{"gene":"UPF2","run_date":"2026-06-10T10:51:56","timeline":{"discoveries":[{"year":2007,"finding":"UPF2 and UPF3b cooperatively stimulate both ATPase and RNA helicase activities of UPF1 within a heptameric complex assembled on RNA with the EJC core (eIF4AIII, MAGOH, Y14). The EJC proteins provide a composite binding site for UPF3b that bridges to UPF2 and UPF1.","method":"In vitro reconstitution of recombinant EJC core + UPF1/2/3b complex on RNA; ATPase and helicase activity assays","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with recombinant proteins, functional helicase/ATPase assays, multiple orthogonal methods","pmids":["18066079"],"is_preprint":false},{"year":2011,"finding":"Crystal structures of Upf1 with and without its CH domain reveal that in isolation Upf1 clamps onto RNA. Upon UPF2 binding, the regulatory CH domain of Upf1 undergoes a large conformational change that causes the catalytic helicase domain to bind RNA less extensively, switching Upf1 from an RNA-clamping mode to an RNA-unwinding mode.","method":"Crystal structures of Upf1 with ADP:AlF4⁻ and RNA (transition-state analogue), with and without CH domain; biochemical ATPase/helicase assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures plus functional biochemical assays, multiple orthogonal methods in one rigorous study","pmids":["21419344"],"is_preprint":false},{"year":2001,"finding":"Human UPF2 interacts with hUPF1, hUPF3b-X, and hUPF3 via defined protein domains. hUPF2 localizes primarily to the cytoplasm (with hUPF1), while hUPF3b-X localizes primarily to nuclei and shuttles.","method":"Co-immunoprecipitation of epitope-tagged proteins in HeLa cells; indirect immunofluorescence; domain mapping by deletion constructs","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — reciprocal Co-IP with domain mapping plus localization by immunofluorescence, single lab but multiple orthogonal methods","pmids":["11113196"],"is_preprint":false},{"year":2004,"finding":"Crystal structure of the UPF2 MIF4G domain in complex with the UPF3b RNP domain at 1.95 Å reveals that the protein-protein interface is mediated by highly conserved charged residues; the UPF3b RNP beta-sheet surface (normally used for nucleic acid binding) is used for protein-protein interaction and does not bind RNA, whereas UPF2 retains RNA-binding capacity.","method":"X-ray crystallography (1.95 Å); RNA binding assays; mutational analysis","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional binding assays and mutagenesis in one rigorous study","pmids":["15004547"],"is_preprint":false},{"year":2005,"finding":"CBP80 interacts with UPF1 and promotes the interaction of UPF1 with UPF2 during NMD of CBP80-bound (pioneer round) mRNAs, but does not promote UPF1 interaction with Staufen1 in SMD.","method":"Co-immunoprecipitation in mammalian cells; NMD reporter assays; siRNA knockdown","journal":"Nature structural & molecular biology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — Co-IP plus functional NMD assays, single lab, two orthogonal methods","pmids":["16186820"],"is_preprint":false},{"year":2006,"finding":"Crystal structure (3 Å) of the UPF1 cysteine-histidine-rich (CH) domain reveals a unique combination of three zinc-binding motifs in two tandem modules related to RING-box and U-box domains. Mutational analysis identifies two distinct conserved surface regions of UPF1 CH domain that mediate interaction with UPF2.","method":"X-ray crystallography (3 Å); site-directed mutagenesis; binding assays","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus mutagenesis defining interaction residues, rigorous single study","pmids":["16931876"],"is_preprint":false},{"year":2013,"finding":"Crystal structures of UPF2 MIF4G-1 and MIF4G-2 show N-terminal capping helices essential for MIF4G core stabilization; MIF4G-2 interacts with MIF4G-3, forming a rigid assembly. MIF4G-3 is the binding site and in vitro substrate of SMG1 kinase, and a ternary UPF2 MIF4G-3/UPF3b/SMG1 complex forms in vitro. MIF4G-1 and MIF4G-2 have an essential scaffolding role for NMD, while MIF4G-3 plus the UPF1-binding region is the minimal module required to trigger NMD.","method":"Crystal structures; in vitro kinase assay (SMG1 phosphorylation of UPF2 MIF4G-3); in vitro complex assembly; in vivo complementation assays; tethering assays","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures combined with in vitro kinase/binding assays and in vivo functional complementation, multiple orthogonal methods","pmids":["24271394"],"is_preprint":false},{"year":2014,"finding":"SMG1C (SMG1-SMG8-SMG9) recruits UPF1 and UPF2 to distinct sites near the kinase domain. UPF2 binds SMG1 at its FRB domain in a UPF1-independent manner. UPF2 can be transferred to UPF1 within SMG1C, inducing UPF2-dependent conformational changes that activate UPF1 within an SMG1C-UPF1-UPF2 complex.","method":"Electron microscopy; in vivo and in vitro interaction analyses; competition experiments; mutagenesis","journal":"Structure","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EM structures combined with in vivo/vitro binding assays and mutagenesis, single lab","pmids":["25002321"],"is_preprint":false},{"year":2014,"finding":"Crystal structure of the N-terminal mIF4G domain of yeast Upf2 reveals a highly conserved region essential for NMD that is independent of Upf2 binding sites for Upf1 and Upf3. Mutations in this region inactivate NMD and disrupt Upf2 binding to Dbp6, a DEAD-box helicase, suggesting Upf2 acts as a platform for additional NMD factors.","method":"X-ray crystallography; site-directed mutagenesis; NMD functional assays; co-immunoprecipitation","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — crystal structure plus mutagenesis and functional assays, single lab","pmids":["25277656"],"is_preprint":false},{"year":2016,"finding":"UPF2 directly interacts with eukaryotic release factor eRF3, associates with the SURF complex and ribosomes in cells in a UPF3-independent manner. The eRF3 binding site maps to the C-terminal part of UPF2, overlapping partially with the UPF3b-binding site. UPF2 binds UPF3b more strongly than eRF3, and UPF3b interaction interferes with UPF2-eRF3 complex assembly.","method":"Biochemical binding assays; electron microscopy of UPF2-eRF3 complex; deletion mapping; co-immunoprecipitation from cells","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EM structure plus biochemical binding assays and in-cell Co-IP, single lab, multiple orthogonal methods","pmids":["26740584"],"is_preprint":false},{"year":2002,"finding":"In yeast, deletion of UPF2 (or UPF1 and UPF3) enhances synthesis of CPSase A encoded by CPA1, an effect that depends on the presence of the CPA1 uORF, showing that the NMD complex destabilizes the 5' end of the CPA1 mRNA and that NMD cooperates with arginine-mediated translational repression.","method":"Yeast genetic deletion analysis; enzymatic activity assays; reporter assays with uORF mutants","journal":"Current genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with functional metabolic readout, yeast model organism, single lab","pmids":["12172963"],"is_preprint":false},{"year":2006,"finding":"Upf1 and Upf2 associate with NMD-sensitive AUF1 3'-UTR splice variant mRNAs in cells (RNP immunoprecipitation). Knockdown of Upf1/Upf2 by RNAi specifically stabilizes NMD-sensitive AUF1 mRNA variants containing exon-exon junctions >50 nt downstream of the stop codon, providing evidence that NMD and ARE-mediated decay pathways are linked.","method":"siRNA knockdown of Upf1/Upf2; RT-qPCR mRNA stability assays; RNP immunoprecipitation; dominant-negative Upf1 transfection","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — RNP-IP plus RNAi functional assays with multiple orthogonal approaches, single lab","pmids":["17000771"],"is_preprint":false},{"year":2011,"finding":"In Drosophila, loss-of-function of upf1 and upf2 inhibits cell growth and induces apoptosis through a Upf3-independent pathway. A mutant Upf2 unable to bind Upf3 still causes lethality, while disruption of Upf2-Upf1 interaction causes death, indicating that the Upf2-Upf1 interaction (not Upf2-Upf3) is essential for viability and NMD of most targets.","method":"Drosophila loss-of-function genetics; epistasis analysis with upf3 mutants; Upf2 binding-domain mutants; cell growth and apoptosis assays","journal":"RNA (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with multiple mutant combinations in Drosophila, single lab","pmids":["21317294"],"is_preprint":false},{"year":2022,"finding":"Binding of UPF2 to UPF1 drastically reduces UPF1's affinity for RNA, causing release of bound RNA through an allosteric mechanism (not direct competition for RNA binding), mediated by the conformational change in UPF1 induced upon UPF2 binding.","method":"Biochemical binding assays (fluorescence anisotropy, filter binding); biophysical methods; in vitro RNA-release assays; mutational analysis","journal":"RNA (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — multiple orthogonal biochemical and biophysical methods establishing allosteric mechanism, single lab","pmids":["36456182"],"is_preprint":false},{"year":2022,"finding":"Crystal and cryo-EM structures of UPF2 MIF4GIII in complex with UPF3B or UPF3A reveal unexpectedly intimate binding interfaces. UPF3B disease-causing mutation Y160D in the NOPS-L domain displaces Y160 from a hydrophobic cleft in UPF2, reducing binding affinity ~40-fold. UPF3A binds UPF2 with ~10-fold higher affinity than UPF3B via NOPS-L residues, explaining competitive binding and compensatory upregulation.","method":"X-ray crystallography and cryo-EM structures; isothermal titration calorimetry / binding affinity measurements; mutagenesis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal/cryo-EM structures plus quantitative binding measurements and mutagenesis, multiple orthogonal methods","pmids":["35640974"],"is_preprint":false},{"year":2024,"finding":"The SMG6 endonuclease contains a conserved short linear motif that binds the UPF1 CH domain (the same domain that binds UPF2), making SMG6 and UPF2 binding to UPF1 mutually exclusive. Cryo-EM data suggest that distinct SMG6-containing and UPF2-containing NMD complexes are dictated by different conformational states linked to UPF1's RNA-binding status.","method":"Mass spectrometry; cryo-EM; biochemical interaction assays; competition experiments; mutagenesis","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — cryo-EM plus mass spectrometry and biochemical competition assays with mutagenesis, multiple orthogonal methods","pmids":["38709891"],"is_preprint":false},{"year":2025,"finding":"UPF2 binds RNA dynamically: MIF4G-1 and MIF4G-3 are the main RNA/DNA-binding modules; MIF4G-3 has RNA annealing activity; full-length UPF2 unfolds a reporter hairpin RNA structure. UPF2 preferentially binds and stabilizes single-stranded RNA in a sequence-independent manner and undergoes a conformational change upon ssRNA binding.","method":"Nucleic acid binding assays; RNA annealing/unfolding assays; biochemical and biophysical methods; domain deletion analysis","journal":"RNA (New York, N.Y.)","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — multiple biochemical/biophysical assays with domain mapping, single lab","pmids":["40246535"],"is_preprint":false},{"year":2025,"finding":"UPF2 binds the exoribonuclease 3'hExo (involved in histone mRNA decay), and UPF2-mediated activation of UPF1 overrides the inhibitory effect of SLBP on UPF1's unwinding activity during histone mRNA degradation.","method":"Direct interaction assays (recombinant proteins); in vitro helicase/unwinding assays; functional mRNA decay assays in cells","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical interaction and functional assays, preprint, single lab","pmids":[],"is_preprint":true},{"year":2025,"finding":"In yeast, Dcp2 and Upf2 compete for the same binding site in the Upf1 N-terminal CH domain, accounting for the existence of two mutually exclusive Upf1-containing complexes (Upf1-Upf2/3 surveillance complex and Upf1-decapping complex).","method":"Recombinant protein interaction assays; competition binding experiments; pulldowns","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical competition assays with recombinant proteins, preprint, single lab","pmids":[],"is_preprint":true},{"year":2025,"finding":"SARS-CoV-2 nucleocapsid protein (Np) binds directly to UPF2, disrupting formation of the UPF1/UPF2 complex and negating the stimulatory effect of UPF2 on UPF1 catalytic activity, thereby inhibiting cellular NMD.","method":"Biochemical and biophysical binding assays; ATPase/helicase activity assays; cellular NMD reporter assays","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical/biophysical methods plus cellular assays, single peer-reviewed study","pmids":["39831305"],"is_preprint":false},{"year":2016,"finding":"Upf2 protein is present in both cytoplasmic and nucleoplasmic fractions of human cells. Upf2 interacts with the EJC core factor RBM8A (Y14) not only in the cytoplasm but also in the intranuclear region, as shown by proximity ligation assay.","method":"Subcellular fractionation; western blotting; immunofluorescence; in situ proximity ligation assay; RNase treatment","journal":"Molecular medicine reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single method per finding (fractionation + PLA), no direct functional consequence linked to nuclear localization","pmids":["27221324"],"is_preprint":false},{"year":2013,"finding":"Proteasome inhibitors cause accumulation of UPF2 protein levels in cells; knockdown of SMG1 also upregulates UPF2 protein levels; these effects are additive, suggesting UPS and SMG1 regulate UPF2 protein stability via distinct pathways.","method":"Proteasome inhibitor treatment; siRNA knockdown of SMG1; western blotting","journal":"International journal of oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, pharmacological inhibitor and knockdown without direct mechanistic identification of the E3 ligase or modification site","pmids":["24173962"],"is_preprint":false},{"year":2002,"finding":"hUPF2-dependent NMD pathway operating in the nuclear fraction of cells is required for down-regulation of PTC-bearing TCR-beta transcripts; this requires translation features (initiator ATG, scanning) suggesting cytoplasmic ribosomes act on nuclear-associated mRNAs.","method":"Antisense hUPF2 knockdown; TCR-beta PTC reporter assays; nuclear/cytoplasmic fractionation; inhibition of translation initiation","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — antisense knockdown plus multiple reporter/fractionation assays, single lab","pmids":["11889124"],"is_preprint":false},{"year":2024,"finding":"Chemical cross-linking mass spectrometry reveals that UPF2 acts as a connection bridge between SMG1 and SMG7 in the NMD machinery; UPF2 N-terminal forms most interactions with SMG7; MIF4G-I, II, and III domains contact SMG1 or SMG7.","method":"Chemical cross-linking mass spectrometry (CLMS); structural modeling","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — CLMS is a single method, largely computational modeling, single lab","pmids":["38542156"],"is_preprint":false}],"current_model":"UPF2 is a core NMD scaffold protein that bridges UPF3b (via its MIF4G-3 domain, which contacts UPF3b's RNP/NOPS-L domain) to UPF1 (via its C-terminal region binding the UPF1 CH domain), and upon binding to UPF1 induces a large conformational change that switches UPF1 from an RNA-clamping to an RNA-unwinding mode while simultaneously reducing UPF1's RNA affinity through allostery; additionally, UPF2 itself binds RNA via MIF4G-1 and MIF4G-3 and can unfold RNA structures, acts as a substrate and docking site for SMG1 kinase, interacts with eRF3 and the SURF complex at the ribosome, and competes with SMG6 and Dcp2 for the UPF1 CH domain, placing UPF2 at the center of a network of mutually exclusive complexes that orchestrate NMD initiation and mRNA degradation."},"narrative":{"mechanistic_narrative":"UPF2 is a central scaffold of the nonsense-mediated mRNA decay (NMD) pathway, physically bridging the surveillance factors UPF3b and UPF1 and acting as the conformational switch that activates UPF1 once a premature termination event is recognized [PMID:18066079, PMID:11113196]. Through its modular MIF4G domains it organizes a heptameric surveillance complex assembled on RNA with the exon-junction complex core, in which UPF3b is bridged from the EJC to UPF1, and UPF2/UPF3b cooperatively stimulate the ATPase and helicase activities of UPF1 [PMID:18066079]. Structurally, the UPF2 MIF4G-3 domain engages the UPF3b RNP/NOPS-L surface through an intimate, conserved interface, while MIF4G-1 and MIF4G-2 provide an essential scaffolding role and MIF4G-3 together with the UPF1-binding region constitutes the minimal NMD-triggering module [PMID:15004547, PMID:24271394, PMID:35640974]. UPF2 binds the UPF1 cysteine-histidine-rich (CH) domain, and this contact drives a large conformational change in UPF1 that converts it from an RNA-clamping state to an RNA-unwinding state while allosterically lowering UPF1's RNA affinity to release bound RNA [PMID:21419344, PMID:16931876, PMID:36456182]. Beyond scaffolding, UPF2 is itself a nucleic-acid-binding protein: its MIF4G-1 and MIF4G-3 modules bind and stabilize single-stranded RNA and MIF4G-3 possesses RNA annealing activity, allowing full-length UPF2 to unfold structured RNA [PMID:40246535]. UPF2 also serves as a substrate and docking site for the SMG1 kinase complex, which recruits UPF1 and UPF2 to distinct sites and transfers UPF2 onto UPF1 to activate it, and it contacts the eukaryotic release factor eRF3 and the SURF complex at terminating ribosomes through its C-terminal region that overlaps the UPF3b site [PMID:24271394, PMID:25002321, PMID:26740584]. By occupying the UPF1 CH domain, UPF2 competes with the SMG6 endonuclease and the decapping enzyme Dcp2, placing it at the center of mutually exclusive UPF1-containing complexes that partition surveillance from degradation [PMID:38709891]. Loss of UPF2 stabilizes PTC-bearing transcripts and other NMD substrates, and the UPF2-UPF1 interaction is essential for viability and for decay of most targets [PMID:21317294, PMID:11889124].","teleology":[{"year":2001,"claim":"Established UPF2 as a physical hub of the human NMD machinery by defining its direct, domain-specific contacts with UPF1 and UPF3b and its predominantly cytoplasmic localization.","evidence":"Reciprocal Co-IP of epitope-tagged proteins with deletion mapping and immunofluorescence in HeLa cells","pmids":["11113196"],"confidence":"Medium","gaps":["Did not resolve the structural basis of the interactions","Did not determine the functional consequence of complex assembly on UPF1 activity"]},{"year":2002,"claim":"Demonstrated that UPF2 is functionally required for NMD by showing its depletion stabilizes PTC-bearing transcripts in a translation-dependent manner.","evidence":"Antisense hUPF2 knockdown with TCR-beta PTC reporters, fractionation, and translation-initiation inhibition; yeast UPF2 deletion with CPA1 uORF reporters","pmids":["11889124","12172963"],"confidence":"Medium","gaps":["Did not define how UPF2 promotes decay biochemically","Did not identify the degradation machinery engaged downstream"]},{"year":2004,"claim":"Resolved the molecular basis of the UPF2-UPF3b interaction, showing UPF3b uses its RNP surface for protein contact while UPF2 retains RNA binding.","evidence":"1.95 A crystal structure of UPF2 MIF4G domain bound to UPF3b RNP with RNA-binding and mutational assays","pmids":["15004547"],"confidence":"High","gaps":["Did not address how this interface positions UPF1","Did not establish the role of other MIF4G domains"]},{"year":2006,"claim":"Defined the UPF1 CH domain structure and identified the surfaces it uses to bind UPF2, the contact later shown to be a competitive hub.","evidence":"3 A crystal structure of the UPF1 CH domain with site-directed mutagenesis and binding assays","pmids":["16931876"],"confidence":"High","gaps":["Did not show the conformational consequence of UPF2 binding","Did not address competition with other CH-domain partners"]},{"year":2007,"claim":"Reconstituted the activating mechanism, showing UPF2 and UPF3b cooperatively stimulate UPF1 ATPase and helicase activity within an EJC-bridged surveillance complex.","evidence":"In vitro reconstitution of recombinant EJC core plus UPF1/2/3b on RNA with ATPase and helicase assays","pmids":["18066079"],"confidence":"High","gaps":["Did not provide a structural mechanism for the activation","Did not establish how this couples to downstream decay"]},{"year":2011,"claim":"Provided the structural mechanism of UPF1 activation, showing UPF2 binding triggers a CH-domain conformational change that switches UPF1 from RNA-clamping to RNA-unwinding.","evidence":"Crystal structures of Upf1 with transition-state analogue and RNA, with and without CH domain, plus ATPase/helicase assays","pmids":["21419344"],"confidence":"High","gaps":["Did not quantify the effect on RNA affinity","Did not place the switch within the full SMG1-coupled pathway"]},{"year":2013,"claim":"Dissected the modular architecture of UPF2, assigning scaffolding to MIF4G-1/2 and the NMD-triggering function plus SMG1 substrate role to MIF4G-3.","evidence":"Crystal structures of MIF4G-1 and MIF4G-2, in vitro SMG1 kinase and complex-assembly assays, and in vivo complementation/tethering","pmids":["24271394"],"confidence":"High","gaps":["Did not determine the consequence of MIF4G-3 phosphorylation","Did not define MIF4G-1/2 binding partners that explain the scaffolding role"]},{"year":2014,"claim":"Linked UPF2 to the SMG1 kinase complex, showing it docks on SMG1 independently of UPF1 and is transferred to UPF1 to activate it, and identified a UPF3-independent scaffolding region.","evidence":"EM of SMG1C with in vivo/in vitro interaction and competition assays; yeast Upf2 N-terminal mIF4G crystal structure with NMD assays and Dbp6 Co-IP","pmids":["25002321","25277656"],"confidence":"Medium","gaps":["Did not resolve the order of events during in-cell activation","Mechanism of UPF2 transfer within SMG1C not structurally defined"]},{"year":2016,"claim":"Connected UPF2 to translation termination by demonstrating direct binding to eRF3 and ribosome/SURF association through a C-terminal region overlapping the UPF3b site.","evidence":"Biochemical binding assays, EM of UPF2-eRF3, deletion mapping, and in-cell Co-IP","pmids":["26740584"],"confidence":"Medium","gaps":["Did not establish the in vivo timing of eRF3 versus UPF3b engagement","Functional consequence of eRF3 binding for NMD not directly tested"]},{"year":2022,"claim":"Refined the activation mechanism by showing UPF2 binding allosterically lowers UPF1 RNA affinity to release RNA, and resolved how UPF3 isoform binding competition and a disease mutation tune the UPF2 interface.","evidence":"Fluorescence anisotropy/filter binding RNA-release assays; crystal and cryo-EM structures of MIF4GIII with UPF3B/UPF3A and ITC affinity measurements with mutagenesis","pmids":["36456182","35640974"],"confidence":"High","gaps":["Did not link RNA release timing to substrate commitment in cells","Functional outcome of UPF3A/UPF3B competition on target selection not fully resolved"]},{"year":2024,"claim":"Established UPF2 as the determinant of mutually exclusive UPF1 complexes by showing SMG6 binds the same CH domain as UPF2, partitioning surveillance from endonucleolytic decay.","evidence":"Mass spectrometry, cryo-EM, and biochemical competition/mutagenesis assays","pmids":["38709891"],"confidence":"High","gaps":["Did not define the in-cell trigger that switches between complexes","Conformational coupling to UPF1 RNA status inferred rather than directly imaged"]},{"year":2025,"claim":"Characterized UPF2 as an intrinsic RNA-binding/remodeling protein and extended its role to histone mRNA decay and viral antagonism, while implicating Dcp2 competition for the UPF1 CH domain.","evidence":"Nucleic-acid binding/annealing/unfolding assays with domain mapping; recombinant interaction and helicase assays for 3'hExo/SLBP; binding and NMD reporter assays for SARS-CoV-2 Np; recombinant competition assays for Dcp2 (preprint)","pmids":["40246535","39831305"],"confidence":"Medium","gaps":["3'hExo and Dcp2 findings rest on preprint/single-lab biochemistry awaiting peer-reviewed confirmation","In-cell relevance of UPF2 RNA-remodeling activity not established","Mechanism by which viral Np outcompetes UPF1 in vivo not resolved"]},{"year":null,"claim":"How the competing UPF2-, SMG6-, and Dcp2-containing UPF1 complexes are temporally ordered and switched in cells to commit a transcript to decay remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No in-cell measurement of complex exchange kinetics","No structure of the complete surveillance-to-decay transition","Regulation of UPF2 stability and its impact on pathway flux not mechanistically defined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[3,16]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,2,6]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,13,0]},{"term_id":"GO:0140098","term_label":"catalytic activity, acting on RNA","supporting_discovery_ids":[16]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[22]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,6,22]}],"complexes":["NMD surveillance complex (UPF1-UPF2-UPF3b)","exon junction complex (EJC core)","SURF complex","SMG1C (SMG1-SMG8-SMG9)"],"partners":["UPF1","UPF3B","UPF3A","SMG1","ETF3H (ERF3)","RBM8A","SMG6","DCP2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9HAU5","full_name":"Regulator of nonsense transcripts 2","aliases":["Up-frameshift suppressor 2 homolog","hUpf2"],"length_aa":1272,"mass_kda":147.8,"function":"Involved in nonsense-mediated decay (NMD) of mRNAs containing premature stop codons by associating with the nuclear exon junction complex (EJC). Recruited by UPF3B associated with the EJC core 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 UPF3B stimulates both ATPase and RNA helicase activities of UPF1. Binds spliced mRNA","subcellular_location":"Cytoplasm, perinuclear region; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q9HAU5/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/UPF2","classification":"Common Essential","n_dependent_lines":1189,"n_total_lines":1208,"dependency_fraction":0.984271523178808},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"UPF1","stoichiometry":10.0},{"gene":"ACTR2","stoichiometry":0.2},{"gene":"EIF4A3","stoichiometry":0.2},{"gene":"GSPT1","stoichiometry":0.2},{"gene":"H2AFZ","stoichiometry":0.2},{"gene":"RBM8A","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/UPF2","total_profiled":1310},"omim":[{"mim_id":"615241","title":"TERMINAL DIFFERENTIATION-INDUCED NONCODING RNA; TINCR","url":"https://www.omim.org/entry/615241"},{"mim_id":"607032","title":"SMG1 NONSENSE-MEDIATED mRNA DECAY-ASSOCIATED PI3K-RELATED KINASE; SMG1","url":"https://www.omim.org/entry/607032"},{"mim_id":"606711","title":"SPECIFIC LANGUAGE IMPAIRMENT 1; SLI1","url":"https://www.omim.org/entry/606711"},{"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":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Vesicles","reliability":"Additional"},{"location":"Primary cilium","reliability":"Additional"},{"location":"Primary cilium tip","reliability":"Additional"},{"location":"Cytoplasmic bodies","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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The EJC proteins provide a composite binding site for UPF3b that bridges to UPF2 and UPF1.\",\n      \"method\": \"In vitro reconstitution of recombinant EJC core + UPF1/2/3b complex on RNA; ATPase and helicase activity assays\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with recombinant proteins, functional helicase/ATPase assays, multiple orthogonal methods\",\n      \"pmids\": [\"18066079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Crystal structures of Upf1 with and without its CH domain reveal that in isolation Upf1 clamps onto RNA. Upon UPF2 binding, the regulatory CH domain of Upf1 undergoes a large conformational change that causes the catalytic helicase domain to bind RNA less extensively, switching Upf1 from an RNA-clamping mode to an RNA-unwinding mode.\",\n      \"method\": \"Crystal structures of Upf1 with ADP:AlF4⁻ and RNA (transition-state analogue), with and without CH domain; biochemical ATPase/helicase assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures plus functional biochemical assays, multiple orthogonal methods in one rigorous study\",\n      \"pmids\": [\"21419344\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human UPF2 interacts with hUPF1, hUPF3b-X, and hUPF3 via defined protein domains. hUPF2 localizes primarily to the cytoplasm (with hUPF1), while hUPF3b-X localizes primarily to nuclei and shuttles.\",\n      \"method\": \"Co-immunoprecipitation of epitope-tagged proteins in HeLa cells; indirect immunofluorescence; domain mapping by deletion constructs\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — reciprocal Co-IP with domain mapping plus localization by immunofluorescence, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"11113196\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crystal structure of the UPF2 MIF4G domain in complex with the UPF3b RNP domain at 1.95 Å reveals that the protein-protein interface is mediated by highly conserved charged residues; the UPF3b RNP beta-sheet surface (normally used for nucleic acid binding) is used for protein-protein interaction and does not bind RNA, whereas UPF2 retains RNA-binding capacity.\",\n      \"method\": \"X-ray crystallography (1.95 Å); RNA binding assays; mutational analysis\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional binding assays and mutagenesis in one rigorous study\",\n      \"pmids\": [\"15004547\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"CBP80 interacts with UPF1 and promotes the interaction of UPF1 with UPF2 during NMD of CBP80-bound (pioneer round) mRNAs, but does not promote UPF1 interaction with Staufen1 in SMD.\",\n      \"method\": \"Co-immunoprecipitation in mammalian cells; NMD reporter assays; siRNA knockdown\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — Co-IP plus functional NMD assays, single lab, two orthogonal methods\",\n      \"pmids\": [\"16186820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Crystal structure (3 Å) of the UPF1 cysteine-histidine-rich (CH) domain reveals a unique combination of three zinc-binding motifs in two tandem modules related to RING-box and U-box domains. Mutational analysis identifies two distinct conserved surface regions of UPF1 CH domain that mediate interaction with UPF2.\",\n      \"method\": \"X-ray crystallography (3 Å); site-directed mutagenesis; binding assays\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus mutagenesis defining interaction residues, rigorous single study\",\n      \"pmids\": [\"16931876\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structures of UPF2 MIF4G-1 and MIF4G-2 show N-terminal capping helices essential for MIF4G core stabilization; MIF4G-2 interacts with MIF4G-3, forming a rigid assembly. MIF4G-3 is the binding site and in vitro substrate of SMG1 kinase, and a ternary UPF2 MIF4G-3/UPF3b/SMG1 complex forms in vitro. MIF4G-1 and MIF4G-2 have an essential scaffolding role for NMD, while MIF4G-3 plus the UPF1-binding region is the minimal module required to trigger NMD.\",\n      \"method\": \"Crystal structures; in vitro kinase assay (SMG1 phosphorylation of UPF2 MIF4G-3); in vitro complex assembly; in vivo complementation assays; tethering assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures combined with in vitro kinase/binding assays and in vivo functional complementation, multiple orthogonal methods\",\n      \"pmids\": [\"24271394\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"SMG1C (SMG1-SMG8-SMG9) recruits UPF1 and UPF2 to distinct sites near the kinase domain. UPF2 binds SMG1 at its FRB domain in a UPF1-independent manner. UPF2 can be transferred to UPF1 within SMG1C, inducing UPF2-dependent conformational changes that activate UPF1 within an SMG1C-UPF1-UPF2 complex.\",\n      \"method\": \"Electron microscopy; in vivo and in vitro interaction analyses; competition experiments; mutagenesis\",\n      \"journal\": \"Structure\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EM structures combined with in vivo/vitro binding assays and mutagenesis, single lab\",\n      \"pmids\": [\"25002321\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Crystal structure of the N-terminal mIF4G domain of yeast Upf2 reveals a highly conserved region essential for NMD that is independent of Upf2 binding sites for Upf1 and Upf3. Mutations in this region inactivate NMD and disrupt Upf2 binding to Dbp6, a DEAD-box helicase, suggesting Upf2 acts as a platform for additional NMD factors.\",\n      \"method\": \"X-ray crystallography; site-directed mutagenesis; NMD functional assays; co-immunoprecipitation\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — crystal structure plus mutagenesis and functional assays, single lab\",\n      \"pmids\": [\"25277656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"UPF2 directly interacts with eukaryotic release factor eRF3, associates with the SURF complex and ribosomes in cells in a UPF3-independent manner. The eRF3 binding site maps to the C-terminal part of UPF2, overlapping partially with the UPF3b-binding site. UPF2 binds UPF3b more strongly than eRF3, and UPF3b interaction interferes with UPF2-eRF3 complex assembly.\",\n      \"method\": \"Biochemical binding assays; electron microscopy of UPF2-eRF3 complex; deletion mapping; co-immunoprecipitation from cells\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EM structure plus biochemical binding assays and in-cell Co-IP, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"26740584\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"In yeast, deletion of UPF2 (or UPF1 and UPF3) enhances synthesis of CPSase A encoded by CPA1, an effect that depends on the presence of the CPA1 uORF, showing that the NMD complex destabilizes the 5' end of the CPA1 mRNA and that NMD cooperates with arginine-mediated translational repression.\",\n      \"method\": \"Yeast genetic deletion analysis; enzymatic activity assays; reporter assays with uORF mutants\",\n      \"journal\": \"Current genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with functional metabolic readout, yeast model organism, single lab\",\n      \"pmids\": [\"12172963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Upf1 and Upf2 associate with NMD-sensitive AUF1 3'-UTR splice variant mRNAs in cells (RNP immunoprecipitation). Knockdown of Upf1/Upf2 by RNAi specifically stabilizes NMD-sensitive AUF1 mRNA variants containing exon-exon junctions >50 nt downstream of the stop codon, providing evidence that NMD and ARE-mediated decay pathways are linked.\",\n      \"method\": \"siRNA knockdown of Upf1/Upf2; RT-qPCR mRNA stability assays; RNP immunoprecipitation; dominant-negative Upf1 transfection\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — RNP-IP plus RNAi functional assays with multiple orthogonal approaches, single lab\",\n      \"pmids\": [\"17000771\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In Drosophila, loss-of-function of upf1 and upf2 inhibits cell growth and induces apoptosis through a Upf3-independent pathway. A mutant Upf2 unable to bind Upf3 still causes lethality, while disruption of Upf2-Upf1 interaction causes death, indicating that the Upf2-Upf1 interaction (not Upf2-Upf3) is essential for viability and NMD of most targets.\",\n      \"method\": \"Drosophila loss-of-function genetics; epistasis analysis with upf3 mutants; Upf2 binding-domain mutants; cell growth and apoptosis assays\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with multiple mutant combinations in Drosophila, single lab\",\n      \"pmids\": [\"21317294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Binding of UPF2 to UPF1 drastically reduces UPF1's affinity for RNA, causing release of bound RNA through an allosteric mechanism (not direct competition for RNA binding), mediated by the conformational change in UPF1 induced upon UPF2 binding.\",\n      \"method\": \"Biochemical binding assays (fluorescence anisotropy, filter binding); biophysical methods; in vitro RNA-release assays; mutational analysis\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — multiple orthogonal biochemical and biophysical methods establishing allosteric mechanism, single lab\",\n      \"pmids\": [\"36456182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Crystal and cryo-EM structures of UPF2 MIF4GIII in complex with UPF3B or UPF3A reveal unexpectedly intimate binding interfaces. UPF3B disease-causing mutation Y160D in the NOPS-L domain displaces Y160 from a hydrophobic cleft in UPF2, reducing binding affinity ~40-fold. UPF3A binds UPF2 with ~10-fold higher affinity than UPF3B via NOPS-L residues, explaining competitive binding and compensatory upregulation.\",\n      \"method\": \"X-ray crystallography and cryo-EM structures; isothermal titration calorimetry / binding affinity measurements; mutagenesis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal/cryo-EM structures plus quantitative binding measurements and mutagenesis, multiple orthogonal methods\",\n      \"pmids\": [\"35640974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"The SMG6 endonuclease contains a conserved short linear motif that binds the UPF1 CH domain (the same domain that binds UPF2), making SMG6 and UPF2 binding to UPF1 mutually exclusive. Cryo-EM data suggest that distinct SMG6-containing and UPF2-containing NMD complexes are dictated by different conformational states linked to UPF1's RNA-binding status.\",\n      \"method\": \"Mass spectrometry; cryo-EM; biochemical interaction assays; competition experiments; mutagenesis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — cryo-EM plus mass spectrometry and biochemical competition assays with mutagenesis, multiple orthogonal methods\",\n      \"pmids\": [\"38709891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UPF2 binds RNA dynamically: MIF4G-1 and MIF4G-3 are the main RNA/DNA-binding modules; MIF4G-3 has RNA annealing activity; full-length UPF2 unfolds a reporter hairpin RNA structure. UPF2 preferentially binds and stabilizes single-stranded RNA in a sequence-independent manner and undergoes a conformational change upon ssRNA binding.\",\n      \"method\": \"Nucleic acid binding assays; RNA annealing/unfolding assays; biochemical and biophysical methods; domain deletion analysis\",\n      \"journal\": \"RNA (New York, N.Y.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — multiple biochemical/biophysical assays with domain mapping, single lab\",\n      \"pmids\": [\"40246535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"UPF2 binds the exoribonuclease 3'hExo (involved in histone mRNA decay), and UPF2-mediated activation of UPF1 overrides the inhibitory effect of SLBP on UPF1's unwinding activity during histone mRNA degradation.\",\n      \"method\": \"Direct interaction assays (recombinant proteins); in vitro helicase/unwinding assays; functional mRNA decay assays in cells\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical interaction and functional assays, preprint, single lab\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yeast, Dcp2 and Upf2 compete for the same binding site in the Upf1 N-terminal CH domain, accounting for the existence of two mutually exclusive Upf1-containing complexes (Upf1-Upf2/3 surveillance complex and Upf1-decapping complex).\",\n      \"method\": \"Recombinant protein interaction assays; competition binding experiments; pulldowns\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical competition assays with recombinant proteins, preprint, single lab\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"SARS-CoV-2 nucleocapsid protein (Np) binds directly to UPF2, disrupting formation of the UPF1/UPF2 complex and negating the stimulatory effect of UPF2 on UPF1 catalytic activity, thereby inhibiting cellular NMD.\",\n      \"method\": \"Biochemical and biophysical binding assays; ATPase/helicase activity assays; cellular NMD reporter assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical/biophysical methods plus cellular assays, single peer-reviewed study\",\n      \"pmids\": [\"39831305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Upf2 protein is present in both cytoplasmic and nucleoplasmic fractions of human cells. Upf2 interacts with the EJC core factor RBM8A (Y14) not only in the cytoplasm but also in the intranuclear region, as shown by proximity ligation assay.\",\n      \"method\": \"Subcellular fractionation; western blotting; immunofluorescence; in situ proximity ligation assay; RNase treatment\",\n      \"journal\": \"Molecular medicine reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single method per finding (fractionation + PLA), no direct functional consequence linked to nuclear localization\",\n      \"pmids\": [\"27221324\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Proteasome inhibitors cause accumulation of UPF2 protein levels in cells; knockdown of SMG1 also upregulates UPF2 protein levels; these effects are additive, suggesting UPS and SMG1 regulate UPF2 protein stability via distinct pathways.\",\n      \"method\": \"Proteasome inhibitor treatment; siRNA knockdown of SMG1; western blotting\",\n      \"journal\": \"International journal of oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, pharmacological inhibitor and knockdown without direct mechanistic identification of the E3 ligase or modification site\",\n      \"pmids\": [\"24173962\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"hUPF2-dependent NMD pathway operating in the nuclear fraction of cells is required for down-regulation of PTC-bearing TCR-beta transcripts; this requires translation features (initiator ATG, scanning) suggesting cytoplasmic ribosomes act on nuclear-associated mRNAs.\",\n      \"method\": \"Antisense hUPF2 knockdown; TCR-beta PTC reporter assays; nuclear/cytoplasmic fractionation; inhibition of translation initiation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — antisense knockdown plus multiple reporter/fractionation assays, single lab\",\n      \"pmids\": [\"11889124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Chemical cross-linking mass spectrometry reveals that UPF2 acts as a connection bridge between SMG1 and SMG7 in the NMD machinery; UPF2 N-terminal forms most interactions with SMG7; MIF4G-I, II, and III domains contact SMG1 or SMG7.\",\n      \"method\": \"Chemical cross-linking mass spectrometry (CLMS); structural modeling\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — CLMS is a single method, largely computational modeling, single lab\",\n      \"pmids\": [\"38542156\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"UPF2 is a core NMD scaffold protein that bridges UPF3b (via its MIF4G-3 domain, which contacts UPF3b's RNP/NOPS-L domain) to UPF1 (via its C-terminal region binding the UPF1 CH domain), and upon binding to UPF1 induces a large conformational change that switches UPF1 from an RNA-clamping to an RNA-unwinding mode while simultaneously reducing UPF1's RNA affinity through allostery; additionally, UPF2 itself binds RNA via MIF4G-1 and MIF4G-3 and can unfold RNA structures, acts as a substrate and docking site for SMG1 kinase, interacts with eRF3 and the SURF complex at the ribosome, and competes with SMG6 and Dcp2 for the UPF1 CH domain, placing UPF2 at the center of a network of mutually exclusive complexes that orchestrate NMD initiation and mRNA degradation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"UPF2 is a central scaffold of the nonsense-mediated mRNA decay (NMD) pathway, physically bridging the surveillance factors UPF3b and UPF1 and acting as the conformational switch that activates UPF1 once a premature termination event is recognized [#0, #2]. Through its modular MIF4G domains it organizes a heptameric surveillance complex assembled on RNA with the exon-junction complex core, in which UPF3b is bridged from the EJC to UPF1, and UPF2/UPF3b cooperatively stimulate the ATPase and helicase activities of UPF1 [#0]. Structurally, the UPF2 MIF4G-3 domain engages the UPF3b RNP/NOPS-L surface through an intimate, conserved interface, while MIF4G-1 and MIF4G-2 provide an essential scaffolding role and MIF4G-3 together with the UPF1-binding region constitutes the minimal NMD-triggering module [#3, #6, #14]. UPF2 binds the UPF1 cysteine-histidine-rich (CH) domain, and this contact drives a large conformational change in UPF1 that converts it from an RNA-clamping state to an RNA-unwinding state while allosterically lowering UPF1's RNA affinity to release bound RNA [#1, #5, #13]. Beyond scaffolding, UPF2 is itself a nucleic-acid-binding protein: its MIF4G-1 and MIF4G-3 modules bind and stabilize single-stranded RNA and MIF4G-3 possesses RNA annealing activity, allowing full-length UPF2 to unfold structured RNA [#16]. UPF2 also serves as a substrate and docking site for the SMG1 kinase complex, which recruits UPF1 and UPF2 to distinct sites and transfers UPF2 onto UPF1 to activate it, and it contacts the eukaryotic release factor eRF3 and the SURF complex at terminating ribosomes through its C-terminal region that overlaps the UPF3b site [#6, #7, #9]. By occupying the UPF1 CH domain, UPF2 competes with the SMG6 endonuclease and the decapping enzyme Dcp2, placing it at the center of mutually exclusive UPF1-containing complexes that partition surveillance from degradation [#15, #18]. Loss of UPF2 stabilizes PTC-bearing transcripts and other NMD substrates, and the UPF2-UPF1 interaction is essential for viability and for decay of most targets [#12, #22].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"Established UPF2 as a physical hub of the human NMD machinery by defining its direct, domain-specific contacts with UPF1 and UPF3b and its predominantly cytoplasmic localization.\",\n      \"evidence\": \"Reciprocal Co-IP of epitope-tagged proteins with deletion mapping and immunofluorescence in HeLa cells\",\n      \"pmids\": [\"11113196\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve the structural basis of the interactions\", \"Did not determine the functional consequence of complex assembly on UPF1 activity\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrated that UPF2 is functionally required for NMD by showing its depletion stabilizes PTC-bearing transcripts in a translation-dependent manner.\",\n      \"evidence\": \"Antisense hUPF2 knockdown with TCR-beta PTC reporters, fractionation, and translation-initiation inhibition; yeast UPF2 deletion with CPA1 uORF reporters\",\n      \"pmids\": [\"11889124\", \"12172963\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not define how UPF2 promotes decay biochemically\", \"Did not identify the degradation machinery engaged downstream\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Resolved the molecular basis of the UPF2-UPF3b interaction, showing UPF3b uses its RNP surface for protein contact while UPF2 retains RNA binding.\",\n      \"evidence\": \"1.95 A crystal structure of UPF2 MIF4G domain bound to UPF3b RNP with RNA-binding and mutational assays\",\n      \"pmids\": [\"15004547\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not address how this interface positions UPF1\", \"Did not establish the role of other MIF4G domains\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Defined the UPF1 CH domain structure and identified the surfaces it uses to bind UPF2, the contact later shown to be a competitive hub.\",\n      \"evidence\": \"3 A crystal structure of the UPF1 CH domain with site-directed mutagenesis and binding assays\",\n      \"pmids\": [\"16931876\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not show the conformational consequence of UPF2 binding\", \"Did not address competition with other CH-domain partners\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Reconstituted the activating mechanism, showing UPF2 and UPF3b cooperatively stimulate UPF1 ATPase and helicase activity within an EJC-bridged surveillance complex.\",\n      \"evidence\": \"In vitro reconstitution of recombinant EJC core plus UPF1/2/3b on RNA with ATPase and helicase assays\",\n      \"pmids\": [\"18066079\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not provide a structural mechanism for the activation\", \"Did not establish how this couples to downstream decay\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Provided the structural mechanism of UPF1 activation, showing UPF2 binding triggers a CH-domain conformational change that switches UPF1 from RNA-clamping to RNA-unwinding.\",\n      \"evidence\": \"Crystal structures of Upf1 with transition-state analogue and RNA, with and without CH domain, plus ATPase/helicase assays\",\n      \"pmids\": [\"21419344\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not quantify the effect on RNA affinity\", \"Did not place the switch within the full SMG1-coupled pathway\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Dissected the modular architecture of UPF2, assigning scaffolding to MIF4G-1/2 and the NMD-triggering function plus SMG1 substrate role to MIF4G-3.\",\n      \"evidence\": \"Crystal structures of MIF4G-1 and MIF4G-2, in vitro SMG1 kinase and complex-assembly assays, and in vivo complementation/tethering\",\n      \"pmids\": [\"24271394\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not determine the consequence of MIF4G-3 phosphorylation\", \"Did not define MIF4G-1/2 binding partners that explain the scaffolding role\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Linked UPF2 to the SMG1 kinase complex, showing it docks on SMG1 independently of UPF1 and is transferred to UPF1 to activate it, and identified a UPF3-independent scaffolding region.\",\n      \"evidence\": \"EM of SMG1C with in vivo/in vitro interaction and competition assays; yeast Upf2 N-terminal mIF4G crystal structure with NMD assays and Dbp6 Co-IP\",\n      \"pmids\": [\"25002321\", \"25277656\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not resolve the order of events during in-cell activation\", \"Mechanism of UPF2 transfer within SMG1C not structurally defined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Connected UPF2 to translation termination by demonstrating direct binding to eRF3 and ribosome/SURF association through a C-terminal region overlapping the UPF3b site.\",\n      \"evidence\": \"Biochemical binding assays, EM of UPF2-eRF3, deletion mapping, and in-cell Co-IP\",\n      \"pmids\": [\"26740584\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not establish the in vivo timing of eRF3 versus UPF3b engagement\", \"Functional consequence of eRF3 binding for NMD not directly tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Refined the activation mechanism by showing UPF2 binding allosterically lowers UPF1 RNA affinity to release RNA, and resolved how UPF3 isoform binding competition and a disease mutation tune the UPF2 interface.\",\n      \"evidence\": \"Fluorescence anisotropy/filter binding RNA-release assays; crystal and cryo-EM structures of MIF4GIII with UPF3B/UPF3A and ITC affinity measurements with mutagenesis\",\n      \"pmids\": [\"36456182\", \"35640974\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not link RNA release timing to substrate commitment in cells\", \"Functional outcome of UPF3A/UPF3B competition on target selection not fully resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Established UPF2 as the determinant of mutually exclusive UPF1 complexes by showing SMG6 binds the same CH domain as UPF2, partitioning surveillance from endonucleolytic decay.\",\n      \"evidence\": \"Mass spectrometry, cryo-EM, and biochemical competition/mutagenesis assays\",\n      \"pmids\": [\"38709891\"],\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"Did not define the in-cell trigger that switches between complexes\", \"Conformational coupling to UPF1 RNA status inferred rather than directly imaged\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Characterized UPF2 as an intrinsic RNA-binding/remodeling protein and extended its role to histone mRNA decay and viral antagonism, while implicating Dcp2 competition for the UPF1 CH domain.\",\n      \"evidence\": \"Nucleic-acid binding/annealing/unfolding assays with domain mapping; recombinant interaction and helicase assays for 3'hExo/SLBP; binding and NMD reporter assays for SARS-CoV-2 Np; recombinant competition assays for Dcp2 (preprint)\",\n      \"pmids\": [\"40246535\", \"39831305\"],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"3'hExo and Dcp2 findings rest on preprint/single-lab biochemistry awaiting peer-reviewed confirmation\", \"In-cell relevance of UPF2 RNA-remodeling activity not established\", \"Mechanism by which viral Np outcompetes UPF1 in vivo not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the competing UPF2-, SMG6-, and Dcp2-containing UPF1 complexes are temporally ordered and switched in cells to commit a transcript to decay remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"\",\n      \"gaps\": [\"No in-cell measurement of complex exchange kinetics\", \"No structure of the complete surveillance-to-decay transition\", \"Regulation of UPF2 stability and its impact on pathway flux not mechanistically defined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [3, 16]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 2, 6]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 13, 0]},\n      {\"term_id\": \"GO:0140098\", \"supporting_discovery_ids\": [16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [22]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 6, 22]},\n      {\"term_id\": \"R-HSA-72163\", \"supporting_discovery_ids\": []}\n    ],\n    \"complexes\": [\"NMD surveillance complex (UPF1-UPF2-UPF3b)\", \"exon junction complex (EJC core)\", \"SURF complex\", \"SMG1C (SMG1-SMG8-SMG9)\"],\n    \"partners\": [\"UPF1\", \"UPF3B\", \"UPF3A\", \"SMG1\", \"ETF3H (eRF3)\", \"RBM8A\", \"SMG6\", \"DCP2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}