Affinage

SAFB2

Scaffold attachment factor B2 · UniProt Q14151

Length
953 aa
Mass
107.5 kDa
Annotated
2026-06-10
9 papers in source corpus 8 papers cited in narrative 9 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SAFB2 is a multifunctional scaffold attachment factor that acts at the intersection of transcriptional control, RNA processing, and signaling, distributing between the nucleus and cytoplasm (PMID:12660241). As a transcriptional corepressor it interacts directly with estrogen receptor α (ERα) upon estradiol stimulation and, together with its paralog SAFB1, restricts ERα intranuclear mobility, cooperatively inhibiting ERα-driven transcription and cell proliferation (PMID:12660241, PMID:22566185). SAFB2 assembles into large nuclear complexes containing SAFB1, Sam68, and hnRNPG that persist independently of nucleic acids and negatively regulate alternative splicing of a tra2-beta variable exon (PMID:17200140). In small-RNA biogenesis, SAFB2 functions as an accessory factor of the Microprocessor complex, associating with the N-terminus of DROSHA and enabling stable binding and cleavage of suboptimal stem-loop hairpins within clustered primary miRNA transcripts, acting downstream of ERH in the cluster-assistance pathway and contributing to Microprocessor feedback regulation [PMID:32502422, PMID:bio_10.1101_2025.09.09.675111]. SAFB2 also binds and destabilizes NFAT5 mRNA, thereby suppressing Wnt/β-catenin signaling and restraining proliferation, migration, and invasion in breast cancer cells (PMID:36652182). Consistent with a tissue-specific physiological role, SAFB2-null mice are viable and fertile but display increased testis weight and Sertoli cell number linked to altered androgen receptor function (PMID:26092125).

Mechanistic history

Synthesis pass · year-by-year structured walk · 6 steps
  1. 2003 Medium

    Established SAFB2 as an ERα corepressor and defined its relationship to SAFB1, answering whether the paralog is functionally redundant or distinct.

    Evidence Overexpression/reporter corepressor assays, direct interaction mapping, and subcellular fractionation/imaging

    PMID:12660241

    Open questions at the time
    • Did not define the structural basis of ERα repression
    • Cytoplasmic function of SAFB2 left unexplained
    • Vinexin interaction noted but not followed up
  2. 2007 Medium

    Showed SAFB2 resides in large, stable nuclear core complexes with SAFB1, Sam68, and hnRNPG and regulates alternative splicing, framing it as a scaffold within an RNA-processing assembly rather than a free monomer.

    Evidence Size-exclusion chromatography, monospecific antisera, splicing reporter and nucleic-acid-independence assays

    PMID:17200140

    Open questions at the time
    • Stoichiometry and architecture of the 670 kDa complex unresolved
    • Range of regulated splicing targets not defined
    • No SR-protein interaction detected, leaving the mechanism of splicing repression open
  3. 2012 Medium

    Defined the mechanism of ERα repression by showing SAFB1/SAFB2 reduce ERα intranuclear mobility, explaining how corepression and antiproliferation are achieved.

    Evidence Live-cell fluorescent imaging, reciprocal Co-IP, FRAP, and proliferation assays

    PMID:22566185

    Open questions at the time
    • Does not identify the chromatin or genomic loci where ERα is immobilized
    • Synergy mechanism between paralogs not biochemically defined
  4. 2015 Medium

    Provided in vivo physiological role via a knockout, distinguishing SAFB2 from the lethal/infertile SAFB1 phenotype and linking it to androgen receptor function.

    Evidence SAFB2-null mouse with histology, cell counts, and paralog-specific expression analysis

    PMID:26092125

    Open questions at the time
    • Molecular basis of altered androgen receptor function not established
    • Connection between Sertoli cell phenotype and SAFB2's RNA/transcription roles unclear
  5. 2020 High

    Identified SAFB2 as a Microprocessor accessory factor required for processing suboptimal hairpins in clustered pri-miRNAs, revealing a previously unknown role in miRNA biogenesis.

    Evidence Unbiased CRISPR/Cas9 screen plus biochemical co-factor and miRNA processing assays in clustered contexts (e.g., pri-miR-15a/16-1)

    PMID:32502422

    Open questions at the time
    • Direct contact between SAFB2 and Microprocessor not yet mapped at this stage
    • Determinants of substrate suboptimality recognition undefined
  6. 2025 Medium

    Placed SAFB2 mechanistically within the cluster-assistance pathway, showing it acts downstream of ERH and binds the DROSHA N-terminus to stabilize suboptimal hairpins after Microprocessor transfer.

    Evidence Epistasis genetic tests, biochemical interaction assays, and genome-wide miRNA profiling in mutant cell lines (preprint)

    PMID:bio_10.1101_2025.09.09.675111 PMID:bio_10.1101_2025.09.23.678008

    Open questions at the time
    • Preprint not yet peer-reviewed
    • ERH-SAFB2 interaction is dispensable for cluster assistance, leaving the functional role of that contact unclear
    • Structural detail of SAFB2-DROSHA recognition lacking

Open questions

Synthesis pass · forward-looking unresolved questions
  • How SAFB2's distinct activities — ERα corepression, splicing regulation, mRNA destabilization, and Microprocessor assistance — are integrated, regulated, and partitioned between compartments remains unresolved.
  • No unifying model connecting transcriptional, splicing, and miRNA-processing roles
  • Regulation of SAFB2 nuclear/cytoplasmic partitioning unknown
  • Substrate-recognition code for suboptimal pri-miRNA hairpins undefined

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003723 RNA binding 3 GO:0098772 molecular function regulator activity 2 GO:0140110 transcription regulator activity 2
Localization
GO:0005634 nucleus 3 GO:0005829 cytosol 1
Pathway
R-HSA-74160 Gene expression (Transcription) 2 R-HSA-8953854 Metabolism of RNA 2 R-HSA-162582 Signal Transduction 1
Partners
DROSHAERHESR1KHDRBS1RBMXSAFB1
Complex memberships
Microprocessor complexSAFB1-SAFB2-Sam68-hnRNPG nuclear complex

Evidence

Reading pass · 9 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2003 SAFB2 functions as an estrogen receptor (ERα) corepressor; its overexpression inhibits cell proliferation. SAFB1 and SAFB2 interact directly through a C-terminal domain, resulting in additive transcriptional repression activity. Unlike SAFB1, SAFB2 is found in both cytoplasm and nucleus. Overexpression/reporter assays for corepressor activity; direct protein interaction demonstrated; subcellular localization by fractionation/imaging The Journal of biological chemistry Medium 12660241
2003 SAFB2 interacts with vinexin, a protein involved in linking signaling to the cytoskeleton, consistent with its cytoplasmic localization. Co-immunoprecipitation / interaction assay The Journal of biological chemistry Low 12660241
2007 Endogenous SAFB2 exists in large nuclear complexes (up to 670 kDa), distinct from SAFB1 which is predominantly monomeric or in smaller complexes. Stable core complexes containing SAFB1, SAFB2, Sam68, and hnRNPG exist independently of intact nucleic acids. SAFB2 acts as a negative regulator of a tra2-beta variable exon (alternative splicing). No stable interaction was detected between SAFB proteins and SR or SR-related splicing regulators. Size-exclusion chromatography, novel monospecific antisera, splicing reporter assays, nucleic-acid-independence experiments Journal of cell science Medium 17200140
2012 SAFB1 and SAFB2 colocalize with ERα in the nucleus of living cells after estradiol treatment; co-IP confirmed ERα interaction with both SAFB1 and SAFB2 in the presence of E2. FRAP analysis showed that SAFB1 and SAFB2 each decrease ERα intranuclear mobility, and coexpression causes a synergistic reduction in ERα dynamics, leading to cooperative inhibition of ERα-mediated transcription and cell proliferation. Chimeric fluorescent protein live-cell imaging, Co-immunoprecipitation, Fluorescence Recovery After Photobleaching (FRAP), proliferation assays Journal of cellular biochemistry Medium 22566185
2015 SAFB2-knockout mice are viable and fertile (in contrast to SAFB1-knockout mice), but show significantly increased testis weight associated with an increased number of Sertoli cells. Loss of SAFB2 alters androgen receptor function and expression, implicating SAFB2 in Sertoli cell differentiation and activity. SAFB2-null mouse generation, histological and cell-count analysis, paralog-specific antibodies for expression analysis Disease models & mechanisms Medium 26092125
2020 SAFB2 is an accessory protein of the Microprocessor complex required for processing of suboptimal stem-loop structures in clustered primary miRNA transcripts (e.g., pri-miR-15a requires neighboring pri-miR-16-1 and SAFB2 for efficient cleavage). SAFB2 enables binding and processing of suboptimal Microprocessor substrates in cis within clustered pri-miRNA transcripts. CRISPR/Cas9 screen, biochemical co-factor assays, miRNA processing assays Molecular cell High 32502422
2023 SAFB2 interacts with NFAT5 mRNA (by RIP assay) and destabilizes NFAT5 mRNA, leading to suppression of the Wnt/β-catenin signaling pathway in breast cancer cells. Overexpression of SAFB2 inhibits proliferation, migration, and invasion while promoting apoptosis, effects that are partially rescued by NFAT5 overexpression. RNA immunoprecipitation (RIP), mRNA stability assay (actinomycin D), western blotting, CCK8/colony formation/EdU proliferation assays, wound healing, transwell assays Molecular biotechnology Medium 36652182
2025 Epistatic analysis places SAFB2 downstream of ERH in the cluster assistance pathway: ERH may mediate Microprocessor transfer between hairpins, while SAFB2 (especially SAFB2) mediates recognition and stable binding of a suboptimal miRNA hairpin after Microprocessor transfer. SAFB2 associates with the N-terminus of DROSHA. Both SAFB1/2 and ERH are required for Microprocessor feedback regulation via processing of pri-miR-1306. Mutant cell lines, epistasis genetic tests, biochemical interaction assays, genome-wide miRNA profiling bioRxivpreprint Medium bio_10.1101_2025.09.09.675111
2025 ERH-mediated cluster assistance is independent of its direct association with SAFB2 (as shown by disrupting the ERH-SAFB2 interaction, which did not abolish cluster assistance). Loss of SAFB1/2 and ERH produce overlapping but not identical defects in primary miRNA biogenesis. Both SAFB2 and ERH are required for efficient Microprocessor feedback regulation via pri-miR-1306 processing. SAFB1/2 deletion and ERH deletion cell lines, miRNA transcriptome profiling, protein interaction disruption experiments bioRxivpreprint Medium bio_10.1101_2025.09.23.678008

Source papers

Stage 0 corpus · 9 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2003 SAFB2, a new scaffold attachment factor homolog and estrogen receptor corepressor. The Journal of biological chemistry 81 12660241
2020 SAFB2 Enables the Processing of Suboptimal Stem-Loop Structures in Clustered Primary miRNA Transcripts. Molecular cell 51 32502422
2003 Scaffold attachment factors SAFB1 and SAFB2: Innocent bystanders or critical players in breast tumorigenesis? Journal of cellular biochemistry 48 14587024
2007 Alternative RNA splicing complexes containing the scaffold attachment factor SAFB2. Journal of cell science 41 17200140
2012 SAFB1- and SAFB2-mediated transcriptional repression: relevance to cancer. Biochemical Society transactions 23 22817742
2012 Scaffold attachment factor B (SAFB)1 and SAFB2 cooperatively inhibit the intranuclear mobility and function of ERα. Journal of cellular biochemistry 17 22566185
2008 No germline mutations in supposed tumour suppressor genes SAFB1 and SAFB2 in familial breast cancer with linkage to 19p. BMC medical genetics 12 19077293
2015 Scaffold attachment factor B2 (SAFB2)-null mice reveal non-redundant functions of SAFB2 compared with its paralog, SAFB1. Disease models & mechanisms 7 26092125
2023 SAFB2 Inhibits the Progression of Breast Cancer by Suppressing the Wnt/β-Catenin Signaling Pathway via NFAT5. Molecular biotechnology 4 36652182

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