{"gene":"BLTP2","run_date":"2026-06-09T22:02:44","timeline":{"discoveries":[{"year":2025,"finding":"BLTP2 localizes to endoplasmic reticulum (ER)-plasma membrane (PM) contact sites and regulates PM fluidity by increasing phosphatidylethanolamine (PE) levels in the PM, consistent with a role in transporting PE from ER to PM via a bridge-like lipid transfer mechanism.","method":"Subcellular localization (live imaging/fractionation), lipid profiling (PE measurement), loss-of-function knockdown/knockout with PM fluidity readout, in vivo metastasis model","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — localization established by direct imaging, PE transport supported by lipid profiling and KO phenotype, replicated across peer-reviewed paper (PMID:40579455) and preprint (PMID:38370643) from same lab using multiple orthogonal methods","pmids":["40579455","38370643"],"is_preprint":false},{"year":2025,"finding":"BLTP2 tethers the ER to tubular endosomes only after they become continuous with the PM, and to macropinosomes in the process of fusing with the PM, indicating bulk lipid transport between ER and PM is its key function; absence of BLTP2 causes accumulation of intracellular vacuoles many of which are connected to the PM.","method":"Live-cell imaging, loss-of-function (BLTP2 KO), electron microscopy of intracellular vacuoles, co-localization of ER with PM-connected endosomes/macropinosomes","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal imaging and KO phenotype with structural readout in peer-reviewed paper, supported by preprint with same data (PMID:39974967)","pmids":["40899996","39974967"],"is_preprint":false},{"year":2025,"finding":"PM binding of BLTP2 is mediated by multiple interactions: phosphoinositides, the adaptor proteins FAM102A and FAM102B, and N-BAR domain proteins at membrane-connected tubules.","method":"Co-immunoprecipitation/pulldown of BLTP2 with FAM102A/FAM102B and N-BAR proteins, phosphoinositide binding assays, loss-of-function imaging","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple binding partners identified in single lab study with KO phenotype, but full reconstitution not explicitly described in abstract","pmids":["40899996","39974967"],"is_preprint":false},{"year":2026,"finding":"The conserved adaptor protein Hoi1 (Ybl086c) targets yeast BLTP2-like proteins Fmp27/Hob1 and Hob2 to ER-PM contact sites via two separate Hoi1 domains that interface with α-helical projections decorating the central hydrophobic channel on Fmp27; loss of these interactions disrupts cellular sterol homeostasis.","method":"Genetic identification of Hoi1, domain mapping, co-IP/interaction assays, sterol homeostasis readout in yeast mutants, genetic epistasis in worms and flies","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — domain-level interaction mapping combined with functional sterol phenotype, cross-species epistasis in worms and flies, replicated across peer-reviewed paper (PMID:41746219) and preprint (PMID:39990326)","pmids":["41746219","39990326"],"is_preprint":false},{"year":2026,"finding":"In Drosophila, the conserved adaptor bilbobaggins (bbo) is required for targeting of Hobbit (BLTP2 ortholog) to ER-PM contact sites; loss of bbo phenocopies loss of hobbit. Additionally, cis-acting sequences in the C-terminal tail of Hobbit are independently required for ER-PM targeting, operating as a 'hook and latch' two-step targeting mechanism.","method":"Drosophila genetics (loss-of-function), structure-function analysis of C-terminal sequences, live imaging of Hobbit localization, epistasis","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis plus structure-function in Drosophila, single lab, preprint only","pmids":["42039411"],"is_preprint":true},{"year":2025,"finding":"BLTP2 is indispensable for intraluminal vesicle (ILV)/exosome formation; SCAMP3 recruits BLTP2 to ER-MVB membrane contact sites in a Rab5-dependent manner, and this recruitment is inhibited by NEDD4-mediated ubiquitination of SCAMP3. Depletion of BLTP2 selectively reduces cone-shaped phospholipids including BMP/LBPA and its precursor phosphatidylglycerol (PG) within endosomes, suggesting BLTP2 transfers PG to MVBs for BMP/LBPA synthesis.","method":"BLTP2 KO/knockdown, co-IP of BLTP2 with SCAMP3, Rab5 dependency assay, NEDD4 ubiquitination assay, lipidomic profiling of endosomes, exosome quantification","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — multiple orthogonal methods (co-IP, lipidomics, KO phenotype) in single lab, preprint only, not yet peer-reviewed","pmids":["bio_10.1101_2025.04.17.649455"],"is_preprint":true},{"year":2024,"finding":"In Drosophila, neuronal loss of the BLTP2 ortholog (hobbit) results in severe early-onset locomotor defects without neurodegeneration, while muscle-specific loss impairs synaptogenesis and neurotransmission at the neuromuscular junction (NMJ), establishing a role for BLTP2 in synaptic development.","method":"Tissue-specific RNAi knockdown in Drosophila neurons and muscles, age-dependent locomotor behavioral assays, NMJ morphology and electrophysiology","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — tissue-specific KD with behavioral and electrophysiological readouts, multiple tissues tested, preprint only","pmids":["bio_10.1101_2024.12.30.630795"],"is_preprint":true}],"current_model":"BLTP2 is a bridge-like lipid transfer protein that localizes to ER-plasma membrane (and ER-MVB) contact sites, where it mediates bulk non-vesicular transport of phospholipids (particularly phosphatidylethanolamine and phosphatidylglycerol/BMP precursors) to maintain PM fluidity and endosomal lipid composition; its targeting to ER-PM contacts requires conserved adaptor proteins (Hoi1/bilbobaggins orthologs), phosphoinositide binding, and FAM102A/FAM102B interactions, and its function at the NMJ is required for synaptic development in Drosophila."},"narrative":{"mechanistic_narrative":"BLTP2 is a bridge-like lipid transfer protein that operates at membrane contact sites to mediate bulk non-vesicular phospholipid transport, sustaining the lipid composition of the plasma membrane and endosomal compartments [PMID:40579455, PMID:38370643, PMID:40899996, PMID:39974967]. It localizes to ER-plasma membrane (PM) contact sites, where it raises PM phosphatidylethanolamine levels and thereby regulates PM fluidity, consistent with transfer of PE from the ER to the PM through its hydrophobic channel [PMID:40579455, PMID:38370643]. BLTP2 tethers the ER specifically to tubular endosomes and macropinosomes once these have become continuous with the PM, and its loss causes accumulation of PM-connected intracellular vacuoles, indicating that bulk ER-to-PM lipid transport is its core activity [PMID:40899996, PMID:39974967]. Its recruitment to the PM is achieved through a combination of phosphoinositide binding, the adaptor proteins FAM102A and FAM102B, and N-BAR domain proteins at membrane-connected tubules [PMID:40899996, PMID:39974967]; in evolutionarily conserved orthologs, dedicated adaptors (yeast Hoi1, Drosophila bilbobaggins) engage α-helical projections of the transfer channel or C-terminal cis-acting sequences to target the protein to ER-PM contacts, and loss of this targeting disrupts sterol homeostasis [PMID:41746219, PMID:39990326, PMID:42039411]. At the level of physiology, the Drosophila ortholog hobbit is required for synaptic development and neurotransmission at the neuromuscular junction [PMID:bio_10.1101_2024.12.30.630795].","teleology":[{"year":2025,"claim":"Established that BLTP2 functions as an ER-PM contact-site lipid transfer protein, answering what compartment it acts at and what lipid it moves.","evidence":"Live imaging/fractionation localization plus PE lipid profiling and loss-of-function PM fluidity readout in a metastasis model","pmids":["40579455","38370643"],"confidence":"High","gaps":["Direct in vitro reconstitution of PE transfer through the channel not shown","Selectivity for PE versus other phospholipids not fully defined"]},{"year":2025,"claim":"Defined the topology of BLTP2 tethering, showing it bridges ER to PM-continuous tubular endosomes and macropinosomes, clarifying that bulk ER-PM lipid transport is its key function.","evidence":"Live-cell imaging, BLTP2 KO with electron microscopy of vacuoles, and ER co-localization with PM-connected organelles","pmids":["40899996","39974967"],"confidence":"High","gaps":["Why tethering requires prior PM continuity is unresolved","Quantitative lipid flux at these contacts not measured"]},{"year":2025,"claim":"Identified the molecular determinants of PM recruitment, addressing how BLTP2 is targeted to its site of action.","evidence":"Co-IP/pulldown with FAM102A/FAM102B and N-BAR proteins, phosphoinositide binding assays, and loss-of-function imaging","pmids":["40899996","39974967"],"confidence":"Medium","gaps":["Reconstitution of the recruitment interactions not described","Relative contribution of each binding mode not dissected"]},{"year":2025,"claim":"Extended BLTP2 function to endosomal lipid biology, showing it is required for ILV/exosome formation and supplies PG for BMP/LBPA synthesis at ER-MVB contacts.","evidence":"BLTP2 KO/knockdown, co-IP with SCAMP3, Rab5 dependency, NEDD4 ubiquitination assay, endosomal lipidomics, and exosome quantification (preprint)","pmids":["bio_10.1101_2025.04.17.649455"],"confidence":"Medium","gaps":["Preprint, not yet peer-reviewed","Direct PG transfer to MVBs inferred from lipidomics, not reconstituted","SCAMP3-Rab5-NEDD4 regulatory axis lacks orthogonal validation"]},{"year":2026,"claim":"Resolved the conserved adaptor-based targeting mechanism, showing Hoi1 engages α-helical projections of the Fmp27/Hob channel to localize it to ER-PM contacts and maintain sterol homeostasis.","evidence":"Genetic identification, domain mapping, co-IP, yeast sterol readout, and cross-species epistasis in worms and flies","pmids":["41746219","39990326"],"confidence":"High","gaps":["Mechanistic link between targeting and sterol homeostasis unclear","Whether mammalian FAM102 adaptors act analogously not established"]},{"year":2026,"claim":"Described a two-step 'hook and latch' targeting in Drosophila, showing both the adaptor bilbobaggins and C-terminal cis-acting sequences of Hobbit are required for ER-PM localization.","evidence":"Drosophila loss-of-function genetics, C-terminal structure-function analysis, live imaging, and epistasis (preprint)","pmids":["42039411"],"confidence":"Medium","gaps":["Preprint, single lab","Structural basis of the C-terminal latch not defined","Conservation of bbo function in mammals untested"]},{"year":2024,"claim":"Connected BLTP2 to organismal physiology, showing its ortholog is required for synaptic development and neurotransmission.","evidence":"Tissue-specific RNAi in Drosophila neurons and muscle with locomotor, NMJ morphology, and electrophysiology readouts (preprint)","pmids":["bio_10.1101_2024.12.30.630795"],"confidence":"Medium","gaps":["Preprint, not peer-reviewed","Link between lipid transfer activity and synaptic phenotype not mechanistically established","Mammalian neuronal role untested"]},{"year":null,"claim":"Whether BLTP2's lipid-transfer activity has been directly demonstrated in vitro and how its various contact-site functions (PM fluidity, endosomal BMP synthesis, synaptic development) mechanistically interconnect remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No reconstituted transfer assay establishing direct lipid handoff","Substrate selectivity across PE, PG, and sterol phenotypes not unified","Human disease relevance not addressed in the corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,1,5]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[1,5]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,3]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[1,5]}],"complexes":[],"partners":["FAM102A","FAM102B","SCAMP3","HOI1","BILBOBAGGINS"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q14667","full_name":"Bridge-like lipid transfer protein family member 2","aliases":["Antigen MLAA-22","Breast cancer-overexpressed gene 1 protein","Protein hobbit homolog"],"length_aa":2235,"mass_kda":253.7,"function":"Tube-forming lipid transport protein which binds to phosphatidylinositols and affects phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) distribution","subcellular_location":"Cell membrane; Endoplasmic reticulum membrane; Mitochondrion membrane","url":"https://www.uniprot.org/uniprotkb/Q14667/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/BLTP2","classification":"Not Classified","n_dependent_lines":321,"n_total_lines":1208,"dependency_fraction":0.26572847682119205},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"ACTB","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/BLTP2","total_profiled":1310},"omim":[{"mim_id":"610664","title":"BRIDGE-LIKE LIPID TRANSFER PROTEIN FAMILY, MEMBER 2; BLTP2","url":"https://www.omim.org/entry/610664"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nuclear speckles","reliability":"Approved"},{"location":"Cytosol","reliability":"Approved"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/BLTP2"},"hgnc":{"alias_symbol":["DKFZp686M0843","MGC111488","BCOX1","CT101","BCOX","FMP27","Hob"],"prev_symbol":["KIAA0100"]},"alphafold":{"accession":"Q14667","domains":[{"cath_id":"-","chopping":"1332-1364_1375-1399_1408-1431_1446-1484","consensus_level":"medium","plddt":80.4383,"start":1332,"end":1484},{"cath_id":"-","chopping":"1559-1641_1650-1658_1675-1689_1698-1724_1749-1783","consensus_level":"medium","plddt":77.9158,"start":1559,"end":1783},{"cath_id":"-","chopping":"1796-1888","consensus_level":"high","plddt":75.0685,"start":1796,"end":1888},{"cath_id":"1.10.287","chopping":"886-950","consensus_level":"high","plddt":84.1128,"start":886,"end":950}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14667","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q14667-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q14667-F1-predicted_aligned_error_v6.png","plddt_mean":70.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=BLTP2","jax_strain_url":"https://www.jax.org/strain/search?query=BLTP2"},"sequence":{"accession":"Q14667","fasta_url":"https://rest.uniprot.org/uniprotkb/Q14667.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q14667/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q14667"}},"corpus_meta":[{"pmid":"12668631","id":"PMC_12668631","title":"Provitamin A conversion to retinal via the beta,beta-carotene-15,15'-oxygenase (bcox) is essential for pattern formation and differentiation during zebrafish embryogenesis.","date":"2003","source":"Development (Cambridge, England)","url":"https://pubmed.ncbi.nlm.nih.gov/12668631","citation_count":108,"is_preprint":false},{"pmid":"26338045","id":"PMC_26338045","title":"MicroRNA-195 suppresses tumor cell proliferation and metastasis by directly targeting BCOX1 in prostate carcinoma.","date":"2015","source":"Journal of experimental & clinical cancer research : CR","url":"https://pubmed.ncbi.nlm.nih.gov/26338045","citation_count":47,"is_preprint":false},{"pmid":"16289875","id":"PMC_16289875","title":"Identification of BCOX1, a novel gene overexpressed in breast cancer.","date":"2005","source":"Biochimica et biophysica acta","url":"https://pubmed.ncbi.nlm.nih.gov/16289875","citation_count":24,"is_preprint":false},{"pmid":"15654843","id":"PMC_15654843","title":"Photoreceptor morphology is severely affected in the beta,beta-carotene-15,15'-oxygenase (bcox) zebrafish morphant.","date":"2005","source":"The European journal of neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/15654843","citation_count":18,"is_preprint":false},{"pmid":"40579455","id":"PMC_40579455","title":"The Vps13-like protein BLTP2 regulates phosphatidylethanolamine levels to maintain plasma membrane fluidity and breast cancer aggressiveness.","date":"2025","source":"Nature cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/40579455","citation_count":17,"is_preprint":false},{"pmid":"38370643","id":"PMC_38370643","title":"The Vps13-like protein BLTP2 is pro-survival and regulates phosphatidylethanolamine levels in the plasma membrane to maintain its fluidity and function.","date":"2024","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/38370643","citation_count":9,"is_preprint":false},{"pmid":"40899996","id":"PMC_40899996","title":"Multiple interactions recruit BLTP2 to ER-PM contacts to control plasma membrane dynamics.","date":"2025","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/40899996","citation_count":6,"is_preprint":false},{"pmid":"41746219","id":"PMC_41746219","title":"Hoi1 targets BLTP2 to ER-PM contact sites to regulate lipid homeostasis.","date":"2026","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/41746219","citation_count":3,"is_preprint":false},{"pmid":"39990326","id":"PMC_39990326","title":"Hoi1 targets the yeast BLTP2 protein to ER-PM contact sites to regulate lipid homeostasis.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/39990326","citation_count":2,"is_preprint":false},{"pmid":"39974967","id":"PMC_39974967","title":"Multiple interactions mediate the localization of BLTP2 at ER-PM contacts to control plasma membrane dynamics.","date":"2025","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/39974967","citation_count":1,"is_preprint":false},{"pmid":"40359410","id":"PMC_40359410","title":"The histone deacetylase inhibitor CT-101 flips the switch to fetal hemoglobin expression in sickle cell disease mice.","date":"2025","source":"PloS one","url":"https://pubmed.ncbi.nlm.nih.gov/40359410","citation_count":0,"is_preprint":false},{"pmid":"42029693","id":"PMC_42029693","title":"Building bridges: BLTP2 forms ER-plasma membrane contact sites.","date":"2026","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/42029693","citation_count":0,"is_preprint":false},{"pmid":"42039411","id":"PMC_42039411","title":"Mechanistic dissection of BLTP2 targeting to ER-PM contact sites.","date":"2026","source":"bioRxiv : the preprint server for biology","url":"https://pubmed.ncbi.nlm.nih.gov/42039411","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.04.17.649455","title":"BLTP2 orchestrates lysobisphosphatidic acid synthesis and exosome biogenesis via SCAMP3-dependent ER contacts in tumorigenesis","date":"2025-04-21","source":"bioRxiv","url":"https://doi.org/10.1101/2025.04.17.649455","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2024.12.30.630795","title":"Neurodegenerative and neurodevelopmental roles for bulk lipid transporters  <i>VPS13A</i>  and  <i>BLTP2</i>  in movement disorders","date":"2024-12-30","source":"bioRxiv","url":"https://doi.org/10.1101/2024.12.30.630795","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":8545,"output_tokens":2039,"usd":0.02811,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":8956,"output_tokens":2780,"usd":0.05714,"stage2_stop_reason":"end_turn"},"total_usd":0.08525,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2025,\n      \"finding\": \"BLTP2 localizes to endoplasmic reticulum (ER)-plasma membrane (PM) contact sites and regulates PM fluidity by increasing phosphatidylethanolamine (PE) levels in the PM, consistent with a role in transporting PE from ER to PM via a bridge-like lipid transfer mechanism.\",\n      \"method\": \"Subcellular localization (live imaging/fractionation), lipid profiling (PE measurement), loss-of-function knockdown/knockout with PM fluidity readout, in vivo metastasis model\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — localization established by direct imaging, PE transport supported by lipid profiling and KO phenotype, replicated across peer-reviewed paper (PMID:40579455) and preprint (PMID:38370643) from same lab using multiple orthogonal methods\",\n      \"pmids\": [\"40579455\", \"38370643\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"BLTP2 tethers the ER to tubular endosomes only after they become continuous with the PM, and to macropinosomes in the process of fusing with the PM, indicating bulk lipid transport between ER and PM is its key function; absence of BLTP2 causes accumulation of intracellular vacuoles many of which are connected to the PM.\",\n      \"method\": \"Live-cell imaging, loss-of-function (BLTP2 KO), electron microscopy of intracellular vacuoles, co-localization of ER with PM-connected endosomes/macropinosomes\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal imaging and KO phenotype with structural readout in peer-reviewed paper, supported by preprint with same data (PMID:39974967)\",\n      \"pmids\": [\"40899996\", \"39974967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PM binding of BLTP2 is mediated by multiple interactions: phosphoinositides, the adaptor proteins FAM102A and FAM102B, and N-BAR domain proteins at membrane-connected tubules.\",\n      \"method\": \"Co-immunoprecipitation/pulldown of BLTP2 with FAM102A/FAM102B and N-BAR proteins, phosphoinositide binding assays, loss-of-function imaging\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple binding partners identified in single lab study with KO phenotype, but full reconstitution not explicitly described in abstract\",\n      \"pmids\": [\"40899996\", \"39974967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"The conserved adaptor protein Hoi1 (Ybl086c) targets yeast BLTP2-like proteins Fmp27/Hob1 and Hob2 to ER-PM contact sites via two separate Hoi1 domains that interface with α-helical projections decorating the central hydrophobic channel on Fmp27; loss of these interactions disrupts cellular sterol homeostasis.\",\n      \"method\": \"Genetic identification of Hoi1, domain mapping, co-IP/interaction assays, sterol homeostasis readout in yeast mutants, genetic epistasis in worms and flies\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — domain-level interaction mapping combined with functional sterol phenotype, cross-species epistasis in worms and flies, replicated across peer-reviewed paper (PMID:41746219) and preprint (PMID:39990326)\",\n      \"pmids\": [\"41746219\", \"39990326\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"In Drosophila, the conserved adaptor bilbobaggins (bbo) is required for targeting of Hobbit (BLTP2 ortholog) to ER-PM contact sites; loss of bbo phenocopies loss of hobbit. Additionally, cis-acting sequences in the C-terminal tail of Hobbit are independently required for ER-PM targeting, operating as a 'hook and latch' two-step targeting mechanism.\",\n      \"method\": \"Drosophila genetics (loss-of-function), structure-function analysis of C-terminal sequences, live imaging of Hobbit localization, epistasis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis plus structure-function in Drosophila, single lab, preprint only\",\n      \"pmids\": [\"42039411\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"BLTP2 is indispensable for intraluminal vesicle (ILV)/exosome formation; SCAMP3 recruits BLTP2 to ER-MVB membrane contact sites in a Rab5-dependent manner, and this recruitment is inhibited by NEDD4-mediated ubiquitination of SCAMP3. Depletion of BLTP2 selectively reduces cone-shaped phospholipids including BMP/LBPA and its precursor phosphatidylglycerol (PG) within endosomes, suggesting BLTP2 transfers PG to MVBs for BMP/LBPA synthesis.\",\n      \"method\": \"BLTP2 KO/knockdown, co-IP of BLTP2 with SCAMP3, Rab5 dependency assay, NEDD4 ubiquitination assay, lipidomic profiling of endosomes, exosome quantification\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — multiple orthogonal methods (co-IP, lipidomics, KO phenotype) in single lab, preprint only, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.04.17.649455\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In Drosophila, neuronal loss of the BLTP2 ortholog (hobbit) results in severe early-onset locomotor defects without neurodegeneration, while muscle-specific loss impairs synaptogenesis and neurotransmission at the neuromuscular junction (NMJ), establishing a role for BLTP2 in synaptic development.\",\n      \"method\": \"Tissue-specific RNAi knockdown in Drosophila neurons and muscles, age-dependent locomotor behavioral assays, NMJ morphology and electrophysiology\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — tissue-specific KD with behavioral and electrophysiological readouts, multiple tissues tested, preprint only\",\n      \"pmids\": [\"bio_10.1101_2024.12.30.630795\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"BLTP2 is a bridge-like lipid transfer protein that localizes to ER-plasma membrane (and ER-MVB) contact sites, where it mediates bulk non-vesicular transport of phospholipids (particularly phosphatidylethanolamine and phosphatidylglycerol/BMP precursors) to maintain PM fluidity and endosomal lipid composition; its targeting to ER-PM contacts requires conserved adaptor proteins (Hoi1/bilbobaggins orthologs), phosphoinositide binding, and FAM102A/FAM102B interactions, and its function at the NMJ is required for synaptic development in Drosophila.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"BLTP2 is a bridge-like lipid transfer protein that operates at membrane contact sites to mediate bulk non-vesicular phospholipid transport, sustaining the lipid composition of the plasma membrane and endosomal compartments [#0, #1]. It localizes to ER-plasma membrane (PM) contact sites, where it raises PM phosphatidylethanolamine levels and thereby regulates PM fluidity, consistent with transfer of PE from the ER to the PM through its hydrophobic channel [#0]. BLTP2 tethers the ER specifically to tubular endosomes and macropinosomes once these have become continuous with the PM, and its loss causes accumulation of PM-connected intracellular vacuoles, indicating that bulk ER-to-PM lipid transport is its core activity [#1]. Its recruitment to the PM is achieved through a combination of phosphoinositide binding, the adaptor proteins FAM102A and FAM102B, and N-BAR domain proteins at membrane-connected tubules [#2]; in evolutionarily conserved orthologs, dedicated adaptors (yeast Hoi1, Drosophila bilbobaggins) engage α-helical projections of the transfer channel or C-terminal cis-acting sequences to target the protein to ER-PM contacts, and loss of this targeting disrupts sterol homeostasis [#3, #4]. At the level of physiology, the Drosophila ortholog hobbit is required for synaptic development and neurotransmission at the neuromuscular junction [#6].\",\n  \"teleology\": [\n    {\n      \"year\": 2025,\n      \"claim\": \"Established that BLTP2 functions as an ER-PM contact-site lipid transfer protein, answering what compartment it acts at and what lipid it moves.\",\n      \"evidence\": \"Live imaging/fractionation localization plus PE lipid profiling and loss-of-function PM fluidity readout in a metastasis model\",\n      \"pmids\": [\"40579455\", \"38370643\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct in vitro reconstitution of PE transfer through the channel not shown\", \"Selectivity for PE versus other phospholipids not fully defined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Defined the topology of BLTP2 tethering, showing it bridges ER to PM-continuous tubular endosomes and macropinosomes, clarifying that bulk ER-PM lipid transport is its key function.\",\n      \"evidence\": \"Live-cell imaging, BLTP2 KO with electron microscopy of vacuoles, and ER co-localization with PM-connected organelles\",\n      \"pmids\": [\"40899996\", \"39974967\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why tethering requires prior PM continuity is unresolved\", \"Quantitative lipid flux at these contacts not measured\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identified the molecular determinants of PM recruitment, addressing how BLTP2 is targeted to its site of action.\",\n      \"evidence\": \"Co-IP/pulldown with FAM102A/FAM102B and N-BAR proteins, phosphoinositide binding assays, and loss-of-function imaging\",\n      \"pmids\": [\"40899996\", \"39974967\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Reconstitution of the recruitment interactions not described\", \"Relative contribution of each binding mode not dissected\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extended BLTP2 function to endosomal lipid biology, showing it is required for ILV/exosome formation and supplies PG for BMP/LBPA synthesis at ER-MVB contacts.\",\n      \"evidence\": \"BLTP2 KO/knockdown, co-IP with SCAMP3, Rab5 dependency, NEDD4 ubiquitination assay, endosomal lipidomics, and exosome quantification (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.04.17.649455\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not yet peer-reviewed\", \"Direct PG transfer to MVBs inferred from lipidomics, not reconstituted\", \"SCAMP3-Rab5-NEDD4 regulatory axis lacks orthogonal validation\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Resolved the conserved adaptor-based targeting mechanism, showing Hoi1 engages α-helical projections of the Fmp27/Hob channel to localize it to ER-PM contacts and maintain sterol homeostasis.\",\n      \"evidence\": \"Genetic identification, domain mapping, co-IP, yeast sterol readout, and cross-species epistasis in worms and flies\",\n      \"pmids\": [\"41746219\", \"39990326\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic link between targeting and sterol homeostasis unclear\", \"Whether mammalian FAM102 adaptors act analogously not established\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Described a two-step 'hook and latch' targeting in Drosophila, showing both the adaptor bilbobaggins and C-terminal cis-acting sequences of Hobbit are required for ER-PM localization.\",\n      \"evidence\": \"Drosophila loss-of-function genetics, C-terminal structure-function analysis, live imaging, and epistasis (preprint)\",\n      \"pmids\": [\"42039411\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, single lab\", \"Structural basis of the C-terminal latch not defined\", \"Conservation of bbo function in mammals untested\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Connected BLTP2 to organismal physiology, showing its ortholog is required for synaptic development and neurotransmission.\",\n      \"evidence\": \"Tissue-specific RNAi in Drosophila neurons and muscle with locomotor, NMJ morphology, and electrophysiology readouts (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.12.30.630795\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Link between lipid transfer activity and synaptic phenotype not mechanistically established\", \"Mammalian neuronal role untested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Whether BLTP2's lipid-transfer activity has been directly demonstrated in vitro and how its various contact-site functions (PM fluidity, endosomal BMP synthesis, synaptic development) mechanistically interconnect remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No reconstituted transfer assay establishing direct lipid handoff\", \"Substrate selectivity across PE, PG, and sterol phenotypes not unified\", \"Human disease relevance not addressed in the corpus\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 1, 5]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [1, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 3]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [1, 5]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"FAM102A\", \"FAM102B\", \"SCAMP3\", \"Hoi1\", \"bilbobaggins\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":4,"faith_total":4,"faith_pct":100.0}}