{"gene":"PPP4R3B","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2007,"finding":"The yeast ortholog Psy2 forms a complex with the type 2A-like protein phosphatase Pph3 (Pph3-Psy2); this complex binds and dephosphorylates activated Rad53 during and after MMS-induced DNA damage, and is required for replication fork restart during recovery from DNA damage checkpoint arrest.","method":"Genetic epistasis, biochemical co-purification, in vitro dephosphorylation assay, replication fork analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — biochemical complex identification, in vitro dephosphorylation assay, and genetic epistasis all in one study; independently consistent with multiple follow-up studies","pmids":["17517611"],"is_preprint":false},{"year":2004,"finding":"Yeast Psy2 physically interacts with Wss1 and Tof1 (by yeast two-hybrid), and genetically interacts with WSS1, TOF1, RAD52, and SRS2, placing Psy2 in a pathway that stabilizes or processes stalled/collapsed replication forks.","method":"Yeast two-hybrid, genetic interaction/epistasis analysis","journal":"Nucleic acids research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — yeast two-hybrid only for physical interaction; genetic interactions support pathway placement but no biochemical reconstitution","pmids":["15598824"],"is_preprint":false},{"year":2011,"finding":"In Candida albicans, Psy2 (orf19.3685) forms a functional complex with Pph3 required for dephosphorylation of the checkpoint kinase Rad53 (but not H2AX) during recovery from DNA damage; loss of PSY2 causes hypersensitivity to DNA-damaging agents and persistent filamentous growth under genotoxic stress.","method":"Deletion mutant phenotypic analysis, phosphorylation western blot, flow cytometry","journal":"Eukaryotic cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined cellular and molecular phenotypes, orthogonal methods (flow cytometry + western blot), but single lab","pmids":["21890819"],"is_preprint":false},{"year":2014,"finding":"Yeast Pph3/Psy2 phosphatase complex promotes efficient non-homologous end-joining (NHEJ) of double-strand breaks; its activity in NHEJ is linked to the Rad53 checkpoint and operates in a pathway parallel to Chk1, as shown by Chk1 overexpression rescuing the NHEJ defect of pph3Δ and psy2Δ strains and double mutants showing additive repair defects.","method":"Plasmid and chromosomal NHEJ repair assays, genetic epistasis (double mutant analysis), overexpression rescue","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional repair assays with genetic epistasis and overexpression rescue; single lab but multiple orthogonal approaches","pmids":["24498054"],"is_preprint":false},{"year":2023,"finding":"In rats, a deletion mutation in Smek2 (PPP4R3B ortholog) impairs glycolysis in the liver and causes extremely low sarcosine dehydrogenase (Sardh) expression, leading to hypersarcosinemia and hyperhomocysteinemia, connecting Smek2 to regulation of hepatic sarcosine and homocysteine metabolism.","method":"Congenic rat model, microarray gene expression analysis, metabolite measurement","journal":"Scientific reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — microarray plus congenic rat model; indirect link between Smek2 mutation and metabolic phenotype, no direct biochemical mechanism for how Smek2 controls Sardh expression","pmids":["36810603"],"is_preprint":false},{"year":2020,"finding":"The RNA binding protein Rbm38 reduces a transcription elongation defect of the SMEK2 (PPP4R3B) gene that occurs under splicing-deficient conditions; this requires the N- and C-terminal regions of Rbm38 and its RNA-recognition motif.","method":"Transcription elongation assay under splicing-deficient conditions, Rbm38 domain deletion analysis","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — functional domain dissection with defined molecular readout (transcription elongation), single lab but multiple construct variants tested","pmids":["33233740"],"is_preprint":false},{"year":2025,"finding":"Yeast Psy2 (PP4 regulatory subunit) undergoes CDK-dependent phosphorylation that is essential for DSB repair but not for DNA damage checkpoint inactivation; Psy2 promotes symmetric DSB resection across tandem retrotransposons by reducing phosphorylation of Pif1 and Rrm3 helicases, and its loss causes an asymmetric resection defect that is alleviated by expressing a phosphodeficient H2A variant or depleting the checkpoint adaptor Rad9.","method":"Genetic epistasis, phosphorylation state analysis, resection assays, phosphodeficient mutant rescue, helicase substrate phosphorylation analysis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal genetic and biochemical methods, but preprint not yet peer-reviewed; single lab","pmids":["bio_10.1101_2025.02.19.639064"],"is_preprint":true}],"current_model":"PPP4R3B (SMEK2/PSY2/FLFL2) encodes a regulatory subunit of the PP4 phosphatase complex (PP4/Pph3–Psy2), which dephosphorylates the DNA damage checkpoint kinase Rad53/CHK2 to enable checkpoint recovery, promotes replication fork restart and NHEJ repair after DNA damage, and facilitates DSB resection across repetitive heterochromatic regions by reducing phosphorylation of Pif1 and Rrm3 helicases; Psy2's own CDK-dependent phosphorylation is required for DSB repair but not checkpoint inactivation, and in mammals/rats, Smek2 dysfunction disrupts hepatic glycolysis and sarcosine/homocysteine metabolism."},"narrative":{"mechanistic_narrative":"PPP4R3B (SMEK2/Psy2) is a regulatory subunit of the PP4-type protein phosphatase complex, where it partners with the catalytic subunit Pph3 to govern the DNA damage response [PMID:17517611]. The Pph3-Psy2 complex binds and dephosphorylates the activated checkpoint kinase Rad53 during and after MMS-induced damage, an activity required to restart replication forks during recovery from checkpoint arrest [PMID:17517611], and this Rad53-directed dephosphorylation role is conserved in Candida albicans, where Psy2 loss causes hypersensitivity to genotoxins and persistent filamentous growth [PMID:21890819]. Beyond checkpoint recovery, the complex promotes non-homologous end-joining of double-strand breaks in a pathway parallel to Chk1 [PMID:24498054], and Psy2 itself undergoes CDK-dependent phosphorylation that is dispensable for checkpoint inactivation but essential for DSB repair, where it enforces symmetric resection across tandem retrotransposons by limiting phosphorylation of the Pif1 and Rrm3 helicases [PMID:bio_10.1101_2025.02.19.639064]. In mammalian/rat systems, Smek2 dysfunction has been linked to a distinct physiological role in hepatic glycolysis and sarcosine/homocysteine metabolism via reduced sarcosine dehydrogenase expression [PMID:36810603]. The mammalian molecular mechanism connecting PPP4R3B to these metabolic phenotypes has not been characterized in the available corpus.","teleology":[{"year":2004,"claim":"Before its biochemical role was known, Psy2 was placed in the replication-fork stability pathway by mapping its protein and genetic interaction network.","evidence":"Yeast two-hybrid and genetic epistasis with WSS1, TOF1, RAD52, SRS2 in budding yeast","pmids":["15598824"],"confidence":"Low","gaps":["Physical interactions rest on yeast two-hybrid alone with no biochemical reconstitution","Does not define Psy2's catalytic partner or molecular activity","Fork-processing role inferred genetically, not directly demonstrated"]},{"year":2007,"claim":"Established that Psy2 is a regulatory subunit of the Pph3 phosphatase and that this complex drives checkpoint recovery by dephosphorylating Rad53.","evidence":"Biochemical co-purification, in vitro dephosphorylation assay, and replication fork analysis in yeast","pmids":["17517611"],"confidence":"High","gaps":["Does not resolve how Psy2 confers substrate specificity onto Pph3","Recruitment mechanism to damage sites not defined","Mammalian ortholog activity not tested"]},{"year":2011,"claim":"Showed the Pph3-Psy2 checkpoint-recovery function is conserved beyond budding yeast and selective for Rad53 over H2AX.","evidence":"PSY2 deletion phenotyping, phospho-western blot, and flow cytometry in Candida albicans","pmids":["21890819"],"confidence":"Medium","gaps":["Substrate selectivity mechanism (Rad53 vs H2AX) unexplained","Single lab","Link between dephosphorylation and filamentous-growth phenotype not mechanistically dissected"]},{"year":2014,"claim":"Extended the complex's role from checkpoint recovery to active DSB repair, defining an NHEJ function parallel to the Chk1 pathway.","evidence":"NHEJ repair assays, double-mutant epistasis, and Chk1 overexpression rescue in yeast","pmids":["24498054"],"confidence":"Medium","gaps":["Direct phosphatase substrate in NHEJ not identified","Single lab","Molecular basis of parallelism with Chk1 unresolved"]},{"year":2023,"claim":"Connected the mammalian/rat ortholog to a metabolic role distinct from DNA repair, implicating Smek2 in hepatic glycolysis and sarcosine/homocysteine handling.","evidence":"Congenic rat model with Smek2 deletion, microarray expression profiling, and metabolite measurement","pmids":["36810603"],"confidence":"Low","gaps":["No direct biochemical mechanism linking Smek2 to Sardh expression","Link between Smek2 loss and metabolic phenotype is correlative","Does not establish whether the PP4 phosphatase activity underlies the metabolic defect"]},{"year":2025,"claim":"Distinguished a CDK-regulated DSB-repair function of Psy2 from its checkpoint-inactivation role and identified replicative helicases as relevant phospho-targets controlling resection symmetry.","evidence":"Genetic epistasis, phospho-state analysis, resection assays, phosphodeficient mutant rescue, and helicase phosphorylation analysis in yeast (preprint)","pmids":["bio_10.1101_2025.02.19.639064"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed; single lab","Whether Pif1/Rrm3 are direct Pph3-Psy2 substrates not established","CDK site mapping and its mechanistic effect on complex function incomplete"]},{"year":null,"claim":"How the mammalian PPP4R3B/SMEK2 subunit directs PP4 substrate choice and whether its DNA-repair role and its hepatic metabolic role share a common biochemical mechanism remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No mammalian substrate of the PPP4R3B-containing complex defined in the corpus","Mechanism bridging phosphatase activity and Sardh/metabolic regulation unknown","No structural model of the regulatory subunit's substrate-targeting function"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0]}],"localization":[],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,3,6]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[0,2]}],"complexes":["Pph3-Psy2 (PP4) phosphatase complex"],"partners":["PPH3","RAD53","WSS1","TOF1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q5MIZ7","full_name":"Serine/threonine-protein phosphatase 4 regulatory subunit 3B","aliases":["SMEK homolog 2"],"length_aa":849,"mass_kda":97.5,"function":"Regulatory subunit of serine/threonine-protein phosphatase 4 (PP4). May regulate the activity of PPP4C at centrosomal microtubule organizing centers","subcellular_location":"Cytoplasm; Cytoplasm, cytoskeleton, microtubule organizing center, centrosome; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q5MIZ7/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PPP4R3B","classification":"Not Classified","n_dependent_lines":6,"n_total_lines":1208,"dependency_fraction":0.004966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PPP4R3B","total_profiled":1310},"omim":[{"mim_id":"610352","title":"PROTEIN PHOSPHATASE 4, REGULATORY SUBUNIT 3, BETA; PPP4R3B","url":"https://www.omim.org/entry/610352"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear speckles","reliability":"Supported"},{"location":"Centrosome","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PPP4R3B"},"hgnc":{"alias_symbol":["PSY2","FLFL2","KIAA1387","FLJ31474"],"prev_symbol":["SMEK2"]},"alphafold":{"accession":"Q5MIZ7","domains":[{"cath_id":"2.30.29.30","chopping":"5-113","consensus_level":"high","plddt":89.4889,"start":5,"end":113},{"cath_id":"-","chopping":"133-256","consensus_level":"medium","plddt":93.0341,"start":133,"end":256}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5MIZ7","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q5MIZ7-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q5MIZ7-F1-predicted_aligned_error_v6.png","plddt_mean":75.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PPP4R3B","jax_strain_url":"https://www.jax.org/strain/search?query=PPP4R3B"},"sequence":{"accession":"Q5MIZ7","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5MIZ7.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5MIZ7/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5MIZ7"}},"corpus_meta":[{"pmid":"17517611","id":"PMC_17517611","title":"Pph3-Psy2 is a phosphatase complex required for Rad53 dephosphorylation and replication fork restart during recovery from DNA damage.","date":"2007","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/17517611","citation_count":114,"is_preprint":false},{"pmid":"8245008","id":"PMC_8245008","title":"cDNA cloning, expression during development, and genome mapping of PSY2, a second tomato gene encoding phytoene synthase.","date":"1993","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/8245008","citation_count":81,"is_preprint":false},{"pmid":"15598824","id":"PMC_15598824","title":"Coordinated functions of WSS1, PSY2 and TOF1 in the DNA damage response.","date":"2004","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/15598824","citation_count":29,"is_preprint":false},{"pmid":"21890819","id":"PMC_21890819","title":"Protein phosphatase Pph3 and its regulatory subunit Psy2 regulate Rad53 dephosphorylation and cell morphogenesis during recovery from DNA damage in Candida albicans.","date":"2011","source":"Eukaryotic cell","url":"https://pubmed.ncbi.nlm.nih.gov/21890819","citation_count":26,"is_preprint":false},{"pmid":"24498054","id":"PMC_24498054","title":"Phosphatase complex Pph3/Psy2 is involved in regulation of efficient non-homologous end-joining pathway in the yeast Saccharomyces cerevisiae.","date":"2014","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/24498054","citation_count":17,"is_preprint":false},{"pmid":"23794103","id":"PMC_23794103","title":"The sub-cellular localisation of the potato (Solanum tuberosum L.) carotenoid biosynthetic enzymes, CrtRb2 and PSY2.","date":"2013","source":"Protoplasma","url":"https://pubmed.ncbi.nlm.nih.gov/23794103","citation_count":13,"is_preprint":false},{"pmid":"28779475","id":"PMC_28779475","title":"Molecular analysis of the expression of a crtB transgene and the endogenous psy2-y 1 and psy2-y 2 genes of cassava and their effect on root carotenoid content.","date":"2017","source":"Transgenic research","url":"https://pubmed.ncbi.nlm.nih.gov/28779475","citation_count":7,"is_preprint":false},{"pmid":"33738386","id":"PMC_33738386","title":"Structural and functional features of phytoene synthase isoforms PSY1 and PSY2 in pepper Capsicum annuum L. cultivars.","date":"2020","source":"Vavilovskii zhurnal genetiki i selektsii","url":"https://pubmed.ncbi.nlm.nih.gov/33738386","citation_count":6,"is_preprint":false},{"pmid":"18753676","id":"PMC_18753676","title":"Identification of SMEK2 as a candidate gene for regulation of responsiveness to dietary cholesterol in rats.","date":"2008","source":"Journal of lipid research","url":"https://pubmed.ncbi.nlm.nih.gov/18753676","citation_count":6,"is_preprint":false},{"pmid":"21887656","id":"PMC_21887656","title":"Complete sequence and organization of the Sphingobium chungbukense DJ77 pSY2 plasmid.","date":"2011","source":"Journal of microbiology (Seoul, Korea)","url":"https://pubmed.ncbi.nlm.nih.gov/21887656","citation_count":3,"is_preprint":false},{"pmid":"33233740","id":"PMC_33233740","title":"Rbm38 Reduces the Transcription Elongation Defect of the SMEK2 Gene Caused by Splicing Deficiency.","date":"2020","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/33233740","citation_count":3,"is_preprint":false},{"pmid":"33368035","id":"PMC_33368035","title":"Variability and Phylogeny of the Pepper Phytoene Synthase Paralogs PSY1 and PSY2 in Species of Various Capsicum Complexes.","date":"2020","source":"Doklady. Biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/33368035","citation_count":2,"is_preprint":false},{"pmid":"36810603","id":"PMC_36810603","title":"Mutation in Smek2 regulating hepatic glucose metabolism causes hypersarcosinemia and hyperhomocysteinemia in rats.","date":"2023","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/36810603","citation_count":1,"is_preprint":false},{"pmid":"39196523","id":"PMC_39196523","title":"Circadian Regulation of Expression of Carotenoid Metabolism Genes (PSY2, LCYE, CrtRB1, and NCED1) in Leaves of Tomato Solanum lycopersicum L.","date":"2024","source":"Doklady. Biochemistry and biophysics","url":"https://pubmed.ncbi.nlm.nih.gov/39196523","citation_count":1,"is_preprint":false},{"pmid":"27396123","id":"PMC_27396123","title":"[Effects of Acaulospora spinosa on plant growth and lycopene related genes (psyl and psy2) expression of tomato].","date":"2016","source":"Ying yong sheng tai xue bao = The journal of applied ecology","url":"https://pubmed.ncbi.nlm.nih.gov/27396123","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.02.19.639064","title":"PP4  <sup>Psy2</sup>  regulates Rrm3 and Pif1 helicases to facilitate DSB resection across tandem retrotransposons","date":"2025-02-26","source":"bioRxiv","url":"https://doi.org/10.1101/2025.02.19.639064","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.10.07.617079","title":"Reduced methane emissions in transgenic rice genotypes are associated with altered rhizosphere microbial hydrogen cycling","date":"2024-10-07","source":"bioRxiv","url":"https://doi.org/10.1101/2024.10.07.617079","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.03.28.646041","title":"Bivariate genome-wide association study reveals polygenic contributions to covariance between total carotenoid and dry matter contents in yellow-fleshed cassava (  <i>Manihot esculenta</i>  )","date":"2025-04-03","source":"bioRxiv","url":"https://doi.org/10.1101/2025.03.28.646041","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10358,"output_tokens":1835,"usd":0.029299,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8741,"output_tokens":2455,"usd":0.05254,"stage2_stop_reason":"end_turn"},"total_usd":0.081839,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"The yeast ortholog Psy2 forms a complex with the type 2A-like protein phosphatase Pph3 (Pph3-Psy2); this complex binds and dephosphorylates activated Rad53 during and after MMS-induced DNA damage, and is required for replication fork restart during recovery from DNA damage checkpoint arrest.\",\n      \"method\": \"Genetic epistasis, biochemical co-purification, in vitro dephosphorylation assay, replication fork analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — biochemical complex identification, in vitro dephosphorylation assay, and genetic epistasis all in one study; independently consistent with multiple follow-up studies\",\n      \"pmids\": [\"17517611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Yeast Psy2 physically interacts with Wss1 and Tof1 (by yeast two-hybrid), and genetically interacts with WSS1, TOF1, RAD52, and SRS2, placing Psy2 in a pathway that stabilizes or processes stalled/collapsed replication forks.\",\n      \"method\": \"Yeast two-hybrid, genetic interaction/epistasis analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — yeast two-hybrid only for physical interaction; genetic interactions support pathway placement but no biochemical reconstitution\",\n      \"pmids\": [\"15598824\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In Candida albicans, Psy2 (orf19.3685) forms a functional complex with Pph3 required for dephosphorylation of the checkpoint kinase Rad53 (but not H2AX) during recovery from DNA damage; loss of PSY2 causes hypersensitivity to DNA-damaging agents and persistent filamentous growth under genotoxic stress.\",\n      \"method\": \"Deletion mutant phenotypic analysis, phosphorylation western blot, flow cytometry\",\n      \"journal\": \"Eukaryotic cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined cellular and molecular phenotypes, orthogonal methods (flow cytometry + western blot), but single lab\",\n      \"pmids\": [\"21890819\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Yeast Pph3/Psy2 phosphatase complex promotes efficient non-homologous end-joining (NHEJ) of double-strand breaks; its activity in NHEJ is linked to the Rad53 checkpoint and operates in a pathway parallel to Chk1, as shown by Chk1 overexpression rescuing the NHEJ defect of pph3Δ and psy2Δ strains and double mutants showing additive repair defects.\",\n      \"method\": \"Plasmid and chromosomal NHEJ repair assays, genetic epistasis (double mutant analysis), overexpression rescue\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional repair assays with genetic epistasis and overexpression rescue; single lab but multiple orthogonal approaches\",\n      \"pmids\": [\"24498054\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In rats, a deletion mutation in Smek2 (PPP4R3B ortholog) impairs glycolysis in the liver and causes extremely low sarcosine dehydrogenase (Sardh) expression, leading to hypersarcosinemia and hyperhomocysteinemia, connecting Smek2 to regulation of hepatic sarcosine and homocysteine metabolism.\",\n      \"method\": \"Congenic rat model, microarray gene expression analysis, metabolite measurement\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — microarray plus congenic rat model; indirect link between Smek2 mutation and metabolic phenotype, no direct biochemical mechanism for how Smek2 controls Sardh expression\",\n      \"pmids\": [\"36810603\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The RNA binding protein Rbm38 reduces a transcription elongation defect of the SMEK2 (PPP4R3B) gene that occurs under splicing-deficient conditions; this requires the N- and C-terminal regions of Rbm38 and its RNA-recognition motif.\",\n      \"method\": \"Transcription elongation assay under splicing-deficient conditions, Rbm38 domain deletion analysis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — functional domain dissection with defined molecular readout (transcription elongation), single lab but multiple construct variants tested\",\n      \"pmids\": [\"33233740\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Yeast Psy2 (PP4 regulatory subunit) undergoes CDK-dependent phosphorylation that is essential for DSB repair but not for DNA damage checkpoint inactivation; Psy2 promotes symmetric DSB resection across tandem retrotransposons by reducing phosphorylation of Pif1 and Rrm3 helicases, and its loss causes an asymmetric resection defect that is alleviated by expressing a phosphodeficient H2A variant or depleting the checkpoint adaptor Rad9.\",\n      \"method\": \"Genetic epistasis, phosphorylation state analysis, resection assays, phosphodeficient mutant rescue, helicase substrate phosphorylation analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal genetic and biochemical methods, but preprint not yet peer-reviewed; single lab\",\n      \"pmids\": [\"bio_10.1101_2025.02.19.639064\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"PPP4R3B (SMEK2/PSY2/FLFL2) encodes a regulatory subunit of the PP4 phosphatase complex (PP4/Pph3–Psy2), which dephosphorylates the DNA damage checkpoint kinase Rad53/CHK2 to enable checkpoint recovery, promotes replication fork restart and NHEJ repair after DNA damage, and facilitates DSB resection across repetitive heterochromatic regions by reducing phosphorylation of Pif1 and Rrm3 helicases; Psy2's own CDK-dependent phosphorylation is required for DSB repair but not checkpoint inactivation, and in mammals/rats, Smek2 dysfunction disrupts hepatic glycolysis and sarcosine/homocysteine metabolism.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PPP4R3B (SMEK2/Psy2) is a regulatory subunit of the PP4-type protein phosphatase complex, where it partners with the catalytic subunit Pph3 to govern the DNA damage response [#0]. The Pph3-Psy2 complex binds and dephosphorylates the activated checkpoint kinase Rad53 during and after MMS-induced damage, an activity required to restart replication forks during recovery from checkpoint arrest [#0], and this Rad53-directed dephosphorylation role is conserved in Candida albicans, where Psy2 loss causes hypersensitivity to genotoxins and persistent filamentous growth [#2]. Beyond checkpoint recovery, the complex promotes non-homologous end-joining of double-strand breaks in a pathway parallel to Chk1 [#3], and Psy2 itself undergoes CDK-dependent phosphorylation that is dispensable for checkpoint inactivation but essential for DSB repair, where it enforces symmetric resection across tandem retrotransposons by limiting phosphorylation of the Pif1 and Rrm3 helicases [#6]. In mammalian/rat systems, Smek2 dysfunction has been linked to a distinct physiological role in hepatic glycolysis and sarcosine/homocysteine metabolism via reduced sarcosine dehydrogenase expression [#4]. The mammalian molecular mechanism connecting PPP4R3B to these metabolic phenotypes has not been characterized in the available corpus.\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Before its biochemical role was known, Psy2 was placed in the replication-fork stability pathway by mapping its protein and genetic interaction network.\",\n      \"evidence\": \"Yeast two-hybrid and genetic epistasis with WSS1, TOF1, RAD52, SRS2 in budding yeast\",\n      \"pmids\": [\"15598824\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Physical interactions rest on yeast two-hybrid alone with no biochemical reconstitution\", \"Does not define Psy2's catalytic partner or molecular activity\", \"Fork-processing role inferred genetically, not directly demonstrated\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Established that Psy2 is a regulatory subunit of the Pph3 phosphatase and that this complex drives checkpoint recovery by dephosphorylating Rad53.\",\n      \"evidence\": \"Biochemical co-purification, in vitro dephosphorylation assay, and replication fork analysis in yeast\",\n      \"pmids\": [\"17517611\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Does not resolve how Psy2 confers substrate specificity onto Pph3\", \"Recruitment mechanism to damage sites not defined\", \"Mammalian ortholog activity not tested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Showed the Pph3-Psy2 checkpoint-recovery function is conserved beyond budding yeast and selective for Rad53 over H2AX.\",\n      \"evidence\": \"PSY2 deletion phenotyping, phospho-western blot, and flow cytometry in Candida albicans\",\n      \"pmids\": [\"21890819\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Substrate selectivity mechanism (Rad53 vs H2AX) unexplained\", \"Single lab\", \"Link between dephosphorylation and filamentous-growth phenotype not mechanistically dissected\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Extended the complex's role from checkpoint recovery to active DSB repair, defining an NHEJ function parallel to the Chk1 pathway.\",\n      \"evidence\": \"NHEJ repair assays, double-mutant epistasis, and Chk1 overexpression rescue in yeast\",\n      \"pmids\": [\"24498054\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct phosphatase substrate in NHEJ not identified\", \"Single lab\", \"Molecular basis of parallelism with Chk1 unresolved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Connected the mammalian/rat ortholog to a metabolic role distinct from DNA repair, implicating Smek2 in hepatic glycolysis and sarcosine/homocysteine handling.\",\n      \"evidence\": \"Congenic rat model with Smek2 deletion, microarray expression profiling, and metabolite measurement\",\n      \"pmids\": [\"36810603\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No direct biochemical mechanism linking Smek2 to Sardh expression\", \"Link between Smek2 loss and metabolic phenotype is correlative\", \"Does not establish whether the PP4 phosphatase activity underlies the metabolic defect\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Distinguished a CDK-regulated DSB-repair function of Psy2 from its checkpoint-inactivation role and identified replicative helicases as relevant phospho-targets controlling resection symmetry.\",\n      \"evidence\": \"Genetic epistasis, phospho-state analysis, resection assays, phosphodeficient mutant rescue, and helicase phosphorylation analysis in yeast (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.02.19.639064\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed; single lab\", \"Whether Pif1/Rrm3 are direct Pph3-Psy2 substrates not established\", \"CDK site mapping and its mechanistic effect on complex function incomplete\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the mammalian PPP4R3B/SMEK2 subunit directs PP4 substrate choice and whether its DNA-repair role and its hepatic metabolic role share a common biochemical mechanism remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No mammalian substrate of the PPP4R3B-containing complex defined in the corpus\", \"Mechanism bridging phosphatase activity and Sardh/metabolic regulation unknown\", \"No structural model of the regulatory subunit's substrate-targeting function\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 3, 6]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [\"Pph3-Psy2 (PP4) phosphatase complex\"],\n    \"partners\": [\"Pph3\", \"Rad53\", \"Wss1\", \"Tof1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}