{"gene":"PPP1R18","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2007,"finding":"Phostensin (PPP1R18/KIAA1949) associates with protein phosphatase 1 (PP1) and targets the PP1/phostensin 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 / Strong — reciprocal Co-IP, GST pull-down, and yeast two-hybrid with direct localization imaging; multiple orthogonal methods in one study","pmids":["17374523"],"is_preprint":false},{"year":2009,"finding":"Phostensin caps the pointed ends of actin filaments, decreasing both elongation and depolymerization rates at the pointed end; this was demonstrated using gelsolin-actin seed elongation assays and fluorescent single-filament binding assay.","method":"Actin dynamics assays (gelsolin-actin seeds), fluorescent single filament binding assay","journal":"Biochemical and biophysical research communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical reconstitution with two orthogonal assays (elongation/depolymerization kinetics and single-filament imaging), single lab","pmids":["19622346"],"is_preprint":false},{"year":2012,"finding":"The actin-binding motif of phostensin resides in its C-terminal region (residues 129–155), while pointed-end capping additionally requires N-terminal residues 35–51; the N-terminal region sterically prevents the C-terminus from binding actin filament sides, enabling pointed-end specificity.","method":"Colocalization, F-actin co-sedimentation assay, single filament binding assay, truncation/deletion analysis","journal":"International journal of molecular sciences","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical assays with domain-deletion mapping and multiple orthogonal readouts, single lab","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 from an upstream start site; phostensin-β retains the ability to associate with PP1 and actin filaments and is distinct from (not a degradation product of) phostensin-α (165 aa, ~26 kDa).","method":"5'-RACE, immunoprecipitation combined with shotgun proteomics, SDS-PAGE, functional binding assays","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — immunoprecipitation/proteomics identification of isoform plus binding validation, single lab","pmids":["24434620"],"is_preprint":false},{"year":2011,"finding":"Phostensin is concentrated at the cell periphery and co-localizes with actin filaments in leukocytes; it is expressed in mature but not immature thymic lymphocytes, suggesting a role linked to lymphocyte maturation.","method":"Anti-phostensin monoclonal antibody (PT2) immunofluorescence, immunohistochemistry of lymphatic tissues","journal":"The journal of histochemistry and cytochemistry","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct localization by antibody staining across multiple cell types, single lab, no functional perturbation","pmids":["21804078"],"is_preprint":false},{"year":2018,"finding":"PPP1r18 is identified as an Src-binding protein; it localizes to the nucleus and actin ring in osteoclasts. Overexpression of PPP1r18 inhibits osteoclast terminal differentiation, actin ring formation, and bone resorption, while knockdown promotes these processes. A mutation of the PP1-binding domain of PPP1r18 rescues the inhibitory phenotype of overexpression, indicating that PP1 binding mediates PPP1r18's inhibitory effect on osteoclast function.","method":"Co-immunoprecipitation (Src binding), overexpression and knockdown in osteoclasts, PP1-binding domain mutagenesis, actin ring formation and bone resorption assays, immunofluorescence localization","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — Co-IP identification of binding partner, domain mutagenesis rescue, multiple functional readouts (differentiation, actin ring, bone resorption), single lab with orthogonal methods","pmids":["29158294"],"is_preprint":false},{"year":2020,"finding":"Phostensin associates with EHD1 and EHD4 (Eps15 homology domain-containing proteins); the phostensin/EHD complex co-localizes at endocytic vesicles, and 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 / Moderate — reciprocal Co-IP/pull-down with functional trafficking assay, single lab","pmids":["32800345"],"is_preprint":false},{"year":2022,"finding":"Phostensin (Ptsn) is a component of the Rap1/MRL protein-integrin-talin (MIT) complex in lymphocytes; it mediates dephosphorylation of Rap1, thereby preserving Rap1 activity and membrane localization to stabilize the MIT complex, enabling integrin (αLβ2 and α4β7) activation and lymphocyte trafficking to peripheral lymphoid organs.","method":"Tandem affinity tag-based proteomics (MIT complex isolation), CRISPR/Cas9 PPP1R18 deletion in Jurkat T cells, Ppp1r18-/- mouse model, integrin activation assays, adoptive transfer colitis model","journal":"The Journal of experimental medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — proteomics complex identification, CRISPR KO in human cells, genetic KO mouse with multiple functional readouts (integrin activation, lymphocyte homing, colitis model), multiple orthogonal methods","pmids":["35766979"],"is_preprint":false},{"year":2025,"finding":"Phostensin binds EHD1 and EHD4 via a novel consensus motif (64ILV(X)4(L/V)RL74S) in residues 51–80, not via the canonical NPF motif; alanine substitutions in this motif reduce binding in GST pull-down and far western blotting, and overexpression of a PPP1r18-β mutant with a defective EHD-binding motif fails to attenuate transferrin endocytic recycling, demonstrating that EHD1/EHD4 binding is required for PPP1r18's regulation of endocytic recycling.","method":"GST pull-down assay, far western blotting, site-directed mutagenesis of binding motif, transferrin endocytic trafficking assay in 293T cells","journal":"Journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis-validated binding motif identification with in vitro pull-down/far-western plus functional trafficking rescue experiment, single lab","pmids":["39776131"],"is_preprint":false},{"year":2024,"finding":"PPP1r18 promotes ESCC tumor progression through activation of the calcineurin-mediated ERK pathway rather than through direct binding to PP1 catalytic subunit alpha (PPP1CA); PPP1r18 overexpression enhanced cell proliferation in vitro and in vivo.","method":"PPP1r18 overexpression and knockdown in ESCC cell lines, in vivo xenograft, Western blotting for pathway components, pathway inhibitor experiments","journal":"Carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — functional knockdown/overexpression with pathway western blot analysis and pharmacological rescue, single lab, limited mechanistic resolution of calcineurin connection","pmids":["38715543"],"is_preprint":false}],"current_model":"PPP1R18 (phostensin) is a PP1-regulatory subunit and actin-binding protein that: (1) recruits PP1 to the F-actin cytoskeleton via its PP1-binding domain; (2) caps the pointed ends of actin filaments through an N-terminal positioning domain (residues 35–51) combined with a C-terminal actin-binding motif (residues 129–155), thereby modulating actin dynamics; (3) dephosphorylates Rap1 to stabilize the Rap1/MRL/integrin/talin complex and enable integrin activation and lymphocyte trafficking; (4) regulates osteoclast actin ring formation and bone resorption in a PP1-binding-dependent manner; and (5) associates with EHD1/EHD4 via a novel ILV-RL motif to regulate endocytic recycling."},"narrative":{"mechanistic_narrative":"PPP1R18 (phostensin) is a protein phosphatase 1 (PP1) regulatory subunit and actin-binding protein that couples PP1 activity to the actin cytoskeleton and to actin-dependent cellular processes [PMID:17374523]. It recruits PP1 to F-actin at the cell periphery [PMID:17374523] and acts directly on actin dynamics by capping filament pointed ends, slowing both elongation and depolymerization [PMID:19622346]; this capping requires a C-terminal actin-binding motif (residues 129–155) together with an N-terminal element (residues 35–51) that sterically restricts side-binding and confers pointed-end specificity [PMID:23443105]. The gene encodes both a short (~26 kDa) and an upstream-initiated long (~110 kDa) isoform, both of which retain PP1 and actin-filament binding [PMID:24434620]. Through these activities PPP1R18 controls actin-dependent functions in distinct cell types: in osteoclasts it localizes to the actin ring and suppresses terminal differentiation, actin ring formation, and bone resorption in a PP1-binding-dependent manner [PMID:29158294], and in lymphocytes it forms part of the Rap1/MRL-integrin-talin (MIT) complex, where it dephosphorylates Rap1 to preserve Rap1 activity and membrane localization, enabling integrin activation and lymphocyte trafficking [PMID:35766979]. PPP1R18 additionally binds the endocytic regulators EHD1 and EHD4 through a non-canonical ILV-RL motif (residues 51–80) to attenuate transferrin endocytic recycling [PMID:32800345, PMID:39776131].","teleology":[{"year":2007,"claim":"Established that phostensin physically links PP1 to the actin cytoskeleton, defining it as an actin-targeting PP1 regulatory subunit.","evidence":"Yeast two-hybrid, reciprocal Co-IP, GST pull-down, and immunofluorescence in MDCK cells","pmids":["17374523"],"confidence":"High","gaps":["Did not identify which PP1-dependent substrates are dephosphorylated at the cytoskeleton","Functional consequence of PP1 recruitment to F-actin not tested"]},{"year":2009,"claim":"Resolved how phostensin affects actin directly by showing it caps filament pointed ends and damps their kinetics, establishing a direct cytoskeletal-modulating activity independent of PP1.","evidence":"In vitro gelsolin-actin seed elongation/depolymerization assays and single-filament fluorescence binding","pmids":["19622346"],"confidence":"High","gaps":["Capping demonstrated in vitro only","Cellular consequence of pointed-end capping not addressed"]},{"year":2012,"claim":"Mapped the structural basis of pointed-end specificity to a bipartite arrangement of an N-terminal positioning element and a C-terminal actin-binding motif.","evidence":"F-actin co-sedimentation, single-filament binding, and truncation/deletion analysis","pmids":["23443105"],"confidence":"High","gaps":["No high-resolution structure of the actin-bound complex","Regulation of the N-terminal steric mechanism unknown"]},{"year":2014,"claim":"Showed that KIAA1949 produces a distinct long isoform (phostensin-β) from an upstream start site that retains PP1 and actin binding, indicating isoform-specific functional potential.","evidence":"5'-RACE, immunoprecipitation/shotgun proteomics, SDS-PAGE, binding assays","pmids":["24434620"],"confidence":"Medium","gaps":["Functional differences between α and β isoforms not defined","Tissue-specific isoform expression not characterized"]},{"year":2011,"claim":"Connected phostensin localization to immune cells, showing peripheral actin co-localization and expression restricted to mature thymic lymphocytes.","evidence":"Monoclonal antibody immunofluorescence and immunohistochemistry of lymphatic tissues","pmids":["21804078"],"confidence":"Medium","gaps":["Correlative localization only, no functional perturbation","Link to lymphocyte maturation not mechanistically tested"]},{"year":2017,"claim":"Demonstrated a physiological cytoskeletal role in osteoclasts, where phostensin is an Src-binding protein that suppresses actin ring formation and bone resorption in a PP1-binding-dependent manner.","evidence":"Co-IP, overexpression/knockdown in osteoclasts, PP1-binding domain mutagenesis rescue, actin ring and bone resorption assays","pmids":["29158294"],"confidence":"High","gaps":["Src-phostensin interaction consequence not resolved","PP1 substrate(s) in osteoclasts not identified"]},{"year":2020,"claim":"Identified an endocytic function by linking phostensin to the EHD1/EHD4 recycling machinery and showing it attenuates transferrin trafficking.","evidence":"Co-IP/shotgun proteomics, GST pull-down, immunofluorescence, transferrin trafficking assay","pmids":["32800345"],"confidence":"Medium","gaps":["Binding interface not yet defined at this stage","Whether PP1 activity contributes to endocytic regulation untested"]},{"year":2022,"claim":"Defined a substrate-level mechanism in immunity: phostensin dephosphorylates Rap1 within the MIT complex to sustain integrin activation and lymphocyte trafficking.","evidence":"Tandem-affinity proteomics, CRISPR KO in Jurkat cells, Ppp1r18-/- mouse, integrin activation, adoptive-transfer colitis model","pmids":["35766979"],"confidence":"High","gaps":["Direct enzymatic dephosphorylation of Rap1 by the PP1/phostensin complex not biochemically reconstituted","Relationship between actin-capping and Rap1 regulation unresolved"]},{"year":2025,"claim":"Pinpointed the EHD-binding determinant as a non-canonical ILV-RL motif and proved it is required for regulation of endocytic recycling.","evidence":"GST pull-down, far western blotting, site-directed mutagenesis, transferrin trafficking rescue in 293T cells","pmids":["39776131"],"confidence":"High","gaps":["Structural basis of the non-NPF EHD interaction unknown","Physiological endocytic cargoes beyond transferrin not defined"]},{"year":2024,"claim":"Extended PPP1R18 into oncogenic signaling, showing it promotes ESCC progression via the calcineurin-ERK pathway rather than direct PPP1CA binding.","evidence":"Overexpression/knockdown in ESCC lines, xenograft, pathway Western blots, inhibitor experiments","pmids":["38715543"],"confidence":"Medium","gaps":["Mechanistic link between phostensin and calcineurin not resolved","How a PP1-targeting protein acts independently of PPP1CA here is unclear"]},{"year":null,"claim":"How phostensin's distinct activities — pointed-end actin capping, PP1 recruitment, Rap1 dephosphorylation, and EHD-dependent recycling — are integrated and selectively engaged in different cell types remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model coupling actin capping to PP1 substrate selection","Isoform-specific division of these functions undefined","No structure of phostensin in any of its functional complexes"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,5,7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,7]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,2,4]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,4]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[7]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[6,8]}],"complexes":["Rap1/MRL-integrin-talin (MIT) complex","PP1/phostensin complex"],"partners":["PPP1CA","EHD1","EHD4","SRC","RAP1"],"other_free_text":[]}},"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":22,"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":"42108458","id":"PMC_42108458","title":"PPP1R18-mediated activation of Wnt/β-catenin and EMT is dependent on ERK signaling in clear cell renal cell carcinoma.","date":"2026","source":"Cancer cell international","url":"https://pubmed.ncbi.nlm.nih.gov/42108458","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":8395,"output_tokens":2658,"usd":0.032528,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":9853,"output_tokens":3313,"usd":0.066045,"stage2_stop_reason":"end_turn"},"total_usd":0.098573,"stage1_batch_id":"msgbatch_01CNoDbkChHEkgrJnL7BwtGJ","stage2_batch_id":"msgbatch_016Y4x9E9Wg5VQDANj9quSQD","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2007,\n      \"finding\": \"Phostensin (PPP1R18/KIAA1949) associates with protein phosphatase 1 (PP1) and targets the PP1/phostensin 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 / Strong — reciprocal Co-IP, GST pull-down, and yeast two-hybrid with direct localization imaging; multiple orthogonal methods in one study\",\n      \"pmids\": [\"17374523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Phostensin caps the pointed ends of actin filaments, decreasing both elongation and depolymerization rates at the pointed end; this was demonstrated using gelsolin-actin seed elongation assays and fluorescent single-filament binding assay.\",\n      \"method\": \"Actin dynamics assays (gelsolin-actin seeds), fluorescent single filament binding assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical reconstitution with two orthogonal assays (elongation/depolymerization kinetics and single-filament imaging), single lab\",\n      \"pmids\": [\"19622346\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The actin-binding motif of phostensin resides in its C-terminal region (residues 129–155), while pointed-end capping additionally requires N-terminal residues 35–51; the N-terminal region sterically prevents the C-terminus from binding actin filament sides, enabling pointed-end specificity.\",\n      \"method\": \"Colocalization, F-actin co-sedimentation assay, single filament binding assay, truncation/deletion analysis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical assays with domain-deletion mapping and multiple orthogonal readouts, single lab\",\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 from an upstream start site; phostensin-β retains the ability to associate with PP1 and actin filaments and is distinct from (not a degradation product of) phostensin-α (165 aa, ~26 kDa).\",\n      \"method\": \"5'-RACE, immunoprecipitation combined with shotgun proteomics, SDS-PAGE, functional binding assays\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — immunoprecipitation/proteomics identification of isoform plus binding validation, single lab\",\n      \"pmids\": [\"24434620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Phostensin is concentrated at the cell periphery and co-localizes with actin filaments in leukocytes; it is expressed in mature but not immature thymic lymphocytes, suggesting a role linked to lymphocyte maturation.\",\n      \"method\": \"Anti-phostensin monoclonal antibody (PT2) immunofluorescence, immunohistochemistry of lymphatic tissues\",\n      \"journal\": \"The journal of histochemistry and cytochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct localization by antibody staining across multiple cell types, single lab, no functional perturbation\",\n      \"pmids\": [\"21804078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PPP1r18 is identified as an Src-binding protein; it localizes to the nucleus and actin ring in osteoclasts. Overexpression of PPP1r18 inhibits osteoclast terminal differentiation, actin ring formation, and bone resorption, while knockdown promotes these processes. A mutation of the PP1-binding domain of PPP1r18 rescues the inhibitory phenotype of overexpression, indicating that PP1 binding mediates PPP1r18's inhibitory effect on osteoclast function.\",\n      \"method\": \"Co-immunoprecipitation (Src binding), overexpression and knockdown in osteoclasts, PP1-binding domain mutagenesis, actin ring formation and bone resorption assays, immunofluorescence localization\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP identification of binding partner, domain mutagenesis rescue, multiple functional readouts (differentiation, actin ring, bone resorption), single lab with orthogonal methods\",\n      \"pmids\": [\"29158294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Phostensin associates with EHD1 and EHD4 (Eps15 homology domain-containing proteins); the phostensin/EHD complex co-localizes at endocytic vesicles, and 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 / Moderate — reciprocal Co-IP/pull-down with functional trafficking assay, single lab\",\n      \"pmids\": [\"32800345\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Phostensin (Ptsn) is a component of the Rap1/MRL protein-integrin-talin (MIT) complex in lymphocytes; it mediates dephosphorylation of Rap1, thereby preserving Rap1 activity and membrane localization to stabilize the MIT complex, enabling integrin (αLβ2 and α4β7) activation and lymphocyte trafficking to peripheral lymphoid organs.\",\n      \"method\": \"Tandem affinity tag-based proteomics (MIT complex isolation), CRISPR/Cas9 PPP1R18 deletion in Jurkat T cells, Ppp1r18-/- mouse model, integrin activation assays, adoptive transfer colitis model\",\n      \"journal\": \"The Journal of experimental medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — proteomics complex identification, CRISPR KO in human cells, genetic KO mouse with multiple functional readouts (integrin activation, lymphocyte homing, colitis model), multiple orthogonal methods\",\n      \"pmids\": [\"35766979\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Phostensin binds EHD1 and EHD4 via a novel consensus motif (64ILV(X)4(L/V)RL74S) in residues 51–80, not via the canonical NPF motif; alanine substitutions in this motif reduce binding in GST pull-down and far western blotting, and overexpression of a PPP1r18-β mutant with a defective EHD-binding motif fails to attenuate transferrin endocytic recycling, demonstrating that EHD1/EHD4 binding is required for PPP1r18's regulation of endocytic recycling.\",\n      \"method\": \"GST pull-down assay, far western blotting, site-directed mutagenesis of binding motif, transferrin endocytic trafficking assay in 293T cells\",\n      \"journal\": \"Journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis-validated binding motif identification with in vitro pull-down/far-western plus functional trafficking rescue experiment, single lab\",\n      \"pmids\": [\"39776131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"PPP1r18 promotes ESCC tumor progression through activation of the calcineurin-mediated ERK pathway rather than through direct binding to PP1 catalytic subunit alpha (PPP1CA); PPP1r18 overexpression enhanced cell proliferation in vitro and in vivo.\",\n      \"method\": \"PPP1r18 overexpression and knockdown in ESCC cell lines, in vivo xenograft, Western blotting for pathway components, pathway inhibitor experiments\",\n      \"journal\": \"Carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — functional knockdown/overexpression with pathway western blot analysis and pharmacological rescue, single lab, limited mechanistic resolution of calcineurin connection\",\n      \"pmids\": [\"38715543\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PPP1R18 (phostensin) is a PP1-regulatory subunit and actin-binding protein that: (1) recruits PP1 to the F-actin cytoskeleton via its PP1-binding domain; (2) caps the pointed ends of actin filaments through an N-terminal positioning domain (residues 35–51) combined with a C-terminal actin-binding motif (residues 129–155), thereby modulating actin dynamics; (3) dephosphorylates Rap1 to stabilize the Rap1/MRL/integrin/talin complex and enable integrin activation and lymphocyte trafficking; (4) regulates osteoclast actin ring formation and bone resorption in a PP1-binding-dependent manner; and (5) associates with EHD1/EHD4 via a novel ILV-RL motif to regulate endocytic recycling.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PPP1R18 (phostensin) is a protein phosphatase 1 (PP1) regulatory subunit and actin-binding protein that couples PP1 activity to the actin cytoskeleton and to actin-dependent cellular processes [#0]. It recruits PP1 to F-actin at the cell periphery [#0] and acts directly on actin dynamics by capping filament pointed ends, slowing both elongation and depolymerization [#1]; this capping requires a C-terminal actin-binding motif (residues 129–155) together with an N-terminal element (residues 35–51) that sterically restricts side-binding and confers pointed-end specificity [#2]. The gene encodes both a short (~26 kDa) and an upstream-initiated long (~110 kDa) isoform, both of which retain PP1 and actin-filament binding [#3]. Through these activities PPP1R18 controls actin-dependent functions in distinct cell types: in osteoclasts it localizes to the actin ring and suppresses terminal differentiation, actin ring formation, and bone resorption in a PP1-binding-dependent manner [#5], and in lymphocytes it forms part of the Rap1/MRL-integrin-talin (MIT) complex, where it dephosphorylates Rap1 to preserve Rap1 activity and membrane localization, enabling integrin activation and lymphocyte trafficking [#7]. PPP1R18 additionally binds the endocytic regulators EHD1 and EHD4 through a non-canonical ILV-RL motif (residues 51–80) to attenuate transferrin endocytic recycling [#6, #8].\",\n  \"teleology\": [\n    {\n      \"year\": 2007,\n      \"claim\": \"Established that phostensin physically links PP1 to the actin cytoskeleton, defining it as an actin-targeting PP1 regulatory subunit.\",\n      \"evidence\": \"Yeast two-hybrid, reciprocal Co-IP, GST pull-down, and immunofluorescence in MDCK cells\",\n      \"pmids\": [\"17374523\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify which PP1-dependent substrates are dephosphorylated at the cytoskeleton\", \"Functional consequence of PP1 recruitment to F-actin not tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Resolved how phostensin affects actin directly by showing it caps filament pointed ends and damps their kinetics, establishing a direct cytoskeletal-modulating activity independent of PP1.\",\n      \"evidence\": \"In vitro gelsolin-actin seed elongation/depolymerization assays and single-filament fluorescence binding\",\n      \"pmids\": [\"19622346\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Capping demonstrated in vitro only\", \"Cellular consequence of pointed-end capping not addressed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Mapped the structural basis of pointed-end specificity to a bipartite arrangement of an N-terminal positioning element and a C-terminal actin-binding motif.\",\n      \"evidence\": \"F-actin co-sedimentation, single-filament binding, and truncation/deletion analysis\",\n      \"pmids\": [\"23443105\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of the actin-bound complex\", \"Regulation of the N-terminal steric mechanism unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showed that KIAA1949 produces a distinct long isoform (phostensin-β) from an upstream start site that retains PP1 and actin binding, indicating isoform-specific functional potential.\",\n      \"evidence\": \"5'-RACE, immunoprecipitation/shotgun proteomics, SDS-PAGE, binding assays\",\n      \"pmids\": [\"24434620\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional differences between α and β isoforms not defined\", \"Tissue-specific isoform expression not characterized\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected phostensin localization to immune cells, showing peripheral actin co-localization and expression restricted to mature thymic lymphocytes.\",\n      \"evidence\": \"Monoclonal antibody immunofluorescence and immunohistochemistry of lymphatic tissues\",\n      \"pmids\": [\"21804078\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative localization only, no functional perturbation\", \"Link to lymphocyte maturation not mechanistically tested\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated a physiological cytoskeletal role in osteoclasts, where phostensin is an Src-binding protein that suppresses actin ring formation and bone resorption in a PP1-binding-dependent manner.\",\n      \"evidence\": \"Co-IP, overexpression/knockdown in osteoclasts, PP1-binding domain mutagenesis rescue, actin ring and bone resorption assays\",\n      \"pmids\": [\"29158294\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Src-phostensin interaction consequence not resolved\", \"PP1 substrate(s) in osteoclasts not identified\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified an endocytic function by linking phostensin to the EHD1/EHD4 recycling machinery and showing it attenuates transferrin trafficking.\",\n      \"evidence\": \"Co-IP/shotgun proteomics, GST pull-down, immunofluorescence, transferrin trafficking assay\",\n      \"pmids\": [\"32800345\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Binding interface not yet defined at this stage\", \"Whether PP1 activity contributes to endocytic regulation untested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined a substrate-level mechanism in immunity: phostensin dephosphorylates Rap1 within the MIT complex to sustain integrin activation and lymphocyte trafficking.\",\n      \"evidence\": \"Tandem-affinity proteomics, CRISPR KO in Jurkat cells, Ppp1r18-/- mouse, integrin activation, adoptive-transfer colitis model\",\n      \"pmids\": [\"35766979\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct enzymatic dephosphorylation of Rap1 by the PP1/phostensin complex not biochemically reconstituted\", \"Relationship between actin-capping and Rap1 regulation unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Pinpointed the EHD-binding determinant as a non-canonical ILV-RL motif and proved it is required for regulation of endocytic recycling.\",\n      \"evidence\": \"GST pull-down, far western blotting, site-directed mutagenesis, transferrin trafficking rescue in 293T cells\",\n      \"pmids\": [\"39776131\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the non-NPF EHD interaction unknown\", \"Physiological endocytic cargoes beyond transferrin not defined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Extended PPP1R18 into oncogenic signaling, showing it promotes ESCC progression via the calcineurin-ERK pathway rather than direct PPP1CA binding.\",\n      \"evidence\": \"Overexpression/knockdown in ESCC lines, xenograft, pathway Western blots, inhibitor experiments\",\n      \"pmids\": [\"38715543\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link between phostensin and calcineurin not resolved\", \"How a PP1-targeting protein acts independently of PPP1CA here is unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How phostensin's distinct activities — pointed-end actin capping, PP1 recruitment, Rap1 dephosphorylation, and EHD-dependent recycling — are integrated and selectively engaged in different cell types remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model coupling actin capping to PP1 substrate selection\", \"Isoform-specific division of these functions undefined\", \"No structure of phostensin in any of its functional complexes\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 5, 7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 2, 4]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [7]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [6, 8]}\n    ],\n    \"complexes\": [\"Rap1/MRL-integrin-talin (MIT) complex\", \"PP1/phostensin complex\"],\n    \"partners\": [\"PPP1CA\", \"EHD1\", \"EHD4\", \"SRC\", \"RAP1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}