{"gene":"CASTOR1","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":2016,"finding":"CASTOR1 is a homodimeric cytosolic arginine sensor that binds arginine at the interface of two ACT domains; arginine binding allosterically controls the adjacent GATOR2-binding site, triggering dissociation of CASTOR1 from GATOR2 and downstream activation of mTORC1. Crystal structure at 1.8 Å revealed structural homology to the lysine-binding regulatory domain of prokaryotic aspartate kinases.","method":"X-ray crystallography (1.8 Å), in vitro binding assays, mutagenesis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus mutagenesis plus functional binding assays, highly cited foundational study","pmids":["27487210"],"is_preprint":false},{"year":2016,"finding":"CASTOR1 binds arginine in a pocket carved between its NTD and CTD (ACT) domains; a surface patch on CASTOR1-NTD opposite the arginine-binding site mediates direct physical interaction with GATOR2 subunit Mios; mutation of this patch disrupts CASTOR1's inhibition of GATOR2.","method":"X-ray crystallography, in vitro pull-down assay, normal mode analysis, mutagenesis","journal":"Cell discovery","confidence":"High","confidence_rationale":"Tier 1 — crystal structure, pull-down, and mutagenesis in single study; replicates Nature finding with orthogonal methods","pmids":["28066558"],"is_preprint":false},{"year":2016,"finding":"CASTOR1 comprises four tandem ACT domains arranged like the C-terminal allosteric domains of aspartate kinases; ACT1 and ACT3 mediate homodimerization, while ACT2 and ACT4 form the arginine-binding pocket; key residues validated by mutagenesis and biochemical assays.","method":"X-ray crystallography (2.5 Å), mutagenesis, biochemical binding assays","journal":"Cell discovery","confidence":"High","confidence_rationale":"Tier 1 — independent crystal structure with mutagenesis and biochemical validation, replicating and extending structural findings","pmids":["27648300"],"is_preprint":false},{"year":2018,"finding":"Comparison of arginine-bound and apo crystal structures of CASTOR1 revealed near-identical overall conformations with differences confined to two loop regions, indicating CASTOR1 does not undergo large conformational changes upon arginine binding.","method":"X-ray crystallography (2.05 Å bound; 2.8 Å apo), structural comparison","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 — two independent crystal structures compared in same study","pmids":["30503338"],"is_preprint":false},{"year":2021,"finding":"E3 ubiquitin ligase RNF167 ubiquitinates CASTOR1 with K29-linked polyubiquitin chains, leading to its proteasomal degradation. AKT phosphorylates CASTOR1 at Ser14, which increases CASTOR1 binding to RNF167 and promotes its ubiquitination/degradation, while simultaneously decreasing CASTOR1 affinity for MIOS (GATOR2 subunit), thereby activating mTORC1 independent of arginine levels.","method":"Co-immunoprecipitation, ubiquitination assay, phospho-mutagenesis, in vitro kinase assay, cell-based loss-of-function/gain-of-function","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, mutagenesis at phosphosite, ubiquitin-linkage typing, multiple orthogonal methods in single study","pmids":["33594058"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structure of human GATOR2 bound to CASTOR1 (in absence of arginine) shows that two MIOS WD40 domain β-propellers of the GATOR2 cage engage both subunits of a single CASTOR1 homodimer at a negatively charged MIOS-binding interface distal to the arginine pocket; arginine-triggered loop ordering in CASTOR1 blocks this MIOS-binding interface, explaining how arginine binding switches off CASTOR1-GATOR2 interaction to activate mTORC1.","method":"Cryo-electron microscopy, structural analysis, functional validation","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure of full complex with mechanistic interpretation, published in high-impact journal","pmids":["40715445"],"is_preprint":false},{"year":2019,"finding":"KSHV-encoded miRNAs miR-K4-5p (and miR-K1-5p) directly target the CASTOR1 3'-UTR to suppress CASTOR1 expression, thereby relieving CASTOR1-mediated inhibition of GATOR2 and activating mTORC1; knockdown of these miRNAs restored CASTOR1 expression and attenuated mTORC1 activation.","method":"miRNA target validation, luciferase reporter assay, miRNA knockdown/overexpression, mTORC1 activity assays","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 — direct miRNA targeting with reporter assays and functional rescue, single lab","pmids":["31305263"],"is_preprint":false},{"year":2026,"finding":"CASTOR1 and its paralog CASTOR2 both bind arginine and interact with GATOR2 subunit Mios to inhibit its binding to GATOR1, but CASTOR1 responds to low arginine levels while CASTOR2 responds to high arginine concentrations; arginine binding induces conformational changes at the ACT2-ACT4 interface causing dissociation from Mios.","method":"Biochemical binding assays, structural/conformational analysis, cell-based mTORC1 activity assays (C2C12 cells), mutagenesis","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal biochemical and cell-based methods establishing differential arginine sensitivity and shared mechanism","pmids":["41506264"],"is_preprint":false},{"year":2022,"finding":"CASTOR1 overexpression inhibits mTOR signaling pathway activation, and this inhibition of mTOR mediates CASTOR1's regulatory effect on microglia M1/M2 polarization; treatment with mTOR activator MHY1485 attenuated the anti-M1 effect of CASTOR1 overexpression, placing CASTOR1 upstream of mTOR in microglial polarization.","method":"Overexpression, pharmacological rescue (mTOR activator), cytokine/marker assays in microglia","journal":"Metabolic brain disease","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis via pharmacological rescue places CASTOR1 upstream of mTOR, single lab","pmids":["36454504"],"is_preprint":false},{"year":2025,"finding":"CASTOR1 loss activates mTORC1 signaling (elevated 4EBP1 and S6 phosphorylation) and also augments AKT and ERK activation, revealing crosstalk between the PI3K/AKT/mTORC1 and KRAS/ERK pathways; CASTOR1 genetic ablation in a KRAS-driven GEMM accelerated lung tumor initiation and progression.","method":"Genetically engineered mouse model (KRAS G12D GEMM with CASTOR1 KO), phospho-protein analysis, organoid cultures","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 — in vivo genetic KO with defined signaling readouts, preprint not yet peer-reviewed","pmids":["40313924"],"is_preprint":true}],"current_model":"CASTOR1 is a homodimeric cytosolic arginine sensor composed of four ACT domains that binds arginine at the ACT2-ACT4 interface; in the absence of arginine, CASTOR1 directly binds the MIOS WD40 β-propellers of the GATOR2 complex (as shown by cryo-EM) to sequester GATOR2, thereby allowing GATOR1 to inactivate Rag GTPases and suppress mTORC1 lysosomal recruitment; arginine binding triggers loop ordering in CASTOR1 that occludes the MIOS-binding interface, releasing GATOR2 to disinhibit mTORC1; CASTOR1 activity is additionally regulated by AKT-mediated phosphorylation at Ser14 followed by RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation, providing an arginine-independent mTORC1 activation mechanism."},"narrative":{"teleology":[{"year":2016,"claim":"Establishing that CASTOR1 is the direct arginine sensor upstream of GATOR2–mTORC1 resolved how cells relay arginine sufficiency to mTORC1 and revealed the structural basis: a homodimer with four ACT domains per subunit, where arginine binds at the ACT2–ACT4 interface and allosterically controls a GATOR2 (MIOS)-binding surface on the opposite face.","evidence":"Three independent crystal structures (1.8–2.5 Å), mutagenesis, and in vitro binding assays across three groups","pmids":["27487210","28066558","27648300"],"confidence":"High","gaps":["No structure of the CASTOR1–GATOR2 complex was available to define the binding interface at residue resolution","Whether arginine binding causes large-scale conformational change or local rearrangement was unresolved","Functional consequence of CASTOR1 regulation in vivo was not yet tested"]},{"year":2018,"claim":"Comparison of arginine-bound and apo crystal structures demonstrated that CASTOR1 does not undergo a global conformational change upon arginine binding, restricting the allosteric switch to local loop reordering near the ligand pocket.","evidence":"Crystal structures of apo (2.8 Å) and arginine-bound (2.05 Å) CASTOR1 compared in the same study","pmids":["30503338"],"confidence":"High","gaps":["Which specific loops mediate MIOS occlusion remained undefined","No full complex structure to visualize how loop changes translate into loss of GATOR2 binding"]},{"year":2019,"claim":"The discovery that KSHV-encoded miRNAs directly suppress CASTOR1 expression to activate mTORC1 established that viral pathogens exploit CASTOR1 downregulation as a strategy to hijack nutrient signaling.","evidence":"miRNA target validation via luciferase reporters, miRNA knockdown/overexpression with mTORC1 readouts in KSHV-infected cells","pmids":["31305263"],"confidence":"Medium","gaps":["Single-lab study; independent replication in other viral systems not performed","Whether endogenous host miRNAs similarly regulate CASTOR1 was not explored"]},{"year":2021,"claim":"Identification of AKT-mediated Ser14 phosphorylation followed by RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1 revealed an arginine-independent mechanism for mTORC1 activation through growth-factor signaling.","evidence":"Reciprocal co-immunoprecipitation, in vitro kinase assay, ubiquitin-linkage typing, phospho-mutant analysis in cells","pmids":["33594058"],"confidence":"High","gaps":["Physiological contexts in which AKT-mediated CASTOR1 degradation dominates over arginine sensing are unclear","Whether other kinases or E3 ligases participate in CASTOR1 turnover was not tested"]},{"year":2025,"claim":"Cryo-EM of the GATOR2–CASTOR1 complex resolved how two MIOS WD40 β-propellers engage both subunits of a CASTOR1 homodimer and how arginine-induced loop ordering sterically blocks this interface, providing a complete structural mechanism for the arginine-dependent switch.","evidence":"Cryo-electron microscopy of human GATOR2 bound to CASTOR1 (arginine-free), with functional validation","pmids":["40715445"],"confidence":"High","gaps":["Dynamic intermediates of CASTOR1 dissociation from GATOR2 upon arginine binding have not been captured","Whether CASTOR1 regulates GATOR2 enzymatic activity or only its interaction with GATOR1 is unresolved"]},{"year":2026,"claim":"Discovery that CASTOR2, a CASTOR1 paralog, shares the same GATOR2-inhibitory mechanism but responds to high rather than low arginine concentrations established a two-sensor system that tunes mTORC1 across a broad range of arginine availability.","evidence":"Biochemical binding assays, mutagenesis, and mTORC1 activity measurements in C2C12 cells comparing CASTOR1 and CASTOR2","pmids":["41506264"],"confidence":"High","gaps":["The physiological significance of dual-sensor regulation in specific tissues or developmental contexts is unexplored","Whether CASTOR1–CASTOR2 heterodimers form and function is unknown"]},{"year":null,"claim":"How CASTOR1 integrates with parallel amino-acid sensing branches (e.g., Sestrin2/leucine, SAMTOR/SAM) at the level of GATOR2 complex stoichiometry and dynamics, and the in vivo consequences of CASTOR1 loss in normal physiology and disease, remain incompletely understood.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of GATOR2 simultaneously engaged by multiple upstream sensors","Conditional knockout studies in specific tissues (beyond KRAS-driven lung tumors) have not been reported in peer-reviewed literature","Whether CASTOR1 has functions independent of GATOR2–mTORC1 signaling is untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[0,1,2,3,7]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,4,5]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,1,4,5,7]}],"complexes":[],"partners":["MIOS","RNF167","AKT1","CASTOR2"],"other_free_text":[]},"mechanistic_narrative":"CASTOR1 is a cytosolic arginine sensor that couples intracellular arginine availability to mTORC1 activation through the GATOR2–GATOR1–Rag GTPase signaling axis. Structurally, CASTOR1 is a homodimer built from four tandem ACT domains per subunit, with ACT1/ACT3 mediating dimerization and ACT2/ACT4 forming the arginine-binding pocket at their interface; in the arginine-free state, a negatively charged surface on CASTOR1 engages two WD40 β-propellers of the GATOR2 subunit MIOS, sequestering GATOR2 and thereby permitting GATOR1-mediated inactivation of Rag GTPases and suppression of mTORC1 [PMID:27487210, PMID:28066558, PMID:40715445]. Arginine binding induces ordering of specific loop regions that occlude the MIOS-binding interface, releasing GATOR2 to activate mTORC1, without requiring large-scale conformational rearrangement of the protein [PMID:30503338, PMID:40715445]. An arginine-independent regulatory layer exists in which AKT phosphorylates CASTOR1 at Ser14, promoting RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1, thereby activating mTORC1 even under low-arginine conditions [PMID:33594058]."},"prefetch_data":{"uniprot":{"accession":"Q8WTX7","full_name":"Cytosolic arginine sensor for mTORC1 subunit 1","aliases":["Cellular arginine sensor for mTORC1 protein 1","GATS-like protein 3"],"length_aa":329,"mass_kda":36.3,"function":"Functions as an intracellular arginine sensor within the amino acid-sensing branch of the TORC1 signaling pathway (PubMed:26972053, PubMed:27487210, PubMed:33594058). As a homodimer or a heterodimer with CASTOR2, binds and inhibits the GATOR subcomplex GATOR2 and thereby mTORC1 (PubMed:26972053, PubMed:27487210, PubMed:33594058). Binding of arginine to CASTOR1 allosterically disrupts the interaction of CASTOR1-containing dimers with GATOR2 which can in turn activate mTORC1 and the TORC1 signaling pathway (PubMed:26972053, PubMed:27487210, PubMed:33594058)","subcellular_location":"Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/Q8WTX7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CASTOR1","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CASTOR1","total_profiled":1310},"omim":[{"mim_id":"620307","title":"WD REPEAT-CONTAINING PROTEIN 24; WDR24","url":"https://www.omim.org/entry/620307"},{"mim_id":"617418","title":"WD REPEAT-CONTAINING PROTEIN 59; WDR59","url":"https://www.omim.org/entry/617418"},{"mim_id":"617034","title":"CELLULAR ARGININE SENSOR FOR MTORC1 PROTEIN 1; CASTOR1","url":"https://www.omim.org/entry/617034"},{"mim_id":"617033","title":"CELLULAR ARGININE SENSOR FOR MTORC1 PROTEIN 2; CASTOR2","url":"https://www.omim.org/entry/617033"},{"mim_id":"601231","title":"MECHANISTIC TARGET OF RAPAMYCIN; MTOR","url":"https://www.omim.org/entry/601231"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"esophagus","ntpm":50.5}],"url":"https://www.proteinatlas.org/search/CASTOR1"},"hgnc":{"alias_symbol":[],"prev_symbol":["GATSL3"]},"alphafold":{"accession":"Q8WTX7","domains":[{"cath_id":"3.30.2130.10","chopping":"1-156_172-209_223-329","consensus_level":"medium","plddt":91.7732,"start":1,"end":329}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8WTX7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8WTX7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8WTX7-F1-predicted_aligned_error_v6.png","plddt_mean":88.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CASTOR1","jax_strain_url":"https://www.jax.org/strain/search?query=CASTOR1"},"sequence":{"accession":"Q8WTX7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8WTX7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8WTX7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8WTX7"}},"corpus_meta":[{"pmid":"27487210","id":"PMC_27487210","title":"Mechanism of arginine sensing by CASTOR1 upstream of mTORC1.","date":"2016","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/27487210","citation_count":256,"is_preprint":false},{"pmid":"28066558","id":"PMC_28066558","title":"Structural mechanism for the arginine sensing and regulation of CASTOR1 in the mTORC1 signaling pathway.","date":"2016","source":"Cell discovery","url":"https://pubmed.ncbi.nlm.nih.gov/28066558","citation_count":47,"is_preprint":false},{"pmid":"33594058","id":"PMC_33594058","title":"RNF167 activates mTORC1 and promotes tumorigenesis by targeting CASTOR1 for ubiquitination and degradation.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33594058","citation_count":41,"is_preprint":false},{"pmid":"31305263","id":"PMC_31305263","title":"Kaposi sarcoma-associated herpesvirus miRNAs suppress CASTOR1-mediated mTORC1 inhibition to promote tumorigenesis.","date":"2019","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/31305263","citation_count":35,"is_preprint":false},{"pmid":"27648300","id":"PMC_27648300","title":"Structural insight into the arginine-binding specificity of CASTOR1 in amino acid-dependent mTORC1 signaling.","date":"2016","source":"Cell discovery","url":"https://pubmed.ncbi.nlm.nih.gov/27648300","citation_count":34,"is_preprint":false},{"pmid":"38552976","id":"PMC_38552976","title":"L-arginine alleviates heat stress-induced mammary gland injury through modulating CASTOR1-mTORC1 axis mediated mitochondrial homeostasis.","date":"2024","source":"The Science of the total environment","url":"https://pubmed.ncbi.nlm.nih.gov/38552976","citation_count":14,"is_preprint":false},{"pmid":"30503338","id":"PMC_30503338","title":"Crystal structures of arginine sensor CASTOR1 in arginine-bound and ligand free states.","date":"2018","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/30503338","citation_count":13,"is_preprint":false},{"pmid":"40715445","id":"PMC_40715445","title":"Structural basis for mTORC1 regulation by the CASTOR1-GATOR2 complex.","date":"2025","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/40715445","citation_count":7,"is_preprint":false},{"pmid":"40982001","id":"PMC_40982001","title":"piR-16404 drives ferroptotic liver injury via CASTOR1/mTORC1/GPX4 dysregulation in HepG2 cells and mice: a novel toxicity mechanism of N, N-dimethylformamide.","date":"2025","source":"Archives of toxicology","url":"https://pubmed.ncbi.nlm.nih.gov/40982001","citation_count":7,"is_preprint":false},{"pmid":"35442666","id":"PMC_35442666","title":"Downregulation of CASTOR1 Inhibits Heat-Stress-Induced Apoptosis and Promotes Casein and Lipid Synthesis in Mammary Epithelial Cells.","date":"2022","source":"Journal of agricultural and food chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/35442666","citation_count":6,"is_preprint":false},{"pmid":"36454504","id":"PMC_36454504","title":"Castor1 overexpression regulates microglia M1/M2 polarization via inhibiting mTOR pathway.","date":"2022","source":"Metabolic brain disease","url":"https://pubmed.ncbi.nlm.nih.gov/36454504","citation_count":6,"is_preprint":false},{"pmid":"40234290","id":"PMC_40234290","title":"Upregulating mTOR/S6 K Pathway by CASTOR1 Promotes Astrocyte Proliferation and Myelination in Gpam-/--induced mouse model of cerebral palsy.","date":"2025","source":"Molecular neurobiology","url":"https://pubmed.ncbi.nlm.nih.gov/40234290","citation_count":2,"is_preprint":false},{"pmid":"40470200","id":"PMC_40470200","title":"Structural basis for mTORC1 regulation by the CASTOR1-GATOR2 complex.","date":"2025","source":"Research square","url":"https://pubmed.ncbi.nlm.nih.gov/40470200","citation_count":0,"is_preprint":false},{"pmid":"40313924","id":"PMC_40313924","title":"CASTOR1: A Novel Tumor Suppressor Linking mTORC1 and KRAS Pathways in Tumorigenesis and Resistance to KRAS-Targeted Therapies in Non-Small Cell Lung Cancer.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/40313924","citation_count":0,"is_preprint":false},{"pmid":"41506264","id":"PMC_41506264","title":"CASTOR1 and CASTOR2 respond to different arginine levels to regulate mTORC1 activity.","date":"2026","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/41506264","citation_count":0,"is_preprint":false},{"pmid":"42028441","id":"PMC_42028441","title":"Putative identification of CASTOR1 as one of the targets of ganoderic acid a via thermal proteome profiling and molecular docking.","date":"2026","source":"Frontiers in pharmacology","url":"https://pubmed.ncbi.nlm.nih.gov/42028441","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.04.23.650349","title":"<i>CASTOR1</i> : A Novel Tumor Suppressor Linking mTORC1 and KRAS Pathways in Tumorigenesis and Resistance to KRAS-Targeted Therapies in Non-Small Cell Lung Cancer","date":"2025-04-26","source":"bioRxiv","url":"https://doi.org/10.1101/2025.04.23.650349","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10126,"output_tokens":2552,"usd":0.034329},"stage2":{"model":"claude-opus-4-6","input_tokens":5887,"output_tokens":2135,"usd":0.124215},"total_usd":0.158544,"stage1_batch_id":"msgbatch_011kfMfE81o6CTcyhVUqumuW","stage2_batch_id":"msgbatch_01DTputS6WNkH745x1nVSoBo","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2016,\n      \"finding\": \"CASTOR1 is a homodimeric cytosolic arginine sensor that binds arginine at the interface of two ACT domains; arginine binding allosterically controls the adjacent GATOR2-binding site, triggering dissociation of CASTOR1 from GATOR2 and downstream activation of mTORC1. Crystal structure at 1.8 Å revealed structural homology to the lysine-binding regulatory domain of prokaryotic aspartate kinases.\",\n      \"method\": \"X-ray crystallography (1.8 Å), in vitro binding assays, mutagenesis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus mutagenesis plus functional binding assays, highly cited foundational study\",\n      \"pmids\": [\"27487210\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CASTOR1 binds arginine in a pocket carved between its NTD and CTD (ACT) domains; a surface patch on CASTOR1-NTD opposite the arginine-binding site mediates direct physical interaction with GATOR2 subunit Mios; mutation of this patch disrupts CASTOR1's inhibition of GATOR2.\",\n      \"method\": \"X-ray crystallography, in vitro pull-down assay, normal mode analysis, mutagenesis\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure, pull-down, and mutagenesis in single study; replicates Nature finding with orthogonal methods\",\n      \"pmids\": [\"28066558\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CASTOR1 comprises four tandem ACT domains arranged like the C-terminal allosteric domains of aspartate kinases; ACT1 and ACT3 mediate homodimerization, while ACT2 and ACT4 form the arginine-binding pocket; key residues validated by mutagenesis and biochemical assays.\",\n      \"method\": \"X-ray crystallography (2.5 Å), mutagenesis, biochemical binding assays\",\n      \"journal\": \"Cell discovery\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — independent crystal structure with mutagenesis and biochemical validation, replicating and extending structural findings\",\n      \"pmids\": [\"27648300\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Comparison of arginine-bound and apo crystal structures of CASTOR1 revealed near-identical overall conformations with differences confined to two loop regions, indicating CASTOR1 does not undergo large conformational changes upon arginine binding.\",\n      \"method\": \"X-ray crystallography (2.05 Å bound; 2.8 Å apo), structural comparison\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — two independent crystal structures compared in same study\",\n      \"pmids\": [\"30503338\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"E3 ubiquitin ligase RNF167 ubiquitinates CASTOR1 with K29-linked polyubiquitin chains, leading to its proteasomal degradation. AKT phosphorylates CASTOR1 at Ser14, which increases CASTOR1 binding to RNF167 and promotes its ubiquitination/degradation, while simultaneously decreasing CASTOR1 affinity for MIOS (GATOR2 subunit), thereby activating mTORC1 independent of arginine levels.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitination assay, phospho-mutagenesis, in vitro kinase assay, cell-based loss-of-function/gain-of-function\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, mutagenesis at phosphosite, ubiquitin-linkage typing, multiple orthogonal methods in single study\",\n      \"pmids\": [\"33594058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structure of human GATOR2 bound to CASTOR1 (in absence of arginine) shows that two MIOS WD40 domain β-propellers of the GATOR2 cage engage both subunits of a single CASTOR1 homodimer at a negatively charged MIOS-binding interface distal to the arginine pocket; arginine-triggered loop ordering in CASTOR1 blocks this MIOS-binding interface, explaining how arginine binding switches off CASTOR1-GATOR2 interaction to activate mTORC1.\",\n      \"method\": \"Cryo-electron microscopy, structural analysis, functional validation\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure of full complex with mechanistic interpretation, published in high-impact journal\",\n      \"pmids\": [\"40715445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"KSHV-encoded miRNAs miR-K4-5p (and miR-K1-5p) directly target the CASTOR1 3'-UTR to suppress CASTOR1 expression, thereby relieving CASTOR1-mediated inhibition of GATOR2 and activating mTORC1; knockdown of these miRNAs restored CASTOR1 expression and attenuated mTORC1 activation.\",\n      \"method\": \"miRNA target validation, luciferase reporter assay, miRNA knockdown/overexpression, mTORC1 activity assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct miRNA targeting with reporter assays and functional rescue, single lab\",\n      \"pmids\": [\"31305263\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"CASTOR1 and its paralog CASTOR2 both bind arginine and interact with GATOR2 subunit Mios to inhibit its binding to GATOR1, but CASTOR1 responds to low arginine levels while CASTOR2 responds to high arginine concentrations; arginine binding induces conformational changes at the ACT2-ACT4 interface causing dissociation from Mios.\",\n      \"method\": \"Biochemical binding assays, structural/conformational analysis, cell-based mTORC1 activity assays (C2C12 cells), mutagenesis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal biochemical and cell-based methods establishing differential arginine sensitivity and shared mechanism\",\n      \"pmids\": [\"41506264\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"CASTOR1 overexpression inhibits mTOR signaling pathway activation, and this inhibition of mTOR mediates CASTOR1's regulatory effect on microglia M1/M2 polarization; treatment with mTOR activator MHY1485 attenuated the anti-M1 effect of CASTOR1 overexpression, placing CASTOR1 upstream of mTOR in microglial polarization.\",\n      \"method\": \"Overexpression, pharmacological rescue (mTOR activator), cytokine/marker assays in microglia\",\n      \"journal\": \"Metabolic brain disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis via pharmacological rescue places CASTOR1 upstream of mTOR, single lab\",\n      \"pmids\": [\"36454504\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CASTOR1 loss activates mTORC1 signaling (elevated 4EBP1 and S6 phosphorylation) and also augments AKT and ERK activation, revealing crosstalk between the PI3K/AKT/mTORC1 and KRAS/ERK pathways; CASTOR1 genetic ablation in a KRAS-driven GEMM accelerated lung tumor initiation and progression.\",\n      \"method\": \"Genetically engineered mouse model (KRAS G12D GEMM with CASTOR1 KO), phospho-protein analysis, organoid cultures\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vivo genetic KO with defined signaling readouts, preprint not yet peer-reviewed\",\n      \"pmids\": [\"40313924\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"CASTOR1 is a homodimeric cytosolic arginine sensor composed of four ACT domains that binds arginine at the ACT2-ACT4 interface; in the absence of arginine, CASTOR1 directly binds the MIOS WD40 β-propellers of the GATOR2 complex (as shown by cryo-EM) to sequester GATOR2, thereby allowing GATOR1 to inactivate Rag GTPases and suppress mTORC1 lysosomal recruitment; arginine binding triggers loop ordering in CASTOR1 that occludes the MIOS-binding interface, releasing GATOR2 to disinhibit mTORC1; CASTOR1 activity is additionally regulated by AKT-mediated phosphorylation at Ser14 followed by RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation, providing an arginine-independent mTORC1 activation mechanism.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CASTOR1 is a cytosolic arginine sensor that couples intracellular arginine availability to mTORC1 activation through the GATOR2–GATOR1–Rag GTPase signaling axis. Structurally, CASTOR1 is a homodimer built from four tandem ACT domains per subunit, with ACT1/ACT3 mediating dimerization and ACT2/ACT4 forming the arginine-binding pocket at their interface; in the arginine-free state, a negatively charged surface on CASTOR1 engages two WD40 β-propellers of the GATOR2 subunit MIOS, sequestering GATOR2 and thereby permitting GATOR1-mediated inactivation of Rag GTPases and suppression of mTORC1 [PMID:27487210, PMID:28066558, PMID:40715445]. Arginine binding induces ordering of specific loop regions that occlude the MIOS-binding interface, releasing GATOR2 to activate mTORC1, without requiring large-scale conformational rearrangement of the protein [PMID:30503338, PMID:40715445]. An arginine-independent regulatory layer exists in which AKT phosphorylates CASTOR1 at Ser14, promoting RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1, thereby activating mTORC1 even under low-arginine conditions [PMID:33594058].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Establishing that CASTOR1 is the direct arginine sensor upstream of GATOR2–mTORC1 resolved how cells relay arginine sufficiency to mTORC1 and revealed the structural basis: a homodimer with four ACT domains per subunit, where arginine binds at the ACT2–ACT4 interface and allosterically controls a GATOR2 (MIOS)-binding surface on the opposite face.\",\n      \"evidence\": \"Three independent crystal structures (1.8–2.5 Å), mutagenesis, and in vitro binding assays across three groups\",\n      \"pmids\": [\"27487210\", \"28066558\", \"27648300\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No structure of the CASTOR1–GATOR2 complex was available to define the binding interface at residue resolution\",\n        \"Whether arginine binding causes large-scale conformational change or local rearrangement was unresolved\",\n        \"Functional consequence of CASTOR1 regulation in vivo was not yet tested\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Comparison of arginine-bound and apo crystal structures demonstrated that CASTOR1 does not undergo a global conformational change upon arginine binding, restricting the allosteric switch to local loop reordering near the ligand pocket.\",\n      \"evidence\": \"Crystal structures of apo (2.8 Å) and arginine-bound (2.05 Å) CASTOR1 compared in the same study\",\n      \"pmids\": [\"30503338\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which specific loops mediate MIOS occlusion remained undefined\",\n        \"No full complex structure to visualize how loop changes translate into loss of GATOR2 binding\"\n      ]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"The discovery that KSHV-encoded miRNAs directly suppress CASTOR1 expression to activate mTORC1 established that viral pathogens exploit CASTOR1 downregulation as a strategy to hijack nutrient signaling.\",\n      \"evidence\": \"miRNA target validation via luciferase reporters, miRNA knockdown/overexpression with mTORC1 readouts in KSHV-infected cells\",\n      \"pmids\": [\"31305263\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single-lab study; independent replication in other viral systems not performed\",\n        \"Whether endogenous host miRNAs similarly regulate CASTOR1 was not explored\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of AKT-mediated Ser14 phosphorylation followed by RNF167-catalyzed K29-linked polyubiquitination and proteasomal degradation of CASTOR1 revealed an arginine-independent mechanism for mTORC1 activation through growth-factor signaling.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, in vitro kinase assay, ubiquitin-linkage typing, phospho-mutant analysis in cells\",\n      \"pmids\": [\"33594058\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Physiological contexts in which AKT-mediated CASTOR1 degradation dominates over arginine sensing are unclear\",\n        \"Whether other kinases or E3 ligases participate in CASTOR1 turnover was not tested\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Cryo-EM of the GATOR2–CASTOR1 complex resolved how two MIOS WD40 β-propellers engage both subunits of a CASTOR1 homodimer and how arginine-induced loop ordering sterically blocks this interface, providing a complete structural mechanism for the arginine-dependent switch.\",\n      \"evidence\": \"Cryo-electron microscopy of human GATOR2 bound to CASTOR1 (arginine-free), with functional validation\",\n      \"pmids\": [\"40715445\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Dynamic intermediates of CASTOR1 dissociation from GATOR2 upon arginine binding have not been captured\",\n        \"Whether CASTOR1 regulates GATOR2 enzymatic activity or only its interaction with GATOR1 is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Discovery that CASTOR2, a CASTOR1 paralog, shares the same GATOR2-inhibitory mechanism but responds to high rather than low arginine concentrations established a two-sensor system that tunes mTORC1 across a broad range of arginine availability.\",\n      \"evidence\": \"Biochemical binding assays, mutagenesis, and mTORC1 activity measurements in C2C12 cells comparing CASTOR1 and CASTOR2\",\n      \"pmids\": [\"41506264\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The physiological significance of dual-sensor regulation in specific tissues or developmental contexts is unexplored\",\n        \"Whether CASTOR1–CASTOR2 heterodimers form and function is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CASTOR1 integrates with parallel amino-acid sensing branches (e.g., Sestrin2/leucine, SAMTOR/SAM) at the level of GATOR2 complex stoichiometry and dynamics, and the in vivo consequences of CASTOR1 loss in normal physiology and disease, remain incompletely understood.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No structural model of GATOR2 simultaneously engaged by multiple upstream sensors\",\n        \"Conditional knockout studies in specific tissues (beyond KRAS-driven lung tumors) have not been reported in peer-reviewed literature\",\n        \"Whether CASTOR1 has functions independent of GATOR2–mTORC1 signaling is untested\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [0, 1, 2, 3, 7]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 4, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 4, 5, 7]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"MIOS\",\n      \"RNF167\",\n      \"AKT1\",\n      \"CASTOR2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}