{"gene":"KPTN","run_date":"2026-04-28T18:30:27","timeline":{"discoveries":[{"year":1999,"finding":"Kaptin (KPTN/2E4) binds to filamentous (F)-actin via F-actin affinity chromatography and is eluted from F-actin affinity columns and extracted from cells with ATP, indicating an ATP-sensitive actin association. It localizes to the leading edge of platelets and lamellipodia of motile fibroblasts, and to the tips of elongating stereocilia of the inner ear, suggesting a role in actin polymerization dynamics.","method":"F-actin affinity chromatography, immunofluorescence localization in platelets, fibroblasts, and inner ear stereocilia","journal":"European journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct biochemical binding assay plus multiple localization experiments; single lab","pmids":["10099934"],"is_preprint":false},{"year":2000,"finding":"KPTN extends beyond the barbed ends of actin filaments at the tips of stereocilia, consistent with a role at sites of actin polymerization. KPTN was mapped to chromosome 19q13.4 by FISH and radiation hybrid mapping.","method":"Double-label immunofluorescence, FISH, radiation hybrid mapping","journal":"Annals of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization beyond actin barbed ends demonstrated by immunofluorescence","pmids":["11409409"],"is_preprint":false},{"year":2013,"finding":"Loss-of-function mutations in KPTN cause macrocephaly, neurodevelopmental delay, and seizures. Endogenous and GFP-tagged kaptin associates with dynamic actin cytoskeletal structures in primary neuronal cell cultures, and this association is lost upon introduction of the identified disease-causing mutations, establishing kaptin as crucial for neuromorphogenesis.","method":"Linkage analysis, whole-exome sequencing, immunofluorescence in primary neuronal cultures with wild-type and mutant GFP-tagged kaptin","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — genetic identification plus functional mutagenesis showing loss of actin association, multiple families","pmids":["24239382"],"is_preprint":false},{"year":2023,"finding":"KPTN is a component of the mTOR regulatory complex KICSTOR. Kptn-/- mice and human iPSC-derived models show transcriptional and biochemical evidence of elevated mTORC1 signaling. Rapamycin treatment in Kptn-/- mice reduces the increased mTOR signaling, confirming that KPTN normally suppresses mTORC1 activity and that the downstream signaling is rapamycin-sensitive.","method":"Mouse knockout (Kptn-/-), iPSC differentiation models, Western blot/biochemical mTOR pathway analysis, rapamycin treatment rescue experiment","journal":"Brain : a journal of neurology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal models (mouse KO and human stem cells), rescue with rapamycin, replicated across labs","pmids":["37437211"],"is_preprint":false},{"year":2024,"finding":"OTUD3 is a deubiquitinase for KPTN. OTUD3 interacts with KPTN via its OTU domain and mediates deubiquitination of KPTN at lysine residue 49. This ubiquitination is a non-degradative, function-regulating modification. OTUD3-mediated deubiquitination of KPTN suppresses mTORC1 signaling and promotes GATOR1 lysosomal localization in a KPTN-dependent manner.","method":"In vivo ubiquitination assay, Co-immunoprecipitation, CRISPR/Cas9 knockout, immunofluorescence, NMR, cell proliferation assay","journal":"Frontiers in pharmacology","confidence":"High","confidence_rationale":"Tier 1-2 — in vivo ubiquitination assay with site-specific mapping (K49), deubiquitinase activity confirmed, domain mapping, multiple orthogonal methods","pmids":["38288086"],"is_preprint":false},{"year":2025,"finding":"FBXO2 directly interacts with KPTN via its F-box-associated domain and promotes K48- and K63-linked polyubiquitination of KPTN at lysine residues 49, 67, 262, and 265. This ubiquitination disrupts KPTN's interaction with ITFG2 and SZT2 while enhancing its interaction with C12orf66, thereby impairing KICSTOR's ability to recruit the GATOR1 complex (DEPDC5, NPRL2, NPRL3) to the lysosomal surface, leading to enhanced mTORC1 signaling.","method":"Co-immunoprecipitation, ubiquitination assays with K48/K63 linkage-specific analysis, site-directed mutagenesis of ubiquitination sites, pulldown assays for interaction mapping, GATOR1 lysosomal recruitment assay","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including site-specific mutagenesis, linkage-specific ubiquitination, protein interaction mapping, and functional consequence on GATOR1 recruitment","pmids":["41401028"],"is_preprint":false},{"year":2026,"finding":"CRISPR/Cas9 Kptn knockout in vitro induces mTOR activation and an mTOR-dependent increase in cell size. Kptn-/- mice exhibit increased cortical mTOR signaling reducible by rapamycin. Focal CRISPR/Cas9 Kptn knockout in cortex via in utero electroporation results in white matter heterotopic neurons, establishing a role for KPTN in cortical neuron positioning during development.","method":"CRISPR/Cas9 knockout in vitro and in vivo, Western blot for mTOR activation, rapamycin rescue, in utero electroporation, histological analysis","journal":"Annals of neurology","confidence":"High","confidence_rationale":"Tier 2 — multiple in vitro and in vivo models, rescue with rapamycin, focal knockout revealing cytoarchitecture defect","pmids":["41696790"],"is_preprint":false},{"year":2025,"finding":"CRISPR/Cas9 Kptn knockout in N2a cells results in mTOR-dependent multi-cell aggregate formation within 24-48 hours of plating, abolished by rapamycin treatment. Proteomic analysis of aggregates revealed altered expression of adhesion molecules (e.g., contactin-3) and cytoskeletal proteins (e.g., stathmin-2), implicating these as downstream effectors of mTOR-driven aggregation.","method":"CRISPR/Cas9 knockout, Western blot (phospho-S6), timelapse live-cell imaging, rapamycin treatment, LC-MS/MS proteomics","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — direct KO with live imaging and proteomics, rapamycin rescue; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.11.02.685388"],"is_preprint":true}],"current_model":"KPTN (kaptin) is an actin-binding protein and a core component of the KICSTOR complex that acts as a critical negative regulator of mTORC1 signaling by recruiting the GATOR1 complex to the lysosomal surface; its function is modulated by non-degradative ubiquitination (promoted by FBXO2 at K49/K67/K262/K265 and reversed by OTUD3 deubiquitinase activity), and loss of KPTN leads to mTORC1 hyperactivation, abnormal cell growth, white matter heterotopic neurons, and the neurodevelopmental features of KPTN-related disorder."},"narrative":{"teleology":[{"year":1999,"claim":"The initial molecular identity of KPTN was established as an actin-binding protein with ATP-sensitive F-actin association, placing it at dynamic actin structures including stereocilia tips and lamellipodia.","evidence":"F-actin affinity chromatography and immunofluorescence in platelets, fibroblasts, and inner ear hair cells","pmids":["10099934"],"confidence":"Medium","gaps":["Single lab; biochemical binding parameters (Kd, stoichiometry) not determined","Functional consequence of actin binding not tested by loss-of-function","Mechanism of ATP-dependent dissociation unresolved"]},{"year":2000,"claim":"Precise sub-stereociliary localization beyond barbed ends strengthened the model that KPTN acts at sites of actin monomer addition, and its chromosomal locus was mapped to 19q13.4.","evidence":"Double-label immunofluorescence and FISH/radiation hybrid mapping","pmids":["11409409"],"confidence":"Medium","gaps":["No direct demonstration that KPTN promotes or caps actin polymerization","No loss-of-function data in stereocilia"]},{"year":2013,"claim":"Establishing KPTN as a disease gene, loss-of-function mutations were shown to cause macrocephaly, neurodevelopmental delay, and seizures, and the pathogenic mutations abolished KPTN's association with actin in neurons, linking actin binding to neuromorphogenesis.","evidence":"Whole-exome sequencing in multiple families, GFP-tagged wild-type and mutant KPTN immunofluorescence in primary neurons","pmids":["24239382"],"confidence":"High","gaps":["Mechanism connecting loss of actin binding to macrocephaly unknown","mTOR pathway involvement not yet suspected","Animal model not yet generated"]},{"year":2023,"claim":"A transformative shift in understanding occurred when KPTN was recognized as a KICSTOR subunit: Kptn knockout mice and human iPSC models showed mTORC1 hyperactivation that was correctable by rapamycin, reframing KPTN's primary disease-relevant function from cytoskeletal to mTOR signaling.","evidence":"Kptn−/− mouse, iPSC-derived models, Western blot of mTOR pathway, rapamycin rescue","pmids":["37437211"],"confidence":"High","gaps":["How KPTN within KICSTOR mechanistically recruits GATOR1 not resolved","Relative contributions of actin-binding vs. mTOR-regulatory roles to pathology unclear","Post-translational regulation of KPTN not yet explored"]},{"year":2024,"claim":"The discovery that OTUD3 deubiquitinates KPTN at K49 revealed that non-degradative ubiquitination is a regulatory switch controlling KPTN function: OTUD3-mediated deubiquitination promotes GATOR1 lysosomal localization and mTORC1 suppression.","evidence":"In vivo ubiquitination assay with K49 site mapping, co-IP, CRISPR KO, NMR domain analysis","pmids":["38288086"],"confidence":"High","gaps":["E3 ligase responsible for K49 ubiquitination not yet identified in this study","Physiological stimuli triggering ubiquitination/deubiquitination cycles unknown","In vivo relevance of OTUD3–KPTN axis not tested"]},{"year":2025,"claim":"FBXO2 was identified as the E3 ligase that ubiquitinates KPTN at four lysines (K49, K67, K262, K265) with K48/K63 linkages, and this modification selectively disrupts KPTN interactions with ITFG2 and SZT2 while enhancing C12orf66 binding, thereby mechanistically explaining how ubiquitination impairs GATOR1 lysosomal recruitment.","evidence":"Co-IP, linkage-specific ubiquitination assays, site-directed mutagenesis, GATOR1 recruitment assays","pmids":["41401028"],"confidence":"High","gaps":["Signals activating FBXO2-mediated KPTN ubiquitination (e.g., amino acid sensing) not determined","Structural basis for how ubiquitination selectively remodels KICSTOR subunit interactions unknown","Interplay between FBXO2 and OTUD3 in dynamic regulation not characterized"]},{"year":2026,"claim":"Focal Kptn knockout in developing cortex via in utero electroporation demonstrated that KPTN loss causes white matter heterotopic neurons, directly establishing KPTN as a regulator of cortical neuron positioning during development through mTOR-dependent mechanisms.","evidence":"CRISPR/Cas9 in vitro and in vivo KO, rapamycin rescue, in utero electroporation, histology","pmids":["41696790"],"confidence":"High","gaps":["Whether heterotopia results from migration defect vs. aberrant proliferation not distinguished","Cell-type specificity of KPTN requirement in cortical development not resolved","Whether actin-binding function contributes to the migration phenotype untested"]},{"year":null,"claim":"The relative contributions of KPTN's actin-binding and KICSTOR/mTOR-regulatory functions remain unresolved, as does the structural basis of KPTN within the KICSTOR complex and the upstream signals that dynamically control its ubiquitination state.","evidence":"","pmids":[],"confidence":"Low","gaps":["No separation-of-function mutants distinguishing actin-binding from KICSTOR roles","No high-resolution structure of KPTN or KICSTOR","Upstream nutrient/stress signals regulating FBXO2–OTUD3 balance on KPTN unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,4,5,6]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[4,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[3,4,5,6]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[2,6]}],"complexes":["KICSTOR"],"partners":["ITFG2","SZT2","C12ORF66","FBXO2","OTUD3","DEPDC5","NPRL2","NPRL3"],"other_free_text":[]},"mechanistic_narrative":"KPTN (kaptin) is an actin-binding protein and core subunit of the KICSTOR complex that functions as a negative regulator of mTORC1 signaling by facilitating recruitment of the GATOR1 complex to the lysosomal surface. KPTN binds F-actin in an ATP-sensitive manner and localizes to sites of active actin polymerization, including stereocilia tips and lamellipodia; disease-causing mutations abolish this actin association and disrupt neuromorphogenesis [PMID:10099934, PMID:24239382]. KPTN's regulatory function within KICSTOR is modulated by non-degradative ubiquitination: FBXO2 promotes K48/K63-linked polyubiquitination at K49, K67, K262, and K265, disrupting KPTN–ITFG2/SZT2 interactions and impairing GATOR1 lysosomal recruitment, while OTUD3 reverses this modification to restore mTORC1 suppression [PMID:41401028, PMID:38288086]. Loss-of-function mutations in KPTN cause a neurodevelopmental disorder characterized by macrocephaly, seizures, mTORC1 hyperactivation, increased cell size, and white matter heterotopic neurons, phenotypes that are rescued by rapamycin [PMID:24239382, PMID:37437211, PMID:41696790]."},"prefetch_data":{"uniprot":{"accession":"Q9Y664","full_name":"KICSTOR complex protein kaptin","aliases":["Actin-associated protein 2E4"],"length_aa":436,"mass_kda":48.1,"function":"As part of the KICSTOR complex functions in the amino acid-sensing branch of the TORC1 signaling pathway. Recruits, in an amino acid-independent manner, the GATOR1 complex to the lysosomal membranes and allows its interaction with GATOR2 and the RAG GTPases. Functions upstream of the RAG GTPases and is required to negatively regulate mTORC1 signaling in absence of amino acids. In absence of the KICSTOR complex mTORC1 is constitutively localized to the lysosome and activated. The KICSTOR complex is also probably involved in the regulation of mTORC1 by glucose","subcellular_location":"Lysosome membrane; Cell projection, lamellipodium; Cell projection, stereocilium","url":"https://www.uniprot.org/uniprotkb/Q9Y664/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/KPTN","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/KPTN","total_profiled":1310},"omim":[{"mim_id":"621100","title":"INTELLECTUAL DEVELOPMENTAL DISORDER, AUTOSOMAL RECESSIVE 83; MRT83","url":"https://www.omim.org/entry/621100"},{"mim_id":"617421","title":"INTEGRIN-ALPHA FG-GAP REPEAT-CONTAINING PROTEIN 2; ITFG2","url":"https://www.omim.org/entry/617421"},{"mim_id":"617420","title":"KICSTOR SUBUNIT 2; KICS2","url":"https://www.omim.org/entry/617420"},{"mim_id":"615637","title":"INTELLECTUAL DEVELOPMENTAL DISORDER, AUTOSOMAL RECESSIVE 41; MRT41","url":"https://www.omim.org/entry/615637"},{"mim_id":"615620","title":"KAPTIN; KPTN","url":"https://www.omim.org/entry/615620"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"},{"location":"Vesicles","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/KPTN"},"hgnc":{"alias_symbol":["2E4","KICS4"],"prev_symbol":[]},"alphafold":{"accession":"Q9Y664","domains":[{"cath_id":"2.130.10.10","chopping":"195-261_271-362","consensus_level":"medium","plddt":94.2719,"start":195,"end":362}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y664","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y664-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9Y664-F1-predicted_aligned_error_v6.png","plddt_mean":88.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=KPTN","jax_strain_url":"https://www.jax.org/strain/search?query=KPTN"},"sequence":{"accession":"Q9Y664","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9Y664.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9Y664/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9Y664"}},"corpus_meta":[{"pmid":"24239382","id":"PMC_24239382","title":"Mutations in KPTN cause macrocephaly, neurodevelopmental delay, and seizures.","date":"2013","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/24239382","citation_count":47,"is_preprint":false},{"pmid":"25847626","id":"PMC_25847626","title":"Novel homozygous mutation in KPTN gene causing a familial intellectual disability-macrocephaly syndrome.","date":"2015","source":"American journal of medical genetics. Part A","url":"https://pubmed.ncbi.nlm.nih.gov/25847626","citation_count":33,"is_preprint":false},{"pmid":"10099934","id":"PMC_10099934","title":"2E4 (kaptin): a novel actin-associated protein from human blood platelets found in lamellipodia and the tips of the stereocilia of the inner ear.","date":"1999","source":"European journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/10099934","citation_count":26,"is_preprint":false},{"pmid":"25596549","id":"PMC_25596549","title":"The mAb against adipocyte fatty acid-binding protein 2E4 attenuates the inflammation in the mouse model of high-fat diet-induced obesity via toll-like receptor 4 pathway.","date":"2015","source":"Molecular and cellular endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/25596549","citation_count":26,"is_preprint":false},{"pmid":"11409409","id":"PMC_11409409","title":"2E4/Kaptin (KPTN)--a candidate gene for the hearing loss locus, DFNA4.","date":"2000","source":"Annals of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/11409409","citation_count":19,"is_preprint":false},{"pmid":"37437211","id":"PMC_37437211","title":"Models of KPTN-related disorder implicate mTOR signalling in cognitive and overgrowth phenotypes.","date":"2023","source":"Brain : a journal of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/37437211","citation_count":10,"is_preprint":false},{"pmid":"32358097","id":"PMC_32358097","title":"Pathogenic variants in KPTN gene identified by clinical whole-genome sequencing.","date":"2020","source":"Cold Spring Harbor molecular case studies","url":"https://pubmed.ncbi.nlm.nih.gov/32358097","citation_count":8,"is_preprint":false},{"pmid":"31999056","id":"PMC_31999056","title":"KPTN gene homozygous variant-related syndrome in the northeast of Brazil: A case report.","date":"2020","source":"American journal of medical genetics. Part A","url":"https://pubmed.ncbi.nlm.nih.gov/31999056","citation_count":7,"is_preprint":false},{"pmid":"29795597","id":"PMC_29795597","title":"Antibacterial Effect of (2E,2E)-4,4-Trisulfanediylbis(but-2-enoic acid) against Staphylococcus aureus.","date":"2018","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/29795597","citation_count":6,"is_preprint":false},{"pmid":"38288086","id":"PMC_38288086","title":"OTUD3 suppresses the mTORC1 signaling by deubiquitinating KPTN.","date":"2024","source":"Frontiers in pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/38288086","citation_count":4,"is_preprint":false},{"pmid":"36703628","id":"PMC_36703628","title":"Case report: KPTN gene-related syndrome associated with a spectrum of neurodevelopmental anomalies including severe epilepsy.","date":"2023","source":"Frontiers in neurology","url":"https://pubmed.ncbi.nlm.nih.gov/36703628","citation_count":4,"is_preprint":false},{"pmid":"31695071","id":"PMC_31695071","title":"Structural Features of a Conformation-dependent Antigen Epitope on ORFV-B2L Recognized by the 2E4 mAb.","date":"2019","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/31695071","citation_count":3,"is_preprint":false},{"pmid":"37311648","id":"PMC_37311648","title":"Nonsense variant in a consanguineous family expands the phenotype of KPTN gene-related syndrome to include hearing impairment.","date":"2023","source":"Clinical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/37311648","citation_count":2,"is_preprint":false},{"pmid":"41401028","id":"PMC_41401028","title":"FBXO2-mediated KPTN ubiquitination promotes amino acid-dependent mTORC1 signaling and tumor growth.","date":"2025","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/41401028","citation_count":1,"is_preprint":false},{"pmid":"41696790","id":"PMC_41696790","title":"The Clinical Spectrum and Neurodevelopmental Pathogenesis of KPTN-Related Disorder in a Mouse Model.","date":"2026","source":"Annals of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/41696790","citation_count":0,"is_preprint":false},{"pmid":"41357736","id":"PMC_41357736","title":"Kaptin-Actin Binding Protein (KPTN)-Related Disorder: A Case Report of Two Siblings Harboring a Novel KPTN Mutation.","date":"2025","source":"Cureus","url":"https://pubmed.ncbi.nlm.nih.gov/41357736","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.11.02.685388","title":"mTOR pathway gene knockout results in mTOR-dependent cellular aggregation","date":"2025-11-04","source":"bioRxiv","url":"https://doi.org/10.1101/2025.11.02.685388","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9186,"output_tokens":2248,"usd":0.030639},"stage2":{"model":"claude-opus-4-6","input_tokens":5519,"output_tokens":2293,"usd":0.12738},"total_usd":0.158019,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1999,\n      \"finding\": \"Kaptin (KPTN/2E4) binds to filamentous (F)-actin via F-actin affinity chromatography and is eluted from F-actin affinity columns and extracted from cells with ATP, indicating an ATP-sensitive actin association. It localizes to the leading edge of platelets and lamellipodia of motile fibroblasts, and to the tips of elongating stereocilia of the inner ear, suggesting a role in actin polymerization dynamics.\",\n      \"method\": \"F-actin affinity chromatography, immunofluorescence localization in platelets, fibroblasts, and inner ear stereocilia\",\n      \"journal\": \"European journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical binding assay plus multiple localization experiments; single lab\",\n      \"pmids\": [\"10099934\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"KPTN extends beyond the barbed ends of actin filaments at the tips of stereocilia, consistent with a role at sites of actin polymerization. KPTN was mapped to chromosome 19q13.4 by FISH and radiation hybrid mapping.\",\n      \"method\": \"Double-label immunofluorescence, FISH, radiation hybrid mapping\",\n      \"journal\": \"Annals of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization beyond actin barbed ends demonstrated by immunofluorescence\",\n      \"pmids\": [\"11409409\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Loss-of-function mutations in KPTN cause macrocephaly, neurodevelopmental delay, and seizures. Endogenous and GFP-tagged kaptin associates with dynamic actin cytoskeletal structures in primary neuronal cell cultures, and this association is lost upon introduction of the identified disease-causing mutations, establishing kaptin as crucial for neuromorphogenesis.\",\n      \"method\": \"Linkage analysis, whole-exome sequencing, immunofluorescence in primary neuronal cultures with wild-type and mutant GFP-tagged kaptin\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic identification plus functional mutagenesis showing loss of actin association, multiple families\",\n      \"pmids\": [\"24239382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"KPTN is a component of the mTOR regulatory complex KICSTOR. Kptn-/- mice and human iPSC-derived models show transcriptional and biochemical evidence of elevated mTORC1 signaling. Rapamycin treatment in Kptn-/- mice reduces the increased mTOR signaling, confirming that KPTN normally suppresses mTORC1 activity and that the downstream signaling is rapamycin-sensitive.\",\n      \"method\": \"Mouse knockout (Kptn-/-), iPSC differentiation models, Western blot/biochemical mTOR pathway analysis, rapamycin treatment rescue experiment\",\n      \"journal\": \"Brain : a journal of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal models (mouse KO and human stem cells), rescue with rapamycin, replicated across labs\",\n      \"pmids\": [\"37437211\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"OTUD3 is a deubiquitinase for KPTN. OTUD3 interacts with KPTN via its OTU domain and mediates deubiquitination of KPTN at lysine residue 49. This ubiquitination is a non-degradative, function-regulating modification. OTUD3-mediated deubiquitination of KPTN suppresses mTORC1 signaling and promotes GATOR1 lysosomal localization in a KPTN-dependent manner.\",\n      \"method\": \"In vivo ubiquitination assay, Co-immunoprecipitation, CRISPR/Cas9 knockout, immunofluorescence, NMR, cell proliferation assay\",\n      \"journal\": \"Frontiers in pharmacology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vivo ubiquitination assay with site-specific mapping (K49), deubiquitinase activity confirmed, domain mapping, multiple orthogonal methods\",\n      \"pmids\": [\"38288086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FBXO2 directly interacts with KPTN via its F-box-associated domain and promotes K48- and K63-linked polyubiquitination of KPTN at lysine residues 49, 67, 262, and 265. This ubiquitination disrupts KPTN's interaction with ITFG2 and SZT2 while enhancing its interaction with C12orf66, thereby impairing KICSTOR's ability to recruit the GATOR1 complex (DEPDC5, NPRL2, NPRL3) to the lysosomal surface, leading to enhanced mTORC1 signaling.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assays with K48/K63 linkage-specific analysis, site-directed mutagenesis of ubiquitination sites, pulldown assays for interaction mapping, GATOR1 lysosomal recruitment assay\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including site-specific mutagenesis, linkage-specific ubiquitination, protein interaction mapping, and functional consequence on GATOR1 recruitment\",\n      \"pmids\": [\"41401028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CRISPR/Cas9 Kptn knockout in vitro induces mTOR activation and an mTOR-dependent increase in cell size. Kptn-/- mice exhibit increased cortical mTOR signaling reducible by rapamycin. Focal CRISPR/Cas9 Kptn knockout in cortex via in utero electroporation results in white matter heterotopic neurons, establishing a role for KPTN in cortical neuron positioning during development.\",\n      \"method\": \"CRISPR/Cas9 knockout in vitro and in vivo, Western blot for mTOR activation, rapamycin rescue, in utero electroporation, histological analysis\",\n      \"journal\": \"Annals of neurology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple in vitro and in vivo models, rescue with rapamycin, focal knockout revealing cytoarchitecture defect\",\n      \"pmids\": [\"41696790\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CRISPR/Cas9 Kptn knockout in N2a cells results in mTOR-dependent multi-cell aggregate formation within 24-48 hours of plating, abolished by rapamycin treatment. Proteomic analysis of aggregates revealed altered expression of adhesion molecules (e.g., contactin-3) and cytoskeletal proteins (e.g., stathmin-2), implicating these as downstream effectors of mTOR-driven aggregation.\",\n      \"method\": \"CRISPR/Cas9 knockout, Western blot (phospho-S6), timelapse live-cell imaging, rapamycin treatment, LC-MS/MS proteomics\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct KO with live imaging and proteomics, rapamycin rescue; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.11.02.685388\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"KPTN (kaptin) is an actin-binding protein and a core component of the KICSTOR complex that acts as a critical negative regulator of mTORC1 signaling by recruiting the GATOR1 complex to the lysosomal surface; its function is modulated by non-degradative ubiquitination (promoted by FBXO2 at K49/K67/K262/K265 and reversed by OTUD3 deubiquitinase activity), and loss of KPTN leads to mTORC1 hyperactivation, abnormal cell growth, white matter heterotopic neurons, and the neurodevelopmental features of KPTN-related disorder.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"KPTN (kaptin) is an actin-binding protein and core subunit of the KICSTOR complex that functions as a negative regulator of mTORC1 signaling by facilitating recruitment of the GATOR1 complex to the lysosomal surface. KPTN binds F-actin in an ATP-sensitive manner and localizes to sites of active actin polymerization, including stereocilia tips and lamellipodia; disease-causing mutations abolish this actin association and disrupt neuromorphogenesis [PMID:10099934, PMID:24239382]. KPTN's regulatory function within KICSTOR is modulated by non-degradative ubiquitination: FBXO2 promotes K48/K63-linked polyubiquitination at K49, K67, K262, and K265, disrupting KPTN–ITFG2/SZT2 interactions and impairing GATOR1 lysosomal recruitment, while OTUD3 reverses this modification to restore mTORC1 suppression [PMID:41401028, PMID:38288086]. Loss-of-function mutations in KPTN cause a neurodevelopmental disorder characterized by macrocephaly, seizures, mTORC1 hyperactivation, increased cell size, and white matter heterotopic neurons, phenotypes that are rescued by rapamycin [PMID:24239382, PMID:37437211, PMID:41696790].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"The initial molecular identity of KPTN was established as an actin-binding protein with ATP-sensitive F-actin association, placing it at dynamic actin structures including stereocilia tips and lamellipodia.\",\n      \"evidence\": \"F-actin affinity chromatography and immunofluorescence in platelets, fibroblasts, and inner ear hair cells\",\n      \"pmids\": [\"10099934\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab; biochemical binding parameters (Kd, stoichiometry) not determined\", \"Functional consequence of actin binding not tested by loss-of-function\", \"Mechanism of ATP-dependent dissociation unresolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Precise sub-stereociliary localization beyond barbed ends strengthened the model that KPTN acts at sites of actin monomer addition, and its chromosomal locus was mapped to 19q13.4.\",\n      \"evidence\": \"Double-label immunofluorescence and FISH/radiation hybrid mapping\",\n      \"pmids\": [\"11409409\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct demonstration that KPTN promotes or caps actin polymerization\", \"No loss-of-function data in stereocilia\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Establishing KPTN as a disease gene, loss-of-function mutations were shown to cause macrocephaly, neurodevelopmental delay, and seizures, and the pathogenic mutations abolished KPTN's association with actin in neurons, linking actin binding to neuromorphogenesis.\",\n      \"evidence\": \"Whole-exome sequencing in multiple families, GFP-tagged wild-type and mutant KPTN immunofluorescence in primary neurons\",\n      \"pmids\": [\"24239382\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism connecting loss of actin binding to macrocephaly unknown\", \"mTOR pathway involvement not yet suspected\", \"Animal model not yet generated\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"A transformative shift in understanding occurred when KPTN was recognized as a KICSTOR subunit: Kptn knockout mice and human iPSC models showed mTORC1 hyperactivation that was correctable by rapamycin, reframing KPTN's primary disease-relevant function from cytoskeletal to mTOR signaling.\",\n      \"evidence\": \"Kptn−/− mouse, iPSC-derived models, Western blot of mTOR pathway, rapamycin rescue\",\n      \"pmids\": [\"37437211\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How KPTN within KICSTOR mechanistically recruits GATOR1 not resolved\", \"Relative contributions of actin-binding vs. mTOR-regulatory roles to pathology unclear\", \"Post-translational regulation of KPTN not yet explored\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"The discovery that OTUD3 deubiquitinates KPTN at K49 revealed that non-degradative ubiquitination is a regulatory switch controlling KPTN function: OTUD3-mediated deubiquitination promotes GATOR1 lysosomal localization and mTORC1 suppression.\",\n      \"evidence\": \"In vivo ubiquitination assay with K49 site mapping, co-IP, CRISPR KO, NMR domain analysis\",\n      \"pmids\": [\"38288086\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase responsible for K49 ubiquitination not yet identified in this study\", \"Physiological stimuli triggering ubiquitination/deubiquitination cycles unknown\", \"In vivo relevance of OTUD3–KPTN axis not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"FBXO2 was identified as the E3 ligase that ubiquitinates KPTN at four lysines (K49, K67, K262, K265) with K48/K63 linkages, and this modification selectively disrupts KPTN interactions with ITFG2 and SZT2 while enhancing C12orf66 binding, thereby mechanistically explaining how ubiquitination impairs GATOR1 lysosomal recruitment.\",\n      \"evidence\": \"Co-IP, linkage-specific ubiquitination assays, site-directed mutagenesis, GATOR1 recruitment assays\",\n      \"pmids\": [\"41401028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Signals activating FBXO2-mediated KPTN ubiquitination (e.g., amino acid sensing) not determined\", \"Structural basis for how ubiquitination selectively remodels KICSTOR subunit interactions unknown\", \"Interplay between FBXO2 and OTUD3 in dynamic regulation not characterized\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Focal Kptn knockout in developing cortex via in utero electroporation demonstrated that KPTN loss causes white matter heterotopic neurons, directly establishing KPTN as a regulator of cortical neuron positioning during development through mTOR-dependent mechanisms.\",\n      \"evidence\": \"CRISPR/Cas9 in vitro and in vivo KO, rapamycin rescue, in utero electroporation, histology\",\n      \"pmids\": [\"41696790\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether heterotopia results from migration defect vs. aberrant proliferation not distinguished\", \"Cell-type specificity of KPTN requirement in cortical development not resolved\", \"Whether actin-binding function contributes to the migration phenotype untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The relative contributions of KPTN's actin-binding and KICSTOR/mTOR-regulatory functions remain unresolved, as does the structural basis of KPTN within the KICSTOR complex and the upstream signals that dynamically control its ubiquitination state.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No separation-of-function mutants distinguishing actin-binding from KICSTOR roles\", \"No high-resolution structure of KPTN or KICSTOR\", \"Upstream nutrient/stress signals regulating FBXO2–OTUD3 balance on KPTN unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 4, 5, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [3, 4, 5, 6]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [2, 6]}\n    ],\n    \"complexes\": [\"KICSTOR\"],\n    \"partners\": [\"ITFG2\", \"SZT2\", \"C12orf66\", \"FBXO2\", \"OTUD3\", \"DEPDC5\", \"NPRL2\", \"NPRL3\"],\n    \"other_free_text\": []\n  }\n}\n```"}