{"gene":"IPO5","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":1997,"finding":"RanBP5 (IPO5) was identified as a novel 124-kDa Ran-binding protein related to importin-beta. It binds the GTP-bound form of Ran, stabilizing it against GTP hydrolysis and nucleotide exchange. RanBP5 forms a trimeric RanBP1-Ran-RanBP5 complex, with RanBP1 capable of relieving GAP resistance of the RanBP5-RanGTP complex. RanBP5 is a predominantly cytoplasmic protein that can bind to nuclear pore complexes.","method":"Overlay assay with RanGTP, yeast two-hybrid with RanBP1 as bait, biochemical binding assays, subcellular fractionation","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal biochemical methods (overlay assay, two-hybrid, in vitro binding assays, fractionation) in a single rigorous study characterizing the founding properties of this protein","pmids":["9271386"],"is_preprint":false},{"year":1998,"finding":"RanBP5 (IPO5) directly binds and imports ribosomal proteins in mammalian cells. Ribosomal proteins L23a, S7, and L5 can each be imported alternatively by RanBP5 (alongside importin-beta, transportin, and RanBP7). RanBP5 binds to a very basic region of rpL23a that may represent an archetypal import signal.","method":"In vitro nuclear import assays in mammalian cells, direct binding assays between import receptors and ribosomal proteins","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro reconstituted import assays with defined substrates, replicated across multiple ribosomal proteins","pmids":["9687515"],"is_preprint":false},{"year":2012,"finding":"IPO5 facilitates nuclear import of CPEB3 by binding directly to the RRM1 domain of CPEB3. NMDAR signaling increases RanBP1 expression and reduces cytoplasmic GTP-bound Ran levels, which enhances the CPEB3-IPO5 interaction and accelerates nuclear import of CPEB3 in neurons.","method":"Co-immunoprecipitation, domain mapping, subcellular fractionation, NMDAR stimulation experiments in neurons","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — Co-IP with domain mapping and functional NMDAR stimulation experiments, single lab but multiple methods","pmids":["22730302"],"is_preprint":false},{"year":2015,"finding":"IPO5 interacts with RNA-binding protein Musashi-1 (MSI1) and facilitates its nuclear translocation to the transcriptionally silenced XY chromatin domain in meiotic pachytene spermatocytes, resulting in release of MSI1 RNA-binding targets.","method":"Co-immunoprecipitation, immunofluorescence, transgenic overexpression model in mouse testis","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP and localization data with functional context in spermatocytes, single lab, multiple methods","pmids":["25782991"],"is_preprint":false},{"year":2016,"finding":"IPO5 forms a stable, stoichiometric complex with the influenza A virus PA-PB1 heterodimer. The PA-PB1-RanBP5 complex can be modelled by SAXS and is no longer capable of 5'-vRNA binding, suggesting that nuclear import and RNA binding by the viral polymerase are mutually exclusive.","method":"Reconstitution of protein complexes, SAXS, in vitro RNA binding assays","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with structural analysis (SAXS) and functional RNA binding assay, single lab but multiple orthogonal methods","pmids":["27095520"],"is_preprint":false},{"year":2019,"finding":"IPO5 binds the NLS sequence of RASAL2 and mediates its nuclear translocation in colorectal cancer cells, which induces RAS signal activation and promotes cancer progression.","method":"Mass spectrometry, co-immunoprecipitation, subcellular fractionation, immunofluorescence, NLS mapping","journal":"Journal of experimental & clinical cancer research","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — Co-IP combined with mass spectrometry, subcellular fractionation, and NLS mapping, single lab","pmids":["31288861"],"is_preprint":false},{"year":2020,"finding":"Crystal structures of two distinct isoforms of ligand-free human RanBP5 (IPO5) were solved, confirming the general architecture and mechanism of the IMB3 karyopherin-β subfamily while highlighting differences with the yeast orthologue Kap121p. Point mutations designed based on NLS docking suppress influenza PA-PB1 binding to RanBP5 in a binary protein complementation assay.","method":"X-ray crystallography, NLS docking, point mutagenesis, binary protein complementation assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure combined with mutagenesis and functional binding assay, single lab but multiple orthogonal methods","pmids":["32222384"],"is_preprint":false},{"year":2022,"finding":"IPO5 directly binds the capsid protein (Cap) of porcine circovirus type 2 (PCV2), with the N-terminal arginine24 of Cap being critical for binding to proline709 of IPO5. IPO5 promotes nuclear import of incoming PCV2 virions and also stabilizes the Cap protein against proteasome-mediated degradation.","method":"Co-immunoprecipitation combined with mass spectrometry, GST pulldown, site-directed mutagenesis, knockdown experiments, cycloheximide chase, proteasome inhibitor rescue","journal":"Journal of virology","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — GST pulldown reconstitution, mutagenesis identifying specific residues, and multiple functional assays (nuclear import, stability) in single rigorous study","pmids":["36409110"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM structure at 3.2 Å resolution of the RanBP5-PB1 NLS domain complex reveals that the NLS domain of influenza PB1 lacks secondary structure and interacts with RanBP5 in a wrapped conformation. Biochemical analyses of mutants identified critical amino acid residues mediating complex binding.","method":"Cryo-EM structure determination at 3.2 Å, biochemical mutagenesis of binding interface","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with mutagenesis validation, single lab but Tier 1 structural method with functional verification","pmids":["39536408"],"is_preprint":false},{"year":2025,"finding":"Global IPO5 knockout in mice is lethal at early embryogenesis (no null embryos at E12.5). Germline-specific deletion using VasaCre causes complete loss of germ cells in adult testes but does not affect oocyte development or female fertility. Stra8Cre-directed IPO5 deletion causes meiotic failure. Novel IPO5-binding proteins identified by immunoprecipitation/mass spectrometry include SFPQ in fetal testes and XPO2 (exportin 2) in spermatocytes and spermatids.","method":"Conditional knockout mice (CMVCre, VasaCre, Stra8Cre), immunoprecipitation followed by mass spectrometry","journal":"Biology of reproduction","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic knockout with multiple Cre lines showing specific phenotypic readouts, combined with IP-MS for cargo identification","pmids":["40652294"],"is_preprint":false}],"current_model":"IPO5 (RanBP5/KPNB3) is an importin-beta family nuclear transport receptor that directly binds RanGTP and nuclear pore complexes; it imports diverse cargoes including ribosomal proteins (via basic NLS regions), transcription factors, RNA-binding proteins (CPEB3, MSI1), and signaling proteins (RASAL2) by recognizing their NLS sequences, with its cargo interactions regulated by the Ran-GTP gradient; it is essential for early embryogenesis and male germline development in vivo, and is exploited by viruses (influenza A PA-PB1, PCV2 Cap) for nuclear trafficking of their components."},"narrative":{"mechanistic_narrative":"IPO5 (RanBP5/KPNB3) is an importin-beta family nuclear transport receptor that mediates the nuclear import of diverse cargoes by recognizing basic NLS sequences, coupling cargo binding and release to the RanGTP gradient [PMID:9271386, PMID:9687515]. It was founded as a 124-kDa Ran-binding protein that binds RanGTP, stabilizes it against hydrolysis and exchange, associates with nuclear pore complexes, and assembles into a RanBP1-Ran-RanBP5 complex [PMID:9271386]. As an import receptor it carries ribosomal proteins (L23a, S7, L5), binding the very basic region of rpL23a as an archetypal import signal [PMID:9687515], and it imports RNA-binding and signaling cargoes including CPEB3 (via its RRM1 domain) [PMID:22730302], Musashi-1 [PMID:25782991], and RASAL2, whose nuclear translocation activates RAS signaling in colorectal cancer cells [PMID:31288861]. Crystal and cryo-EM structures of human RanBP5 define the IMB3 karyopherin-beta architecture and show that cargo NLS elements, such as the unstructured influenza PB1 NLS, are engaged in a wrapped conformation [PMID:32222384, PMID:39536408]. IPO5 is essential in vivo: global knockout causes early embryonic lethality and germline-specific deletion ablates germ cells and causes meiotic failure [PMID:40652294]. Viruses exploit IPO5 for nuclear trafficking, with the influenza A PA-PB1 heterodimer forming a stable complex in which polymerase RNA binding and import are mutually exclusive [PMID:27095520], and PCV2 capsid both imported and stabilized against proteasomal degradation by IPO5 [PMID:36409110].","teleology":[{"year":1997,"claim":"Established IPO5 as a Ran-binding member of the importin-beta family, answering whether this novel protein operates within the Ran-dependent transport system.","evidence":"Overlay assay with RanGTP, yeast two-hybrid, in vitro binding and subcellular fractionation","pmids":["9271386"],"confidence":"High","gaps":["No cargo identified at this stage","Directionality of transport (import vs export) not defined"]},{"year":1998,"claim":"Showed IPO5 acts as a bona fide import receptor for ribosomal proteins, identifying a basic NLS as the recognition signal and defining its physiological cargo class.","evidence":"In vitro reconstituted nuclear import assays and direct receptor-cargo binding in mammalian cells","pmids":["9687515"],"confidence":"High","gaps":["Redundancy with importin-beta, transportin and RanBP7 leaves cargo specificity unresolved","No structural basis for NLS recognition"]},{"year":2012,"claim":"Linked IPO5 cargo handling to physiological signaling by showing NMDAR-driven RanBP1 changes modulate the CPEB3-IPO5 interaction, establishing regulated, signal-responsive import.","evidence":"Co-IP, domain mapping (RRM1) and NMDAR stimulation in neurons","pmids":["22730302"],"confidence":"Medium","gaps":["Single lab","In vivo neuronal relevance not tested genetically"]},{"year":2015,"claim":"Extended the cargo repertoire to Musashi-1 and connected IPO5 to germline chromatin biology, showing its import activity has tissue-specific functional consequences.","evidence":"Co-IP, immunofluorescence and transgenic overexpression in mouse testis","pmids":["25782991"],"confidence":"Medium","gaps":["Overexpression-based, not loss-of-function","NLS within MSI1 not mapped"]},{"year":2016,"claim":"Defined a viral hijacking mechanism by showing IPO5 binds the influenza PA-PB1 heterodimer and that import and polymerase RNA binding are mutually exclusive.","evidence":"Reconstituted complexes, SAXS modelling and in vitro RNA-binding assays","pmids":["27095520"],"confidence":"High","gaps":["No atomic-resolution interface","Cellular import dynamics inferred from in vitro data"]},{"year":2019,"claim":"Connected IPO5-mediated import to oncogenic signaling by demonstrating it carries the RASAL2 NLS to activate RAS signaling in colorectal cancer.","evidence":"Mass spectrometry, Co-IP, fractionation, immunofluorescence and NLS mapping","pmids":["31288861"],"confidence":"Medium","gaps":["Single lab","Causality between import and tumor progression not genetically dissected"]},{"year":2020,"claim":"Provided the first structural framework for human RanBP5, confirming IMB3 subfamily architecture and validating NLS docking through mutations that disrupt influenza PA-PB1 binding.","evidence":"X-ray crystallography of two ligand-free isoforms with point mutagenesis and protein complementation","pmids":["32222384"],"confidence":"High","gaps":["Structures are ligand-free","Cargo-bound conformations not captured here"]},{"year":2022,"claim":"Revealed a dual function beyond transport by showing IPO5 imports PCV2 virions and stabilizes its capsid against proteasomal degradation, defining specific contact residues.","evidence":"GST pulldown, site-directed mutagenesis, knockdown, cycloheximide chase and proteasome inhibitor rescue","pmids":["36409110"],"confidence":"High","gaps":["Mechanism of capsid stabilization not resolved","Whether stabilization extends to host cargoes unknown"]},{"year":2024,"claim":"Resolved how IPO5 engages an unstructured cargo NLS by cryo-EM, showing the influenza PB1 NLS is wrapped by RanBP5 and identifying critical interface residues.","evidence":"Cryo-EM at 3.2 Å of the RanBP5-PB1 NLS complex with interface mutagenesis","pmids":["39536408"],"confidence":"High","gaps":["Structure limited to a viral NLS fragment","Host cargo recognition modes not directly compared"]},{"year":2025,"claim":"Defined the in vivo requirement for IPO5, showing it is essential for early embryogenesis and male germline/meiotic progression, and identified new binding partners.","evidence":"Conditional knockout mice (CMVCre, VasaCre, Stra8Cre) and IP-MS identifying SFPQ and XPO2","pmids":["40652294"],"confidence":"High","gaps":["Cargoes responsible for the meiotic/embryonic phenotypes not established","Functional role of SFPQ and XPO2 interactions not characterized"]},{"year":null,"claim":"How IPO5 achieves cargo selectivity among overlapping importin-beta family receptors and which specific cargoes drive its essential developmental and germline functions remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No unified determinant of cargo specificity defined","Cargoes mediating embryonic lethality and meiotic failure unidentified","Physiological role of capsid-stabilization activity for endogenous substrates unknown"]}],"mechanism_profile":{"molecular_activity":[],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[1,2,5]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[0,1,5]},{"term_id":"R-HSA-1474165","term_label":"Reproduction","supporting_discovery_ids":[9]}],"complexes":["RanBP1-Ran-RanBP5 complex","influenza A PA-PB1-RanBP5 complex"],"partners":["RAN","RANBP1","CPEB3","MSI1","RASAL2","SFPQ","XPO2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O00410","full_name":"Importin-5","aliases":["Importin subunit beta-3","Karyopherin beta-3","Ran-binding protein 5","RanBP5"],"length_aa":1097,"mass_kda":123.6,"function":"Functions in nuclear protein import as nuclear transport receptor. Serves as receptor for nuclear localization signals (NLS) in cargo substrates. Is thought to mediate docking of the importin/substrate complex to the nuclear pore complex (NPC) through binding to nucleoporin and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism. At the nucleoplasmic side of the NPC, Ran binds to the importin, the importin/substrate complex dissociates and importin is re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran. The directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP- and GDP-bound forms of Ran between the cytoplasm and nucleus (By similarity). Mediates the nuclear import of ribosomal proteins RPL23A, RPS7 and RPL5 (PubMed:11682607, PubMed:9687515). In vitro, mediates nuclear import of H2A, H2B, H3 and H4 histones. Binds to CPEB3 and mediates its nuclear import following neuronal stimulation (By similarity). In case of HIV-1 infection, binds and mediates the nuclear import of HIV-1 Rev","subcellular_location":"Cytoplasm; Nucleus; Nucleus, nucleolus","url":"https://www.uniprot.org/uniprotkb/O00410/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/IPO5","classification":"Not Classified","n_dependent_lines":212,"n_total_lines":1208,"dependency_fraction":0.17549668874172186},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000065150","cell_line_id":"CID001554","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"nucleoplasm","grade":3}],"interactors":[{"gene":"NOC4L","stoichiometry":10.0},{"gene":"GNL3","stoichiometry":4.0},{"gene":"RPL29","stoichiometry":4.0},{"gene":"RPL13","stoichiometry":4.0},{"gene":"NOP14","stoichiometry":4.0},{"gene":"NAP1L4","stoichiometry":4.0},{"gene":"NAP1L1","stoichiometry":4.0},{"gene":"C19ORF53","stoichiometry":0.2},{"gene":"CAPZB","stoichiometry":0.2},{"gene":"CSNK2A2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001554","total_profiled":1310},"omim":[{"mim_id":"610889","title":"IMPORTIN 11; IPO11","url":"https://www.omim.org/entry/610889"},{"mim_id":"602008","title":"IMPORTIN 5; IPO5","url":"https://www.omim.org/entry/602008"},{"mim_id":"600876","title":"SYNTAXIN 3; STX3","url":"https://www.omim.org/entry/600876"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear membrane","reliability":"Supported"},{"location":"Golgi apparatus","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"testis","ntpm":196.2}],"url":"https://www.proteinatlas.org/search/IPO5"},"hgnc":{"alias_symbol":["IMB3","MGC2068","PSE1"],"prev_symbol":["KPNB3","RANBP5"]},"alphafold":{"accession":"O00410","domains":[{"cath_id":"-","chopping":"1-97","consensus_level":"medium","plddt":87.4652,"start":1,"end":97},{"cath_id":"1.25.10","chopping":"797-971","consensus_level":"medium","plddt":95.7755,"start":797,"end":971},{"cath_id":"-","chopping":"1000-1097","consensus_level":"medium","plddt":92.1841,"start":1000,"end":1097},{"cath_id":"1.25.40","chopping":"161-296","consensus_level":"medium","plddt":94.7414,"start":161,"end":296},{"cath_id":"1.25.40","chopping":"494-500_523-634","consensus_level":"medium","plddt":87.7493,"start":494,"end":634}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O00410","model_url":"https://alphafold.ebi.ac.uk/files/AF-O00410-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O00410-F1-predicted_aligned_error_v6.png","plddt_mean":92.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=IPO5","jax_strain_url":"https://www.jax.org/strain/search?query=IPO5"},"sequence":{"accession":"O00410","fasta_url":"https://rest.uniprot.org/uniprotkb/O00410.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O00410/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O00410"}},"corpus_meta":[{"pmid":"9687515","id":"PMC_9687515","title":"Importin beta, transportin, RanBP5 and RanBP7 mediate nuclear import of ribosomal proteins in mammalian cells.","date":"1998","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/9687515","citation_count":432,"is_preprint":false},{"pmid":"9732266","id":"PMC_9732266","title":"Phosphorylation regulates association of the transcription factor Pho4 with its import receptor Pse1/Kap121.","date":"1998","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/9732266","citation_count":206,"is_preprint":false},{"pmid":"9271386","id":"PMC_9271386","title":"Ran-binding protein 5 (RanBP5) is related to the nuclear transport factor importin-beta but interacts differently with RanBP1.","date":"1997","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/9271386","citation_count":75,"is_preprint":false},{"pmid":"10191220","id":"PMC_10191220","title":"Multidrug-resistant human and animal Salmonella typhimurium isolates in France belong predominantly to a DT104 clone with the chromosome- and integron-encoded beta-lactamase PSE-1.","date":"1999","source":"The Journal of infectious diseases","url":"https://pubmed.ncbi.nlm.nih.gov/10191220","citation_count":63,"is_preprint":false},{"pmid":"2125285","id":"PMC_2125285","title":"The human U1 snRNA promoter correctly initiates transcription in vitro and is activated by PSE1.","date":"1990","source":"Genes & development","url":"https://pubmed.ncbi.nlm.nih.gov/2125285","citation_count":47,"is_preprint":false},{"pmid":"1804019","id":"PMC_1804019","title":"Sequence of the PSE-1 beta-lactamase gene.","date":"1991","source":"Antimicrobial agents and chemotherapy","url":"https://pubmed.ncbi.nlm.nih.gov/1804019","citation_count":44,"is_preprint":false},{"pmid":"22730302","id":"PMC_22730302","title":"NMDAR signaling facilitates the IPO5-mediated nuclear import of CPEB3.","date":"2012","source":"Nucleic acids 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It binds the GTP-bound form of Ran, stabilizing it against GTP hydrolysis and nucleotide exchange. RanBP5 forms a trimeric RanBP1-Ran-RanBP5 complex, with RanBP1 capable of relieving GAP resistance of the RanBP5-RanGTP complex. RanBP5 is a predominantly cytoplasmic protein that can bind to nuclear pore complexes.\",\n      \"method\": \"Overlay assay with RanGTP, yeast two-hybrid with RanBP1 as bait, biochemical binding assays, subcellular fractionation\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal biochemical methods (overlay assay, two-hybrid, in vitro binding assays, fractionation) in a single rigorous study characterizing the founding properties of this protein\",\n      \"pmids\": [\"9271386\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"RanBP5 (IPO5) directly binds and imports ribosomal proteins in mammalian cells. Ribosomal proteins L23a, S7, and L5 can each be imported alternatively by RanBP5 (alongside importin-beta, transportin, and RanBP7). RanBP5 binds to a very basic region of rpL23a that may represent an archetypal import signal.\",\n      \"method\": \"In vitro nuclear import assays in mammalian cells, direct binding assays between import receptors and ribosomal proteins\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro reconstituted import assays with defined substrates, replicated across multiple ribosomal proteins\",\n      \"pmids\": [\"9687515\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"IPO5 facilitates nuclear import of CPEB3 by binding directly to the RRM1 domain of CPEB3. NMDAR signaling increases RanBP1 expression and reduces cytoplasmic GTP-bound Ran levels, which enhances the CPEB3-IPO5 interaction and accelerates nuclear import of CPEB3 in neurons.\",\n      \"method\": \"Co-immunoprecipitation, domain mapping, subcellular fractionation, NMDAR stimulation experiments in neurons\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — Co-IP with domain mapping and functional NMDAR stimulation experiments, single lab but multiple methods\",\n      \"pmids\": [\"22730302\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"IPO5 interacts with RNA-binding protein Musashi-1 (MSI1) and facilitates its nuclear translocation to the transcriptionally silenced XY chromatin domain in meiotic pachytene spermatocytes, resulting in release of MSI1 RNA-binding targets.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence, transgenic overexpression model in mouse testis\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP and localization data with functional context in spermatocytes, single lab, multiple methods\",\n      \"pmids\": [\"25782991\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"IPO5 forms a stable, stoichiometric complex with the influenza A virus PA-PB1 heterodimer. The PA-PB1-RanBP5 complex can be modelled by SAXS and is no longer capable of 5'-vRNA binding, suggesting that nuclear import and RNA binding by the viral polymerase are mutually exclusive.\",\n      \"method\": \"Reconstitution of protein complexes, SAXS, in vitro RNA binding assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with structural analysis (SAXS) and functional RNA binding assay, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"27095520\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"IPO5 binds the NLS sequence of RASAL2 and mediates its nuclear translocation in colorectal cancer cells, which induces RAS signal activation and promotes cancer progression.\",\n      \"method\": \"Mass spectrometry, co-immunoprecipitation, subcellular fractionation, immunofluorescence, NLS mapping\",\n      \"journal\": \"Journal of experimental & clinical cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — Co-IP combined with mass spectrometry, subcellular fractionation, and NLS mapping, single lab\",\n      \"pmids\": [\"31288861\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Crystal structures of two distinct isoforms of ligand-free human RanBP5 (IPO5) were solved, confirming the general architecture and mechanism of the IMB3 karyopherin-β subfamily while highlighting differences with the yeast orthologue Kap121p. Point mutations designed based on NLS docking suppress influenza PA-PB1 binding to RanBP5 in a binary protein complementation assay.\",\n      \"method\": \"X-ray crystallography, NLS docking, point mutagenesis, binary protein complementation assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure combined with mutagenesis and functional binding assay, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"32222384\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"IPO5 directly binds the capsid protein (Cap) of porcine circovirus type 2 (PCV2), with the N-terminal arginine24 of Cap being critical for binding to proline709 of IPO5. IPO5 promotes nuclear import of incoming PCV2 virions and also stabilizes the Cap protein against proteasome-mediated degradation.\",\n      \"method\": \"Co-immunoprecipitation combined with mass spectrometry, GST pulldown, site-directed mutagenesis, knockdown experiments, cycloheximide chase, proteasome inhibitor rescue\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — GST pulldown reconstitution, mutagenesis identifying specific residues, and multiple functional assays (nuclear import, stability) in single rigorous study\",\n      \"pmids\": [\"36409110\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM structure at 3.2 Å resolution of the RanBP5-PB1 NLS domain complex reveals that the NLS domain of influenza PB1 lacks secondary structure and interacts with RanBP5 in a wrapped conformation. Biochemical analyses of mutants identified critical amino acid residues mediating complex binding.\",\n      \"method\": \"Cryo-EM structure determination at 3.2 Å, biochemical mutagenesis of binding interface\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with mutagenesis validation, single lab but Tier 1 structural method with functional verification\",\n      \"pmids\": [\"39536408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Global IPO5 knockout in mice is lethal at early embryogenesis (no null embryos at E12.5). Germline-specific deletion using VasaCre causes complete loss of germ cells in adult testes but does not affect oocyte development or female fertility. Stra8Cre-directed IPO5 deletion causes meiotic failure. Novel IPO5-binding proteins identified by immunoprecipitation/mass spectrometry include SFPQ in fetal testes and XPO2 (exportin 2) in spermatocytes and spermatids.\",\n      \"method\": \"Conditional knockout mice (CMVCre, VasaCre, Stra8Cre), immunoprecipitation followed by mass spectrometry\",\n      \"journal\": \"Biology of reproduction\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic knockout with multiple Cre lines showing specific phenotypic readouts, combined with IP-MS for cargo identification\",\n      \"pmids\": [\"40652294\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IPO5 (RanBP5/KPNB3) is an importin-beta family nuclear transport receptor that directly binds RanGTP and nuclear pore complexes; it imports diverse cargoes including ribosomal proteins (via basic NLS regions), transcription factors, RNA-binding proteins (CPEB3, MSI1), and signaling proteins (RASAL2) by recognizing their NLS sequences, with its cargo interactions regulated by the Ran-GTP gradient; it is essential for early embryogenesis and male germline development in vivo, and is exploited by viruses (influenza A PA-PB1, PCV2 Cap) for nuclear trafficking of their components.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"IPO5 (RanBP5/KPNB3) is an importin-beta family nuclear transport receptor that mediates the nuclear import of diverse cargoes by recognizing basic NLS sequences, coupling cargo binding and release to the RanGTP gradient [#0, #1]. It was founded as a 124-kDa Ran-binding protein that binds RanGTP, stabilizes it against hydrolysis and exchange, associates with nuclear pore complexes, and assembles into a RanBP1-Ran-RanBP5 complex [#0]. As an import receptor it carries ribosomal proteins (L23a, S7, L5), binding the very basic region of rpL23a as an archetypal import signal [#1], and it imports RNA-binding and signaling cargoes including CPEB3 (via its RRM1 domain) [#2], Musashi-1 [#3], and RASAL2, whose nuclear translocation activates RAS signaling in colorectal cancer cells [#5]. Crystal and cryo-EM structures of human RanBP5 define the IMB3 karyopherin-beta architecture and show that cargo NLS elements, such as the unstructured influenza PB1 NLS, are engaged in a wrapped conformation [#6, #8]. IPO5 is essential in vivo: global knockout causes early embryonic lethality and germline-specific deletion ablates germ cells and causes meiotic failure [#9]. Viruses exploit IPO5 for nuclear trafficking, with the influenza A PA-PB1 heterodimer forming a stable complex in which polymerase RNA binding and import are mutually exclusive [#4], and PCV2 capsid both imported and stabilized against proteasomal degradation by IPO5 [#7].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established IPO5 as a Ran-binding member of the importin-beta family, answering whether this novel protein operates within the Ran-dependent transport system.\",\n      \"evidence\": \"Overlay assay with RanGTP, yeast two-hybrid, in vitro binding and subcellular fractionation\",\n      \"pmids\": [\"9271386\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No cargo identified at this stage\", \"Directionality of transport (import vs export) not defined\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Showed IPO5 acts as a bona fide import receptor for ribosomal proteins, identifying a basic NLS as the recognition signal and defining its physiological cargo class.\",\n      \"evidence\": \"In vitro reconstituted nuclear import assays and direct receptor-cargo binding in mammalian cells\",\n      \"pmids\": [\"9687515\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Redundancy with importin-beta, transportin and RanBP7 leaves cargo specificity unresolved\", \"No structural basis for NLS recognition\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Linked IPO5 cargo handling to physiological signaling by showing NMDAR-driven RanBP1 changes modulate the CPEB3-IPO5 interaction, establishing regulated, signal-responsive import.\",\n      \"evidence\": \"Co-IP, domain mapping (RRM1) and NMDAR stimulation in neurons\",\n      \"pmids\": [\"22730302\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"In vivo neuronal relevance not tested genetically\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extended the cargo repertoire to Musashi-1 and connected IPO5 to germline chromatin biology, showing its import activity has tissue-specific functional consequences.\",\n      \"evidence\": \"Co-IP, immunofluorescence and transgenic overexpression in mouse testis\",\n      \"pmids\": [\"25782991\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Overexpression-based, not loss-of-function\", \"NLS within MSI1 not mapped\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Defined a viral hijacking mechanism by showing IPO5 binds the influenza PA-PB1 heterodimer and that import and polymerase RNA binding are mutually exclusive.\",\n      \"evidence\": \"Reconstituted complexes, SAXS modelling and in vitro RNA-binding assays\",\n      \"pmids\": [\"27095520\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic-resolution interface\", \"Cellular import dynamics inferred from in vitro data\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected IPO5-mediated import to oncogenic signaling by demonstrating it carries the RASAL2 NLS to activate RAS signaling in colorectal cancer.\",\n      \"evidence\": \"Mass spectrometry, Co-IP, fractionation, immunofluorescence and NLS mapping\",\n      \"pmids\": [\"31288861\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Causality between import and tumor progression not genetically dissected\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided the first structural framework for human RanBP5, confirming IMB3 subfamily architecture and validating NLS docking through mutations that disrupt influenza PA-PB1 binding.\",\n      \"evidence\": \"X-ray crystallography of two ligand-free isoforms with point mutagenesis and protein complementation\",\n      \"pmids\": [\"32222384\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structures are ligand-free\", \"Cargo-bound conformations not captured here\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Revealed a dual function beyond transport by showing IPO5 imports PCV2 virions and stabilizes its capsid against proteasomal degradation, defining specific contact residues.\",\n      \"evidence\": \"GST pulldown, site-directed mutagenesis, knockdown, cycloheximide chase and proteasome inhibitor rescue\",\n      \"pmids\": [\"36409110\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of capsid stabilization not resolved\", \"Whether stabilization extends to host cargoes unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Resolved how IPO5 engages an unstructured cargo NLS by cryo-EM, showing the influenza PB1 NLS is wrapped by RanBP5 and identifying critical interface residues.\",\n      \"evidence\": \"Cryo-EM at 3.2 Å of the RanBP5-PB1 NLS complex with interface mutagenesis\",\n      \"pmids\": [\"39536408\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure limited to a viral NLS fragment\", \"Host cargo recognition modes not directly compared\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Defined the in vivo requirement for IPO5, showing it is essential for early embryogenesis and male germline/meiotic progression, and identified new binding partners.\",\n      \"evidence\": \"Conditional knockout mice (CMVCre, VasaCre, Stra8Cre) and IP-MS identifying SFPQ and XPO2\",\n      \"pmids\": [\"40652294\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cargoes responsible for the meiotic/embryonic phenotypes not established\", \"Functional role of SFPQ and XPO2 interactions not characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How IPO5 achieves cargo selectivity among overlapping importin-beta family receptors and which specific cargoes drive its essential developmental and germline functions remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No unified determinant of cargo specificity defined\", \"Cargoes mediating embryonic lethality and meiotic failure unidentified\", \"Physiological role of capsid-stabilization activity for endogenous substrates unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008565\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [1, 2, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"R-HSA-1474165\", \"supporting_discovery_ids\": [9]}\n    ],\n    \"complexes\": [\n      \"RanBP1-Ran-RanBP5 complex\",\n      \"influenza A PA-PB1-RanBP5 complex\"\n    ],\n    \"partners\": [\n      \"RAN\",\n      \"RANBP1\",\n      \"CPEB3\",\n      \"MSI1\",\n      \"RASAL2\",\n      \"SFPQ\",\n      \"XPO2\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}