{"gene":"METTL9","run_date":"2026-04-28T18:30:28","timeline":{"discoveries":[{"year":2021,"finding":"METTL9 is a broad-specificity protein histidine methyltransferase that catalyzes 1-methylhistidine (1MH, Nπ-methylhistidine) formation at His-x-His (HxH) motifs in numerous mammalian proteins, including S100A9 and the NDUFB3 subunit of mitochondrial Complex I. METTL9-mediated methylation enhances Complex I-dependent respiration, and 1MH modification of HxH-containing peptides reduces zinc-binding affinity.","method":"Proteomic mass spectrometry of METTL9 knockout mice and human cells, in vitro methyltransferase assays, METTL9 KO mouse generation, Complex I respiration assays, zinc-binding affinity measurements","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (in vitro assay, KO mouse, MS proteomics, functional respiration assay), independently replicated in companion paper","pmids":["33563959"],"is_preprint":false},{"year":2021,"finding":"METTL9 specifically catalyzes Nπ-methylhistidine (N1/1MH) formation on S100A9 at His-107, a zinc-binding site, thereby attenuating S100A9's affinity for zinc. METTL9 does not affect S100A9/S100A8 heterodimer formation. METTL9 does not methylate MYLK2 in vivo despite an HxH motif.","method":"siRNA screening coupled with LC-tandem MS methylhistidine analysis, in vitro methyltransferase assays, in vivo cell-based assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro assay plus MS-based detection, corroborates PMID 33563959 with orthogonal approach","pmids":["34562450"],"is_preprint":false},{"year":2023,"finding":"Structural and biochemical studies revealed that METTL9 specifically methylates the N1 atom of the second histidine in the HxH motif, using the first histidine as a recognition signature. A small 'x' residue fits within a confined substrate pocket; an aspartate residue stabilizes the N3 atom of the target histidine imidazole ring, presenting N1 to SAM for methylation. METTL9 preferentially methylates tandem HxH repeats in a consecutive, C-to-N directional manner.","method":"X-ray crystallography of METTL9–substrate complex, in vitro methyltransferase assays, site-directed mutagenesis of active-site residues","journal":"Cell insight","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with functional biochemical validation and mutagenesis","pmids":["37398635"],"is_preprint":false},{"year":2022,"finding":"In metastatic gastric cancer cells, METTL9 protein localizes predominantly to mitochondria, and METTL9 knockdown significantly reduces mitochondrial Complex I activity, establishing a direct functional link between METTL9 localization and Complex I function.","method":"shRNA-mediated stable knockdown, subcellular fractionation/immunofluorescence for mitochondrial localization, Complex I activity assay","journal":"Biochemistry and biophysics reports","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization experiment tied to functional consequence (Complex I activity), single lab","pmids":["35402738"],"is_preprint":false},{"year":2023,"finding":"METTL9 knockdown reduces SLC7A11 expression (a key ferroptosis suppressor) in hepatocellular carcinoma cells, promoting ferroptosis and inhibiting HCC progression in vitro and in PDX models.","method":"shRNA knockdown, cell viability/migration assays, PDX xenograft model, Western blot for SLC7A11","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 2–3 — loss-of-function with defined cellular phenotype, but mechanism linking methyltransferase activity to SLC7A11 not fully resolved at molecular level","pmids":["38017014"],"is_preprint":false},{"year":2025,"finding":"METTL9 methylates SLC39A7 (ZIP7 zinc transporter) at His45 and His49 residues; this methylation suppresses ferroptosis via the PERK/ATF4 signaling pathway and downstream SLC7A11-mediated cystine import and glutathione synthesis, reducing ROS and inhibiting adipogenic differentiation of mesenchymal stem cells.","method":"In vitro methyltransferase assay on SLC39A7 peptides, site-directed mutagenesis, METTL9 overexpression/knockdown, adipogenic differentiation assays, OVX mouse model","journal":"Molecular medicine (Cambridge, Mass.)","confidence":"Medium","confidence_rationale":"Tier 2 — direct substrate identification with mutagenesis and pathway readout, single lab","pmids":["40414869"],"is_preprint":false},{"year":2025,"finding":"METTL9 plays a conserved role in vertebrate neurogenesis that is largely independent of its catalytic (methyltransferase) activity. METTL9 interacts with key regulators of cellular transport, endocytosis, and Golgi integrity; Mettl9 KO mouse embryonic stem cells show Golgi fragmentation. METTL9 modulates the secretory pathway to support neural development.","method":"Complete Mettl9 KO, inducible Degron, and catalytically inactive knock-in mouse ES cell lines; Xenopus laevis mettl9 knockdown; multi-omics; Co-IP for interactors; Golgi morphology imaging","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 — multiple genetic models (KO, catalytic dead knock-in, Degron) plus multi-omics and orthogonal organism validation","pmids":["40745158"],"is_preprint":false},{"year":2024,"finding":"METTL9 promotes histidine methylation of NF-κB RELA, resulting in inhibition of NLRP3 transcription and suppression of neuronal pyroptosis in a Parkinson's disease mouse model.","method":"MPTP-induced PD mouse model, METTL9 gain/loss-of-function, Western blot, luciferase reporter assay, ChIP assay for RELA binding to NLRP3 promoter","journal":"Critical reviews in eukaryotic gene expression","confidence":"Low","confidence_rationale":"Tier 3 — indirect evidence for methylation of NF-κB RELA; direct histidine methylation of RELA not biochemically confirmed; single lab","pmids":["39072406"],"is_preprint":false},{"year":2025,"finding":"METTL9 orthologues across eukaryotes retain in vitro methyltransferase activity on HxH-motif substrates (ARMC6, DNAJB12), but Drosophila and Ostreococcus tauri METTL9 show distinct substrate specificities compared to human METTL9. The X-ray structure of OtMETTL9 revealed structural differences from human METTL9 explaining its distinct substrate preference. C. elegans METTL9, previously thought to be a DNA MTase, has protein methylation activity.","method":"In vitro methyltransferase assays with recombinant proteins and peptide arrays, X-ray crystallography of OtMETTL9","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus in vitro enzymatic characterization across multiple species with systematic peptide array","pmids":["40451431"],"is_preprint":false},{"year":2026,"finding":"A first-in-class small-molecule inhibitor (METTL9i) binds within the SAM-binding pocket of METTL9 (IC50 = 0.067 µM), selectively inhibiting METTL9 over other methyltransferases and reducing global 1-methylhistidine levels in cells.","method":"In vitro enzyme inhibition assay, X-ray crystallography of METTL9i–METTL9 complex, cellular target engagement assay, global 1-MH proteomics","journal":"Angewandte Chemie (International ed. in English)","confidence":"High","confidence_rationale":"Tier 1 — crystal structure of inhibitor–enzyme complex, in vitro IC50, cellular 1-MH reduction confirmed by proteomics","pmids":["41870122"],"is_preprint":false},{"year":2025,"finding":"METTL9 binds to SLC7A11 protein and enhances its stability by reducing its degradation, thereby regulating ferroptosis in HCC. This interaction was confirmed by Co-IP and operates independently of GPX4.","method":"Co-immunoprecipitation (Co-IP), RNA sequencing, CUT&Tag analysis showing SIX2 directly regulates METTL9 expression, overexpression/knockdown functional assays","journal":"NPJ precision oncology","confidence":"Medium","confidence_rationale":"Tier 3 — reciprocal Co-IP showing METTL9–SLC7A11 interaction with functional consequence, single lab","pmids":["40523929"],"is_preprint":false}],"current_model":"METTL9 is a eukaryotic seven-β-strand methyltransferase that transfers a methyl group from SAM to the N1 atom of the second histidine within His-x-His (HxH) motifs—using an active-site aspartate to orient the substrate—thereby generating 1-methylhistidine (1MH) on a broad array of proteins including S100A9, NDUFB3 (Complex I), SLC39A7, and ARMC6; this modification reduces zinc-binding affinity of target sites, enhances mitochondrial Complex I respiration, and modulates ferroptosis resistance, while METTL9 also sustains vertebrate neurogenesis through a catalysis-independent role in maintaining Golgi/secretory pathway integrity."},"narrative":{"teleology":[{"year":2021,"claim":"The enzymatic function of METTL9 was unknown; proteomic and biochemical analyses established it as the major mammalian histidine methyltransferase generating 1-methylhistidine at HxH motifs, with substrates including S100A9 and NDUFB3, linking it to Complex I respiration and zinc-binding regulation.","evidence":"METTL9 KO mice, proteomic MS, in vitro methyltransferase assays, Complex I respiration and zinc-binding measurements","pmids":["33563959","34562450"],"confidence":"High","gaps":["Identity of the full in vivo substrate repertoire beyond identified targets","Mechanism by which 1MH modification enhances Complex I activity not resolved at structural level","Selectivity rules explaining why some HxH-containing proteins (e.g. MYLK2) are not methylated in vivo"]},{"year":2022,"claim":"Where METTL9 acts within cells was unclear; subcellular fractionation showed predominant mitochondrial localization in gastric cancer cells, directly coupling METTL9 presence to Complex I activity.","evidence":"Subcellular fractionation, immunofluorescence, and Complex I activity assay in shRNA-knockdown gastric cancer cells","pmids":["35402738"],"confidence":"Medium","gaps":["Localization studied only in one cancer cell type","Whether mitochondrial targeting requires a signal peptide or import machinery not determined","Relative distribution across other compartments not systematically quantified"]},{"year":2023,"claim":"How METTL9 achieves N1-specific methylation was structurally unknown; the crystal structure of METTL9 bound to substrate revealed that an active-site aspartate stabilizes N3 of the target histidine imidazole, presenting N1 to SAM, with the first histidine serving as a recognition element and a confined pocket selecting for small intervening residues.","evidence":"X-ray crystallography of METTL9–substrate complex, site-directed mutagenesis, in vitro assays on tandem HxH repeats","pmids":["37398635"],"confidence":"High","gaps":["Full-length protein substrate complexes not crystallized","Structural basis for C-to-N processivity on tandem repeats not fully defined"]},{"year":2023,"claim":"Whether METTL9 influenced cell death pathways was unexplored; knockdown in HCC cells reduced SLC7A11 expression and promoted ferroptosis, linking METTL9 to ferroptosis resistance.","evidence":"shRNA knockdown, cell viability/migration assays, PDX xenograft models","pmids":["38017014"],"confidence":"Medium","gaps":["Whether SLC7A11 regulation depends on METTL9 catalytic activity not established","No direct methylation of SLC7A11 or its transcriptional regulators demonstrated"]},{"year":2025,"claim":"The molecular mechanism connecting METTL9 to ferroptosis was clarified: METTL9 methylates SLC39A7 (ZIP7) at His45/49, suppressing ferroptosis through PERK/ATF4-mediated SLC7A11 induction and glutathione synthesis.","evidence":"In vitro methyltransferase assay on SLC39A7 peptides, site-directed mutagenesis, OVX mouse model, adipogenic differentiation assays","pmids":["40414869"],"confidence":"Medium","gaps":["How histidine methylation of SLC39A7 triggers PERK/ATF4 signaling is mechanistically unclear","Single-lab finding awaiting independent replication"]},{"year":2025,"claim":"Whether METTL9 functions solely through its catalytic activity was untested; genetic dissection in mouse ESCs and Xenopus revealed a conserved catalysis-independent role in neurogenesis, with METTL9 maintaining Golgi integrity and secretory pathway function through interaction with transport and endocytic regulators.","evidence":"METTL9 KO, catalytically dead knock-in, Degron mouse ESC lines; Xenopus knockdown; multi-omics; Co-IP; Golgi morphology imaging","pmids":["40745158"],"confidence":"High","gaps":["Identity of the specific Golgi/transport interactors mediating the non-catalytic function not fully characterized","Whether the neurogenesis role extends to adult neurogenesis not tested"]},{"year":2025,"claim":"Evolutionary conservation and divergence of METTL9 substrate specificity was uncharted; cross-species analysis showed conserved HxH methylation activity but distinct substrate preferences between human, Drosophila, and algal orthologues, explained by structural differences in the substrate pocket.","evidence":"In vitro methyltransferase assays with recombinant orthologues, peptide arrays, X-ray crystallography of O. tauri METTL9","pmids":["40451431"],"confidence":"High","gaps":["In vivo substrates of non-mammalian METTL9 orthologues not determined","Functional consequences of species-specific substrate selectivity unknown"]},{"year":2025,"claim":"METTL9 was shown to physically bind SLC7A11 and stabilize it against degradation, providing a direct protein-level mechanism for METTL9-dependent ferroptosis suppression in HCC independent of GPX4.","evidence":"Reciprocal Co-IP, RNA-seq, overexpression/knockdown functional assays in HCC cells","pmids":["40523929"],"confidence":"Medium","gaps":["Whether the METTL9–SLC7A11 interaction depends on methyltransferase activity not tested","Single-lab Co-IP without domain mapping or structural characterization of the interaction"]},{"year":2026,"claim":"Pharmacological targeting of METTL9 was previously impossible; a first-in-class small-molecule inhibitor (IC50 = 67 nM) was developed that occupies the SAM-binding pocket and selectively reduces global cellular 1MH levels, providing a chemical tool to dissect METTL9 biology.","evidence":"X-ray crystallography of inhibitor–METTL9 complex, in vitro IC50 determination, selectivity profiling, cellular 1MH proteomics","pmids":["41870122"],"confidence":"High","gaps":["In vivo pharmacokinetics and efficacy not reported","Effects of inhibitor on non-catalytic METTL9 functions not assessed"]},{"year":null,"claim":"Key unresolved questions include: whether METTL9's ferroptosis-related effects operate through its catalytic activity or non-catalytic protein stabilization of SLC7A11; the structural basis and in vivo significance of the catalysis-independent Golgi/neurogenesis role; and the full physiological substrate repertoire in different tissues.","evidence":"","pmids":[],"confidence":"High","gaps":["Catalytic vs. non-catalytic contributions to ferroptosis not genetically separated","Golgi-maintaining mechanism and relevant interactors not biochemically reconstituted","Tissue-specific in vivo substrate profiles not mapped"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2,5,8]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[3]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,5,8]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[6]},{"term_id":"R-HSA-5357801","term_label":"Programmed Cell Death","supporting_discovery_ids":[4,5,10]}],"complexes":[],"partners":["S100A9","NDUFB3","SLC39A7","ARMC6","SLC7A11","DNAJB12"],"other_free_text":[]},"mechanistic_narrative":"METTL9 is a seven-β-strand methyltransferase that catalyzes N1 (Nπ) methylation of histidine residues within His-x-His (HxH) motifs across a broad substrate repertoire, generating 1-methylhistidine (1MH) on proteins including S100A9, NDUFB3 (Complex I subunit), SLC39A7, and ARMC6 [PMID:33563959, PMID:34562450, PMID:40451431]. Crystal structures show that an active-site aspartate orients the target imidazole ring by engaging N3, presenting N1 to the SAM methyl donor, with the first histidine of the HxH dipeptide serving as a recognition element and methylation proceeding C-to-N through tandem repeats [PMID:37398635]. Functionally, METTL9-mediated methylation reduces zinc-binding affinity of target HxH sites, enhances mitochondrial Complex I respiration, and suppresses ferroptosis via SLC39A7/PERK/ATF4-dependent upregulation of SLC7A11 [PMID:33563959, PMID:40414869, PMID:38017014]. Beyond its catalytic role, METTL9 sustains vertebrate neurogenesis through a catalysis-independent function in maintaining Golgi and secretory pathway integrity [PMID:40745158]."},"prefetch_data":{"uniprot":{"accession":"Q9H1A3","full_name":"Protein-L-histidine N-pros-methyltransferase","aliases":["DORA reverse strand protein","DREV","DREV1","Methyltransferase-like protein 9","hMETTL9"],"length_aa":318,"mass_kda":36.5,"function":"Protein-histidine N-methyltransferase that specifically catalyzes 1-methylhistidine (pros-methylhistidine) methylation of target proteins (PubMed:33563959, PubMed:34562450, PubMed:37015930, PubMed:37398635). Specifically methylates the second His of proteins with a His-x-His (HxH) motif (where 'x' is preferably a small amino acid), while exploiting the first one as a recognition signature (PubMed:37398635). Catalyzes methylation of target proteins such as S100A9, NDUFB3, SLC39A5, SLC39A7, ARMC6 and DNAJB12; 1-methylhistidine modification may affect the binding of zinc and other metals to its target proteins (PubMed:33563959, PubMed:34562450, PubMed:37015930, PubMed:37398635). Constitutes the main methyltransferase for the 1-methylhistidine modification in cell (PubMed:33563959)","subcellular_location":"Endoplasmic reticulum; Mitochondrion","url":"https://www.uniprot.org/uniprotkb/Q9H1A3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/METTL9","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/METTL9","total_profiled":1310},"omim":[{"mim_id":"609388","title":"METHYLTRANSFERASE-LIKE 9; METTL9","url":"https://www.omim.org/entry/609388"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Cell Junctions","reliability":"Approved"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/METTL9"},"hgnc":{"alias_symbol":["DREV1"],"prev_symbol":[]},"alphafold":{"accession":"Q9H1A3","domains":[{"cath_id":"3.40.50.150","chopping":"56-316","consensus_level":"high","plddt":88.4314,"start":56,"end":316}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H1A3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H1A3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9H1A3-F1-predicted_aligned_error_v6.png","plddt_mean":83.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=METTL9","jax_strain_url":"https://www.jax.org/strain/search?query=METTL9"},"sequence":{"accession":"Q9H1A3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9H1A3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9H1A3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9H1A3"}},"corpus_meta":[{"pmid":"33563959","id":"PMC_33563959","title":"The methyltransferase METTL9 mediates pervasive 1-methylhistidine modification in mammalian proteomes.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33563959","citation_count":88,"is_preprint":false},{"pmid":"38017014","id":"PMC_38017014","title":"METTL9-SLC7A11 axis promotes hepatocellular carcinoma progression through ferroptosis inhibition.","date":"2023","source":"Cell death discovery","url":"https://pubmed.ncbi.nlm.nih.gov/38017014","citation_count":23,"is_preprint":false},{"pmid":"36803376","id":"PMC_36803376","title":"Circular RNA METTL9 contributes to neuroinflammation following traumatic brain injury by complexing with astrocytic SND1.","date":"2023","source":"Journal of neuroinflammation","url":"https://pubmed.ncbi.nlm.nih.gov/36803376","citation_count":20,"is_preprint":false},{"pmid":"34562450","id":"PMC_34562450","title":"siRNA screening identifies METTL9 as a histidine Nπ-methyltransferase that targets the proinflammatory protein S100A9.","date":"2021","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/34562450","citation_count":18,"is_preprint":false},{"pmid":"37398635","id":"PMC_37398635","title":"Molecular basis for protein histidine N1-specific methylation of the \"His-x-His\" motifs by METTL9.","date":"2023","source":"Cell insight","url":"https://pubmed.ncbi.nlm.nih.gov/37398635","citation_count":14,"is_preprint":false},{"pmid":"37158456","id":"PMC_37158456","title":"METTL9 derived circular RNA circ-METTL9 sponges miR-551b-5p to accelerate colorectal cancer progression by upregulating CDK6.","date":"2023","source":"Carcinogenesis","url":"https://pubmed.ncbi.nlm.nih.gov/37158456","citation_count":11,"is_preprint":false},{"pmid":"35402738","id":"PMC_35402738","title":"Elevated METTL9 is associated with peritoneal dissemination in human scirrhous gastric cancers.","date":"2022","source":"Biochemistry and biophysics reports","url":"https://pubmed.ncbi.nlm.nih.gov/35402738","citation_count":9,"is_preprint":false},{"pmid":"40414869","id":"PMC_40414869","title":"METTL9 mediated N1-Histidine methylation of SLC39A7 confers ferroptosis resistance and inhibits adipogenic differentiation in mesenchymal stem cells.","date":"2025","source":"Molecular medicine (Cambridge, Mass.)","url":"https://pubmed.ncbi.nlm.nih.gov/40414869","citation_count":5,"is_preprint":false},{"pmid":"40745158","id":"PMC_40745158","title":"METTL9 sustains vertebrate neural development primarily via non-catalytic functions.","date":"2025","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/40745158","citation_count":3,"is_preprint":false},{"pmid":"39072406","id":"PMC_39072406","title":"Electroacupuncture Alleviates Parkinson's Disease by Promoting METTL9-Catalyzed Histidine Methylation of Nuclear Factor-κВ.","date":"2024","source":"Critical reviews in eukaryotic gene expression","url":"https://pubmed.ncbi.nlm.nih.gov/39072406","citation_count":2,"is_preprint":false},{"pmid":"40451431","id":"PMC_40451431","title":"Orthologues of the human protein histidine methyltransferase METTL9 display distinct substrate specificities.","date":"2025","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/40451431","citation_count":1,"is_preprint":false},{"pmid":"40523929","id":"PMC_40523929","title":"Targeting SIX2 as a novel sensitization strategy of sorafenib treatment on advanced hepatocellular carcinoma through modulating METTL9-SLC7A11 axis.","date":"2025","source":"NPJ precision oncology","url":"https://pubmed.ncbi.nlm.nih.gov/40523929","citation_count":0,"is_preprint":false},{"pmid":"41900964","id":"PMC_41900964","title":"The Protein Histidine Methyltransferase METTL9-From Mechanism to Biological Function.","date":"2026","source":"Life (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/41900964","citation_count":0,"is_preprint":false},{"pmid":"41870122","id":"PMC_41870122","title":"Discovery of a Potent and Selective Cell-Active Inhibitor of Histidine-N1 Methyltransferase METTL9.","date":"2026","source":"Angewandte Chemie (International ed. in English)","url":"https://pubmed.ncbi.nlm.nih.gov/41870122","citation_count":0,"is_preprint":false},{"pmid":"41539299","id":"PMC_41539299","title":"METTL9 tests Candida's mettle by limiting metal acquisition.","date":"2026","source":"Cell host & microbe","url":"https://pubmed.ncbi.nlm.nih.gov/41539299","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8484,"output_tokens":2901,"usd":0.034484},"stage2":{"model":"claude-opus-4-6","input_tokens":6271,"output_tokens":2626,"usd":0.145508},"total_usd":0.179992,"stage1_batch_id":"msgbatch_01Mgbec4vM6c9vXht7JVJzss","stage2_batch_id":"msgbatch_01JkuuH5RB2BdJNsgnhjbC6s","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2021,\n      \"finding\": \"METTL9 is a broad-specificity protein histidine methyltransferase that catalyzes 1-methylhistidine (1MH, Nπ-methylhistidine) formation at His-x-His (HxH) motifs in numerous mammalian proteins, including S100A9 and the NDUFB3 subunit of mitochondrial Complex I. METTL9-mediated methylation enhances Complex I-dependent respiration, and 1MH modification of HxH-containing peptides reduces zinc-binding affinity.\",\n      \"method\": \"Proteomic mass spectrometry of METTL9 knockout mice and human cells, in vitro methyltransferase assays, METTL9 KO mouse generation, Complex I respiration assays, zinc-binding affinity measurements\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (in vitro assay, KO mouse, MS proteomics, functional respiration assay), independently replicated in companion paper\",\n      \"pmids\": [\"33563959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"METTL9 specifically catalyzes Nπ-methylhistidine (N1/1MH) formation on S100A9 at His-107, a zinc-binding site, thereby attenuating S100A9's affinity for zinc. METTL9 does not affect S100A9/S100A8 heterodimer formation. METTL9 does not methylate MYLK2 in vivo despite an HxH motif.\",\n      \"method\": \"siRNA screening coupled with LC-tandem MS methylhistidine analysis, in vitro methyltransferase assays, in vivo cell-based assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro assay plus MS-based detection, corroborates PMID 33563959 with orthogonal approach\",\n      \"pmids\": [\"34562450\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Structural and biochemical studies revealed that METTL9 specifically methylates the N1 atom of the second histidine in the HxH motif, using the first histidine as a recognition signature. A small 'x' residue fits within a confined substrate pocket; an aspartate residue stabilizes the N3 atom of the target histidine imidazole ring, presenting N1 to SAM for methylation. METTL9 preferentially methylates tandem HxH repeats in a consecutive, C-to-N directional manner.\",\n      \"method\": \"X-ray crystallography of METTL9–substrate complex, in vitro methyltransferase assays, site-directed mutagenesis of active-site residues\",\n      \"journal\": \"Cell insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with functional biochemical validation and mutagenesis\",\n      \"pmids\": [\"37398635\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"In metastatic gastric cancer cells, METTL9 protein localizes predominantly to mitochondria, and METTL9 knockdown significantly reduces mitochondrial Complex I activity, establishing a direct functional link between METTL9 localization and Complex I function.\",\n      \"method\": \"shRNA-mediated stable knockdown, subcellular fractionation/immunofluorescence for mitochondrial localization, Complex I activity assay\",\n      \"journal\": \"Biochemistry and biophysics reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiment tied to functional consequence (Complex I activity), single lab\",\n      \"pmids\": [\"35402738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"METTL9 knockdown reduces SLC7A11 expression (a key ferroptosis suppressor) in hepatocellular carcinoma cells, promoting ferroptosis and inhibiting HCC progression in vitro and in PDX models.\",\n      \"method\": \"shRNA knockdown, cell viability/migration assays, PDX xenograft model, Western blot for SLC7A11\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — loss-of-function with defined cellular phenotype, but mechanism linking methyltransferase activity to SLC7A11 not fully resolved at molecular level\",\n      \"pmids\": [\"38017014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"METTL9 methylates SLC39A7 (ZIP7 zinc transporter) at His45 and His49 residues; this methylation suppresses ferroptosis via the PERK/ATF4 signaling pathway and downstream SLC7A11-mediated cystine import and glutathione synthesis, reducing ROS and inhibiting adipogenic differentiation of mesenchymal stem cells.\",\n      \"method\": \"In vitro methyltransferase assay on SLC39A7 peptides, site-directed mutagenesis, METTL9 overexpression/knockdown, adipogenic differentiation assays, OVX mouse model\",\n      \"journal\": \"Molecular medicine (Cambridge, Mass.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct substrate identification with mutagenesis and pathway readout, single lab\",\n      \"pmids\": [\"40414869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"METTL9 plays a conserved role in vertebrate neurogenesis that is largely independent of its catalytic (methyltransferase) activity. METTL9 interacts with key regulators of cellular transport, endocytosis, and Golgi integrity; Mettl9 KO mouse embryonic stem cells show Golgi fragmentation. METTL9 modulates the secretory pathway to support neural development.\",\n      \"method\": \"Complete Mettl9 KO, inducible Degron, and catalytically inactive knock-in mouse ES cell lines; Xenopus laevis mettl9 knockdown; multi-omics; Co-IP for interactors; Golgi morphology imaging\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple genetic models (KO, catalytic dead knock-in, Degron) plus multi-omics and orthogonal organism validation\",\n      \"pmids\": [\"40745158\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"METTL9 promotes histidine methylation of NF-κB RELA, resulting in inhibition of NLRP3 transcription and suppression of neuronal pyroptosis in a Parkinson's disease mouse model.\",\n      \"method\": \"MPTP-induced PD mouse model, METTL9 gain/loss-of-function, Western blot, luciferase reporter assay, ChIP assay for RELA binding to NLRP3 promoter\",\n      \"journal\": \"Critical reviews in eukaryotic gene expression\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — indirect evidence for methylation of NF-κB RELA; direct histidine methylation of RELA not biochemically confirmed; single lab\",\n      \"pmids\": [\"39072406\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"METTL9 orthologues across eukaryotes retain in vitro methyltransferase activity on HxH-motif substrates (ARMC6, DNAJB12), but Drosophila and Ostreococcus tauri METTL9 show distinct substrate specificities compared to human METTL9. The X-ray structure of OtMETTL9 revealed structural differences from human METTL9 explaining its distinct substrate preference. C. elegans METTL9, previously thought to be a DNA MTase, has protein methylation activity.\",\n      \"method\": \"In vitro methyltransferase assays with recombinant proteins and peptide arrays, X-ray crystallography of OtMETTL9\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus in vitro enzymatic characterization across multiple species with systematic peptide array\",\n      \"pmids\": [\"40451431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"A first-in-class small-molecule inhibitor (METTL9i) binds within the SAM-binding pocket of METTL9 (IC50 = 0.067 µM), selectively inhibiting METTL9 over other methyltransferases and reducing global 1-methylhistidine levels in cells.\",\n      \"method\": \"In vitro enzyme inhibition assay, X-ray crystallography of METTL9i–METTL9 complex, cellular target engagement assay, global 1-MH proteomics\",\n      \"journal\": \"Angewandte Chemie (International ed. in English)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure of inhibitor–enzyme complex, in vitro IC50, cellular 1-MH reduction confirmed by proteomics\",\n      \"pmids\": [\"41870122\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"METTL9 binds to SLC7A11 protein and enhances its stability by reducing its degradation, thereby regulating ferroptosis in HCC. This interaction was confirmed by Co-IP and operates independently of GPX4.\",\n      \"method\": \"Co-immunoprecipitation (Co-IP), RNA sequencing, CUT&Tag analysis showing SIX2 directly regulates METTL9 expression, overexpression/knockdown functional assays\",\n      \"journal\": \"NPJ precision oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — reciprocal Co-IP showing METTL9–SLC7A11 interaction with functional consequence, single lab\",\n      \"pmids\": [\"40523929\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"METTL9 is a eukaryotic seven-β-strand methyltransferase that transfers a methyl group from SAM to the N1 atom of the second histidine within His-x-His (HxH) motifs—using an active-site aspartate to orient the substrate—thereby generating 1-methylhistidine (1MH) on a broad array of proteins including S100A9, NDUFB3 (Complex I), SLC39A7, and ARMC6; this modification reduces zinc-binding affinity of target sites, enhances mitochondrial Complex I respiration, and modulates ferroptosis resistance, while METTL9 also sustains vertebrate neurogenesis through a catalysis-independent role in maintaining Golgi/secretory pathway integrity.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"METTL9 is a seven-β-strand methyltransferase that catalyzes N1 (Nπ) methylation of histidine residues within His-x-His (HxH) motifs across a broad substrate repertoire, generating 1-methylhistidine (1MH) on proteins including S100A9, NDUFB3 (Complex I subunit), SLC39A7, and ARMC6 [PMID:33563959, PMID:34562450, PMID:40451431]. Crystal structures show that an active-site aspartate orients the target imidazole ring by engaging N3, presenting N1 to the SAM methyl donor, with the first histidine of the HxH dipeptide serving as a recognition element and methylation proceeding C-to-N through tandem repeats [PMID:37398635]. Functionally, METTL9-mediated methylation reduces zinc-binding affinity of target HxH sites, enhances mitochondrial Complex I respiration, and suppresses ferroptosis via SLC39A7/PERK/ATF4-dependent upregulation of SLC7A11 [PMID:33563959, PMID:40414869, PMID:38017014]. Beyond its catalytic role, METTL9 sustains vertebrate neurogenesis through a catalysis-independent function in maintaining Golgi and secretory pathway integrity [PMID:40745158].\",\n  \"teleology\": [\n    {\n      \"year\": 2021,\n      \"claim\": \"The enzymatic function of METTL9 was unknown; proteomic and biochemical analyses established it as the major mammalian histidine methyltransferase generating 1-methylhistidine at HxH motifs, with substrates including S100A9 and NDUFB3, linking it to Complex I respiration and zinc-binding regulation.\",\n      \"evidence\": \"METTL9 KO mice, proteomic MS, in vitro methyltransferase assays, Complex I respiration and zinc-binding measurements\",\n      \"pmids\": [\"33563959\", \"34562450\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the full in vivo substrate repertoire beyond identified targets\", \"Mechanism by which 1MH modification enhances Complex I activity not resolved at structural level\", \"Selectivity rules explaining why some HxH-containing proteins (e.g. MYLK2) are not methylated in vivo\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Where METTL9 acts within cells was unclear; subcellular fractionation showed predominant mitochondrial localization in gastric cancer cells, directly coupling METTL9 presence to Complex I activity.\",\n      \"evidence\": \"Subcellular fractionation, immunofluorescence, and Complex I activity assay in shRNA-knockdown gastric cancer cells\",\n      \"pmids\": [\"35402738\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Localization studied only in one cancer cell type\", \"Whether mitochondrial targeting requires a signal peptide or import machinery not determined\", \"Relative distribution across other compartments not systematically quantified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"How METTL9 achieves N1-specific methylation was structurally unknown; the crystal structure of METTL9 bound to substrate revealed that an active-site aspartate stabilizes N3 of the target histidine imidazole, presenting N1 to SAM, with the first histidine serving as a recognition element and a confined pocket selecting for small intervening residues.\",\n      \"evidence\": \"X-ray crystallography of METTL9–substrate complex, site-directed mutagenesis, in vitro assays on tandem HxH repeats\",\n      \"pmids\": [\"37398635\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length protein substrate complexes not crystallized\", \"Structural basis for C-to-N processivity on tandem repeats not fully defined\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Whether METTL9 influenced cell death pathways was unexplored; knockdown in HCC cells reduced SLC7A11 expression and promoted ferroptosis, linking METTL9 to ferroptosis resistance.\",\n      \"evidence\": \"shRNA knockdown, cell viability/migration assays, PDX xenograft models\",\n      \"pmids\": [\"38017014\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether SLC7A11 regulation depends on METTL9 catalytic activity not established\", \"No direct methylation of SLC7A11 or its transcriptional regulators demonstrated\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"The molecular mechanism connecting METTL9 to ferroptosis was clarified: METTL9 methylates SLC39A7 (ZIP7) at His45/49, suppressing ferroptosis through PERK/ATF4-mediated SLC7A11 induction and glutathione synthesis.\",\n      \"evidence\": \"In vitro methyltransferase assay on SLC39A7 peptides, site-directed mutagenesis, OVX mouse model, adipogenic differentiation assays\",\n      \"pmids\": [\"40414869\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"How histidine methylation of SLC39A7 triggers PERK/ATF4 signaling is mechanistically unclear\", \"Single-lab finding awaiting independent replication\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Whether METTL9 functions solely through its catalytic activity was untested; genetic dissection in mouse ESCs and Xenopus revealed a conserved catalysis-independent role in neurogenesis, with METTL9 maintaining Golgi integrity and secretory pathway function through interaction with transport and endocytic regulators.\",\n      \"evidence\": \"METTL9 KO, catalytically dead knock-in, Degron mouse ESC lines; Xenopus knockdown; multi-omics; Co-IP; Golgi morphology imaging\",\n      \"pmids\": [\"40745158\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific Golgi/transport interactors mediating the non-catalytic function not fully characterized\", \"Whether the neurogenesis role extends to adult neurogenesis not tested\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Evolutionary conservation and divergence of METTL9 substrate specificity was uncharted; cross-species analysis showed conserved HxH methylation activity but distinct substrate preferences between human, Drosophila, and algal orthologues, explained by structural differences in the substrate pocket.\",\n      \"evidence\": \"In vitro methyltransferase assays with recombinant orthologues, peptide arrays, X-ray crystallography of O. tauri METTL9\",\n      \"pmids\": [\"40451431\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo substrates of non-mammalian METTL9 orthologues not determined\", \"Functional consequences of species-specific substrate selectivity unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"METTL9 was shown to physically bind SLC7A11 and stabilize it against degradation, providing a direct protein-level mechanism for METTL9-dependent ferroptosis suppression in HCC independent of GPX4.\",\n      \"evidence\": \"Reciprocal Co-IP, RNA-seq, overexpression/knockdown functional assays in HCC cells\",\n      \"pmids\": [\"40523929\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether the METTL9–SLC7A11 interaction depends on methyltransferase activity not tested\", \"Single-lab Co-IP without domain mapping or structural characterization of the interaction\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Pharmacological targeting of METTL9 was previously impossible; a first-in-class small-molecule inhibitor (IC50 = 67 nM) was developed that occupies the SAM-binding pocket and selectively reduces global cellular 1MH levels, providing a chemical tool to dissect METTL9 biology.\",\n      \"evidence\": \"X-ray crystallography of inhibitor–METTL9 complex, in vitro IC50 determination, selectivity profiling, cellular 1MH proteomics\",\n      \"pmids\": [\"41870122\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo pharmacokinetics and efficacy not reported\", \"Effects of inhibitor on non-catalytic METTL9 functions not assessed\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include: whether METTL9's ferroptosis-related effects operate through its catalytic activity or non-catalytic protein stabilization of SLC7A11; the structural basis and in vivo significance of the catalysis-independent Golgi/neurogenesis role; and the full physiological substrate repertoire in different tissues.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Catalytic vs. non-catalytic contributions to ferroptosis not genetically separated\", \"Golgi-maintaining mechanism and relevant interactors not biochemically reconstituted\", \"Tissue-specific in vivo substrate profiles not mapped\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2, 5, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 5, 8]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [4, 5, 10]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"S100A9\",\n      \"NDUFB3\",\n      \"SLC39A7\",\n      \"ARMC6\",\n      \"SLC7A11\",\n      \"DNAJB12\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}