{"gene":"UPF3B","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2003,"finding":"hUpf3b contains a conserved C-terminal domain that mediates direct interaction with the EJC protein Y14; tethered function analysis showed the Y14/hUpf3b interaction is essential for NMD, while the hUpf3b–hUpf2 interaction is not strictly required for NMD activity when hUpf3b is tethered, though hUpf2 is still necessary for NMD induced by tethered Y14.","method":"Co-immunoprecipitation, tethered function (lambdaN/boxB) assay, RNAi knockdown and siRNA-rescue experiments","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal binding assays combined with tethered-function NMD activity readouts and siRNA rescue, multiple orthogonal methods in one study","pmids":["12718880"],"is_preprint":false},{"year":2006,"finding":"Induction of NMD by hUpf3b requires interaction with EJC components Y14, Magoh, BTZ, and eIF4AIII via the C-terminal domain; stimulation of translation by hUpf3b is independent of this EJC interaction and is determined by other protein regions, indicating distinct downstream pathways for NMD versus translation stimulation.","method":"LambdaN/boxB tethered function assay, domain-swap and truncation analysis of hUpf3a vs hUpf3b","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 / Moderate — tethered-function assay with domain-swap mutants dissecting two separable activities in a single study, single lab but multiple orthogonal constructs","pmids":["16601204"],"is_preprint":false},{"year":2010,"finding":"Crystal structure (3.4 Å) of a minimal UPF3b–EJC assembly (UPF3b C-terminal domain + MAGO + Y14 + eIF4AIII + Barentsz + RNA + AMP-PNP) showed that UPF3b binds with its C-terminal domain stretched over a composite surface formed by eIF4AIII, MAGO, and Y14; NMD-impairing mutations map to the core interacting surfaces, and differences between UPF3b and UPF3a map to peripheral interacting residues.","method":"X-ray crystallography at 3.4 Å resolution with functional validation by mutagenesis of interface residues","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — atomic-resolution crystal structure combined with mutagenesis of NMD-relevant interface residues","pmids":["20479275"],"is_preprint":false},{"year":2016,"finding":"UPF3A acts primarily as a potent NMD inhibitor that stabilizes hundreds of transcripts, while UPF3B is the critical NMD activator; UPF3A acquired repressor activity through impairment of a critical EJC-interaction domain present in UPF3B; mice conditionally lacking UPF3A exhibit hyper-NMD with defects in embryogenesis and gametogenesis.","method":"Loss-of-function mouse genetics (conditional knockout), in vitro NMD reporter assays, transcriptome profiling","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo conditional KO combined with in vitro NMD assays and transcriptome-wide evidence, replicated across multiple contexts","pmids":["27040500"],"is_preprint":false},{"year":2017,"finding":"Using a fully reconstituted in vitro translation system, UPF3B was found to (i) directly interact with eukaryotic release factors (eRFs), (ii) delay translation termination under conditions mimicking premature termination, and (iii) dissociate post-termination ribosomal complexes lacking nascent peptide; UPF1 and ribosomes were identified as new interaction partners of UPF3B.","method":"Fully reconstituted in vitro translation/termination assay, direct binding assays with release factors and ribosomes","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro system with multiple biochemical readouts (eRF interaction, termination kinetics, ribosome dissociation) in a single rigorous study","pmids":["28899899"],"is_preprint":false},{"year":2022,"finding":"Crystal structures of UPF2's MIF4GIII domain in complex with either UPF3B or UPF3A revealed intimate binding interfaces; UPF3B's disease-causing mutation Y160D displaces Y160 from a hydrophobic cleft in UPF2, reducing binding affinity ~40-fold; UPF3A (upregulated in UPF3B-Y160D patients) binds UPF2 with ~10-fold higher affinity than UPF3B via NOPS-L residues; the middle domain (RRM-L and NOPS-L) is essential for RNA/ribosome-binding, RNA-induced oligomerization, and UPF2 interaction.","method":"X-ray crystallography of UPF3B–UPF2 and UPF3A–UPF2 complexes, isothermal titration calorimetry (binding affinity measurements), mutagenesis of interface residues","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structures with quantitative binding affinity measurements and disease-mutation validation in a single study","pmids":["35640974"],"is_preprint":false},{"year":2022,"finding":"NMD remains partially active in human HCT116 cells lacking both UPF3 paralogs; the EJC-binding domain of UPF3 paralogs is dispensable for NMD activation; instead, the conserved 'mid' domain of UPF3 paralogs is consequential for NMD activity; UPF3A strongly activates NMD in cells lacking UPF3B, demonstrating functional redundancy.","method":"CRISPR/Cas9 knockout of UPF3A, UPF3B, or both; complementation with domain-deletion and EJC-binding-deficient mutants; NMD reporter assays and RNA-seq","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent studies (PMIDs 35451102 and 35451084) using CRISPR knockouts and complementation with domain mutants reach convergent conclusions","pmids":["35451102","35451084"],"is_preprint":false},{"year":2022,"finding":"Co-depletion of UPF3A and UPF3B markedly inhibits global NMD with transcriptome-wide upregulation of NMD substrates; EJC-binding-deficient or UPF2-binding-deficient UPF3B largely retains NMD activity when tested alone, but combinations of middle-domain deletions with these mutations show additive/synergistic NMD loss, indicating the middle domain is a critical functional unit.","method":"CRISPR/Cas9 knockout; NMD reporter assays; domain-deletion and point-mutant rescue experiments; transcriptome-wide RNA-seq","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — combinatorial knockout and domain-mutant rescue in human cells with transcriptome-wide validation, replicated by independent study","pmids":["35451084"],"is_preprint":false},{"year":2004,"finding":"UPF3B-mediated mRNA degradation occurs exclusively in the cytoplasm; nuclear mRNA export is required for UPF3B-mediated NMD even though the UPF3B fusion protein used is a nucleocytoplasmic shuttling protein predominantly localized to the nucleus.","method":"Tethered UPF3B combined with retroviral Rev/RRE-based nuclear export regulation; subcellular localization by fluorescence microscopy","journal":"RNA biology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — functional tethering combined with nuclear export block provides clear mechanistic conclusion, but single lab, single study","pmids":["17194930"],"is_preprint":false},{"year":2013,"finding":"SATB2 directly binds the UPF3B promoter (shown by ChIP) and activates UPF3B transcription (shown by luciferase reporter assay); siRNA knockdown of SATB2 in HEK293 cells and Satb2-knockout mouse embryonic tissue both showed significantly decreased UPF3B expression, placing SATB2 as a transcriptional activator upstream of UPF3B.","method":"Chromatin immunoprecipitation (ChIP), luciferase reporter assay, siRNA knockdown in HEK293 cells, Satb2-knockout mouse embryonic tissue analysis","journal":"Human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP plus reporter assay plus in vivo KO validation, multiple orthogonal methods but single lab","pmids":["23925499"],"is_preprint":false},{"year":2011,"finding":"In UPF3B-null patient lymphoblastoid cells, UPF3A protein (but not mRNA) is stabilized in proportion inversely correlated with phenotype severity, demonstrating that UPF3B normally competes with UPF3A for UPF2 binding and that free UPF3A is unstable; UPF3A partially compensates for loss of UPF3B function.","method":"Western blot and qRT-PCR from patient-derived lymphoblastoid cell lines with UPF3B loss-of-function mutations; correlation analysis across multiple patients","journal":"Molecular psychiatry","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — protein/mRNA quantification in patient cells across multiple families, convergent with genetic evidence, but no direct biochemical binding competition assay","pmids":["22182939"],"is_preprint":false},{"year":2009,"finding":"UPF3B protein is expressed in neurons and localizes to dendritic spines in mouse primary hippocampal neurons as determined by immunofluorescence.","method":"Immunofluorescence/immunolocalization in mouse primary hippocampal neurons","journal":"Molecular psychiatry","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single-lab localization study without functional consequence directly tested for the dendritic spine localization","pmids":["19238151"],"is_preprint":false},{"year":2013,"finding":"Loss of Upf3b-NMD in neural progenitor cells results in expansion of cell numbers at the expense of differentiation; in primary hippocampal neurons, loss of Upf3b-NMD causes subtle neurite growth defects; Upf3b subcellular localization is regulated during development.","method":"siRNA knockdown of Upf3b in neural progenitor cells and primary hippocampal neurons; cell proliferation and differentiation assays; immunocytochemistry for subcellular localization","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with defined proliferation/differentiation phenotype in relevant cell type, complemented by localization data, single lab","pmids":["23821644"],"is_preprint":false},{"year":2015,"finding":"UPF3B missense mutations associated with neurodevelopmental disorders reduce NMD activity in tethered function assays; expression of missense mutant UPF3B disturbs neuronal differentiation and reduces neurite branching complexity; NMD is downregulated during neural stem cell differentiation with concurrent downregulation of UPF3B and UPF1.","method":"Tethered function NMD assay with mutant UPF3B constructs; GFP-tagged UPF3B localization in neural stem cells and neurons; neurite complexity assay; NMD inhibitor (Amlexanox) treatment","journal":"Molecular brain","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — tethered-function assay establishes reduced NMD activity of mutants; neuronal phenotype supported by pharmacological NMD inhibition as orthogonal approach; single lab","pmids":["26012578"],"is_preprint":false},{"year":2017,"finding":"Upf3b-null mice display deficits in fear-conditioned learning, profound impairment in prepulse inhibition, and deficient dendritic spine maturation in cortical pyramidal neurons in vivo; neural stem cells from Upf3b-null mice have impaired differentiation capacity.","method":"Upf3b-null mouse generation; behavioral testing (fear conditioning, Morris water maze, PPI); in vivo dendritic spine morphology; neural stem cell differentiation assay; RNA-seq of frontal cortex","journal":"Molecular psychiatry","confidence":"High","confidence_rationale":"Tier 2 / Strong — germline knockout mouse with multiple independent behavioral and cellular phenotypic readouts, confirmed by RNA-seq identification of direct NMD target transcripts","pmids":["28948974"],"is_preprint":false},{"year":2024,"finding":"UPF3B directly interacts with IRE1α (the ER stress sensor); this interaction inhibits IRE1α kinase activity, abolishes autophosphorylation, and reduces IRE1α clustering; the disease-causing mutation UPF3BY160D abolishes the UPF3B–IRE1α interaction; phosphorylation of UPF3B at Thr169 abolishes its interaction with UPF2.","method":"Co-immunoprecipitation, kinase activity assay, autophosphorylation assay, IRE1α clustering assay, site-directed mutagenesis (Y160D, T169 phosphomimetic)","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — Co-IP plus kinase/autophosphorylation assay with mutagenesis, single lab, no structural validation","pmids":["39138189"],"is_preprint":false},{"year":2024,"finding":"UPF3B binds the 3'-UTR of CDH1 mRNA to promote its degradation (via the truncated splice variant UPF3B-S generated by HnRNPR-dependent exon 8 exclusion); UPF3B-S overexpression enhances dephosphorylation of LATS1 and nuclear accumulation of YAP1, activating Hippo signaling.","method":"RNA immunoprecipitation (RIP), in vitro and in vivo HCC models with UPF3B-S knockdown/overexpression, Basescope assay","journal":"Journal of advanced research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — describes a truncated splice variant (UPF3B-S), not full-length canonical UPF3B; mechanisms attributed specifically to UPF3B-S; single lab, limited orthogonal validation of molecular mechanism","pmids":["38402949"],"is_preprint":false},{"year":2024,"finding":"UPF3B binds to PPP2R2C mRNA (a regulatory subunit of PP2A) and promotes its degradation, activating the PI3K/AKT/mTOR pathway; E2F6 transcription factor directly binds the UPF3B promoter and activates its transcription.","method":"Co-immunoprecipitation, RNA-binding/pulldown assays, mRNA stability assays, luciferase reporter assay for E2F6-UPF3B promoter, in vivo and in vitro HCC proliferation assays","journal":"Cancer science","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, primarily Co-IP and reporter assays in a cancer context, limited orthogonal validation","pmids":["38889220"],"is_preprint":false},{"year":2025,"finding":"RBM8A interacts with UPF3B (shown by co-immunoprecipitation) to jointly regulate the stability of BBC3 (PUMA) mRNA, promoting its degradation in gastric cancer cells.","method":"Co-immunoprecipitation; RNA immunoprecipitation-seq; RNA pulldown; Actinomycin D mRNA stability assay","journal":"International journal of molecular medicine","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP identifying RBM8A–UPF3B interaction; functional consequence attributed to the complex, single lab, no domain-level mechanistic dissection of UPF3B's role","pmids":["40613240"],"is_preprint":false},{"year":2023,"finding":"In cardiac sarcomeres, UPF3B (but not UPF1 or UPF2) localizes specifically to Z-discs, the presumed site of sarcomeric protein translation, suggesting a role for UPF3B-dependent NMD at the site of initial translation.","method":"Immunofluorescence localization in cardiac tissue from HCM patients and controls; RNA-seq gene set enrichment analysis","journal":"Journal of molecular and cellular cardiology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — localization by immunofluorescence in cardiac tissue, single lab, no direct functional consequence of Z-disc localization experimentally tested","pmids":["37797718"],"is_preprint":false}],"current_model":"UPF3B is a core NMD factor that bridges the exon junction complex (EJC) to the UPF surveillance machinery via its C-terminal domain binding to a composite surface on MAGO/Y14/eIF4AIII (crystal structure resolved) and via its middle NOPS-L domain binding to UPF2 (crystal structure with disease-mutation Y160D reducing affinity ~40-fold); beyond its EJC-adaptor role, UPF3B directly interacts with eukaryotic release factors and ribosomes to delay premature translation termination and promote post-termination ribosome dissociation (reconstituted in vitro), and also interacts with IRE1α to suppress ER stress; its paralog UPF3A is a functional redundant NMD activator whose protein stability is controlled by competition with UPF3B for UPF2 binding, and which can act as an NMD repressor in specific developmental contexts; loss of UPF3B in mice causes dendritic spine immaturity, impaired neural stem cell differentiation, and behavioral deficits consistent with its human intellectual disability phenotype."},"narrative":{"mechanistic_narrative":"UPF3B is a core nonsense-mediated mRNA decay (NMD) factor that couples the exon junction complex (EJC) to the UPF surveillance machinery and contributes directly to translation termination quality control [PMID:12718880, PMID:28899899]. Through its conserved C-terminal domain it binds the EJC, docking across a composite surface formed by eIF4AIII, MAGO, and Y14 as resolved by crystallography, with NMD-impairing mutations mapping to the core interface [PMID:12718880, PMID:20479275]. Its middle region (RRM-L plus NOPS-L) engages UPF2 via a hydrophobic cleft on the MIF4GIII domain and is the critical functional unit for RNA/ribosome binding and NMD activity; the EJC-binding domain itself is dispensable when the middle domain is intact, and combined disruption of both modules synergistically abolishes decay [PMID:35640974, PMID:35451102, PMID:35451084]. Beyond its adaptor role, UPF3B directly interacts with eukaryotic release factors, UPF1, and the ribosome, delaying premature translation termination and dissociating post-termination ribosomal complexes in a reconstituted system [PMID:28899899]. UPF3B activity is controlled by its paralog UPF3A: UPF3A binds UPF2 with higher affinity, competes with UPF3B for this interaction, is unstable when free, and functions as a redundant NMD activator (and context-dependent repressor) such that loss of UPF3B stabilizes UPF3A and is partially compensated [PMID:27040500, PMID:35640974, PMID:22182939]. UPF3B also binds and inhibits the ER stress sensor IRE1α, suppressing its kinase activity, autophosphorylation, and clustering [PMID:39138189]. Loss of UPF3B impairs neural stem cell differentiation and dendritic spine maturation and produces learning and sensorimotor-gating deficits in mice, consistent with its causal role in X-linked intellectual disability, where missense mutations such as Y160D reduce NMD activity and weaken UPF2 binding ~40-fold [PMID:35640974, PMID:26012578, PMID:28948974].","teleology":[{"year":2003,"claim":"Established that UPF3B physically links the EJC to NMD, defining which of its protein contacts are essential for decay.","evidence":"Co-IP and lambdaN/boxB tethered-function NMD assays with RNAi rescue in human cells","pmids":["12718880"],"confidence":"High","gaps":["Did not resolve the structural basis of the Y14 interaction","Roles of the middle domain and UPF2 contact in unmodified contexts unaddressed"]},{"year":2006,"claim":"Separated UPF3B's NMD-promoting activity from a distinct translation-stimulatory activity, showing the two map to different protein regions.","evidence":"Tethered-function assays with domain-swap and truncation mutants comparing UPF3a and UPF3b","pmids":["16601204"],"confidence":"High","gaps":["Molecular basis of translation stimulation not identified","Did not test endogenous targets"]},{"year":2010,"claim":"Provided the atomic-resolution picture of how UPF3B's C-terminal domain docks onto the EJC, explaining NMD-impairing mutations.","evidence":"X-ray structure (3.4 Å) of UPF3b C-terminal domain with eIF4AIII/MAGO/Y14/Barentsz/RNA plus interface mutagenesis","pmids":["20479275"],"confidence":"High","gaps":["Structure limited to C-terminal domain; middle/UPF2 interface not captured","Did not address full-length conformational dynamics"]},{"year":2016,"claim":"Reframed UPF3A versus UPF3B as functionally divergent paralogs, with UPF3A acting largely as an NMD inhibitor due to a weakened EJC-interaction domain.","evidence":"Conditional UPF3A knockout mice, in vitro NMD reporters, and transcriptome profiling","pmids":["27040500"],"confidence":"High","gaps":["Tension with later evidence that UPF3A activates NMD in UPF3B-null cells","Context-dependence of repressor versus activator role not fully resolved"]},{"year":2017,"claim":"Demonstrated a direct biochemical role for UPF3B at the ribosome, distinct from its EJC-adaptor function.","evidence":"Fully reconstituted in vitro translation/termination system with release-factor and ribosome binding assays","pmids":["28899899"],"confidence":"High","gaps":["In-cell contribution of termination delay to NMD target selection not quantified","Structural basis of eRF/ribosome contacts undetermined"]},{"year":2022,"claim":"Defined the UPF3B/UPF3A–UPF2 interface structurally and quantitatively, explaining how the disease mutation Y160D weakens UPF2 binding and how UPF3A out-competes UPF3B.","evidence":"Crystal structures of UPF2 MIF4GIII with UPF3B or UPF3A, ITC affinity measurements, interface mutagenesis","pmids":["35640974"],"confidence":"High","gaps":["Did not directly link affinity changes to in-cell NMD output across substrates","Functional role of RNA-induced oligomerization not fully resolved"]},{"year":2022,"claim":"Established that the middle domain, not the EJC-binding domain, is the essential functional unit of UPF3 paralogs and that UPF3A and UPF3B are functionally redundant for NMD.","evidence":"CRISPR single and double knockouts in HCT116/human cells with domain-deletion and point-mutant complementation, NMD reporters and RNA-seq (two convergent EMBO J studies)","pmids":["35451102","35451084"],"confidence":"High","gaps":["Residual EJC-independent NMD mechanism in double knockouts not fully defined","Precise biochemical activity of the middle domain in decay unclear"]},{"year":2004,"claim":"Localized UPF3B-mediated decay to the cytoplasm and showed nuclear mRNA export is a prerequisite despite the protein's predominantly nuclear steady-state localization.","evidence":"Tethered UPF3B with Rev/RRE-regulated export plus fluorescence localization","pmids":["17194930"],"confidence":"Medium","gaps":["Single-lab, single-study","Did not define the trigger for cytoplasmic decay onset"]},{"year":2013,"claim":"Placed UPF3B downstream of a defined transcriptional activator, SATB2, linking NMD-factor dosage to a developmental regulator.","evidence":"ChIP, luciferase reporter, siRNA knockdown in HEK293, and Satb2-knockout mouse tissue","pmids":["23925499"],"confidence":"Medium","gaps":["Single lab","Physiological impact of SATB2-driven UPF3B levels on NMD output untested"]},{"year":2011,"claim":"Showed that UPF3A protein stability is governed by competition with UPF3B for UPF2, providing a buffering mechanism for UPF3B loss.","evidence":"Western blot and qRT-PCR in UPF3B-null patient lymphoblastoid lines across multiple families","pmids":["22182939"],"confidence":"Medium","gaps":["No direct in vitro binding-competition assay in this study","Correlation with phenotype severity not mechanistically dissected"]},{"year":2013,"claim":"Connected UPF3B-dependent NMD to neural progenitor fate, showing its loss expands progenitors at the expense of differentiation.","evidence":"siRNA knockdown in neural progenitors and hippocampal neurons with proliferation/differentiation and localization assays","pmids":["23821644"],"confidence":"Medium","gaps":["Knockdown rather than complete loss","Direct NMD targets driving the phenotype not identified here"]},{"year":2015,"claim":"Linked disease-associated UPF3B missense mutations to reduced NMD activity and impaired neuronal differentiation, and showed NMD is downregulated during neural differentiation.","evidence":"Tethered-function NMD assays with mutant constructs, neurite complexity assays, and pharmacological NMD inhibition in neural stem cells","pmids":["26012578"],"confidence":"Medium","gaps":["Single lab","Causal NMD targets for neurite phenotype not defined"]},{"year":2017,"claim":"Provided in vivo evidence that UPF3B loss causes dendritic spine immaturity, impaired neural stem cell differentiation, and behavioral deficits modeling the human disorder.","evidence":"Upf3b-null mice with behavioral testing, in vivo spine morphology, NSC differentiation, and frontal cortex RNA-seq","pmids":["28948974"],"confidence":"High","gaps":["Which dysregulated NMD targets are causal for each behavioral phenotype unresolved","Cell-type-specific contributions not dissected"]},{"year":2024,"claim":"Identified a non-NMD function for UPF3B as a direct inhibitor of the ER stress sensor IRE1α, and a phospho-switch (Thr169) controlling UPF2 binding.","evidence":"Co-IP, kinase and autophosphorylation assays, clustering assays, and Y160D/T169 mutagenesis","pmids":["39138189"],"confidence":"Medium","gaps":["No structural validation of the UPF3B–IRE1α interface","Single lab; physiological relevance of the T169 phospho-switch untested in vivo"]},{"year":2024,"claim":"Reported UPF3B (and a truncated UPF3B-S variant) acting on specific cancer-relevant transcripts to modulate Hippo and PI3K/AKT/mTOR signaling.","evidence":"RIP, mRNA stability assays, reporter assays, and HCC models for CDH1/PPP2R2C regulation","pmids":["38402949","38889220"],"confidence":"Low","gaps":["Mechanisms partly attributed to a truncated variant rather than canonical UPF3B; limited orthogonal validation","Generality beyond cancer context unknown"]},{"year":2025,"claim":"Reported UPF3B partnering with RBM8A to regulate BBC3/PUMA mRNA stability in gastric cancer.","evidence":"Co-IP, RIP-seq, RNA pulldown, and Actinomycin D mRNA stability assay","pmids":["40613240"],"confidence":"Low","gaps":["Single Co-IP without domain-level dissection of UPF3B's role","Functional consequence attributed to the complex, not UPF3B specifically"]},{"year":null,"claim":"How UPF3B's reconstituted termination/ribosome-dissociation activity, its EJC-independent middle-domain function, and its non-NMD interactions (IRE1α, tissue-specific localization) are integrated into substrate-specific decay in vivo remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No integrated model linking termination delay to in-cell target selection","Structural basis of non-EJC interactions undefined","Tissue-specific roles (e.g., cardiac Z-disc localization) functionally untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[5,7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[4]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[15]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[8]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[8]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,4,6]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[15]}],"complexes":["exon junction complex (EJC)"],"partners":["UPF2","Y14","MAGOH","EIF4A3","UPF1","UPF3A","IRE1Α","RBM8A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BZI7","full_name":"Regulator of nonsense transcripts 3B","aliases":["Nonsense mRNA reducing factor 3B","Up-frameshift suppressor 3 homolog B","hUpf3B","Up-frameshift suppressor 3 homolog on chromosome X","hUpf3p-X"],"length_aa":483,"mass_kda":57.8,"function":"Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons by associating with the nuclear exon junction complex (EJC) and serving as link between the EJC core and NMD machinery. Recruits UPF2 at the cytoplasmic side of the nuclear envelope and the subsequent formation of an UPF1-UPF2-UPF3 surveillance complex (including UPF1 bound to release factors at the stalled ribosome) is believed to activate NMD. In cooperation with UPF2 stimulates both ATPase and RNA helicase activities of UPF1. Binds spliced mRNA upstream of exon-exon junctions. In vitro, stimulates translation; the function is independent of association with UPF2 and components of the EJC core","subcellular_location":"Nucleus; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9BZI7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/UPF3B","classification":"Not Classified","n_dependent_lines":26,"n_total_lines":1208,"dependency_fraction":0.02152317880794702},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"UPF1","stoichiometry":4.0}],"url":"https://opencell.sf.czbiohub.org/search/UPF3B","total_profiled":1310},"omim":[{"mim_id":"617958","title":"INTERACTOR OF LITTLE ELONGATION COMPLEX ELL SUBUNIT 1; ICE1","url":"https://www.omim.org/entry/617958"},{"mim_id":"612313","title":"GLASS SYNDROME; GLASS","url":"https://www.omim.org/entry/612313"},{"mim_id":"608148","title":"SPECIAL AT-RICH SEQUENCE-BINDING PROTEIN 2; SATB2","url":"https://www.omim.org/entry/608148"},{"mim_id":"606447","title":"RNA-BINDING PROTEIN S1; RNPS1","url":"https://www.omim.org/entry/606447"},{"mim_id":"605530","title":"UPF3A REGULATOR OF NONSENSE-MEDIATED mRNA DECAY; UPF3A","url":"https://www.omim.org/entry/605530"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Cytosol","reliability":"Enhanced"},{"location":"Nucleoplasm","reliability":"Additional"},{"location":"Nucleoli","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/UPF3B"},"hgnc":{"alias_symbol":["RENT3B","UPF3X","HUPF3B","MRX82"],"prev_symbol":["MRX62","UPF3BP1","UPF3BP2","UPF3BP3"]},"alphafold":{"accession":"Q9BZI7","domains":[{"cath_id":"3.30.70.330","chopping":"51-132","consensus_level":"high","plddt":88.9034,"start":51,"end":132},{"cath_id":"4.10.290","chopping":"169-242","consensus_level":"medium","plddt":70.632,"start":169,"end":242}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BZI7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BZI7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BZI7-F1-predicted_aligned_error_v6.png","plddt_mean":65.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=UPF3B","jax_strain_url":"https://www.jax.org/strain/search?query=UPF3B"},"sequence":{"accession":"Q9BZI7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BZI7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BZI7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BZI7"}},"corpus_meta":[{"pmid":"12718880","id":"PMC_12718880","title":"Y14 and hUpf3b form an NMD-activating complex.","date":"2003","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/12718880","citation_count":251,"is_preprint":false},{"pmid":"17704778","id":"PMC_17704778","title":"Mutations in UPF3B, a member of the nonsense-mediated mRNA decay complex, cause syndromic and nonsyndromic mental retardation.","date":"2007","source":"Nature genetics","url":"https://pubmed.ncbi.nlm.nih.gov/17704778","citation_count":219,"is_preprint":false},{"pmid":"27040500","id":"PMC_27040500","title":"The Antagonistic Gene Paralogs Upf3a and Upf3b Govern Nonsense-Mediated RNA Decay.","date":"2016","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/27040500","citation_count":133,"is_preprint":false},{"pmid":"16601204","id":"PMC_16601204","title":"Functions of hUpf3a and hUpf3b in nonsense-mediated mRNA decay and translation.","date":"2006","source":"RNA (New York, 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associated with cMyBP-C haploinsufficiency in hypertrophic cardiomyopathy patients.","date":"2023","source":"Journal of molecular and cellular cardiology","url":"https://pubmed.ncbi.nlm.nih.gov/37797718","citation_count":5,"is_preprint":false},{"pmid":"15526294","id":"PMC_15526294","title":"Localization of MRX82: a new nonsyndromic X-linked mental retardation locus to Xq24-q25 in a Basque family.","date":"2004","source":"American journal of medical genetics. 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tethered function analysis showed the Y14/hUpf3b interaction is essential for NMD, while the hUpf3b–hUpf2 interaction is not strictly required for NMD activity when hUpf3b is tethered, though hUpf2 is still necessary for NMD induced by tethered Y14.\",\n      \"method\": \"Co-immunoprecipitation, tethered function (lambdaN/boxB) assay, RNAi knockdown and siRNA-rescue experiments\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal binding assays combined with tethered-function NMD activity readouts and siRNA rescue, multiple orthogonal methods in one study\",\n      \"pmids\": [\"12718880\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Induction of NMD by hUpf3b requires interaction with EJC components Y14, Magoh, BTZ, and eIF4AIII via the C-terminal domain; stimulation of translation by hUpf3b is independent of this EJC interaction and is determined by other protein regions, indicating distinct downstream pathways for NMD versus translation stimulation.\",\n      \"method\": \"LambdaN/boxB tethered function assay, domain-swap and truncation analysis of hUpf3a vs hUpf3b\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tethered-function assay with domain-swap mutants dissecting two separable activities in a single study, single lab but multiple orthogonal constructs\",\n      \"pmids\": [\"16601204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Crystal structure (3.4 Å) of a minimal UPF3b–EJC assembly (UPF3b C-terminal domain + MAGO + Y14 + eIF4AIII + Barentsz + RNA + AMP-PNP) showed that UPF3b binds with its C-terminal domain stretched over a composite surface formed by eIF4AIII, MAGO, and Y14; NMD-impairing mutations map to the core interacting surfaces, and differences between UPF3b and UPF3a map to peripheral interacting residues.\",\n      \"method\": \"X-ray crystallography at 3.4 Å resolution with functional validation by mutagenesis of interface residues\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — atomic-resolution crystal structure combined with mutagenesis of NMD-relevant interface residues\",\n      \"pmids\": [\"20479275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"UPF3A acts primarily as a potent NMD inhibitor that stabilizes hundreds of transcripts, while UPF3B is the critical NMD activator; UPF3A acquired repressor activity through impairment of a critical EJC-interaction domain present in UPF3B; mice conditionally lacking UPF3A exhibit hyper-NMD with defects in embryogenesis and gametogenesis.\",\n      \"method\": \"Loss-of-function mouse genetics (conditional knockout), in vitro NMD reporter assays, transcriptome profiling\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo conditional KO combined with in vitro NMD assays and transcriptome-wide evidence, replicated across multiple contexts\",\n      \"pmids\": [\"27040500\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Using a fully reconstituted in vitro translation system, UPF3B was found to (i) directly interact with eukaryotic release factors (eRFs), (ii) delay translation termination under conditions mimicking premature termination, and (iii) dissociate post-termination ribosomal complexes lacking nascent peptide; UPF1 and ribosomes were identified as new interaction partners of UPF3B.\",\n      \"method\": \"Fully reconstituted in vitro translation/termination assay, direct binding assays with release factors and ribosomes\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro system with multiple biochemical readouts (eRF interaction, termination kinetics, ribosome dissociation) in a single rigorous study\",\n      \"pmids\": [\"28899899\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Crystal structures of UPF2's MIF4GIII domain in complex with either UPF3B or UPF3A revealed intimate binding interfaces; UPF3B's disease-causing mutation Y160D displaces Y160 from a hydrophobic cleft in UPF2, reducing binding affinity ~40-fold; UPF3A (upregulated in UPF3B-Y160D patients) binds UPF2 with ~10-fold higher affinity than UPF3B via NOPS-L residues; the middle domain (RRM-L and NOPS-L) is essential for RNA/ribosome-binding, RNA-induced oligomerization, and UPF2 interaction.\",\n      \"method\": \"X-ray crystallography of UPF3B–UPF2 and UPF3A–UPF2 complexes, isothermal titration calorimetry (binding affinity measurements), mutagenesis of interface residues\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structures with quantitative binding affinity measurements and disease-mutation validation in a single study\",\n      \"pmids\": [\"35640974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"NMD remains partially active in human HCT116 cells lacking both UPF3 paralogs; the EJC-binding domain of UPF3 paralogs is dispensable for NMD activation; instead, the conserved 'mid' domain of UPF3 paralogs is consequential for NMD activity; UPF3A strongly activates NMD in cells lacking UPF3B, demonstrating functional redundancy.\",\n      \"method\": \"CRISPR/Cas9 knockout of UPF3A, UPF3B, or both; complementation with domain-deletion and EJC-binding-deficient mutants; NMD reporter assays and RNA-seq\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent studies (PMIDs 35451102 and 35451084) using CRISPR knockouts and complementation with domain mutants reach convergent conclusions\",\n      \"pmids\": [\"35451102\", \"35451084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Co-depletion of UPF3A and UPF3B markedly inhibits global NMD with transcriptome-wide upregulation of NMD substrates; EJC-binding-deficient or UPF2-binding-deficient UPF3B largely retains NMD activity when tested alone, but combinations of middle-domain deletions with these mutations show additive/synergistic NMD loss, indicating the middle domain is a critical functional unit.\",\n      \"method\": \"CRISPR/Cas9 knockout; NMD reporter assays; domain-deletion and point-mutant rescue experiments; transcriptome-wide RNA-seq\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — combinatorial knockout and domain-mutant rescue in human cells with transcriptome-wide validation, replicated by independent study\",\n      \"pmids\": [\"35451084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"UPF3B-mediated mRNA degradation occurs exclusively in the cytoplasm; nuclear mRNA export is required for UPF3B-mediated NMD even though the UPF3B fusion protein used is a nucleocytoplasmic shuttling protein predominantly localized to the nucleus.\",\n      \"method\": \"Tethered UPF3B combined with retroviral Rev/RRE-based nuclear export regulation; subcellular localization by fluorescence microscopy\",\n      \"journal\": \"RNA biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — functional tethering combined with nuclear export block provides clear mechanistic conclusion, but single lab, single study\",\n      \"pmids\": [\"17194930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"SATB2 directly binds the UPF3B promoter (shown by ChIP) and activates UPF3B transcription (shown by luciferase reporter assay); siRNA knockdown of SATB2 in HEK293 cells and Satb2-knockout mouse embryonic tissue both showed significantly decreased UPF3B expression, placing SATB2 as a transcriptional activator upstream of UPF3B.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), luciferase reporter assay, siRNA knockdown in HEK293 cells, Satb2-knockout mouse embryonic tissue analysis\",\n      \"journal\": \"Human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus reporter assay plus in vivo KO validation, multiple orthogonal methods but single lab\",\n      \"pmids\": [\"23925499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In UPF3B-null patient lymphoblastoid cells, UPF3A protein (but not mRNA) is stabilized in proportion inversely correlated with phenotype severity, demonstrating that UPF3B normally competes with UPF3A for UPF2 binding and that free UPF3A is unstable; UPF3A partially compensates for loss of UPF3B function.\",\n      \"method\": \"Western blot and qRT-PCR from patient-derived lymphoblastoid cell lines with UPF3B loss-of-function mutations; correlation analysis across multiple patients\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — protein/mRNA quantification in patient cells across multiple families, convergent with genetic evidence, but no direct biochemical binding competition assay\",\n      \"pmids\": [\"22182939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"UPF3B protein is expressed in neurons and localizes to dendritic spines in mouse primary hippocampal neurons as determined by immunofluorescence.\",\n      \"method\": \"Immunofluorescence/immunolocalization in mouse primary hippocampal neurons\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single-lab localization study without functional consequence directly tested for the dendritic spine localization\",\n      \"pmids\": [\"19238151\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Loss of Upf3b-NMD in neural progenitor cells results in expansion of cell numbers at the expense of differentiation; in primary hippocampal neurons, loss of Upf3b-NMD causes subtle neurite growth defects; Upf3b subcellular localization is regulated during development.\",\n      \"method\": \"siRNA knockdown of Upf3b in neural progenitor cells and primary hippocampal neurons; cell proliferation and differentiation assays; immunocytochemistry for subcellular localization\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with defined proliferation/differentiation phenotype in relevant cell type, complemented by localization data, single lab\",\n      \"pmids\": [\"23821644\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"UPF3B missense mutations associated with neurodevelopmental disorders reduce NMD activity in tethered function assays; expression of missense mutant UPF3B disturbs neuronal differentiation and reduces neurite branching complexity; NMD is downregulated during neural stem cell differentiation with concurrent downregulation of UPF3B and UPF1.\",\n      \"method\": \"Tethered function NMD assay with mutant UPF3B constructs; GFP-tagged UPF3B localization in neural stem cells and neurons; neurite complexity assay; NMD inhibitor (Amlexanox) treatment\",\n      \"journal\": \"Molecular brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tethered-function assay establishes reduced NMD activity of mutants; neuronal phenotype supported by pharmacological NMD inhibition as orthogonal approach; single lab\",\n      \"pmids\": [\"26012578\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Upf3b-null mice display deficits in fear-conditioned learning, profound impairment in prepulse inhibition, and deficient dendritic spine maturation in cortical pyramidal neurons in vivo; neural stem cells from Upf3b-null mice have impaired differentiation capacity.\",\n      \"method\": \"Upf3b-null mouse generation; behavioral testing (fear conditioning, Morris water maze, PPI); in vivo dendritic spine morphology; neural stem cell differentiation assay; RNA-seq of frontal cortex\",\n      \"journal\": \"Molecular psychiatry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — germline knockout mouse with multiple independent behavioral and cellular phenotypic readouts, confirmed by RNA-seq identification of direct NMD target transcripts\",\n      \"pmids\": [\"28948974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UPF3B directly interacts with IRE1α (the ER stress sensor); this interaction inhibits IRE1α kinase activity, abolishes autophosphorylation, and reduces IRE1α clustering; the disease-causing mutation UPF3BY160D abolishes the UPF3B–IRE1α interaction; phosphorylation of UPF3B at Thr169 abolishes its interaction with UPF2.\",\n      \"method\": \"Co-immunoprecipitation, kinase activity assay, autophosphorylation assay, IRE1α clustering assay, site-directed mutagenesis (Y160D, T169 phosphomimetic)\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — Co-IP plus kinase/autophosphorylation assay with mutagenesis, single lab, no structural validation\",\n      \"pmids\": [\"39138189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UPF3B binds the 3'-UTR of CDH1 mRNA to promote its degradation (via the truncated splice variant UPF3B-S generated by HnRNPR-dependent exon 8 exclusion); UPF3B-S overexpression enhances dephosphorylation of LATS1 and nuclear accumulation of YAP1, activating Hippo signaling.\",\n      \"method\": \"RNA immunoprecipitation (RIP), in vitro and in vivo HCC models with UPF3B-S knockdown/overexpression, Basescope assay\",\n      \"journal\": \"Journal of advanced research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — describes a truncated splice variant (UPF3B-S), not full-length canonical UPF3B; mechanisms attributed specifically to UPF3B-S; single lab, limited orthogonal validation of molecular mechanism\",\n      \"pmids\": [\"38402949\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"UPF3B binds to PPP2R2C mRNA (a regulatory subunit of PP2A) and promotes its degradation, activating the PI3K/AKT/mTOR pathway; E2F6 transcription factor directly binds the UPF3B promoter and activates its transcription.\",\n      \"method\": \"Co-immunoprecipitation, RNA-binding/pulldown assays, mRNA stability assays, luciferase reporter assay for E2F6-UPF3B promoter, in vivo and in vitro HCC proliferation assays\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, primarily Co-IP and reporter assays in a cancer context, limited orthogonal validation\",\n      \"pmids\": [\"38889220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"RBM8A interacts with UPF3B (shown by co-immunoprecipitation) to jointly regulate the stability of BBC3 (PUMA) mRNA, promoting its degradation in gastric cancer cells.\",\n      \"method\": \"Co-immunoprecipitation; RNA immunoprecipitation-seq; RNA pulldown; Actinomycin D mRNA stability assay\",\n      \"journal\": \"International journal of molecular medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP identifying RBM8A–UPF3B interaction; functional consequence attributed to the complex, single lab, no domain-level mechanistic dissection of UPF3B's role\",\n      \"pmids\": [\"40613240\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In cardiac sarcomeres, UPF3B (but not UPF1 or UPF2) localizes specifically to Z-discs, the presumed site of sarcomeric protein translation, suggesting a role for UPF3B-dependent NMD at the site of initial translation.\",\n      \"method\": \"Immunofluorescence localization in cardiac tissue from HCM patients and controls; RNA-seq gene set enrichment analysis\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — localization by immunofluorescence in cardiac tissue, single lab, no direct functional consequence of Z-disc localization experimentally tested\",\n      \"pmids\": [\"37797718\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UPF3B is a core NMD factor that bridges the exon junction complex (EJC) to the UPF surveillance machinery via its C-terminal domain binding to a composite surface on MAGO/Y14/eIF4AIII (crystal structure resolved) and via its middle NOPS-L domain binding to UPF2 (crystal structure with disease-mutation Y160D reducing affinity ~40-fold); beyond its EJC-adaptor role, UPF3B directly interacts with eukaryotic release factors and ribosomes to delay premature translation termination and promote post-termination ribosome dissociation (reconstituted in vitro), and also interacts with IRE1α to suppress ER stress; its paralog UPF3A is a functional redundant NMD activator whose protein stability is controlled by competition with UPF3B for UPF2 binding, and which can act as an NMD repressor in specific developmental contexts; loss of UPF3B in mice causes dendritic spine immaturity, impaired neural stem cell differentiation, and behavioral deficits consistent with its human intellectual disability phenotype.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UPF3B is a core nonsense-mediated mRNA decay (NMD) factor that couples the exon junction complex (EJC) to the UPF surveillance machinery and contributes directly to translation termination quality control [#0, #4]. Through its conserved C-terminal domain it binds the EJC, docking across a composite surface formed by eIF4AIII, MAGO, and Y14 as resolved by crystallography, with NMD-impairing mutations mapping to the core interface [#0, #2]. Its middle region (RRM-L plus NOPS-L) engages UPF2 via a hydrophobic cleft on the MIF4GIII domain and is the critical functional unit for RNA/ribosome binding and NMD activity; the EJC-binding domain itself is dispensable when the middle domain is intact, and combined disruption of both modules synergistically abolishes decay [#5, #6, #7]. Beyond its adaptor role, UPF3B directly interacts with eukaryotic release factors, UPF1, and the ribosome, delaying premature translation termination and dissociating post-termination ribosomal complexes in a reconstituted system [#4]. UPF3B activity is controlled by its paralog UPF3A: UPF3A binds UPF2 with higher affinity, competes with UPF3B for this interaction, is unstable when free, and functions as a redundant NMD activator (and context-dependent repressor) such that loss of UPF3B stabilizes UPF3A and is partially compensated [#3, #5, #10]. UPF3B also binds and inhibits the ER stress sensor IRE1\\u03b1, suppressing its kinase activity, autophosphorylation, and clustering [#15]. Loss of UPF3B impairs neural stem cell differentiation and dendritic spine maturation and produces learning and sensorimotor-gating deficits in mice, consistent with its causal role in X-linked intellectual disability, where missense mutations such as Y160D reduce NMD activity and weaken UPF2 binding ~40-fold [#5, #13, #14].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Established that UPF3B physically links the EJC to NMD, defining which of its protein contacts are essential for decay.\",\n      \"evidence\": \"Co-IP and lambdaN/boxB tethered-function NMD assays with RNAi rescue in human cells\",\n      \"pmids\": [\"12718880\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the structural basis of the Y14 interaction\", \"Roles of the middle domain and UPF2 contact in unmodified contexts unaddressed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Separated UPF3B's NMD-promoting activity from a distinct translation-stimulatory activity, showing the two map to different protein regions.\",\n      \"evidence\": \"Tethered-function assays with domain-swap and truncation mutants comparing UPF3a and UPF3b\",\n      \"pmids\": [\"16601204\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of translation stimulation not identified\", \"Did not test endogenous targets\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Provided the atomic-resolution picture of how UPF3B's C-terminal domain docks onto the EJC, explaining NMD-impairing mutations.\",\n      \"evidence\": \"X-ray structure (3.4 \\u00c5) of UPF3b C-terminal domain with eIF4AIII/MAGO/Y14/Barentsz/RNA plus interface mutagenesis\",\n      \"pmids\": [\"20479275\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure limited to C-terminal domain; middle/UPF2 interface not captured\", \"Did not address full-length conformational dynamics\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Reframed UPF3A versus UPF3B as functionally divergent paralogs, with UPF3A acting largely as an NMD inhibitor due to a weakened EJC-interaction domain.\",\n      \"evidence\": \"Conditional UPF3A knockout mice, in vitro NMD reporters, and transcriptome profiling\",\n      \"pmids\": [\"27040500\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tension with later evidence that UPF3A activates NMD in UPF3B-null cells\", \"Context-dependence of repressor versus activator role not fully resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated a direct biochemical role for UPF3B at the ribosome, distinct from its EJC-adaptor function.\",\n      \"evidence\": \"Fully reconstituted in vitro translation/termination system with release-factor and ribosome binding assays\",\n      \"pmids\": [\"28899899\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In-cell contribution of termination delay to NMD target selection not quantified\", \"Structural basis of eRF/ribosome contacts undetermined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined the UPF3B/UPF3A\\u2013UPF2 interface structurally and quantitatively, explaining how the disease mutation Y160D weakens UPF2 binding and how UPF3A out-competes UPF3B.\",\n      \"evidence\": \"Crystal structures of UPF2 MIF4GIII with UPF3B or UPF3A, ITC affinity measurements, interface mutagenesis\",\n      \"pmids\": [\"35640974\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not directly link affinity changes to in-cell NMD output across substrates\", \"Functional role of RNA-induced oligomerization not fully resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Established that the middle domain, not the EJC-binding domain, is the essential functional unit of UPF3 paralogs and that UPF3A and UPF3B are functionally redundant for NMD.\",\n      \"evidence\": \"CRISPR single and double knockouts in HCT116/human cells with domain-deletion and point-mutant complementation, NMD reporters and RNA-seq (two convergent EMBO J studies)\",\n      \"pmids\": [\"35451102\", \"35451084\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Residual EJC-independent NMD mechanism in double knockouts not fully defined\", \"Precise biochemical activity of the middle domain in decay unclear\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Localized UPF3B-mediated decay to the cytoplasm and showed nuclear mRNA export is a prerequisite despite the protein's predominantly nuclear steady-state localization.\",\n      \"evidence\": \"Tethered UPF3B with Rev/RRE-regulated export plus fluorescence localization\",\n      \"pmids\": [\"17194930\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab, single-study\", \"Did not define the trigger for cytoplasmic decay onset\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Placed UPF3B downstream of a defined transcriptional activator, SATB2, linking NMD-factor dosage to a developmental regulator.\",\n      \"evidence\": \"ChIP, luciferase reporter, siRNA knockdown in HEK293, and Satb2-knockout mouse tissue\",\n      \"pmids\": [\"23925499\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Physiological impact of SATB2-driven UPF3B levels on NMD output untested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showed that UPF3A protein stability is governed by competition with UPF3B for UPF2, providing a buffering mechanism for UPF3B loss.\",\n      \"evidence\": \"Western blot and qRT-PCR in UPF3B-null patient lymphoblastoid lines across multiple families\",\n      \"pmids\": [\"22182939\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct in vitro binding-competition assay in this study\", \"Correlation with phenotype severity not mechanistically dissected\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Connected UPF3B-dependent NMD to neural progenitor fate, showing its loss expands progenitors at the expense of differentiation.\",\n      \"evidence\": \"siRNA knockdown in neural progenitors and hippocampal neurons with proliferation/differentiation and localization assays\",\n      \"pmids\": [\"23821644\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Knockdown rather than complete loss\", \"Direct NMD targets driving the phenotype not identified here\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Linked disease-associated UPF3B missense mutations to reduced NMD activity and impaired neuronal differentiation, and showed NMD is downregulated during neural differentiation.\",\n      \"evidence\": \"Tethered-function NMD assays with mutant constructs, neurite complexity assays, and pharmacological NMD inhibition in neural stem cells\",\n      \"pmids\": [\"26012578\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Causal NMD targets for neurite phenotype not defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Provided in vivo evidence that UPF3B loss causes dendritic spine immaturity, impaired neural stem cell differentiation, and behavioral deficits modeling the human disorder.\",\n      \"evidence\": \"Upf3b-null mice with behavioral testing, in vivo spine morphology, NSC differentiation, and frontal cortex RNA-seq\",\n      \"pmids\": [\"28948974\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which dysregulated NMD targets are causal for each behavioral phenotype unresolved\", \"Cell-type-specific contributions not dissected\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified a non-NMD function for UPF3B as a direct inhibitor of the ER stress sensor IRE1\\u03b1, and a phospho-switch (Thr169) controlling UPF2 binding.\",\n      \"evidence\": \"Co-IP, kinase and autophosphorylation assays, clustering assays, and Y160D/T169 mutagenesis\",\n      \"pmids\": [\"39138189\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural validation of the UPF3B\\u2013IRE1\\u03b1 interface\", \"Single lab; physiological relevance of the T169 phospho-switch untested in vivo\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Reported UPF3B (and a truncated UPF3B-S variant) acting on specific cancer-relevant transcripts to modulate Hippo and PI3K/AKT/mTOR signaling.\",\n      \"evidence\": \"RIP, mRNA stability assays, reporter assays, and HCC models for CDH1/PPP2R2C regulation\",\n      \"pmids\": [\"38402949\", \"38889220\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Mechanisms partly attributed to a truncated variant rather than canonical UPF3B; limited orthogonal validation\", \"Generality beyond cancer context unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Reported UPF3B partnering with RBM8A to regulate BBC3/PUMA mRNA stability in gastric cancer.\",\n      \"evidence\": \"Co-IP, RIP-seq, RNA pulldown, and Actinomycin D mRNA stability assay\",\n      \"pmids\": [\"40613240\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single Co-IP without domain-level dissection of UPF3B's role\", \"Functional consequence attributed to the complex, not UPF3B specifically\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How UPF3B's reconstituted termination/ribosome-dissociation activity, its EJC-independent middle-domain function, and its non-NMD interactions (IRE1\\u03b1, tissue-specific localization) are integrated into substrate-specific decay in vivo remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No integrated model linking termination delay to in-cell target selection\", \"Structural basis of non-EJC interactions undefined\", \"Tissue-specific roles (e.g., cardiac Z-disc localization) functionally untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 4, 6]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"complexes\": [\"exon junction complex (EJC)\"],\n    \"partners\": [\"UPF2\", \"Y14\", \"MAGOH\", \"EIF4A3\", \"UPF1\", \"UPF3A\", \"IRE1\\u03b1\", \"RBM8A\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}