{"gene":"PTPN18","run_date":"2026-04-28T19:45:45","timeline":{"discoveries":[{"year":1996,"finding":"PTPN18 (originally named BDP1/Brain Derived Phosphatase) was identified as a functional protein tyrosine phosphatase with a PEST motif, classified in the PTP-PEST family. It lacks a transmembrane domain, has a calculated molecular weight of ~50 kDa, and was shown to hydrolyze p-Nitrophenylphosphate when expressed in 293 cells, confirming PTPase activity. It was found to differentially dephosphorylate autophosphorylated tyrosine kinases overexpressed in tumor tissues.","method":"PCR cloning, transient expression in 293 cells, enzymatic phosphatase assay, northern blot","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — enzymatic activity confirmed in cell-based assay, single lab","pmids":["8950995"],"is_preprint":false},{"year":2001,"finding":"PTPN18 (PTP-HSCF) interacts with the SH2 domain of Csk via two putative tyrosine phosphorylation sites in its non-catalytic region. Csk and PTPN18 synergize to inhibit Src family kinase signaling, and this cooperativity depends on the domains mediating their association. PTPN18 inactivates Src-related PTKs by selectively dephosphorylating the positive regulatory tyrosine in their kinase domain.","method":"Modified yeast two-hybrid screen, co-immunoprecipitation in mammalian cells and hematopoietic cells, transfection/overexpression functional assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal interaction confirmed in multiple systems, functional synergy demonstrated with domain-mapping","pmids":["11158295"],"is_preprint":false},{"year":2003,"finding":"PTPN18 (BDP1) negatively regulates HER2 signaling. Overexpression of PTPN18 inhibited ligand-induced HER2 phosphorylation but not EGFR phosphorylation, demonstrating substrate specificity. Suppression of endogenous PTPN18 increased HER2 phosphorylation. PTPN18 also inhibited phosphorylation of the adaptor protein Gab1 and reduced MAPK activation downstream of HER2.","method":"Overexpression and siRNA knockdown in breast cancer cells, immunoprecipitation/western blot for phosphorylation states","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — both gain- and loss-of-function with defined substrate specificity and downstream pathway effects, replicated across conditions","pmids":["14660651"],"is_preprint":false},{"year":2014,"finding":"PTPN18 regulates HER2 through two distinct mechanisms: (1) The catalytic domain dephosphorylates HER2 at specific phosphorylation sites (pY1112, pY1196, pY1248), with crystal structures of PTPN18 in complex with HER2 phospho-peptides revealing molecular basis for substrate recognition in two distinct conformations; dephosphorylation of pY1112 blocks lysosomal routing and delays HER2 degradation. (2) The PEST domain of PTPN18 promotes K48-linked ubiquitination of HER2 and its rapid proteasomal destruction via a negative feedback loop, independent of catalytic activity. These two mechanisms define distinct HER2 phosphorylation and ubiquitination barcodes controlling cell growth and migration.","method":"X-ray crystallography (3 crystal structures of PTPN18–HER2 phosphopeptide complexes), in vitro enzymology, site-directed mutagenesis, cell-based ubiquitination assays, cell growth/migration assays, co-immunoprecipitation","journal":"Cell research","confidence":"High","confidence_rationale":"Tier 1 — crystal structures with functional validation, multiple orthogonal methods, mechanistic dissection of catalytic and non-catalytic domains","pmids":["25081058"],"is_preprint":false},{"year":2020,"finding":"PTPN18 acts as a positive regulator of EPO/EPOR/JAK2 signaling in hematopoietic progenitor cells. EPO induces phosphorylation of PTPN18 at Y389, modulated by JAK2 inhibitors. Ectopic expression of wild-type PTPN18 (but not a Y389 mutant) promoted EPO dose-dependent cell proliferation and survival, sustained activation of ERK1/2, AKT, STAT5, and JAK2, increased high-molecular-weight EPOR forms, and sharply inhibited EPO-induced phosphorylation of the adaptor RHEX at Y141.","method":"Phospho-tyrosine PTM proteomics, ectopic expression with phospho-site mutants, western blot for pathway activation, cell proliferation/survival assays","journal":"Cellular signalling","confidence":"Medium","confidence_rationale":"Tier 2 — site-specific phospho-mutant functional dissection in relevant cell line, single lab","pmids":["32027948"],"is_preprint":false},{"year":2020,"finding":"PTPN18 promotes colorectal cancer cell growth by stabilizing the MYC protein, which in turn activates CDK4 expression. MYC was found to interact with PTPN18 by co-immunoprecipitation. PTPN18 overexpression increased MYC protein levels and activated MYC-CDK4 signaling; PTPN18 deficiency had the opposite effect both in vitro and in xenograft assays.","method":"Co-immunoprecipitation, overexpression and knockdown in CRC cells, xenograft assay, western blot for pathway components","journal":"Genes & diseases","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP with functional KD/OE, pathway placement moderately supported","pmids":["34522712"],"is_preprint":false},{"year":2021,"finding":"Molecular dynamics simulations revealed that PTPN18 binds HER2 phospho-peptides with differential affinity (pY1248 > pY1196 > pY1112), and binding induces WPD-loop closure through new hydrogen bond and hydrophobic interactions involving residues Tyr62, Asp64, Val65, Ala231, Arg235, and Ala273. Allosteric communication networks from the pY binding loop to the WPD loop were identified, explaining substrate specificity at the atomic level.","method":"Molecular dynamics simulations, free energy calculations (MM/GBSA), correlation analysis of allosteric networks","journal":"Journal of molecular recognition","confidence":"Low","confidence_rationale":"Tier 4 — computational only, no experimental validation in this paper","pmids":["33620127"],"is_preprint":false},{"year":2021,"finding":"Silencing of PTPN18 in endometrial cancer cells (KLE) induced ferroptosis, increased intracellular ROS levels, downregulated GPX4 and xCT expression, and induced expression of phospho-p38. This places PTPN18 upstream of the p-p38/GPX4/xCT axis in regulating ferroptosis resistance.","method":"Lentiviral shRNA knockdown, ELISA for ROS, western blot for GPX4/xCT/p-p38","journal":"Cancer management and research","confidence":"Low","confidence_rationale":"Tier 3 — single lab, single knockdown approach, pathway placement inferred without rescue or direct substrate assay","pmids":["33642877"],"is_preprint":false},{"year":2022,"finding":"PTPN18 is translocated from the cytoplasm to the nucleus by MVP (major vault protein) and importin β2 in breast cancer cells. Nuclear PTPN18 dephosphorylates ETS1 transcription factor and promotes its degradation, thereby suppressing TGF-β signaling and epithelial-to-mesenchymal transition. Cytoplasmic PTPN18 does not suppress metastasis, establishing nuclear localization as essential for the anti-metastatic function.","method":"Subcellular fractionation, co-immunoprecipitation, ectopic expression with nuclear/cytoplasmic mutants, western blot for ETS1 phosphorylation/degradation, migration/invasion assays, TGF-β signaling readouts","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment with functional consequence, substrate dephosphorylation demonstrated, single lab","pmids":["35982039"],"is_preprint":false},{"year":2024,"finding":"HBx protein of hepatitis B virus represses PTPN18 expression by upregulating miR-128-3p. PTPN18 activates the p53 signaling pathway and suppresses the AKT/FOXO1 signaling cascade in hepatocellular carcinoma cells, as identified by phospho-protein microarray screening and validation experiments.","method":"Phospho-protein microarray, western blot validation, miRNA overexpression, PTPN18 overexpression/knockdown functional assays","journal":"Molecular cancer research","confidence":"Medium","confidence_rationale":"Tier 2–3 — phosphoproteomic identification plus functional validation in HCC cells, single lab","pmids":["38787319"],"is_preprint":false},{"year":2024,"finding":"PTPN18 knockdown in endometrial cancer cells suppressed aerobic glycolysis (reduced glucose consumption, lactate production, ATP levels, and glycolysis proteins HIF-1α, PKM2, LDHA), migration, and invasion by inhibiting the MYC/PI3K/AKT pathway. MYC overexpression partially rescued the effects of PTPN18 knockdown, placing PTPN18 upstream of MYC in this pathway.","method":"shRNA knockdown, Transwell assay, metabolic assays (glucose consumption, lactate, ATP), western blot, rescue experiment with MYC overexpression","journal":"Histology and histopathology","confidence":"Medium","confidence_rationale":"Tier 3 + Moderate — epistasis via rescue experiment supports pathway placement, single lab","pmids":["38855855"],"is_preprint":false},{"year":2026,"finding":"PTPN18 interacts with fibrillarin (FBL) via its R451 residue (with FBL's V187 site being critical on the other side), as identified by mass spectrometry and confirmed by peptide mapping and co-immunoprecipitation. PTPN18 dephosphorylates FBL at Y313 and promotes FBL degradation via the ubiquitin-proteasome pathway. Through FBL regulation, PTPN18 affects MAPK signaling, rRNA 2'-O-methylation, histone H2AQ104 methylation, and RNA synthesis. The PTPN18 R451A mutant loses these activities.","method":"Mass spectrometry interactome, peptide mapping, co-immunoprecipitation, site-directed mutagenesis (R451A), phosphatase assay for FBL-Y313, ubiquitination assay, downstream methylation and signaling readouts","journal":"Cell death & disease","confidence":"High","confidence_rationale":"Tier 1–2 — MS identification plus mutagenesis, direct dephosphorylation of substrate demonstrated, multiple downstream functional readouts","pmids":["41559020"],"is_preprint":false},{"year":2026,"finding":"PTPN18 binds cyclin E1 (by co-immunoprecipitation) and promotes its degradation via the ubiquitin-proteasome pathway, leading to cell cycle arrest in breast cancer cells. PTPN18 also regulates CDK inhibitors p21 (CDKN1A) and p27 (CDKN1B) through the PI3K/AKT signaling pathway. Cyclin E2 addition did not displace PTPN18 binding to cyclin E1, indicating specificity.","method":"Co-immunoprecipitation, ubiquitin-proteasome pathway assay, western blot, cell cycle analysis, PI3K/AKT inhibitor experiments","journal":"International journal of oncology","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct interaction and proteasomal degradation shown, pathway placement via inhibitor treatment, single lab","pmids":["41480706"],"is_preprint":false}],"current_model":"PTPN18 is a PEST-domain non-receptor protein tyrosine phosphatase that negatively regulates HER2 signaling through dual mechanisms — its catalytic domain selectively dephosphorylates specific HER2 phosphotyrosine sites (characterized by crystal structures) to modulate downstream growth/migration signaling, while its PEST domain promotes K48-linked ubiquitination and proteasomal HER2 degradation; additionally, PTPN18 dephosphorylates Src-family kinases (via Csk scaffolding), ETS1 (nuclear, suppressing TGF-β/EMT), and fibrillarin (regulating rRNA methylation and MAPK signaling), and acts as a positive regulator of EPO/JAK2 signaling in hematopoietic progenitors, with subcellular localization (cytoplasmic vs. nuclear, mediated by MVP/importin β2) determining functional specificity."},"narrative":{"teleology":[{"year":1996,"claim":"Identification of PTPN18 as an active non-receptor PTPase with a PEST motif established a new PEST-family phosphatase capable of dephosphorylating autophosphorylated tyrosine kinases overrepresented in tumors.","evidence":"PCR cloning and phosphatase assay in transfected 293 cells","pmids":["8950995"],"confidence":"Medium","gaps":["Physiological substrates not identified","In vivo relevance not tested","Single-lab characterization"]},{"year":2001,"claim":"Demonstration that PTPN18 physically associates with Csk via SH2-domain interactions and synergizes with it to inactivate Src-family kinases established PTPN18 as a scaffold-dependent negative regulator of Src signaling.","evidence":"Yeast two-hybrid, co-immunoprecipitation in mammalian and hematopoietic cells, domain-mapping functional assays","pmids":["11158295"],"confidence":"High","gaps":["Identity of the specific Src-family member(s) dephosphorylated in vivo unclear","Structural basis of Csk–PTPN18 interface not resolved"]},{"year":2003,"claim":"Gain- and loss-of-function experiments showed PTPN18 selectively dephosphorylates HER2 (not EGFR) and attenuates downstream Gab1/MAPK signaling, defining PTPN18 as a specific negative regulator of the HER2 axis.","evidence":"Overexpression and siRNA knockdown in breast cancer cells, phosphorylation-state immunoblotting","pmids":["14660651"],"confidence":"High","gaps":["Specific HER2 phosphotyrosine sites not yet mapped","Mechanism of substrate selectivity for HER2 over EGFR unknown"]},{"year":2014,"claim":"Crystal structures of PTPN18 bound to HER2 phosphopeptides revealed the molecular basis of site-selective dephosphorylation (pY1112, pY1196, pY1248), while a distinct PEST-domain-mediated K48-ubiquitination mechanism for HER2 degradation was uncovered, showing that catalytic and non-catalytic domains impose distinct signaling barcodes on HER2.","evidence":"X-ray crystallography (three complexes), in vitro enzymology, mutagenesis, ubiquitination assays, cell growth/migration assays","pmids":["25081058"],"confidence":"High","gaps":["E3 ligase recruited by the PEST domain not identified","In vivo confirmation of dual-mechanism regulation lacking"]},{"year":2020,"claim":"Discovery that EPO-induced Y389 phosphorylation converts PTPN18 into a positive regulator of JAK2/STAT5/ERK signaling in hematopoietic progenitors revealed context-dependent signaling roles beyond its canonical tumor-suppressive functions.","evidence":"Phosphoproteomics, phospho-site mutants, proliferation/survival assays in hematopoietic cells","pmids":["32027948"],"confidence":"Medium","gaps":["Direct substrate in EPO pathway not identified (RHEX dephosphorylation implied but not proven direct)","Single cell-line system","Kinase responsible for Y389 phosphorylation not definitively confirmed"]},{"year":2020,"claim":"Co-immunoprecipitation linked PTPN18 to MYC protein stabilization and CDK4 activation in colorectal cancer, suggesting an oncogenic role in certain tissue contexts.","evidence":"Co-IP, overexpression/knockdown in CRC cells, xenograft assay","pmids":["34522712"],"confidence":"Medium","gaps":["Single Co-IP without reciprocal validation or domain-mapping","Whether PTPN18 dephosphorylates MYC directly or acts indirectly not resolved","Mechanism of MYC stabilization not defined"]},{"year":2022,"claim":"Identification of MVP/importin-β2-mediated nuclear translocation of PTPN18 and its dephosphorylation of ETS1 to suppress TGF-β/EMT showed that subcellular compartmentalization determines PTPN18's functional output, with nuclear localization essential for anti-metastatic activity.","evidence":"Subcellular fractionation, nuclear/cytoplasmic mutant constructs, ETS1 phosphorylation/degradation assays, migration/invasion assays in breast cancer cells","pmids":["35982039"],"confidence":"Medium","gaps":["ETS1 phosphosite targeted by PTPN18 not mapped","Whether MVP-dependent transport is regulated by signaling cues unknown","Single-lab finding"]},{"year":2024,"claim":"Phosphoproteomic screening in hepatocellular carcinoma cells placed PTPN18 as an activator of p53 and a suppressor of AKT/FOXO1 signaling, and identified HBx-driven miR-128-3p upregulation as a mechanism of PTPN18 silencing during HBV infection.","evidence":"Phospho-protein microarray, western blot validation, miRNA and PTPN18 overexpression/knockdown in HCC cells","pmids":["38787319"],"confidence":"Medium","gaps":["Direct PTPN18 substrates in the p53 and AKT pathways not identified","In vivo HBV context not tested"]},{"year":2026,"claim":"Mass spectrometry interactome analysis and mutagenesis defined fibrillarin (FBL) as a direct PTPN18 substrate dephosphorylated at Y313, linking PTPN18 to rRNA 2′-O-methylation, histone H2AQ104 methylation, and MAPK signaling — extending its catalytic activity to nucleolar RNA-processing machinery.","evidence":"MS interactome, peptide mapping, co-IP, R451A mutagenesis, phosphatase assay for FBL-Y313, ubiquitination assay, methylation readouts","pmids":["41559020"],"confidence":"High","gaps":["Physiological conditions triggering PTPN18–FBL interaction not defined","Impact on ribosome biogenesis and translation not directly measured"]},{"year":2026,"claim":"PTPN18 was shown to bind cyclin E1 specifically and promote its proteasomal degradation, causing cell cycle arrest and regulating p21/p27 through PI3K/AKT signaling in breast cancer cells.","evidence":"Co-immunoprecipitation, ubiquitin-proteasome pathway assay, cell cycle analysis, PI3K/AKT inhibitor rescue","pmids":["41480706"],"confidence":"Medium","gaps":["Whether PTPN18 dephosphorylates cyclin E1 directly or acts as a scaffold for an E3 ligase not resolved","Single-lab finding"]},{"year":null,"claim":"The E3 ubiquitin ligase(s) recruited by the PTPN18 PEST domain to promote K48-ubiquitination of HER2 (and potentially other substrates) remain unidentified, and the regulatory signals that switch PTPN18 between tumor-suppressive and context-dependent pro-proliferative roles are not understood.","evidence":"","pmids":[],"confidence":"High","gaps":["E3 ligase partner for PEST-domain-mediated ubiquitination unknown","Structural basis for nuclear vs. cytoplasmic substrate selectivity unresolved","No knockout mouse phenotype reported"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,2,3,8,11]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,3,4,12]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,3,8]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[8,11]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[1,2,3,4,9]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[3,11,12]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[12]}],"complexes":[],"partners":["CSK","ERBB2","ETS1","FBL","MYC","MVP","CCNE1"],"other_free_text":[]},"mechanistic_narrative":"PTPN18 is a PEST-domain-containing non-receptor protein tyrosine phosphatase that serves as a negative regulator of receptor tyrosine kinase and Src-family kinase signaling, while also controlling protein stability through ubiquitin-dependent degradation mechanisms. Its catalytic domain selectively dephosphorylates HER2 at pY1112, pY1196, and pY1248 — with crystal structures defining substrate recognition — to attenuate MAPK and growth signaling, while its PEST domain independently promotes K48-linked ubiquitination and proteasomal destruction of HER2 [PMID:14660651, PMID:25081058]. PTPN18 also cooperates with Csk to inactivate Src-family kinases by dephosphorylating their positive-regulatory tyrosine [PMID:11158295], dephosphorylates the transcription factor ETS1 in the nucleus (following MVP/importin-β2-mediated translocation) to suppress TGF-β-driven epithelial-to-mesenchymal transition [PMID:35982039], and dephosphorylates fibrillarin at Y313 to regulate rRNA 2′-O-methylation and MAPK signaling [PMID:41559020]. In hematopoietic progenitors, EPO-induced phosphorylation of PTPN18 at Y389 converts it into a positive regulator of JAK2/STAT5/ERK signaling and cell survival [PMID:32027948]."},"prefetch_data":{"uniprot":{"accession":"Q99952","full_name":"Tyrosine-protein phosphatase non-receptor type 18","aliases":["Brain-derived phosphatase"],"length_aa":460,"mass_kda":50.5,"function":"Differentially dephosphorylate autophosphorylated tyrosine kinases which are known to be overexpressed in tumor tissues","subcellular_location":"Nucleus; Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q99952/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PTPN18","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"POLR1C","stoichiometry":0.2},{"gene":"POLR2E","stoichiometry":0.2},{"gene":"POLR2F","stoichiometry":0.2},{"gene":"POLR2H","stoichiometry":0.2},{"gene":"POLR2K","stoichiometry":0.2},{"gene":"POLR3A","stoichiometry":0.2},{"gene":"POLR3B","stoichiometry":0.2},{"gene":"POLR3D","stoichiometry":0.2},{"gene":"POLR3E","stoichiometry":0.2},{"gene":"POLR3F","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PTPN18","total_profiled":1310},"omim":[{"mim_id":"606587","title":"PROTEIN-TYROSINE PHOSPHATASE, NONRECEPTOR-TYPE, 18; PTPN18","url":"https://www.omim.org/entry/606587"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Intermediate filaments","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PTPN18"},"hgnc":{"alias_symbol":["BDP1"],"prev_symbol":[]},"alphafold":{"accession":"Q99952","domains":[{"cath_id":"3.90.190.10","chopping":"4-18_44-300","consensus_level":"high","plddt":96.8035,"start":4,"end":300}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99952","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q99952-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q99952-F1-predicted_aligned_error_v6.png","plddt_mean":76.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PTPN18","jax_strain_url":"https://www.jax.org/strain/search?query=PTPN18"},"sequence":{"accession":"Q99952","fasta_url":"https://rest.uniprot.org/uniprotkb/Q99952.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q99952/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99952"}},"corpus_meta":[{"pmid":"25081058","id":"PMC_25081058","title":"The catalytic region and PEST 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biology","url":"https://pubmed.ncbi.nlm.nih.gov/40355756","citation_count":9,"is_preprint":false,"source_track":"gene2pubmed"}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":17254,"output_tokens":2858,"usd":0.047316},"stage2":{"model":"claude-opus-4-6","input_tokens":6205,"output_tokens":2037,"usd":0.122925},"total_usd":0.411803,"stage1_batch_id":"msgbatch_01SUR8NDzVrNmyszWyJeCct7","stage2_batch_id":"msgbatch_01G7Yfutko3cDWzejrwH5AMx","note":"batch pricing = 50% of standard","round2_stage1":{"model":"claude-sonnet-4-6","input_tokens":30866,"output_tokens":4418,"usd":0.079434},"round2_rules_fired":"R2","round2_stage2":{"model":"claude-opus-4-6","input_tokens":7397,"output_tokens":2844,"usd":0.162128}},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2014,\n      \"finding\": \"PTPN18 catalytic domain dephosphorylates HER2 at pY1112, blocking lysosomal routing and delaying HER2 degradation, while the PTPN18 PEST domain promotes K48-linked HER2 ubiquitination and rapid proteasomal destruction. Crystal structures of PTPN18 in complex with HER2 phospho-peptides revealed the molecular basis for selective recognition of specific HER2 phosphorylation sites in two distinct conformations.\",\n      \"method\": \"Crystal structures of PTPN18–HER2 phospho-peptide complexes, enzymologic characterization, mutagenesis, cell-based ubiquitination/degradation assays, migration/growth assays\",\n      \"journal\": \"Cell Research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures + in vitro enzymology + mutagenesis + multiple cellular readouts in single study\",\n      \"pmids\": [\"25081058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PTPN18 (BDP1 phosphatase) selectively dephosphorylates HER2 but not EGFR upon ligand stimulation; overexpression inhibits HER2 activation and downstream Gab1 phosphorylation and MAPK activation, while suppression of endogenous PTPN18 increases HER2 phosphorylation.\",\n      \"method\": \"Overexpression and siRNA knockdown in breast cancer cell lines, Western blot for HER2/Gab1/MAPK phosphorylation\",\n      \"journal\": \"Journal of Biological Chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KD/OE with defined signaling phenotype, replicated in two directions (OE and KD), single lab\",\n      \"pmids\": [\"14660651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"PTPN18 (BDP1) is a functional protein tyrosine phosphatase containing a PEST motif that differentially dephosphorylates autophosphorylated tyrosine kinases overexpressed in tumor tissues, as demonstrated by hydrolysis of p-Nitrophenylphosphate in transfected 293 cells.\",\n      \"method\": \"Transient expression in HEK293 cells, phosphatase activity assay with p-Nitrophenylphosphate, differential dephosphorylation of receptor tyrosine kinases\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro enzymatic activity demonstrated with functional domain context; single lab\",\n      \"pmids\": [\"8950995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Molecular dynamics simulations confirmed that three HER2 phospho-peptides (pY1248, pY1196, pY1112) bind the PTPN18 catalytic domain with differential affinities; binding causes WPD-loop closure mediated by new hydrogen bonds and hydrophobic interactions involving residues Tyr62, Asp64, Val65, Ala231, Arg235, and Ala273, with allosteric communication from the pY binding loop to the WPD loop.\",\n      \"method\": \"Molecular dynamics simulation and free energy (MM-GBSA) calculations based on crystal structures\",\n      \"journal\": \"Journal of Molecular Recognition\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational only, no experimental validation in this paper\",\n      \"pmids\": [\"33620127\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PTPN18 is translocated from the cytoplasm to the nucleus by MVP (major vault protein) and importin β2; nuclear PTPN18 dephosphorylates ETS1 and promotes its degradation, suppressing TGF-β signaling and EMT, whereas cytoplasmic PTPN18 does not have this anti-metastatic effect.\",\n      \"method\": \"Ectopic expression, subcellular fractionation, co-immunoprecipitation, Western blot for ETS1 phosphorylation/degradation, EMT markers, migration/invasion assays\",\n      \"journal\": \"Cell Death & Disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with functional consequence, Co-IP for MVP/importin β2 interaction, dephosphorylation of ETS1 shown; single lab\",\n      \"pmids\": [\"35982039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PTPN18 acts as a positive regulator of EPOR/JAK2 signaling in erythroid progenitors; ectopic PTPN18 promotes EPO-dependent ERK1/2, AKT, STAT5, and JAK2 activation, increases high-molecular-weight EPOR forms, and inhibits RHEX-pY141 phosphorylation, all dependent on a JAK2-regulated PTPN18-pY389 site.\",\n      \"method\": \"Phospho-tyrosine proteomics, ectopic expression of wt and Y389 mutant PTPN18, EPO-stimulated proliferation/survival assays, Western blot for downstream signaling\",\n      \"journal\": \"Cellular Signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — phosphoproteomics + site-specific mutagenesis + multiple downstream readouts; single lab\",\n      \"pmids\": [\"32027948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Silencing of PTPN18 in endometrial cancer cells (KLE) induces ferroptosis by increasing intracellular ROS, downregulating GPX4 and xCT, and upregulating p-p38, placing PTPN18 upstream of the p-p38/GPX4/xCT axis.\",\n      \"method\": \"Lentiviral shRNA knockdown, ELISA for ROS, Western blot for GPX4, xCT, and p-p38\",\n      \"journal\": \"Cancer Management and Research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — KD with phenotypic readout but no direct biochemical link established; single lab, single method set\",\n      \"pmids\": [\"33642877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PTPN18 activates the MYC signaling pathway in colorectal cancer cells, stabilizes MYC protein, and thereby enhances CDK4 expression to promote cell cycle progression; MYC was shown to interact with PTPN18 by co-immunoprecipitation.\",\n      \"method\": \"Overexpression and knockdown in CRC cell lines, co-immunoprecipitation, Western blot for MYC/CDK4, xenograft assay\",\n      \"journal\": \"Genes & Diseases\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP showing interaction; mechanism of MYC stabilization not biochemically resolved; single lab\",\n      \"pmids\": [\"34522712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PTPN18 activates the p53 signaling pathway and suppresses the AKT/FOXO1 signaling cascade in hepatocellular carcinoma cells; HBx protein represses PTPN18 expression by upregulating miR-128-3p, identified via phospho-protein microarray screening.\",\n      \"method\": \"Phospho-protein microarray, Western blot for p53/AKT/FOXO1 pathway components, miRNA overexpression, HCC cell functional assays\",\n      \"journal\": \"Molecular Cancer Research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — microarray-driven pathway assignment with Western blot validation; upstream miR-128-3p mechanism shown; single lab\",\n      \"pmids\": [\"38787319\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PTPN18 interacts with fibrillarin (FBL) via its R451 residue (with FBL V187 as the complementary site), dephosphorylates FBL at Y313, and promotes FBL ubiquitin-proteasome degradation, thereby suppressing MAPK signaling, rRNA 2'-O methylation, and histone H2AQ104 methylation.\",\n      \"method\": \"Mass spectrometry, peptide mapping, co-immunoprecipitation, site-directed mutagenesis (R451A), Western blot for FBL phosphorylation and ubiquitination, methylation assays\",\n      \"journal\": \"Cell Death & Disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — MS-identified interactor, reciprocal binding sites mapped, dephosphorylation and ubiquitination demonstrated with mutagenesis; single lab but multiple orthogonal methods\",\n      \"pmids\": [\"41559020\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PTPN18 binds cyclin E1 (shown by co-immunoprecipitation) and promotes its degradation via the ubiquitin-proteasome pathway, leading to cell cycle arrest; downstream, PTPN18 regulates CDKN1A and CDKN1B expression through the PI3K/AKT signaling pathway.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitin-proteasome inhibitor treatment, Western blot for cyclin E/CDK inhibitors, proliferation and apoptosis assays\",\n      \"journal\": \"International Journal of Oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP, proteasome pathway inferred pharmacologically; single lab\",\n      \"pmids\": [\"41480706\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTPN18 is a PEST-domain-containing non-receptor protein tyrosine phosphatase that selectively dephosphorylates specific phosphotyrosine sites on substrates including HER2 (pY1112, pY1196, pY1248), ETS1, fibrillarin (Y313), and cyclin E; its catalytic domain blocks lysosomal HER2 routing while its PEST domain independently promotes K48-linked ubiquitination and proteasomal degradation of HER2 and other substrates, and nuclear-translocated PTPN18 (shuttled by MVP/importin β2) suppresses TGF-β/EMT signaling by targeting ETS1, collectively acting as a context-dependent regulator of receptor tyrosine kinase and transcription factor activity in breast cancer, hematopoietic, and other cell types.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nLooking at the corpus, I need to classify each paper:\n\n**PTPN18-relevant papers (KEEP):**\n- [1] PMID:25081058 — PTPN18/HER2 mechanism ✓\n- [6] PMID:14660651 — BDP1 = old name for PTPN18 (PEST-type PTP, HER2 regulation) ✓\n- [8] PMID:33642877 — PTPN18 ferroptosis/p38 ✓\n- [9] PMID:35982039 — PTPN18 nuclear import/ETS1 ✓\n- [10] PMID:8950995 — BDP1 (PTPN18) characterization ✓\n- [11] PMID:31034093 — PTPN18 endometrial cancer KD ✓\n- [14] PMID:34522712 — PTPN18/MYC/CDK4 ✓\n- [16] PMID:32027948 — PTPN18/EPO/JAK2 ✓\n- [20] PMID:33620127 — PTPN18/HER2 MD simulations ✓\n- [26] PMID:38787319 — PTPN18/HBx/p53/AKT ✓\n- [29] PMID:38855855 — PTPN18/MYC/PI3K/AKT ✓\n- [30] PMID:35310041 — PTPN18/PI3K/AKT ovarian ✓\n- [32] PMID:41559020 — PTPN18/FBL interaction ✓\n- [33] PMID:41480706 — PTPN18/cyclin E ✓\n\n**EXCLUDE (alias collision — BDP1 as RNA Pol III subunit, or unrelated):**\n- Papers [2],[3],[4],[5],[7],[12],[13],[15],[17],[18],[19],[21],[22],[23],[24],[25],[27],[28],[34],[35] — these describe the unrelated BDP1 (RNA Pol III TFIIIB subunit) or Plasmodium BDP1 bromodomain protein, completely different gene/protein\n\n**From gene2pubmed additional papers — EXCLUDE** most as they don't provide direct PTPN18 mechanistic findings (interactome screens, expression studies, etc.), EXCEPT:\n- PMID:11158295 — PTP-HSCF (=PTPN18) interaction with Csk SH2 domain, Src kinase dephosphorylation ✓ KEEP\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1996,\n      \"finding\": \"PTPN18 (originally named BDP1/Brain Derived Phosphatase) was identified as a functional protein tyrosine phosphatase with a PEST motif, classified in the PTP-PEST family. It lacks a transmembrane domain, has a calculated molecular weight of ~50 kDa, and was shown to hydrolyze p-Nitrophenylphosphate when expressed in 293 cells, confirming PTPase activity. It was found to differentially dephosphorylate autophosphorylated tyrosine kinases overexpressed in tumor tissues.\",\n      \"method\": \"PCR cloning, transient expression in 293 cells, enzymatic phosphatase assay, northern blot\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — enzymatic activity confirmed in cell-based assay, single lab\",\n      \"pmids\": [\"8950995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"PTPN18 (PTP-HSCF) interacts with the SH2 domain of Csk via two putative tyrosine phosphorylation sites in its non-catalytic region. Csk and PTPN18 synergize to inhibit Src family kinase signaling, and this cooperativity depends on the domains mediating their association. PTPN18 inactivates Src-related PTKs by selectively dephosphorylating the positive regulatory tyrosine in their kinase domain.\",\n      \"method\": \"Modified yeast two-hybrid screen, co-immunoprecipitation in mammalian cells and hematopoietic cells, transfection/overexpression functional assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal interaction confirmed in multiple systems, functional synergy demonstrated with domain-mapping\",\n      \"pmids\": [\"11158295\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"PTPN18 (BDP1) negatively regulates HER2 signaling. Overexpression of PTPN18 inhibited ligand-induced HER2 phosphorylation but not EGFR phosphorylation, demonstrating substrate specificity. Suppression of endogenous PTPN18 increased HER2 phosphorylation. PTPN18 also inhibited phosphorylation of the adaptor protein Gab1 and reduced MAPK activation downstream of HER2.\",\n      \"method\": \"Overexpression and siRNA knockdown in breast cancer cells, immunoprecipitation/western blot for phosphorylation states\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — both gain- and loss-of-function with defined substrate specificity and downstream pathway effects, replicated across conditions\",\n      \"pmids\": [\"14660651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"PTPN18 regulates HER2 through two distinct mechanisms: (1) The catalytic domain dephosphorylates HER2 at specific phosphorylation sites (pY1112, pY1196, pY1248), with crystal structures of PTPN18 in complex with HER2 phospho-peptides revealing molecular basis for substrate recognition in two distinct conformations; dephosphorylation of pY1112 blocks lysosomal routing and delays HER2 degradation. (2) The PEST domain of PTPN18 promotes K48-linked ubiquitination of HER2 and its rapid proteasomal destruction via a negative feedback loop, independent of catalytic activity. These two mechanisms define distinct HER2 phosphorylation and ubiquitination barcodes controlling cell growth and migration.\",\n      \"method\": \"X-ray crystallography (3 crystal structures of PTPN18–HER2 phosphopeptide complexes), in vitro enzymology, site-directed mutagenesis, cell-based ubiquitination assays, cell growth/migration assays, co-immunoprecipitation\",\n      \"journal\": \"Cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structures with functional validation, multiple orthogonal methods, mechanistic dissection of catalytic and non-catalytic domains\",\n      \"pmids\": [\"25081058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PTPN18 acts as a positive regulator of EPO/EPOR/JAK2 signaling in hematopoietic progenitor cells. EPO induces phosphorylation of PTPN18 at Y389, modulated by JAK2 inhibitors. Ectopic expression of wild-type PTPN18 (but not a Y389 mutant) promoted EPO dose-dependent cell proliferation and survival, sustained activation of ERK1/2, AKT, STAT5, and JAK2, increased high-molecular-weight EPOR forms, and sharply inhibited EPO-induced phosphorylation of the adaptor RHEX at Y141.\",\n      \"method\": \"Phospho-tyrosine PTM proteomics, ectopic expression with phospho-site mutants, western blot for pathway activation, cell proliferation/survival assays\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — site-specific phospho-mutant functional dissection in relevant cell line, single lab\",\n      \"pmids\": [\"32027948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PTPN18 promotes colorectal cancer cell growth by stabilizing the MYC protein, which in turn activates CDK4 expression. MYC was found to interact with PTPN18 by co-immunoprecipitation. PTPN18 overexpression increased MYC protein levels and activated MYC-CDK4 signaling; PTPN18 deficiency had the opposite effect both in vitro and in xenograft assays.\",\n      \"method\": \"Co-immunoprecipitation, overexpression and knockdown in CRC cells, xenograft assay, western blot for pathway components\",\n      \"journal\": \"Genes & diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP with functional KD/OE, pathway placement moderately supported\",\n      \"pmids\": [\"34522712\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Molecular dynamics simulations revealed that PTPN18 binds HER2 phospho-peptides with differential affinity (pY1248 > pY1196 > pY1112), and binding induces WPD-loop closure through new hydrogen bond and hydrophobic interactions involving residues Tyr62, Asp64, Val65, Ala231, Arg235, and Ala273. Allosteric communication networks from the pY binding loop to the WPD loop were identified, explaining substrate specificity at the atomic level.\",\n      \"method\": \"Molecular dynamics simulations, free energy calculations (MM/GBSA), correlation analysis of allosteric networks\",\n      \"journal\": \"Journal of molecular recognition\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational only, no experimental validation in this paper\",\n      \"pmids\": [\"33620127\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Silencing of PTPN18 in endometrial cancer cells (KLE) induced ferroptosis, increased intracellular ROS levels, downregulated GPX4 and xCT expression, and induced expression of phospho-p38. This places PTPN18 upstream of the p-p38/GPX4/xCT axis in regulating ferroptosis resistance.\",\n      \"method\": \"Lentiviral shRNA knockdown, ELISA for ROS, western blot for GPX4/xCT/p-p38\",\n      \"journal\": \"Cancer management and research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, single knockdown approach, pathway placement inferred without rescue or direct substrate assay\",\n      \"pmids\": [\"33642877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PTPN18 is translocated from the cytoplasm to the nucleus by MVP (major vault protein) and importin β2 in breast cancer cells. Nuclear PTPN18 dephosphorylates ETS1 transcription factor and promotes its degradation, thereby suppressing TGF-β signaling and epithelial-to-mesenchymal transition. Cytoplasmic PTPN18 does not suppress metastasis, establishing nuclear localization as essential for the anti-metastatic function.\",\n      \"method\": \"Subcellular fractionation, co-immunoprecipitation, ectopic expression with nuclear/cytoplasmic mutants, western blot for ETS1 phosphorylation/degradation, migration/invasion assays, TGF-β signaling readouts\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment with functional consequence, substrate dephosphorylation demonstrated, single lab\",\n      \"pmids\": [\"35982039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"HBx protein of hepatitis B virus represses PTPN18 expression by upregulating miR-128-3p. PTPN18 activates the p53 signaling pathway and suppresses the AKT/FOXO1 signaling cascade in hepatocellular carcinoma cells, as identified by phospho-protein microarray screening and validation experiments.\",\n      \"method\": \"Phospho-protein microarray, western blot validation, miRNA overexpression, PTPN18 overexpression/knockdown functional assays\",\n      \"journal\": \"Molecular cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — phosphoproteomic identification plus functional validation in HCC cells, single lab\",\n      \"pmids\": [\"38787319\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PTPN18 knockdown in endometrial cancer cells suppressed aerobic glycolysis (reduced glucose consumption, lactate production, ATP levels, and glycolysis proteins HIF-1α, PKM2, LDHA), migration, and invasion by inhibiting the MYC/PI3K/AKT pathway. MYC overexpression partially rescued the effects of PTPN18 knockdown, placing PTPN18 upstream of MYC in this pathway.\",\n      \"method\": \"shRNA knockdown, Transwell assay, metabolic assays (glucose consumption, lactate, ATP), western blot, rescue experiment with MYC overexpression\",\n      \"journal\": \"Histology and histopathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 + Moderate — epistasis via rescue experiment supports pathway placement, single lab\",\n      \"pmids\": [\"38855855\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PTPN18 interacts with fibrillarin (FBL) via its R451 residue (with FBL's V187 site being critical on the other side), as identified by mass spectrometry and confirmed by peptide mapping and co-immunoprecipitation. PTPN18 dephosphorylates FBL at Y313 and promotes FBL degradation via the ubiquitin-proteasome pathway. Through FBL regulation, PTPN18 affects MAPK signaling, rRNA 2'-O-methylation, histone H2AQ104 methylation, and RNA synthesis. The PTPN18 R451A mutant loses these activities.\",\n      \"method\": \"Mass spectrometry interactome, peptide mapping, co-immunoprecipitation, site-directed mutagenesis (R451A), phosphatase assay for FBL-Y313, ubiquitination assay, downstream methylation and signaling readouts\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — MS identification plus mutagenesis, direct dephosphorylation of substrate demonstrated, multiple downstream functional readouts\",\n      \"pmids\": [\"41559020\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"PTPN18 binds cyclin E1 (by co-immunoprecipitation) and promotes its degradation via the ubiquitin-proteasome pathway, leading to cell cycle arrest in breast cancer cells. PTPN18 also regulates CDK inhibitors p21 (CDKN1A) and p27 (CDKN1B) through the PI3K/AKT signaling pathway. Cyclin E2 addition did not displace PTPN18 binding to cyclin E1, indicating specificity.\",\n      \"method\": \"Co-immunoprecipitation, ubiquitin-proteasome pathway assay, western blot, cell cycle analysis, PI3K/AKT inhibitor experiments\",\n      \"journal\": \"International journal of oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct interaction and proteasomal degradation shown, pathway placement via inhibitor treatment, single lab\",\n      \"pmids\": [\"41480706\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTPN18 is a PEST-domain non-receptor protein tyrosine phosphatase that negatively regulates HER2 signaling through dual mechanisms — its catalytic domain selectively dephosphorylates specific HER2 phosphotyrosine sites (characterized by crystal structures) to modulate downstream growth/migration signaling, while its PEST domain promotes K48-linked ubiquitination and proteasomal HER2 degradation; additionally, PTPN18 dephosphorylates Src-family kinases (via Csk scaffolding), ETS1 (nuclear, suppressing TGF-β/EMT), and fibrillarin (regulating rRNA methylation and MAPK signaling), and acts as a positive regulator of EPO/JAK2 signaling in hematopoietic progenitors, with subcellular localization (cytoplasmic vs. nuclear, mediated by MVP/importin β2) determining functional specificity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PTPN18 is a PEST-domain-containing non-receptor protein tyrosine phosphatase that serves as a context-dependent regulator of receptor tyrosine kinase signaling, transcription factor stability, and protein turnover. Its catalytic domain selectively dephosphorylates HER2 at specific phosphotyrosine sites (pY1112, pY1196, pY1248) to block lysosomal routing, while its PEST domain independently promotes K48-linked ubiquitination and proteasomal degradation of HER2, and analogous dephosphorylation-coupled degradation mechanisms operate on fibrillarin (at Y313) and cyclin E1 [PMID:25081058, PMID:41559020, PMID:41480706]. Nuclear translocation of PTPN18, mediated by MVP and importin β2, enables dephosphorylation and degradation of ETS1, thereby suppressing TGF-β-driven epithelial–mesenchymal transition [PMID:35982039]. In erythroid progenitors, PTPN18 positively regulates EPOR/JAK2 signaling through a JAK2-dependent phosphorylation event at PTPN18-Y389, enhancing downstream ERK1/2, AKT, and STAT5 activation [PMID:32027948].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Establishing that PTPN18 is an active protein tyrosine phosphatase with a PEST motif that can differentially dephosphorylate receptor tyrosine kinases provided the foundational biochemical identity of this enzyme.\",\n      \"evidence\": \"Transient expression in HEK293 cells with pNPP phosphatase assay and differential dephosphorylation of overexpressed tyrosine kinases\",\n      \"pmids\": [\"8950995\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological substrates not identified\", \"PEST domain function not addressed\", \"No structural information\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrating that PTPN18 selectively targets HER2 but not EGFR and attenuates downstream MAPK signaling resolved its primary substrate specificity in breast cancer cells.\",\n      \"evidence\": \"Overexpression and siRNA knockdown in breast cancer cell lines with phospho-Western blotting for HER2, Gab1, and MAPK\",\n      \"pmids\": [\"14660651\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis for HER2-over-EGFR selectivity unknown\", \"No structural data on enzyme–substrate interface\", \"In vivo relevance not tested\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Crystal structures of PTPN18 bound to HER2 phospho-peptides, combined with cellular assays, revealed a dual-mechanism model in which the catalytic domain dephosphorylates specific HER2 sites to block lysosomal degradation while the PEST domain independently promotes K48-linked ubiquitination and proteasomal destruction.\",\n      \"evidence\": \"X-ray crystallography of PTPN18–HER2 phospho-peptide complexes, enzymology, mutagenesis, cell-based ubiquitination and degradation assays\",\n      \"pmids\": [\"25081058\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase recruited by the PEST domain not identified\", \"In vivo cancer model validation lacking\", \"How the two degradation pathways are balanced in physiological contexts is unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identification of PTPN18 as a positive regulator of EPOR/JAK2 signaling through a JAK2-phosphorylated Y389 site expanded its functional repertoire beyond HER2 into hematopoietic signaling.\",\n      \"evidence\": \"Phosphoproteomics, ectopic expression of WT and Y389 mutant PTPN18, EPO-stimulated signaling and proliferation assays in erythroid progenitors\",\n      \"pmids\": [\"32027948\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct phosphatase substrates in the EPOR/JAK2 pathway (besides RHEX) not fully mapped\", \"Loss-of-function in primary erythroid cells not shown\", \"Mechanism by which Y389 phosphorylation activates PTPN18 is unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Showing that MVP and importin β2 shuttle PTPN18 into the nucleus, where it dephosphorylates and degrades ETS1 to suppress TGF-β/EMT signaling, revealed a compartment-specific anti-metastatic mechanism distinct from its cytoplasmic HER2-regulatory role.\",\n      \"evidence\": \"Subcellular fractionation, co-immunoprecipitation for MVP/importin β2, ETS1 phosphorylation/degradation assays, EMT marker and invasion assays in breast cancer cells\",\n      \"pmids\": [\"35982039\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ETS1 phosphorylation site targeted by PTPN18 not mapped\", \"Whether the nuclear function operates independently of the PEST-domain ubiquitination mechanism is unclear\", \"In vivo metastasis model validation lacking\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Identification of fibrillarin as a PTPN18 substrate—dephosphorylated at Y313 and targeted for ubiquitin-proteasomal degradation—linked PTPN18 to rRNA 2'-O-methylation and histone methylation regulation, broadening its role to ribosome biogenesis and epigenetics.\",\n      \"evidence\": \"Mass spectrometry interactomics, reciprocal binding-site mutagenesis (R451A/V187), phospho- and ubiquitination assays, methylation readouts\",\n      \"pmids\": [\"41559020\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PTPN18 regulation of fibrillarin operates in non-cancer settings is unknown\", \"E3 ligase mediating FBL ubiquitination not identified\", \"Single-lab finding awaiting independent replication\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The E3 ubiquitin ligase(s) recruited by the PTPN18 PEST domain to drive substrate ubiquitination remain unidentified, and how PTPN18 integrates its catalytic dephosphorylation with PEST-mediated degradation across diverse substrates in vivo is unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No E3 ligase partner identified for PEST-domain-mediated ubiquitination\", \"No knockout mouse phenotype reported\", \"Structural basis for PEST domain function and substrate handoff unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 4, 5, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 9, 10]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ERBB2\",\n      \"ETS1\",\n      \"MVP\",\n      \"FBL\",\n      \"CCNE1\",\n      \"RHEX\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"PTPN18 is a PEST-domain-containing non-receptor protein tyrosine phosphatase that serves as a negative regulator of receptor tyrosine kinase and Src-family kinase signaling, while also controlling protein stability through ubiquitin-dependent degradation mechanisms. Its catalytic domain selectively dephosphorylates HER2 at pY1112, pY1196, and pY1248 — with crystal structures defining substrate recognition — to attenuate MAPK and growth signaling, while its PEST domain independently promotes K48-linked ubiquitination and proteasomal destruction of HER2 [PMID:14660651, PMID:25081058]. PTPN18 also cooperates with Csk to inactivate Src-family kinases by dephosphorylating their positive-regulatory tyrosine [PMID:11158295], dephosphorylates the transcription factor ETS1 in the nucleus (following MVP/importin-β2-mediated translocation) to suppress TGF-β-driven epithelial-to-mesenchymal transition [PMID:35982039], and dephosphorylates fibrillarin at Y313 to regulate rRNA 2′-O-methylation and MAPK signaling [PMID:41559020]. In hematopoietic progenitors, EPO-induced phosphorylation of PTPN18 at Y389 converts it into a positive regulator of JAK2/STAT5/ERK signaling and cell survival [PMID:32027948].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Identification of PTPN18 as an active non-receptor PTPase with a PEST motif established a new PEST-family phosphatase capable of dephosphorylating autophosphorylated tyrosine kinases overrepresented in tumors.\",\n      \"evidence\": \"PCR cloning and phosphatase assay in transfected 293 cells\",\n      \"pmids\": [\"8950995\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological substrates not identified\", \"In vivo relevance not tested\", \"Single-lab characterization\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Demonstration that PTPN18 physically associates with Csk via SH2-domain interactions and synergizes with it to inactivate Src-family kinases established PTPN18 as a scaffold-dependent negative regulator of Src signaling.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation in mammalian and hematopoietic cells, domain-mapping functional assays\",\n      \"pmids\": [\"11158295\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific Src-family member(s) dephosphorylated in vivo unclear\", \"Structural basis of Csk–PTPN18 interface not resolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Gain- and loss-of-function experiments showed PTPN18 selectively dephosphorylates HER2 (not EGFR) and attenuates downstream Gab1/MAPK signaling, defining PTPN18 as a specific negative regulator of the HER2 axis.\",\n      \"evidence\": \"Overexpression and siRNA knockdown in breast cancer cells, phosphorylation-state immunoblotting\",\n      \"pmids\": [\"14660651\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific HER2 phosphotyrosine sites not yet mapped\", \"Mechanism of substrate selectivity for HER2 over EGFR unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Crystal structures of PTPN18 bound to HER2 phosphopeptides revealed the molecular basis of site-selective dephosphorylation (pY1112, pY1196, pY1248), while a distinct PEST-domain-mediated K48-ubiquitination mechanism for HER2 degradation was uncovered, showing that catalytic and non-catalytic domains impose distinct signaling barcodes on HER2.\",\n      \"evidence\": \"X-ray crystallography (three complexes), in vitro enzymology, mutagenesis, ubiquitination assays, cell growth/migration assays\",\n      \"pmids\": [\"25081058\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase recruited by the PEST domain not identified\", \"In vivo confirmation of dual-mechanism regulation lacking\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery that EPO-induced Y389 phosphorylation converts PTPN18 into a positive regulator of JAK2/STAT5/ERK signaling in hematopoietic progenitors revealed context-dependent signaling roles beyond its canonical tumor-suppressive functions.\",\n      \"evidence\": \"Phosphoproteomics, phospho-site mutants, proliferation/survival assays in hematopoietic cells\",\n      \"pmids\": [\"32027948\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct substrate in EPO pathway not identified (RHEX dephosphorylation implied but not proven direct)\", \"Single cell-line system\", \"Kinase responsible for Y389 phosphorylation not definitively confirmed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Co-immunoprecipitation linked PTPN18 to MYC protein stabilization and CDK4 activation in colorectal cancer, suggesting an oncogenic role in certain tissue contexts.\",\n      \"evidence\": \"Co-IP, overexpression/knockdown in CRC cells, xenograft assay\",\n      \"pmids\": [\"34522712\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single Co-IP without reciprocal validation or domain-mapping\", \"Whether PTPN18 dephosphorylates MYC directly or acts indirectly not resolved\", \"Mechanism of MYC stabilization not defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of MVP/importin-β2-mediated nuclear translocation of PTPN18 and its dephosphorylation of ETS1 to suppress TGF-β/EMT showed that subcellular compartmentalization determines PTPN18's functional output, with nuclear localization essential for anti-metastatic activity.\",\n      \"evidence\": \"Subcellular fractionation, nuclear/cytoplasmic mutant constructs, ETS1 phosphorylation/degradation assays, migration/invasion assays in breast cancer cells\",\n      \"pmids\": [\"35982039\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"ETS1 phosphosite targeted by PTPN18 not mapped\", \"Whether MVP-dependent transport is regulated by signaling cues unknown\", \"Single-lab finding\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Phosphoproteomic screening in hepatocellular carcinoma cells placed PTPN18 as an activator of p53 and a suppressor of AKT/FOXO1 signaling, and identified HBx-driven miR-128-3p upregulation as a mechanism of PTPN18 silencing during HBV infection.\",\n      \"evidence\": \"Phospho-protein microarray, western blot validation, miRNA and PTPN18 overexpression/knockdown in HCC cells\",\n      \"pmids\": [\"38787319\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct PTPN18 substrates in the p53 and AKT pathways not identified\", \"In vivo HBV context not tested\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Mass spectrometry interactome analysis and mutagenesis defined fibrillarin (FBL) as a direct PTPN18 substrate dephosphorylated at Y313, linking PTPN18 to rRNA 2′-O-methylation, histone H2AQ104 methylation, and MAPK signaling — extending its catalytic activity to nucleolar RNA-processing machinery.\",\n      \"evidence\": \"MS interactome, peptide mapping, co-IP, R451A mutagenesis, phosphatase assay for FBL-Y313, ubiquitination assay, methylation readouts\",\n      \"pmids\": [\"41559020\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological conditions triggering PTPN18–FBL interaction not defined\", \"Impact on ribosome biogenesis and translation not directly measured\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"PTPN18 was shown to bind cyclin E1 specifically and promote its proteasomal degradation, causing cell cycle arrest and regulating p21/p27 through PI3K/AKT signaling in breast cancer cells.\",\n      \"evidence\": \"Co-immunoprecipitation, ubiquitin-proteasome pathway assay, cell cycle analysis, PI3K/AKT inhibitor rescue\",\n      \"pmids\": [\"41480706\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether PTPN18 dephosphorylates cyclin E1 directly or acts as a scaffold for an E3 ligase not resolved\", \"Single-lab finding\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The E3 ubiquitin ligase(s) recruited by the PTPN18 PEST domain to promote K48-ubiquitination of HER2 (and potentially other substrates) remain unidentified, and the regulatory signals that switch PTPN18 between tumor-suppressive and context-dependent pro-proliferative roles are not understood.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase partner for PEST-domain-mediated ubiquitination unknown\", \"Structural basis for nuclear vs. cytoplasmic substrate selectivity unresolved\", \"No knockout mouse phenotype reported\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 3, 8, 11]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 3, 4, 12]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 3, 8]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [8, 11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [1, 2, 3, 4, 9]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [3, 11, 12]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [12]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"CSK\",\n      \"ERBB2\",\n      \"ETS1\",\n      \"FBL\",\n      \"MYC\",\n      \"MVP\",\n      \"CCNE1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}