{"gene":"XPOT","run_date":"2026-04-28T23:00:23","timeline":{"discoveries":[{"year":1998,"finding":"Exportin-t (XPOT) was identified as a specific nuclear export receptor for tRNA. It is an importin-beta-related, RanGTP-binding protein with predominantly nuclear localization that shuttles between nucleus and cytoplasm, interacts with nuclear pore complexes, binds tRNA directly and with high affinity, and its cellular concentration in Xenopus oocytes was rate-limiting for export of all tRNAs tested. RanGTP regulates the substrate–exportin-t interaction such that tRNA is preferentially bound in the nucleus and released in the cytoplasm.","method":"Affinity chromatography, microinjection into Xenopus oocytes, RanGTP-binding assays, tRNA-binding assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 — original discovery paper with multiple orthogonal methods (affinity chromatography, binding assays, microinjection); replicated in a concurrent paper (PMID:9857198)","pmids":["9660920"],"is_preprint":false},{"year":1998,"finding":"Exportin-t–RanGTP selectively exports mature tRNAs by making extensive contacts with the backbone of the TψC and acceptor arms of tRNA; accurate 5' and 3' end-processing is required for efficient Xpo-t–RanGTP interaction and nuclear export, whereas aminoacylation is not essential. Intron-containing but end-processed pre-tRNAs can be exported when Xpo-t is present in excess, indicating at least two mechanisms discriminate pre-tRNAs from mature tRNAs.","method":"Chemical and enzymatic footprinting, phosphate modification interference, mutant/precursor tRNA microinjection into Xenopus oocyte nuclei, antibody-blocking experiments","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal structural probing and functional export assays with defined mutants","pmids":["9857198"],"is_preprint":false},{"year":2002,"finding":"The steady-state nuclear localization of Xpo-t depends on its interaction with RanGTP. Two distinct NPC-interaction domains were identified: the N-terminus binds Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner, while the C-terminus binds CAN/Nup214 independently of Ran. These interactions increase the concentration of tRNA export complexes and empty Xpo-t near NPCs, enhancing transport cycle efficiency.","method":"In vitro binding assays with isolated nucleoporins, fluorescence microscopy for steady-state localization, RanGTP mutant analysis","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding assays with defined mutants, clear functional interpretation","pmids":["12138183"],"is_preprint":false},{"year":2009,"finding":"Crystal structures of S. pombe Xpot in complex with tRNA and RanGTP (3.2 Å) and of unbound Xpot (3.1 Å) revealed that Xpot undergoes a large conformational change upon cargo binding, wrapping around the tRNA and specifically contacting the tRNA 5' and 3' ends. This binding mode explains how Xpot recognizes all mature tRNAs while discriminating against improperly processed ones, coupling tRNA export to quality control.","method":"X-ray crystallography (3.2 Å and 3.1 Å resolution crystal structures)","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structures of both nuclear (cargo-bound) and cytosolic (unbound) states; single study with two complementary structures","pmids":["19680239"],"is_preprint":false},{"year":2001,"finding":"Xpo-t/RanGTP can bind tRNA-attached ribozymes (tRNA-Rz) with an extended 3' end both in vitro and in somatic cells, enabling their export to the cytoplasm. This recognition is similar to that of mature tRNAs. An inhibitor present in Xenopus oocyte nuclear extract selectively blocks Xpo-t-dependent export of tRNA-Rz but not mature tRNAs, suggesting a proofreading mechanism in oocytes that is relaxed in somatic cells.","method":"In vitro binding assays, microinjection into somatic cells and Xenopus oocytes, nuclear extract inhibition experiments","journal":"Biomacromolecules","confidence":"Medium","confidence_rationale":"Tier 2-3 — binding and export assays with functional readout, single lab","pmids":["11777397"],"is_preprint":false},{"year":2010,"finding":"Downregulation of XPOT (Xpo-t) in human fibroblasts causes nuclear accumulation of tRNA, leading to reduced mTORC1 activity and upregulated autophagy, demonstrating that subcellular tRNA localization mediated by Xpo-t can regulate intracellular nutritional stress signaling independently of cellular nutritional status.","method":"siRNA knockdown of Xpo-t, tRNA localization assays, mTORC1 activity assay, autophagy measurement in human fibroblasts","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2-3 — loss-of-function with defined molecular phenotype (mTORC1, autophagy), single lab","pmids":["20714220"],"is_preprint":false},{"year":2016,"finding":"Molecular dynamics simulations of Xpot complexes revealed dynamic structural hinges that mediate the nuclear-to-cytosolic conformational transition; post-RanGTP hydrolysis, local conformational changes in Ran and loss of critical contacts at the Xpot/tRNA interface drive tRNA release. HEAT repeat flexibility varies depending on cargo-binding state.","method":"Classical all-atom and accelerated molecular dynamics simulations of free and cargo-bound Xpot","journal":"Biophysical journal","confidence":"Low","confidence_rationale":"Tier 4 — computational study only, no experimental validation","pmids":["27028637"],"is_preprint":false},{"year":2022,"finding":"XPOT mediates nuclear export of NFAT5 under hypotonic conditions in an unconventional, tonicity-dependent manner. siRNA screening and proteomics identified that XPOT drives NFAT5 nuclear export under hypotonicity, and that RUVBL2 (a RuvB-like AAA-type ATPase) is an indispensable chaperone for this process.","method":"siRNA screening, proteomics/mass spectrometry, co-immunoprecipitation, live-cell imaging","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2-3 — siRNA screen, proteomics, and co-IP identify XPOT and RUVBL2 in NFAT5 export; single lab with multiple methods","pmids":["35635291"],"is_preprint":false},{"year":2023,"finding":"XPOT knockdown in triple-negative breast cancer (TNBC) cells inhibited proliferation and caused cytokinesis failure. XPOT was shown to preferentially export a specific tRNA isodecoder (tRNA-Ala-AGC-10-1) from the nucleus, thereby promoting translation of TTC19, a protein required for cytokinesis completion in TNBC cells.","method":"siRNA knockdown, high-throughput tRNA sequencing, codon preferential analysis, protein mass spectrometry, RNA-seq, in vitro cell proliferation assays","journal":"International journal of biological sciences","confidence":"Medium","confidence_rationale":"Tier 2-3 — multiple orthogonal methods (tRNA-seq, proteomics, codon analysis) in single lab establishing cargo selectivity and downstream translation consequence","pmids":["37928256"],"is_preprint":false},{"year":2018,"finding":"XPOT knockdown in hepatocellular carcinoma (HCC) cell lines inhibited tumor proliferation and invasion in vitro and in vivo. Mechanistically, XPOT knockdown caused G0/G1 cell cycle arrest accompanied by downregulation of CDK1, CDK2, CDK4, CyclinA1, CyclinB1, CyclinB2, and CyclinE2.","method":"siRNA knockdown, CCK-8 proliferation assay, wound-healing and Transwell invasion assays, subcutaneous xenograft mouse model, GSEA, western blotting","journal":"Molecular carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 2-3 — loss-of-function with defined molecular phenotype (cell cycle regulators), in vitro and in vivo validation, single lab","pmids":["30334580"],"is_preprint":false},{"year":2025,"finding":"XPOT knockdown in breast cancer cells promoted pyroptosis, as evidenced by increased IL-1β and IL-18 secretion, elevated N-terminal GSDMD cleavage, and upregulation of NLRP3, ASC, and cleaved-caspase-1. The pyroptosis inhibitor azalamellarin N reversed these effects, linking XPOT to suppression of the NLRP3 inflammasome/caspase-1/GSDMD pyroptotic pathway.","method":"siRNA knockdown, Western blotting, ELISA, CCK-8, Transwell, TUNEL assays, pharmacological inhibition","journal":"Central-European journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2-3 — loss-of-function with defined pyroptosis pathway components and pharmacological rescue, single lab","pmids":["41438360"],"is_preprint":false},{"year":2025,"finding":"XPOT knockdown in breast cancer cells inhibited proliferation and invasion; mechanistically, XPOT knockdown reduced activation of the PI3K/AKT/mTOR signaling pathway and downregulated cyclin D and CDK4/6.","method":"siRNA knockdown, CCK-8 assay, Transwell assay, Western blotting for PI3K/AKT/mTOR pathway components","journal":"Journal of inflammation research","confidence":"Low","confidence_rationale":"Tier 3 — single lab, loss-of-function with pathway marker readout but no direct mechanistic link established between tRNA export and PI3K/AKT signaling","pmids":["40416714"],"is_preprint":false}],"current_model":"XPOT (Exportin-t) is an importin-β–family nuclear export receptor that cooperatively binds mature tRNA and RanGTP in the nucleus—recognizing the processed 5' and 3' ends and the TψC/acceptor arm—wraps around the tRNA cargo (as shown by crystal structure), and releases it in the cytoplasm upon RanGTP hydrolysis; it interacts with nucleoporins (Nup153, RanBP2/Nup358, CAN/Nup214) via distinct N- and C-terminal domains to concentrate export complexes at the NPC, and has been shown to also export the transcription factor NFAT5 (requiring RUVBL2 as a chaperone) and to selectively transport specific tRNA isodecoders that drive translation of cytokinesis-essential proteins, with its loss causing cell-cycle arrest, autophagy induction via mTORC1 suppression, and pyroptosis in cancer cells."},"narrative":{"teleology":[{"year":1998,"claim":"The identity of the nuclear tRNA export receptor was unknown; identification of Exportin-t as a RanGTP-dependent, importin-β-family factor that directly binds and rate-limits tRNA export established the core transport mechanism.","evidence":"Affinity chromatography, tRNA-binding assays, microinjection into Xenopus oocytes, and chemical/enzymatic footprinting of tRNA–Xpo-t–RanGTP contacts","pmids":["9660920","9857198"],"confidence":"High","gaps":["Structural basis for simultaneous recognition of all mature tRNAs was not yet resolved","Whether Xpo-t exports non-tRNA cargoes was unknown","Mechanism of tRNA release in the cytoplasm was not defined"]},{"year":2002,"claim":"How Exportin-t navigates the nuclear pore was unclear; mapping of two distinct NPC-binding domains — an N-terminal RanGTP-dependent interaction with Nup153/RanBP2 and a C-terminal Ran-independent interaction with CAN/Nup214 — showed how the transporter concentrates at the pore to enhance export cycle efficiency.","evidence":"In vitro binding assays with isolated nucleoporins, fluorescence microscopy, RanGTP mutant analysis","pmids":["12138183"],"confidence":"High","gaps":["The order and kinetics of nucleoporin engagement during a single translocation cycle were not determined","Contribution of each nucleoporin interaction to in vivo export rates was untested"]},{"year":2009,"claim":"The atomic mechanism by which Exportin-t recognizes all mature tRNAs while rejecting unprocessed precursors was resolved by crystal structures showing that Xpot wraps around tRNA, directly contacting the 5′ and 3′ ends, coupling export to end-processing quality control.","evidence":"X-ray crystallography of S. pombe Xpot–tRNA–RanGTP (3.2 Å) and free Xpot (3.1 Å)","pmids":["19680239"],"confidence":"High","gaps":["Dynamic steps of cargo release upon RanGTP hydrolysis lacked experimental characterization","Whether the same binding mode applies to all isoacceptor tRNAs was not tested"]},{"year":2010,"claim":"The physiological consequences of impaired tRNA export were unknown; XPOT knockdown revealed that nuclear tRNA accumulation suppresses mTORC1 activity and induces autophagy, linking tRNA subcellular distribution to nutrient-sensing signaling.","evidence":"siRNA knockdown of Xpo-t in human fibroblasts, tRNA localization assays, mTORC1 activity and autophagy measurements","pmids":["20714220"],"confidence":"Medium","gaps":["The molecular sensor connecting nuclear tRNA levels to mTORC1 was not identified","Single-lab finding; independent replication in other cell types was lacking"]},{"year":2018,"claim":"Whether XPOT is functionally important in cancer cell proliferation was untested; knockdown in hepatocellular carcinoma cells caused G0/G1 arrest with downregulation of multiple CDKs and cyclins both in vitro and in xenograft models, establishing XPOT as a proliferation dependency.","evidence":"siRNA knockdown, proliferation/invasion assays, subcutaneous xenograft model, Western blotting","pmids":["30334580"],"confidence":"Medium","gaps":["Whether cell-cycle arrest results from global tRNA depletion or selective loss of specific tRNAs was not distinguished","No direct mechanistic link between tRNA export deficiency and CDK/cyclin protein levels was established"]},{"year":2022,"claim":"Whether XPOT exports non-tRNA cargoes was an open question; an siRNA screen and proteomics identified XPOT as the export receptor for transcription factor NFAT5 under hypotonic conditions, with RUVBL2 serving as an essential chaperone, broadening XPOT function beyond tRNA.","evidence":"siRNA screen, mass spectrometry, co-immunoprecipitation, live-cell imaging","pmids":["35635291"],"confidence":"Medium","gaps":["Whether XPOT binds NFAT5 directly or via RUVBL2-bridged interaction was not resolved","The NFAT5 export signal recognized by XPOT was not mapped","Generality of XPOT as a protein exporter for other transcription factors is unknown"]},{"year":2023,"claim":"Whether XPOT exhibits selectivity among tRNA species was unknown; demonstration that XPOT preferentially exports the isodecoder tRNA-Ala-AGC-10-1 to drive translation of the cytokinesis factor TTC19 in TNBC cells revealed cargo-selective export coupled to codon-biased translation.","evidence":"siRNA knockdown, high-throughput tRNA-seq, codon preferential analysis, protein mass spectrometry in TNBC cells","pmids":["37928256"],"confidence":"Medium","gaps":["The structural basis for isodecoder selectivity is unknown","Whether selectivity is intrinsic to XPOT or mediated by adaptor proteins was not addressed","Applicability beyond TNBC cells not tested"]},{"year":2025,"claim":"XPOT knockdown in breast cancer cells activates the NLRP3/caspase-1/GSDMD pyroptosis pathway, an effect reversed by a pyroptosis inhibitor, connecting tRNA export deficiency to inflammatory cell death.","evidence":"siRNA knockdown, Western blotting, ELISA for IL-1β/IL-18, pharmacological rescue with azalamellarin N","pmids":["41438360"],"confidence":"Medium","gaps":["The molecular link between tRNA export loss and NLRP3 inflammasome activation is not defined","Single-lab study; independent confirmation needed","Relevance to non-cancer cell types is untested"]},{"year":null,"claim":"Critical open questions remain: the molecular sensor linking nuclear tRNA accumulation to mTORC1 and inflammasome pathways, the structural basis for isodecoder selectivity, and the full repertoire of non-tRNA protein cargoes of XPOT are unknown.","evidence":"","pmids":[],"confidence":"High","gaps":["No sensor/adaptor connecting tRNA compartmentalization to mTORC1 or pyroptosis has been identified","Structural determinants of isodecoder-selective export are uncharacterized","Systematic identification of XPOT protein cargoes beyond NFAT5 has not been performed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[0,1,3,8]},{"term_id":"GO:0005215","term_label":"transporter activity","supporting_discovery_ids":[0,1,7,8]},{"term_id":"GO:0003924","term_label":"GTPase activity","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,2]},{"term_id":"GO:0005635","term_label":"nuclear envelope","supporting_discovery_ids":[2]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[5]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,1,3,8]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[0,1,2,7,8]}],"complexes":[],"partners":["RANBP2","NUP153","NUP214","NFAT5","RUVBL2","RAN"],"other_free_text":[]},"mechanistic_narrative":"XPOT (Exportin-t) is the principal importin-β-family nuclear export receptor for mature tRNA, coupling tRNA biogenesis quality control to cytoplasmic translation. It forms a ternary complex with RanGTP and tRNA in the nucleus, recognizing the processed 5′ and 3′ ends and the TψC/acceptor arm of mature tRNA through a large conformational wrapping motion, and releases cargo in the cytoplasm upon RanGTP hydrolysis [PMID:9660920, PMID:9857198, PMID:19680239]. Distinct N- and C-terminal domains mediate interactions with nucleoporins Nup153, RanBP2/Nup358, and CAN/Nup214, concentrating export complexes at the nuclear pore [PMID:12138183]. Beyond bulk tRNA export, XPOT selectively transports specific tRNA isodecoders to regulate translation of cytokinesis-essential proteins, exports the transcription factor NFAT5 under hypotonic conditions via a RUVBL2-dependent mechanism, and its depletion suppresses mTORC1 signaling to induce autophagy and triggers NLRP3/caspase-1-dependent pyroptosis in cancer cells [PMID:37928256, PMID:35635291, PMID:20714220, PMID:41438360]."},"prefetch_data":{"uniprot":{"accession":"O43592","full_name":"Exportin-T","aliases":["Exportin(tRNA)","tRNA exportin"],"length_aa":962,"mass_kda":110.0,"function":"Mediates the nuclear export of aminoacylated tRNAs. In the nucleus binds to tRNA and to the GTPase Ran in its active GTP-bound form. Docking of this trimeric complex to the nuclear pore complex (NPC) is mediated through binding to nucleoporins. Upon transit of a nuclear export complex into the cytoplasm, disassembling of the complex and hydrolysis of Ran-GTP to Ran-GDP (induced by RANBP1 and RANGAP1, respectively) cause release of the tRNA from the export receptor. XPOT then return to the nuclear compartment and mediate another round of transport. The directionality of nuclear export is thought to be conferred by an asymmetric distribution of the GTP- and GDP-bound forms of Ran between the cytoplasm and nucleus","subcellular_location":"Nucleus; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/O43592/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/XPOT","classification":"Common Essential","n_dependent_lines":704,"n_total_lines":1208,"dependency_fraction":0.5827814569536424},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000184575","cell_line_id":"CID001583","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"ERF","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001583","total_profiled":1310},"omim":[{"mim_id":"611449","title":"EXPORTIN 4; XPO4","url":"https://www.omim.org/entry/611449"},{"mim_id":"604834","title":"TANK-BINDING KINASE 1; TBK1","url":"https://www.omim.org/entry/604834"},{"mim_id":"603180","title":"EXPORTIN, tRNA; XPOT","url":"https://www.omim.org/entry/603180"},{"mim_id":"177700","title":"GLAUCOMA 1, OPEN ANGLE, P; GLC1P","url":"https://www.omim.org/entry/177700"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Nucleoplasm","reliability":"Enhanced"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/XPOT"},"hgnc":{"alias_symbol":["XPO3"],"prev_symbol":[]},"alphafold":{"accession":"O43592","domains":[{"cath_id":"1.20.1050","chopping":"272-386_397-419","consensus_level":"medium","plddt":94.6911,"start":272,"end":419},{"cath_id":"1.20.930","chopping":"422-548","consensus_level":"medium","plddt":91.176,"start":422,"end":548},{"cath_id":"1.25.40","chopping":"819-962","consensus_level":"medium","plddt":91.1044,"start":819,"end":962}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O43592","model_url":"https://alphafold.ebi.ac.uk/files/AF-O43592-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O43592-F1-predicted_aligned_error_v6.png","plddt_mean":91.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XPOT","jax_strain_url":"https://www.jax.org/strain/search?query=XPOT"},"sequence":{"accession":"O43592","fasta_url":"https://rest.uniprot.org/uniprotkb/O43592.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O43592/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O43592"}},"corpus_meta":[{"pmid":"9857198","id":"PMC_9857198","title":"The 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interaction between Xpo-t-RanGTP and the backbone of the TΨC and acceptor arms of tRNA; accurate 5' and 3' end-processing of tRNA is required for Xpo-t-RanGTP interaction and export, while aminoacylation is not essential; antibody inhibition confirmed Xpo-t is responsible for the majority of tRNA export from Xenopus oocyte nuclei.\",\n      \"method\": \"Xenopus oocyte nuclear export assay with antibody inhibition, chemical and enzymatic footprinting, phosphate modification interference, mutant/precursor tRNA binding assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal biochemical methods, replicated in cellular and in vitro contexts, foundational paper with 179 citations\",\n      \"pmids\": [\"9857198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structures of S. pombe Xpot in complex with tRNA and RanGTP (3.2 Å, nuclear state) and unbound Xpot (3.1 Å, cytosolic state) reveal that Xpot undergoes a large conformational change upon cargo binding, wrapping around the tRNA and specifically contacting the 5' and 3' ends; this binding mode explains recognition of all mature tRNAs while discriminating against improperly processed precursors, coupling tRNA export to quality control.\",\n      \"method\": \"X-ray crystallography (3.2 Å and 3.1 Å structures), structural comparison of nuclear and cytosolic states\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structures of both functional states with mechanistic interpretation, 109 citations\",\n      \"pmids\": [\"19680239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Xpo-t steady-state nuclear localization depends on RanGTP interaction; Xpo-t possesses two distinct NPC interaction domains: the N-terminus binds nucleoporins Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner, while the C-terminus binds CAN/Nup214 independently of Ran; these interactions increase the concentration of tRNA export complexes near NPCs to enhance transport efficiency.\",\n      \"method\": \"In vitro binding assays with purified nucleoporins, localization of Xpo-t variants, RanGTP dependency experiments\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding assays with multiple nucleoporins, RanGTP dependency tested, functional consequence of NPC interaction defined\",\n      \"pmids\": [\"12138183\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Xpo-t/RanGTP binds to tRNA-attached ribozymes (tRNA with extended 3' ends) in vitro and in somatic cells, mediating their nuclear export; this recognition resembles that of mature tRNAs in somatic cells but not Xenopus oocytes, where an inhibitor of the Xpo-t-dependent export pathway acts as a proofreading mechanism.\",\n      \"method\": \"In vitro binding assays (Xpo-t/RanGTP pulldown with tRNA-ribozyme constructs), nuclear export assays in somatic cells and Xenopus oocytes, nuclear extract injection experiments\",\n      \"journal\": \"Biomacromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro binding plus cellular export assays, but single lab\",\n      \"pmids\": [\"11777397\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Downregulation of Xpo-t in human fibroblasts causes nuclear accumulation of tRNAs, reduced mTORC1 activity, and upregulated autophagy, demonstrating that subcellular localization of tRNAs regulated by Xpo-t modulates intracellular stress signaling pathways independently of cellular nutritional status.\",\n      \"method\": \"siRNA knockdown of Xpo-t, tRNA localization assay, mTORC1 activity assay, autophagy measurement in human fibroblasts\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with defined cellular phenotype (mTORC1 and autophagy), single lab\",\n      \"pmids\": [\"20714220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Molecular dynamics simulations on S. pombe Xpot complexes revealed that post-RanGTP hydrolysis leads to local conformational changes in Ran and loss of critical contacts at the Xpot/tRNA interface, identifying dynamic structural hinges (specific HEAT repeats) responsible for the nuclear-to-cytosolic state transition and tRNA release.\",\n      \"method\": \"Classical all-atom and accelerated molecular dynamics simulations on crystal structure-based models of Xpot complexes\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational modeling only, no experimental validation\",\n      \"pmids\": [\"27028637\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Modest overexpression of S. pombe los1+ (Xpo-t ortholog) in sla1-Δ cells suppresses nuclear tRNA export defects, reduction in pre-tRNA levels, amino acid metabolism gene upregulation, and slow growth, placing Xpo-t/Los1p as a direct regulator of the nuclear tRNA export pathway that feeds into the TOR/nutritional stress response.\",\n      \"method\": \"Genetic epistasis (overexpression of los1+ in sla1-Δ background), mRNA profiling, tRNA distribution analysis in S. pombe\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with mRNA profiling and tRNA localization, single lab but multiple orthogonal readouts\",\n      \"pmids\": [\"22160596\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"XPOT mediates nuclear export of NFAT5 under hypotonicity in a tonicity-dependent manner, requiring RUVBL2 as an indispensable chaperone; siRNA screening and proteomics identified XPOT as the exportin responsible for NFAT5 cytoplasmic relocalization under low-tonicity conditions, representing an unconventional nuclear export pathway beyond tRNA transport.\",\n      \"method\": \"siRNA screening, proteomics analysis, co-immunoprecipitation, localization assays in mammalian cells under hypo/hypertonic conditions\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA epistasis screen combined with proteomics and direct interaction assay, single lab\",\n      \"pmids\": [\"35635291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XPOT knockdown in triple-negative breast cancer cells inhibits proliferation and causes cytokinesis failure; high-throughput tRNA sequencing showed XPOT specifically controls nuclear export of a subset of tRNA isodecoders (including tRNA-Ala-AGC-10-1); codon preference and protein mass spectrometry revealed that XPOT-mediated tRNA-Ala-AGC-10-1 export drives translation of TTC19, which is required for cytokinesis.\",\n      \"method\": \"siRNA knockdown, RNA-seq, high-throughput tRNA sequencing, codon preference analysis, protein mass spectrometry, proliferation and cytokinesis assays in TNBC cells\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (tRNA-seq, mass spectrometry, functional assays) in single lab\",\n      \"pmids\": [\"37928256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"siRNA-mediated knockdown of XPOT in HCC cell lines (HepG2 and 7721) inhibits tumor cell proliferation and invasion in vitro and tumor formation in xenograft models; knockdown caused G0/G1 phase arrest associated with downregulation of CDK1, CDK2, CDK4, CyclinA1, CyclinB1, CyclinB2, and CyclinE2.\",\n      \"method\": \"siRNA knockdown, CCK-8 proliferation assay, wound healing/migration assay, subcutaneous xenograft model, flow cytometry cell cycle analysis, western blotting\",\n      \"journal\": \"Molecular carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with defined cellular and in vivo phenotype plus molecular pathway readout, single lab\",\n      \"pmids\": [\"30334580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"XPOT knockdown in BC cells activates pyroptosis, evidenced by increased IL-1β and IL-18 secretion, elevated N-terminal cleavage of GSDMD, and upregulation of NLRP3, ASC, and cleaved-caspase1; the pyroptosis inhibitor azalamellarin N reversed si-XPOT effects, linking XPOT to suppression of the NLRP3 inflammasome/GSDMD pyroptosis pathway in breast cancer cells.\",\n      \"method\": \"siRNA knockdown, ELISA (IL-1β, IL-18), western blotting (GSDMD, NLRP3, ASC, caspase-1), pharmacological rescue with pyroptosis inhibitor in MCF-7 cells\",\n      \"journal\": \"Central-European journal of immunology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single KD approach, phenotypic pathway with pharmacological rescue but no direct mechanistic link between tRNA export and pyroptosis\",\n      \"pmids\": [\"41438360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"XPOT knockdown in BC cells (MDA-MB-468/231) inhibits proliferation and invasion; western blotting showed reduced phosphorylation of PI3K/AKT/mTOR pathway components and downregulation of cyclin D and CDK4/6, suggesting XPOT promotes BC cell cycle progression via the PI3K/AKT/mTOR pathway.\",\n      \"method\": \"siRNA knockdown, CCK-8 proliferation assay, Transwell invasion assay, western blotting of PI3K/AKT/mTOR and cell cycle proteins\",\n      \"journal\": \"Journal of inflammation research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, KD with pathway western blot but no direct mechanistic link between XPOT tRNA export activity and PI3K/AKT signaling\",\n      \"pmids\": [\"40416714\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XPOT (Exportin-t) is a karyopherin-β family nuclear export receptor that forms a cooperative trimeric complex with RanGTP and mature, correctly end-processed tRNAs—contacting their 5' and 3' ends and the TΨC/acceptor arms—to drive tRNA nuclear export through the NPC via interactions with nucleoporins Nup153, RanBP2/Nup358, and CAN/Nup214; RanGTP hydrolysis in the cytoplasm triggers conformational opening and cargo release, and XPOT additionally mediates tonicity-dependent nuclear export of the transcription factor NFAT5 (requiring the chaperone RUVBL2), while its role in selective tRNA isodecoder export controls downstream translation of specific proteins (e.g., TTC19) and thereby influences cancer cell proliferation, cytokinesis, and stress signaling.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1998,\n      \"finding\": \"Exportin-t (XPOT) was identified as a specific nuclear export receptor for tRNA. It is an importin-beta-related, RanGTP-binding protein with predominantly nuclear localization that shuttles between nucleus and cytoplasm, interacts with nuclear pore complexes, binds tRNA directly and with high affinity, and its cellular concentration in Xenopus oocytes was rate-limiting for export of all tRNAs tested. RanGTP regulates the substrate–exportin-t interaction such that tRNA is preferentially bound in the nucleus and released in the cytoplasm.\",\n      \"method\": \"Affinity chromatography, microinjection into Xenopus oocytes, RanGTP-binding assays, tRNA-binding assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — original discovery paper with multiple orthogonal methods (affinity chromatography, binding assays, microinjection); replicated in a concurrent paper (PMID:9857198)\",\n      \"pmids\": [\"9660920\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Exportin-t–RanGTP selectively exports mature tRNAs by making extensive contacts with the backbone of the TψC and acceptor arms of tRNA; accurate 5' and 3' end-processing is required for efficient Xpo-t–RanGTP interaction and nuclear export, whereas aminoacylation is not essential. Intron-containing but end-processed pre-tRNAs can be exported when Xpo-t is present in excess, indicating at least two mechanisms discriminate pre-tRNAs from mature tRNAs.\",\n      \"method\": \"Chemical and enzymatic footprinting, phosphate modification interference, mutant/precursor tRNA microinjection into Xenopus oocyte nuclei, antibody-blocking experiments\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal structural probing and functional export assays with defined mutants\",\n      \"pmids\": [\"9857198\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The steady-state nuclear localization of Xpo-t depends on its interaction with RanGTP. Two distinct NPC-interaction domains were identified: the N-terminus binds Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner, while the C-terminus binds CAN/Nup214 independently of Ran. These interactions increase the concentration of tRNA export complexes and empty Xpo-t near NPCs, enhancing transport cycle efficiency.\",\n      \"method\": \"In vitro binding assays with isolated nucleoporins, fluorescence microscopy for steady-state localization, RanGTP mutant analysis\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding assays with defined mutants, clear functional interpretation\",\n      \"pmids\": [\"12138183\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structures of S. pombe Xpot in complex with tRNA and RanGTP (3.2 Å) and of unbound Xpot (3.1 Å) revealed that Xpot undergoes a large conformational change upon cargo binding, wrapping around the tRNA and specifically contacting the tRNA 5' and 3' ends. This binding mode explains how Xpot recognizes all mature tRNAs while discriminating against improperly processed ones, coupling tRNA export to quality control.\",\n      \"method\": \"X-ray crystallography (3.2 Å and 3.1 Å resolution crystal structures)\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structures of both nuclear (cargo-bound) and cytosolic (unbound) states; single study with two complementary structures\",\n      \"pmids\": [\"19680239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Xpo-t/RanGTP can bind tRNA-attached ribozymes (tRNA-Rz) with an extended 3' end both in vitro and in somatic cells, enabling their export to the cytoplasm. This recognition is similar to that of mature tRNAs. An inhibitor present in Xenopus oocyte nuclear extract selectively blocks Xpo-t-dependent export of tRNA-Rz but not mature tRNAs, suggesting a proofreading mechanism in oocytes that is relaxed in somatic cells.\",\n      \"method\": \"In vitro binding assays, microinjection into somatic cells and Xenopus oocytes, nuclear extract inhibition experiments\",\n      \"journal\": \"Biomacromolecules\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — binding and export assays with functional readout, single lab\",\n      \"pmids\": [\"11777397\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Downregulation of XPOT (Xpo-t) in human fibroblasts causes nuclear accumulation of tRNA, leading to reduced mTORC1 activity and upregulated autophagy, demonstrating that subcellular tRNA localization mediated by Xpo-t can regulate intracellular nutritional stress signaling independently of cellular nutritional status.\",\n      \"method\": \"siRNA knockdown of Xpo-t, tRNA localization assays, mTORC1 activity assay, autophagy measurement in human fibroblasts\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — loss-of-function with defined molecular phenotype (mTORC1, autophagy), single lab\",\n      \"pmids\": [\"20714220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Molecular dynamics simulations of Xpot complexes revealed dynamic structural hinges that mediate the nuclear-to-cytosolic conformational transition; post-RanGTP hydrolysis, local conformational changes in Ran and loss of critical contacts at the Xpot/tRNA interface drive tRNA release. HEAT repeat flexibility varies depending on cargo-binding state.\",\n      \"method\": \"Classical all-atom and accelerated molecular dynamics simulations of free and cargo-bound Xpot\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational study only, no experimental validation\",\n      \"pmids\": [\"27028637\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"XPOT mediates nuclear export of NFAT5 under hypotonic conditions in an unconventional, tonicity-dependent manner. siRNA screening and proteomics identified that XPOT drives NFAT5 nuclear export under hypotonicity, and that RUVBL2 (a RuvB-like AAA-type ATPase) is an indispensable chaperone for this process.\",\n      \"method\": \"siRNA screening, proteomics/mass spectrometry, co-immunoprecipitation, live-cell imaging\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — siRNA screen, proteomics, and co-IP identify XPOT and RUVBL2 in NFAT5 export; single lab with multiple methods\",\n      \"pmids\": [\"35635291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"XPOT knockdown in triple-negative breast cancer (TNBC) cells inhibited proliferation and caused cytokinesis failure. XPOT was shown to preferentially export a specific tRNA isodecoder (tRNA-Ala-AGC-10-1) from the nucleus, thereby promoting translation of TTC19, a protein required for cytokinesis completion in TNBC cells.\",\n      \"method\": \"siRNA knockdown, high-throughput tRNA sequencing, codon preferential analysis, protein mass spectrometry, RNA-seq, in vitro cell proliferation assays\",\n      \"journal\": \"International journal of biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — multiple orthogonal methods (tRNA-seq, proteomics, codon analysis) in single lab establishing cargo selectivity and downstream translation consequence\",\n      \"pmids\": [\"37928256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"XPOT knockdown in hepatocellular carcinoma (HCC) cell lines inhibited tumor proliferation and invasion in vitro and in vivo. Mechanistically, XPOT knockdown caused G0/G1 cell cycle arrest accompanied by downregulation of CDK1, CDK2, CDK4, CyclinA1, CyclinB1, CyclinB2, and CyclinE2.\",\n      \"method\": \"siRNA knockdown, CCK-8 proliferation assay, wound-healing and Transwell invasion assays, subcutaneous xenograft mouse model, GSEA, western blotting\",\n      \"journal\": \"Molecular carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — loss-of-function with defined molecular phenotype (cell cycle regulators), in vitro and in vivo validation, single lab\",\n      \"pmids\": [\"30334580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"XPOT knockdown in breast cancer cells promoted pyroptosis, as evidenced by increased IL-1β and IL-18 secretion, elevated N-terminal GSDMD cleavage, and upregulation of NLRP3, ASC, and cleaved-caspase-1. The pyroptosis inhibitor azalamellarin N reversed these effects, linking XPOT to suppression of the NLRP3 inflammasome/caspase-1/GSDMD pyroptotic pathway.\",\n      \"method\": \"siRNA knockdown, Western blotting, ELISA, CCK-8, Transwell, TUNEL assays, pharmacological inhibition\",\n      \"journal\": \"Central-European journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — loss-of-function with defined pyroptosis pathway components and pharmacological rescue, single lab\",\n      \"pmids\": [\"41438360\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"XPOT knockdown in breast cancer cells inhibited proliferation and invasion; mechanistically, XPOT knockdown reduced activation of the PI3K/AKT/mTOR signaling pathway and downregulated cyclin D and CDK4/6.\",\n      \"method\": \"siRNA knockdown, CCK-8 assay, Transwell assay, Western blotting for PI3K/AKT/mTOR pathway components\",\n      \"journal\": \"Journal of inflammation research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, loss-of-function with pathway marker readout but no direct mechanistic link established between tRNA export and PI3K/AKT signaling\",\n      \"pmids\": [\"40416714\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XPOT (Exportin-t) is an importin-β–family nuclear export receptor that cooperatively binds mature tRNA and RanGTP in the nucleus—recognizing the processed 5' and 3' ends and the TψC/acceptor arm—wraps around the tRNA cargo (as shown by crystal structure), and releases it in the cytoplasm upon RanGTP hydrolysis; it interacts with nucleoporins (Nup153, RanBP2/Nup358, CAN/Nup214) via distinct N- and C-terminal domains to concentrate export complexes at the NPC, and has been shown to also export the transcription factor NFAT5 (requiring RUVBL2 as a chaperone) and to selectively transport specific tRNA isodecoders that drive translation of cytokinesis-essential proteins, with its loss causing cell-cycle arrest, autophagy induction via mTORC1 suppression, and pyroptosis in cancer cells.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"XPOT (Exportin-t) is a karyopherin-β family nuclear transport receptor that mediates the RanGTP-dependent export of mature tRNAs from the nucleus, thereby linking tRNA biogenesis quality control to cytoplasmic translation, nutrient signaling, and cell proliferation. XPOT forms a cooperative trimeric complex with RanGTP and tRNA, contacting the TψC and acceptor arms and specifically recognizing correctly 5′- and 3′-end-processed tRNAs; crystal structures of both the cargo-bound nuclear state and the unliganded cytoplasmic state reveal a large conformational change that wraps around tRNA and explains discrimination against precursor species [PMID:9857198, PMID:19680239]. XPOT docks at the nuclear pore through an N-terminal RanGTP-dependent interaction with Nup153 and RanBP2/Nup358 and a C-terminal Ran-independent interaction with CAN/Nup214, concentrating export complexes at the NPC [PMID:12138183]. Beyond tRNA, XPOT mediates tonicity-dependent nuclear export of the transcription factor NFAT5 in a RUVBL2-dependent manner [PMID:35635291], and its selective export of specific tRNA isodecoders controls translation of proteins such as TTC19 required for cytokinesis in cancer cells [PMID:37928256].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Identification of XPOT as the principal tRNA nuclear export receptor resolved how mature tRNAs transit the NPC, establishing that a karyopherin-β family member forms a cooperative RanGTP–tRNA trimeric complex that recognizes correctly end-processed, but not necessarily aminoacylated, tRNAs.\",\n      \"evidence\": \"Xenopus oocyte export assays with antibody inhibition, chemical/enzymatic footprinting, and phosphate modification interference\",\n      \"pmids\": [\"9857198\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of tRNA recognition not yet determined\",\n        \"Mechanism of cargo release after RanGTP hydrolysis unknown\",\n        \"How XPOT interacts with the NPC not characterized\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Mapping of two distinct NPC-binding domains on XPOT explained how the exportin concentrates tRNA export complexes at the nuclear pore — its N-terminus binds Nup153 and RanBP2/Nup358 in a RanGTP-dependent manner while the C-terminus binds CAN/Nup214 independently of Ran.\",\n      \"evidence\": \"In vitro binding assays with purified nucleoporins, localization of XPOT domain variants, RanGTP dependency tests\",\n      \"pmids\": [\"12138183\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural details of XPOT–nucleoporin interactions unresolved\",\n        \"Relative contribution of each NPC docking site to export kinetics in vivo not measured\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Crystal structures of both the cargo-bound (nuclear) and free (cytosolic) states of Xpot revealed the molecular basis of tRNA recognition and quality control — XPOT wraps around tRNA with specific contacts to 5′ and 3′ ends, and a large conformational change between states explains how RanGTP hydrolysis leads to cargo release.\",\n      \"evidence\": \"X-ray crystallography of S. pombe Xpot–RanGTP–tRNA (3.2 Å) and free Xpot (3.1 Å)\",\n      \"pmids\": [\"19680239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Dynamic mechanism of RanGTP hydrolysis-triggered conformational transition not experimentally captured\",\n        \"No structure of the human ortholog\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Functional knockdown revealed that XPOT-dependent tRNA export controls intracellular stress signaling, as loss of XPOT caused nuclear tRNA accumulation, reduced mTORC1 activity, and increased autophagy independently of nutritional status.\",\n      \"evidence\": \"siRNA knockdown of XPOT in human fibroblasts with mTORC1 and autophagy readouts\",\n      \"pmids\": [\"20714220\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular link between cytoplasmic tRNA levels and mTORC1 activation not defined\",\n        \"Contribution of individual tRNA species to the signaling phenotype unknown\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery that XPOT exports the transcription factor NFAT5 under hypotonicity established a non-tRNA cargo for this exportin and identified RUVBL2 as an essential chaperone for this unconventional export pathway.\",\n      \"evidence\": \"siRNA screen, proteomics, co-immunoprecipitation, and NFAT5 localization assays in mammalian cells under osmotic stress\",\n      \"pmids\": [\"35635291\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"How XPOT recognizes NFAT5 (binding interface) not determined\",\n        \"Whether additional protein cargoes exist is unknown\",\n        \"Role of RUVBL2 ATPase activity in the export complex not tested\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"High-throughput tRNA sequencing demonstrated that XPOT selectively exports a subset of tRNA isodecoders (e.g., tRNA-Ala-AGC-10-1), and this selectivity controls translation of specific mRNAs such as TTC19, linking XPOT to codon-biased translational regulation and cytokinesis in cancer cells.\",\n      \"evidence\": \"tRNA-seq, codon preference analysis, protein mass spectrometry, and cytokinesis assays in triple-negative breast cancer cells after siRNA knockdown\",\n      \"pmids\": [\"37928256\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Structural or sequence determinants that confer isodecoder selectivity not identified\",\n        \"Whether selectivity operates in non-cancer cell types not tested\",\n        \"Direct demonstration that tRNA-Ala-AGC-10-1 is rate-limiting for TTC19 translation in rescue experiments not shown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural and sequence determinants that allow XPOT to discriminate among tRNA isodecoders remain unknown, as does the full scope of non-tRNA protein cargoes and the mechanistic link between cytoplasmic tRNA levels and mTORC1/stress signaling.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structural basis for isodecoder-selective export\",\n        \"Complete inventory of XPOT protein cargoes not established\",\n        \"Mechanism coupling cytoplasmic tRNA abundance to mTORC1 and pyroptosis pathways undefined\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 1, 3]},\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 2, 7, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1, 2, 7, 8]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 8]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"complexes\": [\n      \"XPOT–RanGTP–tRNA trimeric export complex\"\n    ],\n    \"partners\": [\n      \"RAN\",\n      \"NUP153\",\n      \"RANBP2\",\n      \"NUP214\",\n      \"NFAT5\",\n      \"RUVBL2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"XPOT (Exportin-t) is the principal importin-β-family nuclear export receptor for mature tRNA, coupling tRNA biogenesis quality control to cytoplasmic translation. It forms a ternary complex with RanGTP and tRNA in the nucleus, recognizing the processed 5′ and 3′ ends and the TψC/acceptor arm of mature tRNA through a large conformational wrapping motion, and releases cargo in the cytoplasm upon RanGTP hydrolysis [PMID:9660920, PMID:9857198, PMID:19680239]. Distinct N- and C-terminal domains mediate interactions with nucleoporins Nup153, RanBP2/Nup358, and CAN/Nup214, concentrating export complexes at the nuclear pore [PMID:12138183]. Beyond bulk tRNA export, XPOT selectively transports specific tRNA isodecoders to regulate translation of cytokinesis-essential proteins, exports the transcription factor NFAT5 under hypotonic conditions via a RUVBL2-dependent mechanism, and its depletion suppresses mTORC1 signaling to induce autophagy and triggers NLRP3/caspase-1-dependent pyroptosis in cancer cells [PMID:37928256, PMID:35635291, PMID:20714220, PMID:41438360].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"The identity of the nuclear tRNA export receptor was unknown; identification of Exportin-t as a RanGTP-dependent, importin-β-family factor that directly binds and rate-limits tRNA export established the core transport mechanism.\",\n      \"evidence\": \"Affinity chromatography, tRNA-binding assays, microinjection into Xenopus oocytes, and chemical/enzymatic footprinting of tRNA–Xpo-t–RanGTP contacts\",\n      \"pmids\": [\"9660920\", \"9857198\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis for simultaneous recognition of all mature tRNAs was not yet resolved\",\n        \"Whether Xpo-t exports non-tRNA cargoes was unknown\",\n        \"Mechanism of tRNA release in the cytoplasm was not defined\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"How Exportin-t navigates the nuclear pore was unclear; mapping of two distinct NPC-binding domains — an N-terminal RanGTP-dependent interaction with Nup153/RanBP2 and a C-terminal Ran-independent interaction with CAN/Nup214 — showed how the transporter concentrates at the pore to enhance export cycle efficiency.\",\n      \"evidence\": \"In vitro binding assays with isolated nucleoporins, fluorescence microscopy, RanGTP mutant analysis\",\n      \"pmids\": [\"12138183\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The order and kinetics of nucleoporin engagement during a single translocation cycle were not determined\",\n        \"Contribution of each nucleoporin interaction to in vivo export rates was untested\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"The atomic mechanism by which Exportin-t recognizes all mature tRNAs while rejecting unprocessed precursors was resolved by crystal structures showing that Xpot wraps around tRNA, directly contacting the 5′ and 3′ ends, coupling export to end-processing quality control.\",\n      \"evidence\": \"X-ray crystallography of S. pombe Xpot–tRNA–RanGTP (3.2 Å) and free Xpot (3.1 Å)\",\n      \"pmids\": [\"19680239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Dynamic steps of cargo release upon RanGTP hydrolysis lacked experimental characterization\",\n        \"Whether the same binding mode applies to all isoacceptor tRNAs was not tested\"\n      ]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"The physiological consequences of impaired tRNA export were unknown; XPOT knockdown revealed that nuclear tRNA accumulation suppresses mTORC1 activity and induces autophagy, linking tRNA subcellular distribution to nutrient-sensing signaling.\",\n      \"evidence\": \"siRNA knockdown of Xpo-t in human fibroblasts, tRNA localization assays, mTORC1 activity and autophagy measurements\",\n      \"pmids\": [\"20714220\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"The molecular sensor connecting nuclear tRNA levels to mTORC1 was not identified\",\n        \"Single-lab finding; independent replication in other cell types was lacking\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Whether XPOT is functionally important in cancer cell proliferation was untested; knockdown in hepatocellular carcinoma cells caused G0/G1 arrest with downregulation of multiple CDKs and cyclins both in vitro and in xenograft models, establishing XPOT as a proliferation dependency.\",\n      \"evidence\": \"siRNA knockdown, proliferation/invasion assays, subcutaneous xenograft model, Western blotting\",\n      \"pmids\": [\"30334580\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether cell-cycle arrest results from global tRNA depletion or selective loss of specific tRNAs was not distinguished\",\n        \"No direct mechanistic link between tRNA export deficiency and CDK/cyclin protein levels was established\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Whether XPOT exports non-tRNA cargoes was an open question; an siRNA screen and proteomics identified XPOT as the export receptor for transcription factor NFAT5 under hypotonic conditions, with RUVBL2 serving as an essential chaperone, broadening XPOT function beyond tRNA.\",\n      \"evidence\": \"siRNA screen, mass spectrometry, co-immunoprecipitation, live-cell imaging\",\n      \"pmids\": [\"35635291\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether XPOT binds NFAT5 directly or via RUVBL2-bridged interaction was not resolved\",\n        \"The NFAT5 export signal recognized by XPOT was not mapped\",\n        \"Generality of XPOT as a protein exporter for other transcription factors is unknown\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Whether XPOT exhibits selectivity among tRNA species was unknown; demonstration that XPOT preferentially exports the isodecoder tRNA-Ala-AGC-10-1 to drive translation of the cytokinesis factor TTC19 in TNBC cells revealed cargo-selective export coupled to codon-biased translation.\",\n      \"evidence\": \"siRNA knockdown, high-throughput tRNA-seq, codon preferential analysis, protein mass spectrometry in TNBC cells\",\n      \"pmids\": [\"37928256\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"The structural basis for isodecoder selectivity is unknown\",\n        \"Whether selectivity is intrinsic to XPOT or mediated by adaptor proteins was not addressed\",\n        \"Applicability beyond TNBC cells not tested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"XPOT knockdown in breast cancer cells activates the NLRP3/caspase-1/GSDMD pyroptosis pathway, an effect reversed by a pyroptosis inhibitor, connecting tRNA export deficiency to inflammatory cell death.\",\n      \"evidence\": \"siRNA knockdown, Western blotting, ELISA for IL-1β/IL-18, pharmacological rescue with azalamellarin N\",\n      \"pmids\": [\"41438360\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"The molecular link between tRNA export loss and NLRP3 inflammasome activation is not defined\",\n        \"Single-lab study; independent confirmation needed\",\n        \"Relevance to non-cancer cell types is untested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Critical open questions remain: the molecular sensor linking nuclear tRNA accumulation to mTORC1 and inflammasome pathways, the structural basis for isodecoder selectivity, and the full repertoire of non-tRNA protein cargoes of XPOT are unknown.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No sensor/adaptor connecting tRNA compartmentalization to mTORC1 or pyroptosis has been identified\",\n        \"Structural determinants of isodecoder-selective export are uncharacterized\",\n        \"Systematic identification of XPOT protein cargoes beyond NFAT5 has not been performed\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [0, 1, 3, 8]},\n      {\"term_id\": \"GO:0005215\", \"supporting_discovery_ids\": [0, 1, 7, 8]},\n      {\"term_id\": \"GO:0003924\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005635\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0009536\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [5]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 1, 3, 8]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1, 2, 7, 8]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"RANBP2\",\n      \"NUP153\",\n      \"NUP214\",\n      \"NFAT5\",\n      \"RUVBL2\",\n      \"RAN\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}