{"gene":"VPS33B","run_date":"2026-04-28T23:00:23","timeline":{"discoveries":[{"year":2004,"finding":"VPS33B encodes a homolog of class C yeast vacuolar protein sorting gene Vps33 containing a Sec1-like domain, identifying it as a regulator of SNARE-dependent vesicle-to-target membrane fusion.","method":"Positional cloning, sequence homology analysis, identification of germline mutations in ARC syndrome patients","journal":"Nature genetics","confidence":"High","confidence_rationale":"Tier 2 — foundational identification with strong genetic evidence across 14 kindreds, replicated extensively","pmids":["15052268"],"is_preprint":false},{"year":2005,"finding":"VPS33B is essential for platelet alpha-granule biogenesis in megakaryocytes but not required for granule secretion; VPS33B colocalizes with alpha-granule markers and late endosomes/lysosomes in megakaryocytes, and is absent from platelets themselves.","method":"Immunofluorescence microscopy, immunoblotting of ARC patient platelets and megakaryocytes, electron microscopy, platelet aggregation assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods, patient cells and normal controls, replicated by subsequent studies","pmids":["16123220"],"is_preprint":false},{"year":2005,"finding":"Zebrafish vps33b acts downstream of the hnf6/vhnf1 transcription factor pathway to regulate biliary development; vhnf1 directly binds the vps33b promoter to increase its expression.","method":"Morpholino knockdown, epistasis analysis, electrophoretic mobility shift assay (EMSA), overexpression in embryos and mammalian liver cells","journal":"Development (Cambridge, England)","confidence":"High","confidence_rationale":"Tier 1-2 — EMSA demonstrates direct promoter binding, combined with genetic epistasis in zebrafish","pmids":["16284120"],"is_preprint":false},{"year":2008,"finding":"VPS33B deficiency causes defective lamellar granule secretion in keratinocytes; ultrastructural analysis showed lamellar granules entombed in cornified cells rather than being secreted, indicating VPS33B is required for SNARE-mediated vesicle fusion in epidermal differentiation.","method":"VPS33B mutation sequencing, ultrastructural electron microscopy of ARC patient skin, mRNA splicing analysis","journal":"Archives of dermatology","confidence":"Medium","confidence_rationale":"Tier 2 — direct ultrastructural evidence in patient tissue, single lab","pmids":["18347289"],"is_preprint":false},{"year":2012,"finding":"VPS33B interacts with VPS16B (encoded by C14orf133); both proteins colocalize at the trans-Golgi network, late endosomes, and alpha-granules in megakaryocytes, and VPS16B is required for platelet alpha-granule biogenesis.","method":"Yeast two-hybrid screen, mass spectrometry, coimmunoprecipitation, immunofluorescence microscopy, immunoblotting of ARC patient platelets","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including Y2H, MS, Co-IP, and patient phenotyping","pmids":["23002115"],"is_preprint":false},{"year":2012,"finding":"SPE-39 (VIPAS39/VPS16B) undergoes tyrosine phosphorylation and ubiquitination upon EGF stimulation; VPS33B association with SPE-39 inhibits EGF-induced ubiquitination of SPE-39, stabilizing it; SPE-39 and VPS33B have opposing effects on EGF receptor downregulation.","method":"EGF stimulation assays, immunoprecipitation, ubiquitination assays, overexpression/knockdown in COS-7 cells","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 — direct biochemical interaction and functional consequence shown, single lab","pmids":["22677173"],"is_preprint":false},{"year":2013,"finding":"VPS33B pathogenic mutations alter subcellular localization of VPS33B to VIPAS39/SPE-39-positive endosomes and some mutants fragment VIPAS39-positive endosomes, implicating VPS33B in VIPAS39-dependent endosomal maturation or fusion.","method":"Yeast two-hybrid, immunoprecipitation, quantitative fluorescence microscopy","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2-3 — multiple methods but functional readout is localization-based, single lab","pmids":["23918659"],"is_preprint":false},{"year":2015,"finding":"VPS33B depletion in HeLa cells causes accumulation of late endosomes in the perinuclear region, impairs cargo degradation, and blocks delivery of endocytosed BSA-gold from late endosomes to lysosomes, indicating VPS33B is required for late endosomal-lysosomal fusion.","method":"siRNA knockdown, fluorescence microscopy, electron microscopy with BSA-gold endocytosis assay","journal":"Traffic (Copenhagen, Denmark)","confidence":"High","confidence_rationale":"Tier 2 — orthogonal microscopy methods with quantitative cargo tracking, mechanistically defined phenotype","pmids":["26403612"],"is_preprint":false},{"year":2015,"finding":"VPS33B deficiency in a tamoxifen-inducible mouse model causes reduction in platelet alpha-granules, accumulation of large vacuoles in megakaryocytes, reduction in mature type-II multivesicular bodies (MVB II), and a defect in delta-granule secretion and stable aggregate formation under arteriolar shear.","method":"Tamoxifen-inducible conditional knockout, ultrastructural and immunoelectron microscopy, platelet aggregation assays, tail-bleeding assay, FeCl3 thrombosis model","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — in vivo mouse model with multiple functional and ultrastructural readouts","pmids":["25947942"],"is_preprint":false},{"year":2015,"finding":"VPS33B binds directly to the integrin beta subunit; VPS33B overexpression potentiates αIIbβ3 outside-in signaling (but not inside-out signaling); megakaryocyte/platelet-specific VPS33B knockout mice show impaired spreading on fibrinogen, defective clot retraction, and reduced aggregation, placing VPS33B upstream of RhoA-ROCK-MLC and Rac1-dependent pathways.","method":"Direct binding assay, CHO cell overexpression, megakaryocyte/platelet-specific conditional knockout mice, platelet spreading, clot retraction, aggregation assays, FeCl3 thrombosis model","journal":"Circulation","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding demonstrated, conditional KO with multiple functional assays and pathway placement","pmids":["26399659"],"is_preprint":false},{"year":2016,"finding":"VPS33B interacts with the GDI2/RAB11A/RAB27A pathway to regulate trafficking of secretory proteins as exosomes in hematopoietic stem cells; VPS33B co-exists in exosomes with GDI2, VPS16B, FLOT1 and other exosome markers.","method":"VPS33B deletion in mouse and human HSCs, exosome purification and characterization, co-immunoprecipitation, rescue experiments with purified exosomes","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — genetic deletion in two species, Co-IP, rescue experiments, multiple orthogonal approaches","pmids":["27797340"],"is_preprint":false},{"year":2016,"finding":"Drosophila and mammalian Vps33B are required for maturation of phagosomes and endosomes containing pattern recognition receptors (PRRs) following microbial recognition; loss of Vps33B results in enhanced inflammatory signaling due to failure to clear PRR-containing endosomes, while trafficking of non-microbial cargo is unaffected.","method":"Drosophila Vps33B mutants, mammalian cell knockdown, phagocytosis assays, inflammatory mediator quantification, endosomal trafficking assays","journal":"Immunity","confidence":"High","confidence_rationale":"Tier 2 — ortholog study in two organisms, multiple assay types, mechanistically distinct cargo selectivity demonstrated","pmids":["27496733"],"is_preprint":false},{"year":2016,"finding":"VPS33B associates with VIPAS39, alpha-tubulin, and SEC22B; VIPAS39 binds intact VPS33B while alpha-tubulin and SEC22B interact separately with the Sec1-like domains of VPS33B; Vps33b deficiency disrupts redistribution of Vipas39 and Sec22b to proplatelets and interrupts colocalization of Sec22b with Vwf-positive vesicles in megakaryocytes.","method":"Coimmunoprecipitation, mass spectrometry, immunoblotting, pull-down assays, conditional knockout mice, immunofluorescence microscopy","journal":"The Journal of pathology","confidence":"High","confidence_rationale":"Tier 1-2 — pull-down and Co-IP with domain mapping, combined with conditional KO functional data","pmids":["27319744"],"is_preprint":false},{"year":2016,"finding":"VPS33B interacts with Rab11a and Rab25; the p.Gly131Glu ARC/ARKID-causing mutation reduces coimmunoprecipitation and colocalization with Rab11a and Rab25 and fails to rescue trafficking of the collagen-modifying enzyme LH3, linking VPS33B to LH3 trafficking and collagen lysine modification.","method":"Mutagenesis, coimmunoprecipitation, colocalization microscopy, LH3 trafficking rescue assay, urine/fibroblast collagen modification analysis","journal":"The Journal of investigative dermatology","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis combined with Co-IP, trafficking assay, and biochemical validation in patient samples","pmids":["28017832"],"is_preprint":false},{"year":2017,"finding":"VPS33B interacts with RAB11A at recycling endosomes and is required for trafficking of ABC transporters specifically trafficked via Rab11a-positive recycling endosomes to the hepatocyte apical membrane; Vps33b liver-specific knockout causes mislocalisation of these apical proteins, impaired tight junction integrity, and loss of functional bile secretion.","method":"Liver-specific conditional knockout mice, bile/plasma mass spectrometry, immunostaining of apical membrane and tight junction proteins, electron microscopy, adeno-associated virus gene rescue","journal":"Journal of hepatology","confidence":"High","confidence_rationale":"Tier 2 — in vivo conditional KO with multiple readouts and gene rescue experiment","pmids":["28082148"],"is_preprint":false},{"year":2018,"finding":"VPS33B and VIPAR (VPS16B) are required for epidermal lamellar body biogenesis; Vps33b and Vipar knockout mice show abnormal lamellar body morphology, disrupted cargo localisation, impaired stratum corneum formation, and reduced corneocyte/cornified envelope integrity.","method":"Mouse knockouts (Vps33b and Vipar), histology, immunofluorescence, electron microscopy, primary cell isolation","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"High","confidence_rationale":"Tier 2 — two knockout mouse models with multiple orthogonal structural and functional readouts","pmids":["29409756"],"is_preprint":false},{"year":2018,"finding":"VPS33B does not associate with CORVET or HOPS complex subunits but stably interacts with VIPAR; the VPS33B/VIPAR complex is considerably smaller than CORVET/HOPS; VPS33B transiently interacts with CCDC22, a CCC complex member, which does not stably co-fractionate with VPS33B/VIPAR.","method":"BioID proximity biotinylation proteomics, gel filtration of human cell lysates, co-localization analysis","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 2 — BioID interactome complemented by gel filtration, distinguishes stable vs transient interactions","pmids":["29778605"],"is_preprint":false},{"year":2019,"finding":"VPS33B and VPS16B form a distinct small complex with the same hydrodynamic radius as a recombinant VPS33B-VPS16B heterodimer purified from bacteria; this complex localizes to the recycling endosome in megakaryocytes, which is a key intermediate compartment for alpha-granule biogenesis; VPS33B deficiency causes alpha-granule cargo degradation in lysosomes.","method":"CRISPR/Cas9 VPS33B knockout in imMKCLs, recombinant protein purification, size-exclusion chromatography, cargo trafficking pulse-chase mapping, GFP-VPS33B rescue","journal":"Blood advances","confidence":"High","confidence_rationale":"Tier 1-2 — reconstituted recombinant complex, CRISPR KO, cargo tracking, and rescue experiments","pmids":["31501156"],"is_preprint":false},{"year":2019,"finding":"The VPS33B missense variant p.Cys576Arg abolishes interaction with VIPAS39 in vitro, demonstrating the functional importance of this region for VPS33B-VIPAS39 complex formation.","method":"In vitro binding/coimmunoprecipitation assay with mutant constructs","journal":"Human mutation","confidence":"Medium","confidence_rationale":"Tier 3 — single Co-IP with mutant construct, single lab","pmids":["31479177"],"is_preprint":false},{"year":2020,"finding":"Loss of p38b MAP kinase reduces enhanced inflammatory responses and prolongs survival of Vps33B-deficient Drosophila; p38 MAPK modulates endosomal trafficking of the PGRP-LC innate immune receptor and phagocytosis of bacteria, placing p38b downstream of Vps33B in the regulation of innate immune endosomal trafficking.","method":"Drosophila genetic epistasis (Vps33B mutant x p38b mutant), survival assays, constitutively active/dominant negative p38b expression, endosomal trafficking assays, phagocytosis assays","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis in Drosophila ortholog with multiple functional readouts","pmids":["32677257"],"is_preprint":false},{"year":2022,"finding":"VPS16B/VPS33B complex physically associates with Syntaxin 12 (Stx12), a SNARE protein mediating vesicle fusion at endosomes; Stx12 deficiency reduces alpha-granule numbers and alpha-granule protein levels in megakaryocytes; VPS16B/VPS33B also binds CCDC22 (CCC complex), and CCDC22 competes with Stx12 for binding to VPS16B/VPS33B, suggesting a hand-off mechanism at endosomes.","method":"Co-immunoprecipitation, CRISPR knockout of Stx12 and COMMD3/CCDC22 in megakaryocytes, immunofluorescence, flow cytometry, electron microscopy","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — Co-IP for interaction, CRISPR KO for functional validation, competition assay for mechanism, multiple readouts","pmids":["34905616"],"is_preprint":false},{"year":2022,"finding":"Vps33B binds with the lysosomal nutrient-sensing complex (LYNUS) and promotes late endosome-lysosome fusion and clearance of LYNUS-containing late endosomes, thereby suppressing mTORC1 activation; Vps33B deficiency in Treg cells leads to disordered endolysosomal fusion, LYNUS accumulation, elevated mTORC1 activation, and hyper-glycolytic metabolism.","method":"Treg-specific conditional knockout mice, mTORC1 activity assays, endosomal trafficking assays, metabolic profiling, co-immunoprecipitation","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with mechanistic pathway placement (LYNUS/mTORC1), Co-IP for binding, multiple orthogonal readouts","pmids":["35705052"],"is_preprint":false},{"year":2022,"finding":"Stable expression of VPS16B in platelets and megakaryocytes is dependent on VPS33B; loss of VPS33B results in loss of both VPS33B and VPS16B protein expression.","method":"Immunoblotting of platelets from ARC patients with VPS33B nonsense variant, protein expression analysis","journal":"Journal of thrombosis and haemostasis : JTH","confidence":"Medium","confidence_rationale":"Tier 3 — single patient case with immunoblotting, corroborated by prior VPS33B-KO imMKCL data","pmids":["35325493"],"is_preprint":false},{"year":2023,"finding":"Human VPS33B-VPS16B forms a high molecular weight complex (~315 kDa) with a VPS33B:VPS16B stoichiometry of approximately 2:3; the complex has a two-lobed shape with one VPS33B molecule at each end oriented in opposite directions; truncated VPS16B (aa 143-316) is sufficient for complex formation; the ARC-causing L30P mutation disrupts complex formation while S243F and H344D do not.","method":"Yeast expression of recombinant complex, circular dichroism, size-exclusion chromatography-MALS, quantitative immunoblotting, small-angle X-ray scattering, negative-stain electron microscopy, avidin tagging, mutagenesis of ARC variants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted complex with structural characterization by multiple methods, mutagenesis of disease variants","pmids":["37062417"],"is_preprint":false},{"year":2023,"finding":"METTL16 methyltransferase induces m6A modification of VPS33B mRNA, impairing VPS33B transcript stability and thereby reducing VPS33B protein levels; this promotes osteosarcoma progression through the PI3K/AKT pathway.","method":"m6A profiling, METTL16 knockdown/overexpression, mRNA stability assays, VPS33B knockdown rescue experiments, in vivo tumor growth","journal":"Journal of cellular physiology","confidence":"Medium","confidence_rationale":"Tier 2 — m6A modification mechanistically linked to VPS33B mRNA stability with functional rescue, single lab","pmids":["37357526"],"is_preprint":false},{"year":2026,"finding":"VPS33B knockout in proximal tubular epithelial cells (RPTEC-TERT1) causes a 'peeling' phenotype with altered cell adhesion and cell-matrix attachment defects, accompanied by transcriptional changes in adhesion-related genes.","method":"CRISPR/Cas9 knockout, brightfield imaging, immunostaining, RNA sequencing, cell detachment assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 — CRISPR KO with transcriptomic and functional adhesion readouts, single lab, no rescue experiment","pmids":["41686830"],"is_preprint":false}],"current_model":"VPS33B is a Sec1/Munc18 family protein that forms a uniquely bidirectional ~315 kDa complex with VPS16B (VIPAR/VIPAS39) — with 2:3 stoichiometry and one VPS33B at each end — to regulate SNARE-dependent membrane fusion events at recycling endosomes and late endosomes, functioning in trafficking of cargo (including ABC transporters, alpha-granule proteins, LH3, lamellar body contents, and PRR-containing endosomes) through endosomal compartments; it promotes late endosome-lysosome fusion, interacts transiently with the CCC complex (CCDC22) and stably with Rab11A, Rab25, SEC22B, and VIPAS39 to coordinate endosomal entry and exit, and additionally modulates integrin outside-in signaling upstream of RhoA-ROCK and Rac1 pathways and suppresses mTORC1 activation by promoting LYNUS complex clearance through endolysosomal fusion."},"narrative":{"teleology":[{"year":2004,"claim":"Positional cloning of VPS33B as the ARC syndrome gene established that a human Sec1/Munc18 homolog is essential for multiorgan vesicle trafficking, resolving the molecular basis of this Mendelian disorder.","evidence":"Positional cloning and mutation analysis across 14 ARC kindreds","pmids":["15052268"],"confidence":"High","gaps":["No binding partners or specific trafficking step identified","Sec1-like domain function not biochemically characterized"]},{"year":2005,"claim":"Demonstration that VPS33B localizes to alpha-granule compartments and late endosomes in megakaryocytes, and that ARC patient platelets lack alpha-granules, defined the first cell-biological function—granule biogenesis rather than secretion.","evidence":"Immunofluorescence, electron microscopy, and functional platelet assays in ARC patient and control megakaryocytes/platelets","pmids":["16123220"],"confidence":"High","gaps":["Mechanism of granule biogenesis role unknown","Whether VPS33B acts at a specific endosomal sorting step undefined"]},{"year":2005,"claim":"Identification that zebrafish vps33b is transcriptionally regulated by hnf6/vhnf1 and required for biliary development provided the first developmental-genetic context for VPS33B function outside hematopoiesis.","evidence":"Morpholino knockdown, EMSA for promoter binding, epistasis analysis in zebrafish","pmids":["16284120"],"confidence":"High","gaps":["Whether transcriptional regulation is conserved in mammals not tested","Downstream trafficking targets in bile duct cells unidentified"]},{"year":2012,"claim":"Discovery that VPS33B physically interacts with VPS16B (VIPAS39) and that both colocalize at trans-Golgi network, late endosomes, and alpha-granules revealed VPS33B operates as part of a dedicated heteromeric complex, paralleling but distinct from HOPS/CORVET.","evidence":"Yeast two-hybrid, mass spectrometry, co-immunoprecipitation, and immunofluorescence in megakaryocytes; EGF-stimulation and ubiquitination assays in COS-7 cells","pmids":["23002115","22677173"],"confidence":"High","gaps":["Stoichiometry and structure of the complex unknown","Whether VPS33B-VPS16B complex functions independently of HOPS/CORVET not resolved"]},{"year":2015,"claim":"Functional studies in HeLa cells and conditional knockout mice established that VPS33B is required for late endosome–lysosome fusion and that its loss causes perinuclear accumulation of late endosomes, cargo degradation failure, and alpha/delta-granule defects in vivo.","evidence":"siRNA knockdown with BSA-gold cargo tracking in HeLa; tamoxifen-inducible KO mice with ultrastructural and functional platelet assays","pmids":["26403612","25947942"],"confidence":"High","gaps":["SNARE partner mediating the fusion step not identified","Whether VPS33B acts catalytically or as a scaffold at the fusion step unclear"]},{"year":2015,"claim":"Direct binding of VPS33B to integrin β subunits and potentiation of αIIbβ3 outside-in signaling upstream of RhoA-ROCK and Rac1 pathways expanded VPS33B function beyond vesicle trafficking to include integrin signal transduction.","evidence":"Direct binding assay, CHO overexpression, megakaryocyte/platelet-specific conditional KO mice with spreading, clot retraction, and thrombosis assays","pmids":["26399659"],"confidence":"High","gaps":["Structural basis of integrin β–VPS33B interaction unknown","Whether integrin signaling role is independent of vesicle trafficking not fully resolved"]},{"year":2016,"claim":"Studies in Drosophila and mammalian cells demonstrated cargo-selective trafficking: VPS33B is specifically required for maturation of PRR-containing endosomes after microbial recognition, with its loss amplifying inflammatory signaling while non-microbial cargo trafficking remains intact.","evidence":"Drosophila Vps33B mutants and mammalian knockdown, phagocytosis assays, inflammatory mediator quantification","pmids":["27496733"],"confidence":"High","gaps":["How cargo selectivity is achieved mechanistically unknown","Whether selectivity is conferred by VPS33B itself or an adaptor unclear"]},{"year":2016,"claim":"Mapping of VPS33B interactions to Rab11a, Rab25, SEC22B, and alpha-tubulin, with domain-level resolution and functional validation in megakaryocytes, defined a recycling-endosome-centered interaction network and linked VPS33B to LH3 trafficking and collagen modification.","evidence":"Co-IP, mass spectrometry, pull-down with domain constructs, conditional KO mice, LH3 trafficking rescue, patient collagen analysis","pmids":["27319744","28017832","27797340"],"confidence":"High","gaps":["Whether Rab11a/Rab25 binding is direct or via VIPAS39 not fully dissected","Structural basis of Sec1-domain interactions with SEC22B undefined"]},{"year":2017,"claim":"Liver-specific knockout established that VPS33B–Rab11a interaction is required for apical trafficking of ABC transporters and tight junction maintenance in hepatocytes, directly explaining the cholestasis phenotype in ARC syndrome.","evidence":"Liver-specific conditional KO mice with bile/plasma MS, immunostaining, EM, and AAV gene rescue","pmids":["28082148"],"confidence":"High","gaps":["Specific SNARE or tethering factor mediating apical delivery not identified","Whether VPS33B acts at sorting, transport, or fusion at the apical membrane unclear"]},{"year":2018,"claim":"BioID proteomics definitively separated VPS33B from the CORVET/HOPS pathway, confirming it forms a distinct small complex with VIPAS39 that transiently contacts the CCC complex (CCDC22), establishing a unique identity for VPS33B-containing machinery.","evidence":"BioID proximity biotinylation, gel filtration chromatography, colocalization analysis in human cells","pmids":["29778605"],"confidence":"High","gaps":["Function of transient CCC interaction unknown","Whether other transient interactors exist not exhaustively surveyed"]},{"year":2019,"claim":"Reconstitution of recombinant VPS33B–VPS16B heterodimer and CRISPR KO in megakaryocytes pinpointed the recycling endosome as the key compartment where VPS33B acts, with its loss causing alpha-granule cargo to be mis-sorted to lysosomes for degradation.","evidence":"Recombinant protein purification, SEC, CRISPR KO in imMKCLs, cargo pulse-chase, GFP-VPS33B rescue","pmids":["31501156"],"confidence":"High","gaps":["Precise SNARE pairing at recycling endosome not determined","Whether VPS33B directly catalyzes SNARE assembly not tested"]},{"year":2022,"claim":"Identification of Syntaxin 12 as a VPS33B/VPS16B-binding SNARE, and demonstration that CCDC22 competes with Stx12 for complex binding, provided the first evidence of a hand-off mechanism coordinating CCC-mediated retrieval and SNARE-dependent fusion at endosomes.","evidence":"Co-IP, CRISPR KO of Stx12 and CCDC22 in megakaryocytes, competition binding assays, EM","pmids":["34905616"],"confidence":"High","gaps":["Whether competition is direct or allosteric not resolved","Whether this hand-off operates in non-megakaryocyte cell types unknown"]},{"year":2022,"claim":"Discovery that VPS33B promotes clearance of LYNUS complex-bearing late endosomes via endolysosomal fusion, thereby suppressing mTORC1 activation, revealed a new signaling-regulatory role with metabolic consequences in Treg cells.","evidence":"Treg-specific conditional KO mice, mTORC1 activity assays, metabolic profiling, Co-IP","pmids":["35705052"],"confidence":"High","gaps":["Whether LYNUS regulation is a general VPS33B function or Treg-specific not tested","Direct VPS33B–LYNUS binding interface uncharacterized"]},{"year":2023,"claim":"Structural characterization of the VPS33B–VPS16B complex revealed an unexpected 2:3 stoichiometry and bidirectional architecture with VPS33B at each lobe, providing the first low-resolution structural model and showing how ARC mutation L30P disrupts assembly.","evidence":"Recombinant expression in yeast, SEC-MALS, SAXS, negative-stain EM, CD, mutagenesis of ARC variants","pmids":["37062417"],"confidence":"High","gaps":["High-resolution atomic structure not available","Functional significance of bidirectional orientation unknown","How the 2:3 complex engages SNAREs or Rabs structurally undefined"]},{"year":null,"claim":"Key open questions remain: the atomic structure of the VPS33B–VPS16B complex, how VPS33B engages SNAREs to catalyze membrane fusion, the molecular basis of cargo selectivity, and whether the integrin signaling and vesicle trafficking functions are mechanistically coupled.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of VPS33B–VPS16B complex","No reconstituted SNARE-dependent fusion assay with VPS33B","Mechanism of cargo-selective trafficking unresolved","Relationship between integrin signaling and trafficking roles unclear"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,7,9,20,21]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[4,12,16,20]}],"localization":[{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[1,6,7,11,14,17,21]},{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[14,17]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[7,21]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[4]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,7,14,17,20,21]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[11,19]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[1,8,9]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9,21]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[21]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[3,15]}],"complexes":["VPS33B–VIPAS39 complex"],"partners":["VIPAS39","RAB11A","RAB25","SEC22B","STX12","CCDC22","ITGB3","GDI2"],"other_free_text":[]},"mechanistic_narrative":"VPS33B is a Sec1/Munc18-family protein that functions as a master regulator of SNARE-dependent membrane fusion at endosomal compartments, orchestrating the trafficking of diverse cargoes including platelet alpha-granule proteins, ABC transporters, epidermal lamellar body contents, collagen-modifying enzyme LH3, and pattern recognition receptor-containing endosomes [PMID:15052268, PMID:16123220, PMID:28082148, PMID:27496733]. VPS33B forms an obligate ~315 kDa complex with VIPAS39 (VPS16B) at 2:3 stoichiometry—distinct from CORVET/HOPS tethering complexes—that localizes to recycling endosomes and late endosomes, where it engages Rab11A, Rab25, SEC22B, and the SNARE Syntaxin 12, with transient handoff to the CCC complex (CCDC22) coordinating endosomal entry and exit [PMID:29778605, PMID:37062417, PMID:34905616, PMID:28017832]. Beyond vesicle trafficking, VPS33B directly binds integrin β subunits to potentiate outside-in signaling through RhoA-ROCK and Rac1 pathways, and promotes late endosome–lysosome fusion to clear LYNUS complex-bearing endosomes, thereby suppressing mTORC1 activation [PMID:26399659, PMID:35705052]. Loss-of-function mutations in VPS33B cause arthrogryposis–renal dysfunction–cholestasis (ARC) syndrome, a multisystem disorder reflecting the broad requirement for VPS33B-dependent trafficking in hepatocytes, megakaryocytes, keratinocytes, and other cell types [PMID:15052268, PMID:29409756]."},"prefetch_data":{"uniprot":{"accession":"Q9H267","full_name":"Vacuolar protein sorting-associated protein 33B","aliases":[],"length_aa":617,"mass_kda":70.6,"function":"May play a role in vesicle-mediated protein trafficking to lysosomal compartments and in membrane docking/fusion reactions of late endosomes/lysosomes. Required for proper trafficking and targeting of the collagen-modifying enzyme lysyl hydroxylase 3 (LH3) to intracellular collagen (PubMed:28017832). Mediates phagolysosomal fusion in macrophages (PubMed:18474358). Proposed to be involved in endosomal maturation implicating VIPAS39. In epithelial cells, the VPS33B:VIPAS39 complex may play a role in the apical recycling pathway and in the maintenance of the apical-basolateral polarity (PubMed:20190753). Seems to be involved in the sorting of specific cargos from the trans-Golgi network to alpha-granule-destined multivesicular bodies (MVBs) promoting MVBs maturation in megakaryocytes (By similarity)","subcellular_location":"Late endosome membrane; Lysosome membrane; Early endosome; Cytoplasmic vesicle, clathrin-coated vesicle; Recycling endosome","url":"https://www.uniprot.org/uniprotkb/Q9H267/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/VPS33B","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"WASF2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/VPS33B","total_profiled":1310},"omim":[{"mim_id":"620010","title":"CHOLESTASIS, PROGRESSIVE FAMILIAL INTRAHEPATIC, 12; PFIC12","url":"https://www.omim.org/entry/620010"},{"mim_id":"620009","title":"KERATODERMA-ICHTHYOSIS-DEAFNESS SYNDROME, AUTOSOMAL RECESSIVE; KDIDAR","url":"https://www.omim.org/entry/620009"},{"mim_id":"616700","title":"COMM DOMAIN-CONTAINING PROTEIN 3; COMMD3","url":"https://www.omim.org/entry/616700"},{"mim_id":"613404","title":"ARTHROGRYPOSIS, RENAL DYSFUNCTION, AND CHOLESTASIS 2; ARCS2","url":"https://www.omim.org/entry/613404"},{"mim_id":"613401","title":"VPS33B-INTERACTING PROTEIN, APICAL-BASOLATERAL POLARITY REGULATOR, SPE39 HOMOLOG; VIPAS39","url":"https://www.omim.org/entry/613401"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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gastroenterology","url":"https://pubmed.ncbi.nlm.nih.gov/24415890","citation_count":7,"is_preprint":false},{"pmid":"33788346","id":"PMC_33788346","title":"VPS33B interacts with NESG1 to suppress cell growth and cisplatin chemoresistance in ovarian cancer.","date":"2021","source":"Cancer science","url":"https://pubmed.ncbi.nlm.nih.gov/33788346","citation_count":6,"is_preprint":false},{"pmid":"35325493","id":"PMC_35325493","title":"Platelet VPS16B is dependent on VPS33B expression, as determined in two siblings with arthrogryposis, renal dysfunction, and cholestasis syndrome.","date":"2022","source":"Journal of thrombosis and haemostasis : JTH","url":"https://pubmed.ncbi.nlm.nih.gov/35325493","citation_count":6,"is_preprint":false},{"pmid":"31777725","id":"PMC_31777725","title":"A Novel VPS33B Variant Identified by Exome Sequencing in a Patient with Arthrogryposis-Renal Dysfunction-Cholestasis Syndrome.","date":"2019","source":"Pediatric gastroenterology, hepatology & 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1950)","url":"https://pubmed.ncbi.nlm.nih.gov/34732466","citation_count":1,"is_preprint":false},{"pmid":"36338198","id":"PMC_36338198","title":"Two novel mutations in the VPS33B gene in a Chinese patient with arthrogryposis, renal dysfunction and cholestasis syndrome 1: A case report.","date":"2022","source":"World journal of clinical cases","url":"https://pubmed.ncbi.nlm.nih.gov/36338198","citation_count":0,"is_preprint":false},{"pmid":"41138802","id":"PMC_41138802","title":"A novel homozygous splice-site variant in VPS33B identified as a cause of bleeding.","date":"2025","source":"Journal of thrombosis and haemostasis : JTH","url":"https://pubmed.ncbi.nlm.nih.gov/41138802","citation_count":0,"is_preprint":false},{"pmid":"39134495","id":"PMC_39134495","title":"[Rare VPS33B gene mutation combined with GP1BA mutation causes severe decrease in plasma VWF levels: a case report and literature review].","date":"2024","source":"Zhonghua xue ye xue za zhi = Zhonghua xueyexue 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standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2004,\n      \"finding\": \"VPS33B encodes a homolog of class C yeast vacuolar protein sorting gene Vps33 containing a Sec1-like domain, identifying it as a regulator of SNARE-dependent vesicle-to-target membrane fusion.\",\n      \"method\": \"Positional cloning, sequence homology analysis, identification of germline mutations in ARC syndrome patients\",\n      \"journal\": \"Nature genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — foundational identification with strong genetic evidence across 14 kindreds, replicated extensively\",\n      \"pmids\": [\"15052268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"VPS33B is essential for platelet alpha-granule biogenesis in megakaryocytes but not required for granule secretion; VPS33B colocalizes with alpha-granule markers and late endosomes/lysosomes in megakaryocytes, and is absent from platelets themselves.\",\n      \"method\": \"Immunofluorescence microscopy, immunoblotting of ARC patient platelets and megakaryocytes, electron microscopy, platelet aggregation assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods, patient cells and normal controls, replicated by subsequent studies\",\n      \"pmids\": [\"16123220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Zebrafish vps33b acts downstream of the hnf6/vhnf1 transcription factor pathway to regulate biliary development; vhnf1 directly binds the vps33b promoter to increase its expression.\",\n      \"method\": \"Morpholino knockdown, epistasis analysis, electrophoretic mobility shift assay (EMSA), overexpression in embryos and mammalian liver cells\",\n      \"journal\": \"Development (Cambridge, England)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — EMSA demonstrates direct promoter binding, combined with genetic epistasis in zebrafish\",\n      \"pmids\": [\"16284120\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"VPS33B deficiency causes defective lamellar granule secretion in keratinocytes; ultrastructural analysis showed lamellar granules entombed in cornified cells rather than being secreted, indicating VPS33B is required for SNARE-mediated vesicle fusion in epidermal differentiation.\",\n      \"method\": \"VPS33B mutation sequencing, ultrastructural electron microscopy of ARC patient skin, mRNA splicing analysis\",\n      \"journal\": \"Archives of dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct ultrastructural evidence in patient tissue, single lab\",\n      \"pmids\": [\"18347289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"VPS33B interacts with VPS16B (encoded by C14orf133); both proteins colocalize at the trans-Golgi network, late endosomes, and alpha-granules in megakaryocytes, and VPS16B is required for platelet alpha-granule biogenesis.\",\n      \"method\": \"Yeast two-hybrid screen, mass spectrometry, coimmunoprecipitation, immunofluorescence microscopy, immunoblotting of ARC patient platelets\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including Y2H, MS, Co-IP, and patient phenotyping\",\n      \"pmids\": [\"23002115\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"SPE-39 (VIPAS39/VPS16B) undergoes tyrosine phosphorylation and ubiquitination upon EGF stimulation; VPS33B association with SPE-39 inhibits EGF-induced ubiquitination of SPE-39, stabilizing it; SPE-39 and VPS33B have opposing effects on EGF receptor downregulation.\",\n      \"method\": \"EGF stimulation assays, immunoprecipitation, ubiquitination assays, overexpression/knockdown in COS-7 cells\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct biochemical interaction and functional consequence shown, single lab\",\n      \"pmids\": [\"22677173\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"VPS33B pathogenic mutations alter subcellular localization of VPS33B to VIPAS39/SPE-39-positive endosomes and some mutants fragment VIPAS39-positive endosomes, implicating VPS33B in VIPAS39-dependent endosomal maturation or fusion.\",\n      \"method\": \"Yeast two-hybrid, immunoprecipitation, quantitative fluorescence microscopy\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — multiple methods but functional readout is localization-based, single lab\",\n      \"pmids\": [\"23918659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"VPS33B depletion in HeLa cells causes accumulation of late endosomes in the perinuclear region, impairs cargo degradation, and blocks delivery of endocytosed BSA-gold from late endosomes to lysosomes, indicating VPS33B is required for late endosomal-lysosomal fusion.\",\n      \"method\": \"siRNA knockdown, fluorescence microscopy, electron microscopy with BSA-gold endocytosis assay\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal microscopy methods with quantitative cargo tracking, mechanistically defined phenotype\",\n      \"pmids\": [\"26403612\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"VPS33B deficiency in a tamoxifen-inducible mouse model causes reduction in platelet alpha-granules, accumulation of large vacuoles in megakaryocytes, reduction in mature type-II multivesicular bodies (MVB II), and a defect in delta-granule secretion and stable aggregate formation under arteriolar shear.\",\n      \"method\": \"Tamoxifen-inducible conditional knockout, ultrastructural and immunoelectron microscopy, platelet aggregation assays, tail-bleeding assay, FeCl3 thrombosis model\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo mouse model with multiple functional and ultrastructural readouts\",\n      \"pmids\": [\"25947942\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"VPS33B binds directly to the integrin beta subunit; VPS33B overexpression potentiates αIIbβ3 outside-in signaling (but not inside-out signaling); megakaryocyte/platelet-specific VPS33B knockout mice show impaired spreading on fibrinogen, defective clot retraction, and reduced aggregation, placing VPS33B upstream of RhoA-ROCK-MLC and Rac1-dependent pathways.\",\n      \"method\": \"Direct binding assay, CHO cell overexpression, megakaryocyte/platelet-specific conditional knockout mice, platelet spreading, clot retraction, aggregation assays, FeCl3 thrombosis model\",\n      \"journal\": \"Circulation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding demonstrated, conditional KO with multiple functional assays and pathway placement\",\n      \"pmids\": [\"26399659\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"VPS33B interacts with the GDI2/RAB11A/RAB27A pathway to regulate trafficking of secretory proteins as exosomes in hematopoietic stem cells; VPS33B co-exists in exosomes with GDI2, VPS16B, FLOT1 and other exosome markers.\",\n      \"method\": \"VPS33B deletion in mouse and human HSCs, exosome purification and characterization, co-immunoprecipitation, rescue experiments with purified exosomes\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic deletion in two species, Co-IP, rescue experiments, multiple orthogonal approaches\",\n      \"pmids\": [\"27797340\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Drosophila and mammalian Vps33B are required for maturation of phagosomes and endosomes containing pattern recognition receptors (PRRs) following microbial recognition; loss of Vps33B results in enhanced inflammatory signaling due to failure to clear PRR-containing endosomes, while trafficking of non-microbial cargo is unaffected.\",\n      \"method\": \"Drosophila Vps33B mutants, mammalian cell knockdown, phagocytosis assays, inflammatory mediator quantification, endosomal trafficking assays\",\n      \"journal\": \"Immunity\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ortholog study in two organisms, multiple assay types, mechanistically distinct cargo selectivity demonstrated\",\n      \"pmids\": [\"27496733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"VPS33B associates with VIPAS39, alpha-tubulin, and SEC22B; VIPAS39 binds intact VPS33B while alpha-tubulin and SEC22B interact separately with the Sec1-like domains of VPS33B; Vps33b deficiency disrupts redistribution of Vipas39 and Sec22b to proplatelets and interrupts colocalization of Sec22b with Vwf-positive vesicles in megakaryocytes.\",\n      \"method\": \"Coimmunoprecipitation, mass spectrometry, immunoblotting, pull-down assays, conditional knockout mice, immunofluorescence microscopy\",\n      \"journal\": \"The Journal of pathology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — pull-down and Co-IP with domain mapping, combined with conditional KO functional data\",\n      \"pmids\": [\"27319744\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"VPS33B interacts with Rab11a and Rab25; the p.Gly131Glu ARC/ARKID-causing mutation reduces coimmunoprecipitation and colocalization with Rab11a and Rab25 and fails to rescue trafficking of the collagen-modifying enzyme LH3, linking VPS33B to LH3 trafficking and collagen lysine modification.\",\n      \"method\": \"Mutagenesis, coimmunoprecipitation, colocalization microscopy, LH3 trafficking rescue assay, urine/fibroblast collagen modification analysis\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis combined with Co-IP, trafficking assay, and biochemical validation in patient samples\",\n      \"pmids\": [\"28017832\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"VPS33B interacts with RAB11A at recycling endosomes and is required for trafficking of ABC transporters specifically trafficked via Rab11a-positive recycling endosomes to the hepatocyte apical membrane; Vps33b liver-specific knockout causes mislocalisation of these apical proteins, impaired tight junction integrity, and loss of functional bile secretion.\",\n      \"method\": \"Liver-specific conditional knockout mice, bile/plasma mass spectrometry, immunostaining of apical membrane and tight junction proteins, electron microscopy, adeno-associated virus gene rescue\",\n      \"journal\": \"Journal of hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo conditional KO with multiple readouts and gene rescue experiment\",\n      \"pmids\": [\"28082148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"VPS33B and VIPAR (VPS16B) are required for epidermal lamellar body biogenesis; Vps33b and Vipar knockout mice show abnormal lamellar body morphology, disrupted cargo localisation, impaired stratum corneum formation, and reduced corneocyte/cornified envelope integrity.\",\n      \"method\": \"Mouse knockouts (Vps33b and Vipar), histology, immunofluorescence, electron microscopy, primary cell isolation\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — two knockout mouse models with multiple orthogonal structural and functional readouts\",\n      \"pmids\": [\"29409756\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"VPS33B does not associate with CORVET or HOPS complex subunits but stably interacts with VIPAR; the VPS33B/VIPAR complex is considerably smaller than CORVET/HOPS; VPS33B transiently interacts with CCDC22, a CCC complex member, which does not stably co-fractionate with VPS33B/VIPAR.\",\n      \"method\": \"BioID proximity biotinylation proteomics, gel filtration of human cell lysates, co-localization analysis\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — BioID interactome complemented by gel filtration, distinguishes stable vs transient interactions\",\n      \"pmids\": [\"29778605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"VPS33B and VPS16B form a distinct small complex with the same hydrodynamic radius as a recombinant VPS33B-VPS16B heterodimer purified from bacteria; this complex localizes to the recycling endosome in megakaryocytes, which is a key intermediate compartment for alpha-granule biogenesis; VPS33B deficiency causes alpha-granule cargo degradation in lysosomes.\",\n      \"method\": \"CRISPR/Cas9 VPS33B knockout in imMKCLs, recombinant protein purification, size-exclusion chromatography, cargo trafficking pulse-chase mapping, GFP-VPS33B rescue\",\n      \"journal\": \"Blood advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted recombinant complex, CRISPR KO, cargo tracking, and rescue experiments\",\n      \"pmids\": [\"31501156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"The VPS33B missense variant p.Cys576Arg abolishes interaction with VIPAS39 in vitro, demonstrating the functional importance of this region for VPS33B-VIPAS39 complex formation.\",\n      \"method\": \"In vitro binding/coimmunoprecipitation assay with mutant constructs\",\n      \"journal\": \"Human mutation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP with mutant construct, single lab\",\n      \"pmids\": [\"31479177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Loss of p38b MAP kinase reduces enhanced inflammatory responses and prolongs survival of Vps33B-deficient Drosophila; p38 MAPK modulates endosomal trafficking of the PGRP-LC innate immune receptor and phagocytosis of bacteria, placing p38b downstream of Vps33B in the regulation of innate immune endosomal trafficking.\",\n      \"method\": \"Drosophila genetic epistasis (Vps33B mutant x p38b mutant), survival assays, constitutively active/dominant negative p38b expression, endosomal trafficking assays, phagocytosis assays\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in Drosophila ortholog with multiple functional readouts\",\n      \"pmids\": [\"32677257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"VPS16B/VPS33B complex physically associates with Syntaxin 12 (Stx12), a SNARE protein mediating vesicle fusion at endosomes; Stx12 deficiency reduces alpha-granule numbers and alpha-granule protein levels in megakaryocytes; VPS16B/VPS33B also binds CCDC22 (CCC complex), and CCDC22 competes with Stx12 for binding to VPS16B/VPS33B, suggesting a hand-off mechanism at endosomes.\",\n      \"method\": \"Co-immunoprecipitation, CRISPR knockout of Stx12 and COMMD3/CCDC22 in megakaryocytes, immunofluorescence, flow cytometry, electron microscopy\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP for interaction, CRISPR KO for functional validation, competition assay for mechanism, multiple readouts\",\n      \"pmids\": [\"34905616\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Vps33B binds with the lysosomal nutrient-sensing complex (LYNUS) and promotes late endosome-lysosome fusion and clearance of LYNUS-containing late endosomes, thereby suppressing mTORC1 activation; Vps33B deficiency in Treg cells leads to disordered endolysosomal fusion, LYNUS accumulation, elevated mTORC1 activation, and hyper-glycolytic metabolism.\",\n      \"method\": \"Treg-specific conditional knockout mice, mTORC1 activity assays, endosomal trafficking assays, metabolic profiling, co-immunoprecipitation\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with mechanistic pathway placement (LYNUS/mTORC1), Co-IP for binding, multiple orthogonal readouts\",\n      \"pmids\": [\"35705052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Stable expression of VPS16B in platelets and megakaryocytes is dependent on VPS33B; loss of VPS33B results in loss of both VPS33B and VPS16B protein expression.\",\n      \"method\": \"Immunoblotting of platelets from ARC patients with VPS33B nonsense variant, protein expression analysis\",\n      \"journal\": \"Journal of thrombosis and haemostasis : JTH\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single patient case with immunoblotting, corroborated by prior VPS33B-KO imMKCL data\",\n      \"pmids\": [\"35325493\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Human VPS33B-VPS16B forms a high molecular weight complex (~315 kDa) with a VPS33B:VPS16B stoichiometry of approximately 2:3; the complex has a two-lobed shape with one VPS33B molecule at each end oriented in opposite directions; truncated VPS16B (aa 143-316) is sufficient for complex formation; the ARC-causing L30P mutation disrupts complex formation while S243F and H344D do not.\",\n      \"method\": \"Yeast expression of recombinant complex, circular dichroism, size-exclusion chromatography-MALS, quantitative immunoblotting, small-angle X-ray scattering, negative-stain electron microscopy, avidin tagging, mutagenesis of ARC variants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted complex with structural characterization by multiple methods, mutagenesis of disease variants\",\n      \"pmids\": [\"37062417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"METTL16 methyltransferase induces m6A modification of VPS33B mRNA, impairing VPS33B transcript stability and thereby reducing VPS33B protein levels; this promotes osteosarcoma progression through the PI3K/AKT pathway.\",\n      \"method\": \"m6A profiling, METTL16 knockdown/overexpression, mRNA stability assays, VPS33B knockdown rescue experiments, in vivo tumor growth\",\n      \"journal\": \"Journal of cellular physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — m6A modification mechanistically linked to VPS33B mRNA stability with functional rescue, single lab\",\n      \"pmids\": [\"37357526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"VPS33B knockout in proximal tubular epithelial cells (RPTEC-TERT1) causes a 'peeling' phenotype with altered cell adhesion and cell-matrix attachment defects, accompanied by transcriptional changes in adhesion-related genes.\",\n      \"method\": \"CRISPR/Cas9 knockout, brightfield imaging, immunostaining, RNA sequencing, cell detachment assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — CRISPR KO with transcriptomic and functional adhesion readouts, single lab, no rescue experiment\",\n      \"pmids\": [\"41686830\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"VPS33B is a Sec1/Munc18 family protein that forms a uniquely bidirectional ~315 kDa complex with VPS16B (VIPAR/VIPAS39) — with 2:3 stoichiometry and one VPS33B at each end — to regulate SNARE-dependent membrane fusion events at recycling endosomes and late endosomes, functioning in trafficking of cargo (including ABC transporters, alpha-granule proteins, LH3, lamellar body contents, and PRR-containing endosomes) through endosomal compartments; it promotes late endosome-lysosome fusion, interacts transiently with the CCC complex (CCDC22) and stably with Rab11A, Rab25, SEC22B, and VIPAS39 to coordinate endosomal entry and exit, and additionally modulates integrin outside-in signaling upstream of RhoA-ROCK and Rac1 pathways and suppresses mTORC1 activation by promoting LYNUS complex clearance through endolysosomal fusion.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"VPS33B is a Sec1/Munc18-family protein that functions as a master regulator of SNARE-dependent membrane fusion at endosomal compartments, orchestrating the trafficking of diverse cargoes including platelet alpha-granule proteins, ABC transporters, epidermal lamellar body contents, collagen-modifying enzyme LH3, and pattern recognition receptor-containing endosomes [PMID:15052268, PMID:16123220, PMID:28082148, PMID:27496733]. VPS33B forms an obligate ~315 kDa complex with VIPAS39 (VPS16B) at 2:3 stoichiometry—distinct from CORVET/HOPS tethering complexes—that localizes to recycling endosomes and late endosomes, where it engages Rab11A, Rab25, SEC22B, and the SNARE Syntaxin 12, with transient handoff to the CCC complex (CCDC22) coordinating endosomal entry and exit [PMID:29778605, PMID:37062417, PMID:34905616, PMID:28017832]. Beyond vesicle trafficking, VPS33B directly binds integrin β subunits to potentiate outside-in signaling through RhoA-ROCK and Rac1 pathways, and promotes late endosome–lysosome fusion to clear LYNUS complex-bearing endosomes, thereby suppressing mTORC1 activation [PMID:26399659, PMID:35705052]. Loss-of-function mutations in VPS33B cause arthrogryposis–renal dysfunction–cholestasis (ARC) syndrome, a multisystem disorder reflecting the broad requirement for VPS33B-dependent trafficking in hepatocytes, megakaryocytes, keratinocytes, and other cell types [PMID:15052268, PMID:29409756].\",\n  \"teleology\": [\n    {\n      \"year\": 2004,\n      \"claim\": \"Positional cloning of VPS33B as the ARC syndrome gene established that a human Sec1/Munc18 homolog is essential for multiorgan vesicle trafficking, resolving the molecular basis of this Mendelian disorder.\",\n      \"evidence\": \"Positional cloning and mutation analysis across 14 ARC kindreds\",\n      \"pmids\": [\"15052268\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No binding partners or specific trafficking step identified\", \"Sec1-like domain function not biochemically characterized\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstration that VPS33B localizes to alpha-granule compartments and late endosomes in megakaryocytes, and that ARC patient platelets lack alpha-granules, defined the first cell-biological function—granule biogenesis rather than secretion.\",\n      \"evidence\": \"Immunofluorescence, electron microscopy, and functional platelet assays in ARC patient and control megakaryocytes/platelets\",\n      \"pmids\": [\"16123220\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of granule biogenesis role unknown\", \"Whether VPS33B acts at a specific endosomal sorting step undefined\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identification that zebrafish vps33b is transcriptionally regulated by hnf6/vhnf1 and required for biliary development provided the first developmental-genetic context for VPS33B function outside hematopoiesis.\",\n      \"evidence\": \"Morpholino knockdown, EMSA for promoter binding, epistasis analysis in zebrafish\",\n      \"pmids\": [\"16284120\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether transcriptional regulation is conserved in mammals not tested\", \"Downstream trafficking targets in bile duct cells unidentified\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Discovery that VPS33B physically interacts with VPS16B (VIPAS39) and that both colocalize at trans-Golgi network, late endosomes, and alpha-granules revealed VPS33B operates as part of a dedicated heteromeric complex, paralleling but distinct from HOPS/CORVET.\",\n      \"evidence\": \"Yeast two-hybrid, mass spectrometry, co-immunoprecipitation, and immunofluorescence in megakaryocytes; EGF-stimulation and ubiquitination assays in COS-7 cells\",\n      \"pmids\": [\"23002115\", \"22677173\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and structure of the complex unknown\", \"Whether VPS33B-VPS16B complex functions independently of HOPS/CORVET not resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Functional studies in HeLa cells and conditional knockout mice established that VPS33B is required for late endosome–lysosome fusion and that its loss causes perinuclear accumulation of late endosomes, cargo degradation failure, and alpha/delta-granule defects in vivo.\",\n      \"evidence\": \"siRNA knockdown with BSA-gold cargo tracking in HeLa; tamoxifen-inducible KO mice with ultrastructural and functional platelet assays\",\n      \"pmids\": [\"26403612\", \"25947942\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SNARE partner mediating the fusion step not identified\", \"Whether VPS33B acts catalytically or as a scaffold at the fusion step unclear\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Direct binding of VPS33B to integrin β subunits and potentiation of αIIbβ3 outside-in signaling upstream of RhoA-ROCK and Rac1 pathways expanded VPS33B function beyond vesicle trafficking to include integrin signal transduction.\",\n      \"evidence\": \"Direct binding assay, CHO overexpression, megakaryocyte/platelet-specific conditional KO mice with spreading, clot retraction, and thrombosis assays\",\n      \"pmids\": [\"26399659\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of integrin β–VPS33B interaction unknown\", \"Whether integrin signaling role is independent of vesicle trafficking not fully resolved\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Studies in Drosophila and mammalian cells demonstrated cargo-selective trafficking: VPS33B is specifically required for maturation of PRR-containing endosomes after microbial recognition, with its loss amplifying inflammatory signaling while non-microbial cargo trafficking remains intact.\",\n      \"evidence\": \"Drosophila Vps33B mutants and mammalian knockdown, phagocytosis assays, inflammatory mediator quantification\",\n      \"pmids\": [\"27496733\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How cargo selectivity is achieved mechanistically unknown\", \"Whether selectivity is conferred by VPS33B itself or an adaptor unclear\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Mapping of VPS33B interactions to Rab11a, Rab25, SEC22B, and alpha-tubulin, with domain-level resolution and functional validation in megakaryocytes, defined a recycling-endosome-centered interaction network and linked VPS33B to LH3 trafficking and collagen modification.\",\n      \"evidence\": \"Co-IP, mass spectrometry, pull-down with domain constructs, conditional KO mice, LH3 trafficking rescue, patient collagen analysis\",\n      \"pmids\": [\"27319744\", \"28017832\", \"27797340\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Rab11a/Rab25 binding is direct or via VIPAS39 not fully dissected\", \"Structural basis of Sec1-domain interactions with SEC22B undefined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Liver-specific knockout established that VPS33B–Rab11a interaction is required for apical trafficking of ABC transporters and tight junction maintenance in hepatocytes, directly explaining the cholestasis phenotype in ARC syndrome.\",\n      \"evidence\": \"Liver-specific conditional KO mice with bile/plasma MS, immunostaining, EM, and AAV gene rescue\",\n      \"pmids\": [\"28082148\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific SNARE or tethering factor mediating apical delivery not identified\", \"Whether VPS33B acts at sorting, transport, or fusion at the apical membrane unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"BioID proteomics definitively separated VPS33B from the CORVET/HOPS pathway, confirming it forms a distinct small complex with VIPAS39 that transiently contacts the CCC complex (CCDC22), establishing a unique identity for VPS33B-containing machinery.\",\n      \"evidence\": \"BioID proximity biotinylation, gel filtration chromatography, colocalization analysis in human cells\",\n      \"pmids\": [\"29778605\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Function of transient CCC interaction unknown\", \"Whether other transient interactors exist not exhaustively surveyed\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Reconstitution of recombinant VPS33B–VPS16B heterodimer and CRISPR KO in megakaryocytes pinpointed the recycling endosome as the key compartment where VPS33B acts, with its loss causing alpha-granule cargo to be mis-sorted to lysosomes for degradation.\",\n      \"evidence\": \"Recombinant protein purification, SEC, CRISPR KO in imMKCLs, cargo pulse-chase, GFP-VPS33B rescue\",\n      \"pmids\": [\"31501156\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise SNARE pairing at recycling endosome not determined\", \"Whether VPS33B directly catalyzes SNARE assembly not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identification of Syntaxin 12 as a VPS33B/VPS16B-binding SNARE, and demonstration that CCDC22 competes with Stx12 for complex binding, provided the first evidence of a hand-off mechanism coordinating CCC-mediated retrieval and SNARE-dependent fusion at endosomes.\",\n      \"evidence\": \"Co-IP, CRISPR KO of Stx12 and CCDC22 in megakaryocytes, competition binding assays, EM\",\n      \"pmids\": [\"34905616\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether competition is direct or allosteric not resolved\", \"Whether this hand-off operates in non-megakaryocyte cell types unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Discovery that VPS33B promotes clearance of LYNUS complex-bearing late endosomes via endolysosomal fusion, thereby suppressing mTORC1 activation, revealed a new signaling-regulatory role with metabolic consequences in Treg cells.\",\n      \"evidence\": \"Treg-specific conditional KO mice, mTORC1 activity assays, metabolic profiling, Co-IP\",\n      \"pmids\": [\"35705052\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether LYNUS regulation is a general VPS33B function or Treg-specific not tested\", \"Direct VPS33B–LYNUS binding interface uncharacterized\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Structural characterization of the VPS33B–VPS16B complex revealed an unexpected 2:3 stoichiometry and bidirectional architecture with VPS33B at each lobe, providing the first low-resolution structural model and showing how ARC mutation L30P disrupts assembly.\",\n      \"evidence\": \"Recombinant expression in yeast, SEC-MALS, SAXS, negative-stain EM, CD, mutagenesis of ARC variants\",\n      \"pmids\": [\"37062417\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution atomic structure not available\", \"Functional significance of bidirectional orientation unknown\", \"How the 2:3 complex engages SNAREs or Rabs structurally undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions remain: the atomic structure of the VPS33B–VPS16B complex, how VPS33B engages SNAREs to catalyze membrane fusion, the molecular basis of cargo selectivity, and whether the integrin signaling and vesicle trafficking functions are mechanistically coupled.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of VPS33B–VPS16B complex\", \"No reconstituted SNARE-dependent fusion assay with VPS33B\", \"Mechanism of cargo-selective trafficking unresolved\", \"Relationship between integrin signaling and trafficking roles unclear\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 7, 9, 20, 21]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [4, 12, 16, 20]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [1, 6, 7, 11, 14, 17, 21]},\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [14, 17]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [7, 21]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 7, 14, 17, 20, 21]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [11, 19]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [1, 8, 9]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9, 21]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [21]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [3, 15]}\n    ],\n    \"complexes\": [\n      \"VPS33B–VIPAS39 complex\"\n    ],\n    \"partners\": [\n      \"VIPAS39\",\n      \"RAB11A\",\n      \"RAB25\",\n      \"SEC22B\",\n      \"STX12\",\n      \"CCDC22\",\n      \"ITGB3\",\n      \"GDI2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}