP51A1 (lanosterol demethylase) and CYP3A4. CYB5D2 functions as a tumor suppressor in multiple cancer types including breast, cervical, and hepatocellular carcinoma, where its downregulation is associated with disease progression and poor prognosis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0012505 endomembrane system | IBA GO_REF:0000033 | ACCEPT | Summary: CYB5D2 localizes to the endoplasmic reticulum where it co-localizes with cytochrome P450 reductase (CYPOR). This IBA annotation is consistent with experimental evidence showing ER localization, which is part of the endomembrane system. Reason: The annotation is supported by experimental evidence from Bruce and Rybak (2014) showing CYB5D2 localizes to the ER. The IBA annotation based on phylogenetic inference is appropriate given the conservation of this localization across MAPR family members. Supporting Evidence: PMID:24466094 Both CYB5D2 and CYB5D2(D86G) localize to the endoplasmic reticulum file:human/CYB5D2/CYB5D2-deep-research-falcon.md CYB5D2 localizes predominantly to the endoplasmic reticulum/perinuclear region file:human/CYB5D2/CYB5D2-deep-research-perplexity.md See deep research file for comprehensive analysis |
| GO:0016020 membrane | IBA GO_REF:0000033 | ACCEPT | Summary: CYB5D2 associates with the ER membrane where it interacts with membrane-bound proteins including CYPOR. Although CYB5D2 lacks a classical transmembrane domain and is secreted, it localizes to membrane compartments. Reason: The membrane annotation is consistent with ER localization evidence. CYB5D2 colocalizes with membrane-bound CYPOR and associates with ER membranes. Supporting Evidence: PMID:24466094 CYB5D2 co-localizes with cytochrome P450 reductase (CYPOR) |
| GO:0005496 steroid binding | IEA GO_REF:0000117 | ACCEPT | Summary: CYB5D2 contains a cytochrome b5-like heme/steroid-binding domain (InterPro IPR001199) that is characteristic of the MAPR family. While the domain architecture suggests potential steroid binding, direct experimental evidence for steroid binding by CYB5D2 specifically is limited. Reason: The annotation is inferred from the conserved heme/steroid-binding domain. MAPR family members are involved in steroid metabolism and signaling. CYB5D2 affects CYP51A1 levels, which is involved in sterol biosynthesis, suggesting functional connections to steroid/sterol metabolism. Supporting Evidence: PMID:24466094 CYB5D2 knockdown reduced lanosterol demethylase (CYP51A1) levels and rendered HeLa cells sensitive to mevalonate PMID:28396637 The four MAPR proteins PGRMC1, PGRMC2, neuferricin and neudesin share a homologous CYB5-like heme/steroid-binding domain |
| GO:0005576 extracellular region | IEA GO_REF:0000120 | ACCEPT | Summary: CYB5D2/Neuferricin is a secreted protein with an N-terminal signal peptide. Secretion requires intact heme-binding capacity. Reason: UniProt annotates CYB5D2 as secreted, consistent with its signal peptide and experimental evidence. The protein functions as both a secreted neurotrophic factor and an intracellular regulator. Supporting Evidence: PMID:28396637 CYB5D2 can be detected as secreted hemoprotein in some cell lines but also colocalized with POR in the ER file:human/CYB5D2/CYB5D2-deep-research-falcon.md Secreted forms have been observed in heterologous expression systems, and secretion requires intact heme-binding capacity |
| GO:0007399 nervous system development | IEA GO_REF:0000043 | ACCEPT | Summary: CYB5D2/Neuferricin promotes neuronal differentiation in neural precursor cells through heme-dependent mechanisms. This IEA annotation from the UniProt Neurogenesis keyword is well-supported by primary literature. Reason: Multiple studies demonstrate CYB5D2's role in promoting neurogenesis and neuronal differentiation. Supporting Evidence: PMID:28396637 neurotrophic activity that required heme binding and was mediated through mitogen-activated protein (MAP) and phosphatidylinositol 3-kinase (PI-3K) pathways file:human/CYB5D2/CYB5D2-deep-research-falcon.md neurotrophic activity that required heme binding and was mediated through mitogen-activated protein (MAP) and phosphatidylinositol 3-kinase (PI-3K) pathways |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MARK AS OVER ANNOTATED | Summary: High-throughput yeast two-hybrid screening (HuRI) identified CYB5D2 interactions. While 'protein binding' is uninformative, the IPI evidence is valid from a large-scale interactome study. Reason: The 'protein binding' term is too general to be informative about CYB5D2's molecular function. The HuRI study (PMID:32296183) detected interactions via Y2H, but this is from a high-throughput screen without functional validation. More specific binding terms would be preferable if the functional context were known. Supporting Evidence: PMID:32296183 Here, we present HuRI, a systematically generated human protein interactome map with more than 50,000 PPIs of high biophysical quality |
| GO:0020037 heme binding | IEA GO_REF:0000107 | ACCEPT | Summary: CYB5D2 binds type b heme through its conserved cytochrome b5-like domain. Asp86 (D86) is the critical residue for heme coordination; the D86G mutation abolishes heme binding. This is a core molecular function of the protein. Reason: Heme binding is the defining biochemical property of CYB5D2 and is essential for all its biological functions including neurogenesis, cell survival, and CYP enzyme modulation. The D86G mutation that abolishes heme binding also eliminates biological activity. Supporting Evidence: PMID:24466094 only the substitution of glycine (G) at D86 (D86G) within its cytochrome b5 heme-binding (cyt-b5) domain abolished its heme-binding ability PMID:24466094 CYB5D2 binds to type b heme |
| GO:0045666 positive regulation of neuron differentiation | IEA GO_REF:0000107 | ACCEPT | Summary: CYB5D2/Neuferricin selectively promotes neuronal differentiation over astrocyte differentiation in neural precursor cells. This activity requires intact heme-binding capacity and involves activation of MAPK/ERK and PI3K/AKT signaling pathways. Reason: This annotation accurately captures a core biological function of CYB5D2. Multiple studies demonstrate its neurotrophic activity promoting neurogenesis through specific signaling pathway activation. Supporting Evidence: PMID:28396637 neurotrophic activity that required heme binding and was mediated through mitogen-activated protein (MAP) and phosphatidylinositol 3-kinase (PI-3K) pathways file:human/CYB5D2/CYB5D2-deep-research-falcon.md neurotrophic activity that required heme binding and was mediated through mitogen-activated protein (MAP) and phosphatidylinositol 3-kinase (PI-3K) pathways |
| GO:0005783 endoplasmic reticulum | IDA PMID:24466094 CYB5D2 requires heme-binding to regulate HeLa cell growth an... | NEW | Summary: CYB5D2 localizes to the endoplasmic reticulum where it colocalizes with CYPOR and modulates CYP enzyme activities. Both wild-type and heme-binding defective mutant localize to the ER, indicating localization is independent of heme binding. Reason: This is a more specific cellular component term than endomembrane system. Bruce and Rybak (2014) explicitly show ER localization by immunofluorescence and colocalization with ER markers. Supporting Evidence: PMID:24466094 Both CYB5D2 and CYB5D2(D86G) localize to the endoplasmic reticulum PMID:24466094 CYB5D2 co-localizes with cytochrome P450 reductase (CYPOR) |
| GO:0005102 signaling receptor binding | NAS | NEW | Summary: Added to align core_functions with existing annotations. Reason: Core function term not present in existing_annotations. Supporting Evidence: PMID:28396637 neurotrophic activity that required heme binding and was mediated through mitogen-activated protein (MAP) and phosphatidylinositol 3-kinase (PI-3K) pathways |
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Download this section (compressed HTML)Q: What is the receptor or binding partner through which secreted CYB5D2 exerts its neurotrophic effects?
Q: Does CYB5D2 directly interact with cytochrome P450 enzymes or is its effect on CYP activity indirect?
Q: How does CYB5D2 regulate E-cadherin expression to inhibit epithelial-mesenchymal transition?
Experiment: Identify the cell surface receptor mediating CYB5D2 neurotrophic signaling using binding assays
Hypothesis: CYB5D2 binds a specific cell surface receptor to activate ERK/AKT signaling
Experiment: Perform co-immunoprecipitation to validate direct interactions with CYP enzymes
Hypothesis: CYB5D2 physically interacts with CYP51A1 to modulate its stability or activity
Experiment: Generate CYB5D2 knockout mice to study in vivo roles in neurogenesis
Hypothesis: CYB5D2 deficiency impairs neuronal differentiation during brain development
Experiment: Determine crystal structure of CYB5D2 with bound heme to understand heme coordination
Hypothesis: D86 coordinates heme iron through a carboxylate-iron interaction distinct from classical cytochrome b5
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