Affinage

SSR4

Translocon-associated protein subunit delta · UniProt P51571

Length
173 aa
Mass
19.0 kDa
Annotated
2026-06-10
22 papers in source corpus 7 papers cited in narrative 7 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 4/4 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SSR4 is a subunit of the heterotetrameric translocon-associated protein (TRAP) complex in the endoplasmic reticulum membrane, where it supports N-linked glycosylation by acting in concert with the oligosaccharyltransferase complex (PMID:24218363). Loss-of-function mutations abolish SSR4 protein, destabilize other TRAP subunits, and cause protein underglycosylation, with wild-type SSR4 overexpression restoring N-glycosylation efficiency; these defects underlie a congenital disorder of glycosylation, and multiple pathogenic alleles act through aberrant splicing and nonsense-mediated decay (PMID:24218363, PMID:26264460, PMID:39653760, PMID:41210240). Beyond bulk glycosylation, SSR4 interacts with the OST subunit DDOST to regulate substrate-specific N-glycosylation: it controls BAFFR glycosylation to sustain B-cell activation and NF-κB-driven LTα1β2 expression, with B-cell-specific deletion causing peripheral B-cell loss, reduced antibody output, and accumulation of high-mannose immunoglobulins (PMID:42159884). SSR4 is also exploited by viral pathogens: the PRRSV non-structural protein Nsp2 binds SSR4 via its PLP2 and hypervariable domains and stabilizes it, with SSR4 required for full PERK-eIF2α and IRE1α-XBP1 UPR activation that promotes viral replication (PMID:42023815).

Mechanistic history

Synthesis pass · year-by-year structured walk · 6 steps
  1. 2013 High

    Established that SSR4 is a functional TRAP complex subunit directly required for N-glycosylation, answering whether the TRAP complex participates in glycosylation rather than acting solely in translocation.

    Evidence Patient fibroblast analysis with Glyc-ER-GFP reporter, western blot of TRAP subunits, and wild-type SSR4 overexpression rescue

    PMID:24218363

    Open questions at the time
    • Direct biochemical contact between SSR4 and the OST complex not structurally resolved
    • Stoichiometry and architecture of SSR4 within TRAP not defined
  2. 2015 Medium

    Confirmed that SSR4 protein loss, not merely reduced expression, drives the glycosylation defect, linking SSR4 deficiency to underglycosylation of serum transferrin in patients.

    Evidence Western blot of patient-derived samples plus whole-exome sequencing

    PMID:26264460

    Open questions at the time
    • Single lab; molecular mechanism connecting SSR4 loss to transferrin underglycosylation not dissected
    • No reconstitution of glycosylation defect in defined system
  3. 2024 Medium

    Defined a specific molecular route by which a splicing variant abolishes SSR4, showing intron retention via a cryptic donor site generates a premature stop codon and NMD.

    Evidence Minigene splicing assay and mRNA sequencing

    PMID:39653760

    Open questions at the time
    • Single lab; downstream glycosylation consequence inferred rather than measured
    • Generalizability to other SSR4 alleles unaddressed
  4. 2025 Medium

    Extended the spectrum of disease-causing SSR4 splice defects by showing a single deletion produces multiple aberrant exon-4 splice forms yielding truncated proteins.

    Evidence Minigene splicing assay, whole-exome and Sanger sequencing

    PMID:41210240

    Open questions at the time
    • Single lab; functional impact of truncated proteins on glycosylation not quantified
    • Relative abundance of the three splice forms in patient tissue unknown
  5. 2026 Medium

    Moved SSR4 from a general glycosylation factor to a substrate-specific regulator by showing it acts via DDOST to control BAFFR glycosylation and sustain B-cell activation and NF-κB signaling.

    Evidence Co-IP of SSR4-DDOST, B-cell-specific conditional knockout mouse, N-glycan mass spectrometry, NF-κB readouts, and ADCC/CDC antibody assays

    PMID:42159884

    Open questions at the time
    • Single lab; whether BAFFR is a direct vs indirect SSR4 substrate not established
    • Breadth of SSR4 substrate selectivity beyond BAFFR/immunoglobulins unknown
  6. 2026 Medium

    Identified SSR4 as a target hijacked by a viral protein, showing PRRSV Nsp2 binds and stabilizes SSR4 to drive UPR-dependent proviral ER stress.

    Evidence Co-immunoprecipitation with domain mapping, protein half-life assay, and knockdown measuring PRRSV replication and UPR pathway activation

    PMID:42023815

    Open questions at the time
    • Single lab; mechanism by which SSR4 promotes PERK and IRE1α activation not defined
    • Whether SSR4 acts through TRAP/OST in this UPR role unaddressed

Open questions

Synthesis pass · forward-looking unresolved questions
  • How SSR4 confers substrate selectivity within TRAP/OST and how its glycosylation role connects to reported metabolic and oncogenic phenotypes remains unresolved.
  • No structural model of SSR4 within the TRAP-OST assembly
  • Mechanistic basis for SSR4-dependent regulation of NDUFB11/ATP6AP1 and OXPHOS not established by direct interaction data

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0140096 catalytic activity, acting on a protein 2 GO:0060089 molecular transducer activity 1
Localization
GO:0005783 endoplasmic reticulum 1
Pathway
R-HSA-392499 Metabolism of proteins 2 R-HSA-8953897 Cellular responses to stimuli 1
Partners
DDOSTNSP2 (PRRSV)
Complex memberships
TRAP complex

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2013 SSR4 (signal sequence receptor 4) is a subunit of the heterotetrameric TRAP complex; loss-of-function mutation (c.316delT, p.F106Sfs*53) reduces expression of other TRAP complex proteins and causes underglycosylation, and overexpression of wild-type SSR4 partially restores N-glycosylation. This established that the TRAP complex, which binds to the oligosaccharyltransferase complex, is directly involved in N-glycosylation. Patient fibroblast analysis, Glyc-ER-GFP glycosylation reporter assay, western blot of TRAP complex members, wild-type SSR4 overexpression rescue Human molecular genetics High 24218363
2015 Loss-of-function mutations in SSR4 cause complete loss of SSR4 protein (western blot), confirming SSR4 protein is required for maintaining normal N-glycosylation of serum transferrin. Western blot analysis of patient-derived samples, whole-exome sequencing Human mutation Medium 26264460
2024 A canonical splicing variant (c.67+2T>C) in SSR4 induces retention of the first 46 bp of intron 1 via recognition of a downstream GC dinucleotide as a non-canonical cryptic donor splice site, generating a premature termination codon that triggers nonsense-mediated mRNA decay and decreases SSR4 expression. Minigene splicing assay, mRNA sequencing, functional molecular analysis Journal of human genetics Medium 39653760
2025 A splice variant (c.351+1del) in SSR4 produces three abnormal splice forms: 1 bp deletion in 3' end of exon 4, 42 bp deletion in 3' end of exon 4, and skipping of exon 4, all resulting in truncated proteins. Minigene splicing assay, whole-exome sequencing, Sanger sequencing Frontiers in pediatrics Medium 41210240
2026 PRRSV non-structural protein Nsp2 physically interacts with SSR4 via its PLP2 and hypervariable domains, selectively upregulates SSR4 expression by prolonging its protein half-life, and SSR4 is required for full activation of PRRSV-induced ER stress (specifically the PERK-eIF2α and IRE1α-XBP1 axes of the UPR), promoting viral replication. Co-immunoprecipitation (physical interaction), protein half-life assay, functional knockdown studies measuring PRRSV replication and ER stress pathway activation Journal of virology Medium 42023815
2026 SSR4 interacts with DDOST (oligosaccharyltransferase subunit) to regulate BAFFR N-glycosylation, thereby sustaining B-cell activation and LTα1β2 expression via NF-κB signaling; B-cell-specific Ssr4 deletion leads to peripheral B-cell loss, reduced antibody output, and increased high-mannose immunoglobulins, while SSR4 overexpression in CHO cells reduces high-mannose glycans and enhances IgG1 ADCC and CDC. Co-immunoprecipitation (SSR4-DDOST interaction), B-cell-specific conditional knockout mouse model, N-glycan analysis by mass spectrometry, NF-κB signaling readout, antibody functional assays (ADCC, CDC) Advanced science (Weinheim, Baden-Wurttemberg, Germany) Medium 42159884
2025 SSR4 regulates expression of NDUFB11 and ATP6AP1, enhancing mitochondrial respiratory chain complex I and complex V function to promote mitochondrial oxidative phosphorylation and gastric cancer progression. In vitro and in vivo functional studies (cell line knockdown/overexpression), measurement of mitochondrial OXPHOS function, in vivo xenograft models Molecular carcinogenesis Low 40999562

Source papers

Stage 0 corpus · 22 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2013 A new congenital disorder of glycosylation caused by a mutation in SSR4, the signal sequence receptor 4 protein of the TRAP complex. Human molecular genetics 52 24218363
2015 Expanding the Molecular and Clinical Phenotype of SSR4-CDG. Human mutation 28 26264460
2020 Nuclear Ssr4 Is Required for the In Vitro and In Vivo Asexual Cycles and Global Gene Activity of Beauveria bassiana. mSystems 21 32317391
2024 SSR4-CDG, an ultra-rare X-linked congenital disorder of glycosylation affecting the TRAP complex: Review of 22 affected individuals including the first adult patient. Molecular genetics and metabolism 16 38805916
2020 Expanding the phenotype of X-linked SSR4-CDG: Connective tissue implications. Human mutation 14 33300232
2022 Case Report: The novel hemizygous mutation in the SSR4 gene caused congenital disorder of glycosylation type iy: A case study and literature review. Frontiers in genetics 7 36386804
2024 A novel SSR4 variant associated with congenital disorder of glycosylation: a case report and related analysis. Frontiers in genetics 5 39086474
2023 [A case of Congenital disorder of glycosylation due to SSR4 gene deletion]. Zhonghua yi xue yi chuan xue za zhi = Zhonghua yixue yichuanxue zazhi = Chinese journal of medical genetics 4 36854416
2025 Unveiling SSR4: a promising biomarker in esophageal squamous cell carcinoma. Frontiers in immunology 3 40066443
2020 The X-ray crystal structure of the N-terminal domain of Ssr4, a Schizosaccharomyces pombe chromatin-remodelling protein. Acta crystallographica. Section F, Structural biology communications 3 33263569
2024 Intron retention caused by a canonical splicing variant in SSR4-related congenital disorder of glycosylation. Journal of human genetics 2 39653760
2022 [Analysis of SSR4 gene variant in a child with congenital glycosylation type 1y in conjunct with congenital dysplasia of external auditory canal]. Zhonghua yi xue yi chuan xue za zhi = Zhonghua yixue yichuanxue zazhi = Chinese journal of medical genetics 2 35810430
2004 Cloning and molecular characterization of a human ortholog of Monodelphis TRAPD in ultraviolet B-induced melanoma. Melanoma research 2 15057039
2025 Hemizygous contiguous gene deletion within Xq28 that includes BCAP31, ABCD1, SRPK3 and SSR4: case report and literature review. Global medical genetics 1 40662097
2025 SSR4 Promote Gastric Cancer Progression by Regulating Mitochondrial Oxidative Phosphorylation via NDUFB11 and ATP6AP1. Molecular carcinogenesis 1 40999562
2025 Novel SSR4 gene splice variant leads to congenital disorder of glycosylation, type Iy. Frontiers in pediatrics 1 41210240
2007 Rat gene mapping in the post-genome sequencing era: the continued utility of cell hybrids to localize rat genes (Cks2, Ephb4, Fabp5, Il13ra1, Rpl10, Ssr4). Cytogenetic and genome research 1 17268179
2026 The TRAP complex (SSR1-SSR4): mechanistic roles and therapeutic opportunities. Annals of medicine 0 41649879
2026 Early neonatal diagnosis of SSR4-related congenital disorder of glycosylation with severe congenital heart defects: a case report and systematic review. Frontiers in pediatrics 0 41960028
2026 The host protein SSR4 mediates PRRSV-induced endoplasmic reticulum stress via interaction with Nsp2. Journal of virology 0 42023815
2026 SSR4 sustains Tertiary Lymphoid Structures by Regulation Quality Control of N-linked Glycosylation During B-cell Differentiation Into Plasmacyte in Colorectal Cancer. Advanced science (Weinheim, Baden-Wurttemberg, Germany) 0 42159884
2025 Multi-Omics Characterization of a Novel SSR4 Variant in Congenital Disorders of Glycosylation. Metabolites 0 41441028

Missed literature

Know a paper Affinage missed for SSR4? Flag it for the maintainers and the community.

No submissions yet.