{"gene":"PPP1R18","run_date":"2026-04-28T19:45:44","timeline":{"discoveries":[{"year":2007,"finding":"Phostensin (PPP1R18/KIAA1949) directly associates with protein phosphatase 1 (PP1) and targets the PP1 complex to the F-actin cytoskeleton at the cell periphery in MDCK epithelial cells.","method":"Yeast two-hybrid assay, co-immunoprecipitation, GST pull-down assay, immunofluorescence microscopy","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (yeast two-hybrid, Co-IP, GST pulldown, immunofluorescence) in a single study","pmids":["17374523"],"is_preprint":false},{"year":2009,"finding":"Phostensin caps the pointed ends of actin filaments and decreases the elongation and depolymerization rates at pointed ends, functioning as an actin filament pointed-end capping protein that modulates actin dynamics.","method":"Actin dynamics assays with gelsolin-actin seeds, fluorescent single filament binding assay","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution assays with direct biochemical readout, replicated across studies","pmids":["19622346"],"is_preprint":false},{"year":2012,"finding":"The actin-binding motif of phostensin is located in the C-terminal region (residues 129–155), while pointed-end capping additionally requires N-terminal residues 35–51; the protein folds such that the N-terminus sterically blocks C-terminus binding to filament sides, directing phostensin to pointed ends.","method":"Colocalization assay, F-actin co-sedimentation assay, single filament binding assay, truncation and domain analysis","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical reconstitution with domain-mapping mutagenesis using multiple orthogonal assays","pmids":["23443105"],"is_preprint":false},{"year":2014,"finding":"A high-molecular-weight isoform of phostensin (phostensin-β, 613 aa, ~110 kDa) is encoded by KIAA1949; phostensin-β retains the ability to associate with PP1 and actin filaments and is present in multiple cell lines.","method":"Immunoprecipitation combined with shotgun proteomics, 5'-RACE analysis, SDS-PAGE","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 — proteomic identification combined with biochemical validation of PP1 and actin binding","pmids":["24434620"],"is_preprint":false},{"year":2018,"finding":"PPP1r18 was identified as an Src-binding protein; it localizes to the nucleus and actin ring in osteoclasts, and its overexpression inhibits terminal osteoclast differentiation, actin ring formation, and bone resorption in a PP1-binding-domain-dependent manner, while knockdown promotes these processes.","method":"Co-immunoprecipitation with constitutively active Src in SYF cells, immunofluorescence localization, overexpression and knockdown in osteoclasts with PP1-binding domain mutant rescue, bone resorption assay","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP identifying binding partner, domain mutant rescue, and clean KD/KO with defined cellular phenotypes across multiple assays","pmids":["29158294"],"is_preprint":false},{"year":2020,"finding":"Phostensin associates with EHD1 and EHD4 and co-localizes with them at endocytic vesicles; overexpression of phostensin-β attenuates endocytic trafficking of transferrin.","method":"Co-immunoprecipitation combined with shotgun proteomics, GST pull-down assay, immunofluorescence microscopy, transferrin endocytic trafficking assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP/pulldown plus functional trafficking assay in a single study","pmids":["32800345"],"is_preprint":false},{"year":2022,"finding":"Phostensin (Ptsn) is a component of the MIT (MRL protein-integrin-talin) complex and mediates dephosphorylation of Rap1 GTPase, thereby preserving Rap1 activity and membrane localization to stabilize the MIT complex; CRISPR/Cas9 deletion of PPP1R18 markedly suppresses integrin αLβ2 and α4β7 activation in T cells, causes lymphocytosis and reduced peripheral lymphoid tissue population in mice, and reduces T cell capacity to induce colitis.","method":"Tandem affinity tag-based proteomics to isolate MIT complex, CRISPR/Cas9 deletion of PPP1R18 in Jurkat cells and mice, integrin activation assays, flow cytometry, adoptive transfer colitis model","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 — proteomic identification of complex, CRISPR KO with multiple orthogonal functional phenotypes in vitro and in vivo, biochemical demonstration of Rap1 dephosphorylation","pmids":["35766979"],"is_preprint":false},{"year":2025,"finding":"Phostensin binds to EHD1 and EHD4 through a novel consensus motif (residues 51–80, core sequence 64ILV(X)4(L/V)RL74S); mutation of this motif reduces binding and abolishes PPP1r18-mediated attenuation of transferrin endocytic recycling, demonstrating that phostensin regulates endocytic recycling via EHD1/EHD4 association.","method":"GST pull-down assay, far western blotting, site-directed mutagenesis, transferrin endocytic trafficking assay","journal":"Journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 — mutagenesis with biochemical binding assays and functional trafficking readout, single study","pmids":["39776131"],"is_preprint":false}],"current_model":"PPP1R18 (phostensin) is a PP1 regulatory subunit that targets PP1 to the F-actin cytoskeleton via its PP1-binding domain and a C-terminal actin-binding motif that, together with N-terminal residues 35–51, directs the protein to cap actin filament pointed ends and modulate actin dynamics; it also functions as a component of the MIT (MRL-integrin-talin) complex in T cells, where it dephosphorylates Rap1 to sustain integrin activation, and associates with EHD1/EHD4 at endocytic vesicles through a distinct motif (residues 51–80) to regulate endocytic recycling, while in osteoclasts its PP1-dependent activity controls podosome organization and bone resorption."},"narrative":{"teleology":[{"year":2007,"claim":"Establishing the founding identity of PPP1R18: it was unknown how PP1 is recruited to the cortical actin cytoskeleton, and this work showed phostensin directly binds PP1 and targets the holoenzyme to F-actin at the cell periphery, defining it as an actin-directed PP1 regulatory subunit.","evidence":"Yeast two-hybrid, co-immunoprecipitation, GST pull-down, and immunofluorescence in MDCK cells","pmids":["17374523"],"confidence":"High","gaps":["No substrates of the phostensin–PP1 complex identified","Mechanism by which phostensin distinguishes cortical actin from other pools not determined","Physiological consequence of PP1 targeting to actin unknown"]},{"year":2009,"claim":"Resolving how phostensin affects actin filament behavior: reconstitution showed it caps pointed ends and inhibits both elongation and depolymerization there, establishing it as a pointed-end capping protein — a rare activity among mammalian actin regulators.","evidence":"In vitro actin dynamics assays with gelsolin-actin seeds and fluorescent single-filament imaging","pmids":["19622346"],"confidence":"High","gaps":["Whether pointed-end capping depends on PP1 association not tested","In vivo relevance of pointed-end capping not demonstrated","Structural basis of capping unknown"]},{"year":2012,"claim":"Mapping the molecular architecture underlying pointed-end specificity: the C-terminal actin-binding motif (residues 129–155) was separated from N-terminal residues 35–51 required for pointed-end capping, and intramolecular folding was shown to sterically prevent side-binding and direct the protein to pointed ends.","evidence":"Co-sedimentation, single-filament binding, and truncation/domain analysis in vitro","pmids":["23443105"],"confidence":"High","gaps":["Atomic-resolution structure of the intramolecular fold not solved","Regulation of the autoinhibitory fold (e.g. by phosphorylation) not explored","Whether PP1 binding alters actin interaction not addressed"]},{"year":2014,"claim":"Identifying an extended isoform (phostensin-β, 613 aa) expanded the gene's coding repertoire and confirmed it retains PP1 and actin binding, raising the question of isoform-specific functions.","evidence":"Immunoprecipitation/shotgun proteomics and 5'-RACE in multiple cell lines","pmids":["24434620"],"confidence":"Medium","gaps":["Functional distinction between short and long isoforms not established","Tissue-specific expression pattern of phostensin-β not characterized","Not independently confirmed by other groups"]},{"year":2018,"claim":"Demonstrating a physiological role in bone biology: PPP1R18 was shown to localize to the nucleus and actin ring in osteoclasts and to inhibit osteoclast differentiation, actin ring formation, and bone resorption in a PP1-binding-domain-dependent manner, connecting the phosphatase-targeting function to a tissue-level phenotype.","evidence":"Co-IP with active Src, immunofluorescence, overexpression/knockdown with PP1-binding-domain mutant rescue, bone resorption assay in osteoclasts","pmids":["29158294"],"confidence":"High","gaps":["Substrates dephosphorylated by the PPP1R18–PP1 complex in osteoclasts not identified","Whether Src phosphorylation of PPP1R18 regulates its activity not tested","In vivo bone phenotype in knockout animals not reported"]},{"year":2020,"claim":"Revealing a second cellular function beyond actin regulation: phostensin associates with EHD1/EHD4 at endocytic vesicles and its overexpression attenuates transferrin endocytic trafficking, linking PPP1R18 to the endosomal recycling pathway.","evidence":"Co-IP/shotgun proteomics, GST pull-down, immunofluorescence, transferrin trafficking assay","pmids":["32800345"],"confidence":"Medium","gaps":["Binding site on phostensin mediating EHD interaction not mapped","Whether PP1 activity is required for the endocytic function unknown","Physiological consequence of attenuated recycling not explored in vivo"]},{"year":2022,"claim":"Establishing PPP1R18 as a critical regulator of integrin activation in T cells: it was identified as a component of the MIT complex where it dephosphorylates Rap1 to maintain Rap1 activity and membrane localization, with CRISPR knockout causing defective integrin activation, lymphocytosis, and impaired T-cell tissue homing in vivo.","evidence":"Tandem affinity proteomics, CRISPR/Cas9 KO in Jurkat cells and mice, integrin activation assays, flow cytometry, adoptive-transfer colitis model","pmids":["35766979"],"confidence":"High","gaps":["Which specific PP1 catalytic subunit isoform operates within the MIT complex not defined","Whether Rap1 is a direct or indirect substrate of the PPP1R18–PP1 complex not biochemically resolved","Contribution of actin-capping versus Rap1-dephosphorylation activity to T-cell phenotype not dissected"]},{"year":2025,"claim":"Defining the molecular determinants of the endocytic function: a novel EHD-binding motif (residues 51–80) was mapped, and its mutation abolished both EHD binding and the ability to attenuate transferrin recycling, establishing this motif as functionally essential and separable from the actin- and PP1-binding domains.","evidence":"GST pull-down, far-western blotting, site-directed mutagenesis, transferrin trafficking assay","pmids":["39776131"],"confidence":"Medium","gaps":["Whether endocytic and actin-capping functions operate simultaneously or in separate pools not tested","Structural basis of the EHD-binding motif interaction not solved","In vivo physiological consequence of disrupting the EHD-binding motif unknown"]},{"year":null,"claim":"A unified model explaining how PPP1R18 coordinates its multiple interaction surfaces — PP1 binding, pointed-end capping, EHD association, and MIT complex participation — within a single polypeptide, and how these functions are regulated by post-translational modification or isoform expression, remains to be established.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of full-length phostensin exists","Phosphorylation-dependent regulation of domain switching not characterized","Relative physiological importance of each function in different cell types not systematically compared"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,4,6]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1,2]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1,2,4]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[5,7]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[4]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[6]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[5,7]}],"complexes":["MIT complex (MRL-integrin-talin)"],"partners":["PPP1CA","EHD1","EHD4","SRC","RAP1A","ACTA1"],"other_free_text":[]},"mechanistic_narrative":"PPP1R18 (phostensin) is a PP1 regulatory subunit that functions at the interface of actin dynamics, integrin signaling, and endocytic trafficking. It directly binds PP1 and targets the phosphatase to the F-actin cytoskeleton, where its C-terminal actin-binding motif (residues 129–155) together with N-terminal residues 35–51 confer pointed-end capping activity that slows actin filament elongation and depolymerization [PMID:17374523, PMID:19622346, PMID:23443105]. In T cells, PPP1R18 is a component of the MIT (MRL–integrin–talin) complex and sustains integrin activation by mediating Rap1 dephosphorylation to preserve Rap1 membrane localization; CRISPR deletion causes lymphocytosis and impaired T-cell homing in vivo [PMID:35766979]. PPP1R18 also associates with EHD1/EHD4 at endocytic vesicles through a distinct motif (residues 51–80) to regulate transferrin endocytic recycling, and in osteoclasts its PP1-binding-domain-dependent activity controls podosome actin ring formation and bone resorption [PMID:32800345, PMID:39776131, PMID:29158294]."},"prefetch_data":{"uniprot":{"accession":"Q6NYC8","full_name":"Phostensin","aliases":["Protein phosphatase 1 F-actin cytoskeleton-targeting subunit","Protein phosphatase 1 regulatory subunit 18"],"length_aa":613,"mass_kda":67.9,"function":"May target protein phosphatase 1 to F-actin cytoskeleton May target protein phosphatase 1 to F-actin cytoskeleton","subcellular_location":"Cytoplasm, cytoskeleton","url":"https://www.uniprot.org/uniprotkb/Q6NYC8/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PPP1R18","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/PPP1R18","total_profiled":1310},"omim":[{"mim_id":"610990","title":"PROTEIN PHOSPHATASE 1 REGULATORY SUBUNIT 18; PPP1R18","url":"https://www.omim.org/entry/610990"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Plasma membrane","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/PPP1R18"},"hgnc":{"alias_symbol":["phostensin"],"prev_symbol":["KIAA1949"]},"alphafold":{"accession":"Q6NYC8","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6NYC8","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q6NYC8-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q6NYC8-F1-predicted_aligned_error_v6.png","plddt_mean":53.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=PPP1R18","jax_strain_url":"https://www.jax.org/strain/search?query=PPP1R18"},"sequence":{"accession":"Q6NYC8","fasta_url":"https://rest.uniprot.org/uniprotkb/Q6NYC8.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q6NYC8/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q6NYC8"}},"corpus_meta":[{"pmid":"17374523","id":"PMC_17374523","title":"Identification of phostensin, a PP1 F-actin cytoskeleton targeting subunit.","date":"2007","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/17374523","citation_count":35,"is_preprint":false},{"pmid":"20842223","id":"PMC_20842223","title":"Undetectable and Decreased Expression of KIAA1949 (Phostensin) Encoded on Chromosome 6p21.33 in Human Breast Cancers Revealed by Transcriptome Analysis.","date":"2010","source":"Journal of Cancer","url":"https://pubmed.ncbi.nlm.nih.gov/20842223","citation_count":29,"is_preprint":false},{"pmid":"19622346","id":"PMC_19622346","title":"Phostensin caps to the pointed end of actin filaments and modulates actin dynamics.","date":"2009","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/19622346","citation_count":21,"is_preprint":false},{"pmid":"29158294","id":"PMC_29158294","title":"The Actin-Binding Protein PPP1r18 Regulates Maturation, Actin Organization, and Bone Resorption Activity of Osteoclasts.","date":"2018","source":"Molecular and cellular biology","url":"https://pubmed.ncbi.nlm.nih.gov/29158294","citation_count":18,"is_preprint":false},{"pmid":"21804078","id":"PMC_21804078","title":"Immunolocalization of phostensin in lymphatic cells and tissues.","date":"2011","source":"The journal of histochemistry and cytochemistry : official journal of the Histochemistry Society","url":"https://pubmed.ncbi.nlm.nih.gov/21804078","citation_count":9,"is_preprint":false},{"pmid":"23443105","id":"PMC_23443105","title":"Identification and characterization of the actin-binding motif of phostensin.","date":"2012","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/23443105","citation_count":9,"is_preprint":false},{"pmid":"24434620","id":"PMC_24434620","title":"Identification of the high molecular weight isoform of phostensin.","date":"2014","source":"International journal of molecular sciences","url":"https://pubmed.ncbi.nlm.nih.gov/24434620","citation_count":6,"is_preprint":false},{"pmid":"35766979","id":"PMC_35766979","title":"Phostensin enables lymphocyte integrin activation and population of peripheral lymphoid organs.","date":"2022","source":"The Journal of experimental medicine","url":"https://pubmed.ncbi.nlm.nih.gov/35766979","citation_count":3,"is_preprint":false},{"pmid":"32800345","id":"PMC_32800345","title":"Identification of phostensin in association with Eps 15 homology domain-containing protein 1 (EHD1) and EHD4.","date":"2020","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/32800345","citation_count":2,"is_preprint":false},{"pmid":"38715543","id":"PMC_38715543","title":"PPP1r18 promotes tumor progression in esophageal squamous cell carcinoma by regulating the calcineurin-mediated ERK pathway.","date":"2024","source":"Carcinogenesis","url":"https://pubmed.ncbi.nlm.nih.gov/38715543","citation_count":1,"is_preprint":false},{"pmid":"39776131","id":"PMC_39776131","title":"Identification of a novel Eps 15 homology domain-containing protein 1 (EHD1) and EHD4-binding motif in phostensin.","date":"2025","source":"Journal of biochemistry","url":"https://pubmed.ncbi.nlm.nih.gov/39776131","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2024.12.07.625684","title":"Deep analysis of the Major Histocompatibility Complex associations using covariate analysis and haploblocks unravels new mechanisms for the molecular etiology of Elite Control in AIDS","date":"2024-12-10","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.07.625684","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":7057,"output_tokens":2030,"usd":0.025811},"stage2":{"model":"claude-opus-4-6","input_tokens":5339,"output_tokens":2651,"usd":0.139455},"total_usd":0.165266,"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\": 2007,\n      \"finding\": \"Phostensin (PPP1R18/KIAA1949) directly associates with protein phosphatase 1 (PP1) and targets the PP1 complex to the F-actin cytoskeleton at the cell periphery in MDCK epithelial cells.\",\n      \"method\": \"Yeast two-hybrid assay, co-immunoprecipitation, GST pull-down assay, immunofluorescence microscopy\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (yeast two-hybrid, Co-IP, GST pulldown, immunofluorescence) in a single study\",\n      \"pmids\": [\"17374523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Phostensin caps the pointed ends of actin filaments and decreases the elongation and depolymerization rates at pointed ends, functioning as an actin filament pointed-end capping protein that modulates actin dynamics.\",\n      \"method\": \"Actin dynamics assays with gelsolin-actin seeds, fluorescent single filament binding assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution assays with direct biochemical readout, replicated across studies\",\n      \"pmids\": [\"19622346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The actin-binding motif of phostensin is located in the C-terminal region (residues 129–155), while pointed-end capping additionally requires N-terminal residues 35–51; the protein folds such that the N-terminus sterically blocks C-terminus binding to filament sides, directing phostensin to pointed ends.\",\n      \"method\": \"Colocalization assay, F-actin co-sedimentation assay, single filament binding assay, truncation and domain analysis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical reconstitution with domain-mapping mutagenesis using multiple orthogonal assays\",\n      \"pmids\": [\"23443105\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"A high-molecular-weight isoform of phostensin (phostensin-β, 613 aa, ~110 kDa) is encoded by KIAA1949; phostensin-β retains the ability to associate with PP1 and actin filaments and is present in multiple cell lines.\",\n      \"method\": \"Immunoprecipitation combined with shotgun proteomics, 5'-RACE analysis, SDS-PAGE\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — proteomic identification combined with biochemical validation of PP1 and actin binding\",\n      \"pmids\": [\"24434620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PPP1r18 was identified as an Src-binding protein; it localizes to the nucleus and actin ring in osteoclasts, and its overexpression inhibits terminal osteoclast differentiation, actin ring formation, and bone resorption in a PP1-binding-domain-dependent manner, while knockdown promotes these processes.\",\n      \"method\": \"Co-immunoprecipitation with constitutively active Src in SYF cells, immunofluorescence localization, overexpression and knockdown in osteoclasts with PP1-binding domain mutant rescue, bone resorption assay\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP identifying binding partner, domain mutant rescue, and clean KD/KO with defined cellular phenotypes across multiple assays\",\n      \"pmids\": [\"29158294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Phostensin associates with EHD1 and EHD4 and co-localizes with them at endocytic vesicles; overexpression of phostensin-β attenuates endocytic trafficking of transferrin.\",\n      \"method\": \"Co-immunoprecipitation combined with shotgun proteomics, GST pull-down assay, immunofluorescence microscopy, transferrin endocytic trafficking assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP/pulldown plus functional trafficking assay in a single study\",\n      \"pmids\": [\"32800345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Phostensin (Ptsn) is a component of the MIT (MRL protein-integrin-talin) complex and mediates dephosphorylation of Rap1 GTPase, thereby preserving Rap1 activity and membrane localization to stabilize the MIT complex; CRISPR/Cas9 deletion of PPP1R18 markedly suppresses integrin αLβ2 and α4β7 activation in T cells, causes lymphocytosis and reduced peripheral lymphoid tissue population in mice, and reduces T cell capacity to induce colitis.\",\n      \"method\": \"Tandem affinity tag-based proteomics to isolate MIT complex, CRISPR/Cas9 deletion of PPP1R18 in Jurkat cells and mice, integrin activation assays, flow cytometry, adoptive transfer colitis model\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — proteomic identification of complex, CRISPR KO with multiple orthogonal functional phenotypes in vitro and in vivo, biochemical demonstration of Rap1 dephosphorylation\",\n      \"pmids\": [\"35766979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Phostensin binds to EHD1 and EHD4 through a novel consensus motif (residues 51–80, core sequence 64ILV(X)4(L/V)RL74S); mutation of this motif reduces binding and abolishes PPP1r18-mediated attenuation of transferrin endocytic recycling, demonstrating that phostensin regulates endocytic recycling via EHD1/EHD4 association.\",\n      \"method\": \"GST pull-down assay, far western blotting, site-directed mutagenesis, transferrin endocytic trafficking assay\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with biochemical binding assays and functional trafficking readout, single study\",\n      \"pmids\": [\"39776131\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PPP1R18 (phostensin) is a PP1 regulatory subunit that targets PP1 to the F-actin cytoskeleton via its PP1-binding domain and a C-terminal actin-binding motif that, together with N-terminal residues 35–51, directs the protein to cap actin filament pointed ends and modulate actin dynamics; it also functions as a component of the MIT (MRL-integrin-talin) complex in T cells, where it dephosphorylates Rap1 to sustain integrin activation, and associates with EHD1/EHD4 at endocytic vesicles through a distinct motif (residues 51–80) to regulate endocytic recycling, while in osteoclasts its PP1-dependent activity controls podosome organization and bone resorption.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"PPP1R18 (phostensin) is a PP1 regulatory subunit that functions at the interface of actin dynamics, integrin signaling, and endocytic trafficking. It directly binds PP1 and targets the phosphatase to the F-actin cytoskeleton, where its C-terminal actin-binding motif (residues 129–155) together with N-terminal residues 35–51 confer pointed-end capping activity that slows actin filament elongation and depolymerization [PMID:17374523, PMID:19622346, PMID:23443105]. In T cells, PPP1R18 is a component of the MIT (MRL–integrin–talin) complex and sustains integrin activation by mediating Rap1 dephosphorylation to preserve Rap1 membrane localization; CRISPR deletion causes lymphocytosis and impaired T-cell homing in vivo [PMID:35766979]. PPP1R18 also associates with EHD1/EHD4 at endocytic vesicles through a distinct motif (residues 51–80) to regulate transferrin endocytic recycling, and in osteoclasts its PP1-binding-domain-dependent activity controls podosome actin ring formation and bone resorption [PMID:32800345, PMID:39776131, PMID:29158294].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Establishing the founding identity of PPP1R18: it was unknown how PP1 is recruited to the cortical actin cytoskeleton, and this work showed phostensin directly binds PP1 and targets the holoenzyme to F-actin at the cell periphery, defining it as an actin-directed PP1 regulatory subunit.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation, GST pull-down, and immunofluorescence in MDCK cells\",\n      \"pmids\": [\"17374523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No substrates of the phostensin–PP1 complex identified\",\n        \"Mechanism by which phostensin distinguishes cortical actin from other pools not determined\",\n        \"Physiological consequence of PP1 targeting to actin unknown\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolving how phostensin affects actin filament behavior: reconstitution showed it caps pointed ends and inhibits both elongation and depolymerization there, establishing it as a pointed-end capping protein — a rare activity among mammalian actin regulators.\",\n      \"evidence\": \"In vitro actin dynamics assays with gelsolin-actin seeds and fluorescent single-filament imaging\",\n      \"pmids\": [\"19622346\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether pointed-end capping depends on PP1 association not tested\",\n        \"In vivo relevance of pointed-end capping not demonstrated\",\n        \"Structural basis of capping unknown\"\n      ]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapping the molecular architecture underlying pointed-end specificity: the C-terminal actin-binding motif (residues 129–155) was separated from N-terminal residues 35–51 required for pointed-end capping, and intramolecular folding was shown to sterically prevent side-binding and direct the protein to pointed ends.\",\n      \"evidence\": \"Co-sedimentation, single-filament binding, and truncation/domain analysis in vitro\",\n      \"pmids\": [\"23443105\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic-resolution structure of the intramolecular fold not solved\",\n        \"Regulation of the autoinhibitory fold (e.g. by phosphorylation) not explored\",\n        \"Whether PP1 binding alters actin interaction not addressed\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identifying an extended isoform (phostensin-β, 613 aa) expanded the gene's coding repertoire and confirmed it retains PP1 and actin binding, raising the question of isoform-specific functions.\",\n      \"evidence\": \"Immunoprecipitation/shotgun proteomics and 5'-RACE in multiple cell lines\",\n      \"pmids\": [\"24434620\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Functional distinction between short and long isoforms not established\",\n        \"Tissue-specific expression pattern of phostensin-β not characterized\",\n        \"Not independently confirmed by other groups\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Demonstrating a physiological role in bone biology: PPP1R18 was shown to localize to the nucleus and actin ring in osteoclasts and to inhibit osteoclast differentiation, actin ring formation, and bone resorption in a PP1-binding-domain-dependent manner, connecting the phosphatase-targeting function to a tissue-level phenotype.\",\n      \"evidence\": \"Co-IP with active Src, immunofluorescence, overexpression/knockdown with PP1-binding-domain mutant rescue, bone resorption assay in osteoclasts\",\n      \"pmids\": [\"29158294\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Substrates dephosphorylated by the PPP1R18–PP1 complex in osteoclasts not identified\",\n        \"Whether Src phosphorylation of PPP1R18 regulates its activity not tested\",\n        \"In vivo bone phenotype in knockout animals not reported\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Revealing a second cellular function beyond actin regulation: phostensin associates with EHD1/EHD4 at endocytic vesicles and its overexpression attenuates transferrin endocytic trafficking, linking PPP1R18 to the endosomal recycling pathway.\",\n      \"evidence\": \"Co-IP/shotgun proteomics, GST pull-down, immunofluorescence, transferrin trafficking assay\",\n      \"pmids\": [\"32800345\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Binding site on phostensin mediating EHD interaction not mapped\",\n        \"Whether PP1 activity is required for the endocytic function unknown\",\n        \"Physiological consequence of attenuated recycling not explored in vivo\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Establishing PPP1R18 as a critical regulator of integrin activation in T cells: it was identified as a component of the MIT complex where it dephosphorylates Rap1 to maintain Rap1 activity and membrane localization, with CRISPR knockout causing defective integrin activation, lymphocytosis, and impaired T-cell tissue homing in vivo.\",\n      \"evidence\": \"Tandem affinity proteomics, CRISPR/Cas9 KO in Jurkat cells and mice, integrin activation assays, flow cytometry, adoptive-transfer colitis model\",\n      \"pmids\": [\"35766979\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which specific PP1 catalytic subunit isoform operates within the MIT complex not defined\",\n        \"Whether Rap1 is a direct or indirect substrate of the PPP1R18–PP1 complex not biochemically resolved\",\n        \"Contribution of actin-capping versus Rap1-dephosphorylation activity to T-cell phenotype not dissected\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Defining the molecular determinants of the endocytic function: a novel EHD-binding motif (residues 51–80) was mapped, and its mutation abolished both EHD binding and the ability to attenuate transferrin recycling, establishing this motif as functionally essential and separable from the actin- and PP1-binding domains.\",\n      \"evidence\": \"GST pull-down, far-western blotting, site-directed mutagenesis, transferrin trafficking assay\",\n      \"pmids\": [\"39776131\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Whether endocytic and actin-capping functions operate simultaneously or in separate pools not tested\",\n        \"Structural basis of the EHD-binding motif interaction not solved\",\n        \"In vivo physiological consequence of disrupting the EHD-binding motif unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A unified model explaining how PPP1R18 coordinates its multiple interaction surfaces — PP1 binding, pointed-end capping, EHD association, and MIT complex participation — within a single polypeptide, and how these functions are regulated by post-translational modification or isoform expression, remains to be established.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of full-length phostensin exists\",\n        \"Phosphorylation-dependent regulation of domain switching not characterized\",\n        \"Relative physiological importance of each function in different cell types not systematically compared\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 4, 6]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1, 2, 4]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [5, 7]}\n    ],\n    \"complexes\": [\n      \"MIT complex (MRL-integrin-talin)\"\n    ],\n    \"partners\": [\n      \"PPP1CA\",\n      \"EHD1\",\n      \"EHD4\",\n      \"SRC\",\n      \"RAP1A\",\n      \"ACTA1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}