{"gene":"PPP1R9A","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":2002,"finding":"Neurabin I (PPP1R9A) recruits active PP1 via a PP1-docking sequence (457)KIKF(460); mutation of this motif or pharmacological PP1 inhibition abolishes neurabin I-induced filopodium formation and restores stress fibers, establishing that the neurabin I/PP1 complex controls actin rearrangement and spine morphology.","method":"Immune complex phosphatase assays, GFP-fusion cell imaging, site-directed mutagenesis of PP1-binding motif, okadaic acid/calyculin A treatment in Cos7, HEK293, and hippocampal neurons","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (biochemical assay, mutagenesis, live cell imaging, pharmacological inhibition) in a single highly-cited study","pmids":["12052877"],"is_preprint":false},{"year":2002,"finding":"The N-terminal F-actin-binding domain of neurabin I dictates its localization to the actin cytoskeleton and promotes disassembly of stress fibers, while deletion of the C-terminal coiled-coil and SAM domains abolishes dimerization and induces filopodium extension.","method":"GFP-fusion domain deletion analysis and fluorescence microscopy in Cos7, HEK293, and hippocampal neurons","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — direct localization experiments with domain mutants showing specific phenotypic readouts, replicated across multiple cell types","pmids":["12052877"],"is_preprint":false},{"year":2002,"finding":"PKA phosphorylation of neurabin I at serine-461 (within the PP1-binding motif) impairs PP1 binding, and in vitro studies showed the actin-binding domain attenuates PKA phosphorylation of neurabin I; p70S6K is excluded from neurabin I/PP1 complexes and requires displacement of PP1 for its own recruitment to neurabin I.","method":"In vitro kinase assays, co-immunoprecipitation, phospho-site mutagenesis (S461E), immune complex assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro assay with mutagenesis plus reciprocal binding experiments, multiple orthogonal approaches","pmids":["12052877"],"is_preprint":false},{"year":1999,"finding":"Neurabin I binds and inhibits PP1 (Ki = 2.7 nM) via residues 456-460; PKA phosphorylates neurabin I at serine-461 (within the PP1-binding motif), and this phosphorylation significantly reduces PP1 binding and inhibitory potency (~35-fold reduction with S461E phosphomimetic mutant).","method":"Overlay assay, yeast two-hybrid, co-precipitation, co-immunoprecipitation, GST-fusion PP1 inhibition assay, in vitro PKA kinase assay, HPLC-MS phosphosite identification, S461E mutagenesis","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted enzymatic inhibition assay with Ki measurement, phosphosite identified by MS, mutagenesis validation; replicated independently from PMID 12052877","pmids":["10504266"],"is_preprint":false},{"year":2002,"finding":"Neurabin I preferentially recruits PP1gamma1 and PP1alpha isoforms over PP1beta from brain extracts; sequences flanking the conserved PP1-binding motif and C-terminal sequences unique to PP1 isoforms both contribute to this selectivity. In PP1gamma null mice, neurabins show enhanced association with PP1alpha but not PP1beta.","method":"Immunoprecipitation from rat brain extracts, in vitro binding assays with recombinant peptides and chimeric neurabins, PP1/PP2A chimera assays, immunoprecipitation from PP1gamma knockout mouse brain","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal in vitro and in vivo binding assays including knockout mouse validation","pmids":["12016225"],"is_preprint":false},{"year":2007,"finding":"Cdk5 directly phosphorylates neurabin I and controls its association with F-actin; mutation of the Cdk5 phosphorylation site reduces the phenotypic consequences of neurabin I overexpression (altered neuronal morphology and migration defects) both in vitro and in vivo, and neurabin I expression levels affect Rac1 activation in neurons.","method":"In vitro kinase assay, site-directed mutagenesis of Cdk5 site, gain/loss-of-function in cortical and hippocampal neurons, in utero electroporation, Rac1 activation assay","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 — direct kinase assay plus mutagenesis plus in vivo phenotypic rescue, multiple orthogonal methods","pmids":["17699587"],"is_preprint":false},{"year":2006,"finding":"Neurabin I directly interacts with the neuronal GTPase Rac3 (identified by yeast two-hybrid), colocalizes with Rac3 at growth cones, and is required for Rac3-induced neuritogenesis; neurabin I mediates this process by anchoring Rac3 to growth cone F-actin, as a Rac3-binding domain deletion mutant fails to rescue neuritogenesis.","method":"Yeast two-hybrid, co-fractionation, co-localization by light microscopy, antisense oligonucleotide knockdown, rescue with wild-type vs. Rac3-binding domain mutant neurabin I","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 — yeast two-hybrid plus biochemical co-partitioning plus loss-of-function rescue with domain mutant","pmids":["16525023"],"is_preprint":false},{"year":1999,"finding":"Bau, a splice form of neurabin I (PPP1R9A) lacking actin- and p70S6K-binding domains, interacts with the tumor suppressor Bin1 via its U3 region, localizes to the nucleus and cytosol, and suppresses oncogene-mediated cell transformation and tumor cell growth.","method":"Yeast two-hybrid identification, co-immunoprecipitation, subcellular fractionation/localization, transformation suppression assay","journal":"Cell adhesion and communication","confidence":"Medium","confidence_rationale":"Tier 3 — single study with Co-IP and functional assay but limited mechanistic follow-up","pmids":["10427963"],"is_preprint":false},{"year":2018,"finding":"Neurabin I is atypically hyper-N-glycosylated at ASN1277, rendering it immunogenic and acting as a B-cell receptor autoantigen that drives BCR pathway activation and proliferation in primary CNS lymphoma cells.","method":"BCR immunoprecipitation/antigen identification, BCR transfection into lymphoma cell lines, BCR pathway activation assays, epitope-toxin conjugate cytotoxicity assay","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2-3 — direct biochemical identification of glycosylation site and functional BCR activation assay, single study","pmids":["30249786"],"is_preprint":false}],"current_model":"PPP1R9A (neurabin I) is a neuronal F-actin-binding scaffold protein that targets PP1 (preferentially gamma1 and alpha isoforms) to the actin cytoskeleton via a conserved KIKF docking motif (residues 457-460), thereby locally regulating actin dynamics to promote dendritic spine morphogenesis and filopodium formation; this activity is bidirectionally controlled by PKA (which phosphorylates S461 to release PP1) and Cdk5 (which phosphorylates neurabin I to regulate F-actin association), while the protein also scaffolds Rac3 at growth cones to mediate neuritogenesis and, via its splice form Bau, interacts with the tumor suppressor Bin1 in a nuclear/cytosolic context."},"narrative":{"teleology":[{"year":1999,"claim":"Establishing that neurabin I is a potent PP1 inhibitor whose binding is negatively regulated by PKA phosphorylation at Ser461 answered how PP1 activity is locally controlled at actin-rich neuronal structures.","evidence":"Reconstituted PP1 inhibition assay (Ki = 2.7 nM), HPLC-MS phosphosite identification, S461E phosphomimetic mutagenesis","pmids":["10504266"],"confidence":"High","gaps":["Physiological consequences of Ser461 phosphorylation in neurons not tested","Whether PP1 isoform selectivity exists was unknown"]},{"year":1999,"claim":"Identification of Bau, a splice form lacking actin-binding and p70S6K-binding domains, as a Bin1-interacting nuclear/cytosolic protein that suppresses transformation revealed an unexpected non-neuronal tumor-suppressive role for the PPP1R9A locus.","evidence":"Yeast two-hybrid, co-immunoprecipitation, subcellular fractionation, transformation suppression assay","pmids":["10427963"],"confidence":"Medium","gaps":["No independent replication of Bin1 interaction","Mechanism of transformation suppression not defined","Relevance to endogenous tumor suppression in vivo unknown"]},{"year":2002,"claim":"Systematic domain analysis and PP1-motif mutagenesis demonstrated that neurabin I's F-actin-binding domain directs cytoskeletal localization, its KIKF motif recruits catalytically active PP1 to drive filopodium formation, and PKA phosphorylation at Ser461 releases PP1 — establishing the tripartite regulatory circuit of actin-scaffolding, phosphatase targeting, and kinase-mediated disassembly.","evidence":"GFP-fusion domain deletions, immune complex phosphatase assays, site-directed mutagenesis, pharmacological PP1 inhibition (okadaic acid/calyculin A) in COS7, HEK293, and hippocampal neurons","pmids":["12052877"],"confidence":"High","gaps":["In vivo spine phenotype of PP1-binding mutant not shown","p70S6K functional role at neurabin I not further resolved"]},{"year":2002,"claim":"Demonstrating that neurabin I preferentially binds PP1γ1 and PP1α over PP1β, with selectivity conferred by flanking sequences and PP1 C-terminal tails, resolved how specificity among PP1 holoenzymes is achieved at the actin cytoskeleton.","evidence":"Immunoprecipitation from rat brain, recombinant peptide binding assays, PP1 chimera analysis, PP1γ knockout mouse brain co-IP","pmids":["12016225"],"confidence":"High","gaps":["Structural basis of isoform selectivity not resolved","Functional consequence of isoform switching (e.g., in PP1γ KO) on spine morphology not tested"]},{"year":2006,"claim":"Identifying Rac3 as a direct neurabin I partner at growth cones, with a Rac3-binding domain deletion abolishing neuritogenesis rescue, established neurabin I as a scaffold linking small GTPase signaling to actin at the growth cone.","evidence":"Yeast two-hybrid, co-fractionation, co-localization, antisense knockdown and domain-mutant rescue in neurons","pmids":["16525023"],"confidence":"High","gaps":["Whether Rac3 scaffolding is independent of PP1 binding unclear","Downstream effectors of the neurabin I–Rac3 complex not identified"]},{"year":2007,"claim":"Showing that Cdk5 phosphorylates neurabin I to regulate its F-actin association and influence neuronal migration in vivo added a second kinase input (alongside PKA) that bidirectionally tunes neurabin I function, and linked neurabin I to Rac1 activation.","evidence":"In vitro Cdk5 kinase assay, phosphosite mutagenesis, gain/loss-of-function in cortical neurons, in utero electroporation, Rac1 activation assay","pmids":["17699587"],"confidence":"High","gaps":["Exact Cdk5 phosphosite(s) on neurabin I not fully mapped by MS in this study","Relationship between Cdk5 phosphorylation and PP1 binding not tested","Whether Rac1 activation is direct or via intermediate effectors unclear"]},{"year":2018,"claim":"Discovery that hyper-N-glycosylated neurabin I (at Asn1277) serves as a B-cell receptor autoantigen driving BCR signaling and proliferation in primary CNS lymphoma revealed an unanticipated role for post-translationally modified neurabin I in lymphomagenesis.","evidence":"BCR immunoprecipitation/antigen identification, BCR transfection into lymphoma lines, BCR pathway activation assays, epitope-toxin conjugate cytotoxicity","pmids":["30249786"],"confidence":"Medium","gaps":["Single study; not independently replicated","Mechanism generating atypical glycosylation unknown","Whether glyco-neurabin I is presented on normal CNS cell surfaces unclear"]},{"year":null,"claim":"How the multiple kinase inputs (PKA, Cdk5), PP1 isoform selectivity, Rac3 scaffolding, and actin binding are integrated at the structural level to coordinate spine morphogenesis and neuronal migration remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of full-length neurabin I or its PP1 holoenzyme","In vivo conditional knockout phenotype in adult neurons not reported in this literature","Functional interplay between Rac3 and Rac1 signaling through neurabin I not dissected"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,3,4]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[1,5]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,6]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,5,6]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[7]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[7]}],"pathway":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,5]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,5,6]},{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[0,5,6]}],"complexes":[],"partners":["PPP1CC","PPP1CA","RAC3","CDK5","BIN1"],"other_free_text":[]},"mechanistic_narrative":"PPP1R9A (neurabin I) is a neuronal scaffolding protein that couples protein phosphatase 1 (PP1) to the actin cytoskeleton to regulate spine morphogenesis, filopodium formation, and neuritogenesis. Its N-terminal F-actin-binding domain targets it to actin filaments, while a conserved KIKF motif (residues 457–460) recruits PP1 (preferentially PP1γ1 and PP1α) with nanomolar affinity (Ki ≈ 2.7 nM), and the resulting complex drives actin rearrangement essential for filopodium induction [PMID:10504266, PMID:12052877, PMID:12016225]. This PP1-anchoring function is dynamically controlled: PKA phosphorylation of Ser461 within the KIKF motif dissociates PP1, whereas Cdk5 phosphorylation regulates neurabin I's F-actin association and downstream Rac1 activation to influence neuronal morphology and migration [PMID:12052877, PMID:17699587]. Neurabin I also scaffolds the GTPase Rac3 at growth cones to mediate neuritogenesis, functioning as a multivalent organizer of actin-based signaling in developing neurons [PMID:16525023]."},"prefetch_data":{"uniprot":{"accession":"Q9ULJ8","full_name":"Neurabin-1","aliases":["Neurabin-I","Neural tissue-specific F-actin-binding protein I","Protein phosphatase 1 regulatory subunit 9A"],"length_aa":1098,"mass_kda":123.3,"function":"Binds to actin filaments (F-actin) and shows cross-linking activity. Binds along the sides of the F-actin. May be involved in neurite formation. Inhibits protein phosphatase 1-alpha activity (By similarity)","subcellular_location":"Cytoplasm, cytoskeleton; Synapse, synaptosome","url":"https://www.uniprot.org/uniprotkb/Q9ULJ8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PPP1R9A","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CALD1","stoichiometry":0.2},{"gene":"CALM3","stoichiometry":0.2},{"gene":"CAPZB","stoichiometry":0.2},{"gene":"CTTN","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PPP1R9A","total_profiled":1310},"omim":[{"mim_id":"602468","title":"PROTEIN PHOSPHATASE 1, REGULATORY SUBUNIT 9A; PPP1R9A","url":"https://www.omim.org/entry/602468"},{"mim_id":"176914","title":"PROTEIN PHOSPHATASE 1, CATALYTIC SUBUNIT, GAMMA ISOFORM; PPP1CC","url":"https://www.omim.org/entry/176914"},{"mim_id":"143100","title":"HUNTINGTON DISEASE; HD","url":"https://www.omim.org/entry/143100"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"retina","ntpm":17.2}],"url":"https://www.proteinatlas.org/search/PPP1R9A"},"hgnc":{"alias_symbol":["Neurabin-I","KIAA1222","FLJ20068"],"prev_symbol":[]},"alphafold":{"accession":"Q9ULJ8","domains":[{"cath_id":"2.30.42.10","chopping":"485-596","consensus_level":"high","plddt":88.5238,"start":485,"end":596},{"cath_id":"1.10.150.50","chopping":"988-1071","consensus_level":"high","plddt":84.9108,"start":988,"end":1071},{"cath_id":"1.20.5","chopping":"682-814","consensus_level":"medium","plddt":94.659,"start":682,"end":814}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9ULJ8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9ULJ8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9ULJ8-F1-predicted_aligned_error_v6.png","plddt_mean":59.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PPP1R9A","jax_strain_url":"https://www.jax.org/strain/search?query=PPP1R9A"},"sequence":{"accession":"Q9ULJ8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9ULJ8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9ULJ8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9ULJ8"}},"corpus_meta":[{"pmid":"12052877","id":"PMC_12052877","title":"Targeting protein phosphatase 1 (PP1) to the actin cytoskeleton: the neurabin I/PP1 complex regulates cell morphology.","date":"2002","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/12052877","citation_count":101,"is_preprint":false},{"pmid":"12016225","id":"PMC_12016225","title":"The neuronal actin-binding proteins, neurabin I and neurabin II, recruit specific isoforms of protein phosphatase-1 catalytic subunits.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12016225","citation_count":75,"is_preprint":false},{"pmid":"10504266","id":"PMC_10504266","title":"Regulation of neurabin I interaction with protein phosphatase 1 by phosphorylation.","date":"1999","source":"Biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/10504266","citation_count":71,"is_preprint":false},{"pmid":"17699587","id":"PMC_17699587","title":"Neurabin-I is phosphorylated by Cdk5: implications for neuronal morphogenesis and cortical migration.","date":"2007","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/17699587","citation_count":32,"is_preprint":false},{"pmid":"30249786","id":"PMC_30249786","title":"Hyper-N-glycosylated SAMD14 and neurabin-I as driver autoantigens of primary central nervous system lymphoma.","date":"2018","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/30249786","citation_count":29,"is_preprint":false},{"pmid":"25757715","id":"PMC_25757715","title":"Altered prefrontal cortical MARCKS and PPP1R9A mRNA expression in schizophrenia and bipolar disorder.","date":"2015","source":"Schizophrenia research","url":"https://pubmed.ncbi.nlm.nih.gov/25757715","citation_count":25,"is_preprint":false},{"pmid":"16525025","id":"PMC_16525025","title":"Rac3-induced neuritogenesis requires binding to Neurabin I.","date":"2006","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/16525025","citation_count":16,"is_preprint":false},{"pmid":"21675243","id":"PMC_21675243","title":"Assessment of genomic imprinting of PPP1R9A, NAP1L5 and PEG3 in pigs.","date":"2011","source":"Genetika","url":"https://pubmed.ncbi.nlm.nih.gov/21675243","citation_count":11,"is_preprint":false},{"pmid":"21498922","id":"PMC_21498922","title":"Molecular cloning, mRNA expression and imprinting status of PEG3, NAP1L5 and PPP1R9A genes in pig.","date":"2011","source":"Genes & genetic systems","url":"https://pubmed.ncbi.nlm.nih.gov/21498922","citation_count":9,"is_preprint":false},{"pmid":"33282736","id":"PMC_33282736","title":"Integration of the B-Cell Receptor Antigen Neurabin-I/SAMD14 Into an Antibody Format as New Therapeutic Approach for the Treatment of Primary CNS Lymphoma.","date":"2020","source":"Frontiers in oncology","url":"https://pubmed.ncbi.nlm.nih.gov/33282736","citation_count":5,"is_preprint":false},{"pmid":"10427963","id":"PMC_10427963","title":"Bau, a splice form of Neurabin-I that interacts with the tumor suppressor Bin1, inhibits malignant cell transformation.","date":"1999","source":"Cell adhesion and communication","url":"https://pubmed.ncbi.nlm.nih.gov/10427963","citation_count":4,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.09.17.676758","title":"Proteomic and Kinetic Characterization of Prion Seeding in Distinct Human CJD Strains Unveils Early Diagnostic Biomarkers","date":"2025-09-17","source":"bioRxiv","url":"https://doi.org/10.1101/2025.09.17.676758","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7687,"output_tokens":2378,"usd":0.029365},"stage2":{"model":"claude-opus-4-6","input_tokens":5669,"output_tokens":2325,"usd":0.129705},"total_usd":0.15907,"stage1_batch_id":"msgbatch_01SUR8NDzVrNmyszWyJeCct7","stage2_batch_id":"msgbatch_01G7Yfutko3cDWzejrwH5AMx","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"Neurabin I (PPP1R9A) recruits active PP1 via a PP1-docking sequence (457)KIKF(460); mutation of this motif or pharmacological PP1 inhibition abolishes neurabin I-induced filopodium formation and restores stress fibers, establishing that the neurabin I/PP1 complex controls actin rearrangement and spine morphology.\",\n      \"method\": \"Immune complex phosphatase assays, GFP-fusion cell imaging, site-directed mutagenesis of PP1-binding motif, okadaic acid/calyculin A treatment in Cos7, HEK293, and hippocampal neurons\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (biochemical assay, mutagenesis, live cell imaging, pharmacological inhibition) in a single highly-cited study\",\n      \"pmids\": [\"12052877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"The N-terminal F-actin-binding domain of neurabin I dictates its localization to the actin cytoskeleton and promotes disassembly of stress fibers, while deletion of the C-terminal coiled-coil and SAM domains abolishes dimerization and induces filopodium extension.\",\n      \"method\": \"GFP-fusion domain deletion analysis and fluorescence microscopy in Cos7, HEK293, and hippocampal neurons\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — direct localization experiments with domain mutants showing specific phenotypic readouts, replicated across multiple cell types\",\n      \"pmids\": [\"12052877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"PKA phosphorylation of neurabin I at serine-461 (within the PP1-binding motif) impairs PP1 binding, and in vitro studies showed the actin-binding domain attenuates PKA phosphorylation of neurabin I; p70S6K is excluded from neurabin I/PP1 complexes and requires displacement of PP1 for its own recruitment to neurabin I.\",\n      \"method\": \"In vitro kinase assays, co-immunoprecipitation, phospho-site mutagenesis (S461E), immune complex assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro assay with mutagenesis plus reciprocal binding experiments, multiple orthogonal approaches\",\n      \"pmids\": [\"12052877\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Neurabin I binds and inhibits PP1 (Ki = 2.7 nM) via residues 456-460; PKA phosphorylates neurabin I at serine-461 (within the PP1-binding motif), and this phosphorylation significantly reduces PP1 binding and inhibitory potency (~35-fold reduction with S461E phosphomimetic mutant).\",\n      \"method\": \"Overlay assay, yeast two-hybrid, co-precipitation, co-immunoprecipitation, GST-fusion PP1 inhibition assay, in vitro PKA kinase assay, HPLC-MS phosphosite identification, S461E mutagenesis\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted enzymatic inhibition assay with Ki measurement, phosphosite identified by MS, mutagenesis validation; replicated independently from PMID 12052877\",\n      \"pmids\": [\"10504266\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Neurabin I preferentially recruits PP1gamma1 and PP1alpha isoforms over PP1beta from brain extracts; sequences flanking the conserved PP1-binding motif and C-terminal sequences unique to PP1 isoforms both contribute to this selectivity. In PP1gamma null mice, neurabins show enhanced association with PP1alpha but not PP1beta.\",\n      \"method\": \"Immunoprecipitation from rat brain extracts, in vitro binding assays with recombinant peptides and chimeric neurabins, PP1/PP2A chimera assays, immunoprecipitation from PP1gamma knockout mouse brain\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal in vitro and in vivo binding assays including knockout mouse validation\",\n      \"pmids\": [\"12016225\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Cdk5 directly phosphorylates neurabin I and controls its association with F-actin; mutation of the Cdk5 phosphorylation site reduces the phenotypic consequences of neurabin I overexpression (altered neuronal morphology and migration defects) both in vitro and in vivo, and neurabin I expression levels affect Rac1 activation in neurons.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis of Cdk5 site, gain/loss-of-function in cortical and hippocampal neurons, in utero electroporation, Rac1 activation assay\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct kinase assay plus mutagenesis plus in vivo phenotypic rescue, multiple orthogonal methods\",\n      \"pmids\": [\"17699587\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Neurabin I directly interacts with the neuronal GTPase Rac3 (identified by yeast two-hybrid), colocalizes with Rac3 at growth cones, and is required for Rac3-induced neuritogenesis; neurabin I mediates this process by anchoring Rac3 to growth cone F-actin, as a Rac3-binding domain deletion mutant fails to rescue neuritogenesis.\",\n      \"method\": \"Yeast two-hybrid, co-fractionation, co-localization by light microscopy, antisense oligonucleotide knockdown, rescue with wild-type vs. Rac3-binding domain mutant neurabin I\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — yeast two-hybrid plus biochemical co-partitioning plus loss-of-function rescue with domain mutant\",\n      \"pmids\": [\"16525023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Bau, a splice form of neurabin I (PPP1R9A) lacking actin- and p70S6K-binding domains, interacts with the tumor suppressor Bin1 via its U3 region, localizes to the nucleus and cytosol, and suppresses oncogene-mediated cell transformation and tumor cell growth.\",\n      \"method\": \"Yeast two-hybrid identification, co-immunoprecipitation, subcellular fractionation/localization, transformation suppression assay\",\n      \"journal\": \"Cell adhesion and communication\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single study with Co-IP and functional assay but limited mechanistic follow-up\",\n      \"pmids\": [\"10427963\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Neurabin I is atypically hyper-N-glycosylated at ASN1277, rendering it immunogenic and acting as a B-cell receptor autoantigen that drives BCR pathway activation and proliferation in primary CNS lymphoma cells.\",\n      \"method\": \"BCR immunoprecipitation/antigen identification, BCR transfection into lymphoma cell lines, BCR pathway activation assays, epitope-toxin conjugate cytotoxicity assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — direct biochemical identification of glycosylation site and functional BCR activation assay, single study\",\n      \"pmids\": [\"30249786\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PPP1R9A (neurabin I) is a neuronal F-actin-binding scaffold protein that targets PP1 (preferentially gamma1 and alpha isoforms) to the actin cytoskeleton via a conserved KIKF docking motif (residues 457-460), thereby locally regulating actin dynamics to promote dendritic spine morphogenesis and filopodium formation; this activity is bidirectionally controlled by PKA (which phosphorylates S461 to release PP1) and Cdk5 (which phosphorylates neurabin I to regulate F-actin association), while the protein also scaffolds Rac3 at growth cones to mediate neuritogenesis and, via its splice form Bau, interacts with the tumor suppressor Bin1 in a nuclear/cytosolic context.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PPP1R9A (neurabin I) is a neuronal scaffolding protein that couples protein phosphatase 1 (PP1) to the actin cytoskeleton to regulate spine morphogenesis, filopodium formation, and neuritogenesis. Its N-terminal F-actin-binding domain targets it to actin filaments, while a conserved KIKF motif (residues 457–460) recruits PP1 (preferentially PP1γ1 and PP1α) with nanomolar affinity (Ki ≈ 2.7 nM), and the resulting complex drives actin rearrangement essential for filopodium induction [PMID:10504266, PMID:12052877, PMID:12016225]. This PP1-anchoring function is dynamically controlled: PKA phosphorylation of Ser461 within the KIKF motif dissociates PP1, whereas Cdk5 phosphorylation regulates neurabin I's F-actin association and downstream Rac1 activation to influence neuronal morphology and migration [PMID:12052877, PMID:17699587]. Neurabin I also scaffolds the GTPase Rac3 at growth cones to mediate neuritogenesis, functioning as a multivalent organizer of actin-based signaling in developing neurons [PMID:16525023].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing that neurabin I is a potent PP1 inhibitor whose binding is negatively regulated by PKA phosphorylation at Ser461 answered how PP1 activity is locally controlled at actin-rich neuronal structures.\",\n      \"evidence\": \"Reconstituted PP1 inhibition assay (Ki = 2.7 nM), HPLC-MS phosphosite identification, S461E phosphomimetic mutagenesis\",\n      \"pmids\": [\"10504266\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological consequences of Ser461 phosphorylation in neurons not tested\", \"Whether PP1 isoform selectivity exists was unknown\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identification of Bau, a splice form lacking actin-binding and p70S6K-binding domains, as a Bin1-interacting nuclear/cytosolic protein that suppresses transformation revealed an unexpected non-neuronal tumor-suppressive role for the PPP1R9A locus.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation, subcellular fractionation, transformation suppression assay\",\n      \"pmids\": [\"10427963\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No independent replication of Bin1 interaction\", \"Mechanism of transformation suppression not defined\", \"Relevance to endogenous tumor suppression in vivo unknown\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Systematic domain analysis and PP1-motif mutagenesis demonstrated that neurabin I's F-actin-binding domain directs cytoskeletal localization, its KIKF motif recruits catalytically active PP1 to drive filopodium formation, and PKA phosphorylation at Ser461 releases PP1 — establishing the tripartite regulatory circuit of actin-scaffolding, phosphatase targeting, and kinase-mediated disassembly.\",\n      \"evidence\": \"GFP-fusion domain deletions, immune complex phosphatase assays, site-directed mutagenesis, pharmacological PP1 inhibition (okadaic acid/calyculin A) in COS7, HEK293, and hippocampal neurons\",\n      \"pmids\": [\"12052877\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo spine phenotype of PP1-binding mutant not shown\", \"p70S6K functional role at neurabin I not further resolved\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Demonstrating that neurabin I preferentially binds PP1γ1 and PP1α over PP1β, with selectivity conferred by flanking sequences and PP1 C-terminal tails, resolved how specificity among PP1 holoenzymes is achieved at the actin cytoskeleton.\",\n      \"evidence\": \"Immunoprecipitation from rat brain, recombinant peptide binding assays, PP1 chimera analysis, PP1γ knockout mouse brain co-IP\",\n      \"pmids\": [\"12016225\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of isoform selectivity not resolved\", \"Functional consequence of isoform switching (e.g., in PP1γ KO) on spine morphology not tested\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identifying Rac3 as a direct neurabin I partner at growth cones, with a Rac3-binding domain deletion abolishing neuritogenesis rescue, established neurabin I as a scaffold linking small GTPase signaling to actin at the growth cone.\",\n      \"evidence\": \"Yeast two-hybrid, co-fractionation, co-localization, antisense knockdown and domain-mutant rescue in neurons\",\n      \"pmids\": [\"16525023\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Rac3 scaffolding is independent of PP1 binding unclear\", \"Downstream effectors of the neurabin I–Rac3 complex not identified\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showing that Cdk5 phosphorylates neurabin I to regulate its F-actin association and influence neuronal migration in vivo added a second kinase input (alongside PKA) that bidirectionally tunes neurabin I function, and linked neurabin I to Rac1 activation.\",\n      \"evidence\": \"In vitro Cdk5 kinase assay, phosphosite mutagenesis, gain/loss-of-function in cortical neurons, in utero electroporation, Rac1 activation assay\",\n      \"pmids\": [\"17699587\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact Cdk5 phosphosite(s) on neurabin I not fully mapped by MS in this study\", \"Relationship between Cdk5 phosphorylation and PP1 binding not tested\", \"Whether Rac1 activation is direct or via intermediate effectors unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Discovery that hyper-N-glycosylated neurabin I (at Asn1277) serves as a B-cell receptor autoantigen driving BCR signaling and proliferation in primary CNS lymphoma revealed an unanticipated role for post-translationally modified neurabin I in lymphomagenesis.\",\n      \"evidence\": \"BCR immunoprecipitation/antigen identification, BCR transfection into lymphoma lines, BCR pathway activation assays, epitope-toxin conjugate cytotoxicity\",\n      \"pmids\": [\"30249786\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single study; not independently replicated\", \"Mechanism generating atypical glycosylation unknown\", \"Whether glyco-neurabin I is presented on normal CNS cell surfaces unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple kinase inputs (PKA, Cdk5), PP1 isoform selectivity, Rac3 scaffolding, and actin binding are integrated at the structural level to coordinate spine morphogenesis and neuronal migration remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of full-length neurabin I or its PP1 holoenzyme\", \"In vivo conditional knockout phenotype in adult neurons not reported in this literature\", \"Functional interplay between Rac3 and Rac1 signaling through neurabin I not dissected\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 3, 4]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [1, 5]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 5, 6]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 5, 6]},\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [0, 5, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"PPP1CC\", \"PPP1CA\", \"RAC3\", \"CDK5\", \"BIN1\"],\n    \"other_free_text\": []\n  }\n}\n```"}