ERO1B (ERO1-like protein beta, formerly ERO1LB; endoplasmic reticulum oxidoreductin-1 beta) is an ER membrane-associated, FAD-dependent flavoprotein sulfhydryl oxidase (EC 1.8.3.2) and a paralog of ERO1A. It drives oxidative protein folding in the endoplasmic reticulum by reoxidizing the protein disulfide isomerase P4HB/PDI (and, less efficiently, other PDI-family members), regenerating their active-site disulfides so they can catalyze further disulfide-bond formation in secretory proteins; the abstracted electrons are passed via bound FAD to molecular oxygen, producing hydrogen peroxide. It is a peripheral membrane protein on the lumenal side of the ER (and is retained there in part through interaction with ERP44), and forms disulfide-linked homodimers as well as heterodimers with ERO1A. Compared with ERO1A it is intrinsically more active and more loosely regulated, consistent with its enrichment in professional secretory tissues - particularly the pancreatic islets of Langerhans, stomach chief cells and digestive tract - where high oxidative folding capacity is required. It is induced during the unfolded protein response and has been implicated in oxidative proinsulin folding and glucose homeostasis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005789 endoplasmic reticulum membrane | IBA GO_REF:0000033 | ACCEPT | Summary: ERO1B is a peripheral ER membrane protein on the lumenal side; this is its primary site of action, supported experimentally and by phylogenetic inference. Reason: The ER membrane (lumenal side) is the documented site where ERO1B reoxidizes PDI; corroborated by direct subcellular-location evidence. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt SUBCELLULAR LOCATION: Endoplasmic reticulum membrane |
| GO:0005783 endoplasmic reticulum | IEA GO_REF:0000002 | ACCEPT | Summary: ER localization is correct and the principal compartment for ERO1B. Reason: ERO1B is an ER-resident oxidase; directly supported by immunofluorescence and glycosylation evidence. Supporting Evidence: PMID:10818100 the products of the ERO1-Lbeta gene are primarily localized in the ER of mammalian cells |
| GO:0005789 endoplasmic reticulum membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic ER-membrane localization, consistent with experimental and IBA evidence. Reason: Correct compartment; ERO1B is a peripheral ER membrane protein. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Peripheral membrane protein |
| GO:0015035 protein-disulfide reductase activity | IEA GO_REF:0000120 | MARK AS OVER ANNOTATED | Summary: This term asserts a disulfide REDUCTASE activity. ERO1B is mechanistically an OXIDASE that reoxidizes (forms disulfides in) PDI; the reductase label misrepresents the catalytic direction. Reason: ERO1B oxidizes PDI and passes electrons to O2 generating H2O2; the physiological direction is dithiol oxidation, not disulfide reduction, so a reductase annotation is an over-annotation. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Efficiently reoxidizes P4HB/PDI, the enzyme catalyzing protein disulfide formation, in order to allow P4HB to sustain additional rounds of disulfide formation. |
| GO:0016971 flavin-dependent sulfhydryl oxidase activity | IEA GO_REF:0000116 | ACCEPT | Summary: This is the precise core molecular function of ERO1B - a FAD-dependent sulfhydryl oxidase catalyzing dithiol + O2 = disulfide + H2O2 (RHEA:59116). Reason: Matches the catalytic activity and FAD cofactor of ERO1B; supported by EXP evidence (PMID:11707400, PMID:21091435). Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Reaction=[protein]-dithiol + O2 = [protein]-disulfide + H2O2 |
| GO:0016972 thiol oxidase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Thiol oxidase activity is the broader parent of ERO1B's flavin-dependent sulfhydryl oxidase activity; correct but less specific. Reason: Correctly captures ERO1B's oxidase activity; the EXP/IDA versions of the same term confirm it. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt EC=1.8.3.2 |
| GO:0034975 protein folding in endoplasmic reticulum | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: ERO1B drives oxidative protein folding in the ER; this is a valid downstream process outcome of its oxidase activity. Reason: Protein folding in the ER is the biological-process consequence of the oxidase activity rather than ERO1B's direct molecular function. Supporting Evidence: PMID:21091435 drives oxidative protein folding |
| GO:0071949 FAD binding | IEA GO_REF:0000002 | ACCEPT | Summary: ERO1B is a flavoprotein that binds FAD as its cofactor. Reason: FAD is the documented cofactor (PMID:21091435), integral to the oxidase mechanism; FAD binding residues are mapped. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Name=FAD; Xref=ChEBI:CHEBI:57692 |
| GO:0005515 protein binding | IPI PMID:20802462 Disulphide production by Ero1Ξ±-PDI relay is rapid and effect... | KEEP AS NON CORE | Summary: IntAct interaction with PDIA3/ERp57 (P30101). The bare protein binding term is uninformative; it records an interaction within the ER oxidoreductase network. Reason: Records a real physical interaction (PDIA3), but the generic protein binding term is uninformative and the informative function is the oxidase MF; ERO1B's principal PDI substrate is P4HB. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Q86YB8; P30101: PDIA3 |
| GO:0005788 endoplasmic reticulum lumen | IEA GO_REF:0000107 | ACCEPT | Summary: ERO1B acts on the lumenal side of the ER membrane; ER lumen is consistent with its site of action. Reason: ERO1B is a lumenal-side ER protein; ER lumen localization is consistent with its function. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Lumenal side |
| GO:0015036 disulfide oxidoreductase activity | IEA GO_REF:0000107 | ACCEPT | Summary: A directionless parent term for thiol-disulfide oxidoreduction; correctly captures ERO1B's catalytic chemistry without mislabeling it as a reductase. Reason: Accurate (direction-neutral) molecular-function term for an enzyme that interconverts dithiols and disulfides on PDI; subsumed by the more specific sulfhydryl oxidase term. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Oxidoreductase involved in disulfide bond formation in the endoplasmic reticulum. |
| GO:0016020 membrane | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Generic membrane localization; ERO1B is a peripheral membrane protein, so this is consistent but uninformative. Reason: Correct but generic; the specific ER membrane term is preferred. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Peripheral membrane protein |
| GO:0030070 insulin processing | TAS Reactome:R-HSA-264876 | KEEP AS NON CORE | Summary: ERO1B may drive oxidative proinsulin folding in pancreatic beta cells, contributing to insulin processing; a genuine but specialized, tissue-restricted role. Recent proteomic work corroborates ERO1B enrichment in beta cells and its association with insulin biogenesis. Reason: Supported as a plausible specialized function in pancreatic islets (where ERO1B is enriched), but it is a tissue-specific downstream role rather than the core oxidase function. The falcon deep research synthesizes recent proteomic data (axelsson2024) identifying ERO1B as enriched in pancreatic beta cells and implicated in insulin biogenesis, consistent with this annotation; this remains a specialized downstream process rather than the core MF. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt May be involved in oxidative proinsulin folding in pancreatic cells, hence may play a role in glucose homeostasis. file:human/ERO1B/ERO1B-deep-research-falcon.md ERO1B plays a specialized role in supporting the high protein secretory load of pancreatic beta cells, particularly for the folding and maturation of proinsulin file:human/ERO1B/ERO1B-deep-research-falcon.md Single-cell proteomics of human pancreatic islet cells found that ERO1B expression was higher in beta cells |
| GO:0016971 flavin-dependent sulfhydryl oxidase activity | EXP PMID:11707400 Manipulation of oxidative protein folding and PDI redox stat... | ACCEPT | Summary: Experimentally supported FAD-dependent sulfhydryl oxidase activity - ERO1B facilitates disulfide bond formation by oxidizing PDI. Reason: Direct experimental evidence (selective oxidation of PDI); core molecular function. Supporting Evidence: PMID:11707400 both human Ero1-Lalpha and Ero1-Lbeta (hEROs) facilitate disulfide bond formation in immunoglobulin subunits by selectively oxidizing PDI |
| GO:0016971 flavin-dependent sulfhydryl oxidase activity | EXP PMID:21091435 The endoplasmic reticulum sulfhydryl oxidase Ero1Ξ² drives ef... | ACCEPT | Summary: Experimentally supported FAD-dependent sulfhydryl oxidase activity; recombinant ERO1B is twice as active as ERO1A and oxidizes PDI efficiently. ERO1B acts on PDI (not client proteins directly) via a two-step ERO1-PDI relay. Reason: Direct enzymatic-assay evidence for the core oxidase function; ERO1B is a highly active, loosely regulated oxidase. The falcon deep research reinforces that ERO1B's catalytic substrate is reduced PDI, which it re-oxidizes so PDI can in turn introduce disulfide bonds into nascent secretory proteins. Supporting Evidence: PMID:21091435 recombinant human Ero1Ξ² is twice as active as Ero1Ξ± in enzymatic assays file:human/ERO1B/ERO1B-deep-research-falcon.md ERO1B does not directly oxidize client secretory proteins; instead, it functions in a two-step relay system |
| GO:0016972 thiol oxidase activity | EXP PMID:11707400 Manipulation of oxidative protein folding and PDI redox stat... | ACCEPT | Summary: Experimentally supported thiol oxidase activity (parent of the flavin-dependent sulfhydryl oxidase term). Reason: EXP evidence for oxidase activity; correct core function, though the flavin-dependent sulfhydryl oxidase term is the most precise. Supporting Evidence: PMID:11707400 both human Ero1-Lalpha and Ero1-Lbeta (hEROs) facilitate disulfide bond formation in immunoglobulin subunits by selectively oxidizing PDI |
| GO:0005788 endoplasmic reticulum lumen | TAS Reactome:R-HSA-9817575 | ACCEPT | Summary: Reactome ER-lumen localization, consistent with ERO1B's lumenal-side site of action. Reason: Consistent with the documented lumenal-side ER localization. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Lumenal side |
| GO:0015035 protein-disulfide reductase activity | EXP PMID:11707400 Manipulation of oxidative protein folding and PDI redox stat... | MARK AS OVER ANNOTATED | Summary: This Reactome EXP annotation labels ERO1B as a protein-disulfide REDUCTASE. ERO1B physically engages PDI active-site disulfides, but its physiological action is oxidation of PDI (forming disulfides), not reduction; the reductase label is directionally misleading. Reason: The supporting study shows ERO1B/ERO1 oxidizes PDI; the reductase directionality mischaracterizes the enzyme, which is an oxidase. The accurate term is sulfhydryl oxidase / disulfide oxidoreductase activity. Supporting Evidence: PMID:11707400 both human Ero1-Lalpha and Ero1-Lbeta (hEROs) facilitate disulfide bond formation in immunoglobulin subunits by selectively oxidizing PDI |
| GO:0015035 protein-disulfide reductase activity | EXP PMID:16407158 Generating disulfides enzymatically: reaction products and e... | MARK AS OVER ANNOTATED | Summary: Reactome EXP reductase annotation. As above, the ERO1 enzyme generates disulfides (oxidase); the reductase directionality is misleading. Reason: The cited work concerns enzymatic disulfide generation by the ER thiol oxidase Ero1; an oxidase, not a reductase. The reductase term over-annotates the catalytic direction. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Following P4HB reoxidation, passes its electrons to molecular oxygen via FAD |
| GO:0015035 protein-disulfide reductase activity | EXP PMID:21091435 The endoplasmic reticulum sulfhydryl oxidase Ero1Ξ² drives ef... | MARK AS OVER ANNOTATED | Summary: Reactome EXP reductase annotation derived from the Ero1Ξ² characterization, which actually shows ERO1B oxidizes PDI; the reductase directionality is misleading. Reason: The supporting study demonstrates oxidase activity (oxidizing PDI), not disulfide reduction; the reductase term mislabels the catalytic direction. Supporting Evidence: PMID:21091435 Ero1Ξ² oxidizes PDI more efficiently than other PDI family members |
| GO:0034975 protein folding in endoplasmic reticulum | IDA PMID:21091435 The endoplasmic reticulum sulfhydryl oxidase Ero1Ξ² drives ef... | KEEP AS NON CORE | Summary: ERO1B drives oxidative protein folding in the ER; protein folding in the ER is a downstream process of its oxidase activity. Reason: A valid process annotation supported by direct evidence, but downstream of the core oxidase molecular function. Supporting Evidence: PMID:21091435 drives oxidative protein folding |
| GO:0016972 thiol oxidase activity | IDA PMID:21091435 The endoplasmic reticulum sulfhydryl oxidase Ero1Ξ² drives ef... | ACCEPT | Summary: Direct-assay thiol oxidase activity confirmed for recombinant ERO1B. Reason: IDA evidence for the core oxidase activity from enzymatic assays. Supporting Evidence: PMID:21091435 recombinant human Ero1Ξ² is twice as active as Ero1Ξ± in enzymatic assays |
| GO:0005783 endoplasmic reticulum | TAS PMID:10818100 Endoplasmic reticulum oxidoreductin 1-lbeta (ERO1-Lbeta), a ... | ACCEPT | Summary: ERO1-Lbeta is primarily localized in the ER (immunofluorescence, endoglycosidase sensitivity, in vitro translocation). Reason: TAS from the founding characterization directly establishes ER localization. Supporting Evidence: PMID:10818100 the products of the ERO1-Lbeta gene are primarily localized in the ER of mammalian cells |
| GO:0006457 protein folding | TAS PMID:10818100 Endoplasmic reticulum oxidoreductin 1-lbeta (ERO1-Lbeta), a ... | KEEP AS NON CORE | Summary: ERO1-Lbeta generates oxidative conditions in the ER required for disulfide bond formation; protein folding is the downstream process. Reason: A valid process annotation but downstream of the core oxidase molecular function. Supporting Evidence: PMID:10818100 ERO1-Lbeta is involved also in generating oxidative conditions in the ER |
| GO:0016491 oxidoreductase activity | NAS PMID:10818100 Endoplasmic reticulum oxidoreductin 1-lbeta (ERO1-Lbeta), a ... | ACCEPT | Summary: ERO1B is an oxidoreductase; a correct but very general parent term. Reason: Correct high-level molecular function, subsumed by the more specific flavin-dependent sulfhydryl oxidase activity. Supporting Evidence: file:human/ERO1B/ERO1B-uniprot.txt Oxidoreductase involved in disulfide bond formation in the endoplasmic reticulum. |
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Download this section (compressed HTML)Q: How is the division of labor between ERO1B and ERO1A (and PRDX4) established in pancreatic beta cells, and how essential is ERO1B specifically for proinsulin oxidative folding and insulin secretion?
Q: Does the looser regulation of ERO1B relative to ERO1A make secretory tissues more vulnerable to ER oxidative stress, and how is its activity feedback-controlled in vivo?
Experiment: Beta-cell-specific knockout or knockdown of ERO1B (alone and combined with ERO1A) followed by assays of proinsulin folding, insulin secretion and ER redox state to test its role in glucose homeostasis.
Experiment: Comparative enzymology of purified ERO1B versus ERO1A measuring O2 consumption, H2O2 production and PDI-isoform substrate preference, including the regulatory-disulfide mutants (e.g. Cys130) that govern feedback control.
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