{"gene":"ZFYVE27","run_date":"2026-04-28T23:00:24","timeline":{"discoveries":[{"year":2006,"finding":"ZFYVE27 (Protrudin) was identified as a specific spastin-binding protein via yeast two-hybrid screen, validated by in vivo co-immunoprecipitation and co-localization in mammalian cells. A pathogenic missense mutation in ZFYVE27 (SPG33) severely disrupts its interaction with spastin and causes an aberrant intracellular tubular structure pattern.","method":"Yeast two-hybrid, co-immunoprecipitation, co-localization in mammalian cells, mutant functional analysis","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP and co-localization validated yeast two-hybrid; mutation functionally characterized; foundational paper with 113 citations","pmids":["16826525"],"is_preprint":false},{"year":2011,"finding":"ZFYVE27 (Protrudin) self-interacts and oligomerizes into dimer/tetramer forms; the core self-interaction region resides within the third hydrophobic region (HR3). ZFYVE27 is a peripheral membrane protein that binds phosphatidylinositol 3-phosphate. Oligomerization is necessary for neurite extension, as deletion of HR3 causes a dominant-negative effect abolishing protrusion formation.","method":"Yeast two-hybrid, co-immunoprecipitation, co-localization, sucrose gradient centrifugation, subcellular fractionation, Triton X-114 membrane phase separation, lipid-binding assay, dominant-negative expression","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal biochemical methods including fractionation, lipid binding, and dominant-negative functional assay in a single study","pmids":["22216323"],"is_preprint":false},{"year":2013,"finding":"Protrudin (ZFYVE27/SPG33) contains hydrophobic intramembrane hairpin domains, interacts with tubular ER proteins (atlastins and ER-shaping proteins), and functions in ER morphogenesis by regulating the sheet-to-tubule balance. Protrudin also interacts with KIF5 and harbors a Rab-binding domain, a noncanonical FYVE domain, and an FFAT domain.","method":"Co-immunoprecipitation, domain analysis, ER morphology assays, interaction studies with ER-shaping proteins","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 — multiple binding partners validated, domain architecture defined, functional ER morphology assay, 56 citations","pmids":["23969831"],"is_preprint":false},{"year":2014,"finding":"Protrudin localizes predominantly to the tubular ER, promotes formation and stabilization of the tubular ER network when overexpressed, and interacts with other HSP-related proteins including myelin proteolipid protein 1 (SPG2), atlastin-1 (SPG3A), REEP1 (SPG31), REEP5, Kif5A/B/C, and reticulons 1, 3, and 4. The disease-associated mutant Protrudin(G191V) has increased intracellular stability and cells expressing it show increased susceptibility to ER stress. Membrane topology analysis revealed one of three hydrophobic segments forms a hydrophobic hairpin domain.","method":"Proteomics of purified protrudin complexes from transgenic mouse brain, co-immunoprecipitation, membrane topology analysis, ER morphology assay, ER stress assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — in vivo proteomic identification of interactors from transgenic mouse brain, membrane topology, and functional ER stress assay with mutant; multiple orthogonal methods","pmids":["24668814"],"is_preprint":false},{"year":2017,"finding":"SRRM4 regulates alternative splicing of ZFYVE27 (protrudin) pre-mRNA to include a neuron-specific microexon (exon L) encoding seven amino acids. The resulting long isoform (protrudin-L) promotes neurite outgrowth more effectively than the shorter isoform (protrudin-S). SRRM4 recognizes a UGC motif upstream of exon L for this splicing event. Deletion of exon L inhibits neurite outgrowth.","method":"RNA splicing assay, shRNA knockdown of SRRM4, exon deletion mutants, neurite outgrowth assay in Neuro2A and embryonic stem cells","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 — KD with defined cellular phenotype, exon deletion functional rescue, motif characterization; multiple orthogonal approaches","pmids":["28106138"],"is_preprint":false},{"year":2019,"finding":"Protrudin functions as a tethering factor at membrane contact sites (MCSs) between the ER and other organelles. Its pleiotropic molecular functions at MCSs include inactivation of small GTPase Rab11, bending of the ER membrane, and functional interactions with motor protein KIF5 and the ER protein VAP.","method":"Review of experimental findings (domain functional analysis, interaction studies)","journal":"Proceedings of the Japan Academy. Series B, Physical and biological sciences","confidence":"Medium","confidence_rationale":"Tier 3 — review article consolidating experimental findings; mechanistic details referenced from primary studies","pmids":["31406056"],"is_preprint":false},{"year":2020,"finding":"Protrudin-deficient mice display pleiotropic behavioral abnormalities (hyperactivity, depression-like behavior, attention deficits, impaired fear-conditioning memory) but no signs of HSP, suggesting that HSP-associated mutations act via gain-of-toxic-function rather than loss-of-function.","method":"Knockout mouse generation, comprehensive behavioral battery testing","journal":"Molecular brain","confidence":"Medium","confidence_rationale":"Tier 2 — clean KO with defined behavioral phenotypes; pathway placement (gain-of-function vs. loss-of-function) inferred from phenotypic absence of HSP","pmids":["33172474"],"is_preprint":false},{"year":2021,"finding":"Protrudin (ZFYVE27) interacts with PDZD8, an SMP domain-containing protein that acts as a tether at ER-late endosome/lysosome (LE/lys) membrane contact sites and transfers lipids (glycerophospholipids and ceramides) between membranes. This interaction is required for LE/lys positioning and neurite outgrowth.","method":"Co-immunoprecipitation, in vitro lipid transfer assay, lipid-binding assay, organelle positioning assay, neurite outgrowth assay","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro reconstitution of lipid transfer, Co-IP validation of interaction, functional cellular assays with defined phenotypes","pmids":["33912962"],"is_preprint":false},{"year":2022,"finding":"Protrudin (ZFYVE27) regulates FAK activation, endothelial cell migration, and angiogenesis. Knockdown of Protrudin inhibits FAK activation and disrupts polarized phospho-FAK distribution in HUVECs and HAECs, and reduces VEGF-mediated S6K activation via perinuclear mTOR accumulation. Mice with global Protrudin deletion show reduced retinal vascular progression.","method":"siRNA knockdown, phospho-FAK immunofluorescence, tube formation assay, mTOR/S6K signaling assays, protrudin knockout mouse retinal vascular analysis","journal":"Cellular and molecular life sciences : CMLS","confidence":"Medium","confidence_rationale":"Tier 2 — KD with defined cellular signaling phenotypes (FAK, mTOR/S6K) and in vivo KO vascular phenotype; single lab","pmids":["35368213"],"is_preprint":false}],"current_model":"ZFYVE27/Protrudin is an ER-resident peripheral membrane protein containing hydrophobic hairpin domains, a noncanonical FYVE domain (binding PI3P), a Rab-binding domain, and an FFAT domain; it forms oligomers through its HR3 domain, interacts with spastin and multiple HSP-related ER-shaping proteins (atlastins, REEPs, reticulons, KIF5), functions as a tethering factor at ER-organelle membrane contact sites (regulating ER tubular network morphology, Rab11 inactivation, and lipid transfer via PDZD8), promotes neurite outgrowth (with a neuron-specific long isoform generated by SRRM4-dependent alternative splicing), and regulates FAK activation and endothelial cell migration, with disease-associated mutations causing gain-of-toxic-function effects on ER morphology and stress responses."},"narrative":{"teleology":[{"year":2006,"claim":"Identification of ZFYVE27 as a spastin-binding partner and SPG33 disease gene established it as a player in HSP pathogenesis and linked it mechanistically to microtubule-severing via spastin.","evidence":"Yeast two-hybrid screen, reciprocal co-immunoprecipitation, co-localization, and functional analysis of the G191V mutation in mammalian cells","pmids":["16826525"],"confidence":"High","gaps":["Mechanism by which disrupted spastin interaction leads to axonal degeneration unknown","Whether other SPG proteins interact with ZFYVE27 was unexplored","Subcellular compartment and membrane association not yet defined"]},{"year":2011,"claim":"Demonstrating that Protrudin is a peripheral membrane protein that binds PI3P and oligomerizes via HR3 revealed how it associates with membranes and showed oligomerization is functionally required for neurite extension.","evidence":"Sucrose gradient centrifugation, Triton X-114 phase separation, lipid-binding assays, and dominant-negative HR3 deletion in neurite outgrowth assays","pmids":["22216323"],"confidence":"High","gaps":["Stoichiometry of oligomers in vivo not resolved","Which PI3P-containing compartments are relevant for function unclear","Structural basis of HR3-mediated self-association unknown"]},{"year":2013,"claim":"Defining Protrudin's multi-domain architecture (hydrophobic hairpins, FYVE, FFAT, Rab-binding domains) and its interactions with atlastins, reticulons, and KIF5 established it as an ER-shaping protein that regulates ER tubular morphology.","evidence":"Co-immunoprecipitation, domain mapping, and ER morphology assays in mammalian cells","pmids":["23969831"],"confidence":"High","gaps":["Relative contribution of each domain to ER morphogenesis not dissected","Whether Protrudin directly bends membranes or acts via recruitment of other shapers unclear"]},{"year":2014,"claim":"Proteomic identification of Protrudin's interactome from mouse brain confirmed interactions with multiple HSP-related ER proteins in vivo and showed the G191V mutant gains stability and sensitizes cells to ER stress, supporting a gain-of-toxic-function disease model.","evidence":"Purification of Protrudin complexes from transgenic mouse brain, co-immunoprecipitation, membrane topology analysis, ER stress assays","pmids":["24668814"],"confidence":"High","gaps":["Mechanism linking increased mutant stability to ER stress susceptibility unknown","Whether gain-of-function toxicity involves dominant-negative disruption of ER morphology or a neomorphic activity not distinguished"]},{"year":2017,"claim":"Discovery that SRRM4 drives inclusion of a neuron-specific microexon in ZFYVE27, producing a long isoform with enhanced neurite-outgrowth-promoting activity, explained tissue-specific functional regulation of Protrudin.","evidence":"RNA splicing assays, SRRM4 knockdown, exon deletion mutants, and neurite outgrowth assays in Neuro2A and ES cells","pmids":["28106138"],"confidence":"High","gaps":["How the 7-amino-acid microexon insert alters protein interactions or conformation unknown","Relevance of the long isoform to HSP pathogenesis not tested"]},{"year":2020,"claim":"Protrudin-knockout mice exhibit behavioral abnormalities but not HSP, establishing that SPG33 mutations cause disease through gain-of-toxic-function rather than haploinsufficiency.","evidence":"Global knockout mouse with comprehensive behavioral battery testing","pmids":["33172474"],"confidence":"Medium","gaps":["Molecular basis of behavioral abnormalities (hyperactivity, depression-like behavior) not characterized","Whether compensatory mechanisms mask loss-of-function HSP phenotypes in mice not excluded","Single lab study; independent replication pending"]},{"year":2021,"claim":"Identification of PDZD8 as a Protrudin-interacting lipid transfer protein at ER–late endosome/lysosome contact sites revealed a direct mechanism by which Protrudin supports inter-organelle lipid transport and endosome positioning required for neurite outgrowth.","evidence":"Co-immunoprecipitation, in vitro lipid transfer reconstitution, organelle positioning and neurite outgrowth assays","pmids":["33912962"],"confidence":"High","gaps":["Which specific lipid species are most important for neurite outgrowth not determined","Whether Protrudin–PDZD8 contact sites are disrupted by HSP mutations not tested"]},{"year":2022,"claim":"Demonstrating that Protrudin regulates FAK activation, polarized phospho-FAK distribution, and mTOR/S6K signaling in endothelial cells expanded its functional repertoire beyond neurons to vascular biology.","evidence":"siRNA knockdown in HUVECs/HAECs, phospho-FAK immunofluorescence, tube formation assays, mTOR/S6K signaling assays, and retinal vascular analysis in knockout mice","pmids":["35368213"],"confidence":"Medium","gaps":["Mechanism linking ER contact site function to FAK activation not defined","Whether endothelial phenotype involves PDZD8-dependent lipid transfer unknown","Single-lab finding; independent confirmation in additional vascular beds needed"]},{"year":null,"claim":"The structural basis of Protrudin oligomerization, the precise mechanism by which disease mutations confer gain-of-toxic-function, and how Protrudin coordinates lipid transfer with ER morphogenesis in axons remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of Protrudin or its complexes available","Gain-of-toxic-function mechanism (neomorphic vs. dominant-negative) not distinguished","Relative contributions of lipid transfer, ER shaping, and Rab11 inactivation to neurite outgrowth not dissected"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1,7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,5,7]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,3]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[2,3,5]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[7]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[2,3]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[5,8]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[5,7]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[0,6]}],"complexes":[],"partners":["SPAST","PDZD8","KIF5A","ATL1","REEP1","VAPA","RTN4","SRRM4"],"other_free_text":[]},"mechanistic_narrative":"ZFYVE27 (Protrudin) is an ER-resident membrane protein that functions as a tethering factor at ER–organelle membrane contact sites, regulating ER tubular network morphology, endosomal positioning, lipid transfer, and neurite outgrowth. It contains hydrophobic hairpin domains that anchor it in the tubular ER, a noncanonical FYVE domain that binds PI3P, an FFAT domain for VAP interaction, and a Rab-binding domain; it oligomerizes via its HR3 domain, which is required for protrusion formation, and interacts with ER-shaping proteins (atlastins, REEPs, reticulons), KIF5, spastin, and the lipid transfer protein PDZD8 [PMID:22216323, PMID:23969831, PMID:24668814, PMID:33912962]. A neuron-specific long isoform generated by SRRM4-dependent inclusion of a microexon promotes neurite outgrowth more effectively than the shorter isoform, and Protrudin additionally regulates FAK activation and endothelial cell migration [PMID:28106138, PMID:35368213]. A missense mutation (G191V) linked to hereditary spastic paraplegia SPG33 disrupts spastin binding and increases ER stress susceptibility, while Protrudin-knockout mice lack HSP phenotypes, indicating that disease-associated mutations act through gain-of-toxic-function [PMID:16826525, PMID:24668814, PMID:33172474]."},"prefetch_data":{"uniprot":{"accession":"Q5T4F4","full_name":"Protrudin","aliases":["Spastic paraplegia 33 protein","Zinc finger FYVE domain-containing protein 27"],"length_aa":411,"mass_kda":45.8,"function":"Key regulator of RAB11-dependent vesicular trafficking during neurite extension through polarized membrane transport (PubMed:17082457). Promotes axonal elongation and contributes to the establishment of neuronal cell polarity (By similarity). Involved in nerve growth factor-induced neurite formation in VAPA-dependent manner (PubMed:19289470). Contributes to both the formation and stabilization of the tubular ER network (PubMed:24668814). Involved in ER morphogenesis by regulating the sheet-to-tubule balance and possibly the density of tubule interconnections (PubMed:23969831). Acts as an adapter protein and facilitates the interaction of KIF5A with VAPA, VAPB, SURF4, RAB11A, RAB11B and RTN3 and the ZFYVE27-KIF5A complex contributes to the transport of these proteins in neurons. Can induce formation of neurite-like membrane protrusions in non-neuronal cells in a KIF5A/B-dependent manner (PubMed:21976701)","subcellular_location":"Recycling endosome membrane; Endoplasmic reticulum membrane; Cell projection, growth cone membrane","url":"https://www.uniprot.org/uniprotkb/Q5T4F4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ZFYVE27","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"VAPA","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/ZFYVE27","total_profiled":1310},"omim":[{"mim_id":"614235","title":"PDZ DOMAIN-CONTAINING PROTEIN 8; PDZD8","url":"https://www.omim.org/entry/614235"},{"mim_id":"610244","title":"SPASTIC PARAPLEGIA 33, AUTOSOMAL DOMINANT; SPG33","url":"https://www.omim.org/entry/610244"},{"mim_id":"610243","title":"ZINC FINGER FYVE DOMAIN-CONTAINING PROTEIN 27; ZFYVE27","url":"https://www.omim.org/entry/610243"},{"mim_id":"605570","title":"RAS-ASSOCIATED PROTEIN RAB11A; RAB11A","url":"https://www.omim.org/entry/605570"},{"mim_id":"182600","title":"SPASTIC PARAPLEGIA 3, AUTOSOMAL DOMINANT; SPG3A","url":"https://www.omim.org/entry/182600"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Vesicles","reliability":"Approved"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ZFYVE27"},"hgnc":{"alias_symbol":["FLJ32919","SPG33"],"prev_symbol":[]},"alphafold":{"accession":"Q5T4F4","domains":[{"cath_id":"-","chopping":"38-228","consensus_level":"medium","plddt":82.0758,"start":38,"end":228},{"cath_id":"3.30.40.10","chopping":"351-411","consensus_level":"medium","plddt":89.3002,"start":351,"end":411}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T4F4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T4F4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T4F4-F1-predicted_aligned_error_v6.png","plddt_mean":70.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ZFYVE27","jax_strain_url":"https://www.jax.org/strain/search?query=ZFYVE27"},"sequence":{"accession":"Q5T4F4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5T4F4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5T4F4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T4F4"}},"corpus_meta":[{"pmid":"22554690","id":"PMC_22554690","title":"Hereditary spastic paraplegias with autosomal dominant, recessive, X-linked, or maternal trait of inheritance.","date":"2012","source":"Journal of the neurological sciences","url":"https://pubmed.ncbi.nlm.nih.gov/22554690","citation_count":228,"is_preprint":false},{"pmid":"16826525","id":"PMC_16826525","title":"ZFYVE27 (SPG33), a novel spastin-binding protein, is mutated in hereditary spastic paraplegia.","date":"2006","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/16826525","citation_count":113,"is_preprint":false},{"pmid":"29980238","id":"PMC_29980238","title":"Clinical spectrum and genetic landscape for hereditary spastic paraplegias in China.","date":"2018","source":"Molecular neurodegeneration","url":"https://pubmed.ncbi.nlm.nih.gov/29980238","citation_count":70,"is_preprint":false},{"pmid":"23969831","id":"PMC_23969831","title":"Protrudin binds atlastins and endoplasmic reticulum-shaping proteins and regulates network formation.","date":"2013","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/23969831","citation_count":56,"is_preprint":false},{"pmid":"24668814","id":"PMC_24668814","title":"Protrudin regulates endoplasmic reticulum morphology and function associated with the pathogenesis of hereditary spastic paraplegia.","date":"2014","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/24668814","citation_count":53,"is_preprint":false},{"pmid":"33912962","id":"PMC_33912962","title":"PDZD8-mediated lipid transfer at contacts between the ER and late endosomes/lysosomes is required for neurite outgrowth.","date":"2021","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/33912962","citation_count":46,"is_preprint":false},{"pmid":"28106138","id":"PMC_28106138","title":"SRRM4-dependent neuron-specific alternative splicing of protrudin transcripts regulates neurite outgrowth.","date":"2017","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/28106138","citation_count":39,"is_preprint":false},{"pmid":"18413476","id":"PMC_18413476","title":"Lack of spartin protein in Troyer syndrome: a loss-of-function disease mechanism?","date":"2008","source":"Archives of neurology","url":"https://pubmed.ncbi.nlm.nih.gov/18413476","citation_count":33,"is_preprint":false},{"pmid":"30283000","id":"PMC_30283000","title":"Protein-protein interactions reveal key canonical pathways, upstream regulators, interactome domains, and novel targets in ALS.","date":"2018","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/30283000","citation_count":30,"is_preprint":false},{"pmid":"27117784","id":"PMC_27117784","title":"A Subpopulation of Label-Retaining Cells of the Kidney Papilla Regenerates Injured Kidney Medullary Tubules.","date":"2016","source":"Stem cell reports","url":"https://pubmed.ncbi.nlm.nih.gov/27117784","citation_count":19,"is_preprint":false},{"pmid":"22216323","id":"PMC_22216323","title":"Oligomerization of ZFYVE27 (Protrudin) is necessary to promote neurite extension.","date":"2011","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/22216323","citation_count":18,"is_preprint":false},{"pmid":"35368213","id":"PMC_35368213","title":"Protrudin regulates FAK activation, endothelial cell migration and angiogenesis.","date":"2022","source":"Cellular and molecular life sciences : CMLS","url":"https://pubmed.ncbi.nlm.nih.gov/35368213","citation_count":17,"is_preprint":false},{"pmid":"18364116","id":"PMC_18364116","title":"A novel candidate locus on chromosome 11p14.1-p11.2 for autosomal dominant hereditary spastic paraplegia.","date":"2008","source":"Chinese medical journal","url":"https://pubmed.ncbi.nlm.nih.gov/18364116","citation_count":13,"is_preprint":false},{"pmid":"33172474","id":"PMC_33172474","title":"Protrudin-deficient mice manifest depression-like behavior with abnormalities in activity, attention, and cued fear-conditioning.","date":"2020","source":"Molecular brain","url":"https://pubmed.ncbi.nlm.nih.gov/33172474","citation_count":8,"is_preprint":false},{"pmid":"31406056","id":"PMC_31406056","title":"Roles of protrudin at interorganelle membrane contact sites.","date":"2019","source":"Proceedings of the Japan Academy. 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A pathogenic missense mutation in ZFYVE27 (SPG33) severely disrupts its interaction with spastin and causes an aberrant intracellular tubular structure pattern.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, co-localization in mammalian cells, mutant functional analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP and co-localization validated yeast two-hybrid; mutation functionally characterized; foundational paper with 113 citations\",\n      \"pmids\": [\"16826525\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"ZFYVE27 (Protrudin) self-interacts and oligomerizes into dimer/tetramer forms; the core self-interaction region resides within the third hydrophobic region (HR3). ZFYVE27 is a peripheral membrane protein that binds phosphatidylinositol 3-phosphate. Oligomerization is necessary for neurite extension, as deletion of HR3 causes a dominant-negative effect abolishing protrusion formation.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, co-localization, sucrose gradient centrifugation, subcellular fractionation, Triton X-114 membrane phase separation, lipid-binding assay, dominant-negative expression\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal biochemical methods including fractionation, lipid binding, and dominant-negative functional assay in a single study\",\n      \"pmids\": [\"22216323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Protrudin (ZFYVE27/SPG33) contains hydrophobic intramembrane hairpin domains, interacts with tubular ER proteins (atlastins and ER-shaping proteins), and functions in ER morphogenesis by regulating the sheet-to-tubule balance. Protrudin also interacts with KIF5 and harbors a Rab-binding domain, a noncanonical FYVE domain, and an FFAT domain.\",\n      \"method\": \"Co-immunoprecipitation, domain analysis, ER morphology assays, interaction studies with ER-shaping proteins\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple binding partners validated, domain architecture defined, functional ER morphology assay, 56 citations\",\n      \"pmids\": [\"23969831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Protrudin localizes predominantly to the tubular ER, promotes formation and stabilization of the tubular ER network when overexpressed, and interacts with other HSP-related proteins including myelin proteolipid protein 1 (SPG2), atlastin-1 (SPG3A), REEP1 (SPG31), REEP5, Kif5A/B/C, and reticulons 1, 3, and 4. The disease-associated mutant Protrudin(G191V) has increased intracellular stability and cells expressing it show increased susceptibility to ER stress. Membrane topology analysis revealed one of three hydrophobic segments forms a hydrophobic hairpin domain.\",\n      \"method\": \"Proteomics of purified protrudin complexes from transgenic mouse brain, co-immunoprecipitation, membrane topology analysis, ER morphology assay, ER stress assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vivo proteomic identification of interactors from transgenic mouse brain, membrane topology, and functional ER stress assay with mutant; multiple orthogonal methods\",\n      \"pmids\": [\"24668814\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"SRRM4 regulates alternative splicing of ZFYVE27 (protrudin) pre-mRNA to include a neuron-specific microexon (exon L) encoding seven amino acids. The resulting long isoform (protrudin-L) promotes neurite outgrowth more effectively than the shorter isoform (protrudin-S). SRRM4 recognizes a UGC motif upstream of exon L for this splicing event. Deletion of exon L inhibits neurite outgrowth.\",\n      \"method\": \"RNA splicing assay, shRNA knockdown of SRRM4, exon deletion mutants, neurite outgrowth assay in Neuro2A and embryonic stem cells\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KD with defined cellular phenotype, exon deletion functional rescue, motif characterization; multiple orthogonal approaches\",\n      \"pmids\": [\"28106138\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Protrudin functions as a tethering factor at membrane contact sites (MCSs) between the ER and other organelles. Its pleiotropic molecular functions at MCSs include inactivation of small GTPase Rab11, bending of the ER membrane, and functional interactions with motor protein KIF5 and the ER protein VAP.\",\n      \"method\": \"Review of experimental findings (domain functional analysis, interaction studies)\",\n      \"journal\": \"Proceedings of the Japan Academy. Series B, Physical and biological sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review article consolidating experimental findings; mechanistic details referenced from primary studies\",\n      \"pmids\": [\"31406056\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Protrudin-deficient mice display pleiotropic behavioral abnormalities (hyperactivity, depression-like behavior, attention deficits, impaired fear-conditioning memory) but no signs of HSP, suggesting that HSP-associated mutations act via gain-of-toxic-function rather than loss-of-function.\",\n      \"method\": \"Knockout mouse generation, comprehensive behavioral battery testing\",\n      \"journal\": \"Molecular brain\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined behavioral phenotypes; pathway placement (gain-of-function vs. loss-of-function) inferred from phenotypic absence of HSP\",\n      \"pmids\": [\"33172474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Protrudin (ZFYVE27) interacts with PDZD8, an SMP domain-containing protein that acts as a tether at ER-late endosome/lysosome (LE/lys) membrane contact sites and transfers lipids (glycerophospholipids and ceramides) between membranes. This interaction is required for LE/lys positioning and neurite outgrowth.\",\n      \"method\": \"Co-immunoprecipitation, in vitro lipid transfer assay, lipid-binding assay, organelle positioning assay, neurite outgrowth assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro reconstitution of lipid transfer, Co-IP validation of interaction, functional cellular assays with defined phenotypes\",\n      \"pmids\": [\"33912962\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Protrudin (ZFYVE27) regulates FAK activation, endothelial cell migration, and angiogenesis. Knockdown of Protrudin inhibits FAK activation and disrupts polarized phospho-FAK distribution in HUVECs and HAECs, and reduces VEGF-mediated S6K activation via perinuclear mTOR accumulation. Mice with global Protrudin deletion show reduced retinal vascular progression.\",\n      \"method\": \"siRNA knockdown, phospho-FAK immunofluorescence, tube formation assay, mTOR/S6K signaling assays, protrudin knockout mouse retinal vascular analysis\",\n      \"journal\": \"Cellular and molecular life sciences : CMLS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with defined cellular signaling phenotypes (FAK, mTOR/S6K) and in vivo KO vascular phenotype; single lab\",\n      \"pmids\": [\"35368213\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ZFYVE27/Protrudin is an ER-resident peripheral membrane protein containing hydrophobic hairpin domains, a noncanonical FYVE domain (binding PI3P), a Rab-binding domain, and an FFAT domain; it forms oligomers through its HR3 domain, interacts with spastin and multiple HSP-related ER-shaping proteins (atlastins, REEPs, reticulons, KIF5), functions as a tethering factor at ER-organelle membrane contact sites (regulating ER tubular network morphology, Rab11 inactivation, and lipid transfer via PDZD8), promotes neurite outgrowth (with a neuron-specific long isoform generated by SRRM4-dependent alternative splicing), and regulates FAK activation and endothelial cell migration, with disease-associated mutations causing gain-of-toxic-function effects on ER morphology and stress responses.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"ZFYVE27 (Protrudin) is an ER-resident membrane protein that functions as a tethering factor at ER–organelle membrane contact sites, regulating ER tubular network morphology, endosomal positioning, lipid transfer, and neurite outgrowth. It contains hydrophobic hairpin domains that anchor it in the tubular ER, a noncanonical FYVE domain that binds PI3P, an FFAT domain for VAP interaction, and a Rab-binding domain; it oligomerizes via its HR3 domain, which is required for protrusion formation, and interacts with ER-shaping proteins (atlastins, REEPs, reticulons), KIF5, spastin, and the lipid transfer protein PDZD8 [PMID:22216323, PMID:23969831, PMID:24668814, PMID:33912962]. A neuron-specific long isoform generated by SRRM4-dependent inclusion of a microexon promotes neurite outgrowth more effectively than the shorter isoform, and Protrudin additionally regulates FAK activation and endothelial cell migration [PMID:28106138, PMID:35368213]. A missense mutation (G191V) linked to hereditary spastic paraplegia SPG33 disrupts spastin binding and increases ER stress susceptibility, while Protrudin-knockout mice lack HSP phenotypes, indicating that disease-associated mutations act through gain-of-toxic-function [PMID:16826525, PMID:24668814, PMID:33172474].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of ZFYVE27 as a spastin-binding partner and SPG33 disease gene established it as a player in HSP pathogenesis and linked it mechanistically to microtubule-severing via spastin.\",\n      \"evidence\": \"Yeast two-hybrid screen, reciprocal co-immunoprecipitation, co-localization, and functional analysis of the G191V mutation in mammalian cells\",\n      \"pmids\": [\"16826525\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism by which disrupted spastin interaction leads to axonal degeneration unknown\",\n        \"Whether other SPG proteins interact with ZFYVE27 was unexplored\",\n        \"Subcellular compartment and membrane association not yet defined\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrating that Protrudin is a peripheral membrane protein that binds PI3P and oligomerizes via HR3 revealed how it associates with membranes and showed oligomerization is functionally required for neurite extension.\",\n      \"evidence\": \"Sucrose gradient centrifugation, Triton X-114 phase separation, lipid-binding assays, and dominant-negative HR3 deletion in neurite outgrowth assays\",\n      \"pmids\": [\"22216323\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Stoichiometry of oligomers in vivo not resolved\",\n        \"Which PI3P-containing compartments are relevant for function unclear\",\n        \"Structural basis of HR3-mediated self-association unknown\"\n      ]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Defining Protrudin's multi-domain architecture (hydrophobic hairpins, FYVE, FFAT, Rab-binding domains) and its interactions with atlastins, reticulons, and KIF5 established it as an ER-shaping protein that regulates ER tubular morphology.\",\n      \"evidence\": \"Co-immunoprecipitation, domain mapping, and ER morphology assays in mammalian cells\",\n      \"pmids\": [\"23969831\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Relative contribution of each domain to ER morphogenesis not dissected\",\n        \"Whether Protrudin directly bends membranes or acts via recruitment of other shapers unclear\"\n      ]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Proteomic identification of Protrudin's interactome from mouse brain confirmed interactions with multiple HSP-related ER proteins in vivo and showed the G191V mutant gains stability and sensitizes cells to ER stress, supporting a gain-of-toxic-function disease model.\",\n      \"evidence\": \"Purification of Protrudin complexes from transgenic mouse brain, co-immunoprecipitation, membrane topology analysis, ER stress assays\",\n      \"pmids\": [\"24668814\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Mechanism linking increased mutant stability to ER stress susceptibility unknown\",\n        \"Whether gain-of-function toxicity involves dominant-negative disruption of ER morphology or a neomorphic activity not distinguished\"\n      ]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Discovery that SRRM4 drives inclusion of a neuron-specific microexon in ZFYVE27, producing a long isoform with enhanced neurite-outgrowth-promoting activity, explained tissue-specific functional regulation of Protrudin.\",\n      \"evidence\": \"RNA splicing assays, SRRM4 knockdown, exon deletion mutants, and neurite outgrowth assays in Neuro2A and ES cells\",\n      \"pmids\": [\"28106138\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How the 7-amino-acid microexon insert alters protein interactions or conformation unknown\",\n        \"Relevance of the long isoform to HSP pathogenesis not tested\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Protrudin-knockout mice exhibit behavioral abnormalities but not HSP, establishing that SPG33 mutations cause disease through gain-of-toxic-function rather than haploinsufficiency.\",\n      \"evidence\": \"Global knockout mouse with comprehensive behavioral battery testing\",\n      \"pmids\": [\"33172474\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Molecular basis of behavioral abnormalities (hyperactivity, depression-like behavior) not characterized\",\n        \"Whether compensatory mechanisms mask loss-of-function HSP phenotypes in mice not excluded\",\n        \"Single lab study; independent replication pending\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of PDZD8 as a Protrudin-interacting lipid transfer protein at ER–late endosome/lysosome contact sites revealed a direct mechanism by which Protrudin supports inter-organelle lipid transport and endosome positioning required for neurite outgrowth.\",\n      \"evidence\": \"Co-immunoprecipitation, in vitro lipid transfer reconstitution, organelle positioning and neurite outgrowth assays\",\n      \"pmids\": [\"33912962\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Which specific lipid species are most important for neurite outgrowth not determined\",\n        \"Whether Protrudin–PDZD8 contact sites are disrupted by HSP mutations not tested\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrating that Protrudin regulates FAK activation, polarized phospho-FAK distribution, and mTOR/S6K signaling in endothelial cells expanded its functional repertoire beyond neurons to vascular biology.\",\n      \"evidence\": \"siRNA knockdown in HUVECs/HAECs, phospho-FAK immunofluorescence, tube formation assays, mTOR/S6K signaling assays, and retinal vascular analysis in knockout mice\",\n      \"pmids\": [\"35368213\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism linking ER contact site function to FAK activation not defined\",\n        \"Whether endothelial phenotype involves PDZD8-dependent lipid transfer unknown\",\n        \"Single-lab finding; independent confirmation in additional vascular beds needed\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of Protrudin oligomerization, the precise mechanism by which disease mutations confer gain-of-toxic-function, and how Protrudin coordinates lipid transfer with ER morphogenesis in axons remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of Protrudin or its complexes available\",\n        \"Gain-of-toxic-function mechanism (neomorphic vs. dominant-negative) not distinguished\",\n        \"Relative contributions of lipid transfer, ER shaping, and Rab11 inactivation to neurite outgrowth not dissected\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 5, 7]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [2, 3, 5]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [2, 3]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [5, 8]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [5, 7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [0, 6]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SPAST\",\n      \"PDZD8\",\n      \"KIF5A\",\n      \"ATL1\",\n      \"REEP1\",\n      \"VAPA\",\n      \"RTN4\",\n      \"SRRM4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}