{"gene":"PTDSS2","run_date":"2026-06-10T06:43:36","timeline":{"discoveries":[{"year":2000,"finding":"PTDSS2 (PSS2) is localized to mitochondria-associated membranes (MAM) and is largely excluded from the bulk of the endoplasmic reticulum, as determined by immunoblotting of CHO-K1 cells and McArdle cells stably expressing recombinant PSS2.","method":"Subcellular fractionation, immunoblotting of endogenous and recombinant PSS2 in CHO-K1 and hepatoma cell lines","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct fractionation with immunoblotting, replicated in two cell systems (CHO-K1 and McArdle cells expressing recombinant PSS2)","pmids":["10938271"],"is_preprint":false},{"year":1999,"finding":"PTDSS2 (PSS2) uses phosphatidylethanolamine (PE) as its primary substrate for the serine base-exchange reaction to synthesize phosphatidylserine, and its activity is subject to end-product (PS) inhibition, in contrast to PSS1 which is not subject to this feedback regulation.","method":"Stable cDNA expression in McArdle hepatoma and M.9.1.1 CHO cells; in vitro serine exchange assays; phospholipid metabolic labeling; ethanolamine auxotrophy rescue","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro enzymatic assays with cloned protein, replicated across two cell lines with multiple metabolic readouts","pmids":["10432300"],"is_preprint":false},{"year":2009,"finding":"Purified human PTDSS2 (PSS2) catalyzes conversion of PE (but not PC) into PS, and the purified enzyme is inhibited by exogenous PS added to the assay mixture, demonstrating direct feedback inhibition by its product.","method":"Purification of epitope-tagged human PSS2 and in vitro enzymatic assay with defined substrates; PS inhibition assay with purified enzyme","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstituted in vitro with purified protein and defined substrate; substrate specificity and product inhibition confirmed biochemically","pmids":["19014349"],"is_preprint":false},{"year":2002,"finding":"Pss2-deficient mice retain less than 5% of normal serine exchange activity in testis extracts and approximately 10% in brain and liver, demonstrating that PTDSS2 is responsible for the majority of PS serine exchange activity in these tissues; however, phospholipid content is not perturbed and Pss1 expression is not compensatorily up-regulated.","method":"Pss2 knockout mouse generation; in vitro serine exchange assays on tissue extracts; phospholipid content analysis; Northern blotting for Pss1","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean knockout with multiple orthogonal assays (enzymatic activity, phospholipid content, mRNA expression) in multiple tissues","pmids":["12361952"],"is_preprint":false},{"year":2002,"finding":"Pss2 is highly expressed in Sertoli cells of the testis, brown fat, neurons, and myometrium as revealed by beta-galactosidase reporter expressed from Pss2 regulatory sequences in knockout mice; testis weight is reduced and some males are infertile in Pss2-deficient mice.","method":"Beta-galactosidase reporter knock-in at Pss2 locus in mice; histological analysis; fertility assessment","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct reporter localization in knockout mouse, single lab, phenotype tied to loss-of-function","pmids":["12361952"],"is_preprint":false},{"year":2012,"finding":"Purified PTDSS2 (PSS2) shows significant preference for DHA (docosahexaenoic acid, 22:6n-3) at the sn-2 position of PE substrates compared to 18:1 or 20:4, and preferentially produces DHA-containing PS; it utilizes both diacyl PE and PE plasmalogen substrates, but diacyl PE is used ~6-fold more efficiently.","method":"Immunopurification of Flag-tagged PSS2 from HEK cells; in vitro enzyme assay with defined substrate species; also confirmed with PSS2-dependent mutant cell microsomes and PSS2 from multiple cell lines","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1 / Strong — purified enzyme with defined substrates, replicated across multiple cell line preparations and cell microsomes","pmids":["23071296"],"is_preprint":false},{"year":2004,"finding":"Neither PSS1 nor PSS2, nor both together, is specifically required for externalization of phosphatidylserine during apoptosis; CHO cells deficient in PSS1, PSS2, or both show normal PtdSer externalization upon staurosporine-induced apoptosis despite >97% reduction in serine-exchange activity.","method":"Apoptosis induction with staurosporine in PSS1- and/or PSS2-deficient CHO cell lines; annexin V labeling and flow cytometry; DNA fragmentation assay","journal":"Biochimica et biophysica acta","confidence":"High","confidence_rationale":"Tier 2 / Strong — clean genetic loss-of-function with multiple cell lines and orthogonal apoptosis readouts; negative result robustly established","pmids":["14984733"],"is_preprint":false},{"year":2018,"finding":"Knockdown of both PTDSS1 and PTDSS2 decreases Ebola virus (EBOV) production and results in accumulation of virions at the plasma membrane and adjacent intracellular organelles, indicating that PS synthesized by these enzymes is required for efficient EBOV budding.","method":"siRNA knockdown of PTDSS1 and PTDSS2 in infected cells; viral production assay; electron microscopy of virion localization","journal":"The Journal of infectious diseases","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — loss-of-function with defined cellular phenotype but PTDSS1 and PTDSS2 knocked down together, so individual PTDSS2 contribution is not isolated; single lab","pmids":["30289506"],"is_preprint":false},{"year":2022,"finding":"High PTDSS2 expression (in conjunction with low ATP11B expression) is associated with increased nonapoptotic phosphatidylserine on the outer leaflet of the cell membrane, which promotes immunosuppression and breast cancer metastasis; BRCA1 negatively regulates PTDSS2 expression in this axis.","method":"Expression manipulation experiments; PS externalization assays; in vivo mouse metastasis model; tumor microenvironment analysis (myeloid-derived suppressor cells, cytotoxic T cells)","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal in vitro and in vivo methods linking PTDSS2-dependent PS externalization to immune evasion, single lab","pmids":["35025764"],"is_preprint":false},{"year":2023,"finding":"A gain-of-function mutant of PSS2 (R97K), analogous to the Lenz-Majewski syndrome PSS1 mutations, impairs osteoclast formation, multinucleation, and activity, and alters the acyl chain composition of PS and phosphatidylethanolamine and decreases phosphatidylinositol levels.","method":"Expression of PSS2 R97K gain-of-function mutant in osteoclast precursor cells; osteoclast differentiation assays; actin podosome imaging; lipidomic analysis; catalytically inactive mutant as control","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — gain-of-function with catalytically inactive control, multiple phospholipid and cellular phenotype readouts, single lab","pmids":["37714410"],"is_preprint":false},{"year":2007,"finding":"Maternal ethanol exposure significantly attenuates microsomal PS biosynthetic activity and reduces PS (particularly 18:0, 22:6-PS) in developing rat brain cortices without reducing PSS2 mRNA expression, and the 18:0, 22:6-PE substrate is the best substrate for PSS2-dependent PS synthesis.","method":"In vitro microsomal PS biosynthesis assay with deuterium-labeled substrates and ESI-MS; mRNA quantification; RT-PCR","journal":"Journal of neuroscience research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — enzymatic activity measured in microsomes without separating PSS1 vs PSS2 contributions specifically; PSS2 protein could not be probed; single lab","pmids":["17387686"],"is_preprint":false},{"year":2007,"finding":"Over-expression of PSS2 (or PSS1) did not alter the PS level or the effect of DHA on PS increase in either neuronal or non-neuronal cells, and DHA enrichment did not affect PSS2 mRNA levels.","method":"Overexpression of PSS1 and PSS2 in Neuro 2A and non-neuronal cell lines; DHA enrichment; lipid mass spectrometry; RT-PCR","journal":"Journal of molecular neuroscience : MN","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct overexpression experiment with phospholipid quantification and mRNA analysis; negative result for PSS2 overexpression on PS levels robustly established","pmids":["17901548"],"is_preprint":false}],"current_model":"PTDSS2 (PSS2) is a phosphatidylserine synthase localized to mitochondria-associated membranes (MAM) that catalyzes the base-exchange of serine for the head group of phosphatidylethanolamine (but not phosphatidylcholine) to synthesize phosphatidylserine; its activity is subject to direct feedback inhibition by the PS product, it exhibits preferential activity toward DHA-containing PE substrates, it accounts for the majority of serine exchange activity in testis, brain, and liver in vivo, and it contributes to nonapoptotic PS exposure at the plasma membrane with consequences for immune evasion and viral budding, while being dispensable for PS externalization during apoptosis."},"narrative":{"mechanistic_narrative":"PTDSS2 (PSS2) is a phosphatidylserine synthase that produces phosphatidylserine (PS) through a serine base-exchange reaction, contributing to membrane lipid composition with downstream consequences for fertility, immune evasion, and viral budding [PMID:10432300, PMID:12361952]. The purified enzyme converts phosphatidylethanolamine (PE), but not phosphatidylcholine, into PS, and is directly inhibited by its PS product—a feedback regulation that distinguishes it from PSS1 [PMID:10432300, PMID:19014349]. PTDSS2 displays marked substrate selectivity, preferring DHA (22:6n-3) at the sn-2 position of PE and thereby preferentially generating DHA-containing PS, and it favors diacyl PE over PE plasmalogen [PMID:23071296]. The enzyme localizes to mitochondria-associated membranes and is largely excluded from bulk endoplasmic reticulum [PMID:10938271]. In vivo, PTDSS2 accounts for the great majority of serine-exchange activity in testis, brain, and liver, yet its loss leaves bulk phospholipid content unchanged and does not trigger compensatory PSS1 up-regulation; knockout mice show reduced testis weight and male infertility, consistent with high expression in Sertoli cells [PMID:12361952]. PTDSS2-dependent PS synthesis is dispensable for PS externalization during apoptosis [PMID:14984733] but supports nonapoptotic PS exposure at the plasma membrane that promotes immunosuppression and breast cancer metastasis and is required for efficient Ebola virus budding [PMID:30289506, PMID:35025764]. A gain-of-function R97K mutant, analogous to Lenz-Majewski syndrome PSS1 mutations, impairs osteoclast formation and remodels cellular phospholipid composition [PMID:37714410].","teleology":[{"year":1999,"claim":"Established the core catalytic identity of PTDSS2 by showing it uses PE for serine base-exchange and, unlike PSS1, is subject to product feedback inhibition, defining a distinct regulatory mode for this synthase.","evidence":"Stable cDNA expression in hepatoma and CHO cells with in vitro serine-exchange assays, metabolic labeling, and ethanolamine auxotrophy rescue","pmids":["10432300"],"confidence":"High","gaps":["Substrate preference among PE species not yet resolved","Structural basis of product inhibition unknown"]},{"year":2000,"claim":"Resolved where PTDSS2 acts within the cell, placing it at mitochondria-associated membranes rather than bulk ER, situating PS synthesis at an organelle contact site.","evidence":"Subcellular fractionation and immunoblotting of endogenous and recombinant PSS2 in CHO-K1 and McArdle cells","pmids":["10938271"],"confidence":"High","gaps":["Mechanism of MAM targeting not defined","Functional consequence of MAM localization for PS trafficking unaddressed"]},{"year":2002,"claim":"Demonstrated the in vivo contribution of PTDSS2 to tissue serine-exchange activity and its physiological role, showing it accounts for most activity in testis, brain, and liver and that its loss impairs male fertility without altering bulk phospholipid content.","evidence":"Pss2 knockout mouse with tissue serine-exchange assays, phospholipid analysis, beta-galactosidase reporter localization, and fertility assessment","pmids":["12361952"],"confidence":"High","gaps":["Why phospholipid content is buffered despite >90% activity loss is unexplained","Mechanistic link between PS synthesis and Sertoli cell/fertility function not established"]},{"year":2004,"claim":"Tested whether PS synthases drive apoptotic PS externalization and showed they do not, dissociating the bulk PS biosynthetic machinery from the canonical apoptotic 'eat-me' signal.","evidence":"Staurosporine-induced apoptosis in PSS1- and/or PSS2-deficient CHO cells with annexin V flow cytometry and DNA fragmentation","pmids":["14984733"],"confidence":"High","gaps":["Source of PS externalized during apoptosis not identified","Does not address nonapoptotic PS exposure"]},{"year":2009,"claim":"Confirmed substrate specificity and product inhibition directly with purified human enzyme, removing ambiguity from cell-based assays.","evidence":"Purified epitope-tagged human PSS2 in in vitro assays with defined substrates and exogenous PS","pmids":["19014349"],"confidence":"High","gaps":["Kinetic parameters and inhibition constants not detailed here","No structural model of substrate/product binding"]},{"year":2012,"claim":"Defined acyl-chain selectivity, showing PTDSS2 preferentially uses DHA-containing PE and diacyl over plasmalogen PE, explaining how it shapes the DHA-rich PS pool.","evidence":"Immunopurified Flag-tagged PSS2 from HEK cells assayed with defined substrate species, plus mutant cell microsomes","pmids":["23071296"],"confidence":"High","gaps":["Structural determinant of DHA preference unknown","In vivo relevance of acyl preference to specific tissues not tested"]},{"year":2018,"claim":"Linked PTDSS2-generated PS to viral egress by showing combined PTDSS1/PTDSS2 knockdown blocks Ebola virus budding at the plasma membrane.","evidence":"siRNA co-knockdown of PTDSS1 and PTDSS2 with viral production assay and electron microscopy","pmids":["30289506"],"confidence":"Medium","gaps":["PTDSS1 and PTDSS2 knocked down together, so individual PTDSS2 contribution not isolated","Single lab, single virus"]},{"year":2022,"claim":"Connected PTDSS2 expression to nonapoptotic surface PS exposure that drives tumor immune evasion and metastasis, and placed it downstream of BRCA1 regulation.","evidence":"Expression manipulation, PS externalization assays, in vivo mouse metastasis model, and tumor microenvironment analysis","pmids":["35025764"],"confidence":"Medium","gaps":["Mechanism by which BRCA1 represses PTDSS2 not defined","Single lab; direct causality of surface PS in immunosuppression partly correlative"]},{"year":2023,"claim":"Showed a gain-of-function PSS2 mutation analogous to Lenz-Majewski PSS1 mutations remodels cellular phospholipid composition and impairs osteoclast formation, implicating PTDSS2 activity in bone cell biology.","evidence":"Expression of PSS2 R97K gain-of-function mutant in osteoclast precursors with differentiation assays, podosome imaging, and lipidomics; catalytically inactive control","pmids":["37714410"],"confidence":"Medium","gaps":["No human disease attributable to PTDSS2 mutation demonstrated","Mechanism linking lipid changes to osteoclast defect unresolved","Single lab"]},{"year":null,"claim":"How PTDSS2-derived PS is selectively trafficked from MAM to the plasma membrane outer leaflet, and what controls its tissue-specific physiological versus disease-associated roles, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No defined PS transport pathway from MAM to plasma membrane in the corpus","No structural model of the enzyme","Regulators beyond product inhibition and BRCA1 largely uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[1,2,5]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[1,2]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0]},{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1,2,3]}],"complexes":[],"partners":[],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9BVG9","full_name":"Phosphatidylserine synthase 2","aliases":["Serine-exchange enzyme II"],"length_aa":487,"mass_kda":56.3,"function":"Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) is replaced by L-serine (PubMed:19014349). Catalyzes the conversion of phosphatatidylethanolamine and does not act on phosphatidylcholine (PubMed:19014349). Can utilize both phosphatidylethanolamine (PE) plasmalogen and diacyl PE as substrate and the latter is six times better utilized, indicating the importance of an ester linkage at the sn-1 position (By similarity). Although it shows no sn-1 fatty acyl preference, exhibits significant preference towards docosahexaenoic acid (22:6n-3) compared with 18:1 or 20:4 at the sn-2 position (By similarity)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q9BVG9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PTDSS2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000174915","cell_line_id":"CID000350","localizations":[{"compartment":"er","grade":3}],"interactors":[{"gene":"FKBP4","stoichiometry":0.2},{"gene":"FKBP5","stoichiometry":0.2},{"gene":"NUDC","stoichiometry":0.2},{"gene":"RPAP3","stoichiometry":0.2},{"gene":"HIST1H2AJ;HIST1H2AH;HIST1H2AG;H2AFJ","stoichiometry":0.2},{"gene":"H2AFJ","stoichiometry":0.2},{"gene":"HSP90AB1","stoichiometry":0.2},{"gene":"HSP90AA1","stoichiometry":0.2},{"gene":"AHSA1","stoichiometry":0.2},{"gene":"URI1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID000350","total_profiled":1310},"omim":[{"mim_id":"612793","title":"PHOSPHATIDYLSERINE SYNTHASE 2; PTDSS2","url":"https://www.omim.org/entry/612793"},{"mim_id":"612792","title":"PHOSPHATIDYLSERINE SYNTHASE 1; PTDSS1","url":"https://www.omim.org/entry/612792"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PTDSS2"},"hgnc":{"alias_symbol":["PSS2"],"prev_symbol":[]},"alphafold":{"accession":"Q9BVG9","domains":[{"cath_id":"-","chopping":"121-270_304-406","consensus_level":"high","plddt":91.6436,"start":121,"end":406},{"cath_id":"1.20.5","chopping":"409-446","consensus_level":"medium","plddt":88.9511,"start":409,"end":446}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BVG9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BVG9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BVG9-F1-predicted_aligned_error_v6.png","plddt_mean":81.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PTDSS2","jax_strain_url":"https://www.jax.org/strain/search?query=PTDSS2"},"sequence":{"accession":"Q9BVG9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BVG9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BVG9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BVG9"}},"corpus_meta":[{"pmid":"10938271","id":"PMC_10938271","title":"Phosphatidylserine 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\"finding\": \"PTDSS2 (PSS2) is localized to mitochondria-associated membranes (MAM) and is largely excluded from the bulk of the endoplasmic reticulum, as determined by immunoblotting of CHO-K1 cells and McArdle cells stably expressing recombinant PSS2.\",\n      \"method\": \"Subcellular fractionation, immunoblotting of endogenous and recombinant PSS2 in CHO-K1 and hepatoma cell lines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct fractionation with immunoblotting, replicated in two cell systems (CHO-K1 and McArdle cells expressing recombinant PSS2)\",\n      \"pmids\": [\"10938271\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"PTDSS2 (PSS2) uses phosphatidylethanolamine (PE) as its primary substrate for the serine base-exchange reaction to synthesize phosphatidylserine, and its activity is subject to end-product (PS) inhibition, in contrast to PSS1 which is not subject to this feedback regulation.\",\n      \"method\": \"Stable cDNA expression in McArdle hepatoma and M.9.1.1 CHO cells; in vitro serine exchange assays; phospholipid metabolic labeling; ethanolamine auxotrophy rescue\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro enzymatic assays with cloned protein, replicated across two cell lines with multiple metabolic readouts\",\n      \"pmids\": [\"10432300\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Purified human PTDSS2 (PSS2) catalyzes conversion of PE (but not PC) into PS, and the purified enzyme is inhibited by exogenous PS added to the assay mixture, demonstrating direct feedback inhibition by its product.\",\n      \"method\": \"Purification of epitope-tagged human PSS2 and in vitro enzymatic assay with defined substrates; PS inhibition assay with purified enzyme\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstituted in vitro with purified protein and defined substrate; substrate specificity and product inhibition confirmed biochemically\",\n      \"pmids\": [\"19014349\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Pss2-deficient mice retain less than 5% of normal serine exchange activity in testis extracts and approximately 10% in brain and liver, demonstrating that PTDSS2 is responsible for the majority of PS serine exchange activity in these tissues; however, phospholipid content is not perturbed and Pss1 expression is not compensatorily up-regulated.\",\n      \"method\": \"Pss2 knockout mouse generation; in vitro serine exchange assays on tissue extracts; phospholipid content analysis; Northern blotting for Pss1\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean knockout with multiple orthogonal assays (enzymatic activity, phospholipid content, mRNA expression) in multiple tissues\",\n      \"pmids\": [\"12361952\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Pss2 is highly expressed in Sertoli cells of the testis, brown fat, neurons, and myometrium as revealed by beta-galactosidase reporter expressed from Pss2 regulatory sequences in knockout mice; testis weight is reduced and some males are infertile in Pss2-deficient mice.\",\n      \"method\": \"Beta-galactosidase reporter knock-in at Pss2 locus in mice; histological analysis; fertility assessment\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct reporter localization in knockout mouse, single lab, phenotype tied to loss-of-function\",\n      \"pmids\": [\"12361952\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Purified PTDSS2 (PSS2) shows significant preference for DHA (docosahexaenoic acid, 22:6n-3) at the sn-2 position of PE substrates compared to 18:1 or 20:4, and preferentially produces DHA-containing PS; it utilizes both diacyl PE and PE plasmalogen substrates, but diacyl PE is used ~6-fold more efficiently.\",\n      \"method\": \"Immunopurification of Flag-tagged PSS2 from HEK cells; in vitro enzyme assay with defined substrate species; also confirmed with PSS2-dependent mutant cell microsomes and PSS2 from multiple cell lines\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — purified enzyme with defined substrates, replicated across multiple cell line preparations and cell microsomes\",\n      \"pmids\": [\"23071296\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Neither PSS1 nor PSS2, nor both together, is specifically required for externalization of phosphatidylserine during apoptosis; CHO cells deficient in PSS1, PSS2, or both show normal PtdSer externalization upon staurosporine-induced apoptosis despite >97% reduction in serine-exchange activity.\",\n      \"method\": \"Apoptosis induction with staurosporine in PSS1- and/or PSS2-deficient CHO cell lines; annexin V labeling and flow cytometry; DNA fragmentation assay\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — clean genetic loss-of-function with multiple cell lines and orthogonal apoptosis readouts; negative result robustly established\",\n      \"pmids\": [\"14984733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Knockdown of both PTDSS1 and PTDSS2 decreases Ebola virus (EBOV) production and results in accumulation of virions at the plasma membrane and adjacent intracellular organelles, indicating that PS synthesized by these enzymes is required for efficient EBOV budding.\",\n      \"method\": \"siRNA knockdown of PTDSS1 and PTDSS2 in infected cells; viral production assay; electron microscopy of virion localization\",\n      \"journal\": \"The Journal of infectious diseases\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — loss-of-function with defined cellular phenotype but PTDSS1 and PTDSS2 knocked down together, so individual PTDSS2 contribution is not isolated; single lab\",\n      \"pmids\": [\"30289506\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"High PTDSS2 expression (in conjunction with low ATP11B expression) is associated with increased nonapoptotic phosphatidylserine on the outer leaflet of the cell membrane, which promotes immunosuppression and breast cancer metastasis; BRCA1 negatively regulates PTDSS2 expression in this axis.\",\n      \"method\": \"Expression manipulation experiments; PS externalization assays; in vivo mouse metastasis model; tumor microenvironment analysis (myeloid-derived suppressor cells, cytotoxic T cells)\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal in vitro and in vivo methods linking PTDSS2-dependent PS externalization to immune evasion, single lab\",\n      \"pmids\": [\"35025764\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"A gain-of-function mutant of PSS2 (R97K), analogous to the Lenz-Majewski syndrome PSS1 mutations, impairs osteoclast formation, multinucleation, and activity, and alters the acyl chain composition of PS and phosphatidylethanolamine and decreases phosphatidylinositol levels.\",\n      \"method\": \"Expression of PSS2 R97K gain-of-function mutant in osteoclast precursor cells; osteoclast differentiation assays; actin podosome imaging; lipidomic analysis; catalytically inactive mutant as control\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — gain-of-function with catalytically inactive control, multiple phospholipid and cellular phenotype readouts, single lab\",\n      \"pmids\": [\"37714410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Maternal ethanol exposure significantly attenuates microsomal PS biosynthetic activity and reduces PS (particularly 18:0, 22:6-PS) in developing rat brain cortices without reducing PSS2 mRNA expression, and the 18:0, 22:6-PE substrate is the best substrate for PSS2-dependent PS synthesis.\",\n      \"method\": \"In vitro microsomal PS biosynthesis assay with deuterium-labeled substrates and ESI-MS; mRNA quantification; RT-PCR\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — enzymatic activity measured in microsomes without separating PSS1 vs PSS2 contributions specifically; PSS2 protein could not be probed; single lab\",\n      \"pmids\": [\"17387686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Over-expression of PSS2 (or PSS1) did not alter the PS level or the effect of DHA on PS increase in either neuronal or non-neuronal cells, and DHA enrichment did not affect PSS2 mRNA levels.\",\n      \"method\": \"Overexpression of PSS1 and PSS2 in Neuro 2A and non-neuronal cell lines; DHA enrichment; lipid mass spectrometry; RT-PCR\",\n      \"journal\": \"Journal of molecular neuroscience : MN\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct overexpression experiment with phospholipid quantification and mRNA analysis; negative result for PSS2 overexpression on PS levels robustly established\",\n      \"pmids\": [\"17901548\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PTDSS2 (PSS2) is a phosphatidylserine synthase localized to mitochondria-associated membranes (MAM) that catalyzes the base-exchange of serine for the head group of phosphatidylethanolamine (but not phosphatidylcholine) to synthesize phosphatidylserine; its activity is subject to direct feedback inhibition by the PS product, it exhibits preferential activity toward DHA-containing PE substrates, it accounts for the majority of serine exchange activity in testis, brain, and liver in vivo, and it contributes to nonapoptotic PS exposure at the plasma membrane with consequences for immune evasion and viral budding, while being dispensable for PS externalization during apoptosis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PTDSS2 (PSS2) is a phosphatidylserine synthase that produces phosphatidylserine (PS) through a serine base-exchange reaction, contributing to membrane lipid composition with downstream consequences for fertility, immune evasion, and viral budding [#1, #3]. The purified enzyme converts phosphatidylethanolamine (PE), but not phosphatidylcholine, into PS, and is directly inhibited by its PS product—a feedback regulation that distinguishes it from PSS1 [#1, #2]. PTDSS2 displays marked substrate selectivity, preferring DHA (22:6n-3) at the sn-2 position of PE and thereby preferentially generating DHA-containing PS, and it favors diacyl PE over PE plasmalogen [#5]. The enzyme localizes to mitochondria-associated membranes and is largely excluded from bulk endoplasmic reticulum [#0]. In vivo, PTDSS2 accounts for the great majority of serine-exchange activity in testis, brain, and liver, yet its loss leaves bulk phospholipid content unchanged and does not trigger compensatory PSS1 up-regulation; knockout mice show reduced testis weight and male infertility, consistent with high expression in Sertoli cells [#3, #4]. PTDSS2-dependent PS synthesis is dispensable for PS externalization during apoptosis [#6] but supports nonapoptotic PS exposure at the plasma membrane that promotes immunosuppression and breast cancer metastasis and is required for efficient Ebola virus budding [#7, #8]. A gain-of-function R97K mutant, analogous to Lenz-Majewski syndrome PSS1 mutations, impairs osteoclast formation and remodels cellular phospholipid composition [#9].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Established the core catalytic identity of PTDSS2 by showing it uses PE for serine base-exchange and, unlike PSS1, is subject to product feedback inhibition, defining a distinct regulatory mode for this synthase.\",\n      \"evidence\": \"Stable cDNA expression in hepatoma and CHO cells with in vitro serine-exchange assays, metabolic labeling, and ethanolamine auxotrophy rescue\",\n      \"pmids\": [\"10432300\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Substrate preference among PE species not yet resolved\", \"Structural basis of product inhibition unknown\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Resolved where PTDSS2 acts within the cell, placing it at mitochondria-associated membranes rather than bulk ER, situating PS synthesis at an organelle contact site.\",\n      \"evidence\": \"Subcellular fractionation and immunoblotting of endogenous and recombinant PSS2 in CHO-K1 and McArdle cells\",\n      \"pmids\": [\"10938271\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of MAM targeting not defined\", \"Functional consequence of MAM localization for PS trafficking unaddressed\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrated the in vivo contribution of PTDSS2 to tissue serine-exchange activity and its physiological role, showing it accounts for most activity in testis, brain, and liver and that its loss impairs male fertility without altering bulk phospholipid content.\",\n      \"evidence\": \"Pss2 knockout mouse with tissue serine-exchange assays, phospholipid analysis, beta-galactosidase reporter localization, and fertility assessment\",\n      \"pmids\": [\"12361952\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why phospholipid content is buffered despite >90% activity loss is unexplained\", \"Mechanistic link between PS synthesis and Sertoli cell/fertility function not established\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Tested whether PS synthases drive apoptotic PS externalization and showed they do not, dissociating the bulk PS biosynthetic machinery from the canonical apoptotic 'eat-me' signal.\",\n      \"evidence\": \"Staurosporine-induced apoptosis in PSS1- and/or PSS2-deficient CHO cells with annexin V flow cytometry and DNA fragmentation\",\n      \"pmids\": [\"14984733\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Source of PS externalized during apoptosis not identified\", \"Does not address nonapoptotic PS exposure\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Confirmed substrate specificity and product inhibition directly with purified human enzyme, removing ambiguity from cell-based assays.\",\n      \"evidence\": \"Purified epitope-tagged human PSS2 in in vitro assays with defined substrates and exogenous PS\",\n      \"pmids\": [\"19014349\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetic parameters and inhibition constants not detailed here\", \"No structural model of substrate/product binding\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Defined acyl-chain selectivity, showing PTDSS2 preferentially uses DHA-containing PE and diacyl over plasmalogen PE, explaining how it shapes the DHA-rich PS pool.\",\n      \"evidence\": \"Immunopurified Flag-tagged PSS2 from HEK cells assayed with defined substrate species, plus mutant cell microsomes\",\n      \"pmids\": [\"23071296\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural determinant of DHA preference unknown\", \"In vivo relevance of acyl preference to specific tissues not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Linked PTDSS2-generated PS to viral egress by showing combined PTDSS1/PTDSS2 knockdown blocks Ebola virus budding at the plasma membrane.\",\n      \"evidence\": \"siRNA co-knockdown of PTDSS1 and PTDSS2 with viral production assay and electron microscopy\",\n      \"pmids\": [\"30289506\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"PTDSS1 and PTDSS2 knocked down together, so individual PTDSS2 contribution not isolated\", \"Single lab, single virus\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Connected PTDSS2 expression to nonapoptotic surface PS exposure that drives tumor immune evasion and metastasis, and placed it downstream of BRCA1 regulation.\",\n      \"evidence\": \"Expression manipulation, PS externalization assays, in vivo mouse metastasis model, and tumor microenvironment analysis\",\n      \"pmids\": [\"35025764\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which BRCA1 represses PTDSS2 not defined\", \"Single lab; direct causality of surface PS in immunosuppression partly correlative\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Showed a gain-of-function PSS2 mutation analogous to Lenz-Majewski PSS1 mutations remodels cellular phospholipid composition and impairs osteoclast formation, implicating PTDSS2 activity in bone cell biology.\",\n      \"evidence\": \"Expression of PSS2 R97K gain-of-function mutant in osteoclast precursors with differentiation assays, podosome imaging, and lipidomics; catalytically inactive control\",\n      \"pmids\": [\"37714410\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No human disease attributable to PTDSS2 mutation demonstrated\", \"Mechanism linking lipid changes to osteoclast defect unresolved\", \"Single lab\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How PTDSS2-derived PS is selectively trafficked from MAM to the plasma membrane outer leaflet, and what controls its tissue-specific physiological versus disease-associated roles, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No defined PS transport pathway from MAM to plasma membrane in the corpus\", \"No structural model of the enzyme\", \"Regulators beyond product inhibition and BRCA1 largely uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [1, 2, 5]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 2, 3]}\n    ],\n    \"complexes\": [],\n    \"partners\": [],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}