{"gene":"NUTF2","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":1995,"finding":"NTF2 (NUTF2) was purified as a cytosolic factor that interacts with the nuclear pore complex glycoprotein p62; it exists as an apparent dimer of ~14-kDa subunits and acts at a relatively late stage of nuclear protein import, subsequent to initial docking of import ligand at the nuclear envelope.","method":"Biochemical complementation of depleted cytosol, flow cytometry-based import assay, recombinant protein rescue","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — original purification with functional reconstitution, replicated by multiple subsequent labs","pmids":["7744965"],"is_preprint":false},{"year":1996,"finding":"X-ray crystallography of rat NTF2 at 1.6 Å resolution revealed an alpha+beta barrel fold with a distinctive hydrophobic cavity homologous to scytalone dehydratase; the cavity contains a putative catalytic Asp-His pair and is a candidate binding site for Ran and nucleoporin p62.","method":"X-ray crystallography","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure, foundational structural result replicated and extended in multiple subsequent structures","pmids":["8757804"],"is_preprint":false},{"year":1996,"finding":"NTF2 binds specifically to RanGDP (but not RanGTP) and to xFxFG repeat-containing domains of nucleoporins p62 and Nsp1p at separate, non-competing binding sites on the dimer.","method":"Affinity pulldown (Sepharose-bound NTF2) from rat liver homogenates; competition binding assays","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal binding assays with defined mutants, replicated in multiple subsequent studies","pmids":["8918934"],"is_preprint":false},{"year":1996,"finding":"The NTF2 gene is essential for viability in S. cerevisiae; temperature-sensitive ntf2 mutants show defects in nuclear protein localization but not poly(A)+ RNA export, and NTF2 protein concentrates at the nuclear envelope.","method":"Yeast genetics (conditional lethal mutants), epitope-tagged localization, human gene complementation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic essentiality, cross-species complementation, and direct localization in one study","pmids":["8702493"],"is_preprint":false},{"year":1996,"finding":"NTF2 nucleotide-specifically binds the GDP-bound form of Ran/TC4 (not GTP-bound), as demonstrated by solution and solid-phase binding assays with radiolabeled nucleotide-preloaded Ran.","method":"Solution and solid-phase binding assays with [γ-32P]GTP- and [3H]GDP-loaded Ran","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct biochemical assay, independently replicated across multiple labs","pmids":["8755535"],"is_preprint":false},{"year":1997,"finding":"Site-directed NTF2 mutants (E42K, D92N/D94N) in conserved residues surrounding the hydrophobic cavity lose GDP-Ran binding while retaining FxFG-nucleoporin binding; these mutants fail to support nuclear protein import in permeabilized cells and are non-viable when substituted for chromosomal yeast NTF2.","method":"Site-directed mutagenesis, crystal structure of E42K mutant, permeabilized-cell import assay, yeast viability assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — mutagenesis guided by crystal structure, functional assays in two systems, cross-validated in vivo","pmids":["9368653"],"is_preprint":false},{"year":1998,"finding":"Crystal structure of the GDP-Ran–NTF2 complex at 2.5 Å resolution showed that the switch II loop of Ran (residues 65–78) interacts with the hydrophobic cavity of NTF2; Phe72 inserts into the cavity, and salt bridges between Ran Lys71/Arg76 and NTF2 Asp92/Asp94/Glu42 account for GDP-vs-GTP selectivity.","method":"X-ray crystallography of the complex","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution co-crystal structure with direct structural explanation of selectivity, confirmed by prior mutagenesis","pmids":["9533885"],"is_preprint":false},{"year":1998,"finding":"NTF2 acts as the transport receptor that mediates nuclear import of RanGDP; it binds RanGDP, docks at NPCs, and translocates the NTF2–RanGDP complex to the nuclear side, where RanGDP is released upon nucleotide exchange to RanGTP (to which NTF2 has no detectable affinity), establishing directionality of Ran import.","method":"In vitro nuclear import reconstitution with purified factors, point-mutant NTF2 that cannot bind RanGDP, digitonin-permeabilized cell assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — reconstituted in vitro system, loss-of-function mutant controls, independently replicated simultaneously by Smith et al. 1998","pmids":["9822603"],"is_preprint":false},{"year":1998,"finding":"NTF2-mediated nuclear import of RanGDP is sufficient (with energy) to drive Ran nuclear accumulation; a dominant-negative importin-beta mutant inhibits Ran import, and a RanGTP-locked mutant (unable to bind NTF2) is not imported.","method":"Digitonin-permeabilized cell nuclear import assay, NTF2 mutant unable to bind Ran, dominant-negative importin-beta","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple genetic/biochemical controls, independent replication of Ribbeck et al. 1998 findings","pmids":["9889103"],"is_preprint":false},{"year":1998,"finding":"NTF2 acts as a GDP dissociation inhibitor (Ran-GDI) for Ran: it dramatically inhibits dissociation of GDP from Ran and blocks subsequent GTP binding catalyzed by RCC1, without affecting GTP dissociation; the GDI activity of NTF2 mutants correlates with their nuclear import activity.","method":"Radiolabeled nucleotide dissociation assays ([3H]GDP, [35S]GTPγS) with recombinant proteins, permeabilized-cell import assay with NTF2 mutants","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct in vitro biochemical assay with multiple nucleotide forms, structure-function correlation with import assay","pmids":["9843686"],"is_preprint":false},{"year":1998,"finding":"The RanQ69L mutant fails to bind NTF2 because the Q69L substitution causes a large conformational change in the switch II loop (residues 68–74), repositioning Lys71, Phe72, and Arg76 that are critical for the Ran–NTF2 interface.","method":"X-ray crystallography of GDP-RanQ69L (2.3 Å), solution binding assays","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with direct binding data confirming loss of interaction","pmids":["9878368"],"is_preprint":false},{"year":1999,"finding":"Interaction between NTF2 and xFxFG-containing nucleoporins is required for nuclear import of RanGDP: the W7A mutation in NTF2 specifically reduces nucleoporin affinity while retaining RanGDP binding, and W7A-NTF2 only weakly stimulates RanGDP import in permeabilized cells and binds less strongly to the NPC central channel.","method":"Crystal structure of W7A-NTF2 (2.5 Å), fluorescence import assay, nuclear envelope binding assay, microinjection into Xenopus oocytes with colloidal gold EM","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — structure-guided mutagenesis, multiple orthogonal functional assays (permeabilized cells, oocyte microinjection, EM)","pmids":["10543952"],"is_preprint":false},{"year":1999,"finding":"Switch II residues Lys71, Phe72, and Arg76 of Ran are all individually essential for interaction with NTF2; mutations F72Y, F72W, and R76E abolish NTF2 binding without major structural change, and these Ran mutants are deficient in stimulating nuclear protein import.","method":"X-ray crystallography of Ran switch II mutants, solution binding assays, permeabilized-cell import assay, yeast viability assay (Gsp1p equivalents)","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — structure + binding + in vitro function + in vivo viability, multiple independent mutations tested","pmids":["10356329"],"is_preprint":false},{"year":2000,"finding":"Cytoplasmic injection of anti-NTF2 monoclonal antibodies causes dramatic relocalization of Ran to the cytoplasm in vivo and inhibits nuclear import of both Ran and NLS-containing proteins in vitro, demonstrating that NTF2 is required to maintain the nuclear concentration of Ran in living cells.","method":"Cytoplasmic microinjection of anti-NTF2 mAbs, fluorescence microscopy, in vitro import assay","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — antibody-mediated loss-of-function in living cells and in vitro, multiple readouts","pmids":["10679025"],"is_preprint":false},{"year":2000,"finding":"In S. cerevisiae, a temperature-sensitive NTF2 mutant that cannot bind Ran fails to import Ran into the nucleus at non-permissive temperature; depletion of nuclear Ran triggers a MAD2 spindle-assembly checkpoint-dependent G2 arrest, linking NTF2-mediated Ran import to cell cycle progression.","method":"Yeast temperature-sensitive genetics, Ran localization assay, epistasis with MAD2 deletion","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis (NTF2ts × mad2Δ), direct Ran localization measurement, functional cell cycle readout","pmids":["10930458"],"is_preprint":false},{"year":2002,"finding":"1.9 Å crystal structure of yeast NTF2-N77Y bound to an FxFG-nucleoporin core revealed that both identical FxFG-binding sites on the NTF2 dimer are formed by residues from each chain; engineered mutations at this interface reduce NPC binding and cause reduced growth and Ran mislocalization in yeast.","method":"X-ray crystallography, yeast mutagenesis, growth assay, Ran localization","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution co-crystal structure with functional validation in vivo","pmids":["12065398"],"is_preprint":false},{"year":2002,"finding":"NTF2 is exported from isolated oocyte nuclei approximately 30 times faster than GFP (a similarly sized inert molecule), demonstrating facilitated NPC translocation dependent on FG-repeat binding.","method":"Optical single transporter recording in isolated Xenopus oocyte nuclei and purified nuclear envelopes","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — direct single-molecule transport measurement in isolated nuclei, single study","pmids":["12189172"],"is_preprint":false},{"year":2002,"finding":"Drosophila NTF2 (DNTF-2) is essential for nuclear import; hypomorphic alleles block nuclear translocation of NF-κB/Rel proteins (Dorsal, Dif, Relish) after infection, impairing antimicrobial peptide gene expression; DNTF-2 directly interacts with Mbo/DNup88, a nucleoporin lacking FG repeats.","method":"Drosophila genetics (hypomorphic alleles, lethality), nuclear localization assay for Rel proteins, direct protein interaction","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo genetic loss-of-function with specific pathway phenotype and novel binding partner identified, single lab","pmids":["11943764"],"is_preprint":false},{"year":2004,"finding":"A novel ATP-dependent activity releases RanGDP from NTF2 (acts as a RanGDF — GDI displacement factor); hydrolyzable ATP enhances GDP dissociation from the Ran–NTF2 complex, and this activity is regulated by phosphorylation (inhibited by a phosphatase inhibitor mixture).","method":"In vitro GDP dissociation assay with hydrolyzable vs. non-hydrolyzable ATP analogues, phosphatase inhibitor treatment","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — direct in vitro biochemical assay, single lab, no identity of the ATPase factor","pmids":["15155737"],"is_preprint":false},{"year":2004,"finding":"A D23A mutation in NTF2 increases affinity for the NPC without gross structural change; residue D23 lies in an evolutionarily conserved region of the NTF2-domain superfamily that functions as an NPC-binding site distinct from the FxFG-binding site.","method":"Computational evolutionary analysis, mutagenesis, NPC-binding assay","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — structural and biochemical analysis with functional binding assay, but single lab and limited structural resolution","pmids":["15522285"],"is_preprint":false},{"year":2006,"finding":"Molecular dynamics simulations identified six FG-repeat peptide binding spots on the NTF2 surface forming a stripe, including two novel sites beyond the four known from X-ray/NMR data, providing a model for how FG repeats recognize NTF2 during NPC transit.","method":"Molecular dynamics simulation (254+ ns), agreement with prior X-ray, mutational, NMR, and computational data","journal":"Journal of molecular biology","confidence":"Low","confidence_rationale":"Tier 4 / Weak — computational only, no direct experimental validation of novel sites in this paper","pmids":["17161424"],"is_preprint":false},{"year":2010,"finding":"Nuclear size scales with NTF2 concentration in Xenopus egg extracts: higher NTF2 reduces nuclear size and selectively reduces import of large cargo molecules. NTF2 and importin-α together account for nuclear size differences between X. laevis and X. tropicalis, both acting through lamin B3 import.","method":"Xenopus egg extract nuclear reconstitution, titration of NTF2 and importin-α, fluorescent import assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reconstituted in vitro system with defined factor titration, two orthogonal phenotypic readouts (nuclear size, import rate), replicated in developmental context","pmids":["20946986"],"is_preprint":false},{"year":2010,"finding":"In senescent cells, NTF2 is sequestered in the nucleus alongside reduced RanGTP restoration (due to ATP deficiency), contributing to failure of actin export via exportin 6.","method":"GST-pulldown (Exp6-RanGTP interaction), cellular fractionation, Exp6 knockdown, H2O2-induced senescence","journal":"Experimental cell research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — indirect evidence for NTF2 sequestration, mechanistic link to actin export is inferred rather than directly demonstrated for NTF2","pmids":["21195711"],"is_preprint":false},{"year":2012,"finding":"Translocation of NTF2 from nucleus to cytoplasm to collect RanGDP is subject to regulation: treatment with polysorbitan monolaurate blocks NTF2 nuclear export (not import), causing cytoplasmic Ran accumulation and inhibiting RanGTP-dependent nuclear export processes; rescue requires NTF2 overexpression. Increased phosphorylation of tyrosine/threonine proteins accompanies this block.","method":"Polysorbitan monolaurate treatment, NTF2 overexpression rescue, tRNA and protein export assays, phosphorylation analysis","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — specific pharmacological block with rescue by NTF2 overexpression, two orthogonal transport readouts, single lab","pmids":["22880006"],"is_preprint":false},{"year":2016,"finding":"NTF2 reduces nuclear pore complex diameter in a Ran-binding-dependent manner in Xenopus extracts; Ran binding to NTF2 is required for NTF2 to inhibit nuclear expansion and large-cargo import. Ectopic NTF2 expression reduces nuclear size in Xenopus embryos and mammalian tissue culture cells.","method":"Xenopus egg and embryo extract nuclear reconstitution, NTF2 mutants deficient in Ran binding, NPC diameter measurement, ectopic NTF2 expression in mammalian cells","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal assays (extract reconstitution, NPC diameter, mammalian cell overexpression), Ran-binding-dependent mechanism defined by mutant","pmids":["26823604"],"is_preprint":false},{"year":2023,"finding":"Native top-down proteomics revealed that EGFR signaling induces dissociation of NUTF2 dimers in breast cancer cells; NUTF2 proteoforms with K4 and K55 post-translational modifications differentially impact NUTF2's inhibition of the estrogen receptor (ER) signaling pathway.","method":"Native top-down proteomics (SEC, nano-LC, FAIMS, multistage MS), identification of endogenous proteoform complexes","journal":"Research square","confidence":"Low","confidence_rationale":"Tier 3 / Weak — preprint, single-lab mass spectrometry discovery, limited functional validation of the specific PTM–ER mechanism","pmids":["37546719"],"is_preprint":true},{"year":2024,"finding":"SANS with site-specific deuteration showed that FG-Nup repeats remain fully disordered upon binding to NTF2 in vitro, with no significant inter-aromatic contacts or interchain linkage in the FG-Nup–NTF2 complex.","method":"Small-angle neutron scattering (SANS) with site-specific deuteration and model fitting","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — novel structural method directly probing the complex, preprint, single study","pmids":[],"is_preprint":true}],"current_model":"NUTF2 (NTF2) is a small homodimeric protein that functions as the dedicated nuclear import receptor for RanGDP: it binds RanGDP via a hydrophobic cavity (engaging the switch II loop of Ran, particularly Phe72, Lys71, and Arg76) and simultaneously engages FxFG-repeat nucleoporins at a separate dimer-interface site to translocate the NTF2–RanGDP complex through the nuclear pore complex; once inside the nucleus, RanGDP is released upon RCC1-catalyzed exchange to RanGTP (for which NTF2 has no affinity), establishing directional Ran import that replenishes the nuclear RanGTP pool required for all importin-β-family transport and for cell cycle progression; additionally, NTF2 acts as a Ran-GDI that stabilizes the GDP-bound state, its nuclear-to-cytoplasmic recycling is subject to phosphorylation-dependent regulation, and NTF2 levels negatively regulate nuclear size by reducing NPC diameter in a Ran-binding-dependent manner."},"narrative":{"mechanistic_narrative":"NUTF2 (NTF2) is a small homodimeric nuclear transport factor that serves as the dedicated import receptor for RanGDP, replenishing the nuclear Ran pool that powers all importin-β-family transport [PMID:7744965, PMID:9822603]. Originally purified as a cytosolic factor acting at a late, post-docking stage of nuclear protein import through its interaction with the nucleoporin p62 [PMID:7744965], NTF2 binds specifically to the GDP-bound form of Ran but not RanGTP, and engages FxFG-repeat nucleoporins at a separate, non-competing site on the dimer [PMID:8918934, PMID:8755535]. Crystallographic and structure-guided mutagenesis studies define both interfaces: the Ran switch II loop (Phe72, Lys71, Arg76) inserts into NTF2's hydrophobic cavity, with salt bridges to NTF2 Asp92/Asp94/Glu42 conferring GDP-versus-GTP selectivity, while a distinct dimer-interface site formed from both chains binds FxFG nucleoporins [PMID:9368653, PMID:9533885, PMID:12065398]. Mutations that abolish Ran binding (E42K, D92N/D94N) or nucleoporin binding (W7A) each fail to support RanGDP import, establishing that both contacts are required for facilitated translocation through the pore [PMID:9368653, PMID:10543952]. The receptor functions directionally: it imports RanGDP, docks at and rapidly transits NPCs, and releases Ran upon RCC1-catalyzed exchange to RanGTP, for which it has no affinity [PMID:9822603, PMID:12189172]; NTF2 additionally acts as a Ran-GDI that stabilizes the GDP-bound state by blocking nucleotide dissociation [PMID:9843686]. This activity is essential for viability and maintains the nuclear Ran concentration in living cells, linking Ran import to cell cycle progression through a spindle-assembly-checkpoint-dependent arrest when disrupted [PMID:8702493, PMID:10679025, PMID:10930458]. Beyond transport, NTF2 concentration negatively regulates nuclear size by reducing NPC diameter in a Ran-binding-dependent manner and selectively limiting import of large cargo [PMID:20946986, PMID:26823604].","teleology":[{"year":1995,"claim":"Established that a distinct cytosolic factor is required for nuclear protein import downstream of initial ligand docking, defining NTF2 as a discrete transport component rather than part of the importin machinery.","evidence":"Biochemical complementation of depleted cytosol with recombinant protein rescue and flow-cytometry import assay","pmids":["7744965"],"confidence":"High","gaps":["Molecular cargo and the precise transport step were not defined","No structural basis for nucleoporin interaction"]},{"year":1996,"claim":"Solved the NTF2 fold and identified its hydrophobic cavity, then showed nucleotide-specific binding to RanGDP and separate FxFG-nucleoporin binding, revealing the bipartite molecular logic of the receptor.","evidence":"X-ray crystallography at 1.6 Å, affinity pulldowns from liver homogenate, competition binding, and GDP/GTP-loaded Ran binding assays; yeast essentiality and human complementation","pmids":["8757804","8918934","8755535","8702493"],"confidence":"High","gaps":["Atomic basis of GDP-vs-GTP selectivity not yet resolved at the complex level","Cargo of NTF2 transport not yet identified as Ran itself"]},{"year":1997,"claim":"Demonstrated by structure-guided mutagenesis that the Ran-binding cavity is functionally separable from nucleoporin binding and is strictly required for import and viability, validating the two-site model in vivo.","evidence":"Site-directed mutants (E42K, D92N/D94N), mutant crystal structure, permeabilized-cell import, and yeast viability assays","pmids":["9368653"],"confidence":"High","gaps":["Direction and recycling of the receptor not yet established"]},{"year":1998,"claim":"Resolved the GDP-Ran–NTF2 co-crystal to explain GDP/GTP selectivity at atomic resolution and established NTF2 as the directional import receptor for RanGDP that also acts as a Ran-GDI.","evidence":"2.5 Å co-crystal structure, in vitro import reconstitution with loss-of-function mutants, digitonin-permeabilized assays, dominant-negative importin-β, and radiolabeled nucleotide dissociation assays","pmids":["9533885","9822603","9889103","9843686"],"confidence":"High","gaps":["Mechanism of RanGDP release on the nuclear side not directly characterized","How NTF2 recycles back to the cytoplasm unresolved"]},{"year":1999,"claim":"Established that both the Ran switch II residues and the NTF2 nucleoporin-binding tryptophan are individually essential for import, mechanistically coupling FG-nucleoporin engagement to productive translocation.","evidence":"Crystal structures of W7A-NTF2 and Ran switch II mutants, fluorescence/permeabilized import assays, Xenopus oocyte microinjection with colloidal gold EM, yeast viability","pmids":["10543952","10356329"],"confidence":"High","gaps":["Full set of FG-binding sites on NTF2 not yet mapped"]},{"year":2000,"claim":"Showed in living cells and yeast that NTF2 maintains the nuclear Ran concentration, and that loss of nuclear Ran arrests the cell cycle through the spindle-assembly checkpoint, connecting Ran import to proliferation.","evidence":"Anti-NTF2 mAb cytoplasmic microinjection, fluorescence microscopy, in vitro import; yeast ts genetics with MAD2 epistasis","pmids":["10679025","10930458"],"confidence":"High","gaps":["Direct mechanism linking Ran depletion to checkpoint activation not dissected"]},{"year":2002,"claim":"Defined the symmetric dual FxFG-binding sites on the dimer and measured facilitated NTF2 transit through the NPC, and identified a non-FG nucleoporin (DNup88) partner in Drosophila tied to NF-κB nuclear import.","evidence":"1.9 Å co-crystal with FxFG core plus yeast functional validation, optical single-transporter recording in isolated nuclei, Drosophila genetics with Rel-protein localization and direct interaction","pmids":["12065398","12189172","11943764"],"confidence":"Medium","gaps":["DNup88 interaction characterized in a single organism","Single-transporter measurements from one study"]},{"year":2004,"claim":"Identified additional regulatory layers: an ATP-dependent phosphorylation-sensitive activity that displaces RanGDP from NTF2, and a conserved non-FxFG NPC-binding surface, refining the model of receptor cycling.","evidence":"In vitro GDP-dissociation assays with hydrolyzable vs non-hydrolyzable ATP and phosphatase inhibitors; evolutionary analysis with D23A NPC-binding mutant","pmids":["15155737","15522285"],"confidence":"Medium","gaps":["Identity of the ATP-dependent displacement factor unknown","Physiological relevance of the second NPC-binding site not established in vivo"]},{"year":2010,"claim":"Revealed a developmental and morphometric role: NTF2 concentration scales inversely with nuclear size and selectively limits large-cargo import, accounting for interspecies nuclear-size differences via lamin B3 import.","evidence":"Xenopus egg extract nuclear reconstitution with titration of NTF2 and importin-α, fluorescent import assays","pmids":["20946986"],"confidence":"High","gaps":["Mechanism by which NTF2 levels constrain import not yet defined at the pore"]},{"year":2016,"claim":"Showed that NTF2 controls nuclear size by reducing NPC diameter in a Ran-binding-dependent manner, mechanistically grounding the size phenotype in pore architecture.","evidence":"Xenopus egg/embryo extract reconstitution with Ran-binding-deficient NTF2 mutants, NPC diameter measurement, ectopic expression in mammalian cells","pmids":["26823604"],"confidence":"High","gaps":["How Ran binding to NTF2 alters NPC diameter molecularly is unresolved"]},{"year":2024,"claim":"Probed the physical nature of the FG-Nup–NTF2 complex, showing FG repeats remain disordered upon binding with no inter-aromatic crosslinking, informing models of how the receptor transits the permeability barrier.","evidence":"Small-angle neutron scattering with site-specific deuteration (preprint)","pmids":[],"confidence":"Medium","gaps":["Preprint, single study","In vitro reconstitution may not capture the crowded NPC environment"]},{"year":null,"claim":"How NTF2 recycling, post-translational regulation, and putative signaling-linked moonlighting functions integrate with its canonical RanGDP import role remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["Identity of the ATP/phosphorylation-dependent displacement factor unknown","Reported EGFR/ER-signaling and senescence roles rest on low-confidence or preprint evidence","Mechanism coupling Ran binding to NPC diameter change undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0,7,8]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[9,18]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[4,6]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]},{"term_id":"GO:0005635","term_label":"nuclear envelope","supporting_discovery_ids":[3]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[13]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[7,8]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[14]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[21,24]}],"complexes":["NTF2 homodimer","NTF2–RanGDP complex"],"partners":["RAN","NUP62","NUP88"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P61970","full_name":"Nuclear transport factor 2","aliases":["Placental protein 15","PP15"],"length_aa":127,"mass_kda":14.5,"function":"Mediates the import of GDP-bound RAN from the cytoplasm into the nucleus which is essential for the function of RAN in cargo receptor-mediated nucleocytoplasmic transport. Thereby, plays indirectly a more general role in cargo receptor-mediated nucleocytoplasmic transport. Interacts with GDP-bound RAN in the cytosol, recruits it to the nuclear pore complex via its interaction with nucleoporins and promotes its nuclear import","subcellular_location":"Cytoplasm, cytosol; Nucleus outer membrane; Nucleus, nuclear pore complex; Nucleus inner membrane; Nucleus, nucleoplasm","url":"https://www.uniprot.org/uniprotkb/P61970/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/NUTF2","classification":"Common Essential","n_dependent_lines":1198,"n_total_lines":1208,"dependency_fraction":0.9917218543046358},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000102898","cell_line_id":"CID001565","localizations":[{"compartment":"nuclear_membrane","grade":3},{"compartment":"cytoplasmic","grade":1},{"compartment":"nucleoplasm","grade":1}],"interactors":[{"gene":"RAN","stoichiometry":10.0},{"gene":"RANGAP1","stoichiometry":0.2},{"gene":"NUP153","stoichiometry":0.2},{"gene":"RANBP2","stoichiometry":0.2},{"gene":"NUP214","stoichiometry":0.2},{"gene":"RCC1","stoichiometry":0.2},{"gene":"SUMO1","stoichiometry":0.2},{"gene":"NUP93","stoichiometry":0.2},{"gene":"UBE2I","stoichiometry":0.2},{"gene":"NUP88","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001565","total_profiled":1310},"omim":[{"mim_id":"620020","title":"G3BP STRESS GRANULE ASSEMBLY FACTOR 2; 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it exists as an apparent dimer of ~14-kDa subunits and acts at a relatively late stage of nuclear protein import, subsequent to initial docking of import ligand at the nuclear envelope.\",\n      \"method\": \"Biochemical complementation of depleted cytosol, flow cytometry-based import assay, recombinant protein rescue\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — original purification with functional reconstitution, replicated by multiple subsequent labs\",\n      \"pmids\": [\"7744965\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"X-ray crystallography of rat NTF2 at 1.6 Å resolution revealed an alpha+beta barrel fold with a distinctive hydrophobic cavity homologous to scytalone dehydratase; the cavity contains a putative catalytic Asp-His pair and is a candidate binding site for Ran and nucleoporin p62.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure, foundational structural result replicated and extended in multiple subsequent structures\",\n      \"pmids\": [\"8757804\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"NTF2 binds specifically to RanGDP (but not RanGTP) and to xFxFG repeat-containing domains of nucleoporins p62 and Nsp1p at separate, non-competing binding sites on the dimer.\",\n      \"method\": \"Affinity pulldown (Sepharose-bound NTF2) from rat liver homogenates; competition binding assays\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal binding assays with defined mutants, replicated in multiple subsequent studies\",\n      \"pmids\": [\"8918934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"The NTF2 gene is essential for viability in S. cerevisiae; temperature-sensitive ntf2 mutants show defects in nuclear protein localization but not poly(A)+ RNA export, and NTF2 protein concentrates at the nuclear envelope.\",\n      \"method\": \"Yeast genetics (conditional lethal mutants), epitope-tagged localization, human gene complementation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic essentiality, cross-species complementation, and direct localization in one study\",\n      \"pmids\": [\"8702493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"NTF2 nucleotide-specifically binds the GDP-bound form of Ran/TC4 (not GTP-bound), as demonstrated by solution and solid-phase binding assays with radiolabeled nucleotide-preloaded Ran.\",\n      \"method\": \"Solution and solid-phase binding assays with [γ-32P]GTP- and [3H]GDP-loaded Ran\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct biochemical assay, independently replicated across multiple labs\",\n      \"pmids\": [\"8755535\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Site-directed NTF2 mutants (E42K, D92N/D94N) in conserved residues surrounding the hydrophobic cavity lose GDP-Ran binding while retaining FxFG-nucleoporin binding; these mutants fail to support nuclear protein import in permeabilized cells and are non-viable when substituted for chromosomal yeast NTF2.\",\n      \"method\": \"Site-directed mutagenesis, crystal structure of E42K mutant, permeabilized-cell import assay, yeast viability assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — mutagenesis guided by crystal structure, functional assays in two systems, cross-validated in vivo\",\n      \"pmids\": [\"9368653\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Crystal structure of the GDP-Ran–NTF2 complex at 2.5 Å resolution showed that the switch II loop of Ran (residues 65–78) interacts with the hydrophobic cavity of NTF2; Phe72 inserts into the cavity, and salt bridges between Ran Lys71/Arg76 and NTF2 Asp92/Asp94/Glu42 account for GDP-vs-GTP selectivity.\",\n      \"method\": \"X-ray crystallography of the complex\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution co-crystal structure with direct structural explanation of selectivity, confirmed by prior mutagenesis\",\n      \"pmids\": [\"9533885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"NTF2 acts as the transport receptor that mediates nuclear import of RanGDP; it binds RanGDP, docks at NPCs, and translocates the NTF2–RanGDP complex to the nuclear side, where RanGDP is released upon nucleotide exchange to RanGTP (to which NTF2 has no detectable affinity), establishing directionality of Ran import.\",\n      \"method\": \"In vitro nuclear import reconstitution with purified factors, point-mutant NTF2 that cannot bind RanGDP, digitonin-permeabilized cell assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reconstituted in vitro system, loss-of-function mutant controls, independently replicated simultaneously by Smith et al. 1998\",\n      \"pmids\": [\"9822603\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"NTF2-mediated nuclear import of RanGDP is sufficient (with energy) to drive Ran nuclear accumulation; a dominant-negative importin-beta mutant inhibits Ran import, and a RanGTP-locked mutant (unable to bind NTF2) is not imported.\",\n      \"method\": \"Digitonin-permeabilized cell nuclear import assay, NTF2 mutant unable to bind Ran, dominant-negative importin-beta\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple genetic/biochemical controls, independent replication of Ribbeck et al. 1998 findings\",\n      \"pmids\": [\"9889103\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"NTF2 acts as a GDP dissociation inhibitor (Ran-GDI) for Ran: it dramatically inhibits dissociation of GDP from Ran and blocks subsequent GTP binding catalyzed by RCC1, without affecting GTP dissociation; the GDI activity of NTF2 mutants correlates with their nuclear import activity.\",\n      \"method\": \"Radiolabeled nucleotide dissociation assays ([3H]GDP, [35S]GTPγS) with recombinant proteins, permeabilized-cell import assay with NTF2 mutants\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct in vitro biochemical assay with multiple nucleotide forms, structure-function correlation with import assay\",\n      \"pmids\": [\"9843686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The RanQ69L mutant fails to bind NTF2 because the Q69L substitution causes a large conformational change in the switch II loop (residues 68–74), repositioning Lys71, Phe72, and Arg76 that are critical for the Ran–NTF2 interface.\",\n      \"method\": \"X-ray crystallography of GDP-RanQ69L (2.3 Å), solution binding assays\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with direct binding data confirming loss of interaction\",\n      \"pmids\": [\"9878368\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Interaction between NTF2 and xFxFG-containing nucleoporins is required for nuclear import of RanGDP: the W7A mutation in NTF2 specifically reduces nucleoporin affinity while retaining RanGDP binding, and W7A-NTF2 only weakly stimulates RanGDP import in permeabilized cells and binds less strongly to the NPC central channel.\",\n      \"method\": \"Crystal structure of W7A-NTF2 (2.5 Å), fluorescence import assay, nuclear envelope binding assay, microinjection into Xenopus oocytes with colloidal gold EM\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — structure-guided mutagenesis, multiple orthogonal functional assays (permeabilized cells, oocyte microinjection, EM)\",\n      \"pmids\": [\"10543952\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Switch II residues Lys71, Phe72, and Arg76 of Ran are all individually essential for interaction with NTF2; mutations F72Y, F72W, and R76E abolish NTF2 binding without major structural change, and these Ran mutants are deficient in stimulating nuclear protein import.\",\n      \"method\": \"X-ray crystallography of Ran switch II mutants, solution binding assays, permeabilized-cell import assay, yeast viability assay (Gsp1p equivalents)\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — structure + binding + in vitro function + in vivo viability, multiple independent mutations tested\",\n      \"pmids\": [\"10356329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cytoplasmic injection of anti-NTF2 monoclonal antibodies causes dramatic relocalization of Ran to the cytoplasm in vivo and inhibits nuclear import of both Ran and NLS-containing proteins in vitro, demonstrating that NTF2 is required to maintain the nuclear concentration of Ran in living cells.\",\n      \"method\": \"Cytoplasmic microinjection of anti-NTF2 mAbs, fluorescence microscopy, in vitro import assay\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — antibody-mediated loss-of-function in living cells and in vitro, multiple readouts\",\n      \"pmids\": [\"10679025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"In S. cerevisiae, a temperature-sensitive NTF2 mutant that cannot bind Ran fails to import Ran into the nucleus at non-permissive temperature; depletion of nuclear Ran triggers a MAD2 spindle-assembly checkpoint-dependent G2 arrest, linking NTF2-mediated Ran import to cell cycle progression.\",\n      \"method\": \"Yeast temperature-sensitive genetics, Ran localization assay, epistasis with MAD2 deletion\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis (NTF2ts × mad2Δ), direct Ran localization measurement, functional cell cycle readout\",\n      \"pmids\": [\"10930458\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"1.9 Å crystal structure of yeast NTF2-N77Y bound to an FxFG-nucleoporin core revealed that both identical FxFG-binding sites on the NTF2 dimer are formed by residues from each chain; engineered mutations at this interface reduce NPC binding and cause reduced growth and Ran mislocalization in yeast.\",\n      \"method\": \"X-ray crystallography, yeast mutagenesis, growth assay, Ran localization\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution co-crystal structure with functional validation in vivo\",\n      \"pmids\": [\"12065398\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"NTF2 is exported from isolated oocyte nuclei approximately 30 times faster than GFP (a similarly sized inert molecule), demonstrating facilitated NPC translocation dependent on FG-repeat binding.\",\n      \"method\": \"Optical single transporter recording in isolated Xenopus oocyte nuclei and purified nuclear envelopes\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — direct single-molecule transport measurement in isolated nuclei, single study\",\n      \"pmids\": [\"12189172\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Drosophila NTF2 (DNTF-2) is essential for nuclear import; hypomorphic alleles block nuclear translocation of NF-κB/Rel proteins (Dorsal, Dif, Relish) after infection, impairing antimicrobial peptide gene expression; DNTF-2 directly interacts with Mbo/DNup88, a nucleoporin lacking FG repeats.\",\n      \"method\": \"Drosophila genetics (hypomorphic alleles, lethality), nuclear localization assay for Rel proteins, direct protein interaction\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo genetic loss-of-function with specific pathway phenotype and novel binding partner identified, single lab\",\n      \"pmids\": [\"11943764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A novel ATP-dependent activity releases RanGDP from NTF2 (acts as a RanGDF — GDI displacement factor); hydrolyzable ATP enhances GDP dissociation from the Ran–NTF2 complex, and this activity is regulated by phosphorylation (inhibited by a phosphatase inhibitor mixture).\",\n      \"method\": \"In vitro GDP dissociation assay with hydrolyzable vs. non-hydrolyzable ATP analogues, phosphatase inhibitor treatment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — direct in vitro biochemical assay, single lab, no identity of the ATPase factor\",\n      \"pmids\": [\"15155737\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A D23A mutation in NTF2 increases affinity for the NPC without gross structural change; residue D23 lies in an evolutionarily conserved region of the NTF2-domain superfamily that functions as an NPC-binding site distinct from the FxFG-binding site.\",\n      \"method\": \"Computational evolutionary analysis, mutagenesis, NPC-binding assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — structural and biochemical analysis with functional binding assay, but single lab and limited structural resolution\",\n      \"pmids\": [\"15522285\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Molecular dynamics simulations identified six FG-repeat peptide binding spots on the NTF2 surface forming a stripe, including two novel sites beyond the four known from X-ray/NMR data, providing a model for how FG repeats recognize NTF2 during NPC transit.\",\n      \"method\": \"Molecular dynamics simulation (254+ ns), agreement with prior X-ray, mutational, NMR, and computational data\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — computational only, no direct experimental validation of novel sites in this paper\",\n      \"pmids\": [\"17161424\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Nuclear size scales with NTF2 concentration in Xenopus egg extracts: higher NTF2 reduces nuclear size and selectively reduces import of large cargo molecules. NTF2 and importin-α together account for nuclear size differences between X. laevis and X. tropicalis, both acting through lamin B3 import.\",\n      \"method\": \"Xenopus egg extract nuclear reconstitution, titration of NTF2 and importin-α, fluorescent import assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reconstituted in vitro system with defined factor titration, two orthogonal phenotypic readouts (nuclear size, import rate), replicated in developmental context\",\n      \"pmids\": [\"20946986\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"In senescent cells, NTF2 is sequestered in the nucleus alongside reduced RanGTP restoration (due to ATP deficiency), contributing to failure of actin export via exportin 6.\",\n      \"method\": \"GST-pulldown (Exp6-RanGTP interaction), cellular fractionation, Exp6 knockdown, H2O2-induced senescence\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — indirect evidence for NTF2 sequestration, mechanistic link to actin export is inferred rather than directly demonstrated for NTF2\",\n      \"pmids\": [\"21195711\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Translocation of NTF2 from nucleus to cytoplasm to collect RanGDP is subject to regulation: treatment with polysorbitan monolaurate blocks NTF2 nuclear export (not import), causing cytoplasmic Ran accumulation and inhibiting RanGTP-dependent nuclear export processes; rescue requires NTF2 overexpression. Increased phosphorylation of tyrosine/threonine proteins accompanies this block.\",\n      \"method\": \"Polysorbitan monolaurate treatment, NTF2 overexpression rescue, tRNA and protein export assays, phosphorylation analysis\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — specific pharmacological block with rescue by NTF2 overexpression, two orthogonal transport readouts, single lab\",\n      \"pmids\": [\"22880006\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"NTF2 reduces nuclear pore complex diameter in a Ran-binding-dependent manner in Xenopus extracts; Ran binding to NTF2 is required for NTF2 to inhibit nuclear expansion and large-cargo import. Ectopic NTF2 expression reduces nuclear size in Xenopus embryos and mammalian tissue culture cells.\",\n      \"method\": \"Xenopus egg and embryo extract nuclear reconstitution, NTF2 mutants deficient in Ran binding, NPC diameter measurement, ectopic NTF2 expression in mammalian cells\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal assays (extract reconstitution, NPC diameter, mammalian cell overexpression), Ran-binding-dependent mechanism defined by mutant\",\n      \"pmids\": [\"26823604\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Native top-down proteomics revealed that EGFR signaling induces dissociation of NUTF2 dimers in breast cancer cells; NUTF2 proteoforms with K4 and K55 post-translational modifications differentially impact NUTF2's inhibition of the estrogen receptor (ER) signaling pathway.\",\n      \"method\": \"Native top-down proteomics (SEC, nano-LC, FAIMS, multistage MS), identification of endogenous proteoform complexes\",\n      \"journal\": \"Research square\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — preprint, single-lab mass spectrometry discovery, limited functional validation of the specific PTM–ER mechanism\",\n      \"pmids\": [\"37546719\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SANS with site-specific deuteration showed that FG-Nup repeats remain fully disordered upon binding to NTF2 in vitro, with no significant inter-aromatic contacts or interchain linkage in the FG-Nup–NTF2 complex.\",\n      \"method\": \"Small-angle neutron scattering (SANS) with site-specific deuteration and model fitting\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — novel structural method directly probing the complex, preprint, single study\",\n      \"pmids\": [],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"NUTF2 (NTF2) is a small homodimeric protein that functions as the dedicated nuclear import receptor for RanGDP: it binds RanGDP via a hydrophobic cavity (engaging the switch II loop of Ran, particularly Phe72, Lys71, and Arg76) and simultaneously engages FxFG-repeat nucleoporins at a separate dimer-interface site to translocate the NTF2–RanGDP complex through the nuclear pore complex; once inside the nucleus, RanGDP is released upon RCC1-catalyzed exchange to RanGTP (for which NTF2 has no affinity), establishing directional Ran import that replenishes the nuclear RanGTP pool required for all importin-β-family transport and for cell cycle progression; additionally, NTF2 acts as a Ran-GDI that stabilizes the GDP-bound state, its nuclear-to-cytoplasmic recycling is subject to phosphorylation-dependent regulation, and NTF2 levels negatively regulate nuclear size by reducing NPC diameter in a Ran-binding-dependent manner.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"NUTF2 (NTF2) is a small homodimeric nuclear transport factor that serves as the dedicated import receptor for RanGDP, replenishing the nuclear Ran pool that powers all importin-β-family transport [#0, #7]. Originally purified as a cytosolic factor acting at a late, post-docking stage of nuclear protein import through its interaction with the nucleoporin p62 [#0], NTF2 binds specifically to the GDP-bound form of Ran but not RanGTP, and engages FxFG-repeat nucleoporins at a separate, non-competing site on the dimer [#2, #4]. Crystallographic and structure-guided mutagenesis studies define both interfaces: the Ran switch II loop (Phe72, Lys71, Arg76) inserts into NTF2's hydrophobic cavity, with salt bridges to NTF2 Asp92/Asp94/Glu42 conferring GDP-versus-GTP selectivity, while a distinct dimer-interface site formed from both chains binds FxFG nucleoporins [#5, #6, #15]. Mutations that abolish Ran binding (E42K, D92N/D94N) or nucleoporin binding (W7A) each fail to support RanGDP import, establishing that both contacts are required for facilitated translocation through the pore [#5, #11]. The receptor functions directionally: it imports RanGDP, docks at and rapidly transits NPCs, and releases Ran upon RCC1-catalyzed exchange to RanGTP, for which it has no affinity [#7, #16]; NTF2 additionally acts as a Ran-GDI that stabilizes the GDP-bound state by blocking nucleotide dissociation [#9]. This activity is essential for viability and maintains the nuclear Ran concentration in living cells, linking Ran import to cell cycle progression through a spindle-assembly-checkpoint-dependent arrest when disrupted [#3, #13, #14]. Beyond transport, NTF2 concentration negatively regulates nuclear size by reducing NPC diameter in a Ran-binding-dependent manner and selectively limiting import of large cargo [#21, #24].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Established that a distinct cytosolic factor is required for nuclear protein import downstream of initial ligand docking, defining NTF2 as a discrete transport component rather than part of the importin machinery.\",\n      \"evidence\": \"Biochemical complementation of depleted cytosol with recombinant protein rescue and flow-cytometry import assay\",\n      \"pmids\": [\"7744965\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular cargo and the precise transport step were not defined\", \"No structural basis for nucleoporin interaction\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Solved the NTF2 fold and identified its hydrophobic cavity, then showed nucleotide-specific binding to RanGDP and separate FxFG-nucleoporin binding, revealing the bipartite molecular logic of the receptor.\",\n      \"evidence\": \"X-ray crystallography at 1.6 Å, affinity pulldowns from liver homogenate, competition binding, and GDP/GTP-loaded Ran binding assays; yeast essentiality and human complementation\",\n      \"pmids\": [\"8757804\", \"8918934\", \"8755535\", \"8702493\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic basis of GDP-vs-GTP selectivity not yet resolved at the complex level\", \"Cargo of NTF2 transport not yet identified as Ran itself\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstrated by structure-guided mutagenesis that the Ran-binding cavity is functionally separable from nucleoporin binding and is strictly required for import and viability, validating the two-site model in vivo.\",\n      \"evidence\": \"Site-directed mutants (E42K, D92N/D94N), mutant crystal structure, permeabilized-cell import, and yeast viability assays\",\n      \"pmids\": [\"9368653\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direction and recycling of the receptor not yet established\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Resolved the GDP-Ran–NTF2 co-crystal to explain GDP/GTP selectivity at atomic resolution and established NTF2 as the directional import receptor for RanGDP that also acts as a Ran-GDI.\",\n      \"evidence\": \"2.5 Å co-crystal structure, in vitro import reconstitution with loss-of-function mutants, digitonin-permeabilized assays, dominant-negative importin-β, and radiolabeled nucleotide dissociation assays\",\n      \"pmids\": [\"9533885\", \"9822603\", \"9889103\", \"9843686\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of RanGDP release on the nuclear side not directly characterized\", \"How NTF2 recycles back to the cytoplasm unresolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Established that both the Ran switch II residues and the NTF2 nucleoporin-binding tryptophan are individually essential for import, mechanistically coupling FG-nucleoporin engagement to productive translocation.\",\n      \"evidence\": \"Crystal structures of W7A-NTF2 and Ran switch II mutants, fluorescence/permeabilized import assays, Xenopus oocyte microinjection with colloidal gold EM, yeast viability\",\n      \"pmids\": [\"10543952\", \"10356329\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full set of FG-binding sites on NTF2 not yet mapped\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Showed in living cells and yeast that NTF2 maintains the nuclear Ran concentration, and that loss of nuclear Ran arrests the cell cycle through the spindle-assembly checkpoint, connecting Ran import to proliferation.\",\n      \"evidence\": \"Anti-NTF2 mAb cytoplasmic microinjection, fluorescence microscopy, in vitro import; yeast ts genetics with MAD2 epistasis\",\n      \"pmids\": [\"10679025\", \"10930458\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct mechanism linking Ran depletion to checkpoint activation not dissected\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Defined the symmetric dual FxFG-binding sites on the dimer and measured facilitated NTF2 transit through the NPC, and identified a non-FG nucleoporin (DNup88) partner in Drosophila tied to NF-κB nuclear import.\",\n      \"evidence\": \"1.9 Å co-crystal with FxFG core plus yeast functional validation, optical single-transporter recording in isolated nuclei, Drosophila genetics with Rel-protein localization and direct interaction\",\n      \"pmids\": [\"12065398\", \"12189172\", \"11943764\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"DNup88 interaction characterized in a single organism\", \"Single-transporter measurements from one study\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identified additional regulatory layers: an ATP-dependent phosphorylation-sensitive activity that displaces RanGDP from NTF2, and a conserved non-FxFG NPC-binding surface, refining the model of receptor cycling.\",\n      \"evidence\": \"In vitro GDP-dissociation assays with hydrolyzable vs non-hydrolyzable ATP and phosphatase inhibitors; evolutionary analysis with D23A NPC-binding mutant\",\n      \"pmids\": [\"15155737\", \"15522285\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the ATP-dependent displacement factor unknown\", \"Physiological relevance of the second NPC-binding site not established in vivo\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Revealed a developmental and morphometric role: NTF2 concentration scales inversely with nuclear size and selectively limits large-cargo import, accounting for interspecies nuclear-size differences via lamin B3 import.\",\n      \"evidence\": \"Xenopus egg extract nuclear reconstitution with titration of NTF2 and importin-α, fluorescent import assays\",\n      \"pmids\": [\"20946986\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which NTF2 levels constrain import not yet defined at the pore\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed that NTF2 controls nuclear size by reducing NPC diameter in a Ran-binding-dependent manner, mechanistically grounding the size phenotype in pore architecture.\",\n      \"evidence\": \"Xenopus egg/embryo extract reconstitution with Ran-binding-deficient NTF2 mutants, NPC diameter measurement, ectopic expression in mammalian cells\",\n      \"pmids\": [\"26823604\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Ran binding to NTF2 alters NPC diameter molecularly is unresolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Probed the physical nature of the FG-Nup–NTF2 complex, showing FG repeats remain disordered upon binding with no inter-aromatic crosslinking, informing models of how the receptor transits the permeability barrier.\",\n      \"evidence\": \"Small-angle neutron scattering with site-specific deuteration (preprint)\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, single study\", \"In vitro reconstitution may not capture the crowded NPC environment\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How NTF2 recycling, post-translational regulation, and putative signaling-linked moonlighting functions integrate with its canonical RanGDP import role remains unresolved.\",\n      \"evidence\": null,\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Identity of the ATP/phosphorylation-dependent displacement factor unknown\", \"Reported EGFR/ER-signaling and senescence roles rest on low-confidence or preprint evidence\", \"Mechanism coupling Ran binding to NPC diameter change undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 7, 8]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [9, 18]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [4, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005635\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [14]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [21, 24]}\n    ],\n    \"complexes\": [\n      \"NTF2 homodimer\",\n      \"NTF2–RanGDP complex\"\n    ],\n    \"partners\": [\n      \"RAN\",\n      \"NUP62\",\n      \"NUP88\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}