{"gene":"SEPHS2","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":1997,"finding":"Mouse SPS2 (SEPHS2) encodes a selenophosphate synthetase that uses selenocysteine (encoded by TGA) at the active site; the cysteine mutant (Sec→Cys) retains selenophosphate synthetase activity as measured by selenide-dependent AMP formation from ATP, confirming the catalytic role of the active-site chalcogen residue.","method":"Baculovirus-insect cell expression, purification of Cys-mutant enzyme, in vitro enzymatic assay (selenide-dependent AMP formation from ATP)","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstituted enzymatic activity with active-site mutagenesis, replicated in subsequent studies","pmids":["9012797"],"is_preprint":false},{"year":1999,"finding":"Mouse SPS2 Cys-mutant complements an E. coli selD (selenophosphate synthetase) deficiency, demonstrating functional equivalence; replacement of the active-site Cys with Ala, Ser, or Thr abolishes complementation, defining Cys (or Sec) as essential for catalysis. Purified SPS2-Cys shows Km(ATP) = 0.75 mM and Vmax = 9.23 nmol/min/mg.","method":"In vivo genetic complementation (selD mutant E. coli), site-directed mutagenesis, enzyme purification and kinetic assay","journal":"Molecules and cells","confidence":"High","confidence_rationale":"Tier 1 — genetic complementation combined with mutagenesis and kinetic characterization","pmids":["10515607"],"is_preprint":false},{"year":2004,"finding":"Human SEPHS2 (Sps2Cys) effectively complements the E. coli selD mutant when selenite is the selenium source, demonstrating that SEPHS2 functions in a selenite assimilation pathway, whereas human SPS1 preferentially recycles L-selenocysteine through a selenocysteine lyase-dependent salvage system.","method":"In vivo complementation assay (selD mutant E. coli), formate dehydrogenase H activity measurement, substrate specificity comparison with L-selenocysteine vs. selenite","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 — genetic complementation with defined substrate specificity, replicated across labs","pmids":["15534230"],"is_preprint":false},{"year":2017,"finding":"SEPHS2 physically interacts with selenocysteine synthase SEPSECS and with SEPHS1; additionally, SEPHS2, SEPHS1, SEPSECS, and SECp43 form oligomers within the selenocysteine biosynthesis machinery in mammalian cells.","method":"Bioluminescence resonance energy transfer (BRET) in mammalian cells, co-immunoprecipitation","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal Co-IP plus BRET in live mammalian cells, single study","pmids":["28414460"],"is_preprint":false},{"year":2025,"finding":"METTL5-mediated 18S rRNA m6A methylation promotes SEPHS2 translation efficiency; METTL5 depletion reduces SEPHS2 protein levels, leading to diminished selenoprotein synthesis, elevated ROS, and apoptosis in multiple myeloma cells, placing SEPHS2 downstream of METTL5 in a translational control pathway.","method":"Genetic knockdown/knockout in vitro and orthotopic xenograft model, ROS measurement, selenoprotein synthesis assay","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 — clean KO with defined cellular phenotype and pathway placement, single lab","pmids":["40750759"],"is_preprint":false},{"year":2026,"finding":"SEPHS2 knockout suppresses oxidative phosphorylation (OXPHOS) and redirects glucose metabolism toward gluconeogenesis and the pentose phosphate pathway (PPP) independently of its selenoprotein biosynthesis function; mechanistically, SEPHS2 loss elevates intracellular NAD+, activating deacetylase SIRT2, which promotes deacetylation-dependent stabilization of the gluconeogenic enzyme PCK1.","method":"OXPHOS-focused genetic screen, SEPHS2 knockout, metabolic flux analysis, NAD+ measurement, SIRT2 activity assay, PCK1 acetylation/stabilization assay, in vivo tumor spread model, PPP inhibitor sensitivity assay","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (genetic KO, metabolomics, biochemical deacetylation assay, in vivo) in a single rigorous study defining a non-canonical SEPHS2 function","pmids":["42035418"],"is_preprint":false},{"year":2024,"finding":"PRDX6 can react with selenide and physically interact with SEPHS2, potentially acting as an alternative selenium delivery system to feed selenide into the SEPHS2-dependent selenophosphate biosynthesis pathway, independent of the canonical SCLY route.","method":"Biochemical interaction assay (PRDX6–selenide reaction), protein–protein interaction with SEPHS2","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — preprint, single-lab biochemical interaction without full mechanistic reconstitution","pmids":["bio_10.1101_2024.06.04.597364"],"is_preprint":true},{"year":2024,"finding":"SEPHS2 was identified as a novel binding partner of the kinase inhibitor dasatinib in live cells using the POST-IT proximity-tagging system, suggesting a previously unrecognized direct interaction between SEPHS2 and dasatinib.","method":"POST-IT non-diffusive proximity tagging (PafA-HaloTag fusion) in live cells and zebrafish embryos","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 3 — preprint, single proximity-tagging method without validation of functional consequence","pmids":["bio_10.1101_2024.09.06.611731"],"is_preprint":true}],"current_model":"SEPHS2 is a selenophosphate synthetase that catalyzes the ATP-dependent synthesis of selenophosphate from selenide using an active-site selenocysteine (or functional Cys surrogate), provides the essential selenium donor for selenocysteine tRNA biosynthesis, physically associates with SEPSECS and SEPHS1 in the selenocysteine incorporation machinery, and—independently of selenoprotein biosynthesis—modulates oxidative phosphorylation by elevating NAD+, activating SIRT2, and stabilizing the gluconeogenic enzyme PCK1 to redirect glucose metabolism toward the pentose phosphate pathway."},"narrative":{"teleology":[{"year":1997,"claim":"Establishing that mammalian SPS2 (SEPHS2) possesses intrinsic selenophosphate synthetase activity and that the active-site selenocysteine (or its Cys surrogate) is catalytically competent answered the fundamental question of whether mammals encode their own selenophosphate-generating enzyme.","evidence":"Baculovirus-expressed purified Cys-mutant enzyme assayed for selenide-dependent AMP formation from ATP","pmids":["9012797"],"confidence":"High","gaps":["Kinetic parameters for the native Sec-containing enzyme were not determined","In vivo role in mammalian selenium metabolism not yet demonstrated"]},{"year":1999,"claim":"Demonstrating that SPS2 functionally complements bacterial selD deficiency and that only Cys/Sec at the active site supports activity resolved which residue is catalytically essential and provided kinetic constants (Km(ATP) = 0.75 mM).","evidence":"Site-directed mutagenesis (Cys→Ala/Ser/Thr), complementation of E. coli selD mutant, purified enzyme kinetics","pmids":["10515607"],"confidence":"High","gaps":["No structural model explaining how the active-site chalcogen participates in catalysis","Role of SEPHS1 versus SEPHS2 not yet distinguished"]},{"year":2004,"claim":"Distinguishing SEPHS2 from SEPHS1 by substrate specificity—SEPHS2 preferentially uses inorganic selenite while SEPHS1 relies on a selenocysteine lyase salvage route—resolved a longstanding question about functional redundancy between the two paralogs.","evidence":"Complementation of E. coli selD mutant with selenite vs. L-selenocysteine, formate dehydrogenase H activity readout","pmids":["15534230"],"confidence":"High","gaps":["Mechanism by which selenite is reduced to selenide upstream of SEPHS2 not defined","Whether SEPHS2 contributes to selenium salvage under physiological conditions unclear"]},{"year":2017,"claim":"Identifying physical interactions between SEPHS2, SEPSECS, and SEPHS1 established that selenophosphate synthesis and selenocysteine biosynthesis occur within a multi-protein complex rather than via freely diffusing intermediates.","evidence":"BRET and co-immunoprecipitation in mammalian cells","pmids":["28414460"],"confidence":"Medium","gaps":["Stoichiometry and architecture of the complex not determined","Functional consequence of complex disruption not tested","Single study; independent replication pending"]},{"year":2025,"claim":"Placing SEPHS2 translation downstream of METTL5-mediated 18S rRNA m6A modification revealed a translational control layer for selenoprotein biosynthesis and explained how METTL5 loss causes selenoprotein deficiency and ROS-mediated apoptosis in myeloma cells.","evidence":"METTL5 KO/KD in myeloma cells and orthotopic xenografts, selenoprotein synthesis and ROS assays","pmids":["40750759"],"confidence":"Medium","gaps":["Whether SEPHS2 mRNA has specific structural features conferring METTL5 sensitivity is unknown","Generalizability beyond multiple myeloma not tested"]},{"year":2026,"claim":"Discovering that SEPHS2 loss suppresses OXPHOS and redirects metabolism through NAD⁺/SIRT2-dependent PCK1 stabilization toward the pentose phosphate pathway established a non-canonical, selenoprotein-independent metabolic function for SEPHS2.","evidence":"OXPHOS genetic screen, SEPHS2 KO, metabolic flux analysis, NAD⁺ measurement, SIRT2 activity and PCK1 acetylation assays, in vivo tumor model","pmids":["42035418"],"confidence":"High","gaps":["How SEPHS2 mechanistically controls NAD⁺ levels remains undefined","Whether the metabolic phenotype reflects a moonlighting enzymatic activity or loss of a metabolite is unresolved","Relevance in non-cancer tissues not examined"]},{"year":null,"claim":"The mechanism by which SEPHS2 regulates intracellular NAD⁺ independently of selenoprotein biosynthesis, and whether SEPHS2 has additional non-canonical substrates or enzymatic activities, remain open questions.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structural model of human SEPHS2 explaining dual functionality","Upstream selenium delivery pathway to SEPHS2 in mammalian cells not fully reconstituted","The relationship between selenophosphate synthesis and NAD⁺ metabolism is mechanistically unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[0,1]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1,2,3]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[5]}],"complexes":["Selenocysteine biosynthesis complex (SEPHS2–SEPSECS–SEPHS1–SECp43)"],"partners":["SEPSECS","SEPHS1","SIRT2","PCK1","METTL5"],"other_free_text":[]},"mechanistic_narrative":"SEPHS2 is a selenophosphate synthetase that catalyzes the ATP-dependent conversion of selenide to selenophosphate, the essential selenium donor for selenocysteine biosynthesis. The enzyme employs an active-site selenocysteine residue (or a functional cysteine surrogate) that is absolutely required for catalysis, as demonstrated by genetic complementation of E. coli selD mutants and in vitro kinetic analysis [PMID:9012797, PMID:10515607]; SEPHS2 preferentially assimilates inorganic selenite as its selenium source, distinguishing it functionally from SEPHS1 [PMID:15534230]. Within mammalian cells, SEPHS2 physically associates with SEPSECS and SEPHS1 in oligomeric complexes that constitute the selenocysteine incorporation machinery [PMID:28414460]. Independent of selenoprotein biosynthesis, SEPHS2 loss suppresses oxidative phosphorylation and elevates intracellular NAD⁺, activating the deacetylase SIRT2 to stabilize the gluconeogenic enzyme PCK1 and redirect glucose flux toward the pentose phosphate pathway [PMID:42035418]."},"prefetch_data":{"uniprot":{"accession":"Q99611","full_name":"Selenide, water dikinase 2","aliases":["Selenium donor protein 2","Selenophosphate synthase 2"],"length_aa":448,"mass_kda":47.3,"function":"Selenophosphate synthase that generates the selenium donor for selenocysteine biosynthesis by catalyzing formation of selenophosphate from selenide and ATP","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q99611/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SEPHS2","classification":"Common Essential","n_dependent_lines":832,"n_total_lines":1208,"dependency_fraction":0.6887417218543046},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SEPHS2","total_profiled":1310},"omim":[{"mim_id":"619597","title":"tRNA SELENOCYSTEINE 1-ASSOCIATED PROTEIN 1; TRNAU1AP","url":"https://www.omim.org/entry/619597"},{"mim_id":"606218","title":"SELENOPHOSPHATE SYNTHETASE 2; SEPHS2","url":"https://www.omim.org/entry/606218"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"liver","ntpm":377.1}],"url":"https://www.proteinatlas.org/search/SEPHS2"},"hgnc":{"alias_symbol":["SPS2","SPS2b"],"prev_symbol":[]},"alphafold":{"accession":"Q99611","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99611","model_url":"","pae_url":"","plddt_mean":null},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SEPHS2","jax_strain_url":"https://www.jax.org/strain/search?query=SEPHS2"},"sequence":{"accession":"Q99611","fasta_url":"https://rest.uniprot.org/uniprotkb/Q99611.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q99611/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q99611"}},"corpus_meta":[{"pmid":"15534230","id":"PMC_15534230","title":"Selenophosphate synthetase genes from lung adenocarcinoma cells: Sps1 for recycling L-selenocysteine and Sps2 for selenite assimilation.","date":"2004","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/15534230","citation_count":63,"is_preprint":false},{"pmid":"9012797","id":"PMC_9012797","title":"Fetal mouse selenophosphate synthetase 2 (SPS2): characterization of the cysteine mutant form overproduced in a baculovirus-insect cell system.","date":"1997","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/9012797","citation_count":47,"is_preprint":false},{"pmid":"31695102","id":"PMC_31695102","title":"Structural analysis of human SEPHS2 protein, a selenocysteine machinery component, over-expressed in triple negative breast cancer.","date":"2019","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/31695102","citation_count":26,"is_preprint":false},{"pmid":"38539903","id":"PMC_38539903","title":"Se Alleviated Pb-Caused Neurotoxicity in Chickens: SPS2-GPx1-GSH-IL-2/IL-17-NO Pathway, Selenoprotein Suppression, Oxidative Stress, and Inflammatory Injury.","date":"2024","source":"Antioxidants (Basel, Switzerland)","url":"https://pubmed.ncbi.nlm.nih.gov/38539903","citation_count":22,"is_preprint":false},{"pmid":"3302678","id":"PMC_3302678","title":"Increased copy number of the 5' end of the SPS2 gene inhibits sporulation of Saccharomyces cerevisiae.","date":"1987","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/3302678","citation_count":21,"is_preprint":false},{"pmid":"28414460","id":"PMC_28414460","title":"Analysis of Novel Interactions between Components of the Selenocysteine Biosynthesis Pathway, SEPHS1, SEPHS2, SEPSECS, and SECp43.","date":"2017","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/28414460","citation_count":18,"is_preprint":false},{"pmid":"36144391","id":"PMC_36144391","title":"Whole-Genome Sequence Analysis of an Endophytic Fungus Alternaria sp. SPS-2 and Its Biosynthetic Potential of Bioactive Secondary Metabolites.","date":"2022","source":"Microorganisms","url":"https://pubmed.ncbi.nlm.nih.gov/36144391","citation_count":11,"is_preprint":false},{"pmid":"10515607","id":"PMC_10515607","title":"Fetal mouse selenophosphate synthetase 2 (SPS2): biological activities of mutant forms in Escherichia coli.","date":"1999","source":"Molecules and cells","url":"https://pubmed.ncbi.nlm.nih.gov/10515607","citation_count":8,"is_preprint":false},{"pmid":"40750759","id":"PMC_40750759","title":"METTL5 regulates SEPHS2-mediated selenoprotein synthesis to promote multiple myeloma survival and progression.","date":"2025","source":"Cell death & disease","url":"https://pubmed.ncbi.nlm.nih.gov/40750759","citation_count":3,"is_preprint":false},{"pmid":"41988776","id":"PMC_41988776","title":"Evaluating SEPHS2 expression and glutathione peroxidase as biomarkers in recurrent pregnancy loss: a new insight.","date":"2026","source":"Biomarkers : biochemical indicators of exposure, response, and susceptibility to chemicals","url":"https://pubmed.ncbi.nlm.nih.gov/41988776","citation_count":0,"is_preprint":false},{"pmid":"42035418","id":"PMC_42035418","title":"SEPHS2 loss reprograms cancer metabolism from oxidative phosphorylation to gluconeogenesis via PCK1 stabilization.","date":"2026","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/42035418","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.09.06.611731","title":"Target protein identification in live cells and organisms with a non-diffusive proximity tagging system","date":"2024-09-08","source":"bioRxiv","url":"https://doi.org/10.1101/2024.09.06.611731","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.06.04.597364","title":"PRDX6 contributes to selenocysteine metabolism and ferroptosis resistance","date":"2024-06-06","source":"bioRxiv","url":"https://doi.org/10.1101/2024.06.04.597364","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7938,"output_tokens":2004,"usd":0.026937},"stage2":{"model":"claude-opus-4-6","input_tokens":5294,"output_tokens":1973,"usd":0.113693},"total_usd":0.14063,"stage1_batch_id":"msgbatch_011rkcQdVV1RaFq9XhcG6df6","stage2_batch_id":"msgbatch_01RzgrJafdF4EWHhj5mNi6dh","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1997,\n      \"finding\": \"Mouse SPS2 (SEPHS2) encodes a selenophosphate synthetase that uses selenocysteine (encoded by TGA) at the active site; the cysteine mutant (Sec→Cys) retains selenophosphate synthetase activity as measured by selenide-dependent AMP formation from ATP, confirming the catalytic role of the active-site chalcogen residue.\",\n      \"method\": \"Baculovirus-insect cell expression, purification of Cys-mutant enzyme, in vitro enzymatic assay (selenide-dependent AMP formation from ATP)\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstituted enzymatic activity with active-site mutagenesis, replicated in subsequent studies\",\n      \"pmids\": [\"9012797\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Mouse SPS2 Cys-mutant complements an E. coli selD (selenophosphate synthetase) deficiency, demonstrating functional equivalence; replacement of the active-site Cys with Ala, Ser, or Thr abolishes complementation, defining Cys (or Sec) as essential for catalysis. Purified SPS2-Cys shows Km(ATP) = 0.75 mM and Vmax = 9.23 nmol/min/mg.\",\n      \"method\": \"In vivo genetic complementation (selD mutant E. coli), site-directed mutagenesis, enzyme purification and kinetic assay\",\n      \"journal\": \"Molecules and cells\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — genetic complementation combined with mutagenesis and kinetic characterization\",\n      \"pmids\": [\"10515607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Human SEPHS2 (Sps2Cys) effectively complements the E. coli selD mutant when selenite is the selenium source, demonstrating that SEPHS2 functions in a selenite assimilation pathway, whereas human SPS1 preferentially recycles L-selenocysteine through a selenocysteine lyase-dependent salvage system.\",\n      \"method\": \"In vivo complementation assay (selD mutant E. coli), formate dehydrogenase H activity measurement, substrate specificity comparison with L-selenocysteine vs. selenite\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic complementation with defined substrate specificity, replicated across labs\",\n      \"pmids\": [\"15534230\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"SEPHS2 physically interacts with selenocysteine synthase SEPSECS and with SEPHS1; additionally, SEPHS2, SEPHS1, SEPSECS, and SECp43 form oligomers within the selenocysteine biosynthesis machinery in mammalian cells.\",\n      \"method\": \"Bioluminescence resonance energy transfer (BRET) in mammalian cells, co-immunoprecipitation\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus BRET in live mammalian cells, single study\",\n      \"pmids\": [\"28414460\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"METTL5-mediated 18S rRNA m6A methylation promotes SEPHS2 translation efficiency; METTL5 depletion reduces SEPHS2 protein levels, leading to diminished selenoprotein synthesis, elevated ROS, and apoptosis in multiple myeloma cells, placing SEPHS2 downstream of METTL5 in a translational control pathway.\",\n      \"method\": \"Genetic knockdown/knockout in vitro and orthotopic xenograft model, ROS measurement, selenoprotein synthesis assay\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotype and pathway placement, single lab\",\n      \"pmids\": [\"40750759\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"SEPHS2 knockout suppresses oxidative phosphorylation (OXPHOS) and redirects glucose metabolism toward gluconeogenesis and the pentose phosphate pathway (PPP) independently of its selenoprotein biosynthesis function; mechanistically, SEPHS2 loss elevates intracellular NAD+, activating deacetylase SIRT2, which promotes deacetylation-dependent stabilization of the gluconeogenic enzyme PCK1.\",\n      \"method\": \"OXPHOS-focused genetic screen, SEPHS2 knockout, metabolic flux analysis, NAD+ measurement, SIRT2 activity assay, PCK1 acetylation/stabilization assay, in vivo tumor spread model, PPP inhibitor sensitivity assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (genetic KO, metabolomics, biochemical deacetylation assay, in vivo) in a single rigorous study defining a non-canonical SEPHS2 function\",\n      \"pmids\": [\"42035418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PRDX6 can react with selenide and physically interact with SEPHS2, potentially acting as an alternative selenium delivery system to feed selenide into the SEPHS2-dependent selenophosphate biosynthesis pathway, independent of the canonical SCLY route.\",\n      \"method\": \"Biochemical interaction assay (PRDX6–selenide reaction), protein–protein interaction with SEPHS2\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — preprint, single-lab biochemical interaction without full mechanistic reconstitution\",\n      \"pmids\": [\"bio_10.1101_2024.06.04.597364\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"SEPHS2 was identified as a novel binding partner of the kinase inhibitor dasatinib in live cells using the POST-IT proximity-tagging system, suggesting a previously unrecognized direct interaction between SEPHS2 and dasatinib.\",\n      \"method\": \"POST-IT non-diffusive proximity tagging (PafA-HaloTag fusion) in live cells and zebrafish embryos\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — preprint, single proximity-tagging method without validation of functional consequence\",\n      \"pmids\": [\"bio_10.1101_2024.09.06.611731\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SEPHS2 is a selenophosphate synthetase that catalyzes the ATP-dependent synthesis of selenophosphate from selenide using an active-site selenocysteine (or functional Cys surrogate), provides the essential selenium donor for selenocysteine tRNA biosynthesis, physically associates with SEPSECS and SEPHS1 in the selenocysteine incorporation machinery, and—independently of selenoprotein biosynthesis—modulates oxidative phosphorylation by elevating NAD+, activating SIRT2, and stabilizing the gluconeogenic enzyme PCK1 to redirect glucose metabolism toward the pentose phosphate pathway.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SEPHS2 is a selenophosphate synthetase that catalyzes the ATP-dependent conversion of selenide to selenophosphate, the essential selenium donor for selenocysteine biosynthesis. The enzyme employs an active-site selenocysteine residue (or a functional cysteine surrogate) that is absolutely required for catalysis, as demonstrated by genetic complementation of E. coli selD mutants and in vitro kinetic analysis [PMID:9012797, PMID:10515607]; SEPHS2 preferentially assimilates inorganic selenite as its selenium source, distinguishing it functionally from SEPHS1 [PMID:15534230]. Within mammalian cells, SEPHS2 physically associates with SEPSECS and SEPHS1 in oligomeric complexes that constitute the selenocysteine incorporation machinery [PMID:28414460]. Independent of selenoprotein biosynthesis, SEPHS2 loss suppresses oxidative phosphorylation and elevates intracellular NAD⁺, activating the deacetylase SIRT2 to stabilize the gluconeogenic enzyme PCK1 and redirect glucose flux toward the pentose phosphate pathway [PMID:42035418].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Establishing that mammalian SPS2 (SEPHS2) possesses intrinsic selenophosphate synthetase activity and that the active-site selenocysteine (or its Cys surrogate) is catalytically competent answered the fundamental question of whether mammals encode their own selenophosphate-generating enzyme.\",\n      \"evidence\": \"Baculovirus-expressed purified Cys-mutant enzyme assayed for selenide-dependent AMP formation from ATP\",\n      \"pmids\": [\"9012797\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Kinetic parameters for the native Sec-containing enzyme were not determined\",\n        \"In vivo role in mammalian selenium metabolism not yet demonstrated\"\n      ]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrating that SPS2 functionally complements bacterial selD deficiency and that only Cys/Sec at the active site supports activity resolved which residue is catalytically essential and provided kinetic constants (Km(ATP) = 0.75 mM).\",\n      \"evidence\": \"Site-directed mutagenesis (Cys→Ala/Ser/Thr), complementation of E. coli selD mutant, purified enzyme kinetics\",\n      \"pmids\": [\"10515607\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No structural model explaining how the active-site chalcogen participates in catalysis\",\n        \"Role of SEPHS1 versus SEPHS2 not yet distinguished\"\n      ]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Distinguishing SEPHS2 from SEPHS1 by substrate specificity—SEPHS2 preferentially uses inorganic selenite while SEPHS1 relies on a selenocysteine lyase salvage route—resolved a longstanding question about functional redundancy between the two paralogs.\",\n      \"evidence\": \"Complementation of E. coli selD mutant with selenite vs. L-selenocysteine, formate dehydrogenase H activity readout\",\n      \"pmids\": [\"15534230\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism by which selenite is reduced to selenide upstream of SEPHS2 not defined\",\n        \"Whether SEPHS2 contributes to selenium salvage under physiological conditions unclear\"\n      ]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Identifying physical interactions between SEPHS2, SEPSECS, and SEPHS1 established that selenophosphate synthesis and selenocysteine biosynthesis occur within a multi-protein complex rather than via freely diffusing intermediates.\",\n      \"evidence\": \"BRET and co-immunoprecipitation in mammalian cells\",\n      \"pmids\": [\"28414460\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Stoichiometry and architecture of the complex not determined\",\n        \"Functional consequence of complex disruption not tested\",\n        \"Single study; independent replication pending\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Placing SEPHS2 translation downstream of METTL5-mediated 18S rRNA m6A modification revealed a translational control layer for selenoprotein biosynthesis and explained how METTL5 loss causes selenoprotein deficiency and ROS-mediated apoptosis in myeloma cells.\",\n      \"evidence\": \"METTL5 KO/KD in myeloma cells and orthotopic xenografts, selenoprotein synthesis and ROS assays\",\n      \"pmids\": [\"40750759\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether SEPHS2 mRNA has specific structural features conferring METTL5 sensitivity is unknown\",\n        \"Generalizability beyond multiple myeloma not tested\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Discovering that SEPHS2 loss suppresses OXPHOS and redirects metabolism through NAD⁺/SIRT2-dependent PCK1 stabilization toward the pentose phosphate pathway established a non-canonical, selenoprotein-independent metabolic function for SEPHS2.\",\n      \"evidence\": \"OXPHOS genetic screen, SEPHS2 KO, metabolic flux analysis, NAD⁺ measurement, SIRT2 activity and PCK1 acetylation assays, in vivo tumor model\",\n      \"pmids\": [\"42035418\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How SEPHS2 mechanistically controls NAD⁺ levels remains undefined\",\n        \"Whether the metabolic phenotype reflects a moonlighting enzymatic activity or loss of a metabolite is unresolved\",\n        \"Relevance in non-cancer tissues not examined\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The mechanism by which SEPHS2 regulates intracellular NAD⁺ independently of selenoprotein biosynthesis, and whether SEPHS2 has additional non-canonical substrates or enzymatic activities, remain open questions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No structural model of human SEPHS2 explaining dual functionality\",\n        \"Upstream selenium delivery pathway to SEPHS2 in mammalian cells not fully reconstituted\",\n        \"The relationship between selenophosphate synthesis and NAD⁺ metabolism is mechanistically unresolved\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1, 2, 3]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5]}\n    ],\n    \"complexes\": [\n      \"Selenocysteine biosynthesis complex (SEPHS2–SEPSECS–SEPHS1–SECp43)\"\n    ],\n    \"partners\": [\n      \"SEPSECS\",\n      \"SEPHS1\",\n      \"SIRT2\",\n      \"PCK1\",\n      \"METTL5\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}