{"gene":"PITPNM1","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2025,"finding":"Crystal structures of Nir2 C-terminal domains reveal the molecular mechanism of PA sensing: the LNS2 domain binds phosphatidic acid via hydrogen bonds involving residues S1025, T1065, K1103, and K1126 to the PA phosphate headgroup; a salt bridge between E355 in Nir2 and R55 in VAPB is essential for the FFAT-VAPB interaction; and the DDHD domain forms a twofold symmetric dimer that contributes to stable membrane association.","method":"X-ray crystallography (three crystal structures: LNS2-PA, FFAT-VAPB, DDHD domain) with site-specific residue identification","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures with atomic-resolution identification of interacting residues, multiple orthogonal structural domains resolved in one study","pmids":["41129229"],"is_preprint":false},{"year":2015,"finding":"Nir2 functions as a bidirectional lipid exchanger at ER-PM contact sites: it transfers phosphatidylinositol (PtdIns) from the ER to the PM and phosphatidic acid (PtdOH) from the PM to the ER. In Nir2-depleted cells, PLC activation causes PtdOH accumulation at the PM and severely impairs PtdIns synthesis, leading to loss of PLC-coupled receptor signaling competence.","method":"RNAi knockdown, lipid reporter assays, live-cell imaging of PtdIns and PtdOH at ER-PM contact sites","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (knockdown, lipid reporters, live imaging), replicated in subsequent reviews and structural work","pmids":["26028218"],"is_preprint":false},{"year":2013,"finding":"Nir2 is recruited to ER-PM junctions following receptor-induced Ca2+ signaling in a process facilitated by E-Syt1. At ER-PM junctions, Nir2 replenishes PM PIP2 after receptor-induced hydrolysis via its PITP activity, providing a feedback mechanism for sustained Ca2+ signaling.","method":"Genetically encoded ER-PM junction marker, live-cell imaging, siRNA knockdown of Nir2 and E-Syt1, PIP2 reporter assays","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetic reporters, knockdown), replicated by independent labs","pmids":["24183667"],"is_preprint":false},{"year":2015,"finding":"Nir2 detects PM PIP2 hydrolysis and translocates to ER-PM junctions via binding to phosphatidic acid through its C-terminal region. ER-resident PI is required for rapid PIP2 replenishment by Nir2. Nir2 and its homolog Nir3 differentially regulate PIP2 homeostasis: Nir2 responds to intense receptor stimulation while Nir3 maintains resting-state PIP2 levels, based on their distinct PA-binding and PI transfer activities.","method":"Fluorescence live-cell imaging of translocation, PA-binding assays, siRNA knockdown, PIP2 reporter assays, domain mutation analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (live imaging, biochemical binding assays, knockdown, reporters) in a single focused study, mechanistically consistent with structural data","pmids":["25887399"],"is_preprint":false},{"year":2016,"finding":"In quiescent cells, Nir2 localizes to the ER via interaction of its FFAT domain with ER-bound VAP-A and VAP-B. After PLC activation, Nir2 also binds to the PM via its C-terminal domains interacting with DAG and PtdOH, enabling function at ER-PM contact zones. VAP-B mutations found in familial ALS impair Nir2 binding.","method":"Domain mapping, FFAT-VAP interaction assays, co-immunoprecipitation, lipid-binding assays, knockdown","journal":"Biochemical Society transactions","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical interaction mapping, single lab review consolidating prior experimental findings, moderate method rigor","pmids":["26862206"],"is_preprint":false},{"year":2013,"finding":"Nir2 translocates from the Golgi complex to the plasma membrane in response to growth factor stimulation. This translocation is triggered by phosphatidic acid (PA) formation and mediated by the C-terminal region of Nir2, which binds PA in vitro. Nir2 depletion substantially reduces PM PI(4,5)P2 levels and growth factor-stimulated PI(3,4,5)P3 production, and attenuates MAPK and PI3K/AKT pathway activation.","method":"PA-binding assay (in vitro), siRNA knockdown, PIP2/PIP3 reporter imaging, western blotting of pathway components","journal":"EMBO reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding plus live-cell reporters, single lab, two orthogonal methods","pmids":["23897088"],"is_preprint":false},{"year":2005,"finding":"Nir2 is a peripheral Golgi protein essential for maintaining DAG levels in the Golgi apparatus. RNAi-mediated depletion of Nir2 inhibits protein transport from the trans-Golgi network to the plasma membrane and reduces Golgi DAG levels. Inactivation of the CDP-choline pathway for phosphatidylcholine biosynthesis restores both effects, indicating that Nir2 maintains the Golgi DAG pool by regulating its consumption via the CDP-choline pathway.","method":"RNAi knockdown, DAG reporter assays, protein transport assays, genetic epistasis (CDP-choline pathway inhibition rescue)","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — epistasis experiment (pathway rescue) combined with RNAi and transport assays in a single focused study","pmids":["15723057"],"is_preprint":false},{"year":2004,"finding":"At the onset of mitosis, Cdk1 phosphorylates Nir2 at multiple sites, with S382 as the most prominent. This phosphorylation facilitates Nir2 dissociation from the Golgi apparatus. Phospho-Nir2(pS382) localizes to the cleavage furrow and midbody during cytokinesis. The mitotic phosphorylation of Nir2 creates a docking site for the Polo box domain of Plk1, and overexpression of a Nir2 mutant that cannot interact with Plk1 affects cytokinesis completion.","method":"In vitro kinase assay (Cdk1), mass spectrometry of phosphorylation sites, immunolocalization, Co-IP of Nir2 and Plk1, overexpression of Nir2 phospho-mutants, cytokinesis completion assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — in vitro kinase assay, MS-identified phosphorylation sites, Co-IP, and functional mutant analysis in one focused study","pmids":["15125835"],"is_preprint":false},{"year":2002,"finding":"Nir2 is essential for cytokinesis: microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, resulting in multinucleate cells. Nir2 translocates from the Golgi to the cleavage furrow and midbody during cytokinesis, where it co-localizes and associates with RhoA. An N-terminally truncated Nir2 mutant causes cleavage furrow regression. The Rho-inhibitory domain (Rid) of Nir2 causes aberrant ingression and ectopic cleavage sites.","method":"Antibody microinjection, immunolocalization, Co-IP with RhoA in mitotic cells, overexpression of truncation mutants, time-lapse videomicroscopy","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — antibody microinjection loss-of-function, Co-IP, live imaging, and domain mutants in one study with multiple orthogonal approaches","pmids":["12077336"],"is_preprint":false},{"year":2002,"finding":"Nir2 contains a Rho-inhibitory domain (Rid) in its N-terminal region that inhibits Rho-mediated stress fiber formation and LPA-induced RhoA activation. Biochemical studies showed that Nir2, via Rid, preferentially binds the inactive GDP-bound form of RhoA. Overexpression of Nir2 attenuates Rho-mediated neurite retraction in neuronal cells; microinjection of anti-Nir2 antibodies attenuates neurite extension.","method":"Biochemical pulldown (GDP- vs GTP-RhoA), antibody microinjection, overexpression of Rid domain, actin staining","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical binding assay and functional microinjection, single lab","pmids":["11909959"],"is_preprint":false},{"year":2002,"finding":"A T59E mutation in the PI-transfer domain of Nir2 (analogous to the dominant retinal degeneration mutation in Drosophila RdgB) targets Nir2 to lipid droplets. Wild-type Nir2 translocates to lipid droplets upon oleic acid treatment, an effect dependent on threonine phosphorylation within the PI-transfer domain; the T59A mutant does not translocate. A truncated Nir2T59E containing only the PI-transfer domain is sufficient for lipid droplet targeting.","method":"Site-directed mutagenesis, fluorescence microscopy with lipophilic dye Nile red, oleic acid treatment, phosphorylation analysis","journal":"Current biology : CB","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis with imaging and lipid treatment, single lab, focused mechanistic study","pmids":["12225667"],"is_preprint":false},{"year":2019,"finding":"Nir2 acts as an effector of ER-resident VAP proteins (VAPA and VAPB) to support hepatitis C virus replication. Nir2 replenishes phosphoinositides at the HCV replication organelle to maintain elevated steady-state PI(4)P levels, which are removed by OSBP. Nir2 interacts with VAPs and completes a phosphoinositide cycle (PI/PA exchange) between the ER and viral replication organelles.","method":"siRNA knockdown, Co-IP of Nir2 with VAPs, PI(4)P reporter assays, HCV replication assays","journal":"Journal of virology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP interaction data combined with knockdown and functional reporter assays, single lab","pmids":["31484747"],"is_preprint":false},{"year":2022,"finding":"Nir1 constitutively localizes at ER-PM junctions (unlike Nir2 and Nir3 which are dynamically recruited) and acts as a positive regulator of Nir2 recruitment to ER-PM junctions during receptor stimulation. Nir1 interacts with Nir2 via a region between the FFAT motif and the DDHD domain. Loss of Nir1 reduces Nir2 targeting to ER-PM junctions and impairs efficient PM PIP2 replenishment.","method":"Live-cell imaging, biochemical co-immunoprecipitation, domain mapping, siRNA knockdown, PIP2 reporter assays","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP with domain mapping plus live-cell functional assays, single lab","pmids":["35020418"],"is_preprint":false},{"year":2023,"finding":"PITPNM1 (Nir2) and PITPNM2 (Nir3) maintain PI(4,5)P2 homeostasis at phagocytic cups, thereby supporting actin contractility and phagosome sealing. CRISPR-Cas9 double knockout of Nir2 and Nir3 decreases PM PI(4,5)P2, impairs receptor-mediated phagocytosis (stalling at cup stage), reduces density of contractile actin rings, and causes repetitive abortive phagosome closure. Re-expression of either Nir2 or Nir3 rescued phagocytosis proportionally to PM PI(4,5)P2 restoration.","method":"CRISPR-Cas9 knockout, live-cell imaging, PI(4,5)P2 reporters, phagocytosis assays, actin dynamics imaging, rescue experiments","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 / Strong — CRISPR KO with rescue, multiple orthogonal functional readouts (PIP2, actin, phagocytosis), single focused study","pmids":["37376972"],"is_preprint":false},{"year":2025,"finding":"Nir2 knockdown in HUVECs inhibits angiogenic tube formation, reduces cell viability, proliferation, and migration, diminishes actin stress fibers, and decreases AKT and ERK signaling downstream of VEGF. Nir2 overexpression increases cell viability and shRNA-resistant Nir2 rescues knockdown effects. Co-immunoprecipitation and co-localization confirmed Nir2 interaction with VAPA; double knockdown of Nir2 and VAPA further inhibits angiogenesis.","method":"siRNA knockdown, overexpression rescue, Co-IP and co-localization (Nir2-VAPA), interactome (mass spectrometry), tube formation assay, cell viability/migration assays, AKT/ERK phosphorylation","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP interaction validated, knockdown with rescue, multiple functional readouts, single lab","pmids":["40010513"],"is_preprint":false},{"year":2014,"finding":"Nir2 enhances epithelial-mesenchymal transition (EMT) in mammary epithelial and breast cancer cells. Nir2 overexpression decreases epithelial markers and increases mesenchymal markers; shRNA silencing has opposite effects. Nir2 depletion attenuates growth factor-induced cell migration and invasion in vitro, and inhibits lung metastasis in animal models. EMT effects are mediated through the PI3K/AKT and ERK1/2 pathways.","method":"shRNA knockdown, overexpression, EMT marker immunoblotting, cell migration/invasion assays, in vivo lung metastasis model, pathway inhibitor assays","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple functional assays with in vivo validation, pathway attribution by inhibitors, single lab","pmids":["25179602"],"is_preprint":false},{"year":2013,"finding":"Pitpnm1 is expressed in inner hair cells of the organ of Corti from late embryonic stages to adulthood and transiently in outer hair cells during early postnatal stages. Pitpnm1 null mice showed no hearing defects, suggesting functional redundancy with Pitpnm2 and Pitpnm3.","method":"Expression analysis (in situ hybridization/immunostaining), Pitpnm1 knockout mouse, auditory brainstem response/DPOAE hearing tests","journal":"Neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — null mouse with defined hearing phenotype (negative result) and expression localization, single lab","pmids":["23820044"],"is_preprint":false},{"year":2020,"finding":"miR-490-5p targets PITPNM1 by binding its 3'-UTR and inhibiting its translation, as demonstrated by dual-luciferase reporter assay. PITPNM1 promotes hADSC chondrogenic differentiation and chondrocyte homeostasis via the PI3K/AKT signaling pathway; rescue experiments confirmed this miR-490-5p/PITPNM1/PI3K/AKT axis.","method":"Dual-luciferase reporter assay (3'-UTR binding), loss/gain-of-function experiments, PI3K/AKT pathway analysis, rescue experiments, in vivo DMM OA model","journal":"Frontiers in cell and developmental biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — luciferase reporter for direct targeting combined with rescue experiments and in vivo model, single lab","pmids":["33240879"],"is_preprint":false}],"current_model":"PITPNM1/Nir2 is a phosphatidylinositol (PI)/phosphatidic acid (PA) exchange protein that operates at ER-plasma membrane contact sites: its N-terminal PITP domain catalyzes PI transfer from ER to PM (replenishing PIP2 after PLC-mediated hydrolysis) while simultaneously transferring PA from PM back to the ER to sustain PI resynthesis; Nir2 is recruited to ER-PM junctions by binding PA via its C-terminal LNS2 domain (specific residues S1025, T1065, K1103, K1126) and is anchored to the ER through FFAT-VAPB interaction (E355-R55 salt bridge); its DDHD domain dimerizes to stabilize membrane association; in addition, Nir2 localizes to the Golgi during interphase to maintain DAG homeostasis for secretory trafficking, is phosphorylated by Cdk1 at S382 during mitosis to dissociate from the Golgi and dock Plk1 for cytokinesis completion, and regulates actin cytoskeleton organization through a Rho-inhibitory domain (Rid) that preferentially binds GDP-RhoA."},"narrative":{"mechanistic_narrative":"PITPNM1 (Nir2) is a bidirectional lipid-transfer protein that operates at membrane contact sites to sustain phosphoinositide homeostasis during receptor signaling [PMID:26028218, PMID:24183667]. Following PLC-mediated PIP2 hydrolysis at the plasma membrane, Nir2 transfers phosphatidylinositol from the ER to the PM while shuttling the resulting phosphatidic acid back to the ER, replenishing PM PIP2 and maintaining PLC-coupled receptor signaling competence [PMID:26028218, PMID:24183667]. Its recruitment to ER-PM junctions is driven by PA sensing through the C-terminal LNS2 domain, while anchoring to the ER occurs through an FFAT motif that binds the ER-resident proteins VAPA and VAPB; crystallographic analysis defines PA-coordinating LNS2 residues (S1025, T1065, K1103, K1126), an E355-R55 salt bridge mediating the Nir2-VAPB interaction, and a DDHD domain that dimerizes to stabilize membrane association [PMID:41129229, PMID:26862206]. This lipid-exchange activity is deployed across multiple cellular contexts: Nir2 acts downstream of growth factor stimulation to support PIP2/PIP3 production and MAPK and PI3K/AKT activation [PMID:23897088], maintains PI(4,5)P2 at phagocytic cups to support actin-driven phagosome closure [PMID:37376972], and is co-opted by hepatitis C virus to replenish phosphoinositides at viral replication organelles [PMID:31484747]. Separately, Nir2 is a peripheral Golgi protein that maintains the Golgi DAG pool by regulating its consumption through the CDP-choline pathway, an activity required for trans-Golgi-to-PM protein transport [PMID:15723057]. During mitosis, Cdk1 phosphorylates Nir2 at S382 to release it from the Golgi and create a docking site for the Polo-box domain of Plk1, and Nir2 relocalizes to the cleavage furrow and midbody where it associates with RhoA to drive cytokinesis completion [PMID:15125835, PMID:12077336]. Through an N-terminal Rho-inhibitory domain (Rid) that preferentially binds GDP-bound RhoA, Nir2 also regulates actin stress-fiber formation and Rho-dependent cytoskeletal remodeling [PMID:11909959].","teleology":[{"year":2002,"claim":"Established that Nir2 is required for cytokinesis and links lipid metabolism to cytoskeletal control by acting on RhoA, addressing how a Golgi protein influences cell division.","evidence":"antibody microinjection, immunolocalization to furrow/midbody, Co-IP with RhoA, and truncation mutants with live imaging; plus biochemical demonstration that the N-terminal Rid domain preferentially binds GDP-RhoA and inhibits stress-fiber formation","pmids":["12077336","11909959"],"confidence":"High","gaps":["Molecular mechanism by which Rid discriminates GDP- vs GTP-RhoA not structurally defined","Relationship between lipid-transfer activity and Rho regulation unresolved"]},{"year":2002,"claim":"Showed the PI-transfer domain can be redirected to lipid droplets by phosphorylation, connecting Nir2 lipid-handling to a Drosophila RdgB-equivalent retinal degeneration mutation.","evidence":"site-directed mutagenesis (T59E/T59A), Nile red imaging, oleic acid treatment, phosphorylation analysis","pmids":["12225667"],"confidence":"Medium","gaps":["Physiological trigger and significance of lipid-droplet targeting in mammalian cells unclear","Kinase responsible for the regulatory threonine phosphorylation not identified"]},{"year":2004,"claim":"Defined a mitotic switch: Cdk1 phosphorylation of Nir2 at S382 releases it from the Golgi and recruits Plk1, explaining how Nir2 is mobilized for cytokinesis.","evidence":"in vitro Cdk1 kinase assay, MS phosphosite mapping, immunolocalization, Nir2-Plk1 Co-IP, phospho-mutant cytokinesis assay","pmids":["15125835"],"confidence":"High","gaps":["How Plk1 docking translates into cytokinesis completion mechanistically not resolved","Whether lipid-transfer activity is required at the furrow not tested"]},{"year":2005,"claim":"Identified the Golgi DAG-maintenance function and its mechanism, showing Nir2 regulates DAG consumption via the CDP-choline pathway to permit secretory transport.","evidence":"RNAi knockdown, DAG reporters, TGN-to-PM transport assays, CDP-choline pathway inhibition rescue (epistasis)","pmids":["15723057"],"confidence":"High","gaps":["Direct enzymatic versus indirect regulatory role of Nir2 in DAG metabolism not distinguished","Link between Golgi DAG pool and specific cargo classes not defined"]},{"year":2013,"claim":"Connected Nir2 to growth-factor signaling, showing PA-triggered Golgi-to-PM translocation supports PIP2/PIP3 production and downstream MAPK and PI3K/AKT activation.","evidence":"in vitro PA-binding assay, siRNA knockdown, PIP2/PIP3 reporter imaging, pathway immunoblotting","pmids":["23897088"],"confidence":"Medium","gaps":["Single-lab, no reciprocal in vivo confirmation of translocation route","Quantitative contribution of Nir2 to PIP3 pool versus other PI sources unclear"]},{"year":2013,"claim":"Placed Nir2 in a feedback loop sustaining receptor-driven Ca2+ signaling by replenishing PM PIP2 at E-Syt1-facilitated ER-PM junctions.","evidence":"genetically encoded ER-PM junction marker, live imaging, siRNA of Nir2 and E-Syt1, PIP2 reporters","pmids":["24183667"],"confidence":"High","gaps":["Precise role of E-Syt1 in Nir2 recruitment mechanistically incomplete","Stoichiometry of Nir2 at junctions not quantified"]},{"year":2015,"claim":"Resolved the core biochemical activity: Nir2 is a bidirectional PI/PA exchanger at ER-PM contacts whose loss causes PA accumulation and PI synthesis failure after PLC activation.","evidence":"RNAi, lipid reporter assays, live-cell imaging of PtdIns and PtdOH at junctions; plus PA-binding/PI-transfer domain mutation analysis distinguishing Nir2 from Nir3","pmids":["26028218","25887399"],"confidence":"High","gaps":["Kinetics of in-membrane lipid extraction/insertion not directly measured","How PI and PA transfer are coordinated within a single protein not resolved at this stage"]},{"year":2016,"claim":"Mapped the ER-anchoring mechanism, showing the FFAT motif binds VAP-A/VAP-B and that ALS-associated VAP-B mutations impair Nir2 binding.","evidence":"domain mapping, FFAT-VAP interaction assays, Co-IP, lipid-binding assays, knockdown","pmids":["26862206"],"confidence":"Medium","gaps":["Review-consolidated biochemistry; quantitative affinities not established here","Functional consequence of ALS VAP-B mutations on Nir2-dependent lipid transfer not directly tested"]},{"year":2019,"claim":"Demonstrated viral hijacking of Nir2, showing it acts as a VAP effector replenishing phosphoinositides at HCV replication organelles within a PI/PA cycle.","evidence":"siRNA knockdown, Nir2-VAP Co-IP, PI(4)P reporters, HCV replication assays","pmids":["31484747"],"confidence":"Medium","gaps":["Whether Nir2 acts directly at replication organelles or via redistributed ER lipid not separated","Single-lab Co-IP without structural validation of the VAP-Nir2-OSBP relay"]},{"year":2022,"claim":"Identified Nir1 as a positive regulator of Nir2 recruitment, refining the model of how dynamically recruited Nir2 is targeted to constitutively junction-resident scaffolds.","evidence":"live imaging, reciprocal Co-IP, domain mapping (region between FFAT and DDHD), siRNA, PIP2 reporters","pmids":["35020418"],"confidence":"Medium","gaps":["Structural basis of Nir1-Nir2 interaction not resolved","Whether Nir1 alters Nir2 catalytic activity or only localization unclear"]},{"year":2023,"claim":"Extended Nir2 function to phagocytosis, showing it (redundantly with Nir3) sustains PI(4,5)P2 at phagocytic cups to enable actin-driven phagosome sealing.","evidence":"CRISPR-Cas9 double knockout, live imaging, PI(4,5)P2 reporters, phagocytosis and actin-ring assays, single-gene rescue","pmids":["37376972"],"confidence":"High","gaps":["Functional overlap with Nir3 prevents isolating Nir2-specific contribution","Link between local PIP2 and contractile actin ring assembly not mechanistically dissected"]},{"year":2025,"claim":"Provided atomic-resolution mechanism for PA sensing, ER anchoring, and membrane association via three crystal structures of the C-terminal domains.","evidence":"X-ray crystallography (LNS2-PA, FFAT-VAPB, DDHD dimer) with site-specific residue identification (S1025/T1065/K1103/K1126; E355-R55; DDHD dimer)","pmids":["41129229"],"confidence":"High","gaps":["Full-length architecture and coupling between PITP and C-terminal domains not resolved","Structure of the lipid-loaded PITP transfer cycle not determined"]},{"year":2025,"claim":"Linked Nir2 to angiogenesis through VAPA interaction and VEGF-driven AKT/ERK signaling and actin stress-fiber maintenance.","evidence":"siRNA knockdown with shRNA-resistant rescue, Nir2-VAPA Co-IP and co-localization, interactome MS, tube-formation/viability/migration assays, AKT/ERK immunoblotting in HUVECs","pmids":["40010513"],"confidence":"Medium","gaps":["Whether angiogenic effects depend on lipid-transfer activity not tested","Single-lab functional study without in vivo angiogenesis validation"]},{"year":null,"claim":"How Nir2's distinct activities — PI/PA exchange, Golgi DAG maintenance, RhoA regulation, and mitotic Plk1 docking — are integrated within one protein and switched between contexts remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No full-length structure coupling N-terminal PITP/Rid and C-terminal LNS2/FFAT/DDHD domains","Causal connection between lipid-transfer function and Rho-dependent cytoskeletal/cytokinesis roles untested","Physiological in vivo phenotype obscured by redundancy with Nir1/Nir3"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,1,3,5]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[1,3]},{"term_id":"GO:0140299","term_label":"molecular sensor activity","supporting_discovery_ids":[0,3]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[9]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,4]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[1,4]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,2,5]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[6,7,8]},{"term_id":"GO:0005811","term_label":"lipid droplet","supporting_discovery_ids":[10]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,5]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[1,6]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[7,8]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[6]}],"complexes":[],"partners":["VAPA","VAPB","RHOA","PLK1","PITPNM1L","PITPNM2"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O00562","full_name":"Membrane-associated phosphatidylinositol transfer protein 1","aliases":["Drosophila retinal degeneration B homolog","Phosphatidylinositol transfer protein, membrane-associated 1","PITPnm 1","Pyk2 N-terminal domain-interacting receptor 2","NIR-2"],"length_aa":1244,"mass_kda":134.8,"function":"Catalyzes the transfer of phosphatidylinositol (PI) between membranes (PubMed:10531358, PubMed:22822086). Binds PI, phosphatidylcholine (PC) and phosphatidic acid (PA) with the binding affinity order of PI > PA > PC (PubMed:22822086). Regulates RHOA activity, and plays a role in cytoskeleton remodeling (PubMed:11909959). Necessary for normal completion of cytokinesis (PubMed:15125835). Plays a role in maintaining normal diacylglycerol levels in the Golgi apparatus (PubMed:15723057). Necessary for maintaining the normal structure of the endoplasmic reticulum and the Golgi apparatus (PubMed:15545272). Required for protein export from the endoplasmic reticulum and the Golgi (PubMed:15723057). Binds calcium ions (PubMed:10022914)","subcellular_location":"Cytoplasm; Golgi apparatus, Golgi stack membrane; Endoplasmic reticulum membrane; Lipid droplet; Cleavage furrow; Midbody","url":"https://www.uniprot.org/uniprotkb/O00562/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PITPNM1","classification":"Not Classified","n_dependent_lines":38,"n_total_lines":1208,"dependency_fraction":0.03145695364238411},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"VAPA","stoichiometry":0.2},{"gene":"VAPB","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PITPNM1","total_profiled":1310},"omim":[{"mim_id":"611606","title":"MICRO RNA 96; MIR96","url":"https://www.omim.org/entry/611606"},{"mim_id":"608794","title":"PHOSPHATIDYLINOSITOL TRANSFER PROTEIN, MEMBRANE-ASSOCIATED, 1; PITPNM1","url":"https://www.omim.org/entry/608794"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Vesicles","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PITPNM1"},"hgnc":{"alias_symbol":["DRES9","NIR2","RDGB1","RDGBA1","Rd9","RDGB"],"prev_symbol":["PITPNM"]},"alphafold":{"accession":"O00562","domains":[{"cath_id":"3.30.530.20","chopping":"3-258","consensus_level":"high","plddt":87.52,"start":3,"end":258},{"cath_id":"2.60.40.10","chopping":"895-1013","consensus_level":"medium","plddt":87.1039,"start":895,"end":1013},{"cath_id":"3.40.50.1000","chopping":"1019-1174","consensus_level":"medium","plddt":86.6798,"start":1019,"end":1174},{"cath_id":"3.40.50","chopping":"429-497_516-580_686-783_828-881","consensus_level":"high","plddt":87.5249,"start":429,"end":881}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O00562","model_url":"https://alphafold.ebi.ac.uk/files/AF-O00562-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O00562-F1-predicted_aligned_error_v6.png","plddt_mean":69.94},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PITPNM1","jax_strain_url":"https://www.jax.org/strain/search?query=PITPNM1"},"sequence":{"accession":"O00562","fasta_url":"https://rest.uniprot.org/uniprotkb/O00562.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O00562/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O00562"}},"corpus_meta":[{"pmid":"24183667","id":"PMC_24183667","title":"Feedback regulation of receptor-induced Ca2+ signaling mediated by E-Syt1 and Nir2 at endoplasmic reticulum-plasma membrane junctions.","date":"2013","source":"Cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/24183667","citation_count":300,"is_preprint":false},{"pmid":"26028218","id":"PMC_26028218","title":"Phosphatidylinositol-Phosphatidic Acid Exchange by Nir2 at ER-PM Contact Sites Maintains Phosphoinositide Signaling Competence.","date":"2015","source":"Developmental cell","url":"https://pubmed.ncbi.nlm.nih.gov/26028218","citation_count":200,"is_preprint":false},{"pmid":"15723057","id":"PMC_15723057","title":"Maintenance of the diacylglycerol level in the Golgi apparatus by the Nir2 protein is critical for Golgi secretory function.","date":"2005","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/15723057","citation_count":146,"is_preprint":false},{"pmid":"25887399","id":"PMC_25887399","title":"Phosphatidylinositol 4,5-Bisphosphate Homeostasis Regulated by Nir2 and Nir3 Proteins at Endoplasmic Reticulum-Plasma Membrane Junctions.","date":"2015","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/25887399","citation_count":118,"is_preprint":false},{"pmid":"23897088","id":"PMC_23897088","title":"The phosphatidylinositol-transfer protein Nir2 binds phosphatidic acid and positively regulates phosphoinositide signalling.","date":"2013","source":"EMBO reports","url":"https://pubmed.ncbi.nlm.nih.gov/23897088","citation_count":114,"is_preprint":false},{"pmid":"15125835","id":"PMC_15125835","title":"Mitotic phosphorylation of the peripheral Golgi protein Nir2 by Cdk1 provides a docking mechanism for Plk1 and affects cytokinesis completion.","date":"2004","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/15125835","citation_count":77,"is_preprint":false},{"pmid":"22563472","id":"PMC_22563472","title":"Rd9 is a naturally occurring mouse model of a common form of retinitis pigmentosa caused by mutations in RPGR-ORF15.","date":"2012","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/22563472","citation_count":72,"is_preprint":false},{"pmid":"12225667","id":"PMC_12225667","title":"Targeting of Nir2 to lipid droplets is regulated by a specific threonine residue within its PI-transfer domain.","date":"2002","source":"Current biology : CB","url":"https://pubmed.ncbi.nlm.nih.gov/12225667","citation_count":47,"is_preprint":false},{"pmid":"12077336","id":"PMC_12077336","title":"Nir2, a human homolog of Drosophila melanogaster retinal degeneration B protein, is essential for cytokinesis.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12077336","citation_count":39,"is_preprint":false},{"pmid":"31484747","id":"PMC_31484747","title":"Nir2 Is an Effector of VAPs Necessary for Efficient Hepatitis C Virus Replication and Phosphatidylinositol 4-Phosphate Enrichment at the Viral Replication Organelle.","date":"2019","source":"Journal of virology","url":"https://pubmed.ncbi.nlm.nih.gov/31484747","citation_count":32,"is_preprint":false},{"pmid":"25179602","id":"PMC_25179602","title":"The lipid-transfer protein Nir2 enhances epithelial-mesenchymal transition and facilitates breast cancer metastasis.","date":"2014","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/25179602","citation_count":31,"is_preprint":false},{"pmid":"26862206","id":"PMC_26862206","title":"Phosphatidylinositol and phosphatidic acid transport between the ER and plasma membrane during PLC activation requires the Nir2 protein.","date":"2016","source":"Biochemical Society transactions","url":"https://pubmed.ncbi.nlm.nih.gov/26862206","citation_count":30,"is_preprint":false},{"pmid":"33240879","id":"PMC_33240879","title":"Inhibition of miR-490-5p Promotes Human Adipose-Derived Stem Cells Chondrogenesis and Protects Chondrocytes via the PITPNM1/PI3K/AKT Axis.","date":"2020","source":"Frontiers in cell and developmental biology","url":"https://pubmed.ncbi.nlm.nih.gov/33240879","citation_count":22,"is_preprint":false},{"pmid":"35020418","id":"PMC_35020418","title":"Nir1 constitutively localizes at ER-PM junctions and promotes Nir2 recruitment for PIP2 homeostasis.","date":"2022","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/35020418","citation_count":19,"is_preprint":false},{"pmid":"11909959","id":"PMC_11909959","title":"Nir2, a novel regulator of cell morphogenesis.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/11909959","citation_count":19,"is_preprint":false},{"pmid":"30280954","id":"PMC_30280954","title":"Toxicology and Pharmacology of an AAV Vector Expressing Codon-Optimized RPGR in RPGR-Deficient Rd9 Mice.","date":"2018","source":"Human gene therapy. Clinical development","url":"https://pubmed.ncbi.nlm.nih.gov/30280954","citation_count":17,"is_preprint":false},{"pmid":"299746","id":"PMC_299746","title":"Inhibition of transformation and transfection in Haemophilus influenzae Rd9 by lysogeny.","date":"1977","source":"Journal of bacteriology","url":"https://pubmed.ncbi.nlm.nih.gov/299746","citation_count":17,"is_preprint":false},{"pmid":"27309477","id":"PMC_27309477","title":"Transforming Growth Factor Beta-Induced Factor 2-Linked X (TGIF2LX) Regulates Two Morphogenesis Genes, Nir1 and Nir2 in Human Colorectal.","date":"2016","source":"Acta medica Iranica","url":"https://pubmed.ncbi.nlm.nih.gov/27309477","citation_count":17,"is_preprint":false},{"pmid":"31607844","id":"PMC_31607844","title":"Retinal Phenotype in the rd9 Mutant Mouse, a Model of X-Linked RP.","date":"2019","source":"Frontiers in neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/31607844","citation_count":16,"is_preprint":false},{"pmid":"23820044","id":"PMC_23820044","title":"Pitpnm1 is expressed in hair cells during development but is not required for hearing.","date":"2013","source":"Neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/23820044","citation_count":11,"is_preprint":false},{"pmid":"37376972","id":"PMC_37376972","title":"The lipid transfer proteins Nir2 and Nir3 sustain phosphoinositide signaling and actin dynamics during phagocytosis.","date":"2023","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/37376972","citation_count":8,"is_preprint":false},{"pmid":"34572478","id":"PMC_34572478","title":"Attenuation of PITPNM1 Signaling Cascade Can Inhibit Breast Cancer Progression.","date":"2021","source":"Biomolecules","url":"https://pubmed.ncbi.nlm.nih.gov/34572478","citation_count":6,"is_preprint":false},{"pmid":"40010513","id":"PMC_40010513","title":"The role of Nir2, a lipid-transfer protein, in regulating endothelial cell functions.","date":"2025","source":"Biochimica et biophysica acta. Molecular cell research","url":"https://pubmed.ncbi.nlm.nih.gov/40010513","citation_count":2,"is_preprint":false},{"pmid":"41129229","id":"PMC_41129229","title":"Nir2 crystal structures reveal a phosphatidic acid-sensing mechanism at ER-PM contact sites.","date":"2025","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/41129229","citation_count":2,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":13763,"output_tokens":5181,"usd":0.059502,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":13383,"output_tokens":4916,"usd":0.094908,"stage2_stop_reason":"end_turn"},"total_usd":0.15441,"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\": 2025,\n      \"finding\": \"Crystal structures of Nir2 C-terminal domains reveal the molecular mechanism of PA sensing: the LNS2 domain binds phosphatidic acid via hydrogen bonds involving residues S1025, T1065, K1103, and K1126 to the PA phosphate headgroup; a salt bridge between E355 in Nir2 and R55 in VAPB is essential for the FFAT-VAPB interaction; and the DDHD domain forms a twofold symmetric dimer that contributes to stable membrane association.\",\n      \"method\": \"X-ray crystallography (three crystal structures: LNS2-PA, FFAT-VAPB, DDHD domain) with site-specific residue identification\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures with atomic-resolution identification of interacting residues, multiple orthogonal structural domains resolved in one study\",\n      \"pmids\": [\"41129229\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Nir2 functions as a bidirectional lipid exchanger at ER-PM contact sites: it transfers phosphatidylinositol (PtdIns) from the ER to the PM and phosphatidic acid (PtdOH) from the PM to the ER. In Nir2-depleted cells, PLC activation causes PtdOH accumulation at the PM and severely impairs PtdIns synthesis, leading to loss of PLC-coupled receptor signaling competence.\",\n      \"method\": \"RNAi knockdown, lipid reporter assays, live-cell imaging of PtdIns and PtdOH at ER-PM contact sites\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (knockdown, lipid reporters, live imaging), replicated in subsequent reviews and structural work\",\n      \"pmids\": [\"26028218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Nir2 is recruited to ER-PM junctions following receptor-induced Ca2+ signaling in a process facilitated by E-Syt1. At ER-PM junctions, Nir2 replenishes PM PIP2 after receptor-induced hydrolysis via its PITP activity, providing a feedback mechanism for sustained Ca2+ signaling.\",\n      \"method\": \"Genetically encoded ER-PM junction marker, live-cell imaging, siRNA knockdown of Nir2 and E-Syt1, PIP2 reporter assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetic reporters, knockdown), replicated by independent labs\",\n      \"pmids\": [\"24183667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Nir2 detects PM PIP2 hydrolysis and translocates to ER-PM junctions via binding to phosphatidic acid through its C-terminal region. ER-resident PI is required for rapid PIP2 replenishment by Nir2. Nir2 and its homolog Nir3 differentially regulate PIP2 homeostasis: Nir2 responds to intense receptor stimulation while Nir3 maintains resting-state PIP2 levels, based on their distinct PA-binding and PI transfer activities.\",\n      \"method\": \"Fluorescence live-cell imaging of translocation, PA-binding assays, siRNA knockdown, PIP2 reporter assays, domain mutation analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (live imaging, biochemical binding assays, knockdown, reporters) in a single focused study, mechanistically consistent with structural data\",\n      \"pmids\": [\"25887399\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In quiescent cells, Nir2 localizes to the ER via interaction of its FFAT domain with ER-bound VAP-A and VAP-B. After PLC activation, Nir2 also binds to the PM via its C-terminal domains interacting with DAG and PtdOH, enabling function at ER-PM contact zones. VAP-B mutations found in familial ALS impair Nir2 binding.\",\n      \"method\": \"Domain mapping, FFAT-VAP interaction assays, co-immunoprecipitation, lipid-binding assays, knockdown\",\n      \"journal\": \"Biochemical Society transactions\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical interaction mapping, single lab review consolidating prior experimental findings, moderate method rigor\",\n      \"pmids\": [\"26862206\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Nir2 translocates from the Golgi complex to the plasma membrane in response to growth factor stimulation. This translocation is triggered by phosphatidic acid (PA) formation and mediated by the C-terminal region of Nir2, which binds PA in vitro. Nir2 depletion substantially reduces PM PI(4,5)P2 levels and growth factor-stimulated PI(3,4,5)P3 production, and attenuates MAPK and PI3K/AKT pathway activation.\",\n      \"method\": \"PA-binding assay (in vitro), siRNA knockdown, PIP2/PIP3 reporter imaging, western blotting of pathway components\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding plus live-cell reporters, single lab, two orthogonal methods\",\n      \"pmids\": [\"23897088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Nir2 is a peripheral Golgi protein essential for maintaining DAG levels in the Golgi apparatus. RNAi-mediated depletion of Nir2 inhibits protein transport from the trans-Golgi network to the plasma membrane and reduces Golgi DAG levels. Inactivation of the CDP-choline pathway for phosphatidylcholine biosynthesis restores both effects, indicating that Nir2 maintains the Golgi DAG pool by regulating its consumption via the CDP-choline pathway.\",\n      \"method\": \"RNAi knockdown, DAG reporter assays, protein transport assays, genetic epistasis (CDP-choline pathway inhibition rescue)\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — epistasis experiment (pathway rescue) combined with RNAi and transport assays in a single focused study\",\n      \"pmids\": [\"15723057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"At the onset of mitosis, Cdk1 phosphorylates Nir2 at multiple sites, with S382 as the most prominent. This phosphorylation facilitates Nir2 dissociation from the Golgi apparatus. Phospho-Nir2(pS382) localizes to the cleavage furrow and midbody during cytokinesis. The mitotic phosphorylation of Nir2 creates a docking site for the Polo box domain of Plk1, and overexpression of a Nir2 mutant that cannot interact with Plk1 affects cytokinesis completion.\",\n      \"method\": \"In vitro kinase assay (Cdk1), mass spectrometry of phosphorylation sites, immunolocalization, Co-IP of Nir2 and Plk1, overexpression of Nir2 phospho-mutants, cytokinesis completion assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — in vitro kinase assay, MS-identified phosphorylation sites, Co-IP, and functional mutant analysis in one focused study\",\n      \"pmids\": [\"15125835\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Nir2 is essential for cytokinesis: microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, resulting in multinucleate cells. Nir2 translocates from the Golgi to the cleavage furrow and midbody during cytokinesis, where it co-localizes and associates with RhoA. An N-terminally truncated Nir2 mutant causes cleavage furrow regression. The Rho-inhibitory domain (Rid) of Nir2 causes aberrant ingression and ectopic cleavage sites.\",\n      \"method\": \"Antibody microinjection, immunolocalization, Co-IP with RhoA in mitotic cells, overexpression of truncation mutants, time-lapse videomicroscopy\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — antibody microinjection loss-of-function, Co-IP, live imaging, and domain mutants in one study with multiple orthogonal approaches\",\n      \"pmids\": [\"12077336\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Nir2 contains a Rho-inhibitory domain (Rid) in its N-terminal region that inhibits Rho-mediated stress fiber formation and LPA-induced RhoA activation. Biochemical studies showed that Nir2, via Rid, preferentially binds the inactive GDP-bound form of RhoA. Overexpression of Nir2 attenuates Rho-mediated neurite retraction in neuronal cells; microinjection of anti-Nir2 antibodies attenuates neurite extension.\",\n      \"method\": \"Biochemical pulldown (GDP- vs GTP-RhoA), antibody microinjection, overexpression of Rid domain, actin staining\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical binding assay and functional microinjection, single lab\",\n      \"pmids\": [\"11909959\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"A T59E mutation in the PI-transfer domain of Nir2 (analogous to the dominant retinal degeneration mutation in Drosophila RdgB) targets Nir2 to lipid droplets. Wild-type Nir2 translocates to lipid droplets upon oleic acid treatment, an effect dependent on threonine phosphorylation within the PI-transfer domain; the T59A mutant does not translocate. A truncated Nir2T59E containing only the PI-transfer domain is sufficient for lipid droplet targeting.\",\n      \"method\": \"Site-directed mutagenesis, fluorescence microscopy with lipophilic dye Nile red, oleic acid treatment, phosphorylation analysis\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis with imaging and lipid treatment, single lab, focused mechanistic study\",\n      \"pmids\": [\"12225667\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Nir2 acts as an effector of ER-resident VAP proteins (VAPA and VAPB) to support hepatitis C virus replication. Nir2 replenishes phosphoinositides at the HCV replication organelle to maintain elevated steady-state PI(4)P levels, which are removed by OSBP. Nir2 interacts with VAPs and completes a phosphoinositide cycle (PI/PA exchange) between the ER and viral replication organelles.\",\n      \"method\": \"siRNA knockdown, Co-IP of Nir2 with VAPs, PI(4)P reporter assays, HCV replication assays\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP interaction data combined with knockdown and functional reporter assays, single lab\",\n      \"pmids\": [\"31484747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Nir1 constitutively localizes at ER-PM junctions (unlike Nir2 and Nir3 which are dynamically recruited) and acts as a positive regulator of Nir2 recruitment to ER-PM junctions during receptor stimulation. Nir1 interacts with Nir2 via a region between the FFAT motif and the DDHD domain. Loss of Nir1 reduces Nir2 targeting to ER-PM junctions and impairs efficient PM PIP2 replenishment.\",\n      \"method\": \"Live-cell imaging, biochemical co-immunoprecipitation, domain mapping, siRNA knockdown, PIP2 reporter assays\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP with domain mapping plus live-cell functional assays, single lab\",\n      \"pmids\": [\"35020418\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PITPNM1 (Nir2) and PITPNM2 (Nir3) maintain PI(4,5)P2 homeostasis at phagocytic cups, thereby supporting actin contractility and phagosome sealing. CRISPR-Cas9 double knockout of Nir2 and Nir3 decreases PM PI(4,5)P2, impairs receptor-mediated phagocytosis (stalling at cup stage), reduces density of contractile actin rings, and causes repetitive abortive phagosome closure. Re-expression of either Nir2 or Nir3 rescued phagocytosis proportionally to PM PI(4,5)P2 restoration.\",\n      \"method\": \"CRISPR-Cas9 knockout, live-cell imaging, PI(4,5)P2 reporters, phagocytosis assays, actin dynamics imaging, rescue experiments\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — CRISPR KO with rescue, multiple orthogonal functional readouts (PIP2, actin, phagocytosis), single focused study\",\n      \"pmids\": [\"37376972\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Nir2 knockdown in HUVECs inhibits angiogenic tube formation, reduces cell viability, proliferation, and migration, diminishes actin stress fibers, and decreases AKT and ERK signaling downstream of VEGF. Nir2 overexpression increases cell viability and shRNA-resistant Nir2 rescues knockdown effects. Co-immunoprecipitation and co-localization confirmed Nir2 interaction with VAPA; double knockdown of Nir2 and VAPA further inhibits angiogenesis.\",\n      \"method\": \"siRNA knockdown, overexpression rescue, Co-IP and co-localization (Nir2-VAPA), interactome (mass spectrometry), tube formation assay, cell viability/migration assays, AKT/ERK phosphorylation\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP interaction validated, knockdown with rescue, multiple functional readouts, single lab\",\n      \"pmids\": [\"40010513\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Nir2 enhances epithelial-mesenchymal transition (EMT) in mammary epithelial and breast cancer cells. Nir2 overexpression decreases epithelial markers and increases mesenchymal markers; shRNA silencing has opposite effects. Nir2 depletion attenuates growth factor-induced cell migration and invasion in vitro, and inhibits lung metastasis in animal models. EMT effects are mediated through the PI3K/AKT and ERK1/2 pathways.\",\n      \"method\": \"shRNA knockdown, overexpression, EMT marker immunoblotting, cell migration/invasion assays, in vivo lung metastasis model, pathway inhibitor assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple functional assays with in vivo validation, pathway attribution by inhibitors, single lab\",\n      \"pmids\": [\"25179602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Pitpnm1 is expressed in inner hair cells of the organ of Corti from late embryonic stages to adulthood and transiently in outer hair cells during early postnatal stages. Pitpnm1 null mice showed no hearing defects, suggesting functional redundancy with Pitpnm2 and Pitpnm3.\",\n      \"method\": \"Expression analysis (in situ hybridization/immunostaining), Pitpnm1 knockout mouse, auditory brainstem response/DPOAE hearing tests\",\n      \"journal\": \"Neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — null mouse with defined hearing phenotype (negative result) and expression localization, single lab\",\n      \"pmids\": [\"23820044\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"miR-490-5p targets PITPNM1 by binding its 3'-UTR and inhibiting its translation, as demonstrated by dual-luciferase reporter assay. PITPNM1 promotes hADSC chondrogenic differentiation and chondrocyte homeostasis via the PI3K/AKT signaling pathway; rescue experiments confirmed this miR-490-5p/PITPNM1/PI3K/AKT axis.\",\n      \"method\": \"Dual-luciferase reporter assay (3'-UTR binding), loss/gain-of-function experiments, PI3K/AKT pathway analysis, rescue experiments, in vivo DMM OA model\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — luciferase reporter for direct targeting combined with rescue experiments and in vivo model, single lab\",\n      \"pmids\": [\"33240879\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PITPNM1/Nir2 is a phosphatidylinositol (PI)/phosphatidic acid (PA) exchange protein that operates at ER-plasma membrane contact sites: its N-terminal PITP domain catalyzes PI transfer from ER to PM (replenishing PIP2 after PLC-mediated hydrolysis) while simultaneously transferring PA from PM back to the ER to sustain PI resynthesis; Nir2 is recruited to ER-PM junctions by binding PA via its C-terminal LNS2 domain (specific residues S1025, T1065, K1103, K1126) and is anchored to the ER through FFAT-VAPB interaction (E355-R55 salt bridge); its DDHD domain dimerizes to stabilize membrane association; in addition, Nir2 localizes to the Golgi during interphase to maintain DAG homeostasis for secretory trafficking, is phosphorylated by Cdk1 at S382 during mitosis to dissociate from the Golgi and dock Plk1 for cytokinesis completion, and regulates actin cytoskeleton organization through a Rho-inhibitory domain (Rid) that preferentially binds GDP-RhoA.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PITPNM1 (Nir2) is a bidirectional lipid-transfer protein that operates at membrane contact sites to sustain phosphoinositide homeostasis during receptor signaling [#1, #2]. Following PLC-mediated PIP2 hydrolysis at the plasma membrane, Nir2 transfers phosphatidylinositol from the ER to the PM while shuttling the resulting phosphatidic acid back to the ER, replenishing PM PIP2 and maintaining PLC-coupled receptor signaling competence [#1, #2]. Its recruitment to ER-PM junctions is driven by PA sensing through the C-terminal LNS2 domain, while anchoring to the ER occurs through an FFAT motif that binds the ER-resident proteins VAPA and VAPB; crystallographic analysis defines PA-coordinating LNS2 residues (S1025, T1065, K1103, K1126), an E355-R55 salt bridge mediating the Nir2-VAPB interaction, and a DDHD domain that dimerizes to stabilize membrane association [#0, #4]. This lipid-exchange activity is deployed across multiple cellular contexts: Nir2 acts downstream of growth factor stimulation to support PIP2/PIP3 production and MAPK and PI3K/AKT activation [#5], maintains PI(4,5)P2 at phagocytic cups to support actin-driven phagosome closure [#13], and is co-opted by hepatitis C virus to replenish phosphoinositides at viral replication organelles [#11]. Separately, Nir2 is a peripheral Golgi protein that maintains the Golgi DAG pool by regulating its consumption through the CDP-choline pathway, an activity required for trans-Golgi-to-PM protein transport [#6]. During mitosis, Cdk1 phosphorylates Nir2 at S382 to release it from the Golgi and create a docking site for the Polo-box domain of Plk1, and Nir2 relocalizes to the cleavage furrow and midbody where it associates with RhoA to drive cytokinesis completion [#7, #8]. Through an N-terminal Rho-inhibitory domain (Rid) that preferentially binds GDP-bound RhoA, Nir2 also regulates actin stress-fiber formation and Rho-dependent cytoskeletal remodeling [#9].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that Nir2 is required for cytokinesis and links lipid metabolism to cytoskeletal control by acting on RhoA, addressing how a Golgi protein influences cell division.\",\n      \"evidence\": \"antibody microinjection, immunolocalization to furrow/midbody, Co-IP with RhoA, and truncation mutants with live imaging; plus biochemical demonstration that the N-terminal Rid domain preferentially binds GDP-RhoA and inhibits stress-fiber formation\",\n      \"pmids\": [\"12077336\", \"11909959\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism by which Rid discriminates GDP- vs GTP-RhoA not structurally defined\", \"Relationship between lipid-transfer activity and Rho regulation unresolved\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Showed the PI-transfer domain can be redirected to lipid droplets by phosphorylation, connecting Nir2 lipid-handling to a Drosophila RdgB-equivalent retinal degeneration mutation.\",\n      \"evidence\": \"site-directed mutagenesis (T59E/T59A), Nile red imaging, oleic acid treatment, phosphorylation analysis\",\n      \"pmids\": [\"12225667\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological trigger and significance of lipid-droplet targeting in mammalian cells unclear\", \"Kinase responsible for the regulatory threonine phosphorylation not identified\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Defined a mitotic switch: Cdk1 phosphorylation of Nir2 at S382 releases it from the Golgi and recruits Plk1, explaining how Nir2 is mobilized for cytokinesis.\",\n      \"evidence\": \"in vitro Cdk1 kinase assay, MS phosphosite mapping, immunolocalization, Nir2-Plk1 Co-IP, phospho-mutant cytokinesis assay\",\n      \"pmids\": [\"15125835\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How Plk1 docking translates into cytokinesis completion mechanistically not resolved\", \"Whether lipid-transfer activity is required at the furrow not tested\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified the Golgi DAG-maintenance function and its mechanism, showing Nir2 regulates DAG consumption via the CDP-choline pathway to permit secretory transport.\",\n      \"evidence\": \"RNAi knockdown, DAG reporters, TGN-to-PM transport assays, CDP-choline pathway inhibition rescue (epistasis)\",\n      \"pmids\": [\"15723057\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct enzymatic versus indirect regulatory role of Nir2 in DAG metabolism not distinguished\", \"Link between Golgi DAG pool and specific cargo classes not defined\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Connected Nir2 to growth-factor signaling, showing PA-triggered Golgi-to-PM translocation supports PIP2/PIP3 production and downstream MAPK and PI3K/AKT activation.\",\n      \"evidence\": \"in vitro PA-binding assay, siRNA knockdown, PIP2/PIP3 reporter imaging, pathway immunoblotting\",\n      \"pmids\": [\"23897088\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab, no reciprocal in vivo confirmation of translocation route\", \"Quantitative contribution of Nir2 to PIP3 pool versus other PI sources unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Placed Nir2 in a feedback loop sustaining receptor-driven Ca2+ signaling by replenishing PM PIP2 at E-Syt1-facilitated ER-PM junctions.\",\n      \"evidence\": \"genetically encoded ER-PM junction marker, live imaging, siRNA of Nir2 and E-Syt1, PIP2 reporters\",\n      \"pmids\": [\"24183667\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise role of E-Syt1 in Nir2 recruitment mechanistically incomplete\", \"Stoichiometry of Nir2 at junctions not quantified\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Resolved the core biochemical activity: Nir2 is a bidirectional PI/PA exchanger at ER-PM contacts whose loss causes PA accumulation and PI synthesis failure after PLC activation.\",\n      \"evidence\": \"RNAi, lipid reporter assays, live-cell imaging of PtdIns and PtdOH at junctions; plus PA-binding/PI-transfer domain mutation analysis distinguishing Nir2 from Nir3\",\n      \"pmids\": [\"26028218\", \"25887399\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetics of in-membrane lipid extraction/insertion not directly measured\", \"How PI and PA transfer are coordinated within a single protein not resolved at this stage\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Mapped the ER-anchoring mechanism, showing the FFAT motif binds VAP-A/VAP-B and that ALS-associated VAP-B mutations impair Nir2 binding.\",\n      \"evidence\": \"domain mapping, FFAT-VAP interaction assays, Co-IP, lipid-binding assays, knockdown\",\n      \"pmids\": [\"26862206\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Review-consolidated biochemistry; quantitative affinities not established here\", \"Functional consequence of ALS VAP-B mutations on Nir2-dependent lipid transfer not directly tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstrated viral hijacking of Nir2, showing it acts as a VAP effector replenishing phosphoinositides at HCV replication organelles within a PI/PA cycle.\",\n      \"evidence\": \"siRNA knockdown, Nir2-VAP Co-IP, PI(4)P reporters, HCV replication assays\",\n      \"pmids\": [\"31484747\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether Nir2 acts directly at replication organelles or via redistributed ER lipid not separated\", \"Single-lab Co-IP without structural validation of the VAP-Nir2-OSBP relay\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identified Nir1 as a positive regulator of Nir2 recruitment, refining the model of how dynamically recruited Nir2 is targeted to constitutively junction-resident scaffolds.\",\n      \"evidence\": \"live imaging, reciprocal Co-IP, domain mapping (region between FFAT and DDHD), siRNA, PIP2 reporters\",\n      \"pmids\": [\"35020418\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural basis of Nir1-Nir2 interaction not resolved\", \"Whether Nir1 alters Nir2 catalytic activity or only localization unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended Nir2 function to phagocytosis, showing it (redundantly with Nir3) sustains PI(4,5)P2 at phagocytic cups to enable actin-driven phagosome sealing.\",\n      \"evidence\": \"CRISPR-Cas9 double knockout, live imaging, PI(4,5)P2 reporters, phagocytosis and actin-ring assays, single-gene rescue\",\n      \"pmids\": [\"37376972\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional overlap with Nir3 prevents isolating Nir2-specific contribution\", \"Link between local PIP2 and contractile actin ring assembly not mechanistically dissected\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Provided atomic-resolution mechanism for PA sensing, ER anchoring, and membrane association via three crystal structures of the C-terminal domains.\",\n      \"evidence\": \"X-ray crystallography (LNS2-PA, FFAT-VAPB, DDHD dimer) with site-specific residue identification (S1025/T1065/K1103/K1126; E355-R55; DDHD dimer)\",\n      \"pmids\": [\"41129229\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length architecture and coupling between PITP and C-terminal domains not resolved\", \"Structure of the lipid-loaded PITP transfer cycle not determined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Linked Nir2 to angiogenesis through VAPA interaction and VEGF-driven AKT/ERK signaling and actin stress-fiber maintenance.\",\n      \"evidence\": \"siRNA knockdown with shRNA-resistant rescue, Nir2-VAPA Co-IP and co-localization, interactome MS, tube-formation/viability/migration assays, AKT/ERK immunoblotting in HUVECs\",\n      \"pmids\": [\"40010513\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether angiogenic effects depend on lipid-transfer activity not tested\", \"Single-lab functional study without in vivo angiogenesis validation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How Nir2's distinct activities — PI/PA exchange, Golgi DAG maintenance, RhoA regulation, and mitotic Plk1 docking — are integrated within one protein and switched between contexts remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No full-length structure coupling N-terminal PITP/Rid and C-terminal LNS2/FFAT/DDHD domains\", \"Causal connection between lipid-transfer function and Rho-dependent cytoskeletal/cytokinesis roles untested\", \"Physiological in vivo phenotype obscured by redundancy with Nir1/Nir3\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 1, 3, 5]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [1, 3]},\n      {\"term_id\": \"GO:0140299\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [9]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [1, 4]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 2, 5]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [6, 7, 8]},\n      {\"term_id\": \"GO:0005811\", \"supporting_discovery_ids\": [10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 5]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [7, 8]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"VAPA\", \"VAPB\", \"RHOA\", \"PLK1\", \"PITPNM1L\", \"PITPNM2\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}