Affinage

OS9

Protein OS-9 · UniProt Q13438

Length
667 aa
Mass
75.6 kDa
Annotated
2026-04-29
35 papers in source corpus 22 papers cited in narrative 21 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

OS9 is an ER-luminal lectin that functions as a substrate receptor in endoplasmic reticulum-associated degradation (ERAD), recognizing misfolded glycoproteins through its mannose 6-phosphate receptor homology (MRH) domain and delivering them to the HRD1 ubiquitin ligase complex for retrotranslocation and proteasomal destruction. The MRH domain adopts a flattened beta-barrel fold with a double-tryptophan motif that specifically binds mannose-trimmed N-glycans on the C-arm (Manα1,6Manα1,6Man), a glycan signal generated by ER mannosidases on terminally misfolded proteins (PMID:19346256, PMID:21172656, PMID:19914915). OS9 associates with SEL1L to form a dimeric claw-like luminal assembly that engages substrates and couples to the HRD1 membrane translocon; OS9 and XTP3-B act redundantly for glycoprotein ERAD and stabilize the SEL1L–HRD1 dislocon, but OS9 uniquely antagonizes XTP3-B-mediated inhibition of non-glycosylated substrate degradation and can itself recognize misfolded polypeptide backbones independently of glycans (PMID:18264092, PMID:29706535, PMID:24910992, PMID:41593065). OS9 is transcriptionally induced by the IRE1/XBP1 unfolded protein response pathway, selectively binds non-native conformers over folding-competent proteins, and also targets hyperglycosylated forms of the ER chaperone GRP94 for lysosomal degradation via its intrinsically disordered C-terminal domain (PMID:18417469, PMID:24899641).

Mechanistic history

Synthesis pass · year-by-year structured walk · 8 steps
  1. 2002 Medium

    Initial identification of OS-9 as an ER-associated protein with specific protein-protein interaction capabilities established it as a factor in the early secretory pathway, though its function remained unclear.

    Evidence Co-IP, subcellular fractionation, and yeast two-hybrid showing OS-9 association with meprin beta cytoplasmic tail during ER-to-Golgi transport

    PMID:12093806

    Open questions at the time
    • No function beyond binding was demonstrated
    • Interaction with meprin beta was not connected to a degradation pathway
    • Lumenal vs. cytoplasmic topology of OS-9 was ambiguous
  2. 2008 High

    Multiple groups converged to establish OS-9 as a bona fide ERAD factor that recognizes misfolded glycoproteins through its MRH domain and delivers them via SEL1L to the HRD1 ubiquitin ligase, resolving the question of why this lectin-like protein resides in the ER.

    Evidence Reciprocal Co-IP, RNAi knockdown, MRH domain mutagenesis, degradation assays for misfolded alpha1-antitrypsin variants; UPR induction analysis showing IRE1/XBP1 transcriptional upregulation; ubiquitination assays demonstrating glycoprotein-specific requirement

    PMID:18264092 PMID:18417469 PMID:19084021

    Open questions at the time
    • Precise glycan determinant recognized by OS-9 was not yet defined
    • Whether OS-9 could recognize non-glycosylated substrates was unresolved
    • Structural basis of MRH domain recognition unknown
  3. 2009 High

    Comprehensive glycan-binding studies defined the precise sugar code read by OS-9: its MRH domain binds N-glycans trimmed of the terminal α1,2-mannose on the C-arm, establishing the molecular logic by which ER mannosidase activity marks proteins for destruction.

    Evidence Frontal affinity chromatography against 92 oligosaccharides with recombinant MRH domain, site-directed mutagenesis, combined with siRNA knockdown and glycan manipulation in cells

    PMID:19346256 PMID:19914915

    Open questions at the time
    • No atomic-resolution structure of the MRH–glycan complex
    • How OS-9 discriminates misfolded from native proteins bearing similar glycans was unclear
  4. 2010 High

    Crystallography of the MRH domain in complex with mannopentaose revealed the structural basis of glycan recognition — a P-type lectin fold with a distinctive double-tryptophan motif — while genetic epistasis showed OS-9/XTP3-B are essential for soluble luminal ERAD but dispensable for membrane-anchored substrates.

    Evidence X-ray crystallography and NMR of MRH domain; siRNA epistasis with defined ERAD-LS vs. ERAD-LM substrates

    PMID:20100910 PMID:21172656

    Open questions at the time
    • No structure of full-length OS-9 or OS-9 in complex with SEL1L/HRD1
    • Mechanism by which membrane anchoring bypasses OS-9 requirement was unexplained
  5. 2011 Medium

    The earlier proposed role of OS-9 in cytoplasmic HIF-1α prolyl hydroxylation was refuted by demonstrating that OS-9 is exclusively ER-luminal and cannot physically contact cytoplasmic PHD enzymes, resolving a major discrepancy in the field.

    Evidence Subcellular fractionation, glycosylation analysis confirming ER-luminal localization, FRET showing no OS-9/PHD2 interaction in living cells

    PMID:21559462

    Open questions at the time
    • The original HIF-1α observation from 2005 was not fully mechanistically explained
    • Whether OS-9 could have indirect effects on HIF signaling via ERAD of pathway components was not tested
  6. 2014 Medium

    OS-9's substrate scope was expanded beyond glycoproteins: it was shown to recognize misfolded polypeptide backbones of non-glycosylated substrates such as sonic hedgehog, and to target hyperglycosylated non-native GRP94 to a lysosomal degradation pathway via its intrinsically disordered C-terminal domain.

    Evidence siRNA knockdown panels with non-glycosylated SHH, Co-IP with glycosylation-deficient substrates; domain mapping showing mammalian-specific C-terminal inserts in OS-9 bind GRP94 middle/C-terminal domains; pulse-chase showing ERAD-independent lysosomal degradation

    PMID:24899641 PMID:24910992 PMID:25193139

    Open questions at the time
    • Structural basis for polypeptide backbone recognition by OS-9 unknown
    • The lysosomal degradation pathway for GRP94 was not further characterized
    • Whether polypeptide vs. glycan recognition involves distinct OS-9 surfaces was unresolved
  7. 2018 High

    CRISPR knockout studies definitively established the epistatic relationship between OS9 and XTP3-B: they are redundant for glycoprotein ERAD and stabilize the SEL1L/HRD1 dislocon, but OS9 uniquely antagonizes XTP3-B's inhibition of non-glycosylated substrate degradation, explaining how relative lectin expression tunes ERAD triage fidelity.

    Evidence CRISPR homozygous deletion of OS9 and XTP3-B individually and in combination, degradation assays with glycosylated and non-glycosylated substrates, dislocon complex stability analysis

    PMID:29706535

    Open questions at the time
    • Molecular mechanism by which XTP3-B inhibits non-glycoprotein ERAD was not determined
    • Whether OS9/XTP3-B ratio is physiologically regulated beyond UPR induction was unknown
  8. 2025 High

    Cryo-EM structure of the full OS9–SEL1L–HRD1 complex revealed how the ERAD machinery is architecturally organized: OS9 and SEL1L form a dimeric claw in the ER lumen for substrate capture while HRD1 dimerizes in the membrane for translocation, and pathogenic SEL1L mutations at the OS9 interface impair complex assembly and function.

    Evidence Cryo-EM structure determination, crosslinking mass spectrometry, site-directed mutagenesis of pathogenic SEL1L-G585D variant, ERAD functional assays

    PMID:40661598 PMID:41593065

    Open questions at the time
    • No structure with a substrate glycoprotein bound in the claw
    • Mechanism of substrate handoff from the luminal claw through the HRD1 channel unresolved
    • Contribution of the OS-9 disordered C-terminal domain to substrate engagement not visualized

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include how OS-9 discriminates misfolded from native proteins bearing similar glycan structures, the structural basis for its glycan-independent polypeptide backbone recognition, and how substrates are handed off from the OS9–SEL1L luminal claw through the HRD1 retrotranslocation channel.
  • No substrate-bound structure of the OS9–SEL1L claw
  • Polypeptide backbone recognition determinants on OS-9 are unmapped
  • Dynamics of substrate threading through HRD1 pore not captured

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 4 GO:0008289 lipid binding 3
Localization
GO:0005783 endoplasmic reticulum 5
Pathway
R-HSA-392499 Metabolism of proteins 6
Partners
GRP94HRD1MEP1BSEL1LTRPV4XTP3-B
Complex memberships
SEL1L-HRD1 ERAD complex

Evidence

Reading pass · 21 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2008 OS-9 is an ER-resident glycoprotein that binds to ERAD substrates (mutant alpha1-antitrypsin) and, through its MRH domain interaction with the SEL1L adaptor, delivers substrates to the Hrd1 E3 ubiquitin ligase complex for degradation. OS-9 also associates with the ER chaperone GRP94 in this process. Co-immunoprecipitation, RNAi knockdown, domain mutagenesis (MRH domain), degradation assays Nature cell biology High 18264092
2005 OS-9 interacts with both HIF-1alpha and HIF-1alpha prolyl hydroxylases (PHDs), promoting oxygen-dependent prolyl hydroxylation of HIF-1alpha, VHL binding, and proteasomal degradation of HIF-1alpha; OS-9 loss-of-function by RNAi increases HIF-1alpha protein levels and HIF-1-mediated transcription under normoxia. Co-immunoprecipitation, RNAi knockdown, gain-of-function overexpression, HIF-1alpha hydroxylation and degradation assays, transcriptional reporter assays Molecular cell Medium 15721254
2009 The MRH domain of human OS-9 specifically binds N-glycans lacking the terminal mannose from the C branch (Man alpha1,6-linked residues on the processed C-arm of high-mannose glycans); this lectin activity is required for targeting misfolded glycoproteins to ERAD in vivo. Frontal affinity chromatography with recombinant MRH domain, site-directed mutagenesis of MRH domain, siRNA knockdown combined with glycan structure manipulation (EDEM3/mannosidase I overexpression), immunoprecipitation The Journal of biological chemistry High 19346256
2010 Crystal structure of the human OS-9 MRH domain complexed with alpha3,alpha6-mannopentaose reveals a flattened beta-barrel P-type lectin fold with a distinctive double tryptophan (WW) motif in the oligosaccharide-binding site that specifically recognizes Manalpha1,6Manalpha1,6Man residues on the processed C-arm of substrate glycans. X-ray crystallography, NMR spectroscopy, biochemical binding assays, mutagenesis Molecular cell High 21172656
2010 Disposal of soluble luminal ERAD-LS substrates is strictly dependent on HRD1, SEL1L, and either OS-9 or XTP3-B acting interchangeably, but these ERAD factors become dispensable when the same substrates are membrane-tethered (ERAD-LM), revealing that OS-9/XTP3-B function is pathway-position-dependent. Genetic epistasis with siRNA knockdowns, pulse-chase degradation assays, cell biology with defined ERAD substrates The Journal of cell biology High 20100910
2008 OS-9.1 and OS-9.2 (splice variants) are transcriptionally induced by the Ire1/Xbp1 ER stress pathway and selectively bind misfolded (but not folding-competent) glycoproteins; they inhibit secretion of non-native conformers and promote ERAD of misfolded substrates. Association with non-glycosylated misfolded proteins is unproductive. Co-immunoprecipitation, siRNA knockdown, overexpression, pulse-chase assays, ER stress induction (Xbp1 pathway analysis) The Journal of biological chemistry High 18417469
2008 OS-9 is required for efficient polyubiquitination of glycosylated ERAD substrates but is not required for ubiquitination or degradation of a non-glycosylated ERAD substrate, suggesting OS-9 transfers N-glycan-bearing misfolded proteins to the ubiquitination machinery. RNAi knockdown, ubiquitination assays, co-immunoprecipitation with ERAD machinery and substrates, pulse-chase degradation Journal of molecular biology Medium 19084021
2007 OS-9 interacts with the cytosolic N-terminal tail of TRPV4, preferentially binding monomeric/immature ER-localized TRPV4, impeding its release from the ER and attenuating its polyubiquitination, thereby acting as an auxiliary protein for TRPV4 maturation. Co-immunoprecipitation, siRNA knockdown, overexpression, plasma membrane trafficking assays, zebrafish in vivo rescue, ubiquitination assays The Journal of biological chemistry Medium 17932042
2002 OS-9 associates with ER membranes exposed to the cytoplasm and transiently interacts with the cytoplasmic carboxyl-terminal tail of meprin beta during ER-to-Golgi transport; only the non-alternatively-spliced form of OS-9 binds meprin beta, implicating the spliced-out segment in binding. Co-immunoprecipitation, subcellular fractionation, yeast two-hybrid, domain deletion analysis, alternative splice variant characterization The Journal of biological chemistry Medium 12093806
2009 The OS-9 MRH domain binds N-glycans containing terminal alpha1,6-linked mannose in the Manalpha1,6(Manalpha1,3)Manalpha1,6(Manalpha1,3)Man structure; trimming of either alpha1,6-linked mannose from the C-arm or alpha1,3-linked mannose from the B-arm abolishes binding. The misfolded NHK alpha1-antitrypsin variant but not wild-type interacts with OS-9 in a sugar-dependent manner. Frontal affinity chromatography (92 oligosaccharides), cell surface binding assays with OS-9 MRH tetramers, site-directed mutagenesis of sugar-binding residues, immunoprecipitation Glycobiology High 19914915
2014 OS-9 preferentially binds a hyperglycosylated (on cryptic N-linked glycan sites), non-native subpopulation of GRP94 and facilitates its degradation via an ERAD-independent, lysosomal-like mechanism; GRP94 glycosylation is essential for OS-9 binding and the C-terminal domain of OS-9 (containing mammalian-specific inserts) is recognized by the middle and C-terminal domains of Grp94. Co-immunoprecipitation, pulse-chase degradation assays, glycosylation mutant analysis, domain mapping Molecular biology of the cell Medium 24899641
2014 OS-9 (but not XTP3-B) is required for ERAD of both glycosylated and non-glycosylated sonic hedgehog (SHH); robust interaction of OS-9 with non-glycosylated SHH indicates that the misfolded polypeptide backbone can function as the predominant recognition signal for OS-9. siRNA knockdown of individual ER lectins, co-immunoprecipitation with non-glycosylated substrate, cycloheximide-chase degradation assays PloS one Medium 24910992
2018 OS9 and XTP3-B redundantly promote glycoprotein ERAD and stabilize the SEL1L/HRD1 dislocon complex; XTP3-B inhibits degradation of non-glycosylated proteins while OS9 antagonizes this inhibition, with their relative expression levels determining triage fidelity. CRISPR homozygous deletion of OS9 and XTP3-B individually and in combination, degradation assays, complex stability analysis Molecular cell High 29706535
2015 OS9 interacts specifically with the immature form of NKCC2 co-transporter in the ER; OS9 overexpression increases NKCC2 proteasomal degradation in an N-glycan-dependent manner (inactivation of OS9 MRH domain has no effect, but NKCC2 N-glycosylation site mutations abolish OS9-induced degradation). Yeast two-hybrid screening, co-immunoprecipitation, immunocytochemistry, siRNA knockdown, pulse-chase and cycloheximide-chase assays, MRH domain mutagenesis, proteasome inhibitor (MG132) The Journal of biological chemistry Medium 26721884
2014 OS-9 lectin delivers mutant neuroserpin (FENIB mutations) to ERAD via recognition of glycan side chains; OS-9 overexpression decreases mutant neuroserpin levels and removal of neuroserpin glycosylation sites increases protein load; OS-9 (but not XTP3-B) is differentially expressed in a FENIB mouse model. Co-immunoprecipitation, overexpression/knockdown, glycosylation site mutagenesis, FENIB mouse model analysis Neurobiology of aging Medium 24795221
2011 OS-9 is localized exclusively in the ER lumen (reticular staining, fractionation, glycosylation tests) and does not physically interact with cytoplasmic PHD2 in vivo as measured by FRET; therefore OS-9 cannot directly interact with HIF prolyl-hydroxylases due to differential subcellular localization. Subcellular fractionation, glycosylation analysis, immunofluorescence, FRET (PHD2-CFP / OS-9-YFP), overexpression and lentiviral knockdown with HIF activity assays PloS one Medium 21559462
2008 OS9 physically interacts with DC-STAMP and both co-localize in the ER; upon TLR-induced DC maturation, DC-STAMP translocates from ER to Golgi while OS9 remains in the ER; the DC-STAMP/OS9 interaction is involved in ER-to-Golgi translocation of DC-STAMP. Yeast two-hybrid, co-immunoprecipitation, confocal colocalization, TLR stimulation experiments in CHO cells and dendritic cells Molecular immunology Low 18952287
1999 OS-9 interacts with N-copine (a two-C2-domain protein) in a Ca2+-dependent manner; the second C2 domain of N-copine binds the carboxy-terminal region of OS-9, as confirmed by yeast two-hybrid and co-immunoprecipitation. Yeast two-hybrid, co-immunoprecipitation, in vitro binding assays with Ca2+ titration FEBS letters Low 10403379
2025 Cryo-EM structure of the core mammalian ERAD complex comprising OS9, SEL1L, and HRD1 reveals a dimeric assembly where SEL1L and OS9 form a claw-like configuration in the ER lumen for substrate engagement, while HRD1 dimerizes in the membrane for substrate translocation; pathogenic SEL1L mutations at the SEL1L-OS9 (Gly585Asp) interface disrupt complex formation and impair ERAD activity. Cryo-EM structure determination, site-directed mutagenesis, crosslinking mass spectrometry, ERAD functional assays Nature communications High 40661598 41593065
2016 Meprin beta cleaves OS-9 in vitro and in vivo during ischemia-reperfusion injury; fragmentation of OS-9 by meprin B occurs in kidney proteins from wild-type but not meprin alphabeta knockout mice subjected to IR injury, and meprin beta transfection prevents OS-9 accumulation under hypoxia mimicry. In vitro protease cleavage assays, ischemia-reperfusion mouse model (wild-type vs. meprin KO), Western blot, cell transfection with hypoxia mimic (CoCl2) International journal of nephrology Medium 27478637
2014 The C-terminal domain of OS-9 in higher eukaryotes contains mammalian-specific inserts that are specifically recognized by the middle and C-terminal domains of Grp94; Grp94 glycosylation is essential for OS-9 binding (allostery via the N-terminal domain); the Grp94-binding domain in OS-9 is intrinsically disordered. Domain deletion mapping, biochemical binding assays, glycosylation analysis of Grp94 interaction requirement Journal of molecular biology Medium 25193139

Source papers

Stage 0 corpus · 35 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2008 OS-9 and GRP94 deliver mutant alpha1-antitrypsin to the Hrd1-SEL1L ubiquitin ligase complex for ERAD. Nature cell biology 419 18264092
2005 OS-9 interacts with hypoxia-inducible factor 1alpha and prolyl hydroxylases to promote oxygen-dependent degradation of HIF-1alpha. Molecular cell 193 15721254
2009 Human OS-9, a lectin required for glycoprotein endoplasmic reticulum-associated degradation, recognizes mannose-trimmed N-glycans. The Journal of biological chemistry 167 19346256
2010 Stringent requirement for HRD1, SEL1L, and OS-9/XTP3-B for disposal of ERAD-LS substrates. The Journal of cell biology 147 20100910
2008 A dual task for the Xbp1-responsive OS-9 variants in the mammalian endoplasmic reticulum: inhibiting secretion of misfolded protein conformers and enhancing their disposal. The Journal of biological chemistry 102 18417469
2014 A novel long non-coding RNA ENST00000480739 suppresses tumour cell invasion by regulating OS-9 and HIF-1α in pancreatic ductal adenocarcinoma. British journal of cancer 95 25314054
2012 Unraveling the function of Arabidopsis thaliana OS9 in the endoplasmic reticulum-associated degradation of glycoproteins. Plant molecular biology 67 22328055
2010 Structural basis for oligosaccharide recognition of misfolded glycoproteins by OS-9 in ER-associated degradation. Molecular cell 67 21172656
2012 The Arabidopsis homolog of the mammalian OS-9 protein plays a key role in the endoplasmic reticulum-associated degradation of misfolded receptor-like kinases. Molecular plant 65 22516478
2007 OS-9 regulates the transit and polyubiquitination of TRPV4 in the endoplasmic reticulum. The Journal of biological chemistry 65 17932042
2016 Quality control of glycoprotein folding and ERAD: the role of N-glycan handling, EDEM1 and OS-9. Histochemistry and cell biology 63 27803995
2008 Mammalian OS-9 is upregulated in response to endoplasmic reticulum stress and facilitates ubiquitination of misfolded glycoproteins. Journal of molecular biology 52 19084021
2009 The sugar-binding ability of human OS-9 and its involvement in ER-associated degradation. Glycobiology 51 19914915
1996 Complete sequence analysis of a gene (OS-9) ubiquitously expressed in human tissues and amplified in sarcomas. Molecular carcinogenesis 48 8634085
2018 Redundant and Antagonistic Roles of XTP3B and OS9 in Decoding Glycan and Non-glycan Degrons in ER-Associated Degradation. Molecular cell 45 29706535
2002 A selective interaction between OS-9 and the carboxyl-terminal tail of meprin beta. The Journal of biological chemistry 38 12093806
1998 Cloning and characterization of three isoforms of OS-9 cDNA and expression of the OS-9 gene in various human tumor cell lines. Journal of biochemistry 36 9562620
2015 OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway. The Journal of biological chemistry 29 26721884
2014 OS-9 facilitates turnover of nonnative GRP94 marked by hyperglycosylation. Molecular biology of the cell 29 24899641
1999 Ca2(+)-dependent interaction of N-copine, a member of the two C2 domain protein family, with OS-9, the product of a gene frequently amplified in osteosarcoma. FEBS letters 24 10403379
2014 EDEM2 and OS-9 are required for ER-associated degradation of non-glycosylated sonic hedgehog. PloS one 23 24910992
2008 OS9 interacts with DC-STAMP and modulates its intracellular localization in response to TLR ligation. Molecular immunology 22 18952287
2014 Lectin OS-9 delivers mutant neuroserpin to endoplasmic reticulum associated degradation in familial encephalopathy with neuroserpin inclusion bodies. Neurobiology of aging 21 24795221
1994 OS9: a novel olfactory gene of Drosophila expressed in two olfactory organs. Journal of neurobiology 17 8021646
2014 Interaction of structural core protein of classical swine fever virus with endoplasmic reticulum-associated degradation pathway protein OS9. Virology 15 25010283
2014 Characterization of the Grp94/OS-9 chaperone-lectin complex. Journal of molecular biology 13 25193139
2015 A Novel Role of OS-9 in the Maintenance of Intestinal Barrier Function from Hypoxia-induced Injury via p38-dependent Pathway. International journal of biological sciences 12 25999789
1997 Genomic organization of the OS-9 gene amplified in human sarcomas. Journal of biochemistry 12 9498564
2011 The function of hypoxia-inducible factor (HIF) is independent of the endoplasmic reticulum protein OS-9. PloS one 10 21559462
2005 OS-9: another piece in the HIF complex story. Molecular cell 7 15721249
2017 The endoplasmic reticulum-associated protein, OS-9, behaves as a lectin in targeting the immature calcium-sensing receptor. Journal of cellular physiology 6 28419469
2025 Structural basis and pathological implications of the dimeric OS9-SEL1L-HRD1 ERAD Core Complex. bioRxiv : the preprint server for biology 3 40661598
2016 Hypoxia Associated Proteolytic Processing of OS-9 by the Metalloproteinase Meprin β. International journal of nephrology 3 27478637
2026 Structural basis and pathological implications of the dimeric OS9-SEL1L-HRD1 ERAD Core Complex. Nature communications 1 41593065
2009 [The cooperation of OS-9 and PHDs in hypoxia-induced pulmonary hypertension of rats]. Zhongguo ying yong sheng li xue za zhi = Zhongguo yingyong shenglixue zazhi = Chinese journal of applied physiology 0 21186601