{"gene":"STXBP2","run_date":"2026-04-28T21:42:57","timeline":{"discoveries":[{"year":2009,"finding":"STXBP2 (Munc18-2) was identified as a binding partner of syntaxin 11 (STX11), and FHL-5 missense mutations in STXBP2 eliminate this interaction, leading to decreased stability of both proteins in patient lymphocytes and markedly reduced NK and CTL degranulation.","method":"Co-immunoprecipitation, patient lymphocyte analysis, CD107 degranulation assay","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP with functional validation in patient cells, replicated independently in same year","pmids":["19804848"],"is_preprint":false},{"year":2009,"finding":"STXBP2 deficiency impairs cytotoxic granule exocytosis in NK cells, a defect rescued by ectopic expression of wild-type STXBP2; syntaxin 11 expression requires the presence of STXBP2, identifying STXBP2 as required at a late step of the secretory pathway for cytotoxic granule release.","method":"Patient lymphoblast analysis, NK cell cytotoxicity assay, ectopic expression rescue experiment","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — loss-of-function with defined phenotype, rescue experiment, orthogonal functional assays, independent replication","pmids":["19884660"],"is_preprint":false},{"year":2000,"finding":"Munc18-2 forms a complex with syntaxin 3 (a t-SNARE at the apical plasma membrane) and overexpression of wild-type Munc18-2 inhibits apical delivery of influenza hemagglutinin in Caco-2 epithelial cells; point mutants unable to bind syntaxin 3 have differential effects on apical transport, indicating Munc18-2 controls apical membrane trafficking via syntaxin 3 interaction and additional syntaxin-independent mechanisms.","method":"Munc18-2 point mutagenesis, co-immunoprecipitation, SNAP-23 displacement assay, influenza HA apical transport assay in Caco-2 cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with functional transport assay and in vitro binding confirmation","pmids":["10788461"],"is_preprint":false},{"year":2003,"finding":"Munc18-2 localizes to secretory granules in mast cells, interacts with syntaxin 2 and syntaxin 3 (not syntaxin 4), redistributes toward lamellipodia upon stimulation and associates with microtubule-aligned granules; overexpression of Munc18-2 or effector-loop-containing peptides inhibits IgE-triggered exocytosis, and disruption of microtubules with nocodazole causes Munc18-2 redistribution and impairs mediator release.","method":"Munc18-2 overexpression, peptide inhibition, immunofluorescence/live imaging, nocodazole treatment, mediator release assay","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — localization with functional consequence, multiple orthogonal perturbation methods","pmids":["12482918"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of human Munc18-2 solved at 2.6 Å resolution; surface mutations causing FHL5 (R39P, L130S, E132A, P334L) map to predicted syntaxin and SNARE binding sites; Munc18-2 binds the N-terminal peptide of STX11 with ~20-fold higher affinity than STX3; upon IL-2 activation, increased STX3 levels favor Munc18-2 binding when STX11 is absent, and Munc18-1 expressed in activated CTL can bind STX11.","method":"X-ray crystallography, surface plasmon resonance binding assay, mutation mapping, patient cell analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — crystal structure with binding affinity measurements and mutagenesis mapping","pmids":["24194549"],"is_preprint":false},{"year":2015,"finding":"STXBP2 R65Q and R65W mutations retain interaction with and stabilization of syntaxin 11 but hinder membrane fusion in vitro by arresting late steps of SNARE-complex assembly; forced expression of these mutants in control CTLs and NK cells diminishes degranulation and cytotoxic activity in a dominant-negative manner.","method":"In vitro membrane fusion assay, SNARE complex assembly assay, forced expression in primary CTLs/NK cells, cytotoxicity/degranulation assay","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution of SNARE fusion + dominant-negative functional validation in primary cells","pmids":["25564401"],"is_preprint":false},{"year":2017,"finding":"Munc18-2 facilitates the transition from lipid-anchored STX11-mediated hemifusion (lipid mixing without content mixing) to complete membrane fusion during CTL lytic granule secretion; wild-type Munc18-2 but not STX11-binding mutants stimulates complete fusion in a reconstituted flipped cell-cell fusion assay, demonstrating Munc18-2 directly promotes SNARE complex assembly beyond its chaperone role.","method":"Reconstituted 'flipped' cell-cell fusion assay measuring lipid and content mixing, Munc18-2 mutant expression","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — reconstituted cell-cell fusion assay with mutagenesis validation distinguishing hemifusion from complete fusion","pmids":["28265073"],"is_preprint":false},{"year":2012,"finding":"Munc18b (STXBP2) is the major Munc18 isoform in platelets and forms complexes with syntaxin 11, SNAP-23, and VAMP-8, as well as Munc13-4 and Rab27; FHL5 patient platelets with biallelic mutations show profoundly defective serotonin, ADP/ATP, platelet factor 4, and lysosomal content secretion, with decreased Munc18b and syntaxin 11 levels.","method":"Co-immunoprecipitation from human platelets, platelet secretion assays (serotonin, ADP/ATP release), patient platelet analysis","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP establishing native complex, functional validation in patient material","pmids":["22791290"],"is_preprint":false},{"year":2013,"finding":"siRNA-mediated silencing of Munc18-2 in mast cells inhibits secretory granule (SG) translocation but not CCL2 chemokine secretion; Munc18-2 (but not STX3) interacts with tubulin in resting cells in a nocodazole-sensitive manner, and this interaction decreases after stimulation, demonstrating Munc18-2 dynamically couples the fusion machinery to the microtubule cytoskeleton.","method":"siRNA knockdown, combined knockdown epistasis, immunogold electron microscopy, co-immunoprecipitation with tubulin, nocodazole perturbation, degranulation assay","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — siRNA knockdown with specific phenotypic readout, Co-IP, localization, and epistasis between Munc18-2 and STX3","pmids":["24323579"],"is_preprint":false},{"year":2017,"finding":"In FHL5 enterocytes, Munc18-2 is required for Slp4a/Stx3 interaction necessary for fusion of cargo vesicles with the apical plasma membrane; loss of Munc18-2 selectively disrupts trafficking of NHE3 and GLUT5 to the brush border while DPPIV transport is unaffected, causing subapical accumulation of cargo vesicles.","method":"CRISPR/Cas9 Munc18-2 knockout in CaCo2 cells, patient biopsy analysis, patient-derived organoid analysis, fluorescence and electron microscopy, co-immunoprecipitation (Slp4a/Stx3)","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 1-2 — CRISPR KO model with mechanistic Co-IP, patient biopsies, and organoids with defined cargo-selective phenotype","pmids":["28724787"],"is_preprint":false},{"year":2015,"finding":"Munc18-2 localizes predominantly to cytolytic granules with low levels at the plasma membrane in CTLs; STX11 localizes to the plasma membrane and its plasma membrane localization is lost in FHL5 CTLs lacking Munc18-2, whereas Munc18-2 localization is unaffected in FHL4 CTLs lacking STX11, demonstrating Munc18-2 chaperones STX11 to the plasma membrane.","method":"Immunofluorescence localization of endogenous proteins in FHL4 and FHL5 patient CTLs, loss-of-function analysis","journal":"Traffic","confidence":"High","confidence_rationale":"Tier 2 — endogenous protein localization in matched patient cell lines with defined functional consequence","pmids":["26771955"],"is_preprint":false},{"year":2005,"finding":"Slp4-a (granuphilin-a) interacts with syntaxin 2/3 in a closed conformation only in the presence of Munc18-2, while Munc18-2 itself does not directly bind Slp4-a; the Slp4-a linker domain (residues 144–354) mediates syntaxin 2/3 binding, and antibody interference with this interaction inhibits isoproterenol-stimulated amylase release from permeabilized parotid acinar cells.","method":"Co-immunoprecipitation in COS-7 cells, deletion analysis, antibody inhibition in streptolysin-O permeabilized cells, amylase release assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP with deletion mapping and functional antibody inhibition, but Munc18-2 involvement is indirect (required for complex formation)","pmids":["16186111"],"is_preprint":false},{"year":2003,"finding":"Munc18-2/syntaxin 3 complexes are spatially excluded from lipid rafts in mast cells, whereas STX3-containing SNARE complexes (with SNAP-23 and VAMP-8) are enriched in rafts, demonstrating that Munc18-2 acts at a step distinct from SNARE complex formation and membrane fusion.","method":"Lipid raft fractionation, Western blotting for SNARE and Munc18 proteins in RBL mast cells","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 3 — biochemical fractionation showing spatial separation, single method without direct functional manipulation","pmids":["12935901"],"is_preprint":false},{"year":2007,"finding":"Munc18-2 knockdown in RBL-2H3 mast cells markedly inhibits degranulation without changing syntaxin expression or Ca2+ mobilization; Munc18-2 interacts with syntaxin 3 (not syntaxin 4) both at the plasma membrane and on secretory granules in vivo, suggesting regulation of both granule-granule and granule-plasma membrane fusion.","method":"Munc18-2 knockdown, fluorescent chimera co-localization/co-immunoprecipitation, single-cell granule imaging, degranulation assay","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 — KD with phenotype, Co-IP binding specificity, but single lab","pmids":["17408745"],"is_preprint":false},{"year":2013,"finding":"FHL-5 neutrophils (with STXBP2/Munc18-2 mutations) have a profound defect in granule mobilization resulting in inadequate bacterial killing, particularly of gram-negative E. coli, identifying STXBP2 as required for neutrophil granule exocytosis and bactericidal function.","method":"Patient neutrophil functional assays (degranulation, bacterial killing assay), comparison of FHL-5 vs. control neutrophils","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 — loss-of-function with specific phenotypic readout in patient cells, single lab","pmids":["23687090"],"is_preprint":false},{"year":2018,"finding":"In mast cells, conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) causes near-complete absence of regulated exocytosis and abolishes compound (homotypic) exocytosis as shown by plasma membrane capacitance recordings and stereological EM analysis; Munc18-2 cKO mice are protected from anaphylaxis.","method":"Conditional knockout mice (Munc18-1, -2, -3 each deleted in mast cells), plasma membrane capacitance recordings, stereological EM, mediator secretion assays, in vivo anaphylaxis model","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — clean conditional KO with multiple orthogonal assays including electrophysiology and EM, in vivo validation","pmids":["29599294"],"is_preprint":false},{"year":2019,"finding":"Munc18-2 (but not Munc18-1 or Munc18-3) conditional knockout in megakaryocytes ablates release of alpha, dense, and lysosomal granules from platelets; Munc18-2-deficient platelets have defective aggregation at low collagen doses, impaired thrombus formation under shear stress, prolonged bleeding times in vivo, and protection against arterial thrombosis.","method":"Conditional knockout mice, platelet granule secretion assays, aggregometry, shear stress thrombus assay, tail bleeding time, arterial thrombosis model","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — conditional KO with multiple orthogonal assays in vitro and in vivo, isoform-specific comparison","pmids":["30696774"],"is_preprint":false},{"year":2011,"finding":"In pancreatic β-cells, Munc18-2 overexpression increases Ca2+ sensitivity for insulin exocytosis and mediates release of fusion-competent granules at lower cytoplasmic Ca2+ concentrations; Munc18-2 localization is distinct from Munc18-1 and does not redistribute in response to glucose stimulation; the Ca2+ sensitivity of exocytosis depends on phosphorylation status of Munc18 proteins.","method":"Overexpression in β-cells, ramp-like caged Ca2+ photorelease, whole-cell patch clamp capacitance recordings, subcellular fractionation/immunofluorescence","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1-2 — electrophysiology with Ca2+ uncaging, overexpression functional assay, single lab","pmids":["21690086"],"is_preprint":false},{"year":2018,"finding":"Loss of Munc18-2/Stxbp2 in intestinal organoids from knockout mice recapitulates MVID pathology (secretory vesicle accumulation, apical tubulovesicular network, microvillus inclusions); the patient variant P477L fails to rescue the phenotype while wild-type human MUNC18-2 fully restores it; microvillus inclusions form dynamically by intracellular maturation or invagination of apical/basolateral membranes.","method":"Munc18-2/Stxbp2-null mouse intestinal organoids, lentiviral rescue with WT vs. P477L variant, confocal and transmission electron microscopy, spinning disc time-lapse microscopy","journal":"Cellular and molecular gastroenterology and hepatology","confidence":"High","confidence_rationale":"Tier 2 — KO organoid model with rescue experiment distinguishing WT from disease variant, live imaging","pmids":["30364784"],"is_preprint":false},{"year":2013,"finding":"STXBP2 mutations cause persistent defective membrane trafficking in intestinal epithelial cells and renal tubular cells (intracytoplasmic PAS-positive granule accumulation, enlarged intracytoplasmic CD10-positive apical vesicles, short microvilli), defects that persist after hematopoietic stem cell transplantation, demonstrating STXBP2 function is cell-intrinsic in epithelium.","method":"Immunohistochemistry, electron microscopy of patient gut and kidney biopsies","journal":"Pediatric blood & cancer","confidence":"Medium","confidence_rationale":"Tier 3 — patient tissue analysis showing localization and trafficking defects, no direct functional manipulation","pmids":["23382066"],"is_preprint":false},{"year":2014,"finding":"N-terminal syntaxin 11 mutations R4A and L58P (in the Habc domain) abolish binding to Munc18-2, demonstrating that both the N-terminus and Habc domain of STX11 are required for Munc18-2 interaction, with similarity to the dynamic binary binding of neuronal syntaxin 1 to Munc18-1.","method":"Ectopic expression, co-immunoprecipitation, functional NK cell degranulation assay in patient cells","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — Co-IP binding mapping with patient cell functional validation, single lab","pmids":["24459464"],"is_preprint":false},{"year":2015,"finding":"STXBP2 is required in erythroid cells: red blood cells from FHL-5 patients express Munc18-2 and show reduced phosphatidylserine exposure after Ca2+ ionophore treatment; cultured erythroblasts from FHL-5 patients display decreased CD235a expression and aberrant morphology, identifying STXBP2 as required for phospholipid scrambling and normal erythropoiesis.","method":"Patient RBC phosphatidylserine exposure assay (Ca2+ ionophore/ionomycin), erythroblast culture with morphological and surface marker analysis","journal":"Experimental hematology","confidence":"Medium","confidence_rationale":"Tier 2 — patient loss-of-function with specific functional readout in erythroid lineage, single lab","pmids":["26320718"],"is_preprint":false},{"year":2020,"finding":"STXBP2-R190C mutation does not alter protein expression, subcellular localization, or STXBP2/STX11 interaction, but forced expression in normal CTLs strongly inhibits degranulation and cytolytic activity in a dominant-negative manner, implicating domain 2 of STXBP2 in stabilizing productive interactions required for granule exocytosis.","method":"Forced expression in primary CTLs, degranulation assay, cytotoxicity assay, Co-immunoprecipitation, immunofluorescence localization","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — dominant-negative functional characterization in primary cells with binding and localization controls, single lab","pmids":["33162974"],"is_preprint":false},{"year":2018,"finding":"STXBP2 and STX11 deficiency is associated with significant reduction in STXBP1 protein and its partner STX1 in patient NK and T cells; functional assays revealed the STXBP1/STX1 axis accounts for up to 50% of NK and CD8+ T-cell cytotoxic activity, suggesting a complementary exocytosis pathway.","method":"Patient cell protein expression analysis, NK/CTL degranulation and cytotoxicity assays, IL-2 stimulation rescue","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 — functional assays in patient and normal cells, but mechanistic pathway assignment partly indirect","pmids":["29599780"],"is_preprint":false}],"current_model":"STXBP2 (Munc18-2) is a SEC/Munc18-family protein that functions as an essential regulator of SNARE-mediated membrane fusion across multiple cell types: it binds and chaperones syntaxin 11 (with ~20-fold higher affinity than syntaxin 3) to the plasma membrane of cytotoxic lymphocytes where it directly promotes SNARE complex assembly and complete lytic granule exocytosis (beyond hemifusion); it interacts with syntaxins 2/3 to mediate secretory granule translocation (via microtubule coupling) and membrane fusion in mast cells and epithelial apical trafficking; it is the sole Munc18 isoform required for regulated exocytosis in platelets; and disease-causing mutations either disrupt syntaxin binding/protein stability or act in a dominant-negative manner to arrest SNARE complex assembly, causing familial hemophagocytic lymphohistiocytosis type 5 and associated enteropathy."},"narrative":{"teleology":[{"year":2000,"claim":"Establishing STXBP2 as a syntaxin 3-binding regulator of polarized epithelial trafficking answered whether Sec1/Munc18 proteins operate beyond neurons—Munc18-2 bound syntaxin 3 and controlled apical delivery in intestinal epithelial cells, with point mutants revealing both syntaxin-dependent and -independent regulatory modes.","evidence":"Mutagenesis, co-immunoprecipitation, and influenza HA apical transport assay in Caco-2 cells","pmids":["10788461"],"confidence":"High","gaps":["Syntaxin-independent mechanism of Munc18-2 action not molecularly defined","No in vivo validation in epithelial tissue"]},{"year":2003,"claim":"Linking STXBP2 to microtubule-dependent granule translocation in mast cells revealed a pre-fusion role: Munc18-2 associated with secretory granules, interacted selectively with syntaxins 2/3, and was excluded from lipid raft-resident SNARE complexes, placing it upstream of productive SNARE assembly.","evidence":"Overexpression/peptide inhibition, live imaging, nocodazole perturbation, lipid raft fractionation in RBL mast cells","pmids":["12482918","12935901"],"confidence":"High","gaps":["Direct Munc18-2–tubulin interaction not yet demonstrated","Mechanism of raft exclusion of Munc18-2/STX3 unclear"]},{"year":2005,"claim":"Demonstrating that Slp4-a binds closed-conformation syntaxins 2/3 only in the presence of Munc18-2 established a tripartite regulatory complex linking Rab effectors to SNARE machinery in exocrine secretion.","evidence":"Co-immunoprecipitation with deletion mapping, antibody inhibition of amylase release in permeabilized parotid acinar cells","pmids":["16186111"],"confidence":"Medium","gaps":["Munc18-2 does not directly bind Slp4-a—structural basis for indirect requirement unknown","Relevance beyond parotid cells not tested"]},{"year":2009,"claim":"Identification of STXBP2 as the FHL5 disease gene and its requirement for syntaxin 11 stability and cytotoxic lymphocyte degranulation established the first direct genetic link between a Sec1/Munc18 protein and immune deficiency.","evidence":"Patient lymphocyte analysis, co-immunoprecipitation, NK/CTL degranulation assays, ectopic rescue in two independent studies","pmids":["19804848","19884660"],"confidence":"High","gaps":["Mechanism by which Munc18-2 stabilizes STX11 not resolved at molecular level","Whether other syntaxins partially compensate in vivo unclear"]},{"year":2012,"claim":"Showing that Munc18-2 is the sole Munc18 isoform forming native complexes with STX11/SNAP-23/VAMP-8 in platelets and that FHL5 platelets have pan-granule secretion failure defined STXBP2 as a non-redundant exocytosis regulator in the megakaryocyte lineage.","evidence":"Co-immunoprecipitation from human platelets, granule secretion assays in FHL5 patient platelets","pmids":["22791290"],"confidence":"High","gaps":["Relative contributions of dense vs. alpha granule defects to bleeding phenotype not dissected","Whether Munc18-2 acts at granule-granule fusion in platelets untested"]},{"year":2013,"claim":"Structural determination of Munc18-2 at 2.6 Å resolution mapped FHL5 mutations to syntaxin and SNARE binding surfaces, and quantitative binding revealed ~20-fold preference for STX11 over STX3, explaining isoform-selective function in lymphocytes.","evidence":"X-ray crystallography, surface plasmon resonance, mutation mapping, patient cell analysis","pmids":["24194549"],"confidence":"High","gaps":["No co-crystal with STX11 obtained","Structural basis for domain 3 loop interactions with SNARE bundle not resolved"]},{"year":2013,"claim":"Munc18-2 knockdown selectively impaired secretory granule translocation (not chemokine secretion) in mast cells through a direct, nocodazole-sensitive interaction with tubulin, establishing a mechanistic link between the fusion machinery and microtubule cytoskeleton.","evidence":"siRNA knockdown, co-immunoprecipitation with tubulin, nocodazole perturbation, immunogold EM in mast cells","pmids":["24323579"],"confidence":"High","gaps":["Tubulin binding site on Munc18-2 not mapped","Whether tubulin interaction is direct or via adaptors not definitively shown"]},{"year":2015,"claim":"Demonstrating that FHL5 mutations R65Q/W arrest late SNARE complex assembly while preserving STX11 binding and stability distinguished a post-chaperoning fusion-promoting function of Munc18-2 and revealed a dominant-negative disease mechanism.","evidence":"In vitro reconstituted membrane fusion, SNARE complex assembly assay, forced expression in primary CTLs/NK cells","pmids":["25564401"],"confidence":"High","gaps":["Exact step of SNARE zippering that is arrested not defined","Whether all domain 1 mutations share the same mechanism untested"]},{"year":2015,"claim":"Endogenous localization studies in FHL4/FHL5 patient CTLs demonstrated that Munc18-2 chaperones STX11 to the plasma membrane, while its own granule localization is STX11-independent, establishing the directionality of the chaperoning relationship.","evidence":"Immunofluorescence of endogenous proteins in matched FHL4 and FHL5 patient CTLs","pmids":["26771955"],"confidence":"High","gaps":["Transport intermediate carrying STX11 from granules to plasma membrane not identified","Whether chaperoning involves co-transport or sequential handoff unknown"]},{"year":2017,"claim":"Reconstituted fusion assays showed Munc18-2 drives the transition from STX11-mediated hemifusion to full content mixing, establishing that its role extends beyond SNARE complex assembly to actively catalyzing fusion pore opening.","evidence":"Flipped cell-cell fusion assay measuring lipid vs. content mixing, Munc18-2 wild-type and mutant expression","pmids":["28265073"],"confidence":"High","gaps":["Whether Munc18-2 engages a specific fusion-pore intermediate structurally unknown","Lipid composition effects on this transition not explored"]},{"year":2017,"claim":"Loss of Munc18-2 in enterocytes disrupted Slp4a/Stx3 interaction and selectively impaired apical trafficking of NHE3 and GLUT5 but not DPPIV, providing a molecular mechanism for FHL5-associated enteropathy.","evidence":"CRISPR/Cas9 knockout in CaCo2 cells, patient biopsies, patient-derived organoids, co-immunoprecipitation","pmids":["28724787"],"confidence":"High","gaps":["Basis for cargo selectivity (NHE3/GLUT5 affected, DPPIV spared) not defined","Whether STX3 or another syntaxin mediates unaffected routes unknown"]},{"year":2018,"claim":"Conditional knockout of Munc18-2 (but not Munc18-1 or -3) in mast cells and megakaryocytes abolished regulated exocytosis in vivo, confirming non-redundant isoform specificity and demonstrating physiological protection against anaphylaxis and arterial thrombosis.","evidence":"Conditional knockout mice with electrophysiology, EM, secretion assays, anaphylaxis and thrombosis models","pmids":["29599294","30696774"],"confidence":"High","gaps":["Structural basis for isoform non-redundancy despite conserved architecture not explained","Whether compound exocytosis requires Munc18-2 catalytic activity or only scaffolding unknown"]},{"year":2018,"claim":"Munc18-2-null intestinal organoids recapitulated microvillus inclusion disease pathology with dynamic microvillus inclusion formation, and rescue with wild-type but not P477L Munc18-2 confirmed variant pathogenicity, establishing an ex vivo disease model.","evidence":"Stxbp2-knockout mouse organoids, lentiviral rescue, confocal/TEM and time-lapse imaging","pmids":["30364784"],"confidence":"High","gaps":["P477L structural effect not characterized","Whether organoid phenotype fully recapitulates in vivo intestinal pathology long-term unclear"]},{"year":null,"claim":"Key unresolved questions include the structural basis of the Munc18-2/STX11 co-complex during SNARE zippering, the molecular determinants of cargo selectivity in epithelial trafficking, and whether the Munc18-2–tubulin interaction reflects a direct binding mode with a defined interface.","evidence":"","pmids":[],"confidence":"Low","gaps":["No co-crystal structure of Munc18-2 with STX11 or SNARE complex","Tubulin binding interface not mapped","Cargo-selective sorting signals downstream of Munc18-2/Stx3 undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[2,5,6,15,16]},{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[0,1,10]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[8]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3,10,15]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,10,13]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[3,10]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,3,6,9,15,16,18]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,1,14]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[7,16]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[9,10,18]}],"complexes":["Munc18-2/STX11 binary complex","Munc18-2/STX3 binary complex","STX11/SNAP-23/VAMP-8/Munc18-2 SNARE complex"],"partners":["STX11","STX3","STX2","SNAP23","VAMP8","SYTL4","UNC13D","TUBA1A"],"other_free_text":[]},"mechanistic_narrative":"STXBP2 (Munc18-2) is a Sec1/Munc18-family protein that functions as a master regulator of SNARE-mediated exocytosis in hematopoietic and epithelial cells by chaperoning cognate syntaxins to target membranes and directly promoting SNARE complex assembly to drive complete membrane fusion. It binds syntaxin 11 with high affinity in cytotoxic lymphocytes and platelets, where it is required for lytic granule and platelet granule exocytosis, and interacts with syntaxins 2 and 3 in mast cells and intestinal epithelial cells to control secretory granule translocation along microtubules and cargo-selective apical membrane trafficking [PMID:19804848, PMID:28265073, PMID:30696774, PMID:24323579, PMID:28724787]. Biallelic loss-of-function mutations in STXBP2 cause familial hemophagocytic lymphohistiocytosis type 5 (FHL5), and specific missense mutations such as R65Q/W and R190C act in a dominant-negative manner by arresting late SNARE complex assembly without abolishing syntaxin binding [PMID:25564401, PMID:33162974]. STXBP2 mutations also produce cell-intrinsic intestinal and renal epithelial trafficking defects recapitulating microvillus inclusion disease pathology, which persists after hematopoietic stem cell transplantation [PMID:23382066, PMID:30364784]."},"prefetch_data":{"uniprot":{"accession":"Q15833","full_name":"Syntaxin-binding protein 2","aliases":["Protein unc-18 homolog 2","Unc18-2","Protein unc-18 homolog B","Unc-18B"],"length_aa":593,"mass_kda":66.5,"function":"Involved in intracellular vesicle trafficking and vesicle fusion with membranes. Contributes to the granule exocytosis machinery through interaction with soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that regulate membrane fusion. Regulates cytotoxic granule exocytosis in natural killer (NK) cells","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q15833/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/STXBP2","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":"STX3","stoichiometry":4.0},{"gene":"STX2","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/STXBP2","total_profiled":1310},"omim":[{"mim_id":"613101","title":"HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS, FAMILIAL, 5, WITH OR WITHOUT MICROVILLUS INCLUSION DISEASE; FHL5","url":"https://www.omim.org/entry/613101"},{"mim_id":"608897","title":"UNC13 HOMOLOG D; UNC13D","url":"https://www.omim.org/entry/608897"},{"mim_id":"605014","title":"SYNTAXIN 11; STX11","url":"https://www.omim.org/entry/605014"},{"mim_id":"603552","title":"HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS, FAMILIAL, 4; FHL4","url":"https://www.omim.org/entry/603552"},{"mim_id":"601717","title":"SYNTAXIN-BINDING PROTEIN 2; STXBP2","url":"https://www.omim.org/entry/601717"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Cytosol","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"lymphoid 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\"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP with functional validation in patient cells, replicated independently in same year\",\n      \"pmids\": [\"19804848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"STXBP2 deficiency impairs cytotoxic granule exocytosis in NK cells, a defect rescued by ectopic expression of wild-type STXBP2; syntaxin 11 expression requires the presence of STXBP2, identifying STXBP2 as required at a late step of the secretory pathway for cytotoxic granule release.\",\n      \"method\": \"Patient lymphoblast analysis, NK cell cytotoxicity assay, ectopic expression rescue experiment\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined phenotype, rescue experiment, orthogonal functional assays, independent replication\",\n      \"pmids\": [\"19884660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Munc18-2 forms a complex with syntaxin 3 (a t-SNARE at the apical plasma membrane) and overexpression of wild-type Munc18-2 inhibits apical delivery of influenza hemagglutinin in Caco-2 epithelial cells; point mutants unable to bind syntaxin 3 have differential effects on apical transport, indicating Munc18-2 controls apical membrane trafficking via syntaxin 3 interaction and additional syntaxin-independent mechanisms.\",\n      \"method\": \"Munc18-2 point mutagenesis, co-immunoprecipitation, SNAP-23 displacement assay, influenza HA apical transport assay in Caco-2 cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with functional transport assay and in vitro binding confirmation\",\n      \"pmids\": [\"10788461\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Munc18-2 localizes to secretory granules in mast cells, interacts with syntaxin 2 and syntaxin 3 (not syntaxin 4), redistributes toward lamellipodia upon stimulation and associates with microtubule-aligned granules; overexpression of Munc18-2 or effector-loop-containing peptides inhibits IgE-triggered exocytosis, and disruption of microtubules with nocodazole causes Munc18-2 redistribution and impairs mediator release.\",\n      \"method\": \"Munc18-2 overexpression, peptide inhibition, immunofluorescence/live imaging, nocodazole treatment, mediator release assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — localization with functional consequence, multiple orthogonal perturbation methods\",\n      \"pmids\": [\"12482918\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Crystal structure of human Munc18-2 solved at 2.6 Å resolution; surface mutations causing FHL5 (R39P, L130S, E132A, P334L) map to predicted syntaxin and SNARE binding sites; Munc18-2 binds the N-terminal peptide of STX11 with ~20-fold higher affinity than STX3; upon IL-2 activation, increased STX3 levels favor Munc18-2 binding when STX11 is absent, and Munc18-1 expressed in activated CTL can bind STX11.\",\n      \"method\": \"X-ray crystallography, surface plasmon resonance binding assay, mutation mapping, patient cell analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure with binding affinity measurements and mutagenesis mapping\",\n      \"pmids\": [\"24194549\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"STXBP2 R65Q and R65W mutations retain interaction with and stabilization of syntaxin 11 but hinder membrane fusion in vitro by arresting late steps of SNARE-complex assembly; forced expression of these mutants in control CTLs and NK cells diminishes degranulation and cytotoxic activity in a dominant-negative manner.\",\n      \"method\": \"In vitro membrane fusion assay, SNARE complex assembly assay, forced expression in primary CTLs/NK cells, cytotoxicity/degranulation assay\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of SNARE fusion + dominant-negative functional validation in primary cells\",\n      \"pmids\": [\"25564401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Munc18-2 facilitates the transition from lipid-anchored STX11-mediated hemifusion (lipid mixing without content mixing) to complete membrane fusion during CTL lytic granule secretion; wild-type Munc18-2 but not STX11-binding mutants stimulates complete fusion in a reconstituted flipped cell-cell fusion assay, demonstrating Munc18-2 directly promotes SNARE complex assembly beyond its chaperone role.\",\n      \"method\": \"Reconstituted 'flipped' cell-cell fusion assay measuring lipid and content mixing, Munc18-2 mutant expression\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted cell-cell fusion assay with mutagenesis validation distinguishing hemifusion from complete fusion\",\n      \"pmids\": [\"28265073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Munc18b (STXBP2) is the major Munc18 isoform in platelets and forms complexes with syntaxin 11, SNAP-23, and VAMP-8, as well as Munc13-4 and Rab27; FHL5 patient platelets with biallelic mutations show profoundly defective serotonin, ADP/ATP, platelet factor 4, and lysosomal content secretion, with decreased Munc18b and syntaxin 11 levels.\",\n      \"method\": \"Co-immunoprecipitation from human platelets, platelet secretion assays (serotonin, ADP/ATP release), patient platelet analysis\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP establishing native complex, functional validation in patient material\",\n      \"pmids\": [\"22791290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"siRNA-mediated silencing of Munc18-2 in mast cells inhibits secretory granule (SG) translocation but not CCL2 chemokine secretion; Munc18-2 (but not STX3) interacts with tubulin in resting cells in a nocodazole-sensitive manner, and this interaction decreases after stimulation, demonstrating Munc18-2 dynamically couples the fusion machinery to the microtubule cytoskeleton.\",\n      \"method\": \"siRNA knockdown, combined knockdown epistasis, immunogold electron microscopy, co-immunoprecipitation with tubulin, nocodazole perturbation, degranulation assay\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — siRNA knockdown with specific phenotypic readout, Co-IP, localization, and epistasis between Munc18-2 and STX3\",\n      \"pmids\": [\"24323579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In FHL5 enterocytes, Munc18-2 is required for Slp4a/Stx3 interaction necessary for fusion of cargo vesicles with the apical plasma membrane; loss of Munc18-2 selectively disrupts trafficking of NHE3 and GLUT5 to the brush border while DPPIV transport is unaffected, causing subapical accumulation of cargo vesicles.\",\n      \"method\": \"CRISPR/Cas9 Munc18-2 knockout in CaCo2 cells, patient biopsy analysis, patient-derived organoid analysis, fluorescence and electron microscopy, co-immunoprecipitation (Slp4a/Stx3)\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — CRISPR KO model with mechanistic Co-IP, patient biopsies, and organoids with defined cargo-selective phenotype\",\n      \"pmids\": [\"28724787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Munc18-2 localizes predominantly to cytolytic granules with low levels at the plasma membrane in CTLs; STX11 localizes to the plasma membrane and its plasma membrane localization is lost in FHL5 CTLs lacking Munc18-2, whereas Munc18-2 localization is unaffected in FHL4 CTLs lacking STX11, demonstrating Munc18-2 chaperones STX11 to the plasma membrane.\",\n      \"method\": \"Immunofluorescence localization of endogenous proteins in FHL4 and FHL5 patient CTLs, loss-of-function analysis\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — endogenous protein localization in matched patient cell lines with defined functional consequence\",\n      \"pmids\": [\"26771955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Slp4-a (granuphilin-a) interacts with syntaxin 2/3 in a closed conformation only in the presence of Munc18-2, while Munc18-2 itself does not directly bind Slp4-a; the Slp4-a linker domain (residues 144–354) mediates syntaxin 2/3 binding, and antibody interference with this interaction inhibits isoproterenol-stimulated amylase release from permeabilized parotid acinar cells.\",\n      \"method\": \"Co-immunoprecipitation in COS-7 cells, deletion analysis, antibody inhibition in streptolysin-O permeabilized cells, amylase release assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP with deletion mapping and functional antibody inhibition, but Munc18-2 involvement is indirect (required for complex formation)\",\n      \"pmids\": [\"16186111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Munc18-2/syntaxin 3 complexes are spatially excluded from lipid rafts in mast cells, whereas STX3-containing SNARE complexes (with SNAP-23 and VAMP-8) are enriched in rafts, demonstrating that Munc18-2 acts at a step distinct from SNARE complex formation and membrane fusion.\",\n      \"method\": \"Lipid raft fractionation, Western blotting for SNARE and Munc18 proteins in RBL mast cells\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — biochemical fractionation showing spatial separation, single method without direct functional manipulation\",\n      \"pmids\": [\"12935901\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Munc18-2 knockdown in RBL-2H3 mast cells markedly inhibits degranulation without changing syntaxin expression or Ca2+ mobilization; Munc18-2 interacts with syntaxin 3 (not syntaxin 4) both at the plasma membrane and on secretory granules in vivo, suggesting regulation of both granule-granule and granule-plasma membrane fusion.\",\n      \"method\": \"Munc18-2 knockdown, fluorescent chimera co-localization/co-immunoprecipitation, single-cell granule imaging, degranulation assay\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KD with phenotype, Co-IP binding specificity, but single lab\",\n      \"pmids\": [\"17408745\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FHL-5 neutrophils (with STXBP2/Munc18-2 mutations) have a profound defect in granule mobilization resulting in inadequate bacterial killing, particularly of gram-negative E. coli, identifying STXBP2 as required for neutrophil granule exocytosis and bactericidal function.\",\n      \"method\": \"Patient neutrophil functional assays (degranulation, bacterial killing assay), comparison of FHL-5 vs. control neutrophils\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with specific phenotypic readout in patient cells, single lab\",\n      \"pmids\": [\"23687090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In mast cells, conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) causes near-complete absence of regulated exocytosis and abolishes compound (homotypic) exocytosis as shown by plasma membrane capacitance recordings and stereological EM analysis; Munc18-2 cKO mice are protected from anaphylaxis.\",\n      \"method\": \"Conditional knockout mice (Munc18-1, -2, -3 each deleted in mast cells), plasma membrane capacitance recordings, stereological EM, mediator secretion assays, in vivo anaphylaxis model\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with multiple orthogonal assays including electrophysiology and EM, in vivo validation\",\n      \"pmids\": [\"29599294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Munc18-2 (but not Munc18-1 or Munc18-3) conditional knockout in megakaryocytes ablates release of alpha, dense, and lysosomal granules from platelets; Munc18-2-deficient platelets have defective aggregation at low collagen doses, impaired thrombus formation under shear stress, prolonged bleeding times in vivo, and protection against arterial thrombosis.\",\n      \"method\": \"Conditional knockout mice, platelet granule secretion assays, aggregometry, shear stress thrombus assay, tail bleeding time, arterial thrombosis model\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — conditional KO with multiple orthogonal assays in vitro and in vivo, isoform-specific comparison\",\n      \"pmids\": [\"30696774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In pancreatic β-cells, Munc18-2 overexpression increases Ca2+ sensitivity for insulin exocytosis and mediates release of fusion-competent granules at lower cytoplasmic Ca2+ concentrations; Munc18-2 localization is distinct from Munc18-1 and does not redistribute in response to glucose stimulation; the Ca2+ sensitivity of exocytosis depends on phosphorylation status of Munc18 proteins.\",\n      \"method\": \"Overexpression in β-cells, ramp-like caged Ca2+ photorelease, whole-cell patch clamp capacitance recordings, subcellular fractionation/immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — electrophysiology with Ca2+ uncaging, overexpression functional assay, single lab\",\n      \"pmids\": [\"21690086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Loss of Munc18-2/Stxbp2 in intestinal organoids from knockout mice recapitulates MVID pathology (secretory vesicle accumulation, apical tubulovesicular network, microvillus inclusions); the patient variant P477L fails to rescue the phenotype while wild-type human MUNC18-2 fully restores it; microvillus inclusions form dynamically by intracellular maturation or invagination of apical/basolateral membranes.\",\n      \"method\": \"Munc18-2/Stxbp2-null mouse intestinal organoids, lentiviral rescue with WT vs. P477L variant, confocal and transmission electron microscopy, spinning disc time-lapse microscopy\",\n      \"journal\": \"Cellular and molecular gastroenterology and hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO organoid model with rescue experiment distinguishing WT from disease variant, live imaging\",\n      \"pmids\": [\"30364784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"STXBP2 mutations cause persistent defective membrane trafficking in intestinal epithelial cells and renal tubular cells (intracytoplasmic PAS-positive granule accumulation, enlarged intracytoplasmic CD10-positive apical vesicles, short microvilli), defects that persist after hematopoietic stem cell transplantation, demonstrating STXBP2 function is cell-intrinsic in epithelium.\",\n      \"method\": \"Immunohistochemistry, electron microscopy of patient gut and kidney biopsies\",\n      \"journal\": \"Pediatric blood & cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — patient tissue analysis showing localization and trafficking defects, no direct functional manipulation\",\n      \"pmids\": [\"23382066\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"N-terminal syntaxin 11 mutations R4A and L58P (in the Habc domain) abolish binding to Munc18-2, demonstrating that both the N-terminus and Habc domain of STX11 are required for Munc18-2 interaction, with similarity to the dynamic binary binding of neuronal syntaxin 1 to Munc18-1.\",\n      \"method\": \"Ectopic expression, co-immunoprecipitation, functional NK cell degranulation assay in patient cells\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP binding mapping with patient cell functional validation, single lab\",\n      \"pmids\": [\"24459464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"STXBP2 is required in erythroid cells: red blood cells from FHL-5 patients express Munc18-2 and show reduced phosphatidylserine exposure after Ca2+ ionophore treatment; cultured erythroblasts from FHL-5 patients display decreased CD235a expression and aberrant morphology, identifying STXBP2 as required for phospholipid scrambling and normal erythropoiesis.\",\n      \"method\": \"Patient RBC phosphatidylserine exposure assay (Ca2+ ionophore/ionomycin), erythroblast culture with morphological and surface marker analysis\",\n      \"journal\": \"Experimental hematology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — patient loss-of-function with specific functional readout in erythroid lineage, single lab\",\n      \"pmids\": [\"26320718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"STXBP2-R190C mutation does not alter protein expression, subcellular localization, or STXBP2/STX11 interaction, but forced expression in normal CTLs strongly inhibits degranulation and cytolytic activity in a dominant-negative manner, implicating domain 2 of STXBP2 in stabilizing productive interactions required for granule exocytosis.\",\n      \"method\": \"Forced expression in primary CTLs, degranulation assay, cytotoxicity assay, Co-immunoprecipitation, immunofluorescence localization\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — dominant-negative functional characterization in primary cells with binding and localization controls, single lab\",\n      \"pmids\": [\"33162974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"STXBP2 and STX11 deficiency is associated with significant reduction in STXBP1 protein and its partner STX1 in patient NK and T cells; functional assays revealed the STXBP1/STX1 axis accounts for up to 50% of NK and CD8+ T-cell cytotoxic activity, suggesting a complementary exocytosis pathway.\",\n      \"method\": \"Patient cell protein expression analysis, NK/CTL degranulation and cytotoxicity assays, IL-2 stimulation rescue\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional assays in patient and normal cells, but mechanistic pathway assignment partly indirect\",\n      \"pmids\": [\"29599780\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STXBP2 (Munc18-2) is a SEC/Munc18-family protein that functions as an essential regulator of SNARE-mediated membrane fusion across multiple cell types: it binds and chaperones syntaxin 11 (with ~20-fold higher affinity than syntaxin 3) to the plasma membrane of cytotoxic lymphocytes where it directly promotes SNARE complex assembly and complete lytic granule exocytosis (beyond hemifusion); it interacts with syntaxins 2/3 to mediate secretory granule translocation (via microtubule coupling) and membrane fusion in mast cells and epithelial apical trafficking; it is the sole Munc18 isoform required for regulated exocytosis in platelets; and disease-causing mutations either disrupt syntaxin binding/protein stability or act in a dominant-negative manner to arrest SNARE complex assembly, causing familial hemophagocytic lymphohistiocytosis type 5 and associated enteropathy.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"STXBP2 (Munc18-2) is a Sec1/Munc18-family protein that functions as a master regulator of SNARE-mediated exocytosis in hematopoietic and epithelial cells by chaperoning cognate syntaxins to target membranes and directly promoting SNARE complex assembly to drive complete membrane fusion. It binds syntaxin 11 with high affinity in cytotoxic lymphocytes and platelets, where it is required for lytic granule and platelet granule exocytosis, and interacts with syntaxins 2 and 3 in mast cells and intestinal epithelial cells to control secretory granule translocation along microtubules and cargo-selective apical membrane trafficking [PMID:19804848, PMID:28265073, PMID:30696774, PMID:24323579, PMID:28724787]. Biallelic loss-of-function mutations in STXBP2 cause familial hemophagocytic lymphohistiocytosis type 5 (FHL5), and specific missense mutations such as R65Q/W and R190C act in a dominant-negative manner by arresting late SNARE complex assembly without abolishing syntaxin binding [PMID:25564401, PMID:33162974]. STXBP2 mutations also produce cell-intrinsic intestinal and renal epithelial trafficking defects recapitulating microvillus inclusion disease pathology, which persists after hematopoietic stem cell transplantation [PMID:23382066, PMID:30364784].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Establishing STXBP2 as a syntaxin 3-binding regulator of polarized epithelial trafficking answered whether Sec1/Munc18 proteins operate beyond neurons—Munc18-2 bound syntaxin 3 and controlled apical delivery in intestinal epithelial cells, with point mutants revealing both syntaxin-dependent and -independent regulatory modes.\",\n      \"evidence\": \"Mutagenesis, co-immunoprecipitation, and influenza HA apical transport assay in Caco-2 cells\",\n      \"pmids\": [\"10788461\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Syntaxin-independent mechanism of Munc18-2 action not molecularly defined\", \"No in vivo validation in epithelial tissue\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Linking STXBP2 to microtubule-dependent granule translocation in mast cells revealed a pre-fusion role: Munc18-2 associated with secretory granules, interacted selectively with syntaxins 2/3, and was excluded from lipid raft-resident SNARE complexes, placing it upstream of productive SNARE assembly.\",\n      \"evidence\": \"Overexpression/peptide inhibition, live imaging, nocodazole perturbation, lipid raft fractionation in RBL mast cells\",\n      \"pmids\": [\"12482918\", \"12935901\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct Munc18-2–tubulin interaction not yet demonstrated\", \"Mechanism of raft exclusion of Munc18-2/STX3 unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstrating that Slp4-a binds closed-conformation syntaxins 2/3 only in the presence of Munc18-2 established a tripartite regulatory complex linking Rab effectors to SNARE machinery in exocrine secretion.\",\n      \"evidence\": \"Co-immunoprecipitation with deletion mapping, antibody inhibition of amylase release in permeabilized parotid acinar cells\",\n      \"pmids\": [\"16186111\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Munc18-2 does not directly bind Slp4-a—structural basis for indirect requirement unknown\", \"Relevance beyond parotid cells not tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identification of STXBP2 as the FHL5 disease gene and its requirement for syntaxin 11 stability and cytotoxic lymphocyte degranulation established the first direct genetic link between a Sec1/Munc18 protein and immune deficiency.\",\n      \"evidence\": \"Patient lymphocyte analysis, co-immunoprecipitation, NK/CTL degranulation assays, ectopic rescue in two independent studies\",\n      \"pmids\": [\"19804848\", \"19884660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which Munc18-2 stabilizes STX11 not resolved at molecular level\", \"Whether other syntaxins partially compensate in vivo unclear\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Showing that Munc18-2 is the sole Munc18 isoform forming native complexes with STX11/SNAP-23/VAMP-8 in platelets and that FHL5 platelets have pan-granule secretion failure defined STXBP2 as a non-redundant exocytosis regulator in the megakaryocyte lineage.\",\n      \"evidence\": \"Co-immunoprecipitation from human platelets, granule secretion assays in FHL5 patient platelets\",\n      \"pmids\": [\"22791290\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of dense vs. alpha granule defects to bleeding phenotype not dissected\", \"Whether Munc18-2 acts at granule-granule fusion in platelets untested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Structural determination of Munc18-2 at 2.6 Å resolution mapped FHL5 mutations to syntaxin and SNARE binding surfaces, and quantitative binding revealed ~20-fold preference for STX11 over STX3, explaining isoform-selective function in lymphocytes.\",\n      \"evidence\": \"X-ray crystallography, surface plasmon resonance, mutation mapping, patient cell analysis\",\n      \"pmids\": [\"24194549\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal with STX11 obtained\", \"Structural basis for domain 3 loop interactions with SNARE bundle not resolved\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Munc18-2 knockdown selectively impaired secretory granule translocation (not chemokine secretion) in mast cells through a direct, nocodazole-sensitive interaction with tubulin, establishing a mechanistic link between the fusion machinery and microtubule cytoskeleton.\",\n      \"evidence\": \"siRNA knockdown, co-immunoprecipitation with tubulin, nocodazole perturbation, immunogold EM in mast cells\",\n      \"pmids\": [\"24323579\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tubulin binding site on Munc18-2 not mapped\", \"Whether tubulin interaction is direct or via adaptors not definitively shown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrating that FHL5 mutations R65Q/W arrest late SNARE complex assembly while preserving STX11 binding and stability distinguished a post-chaperoning fusion-promoting function of Munc18-2 and revealed a dominant-negative disease mechanism.\",\n      \"evidence\": \"In vitro reconstituted membrane fusion, SNARE complex assembly assay, forced expression in primary CTLs/NK cells\",\n      \"pmids\": [\"25564401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Exact step of SNARE zippering that is arrested not defined\", \"Whether all domain 1 mutations share the same mechanism untested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Endogenous localization studies in FHL4/FHL5 patient CTLs demonstrated that Munc18-2 chaperones STX11 to the plasma membrane, while its own granule localization is STX11-independent, establishing the directionality of the chaperoning relationship.\",\n      \"evidence\": \"Immunofluorescence of endogenous proteins in matched FHL4 and FHL5 patient CTLs\",\n      \"pmids\": [\"26771955\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transport intermediate carrying STX11 from granules to plasma membrane not identified\", \"Whether chaperoning involves co-transport or sequential handoff unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Reconstituted fusion assays showed Munc18-2 drives the transition from STX11-mediated hemifusion to full content mixing, establishing that its role extends beyond SNARE complex assembly to actively catalyzing fusion pore opening.\",\n      \"evidence\": \"Flipped cell-cell fusion assay measuring lipid vs. content mixing, Munc18-2 wild-type and mutant expression\",\n      \"pmids\": [\"28265073\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether Munc18-2 engages a specific fusion-pore intermediate structurally unknown\", \"Lipid composition effects on this transition not explored\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Loss of Munc18-2 in enterocytes disrupted Slp4a/Stx3 interaction and selectively impaired apical trafficking of NHE3 and GLUT5 but not DPPIV, providing a molecular mechanism for FHL5-associated enteropathy.\",\n      \"evidence\": \"CRISPR/Cas9 knockout in CaCo2 cells, patient biopsies, patient-derived organoids, co-immunoprecipitation\",\n      \"pmids\": [\"28724787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Basis for cargo selectivity (NHE3/GLUT5 affected, DPPIV spared) not defined\", \"Whether STX3 or another syntaxin mediates unaffected routes unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Conditional knockout of Munc18-2 (but not Munc18-1 or -3) in mast cells and megakaryocytes abolished regulated exocytosis in vivo, confirming non-redundant isoform specificity and demonstrating physiological protection against anaphylaxis and arterial thrombosis.\",\n      \"evidence\": \"Conditional knockout mice with electrophysiology, EM, secretion assays, anaphylaxis and thrombosis models\",\n      \"pmids\": [\"29599294\", \"30696774\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for isoform non-redundancy despite conserved architecture not explained\", \"Whether compound exocytosis requires Munc18-2 catalytic activity or only scaffolding unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Munc18-2-null intestinal organoids recapitulated microvillus inclusion disease pathology with dynamic microvillus inclusion formation, and rescue with wild-type but not P477L Munc18-2 confirmed variant pathogenicity, establishing an ex vivo disease model.\",\n      \"evidence\": \"Stxbp2-knockout mouse organoids, lentiviral rescue, confocal/TEM and time-lapse imaging\",\n      \"pmids\": [\"30364784\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"P477L structural effect not characterized\", \"Whether organoid phenotype fully recapitulates in vivo intestinal pathology long-term unclear\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis of the Munc18-2/STX11 co-complex during SNARE zippering, the molecular determinants of cargo selectivity in epithelial trafficking, and whether the Munc18-2–tubulin interaction reflects a direct binding mode with a defined interface.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No co-crystal structure of Munc18-2 with STX11 or SNARE complex\", \"Tubulin binding interface not mapped\", \"Cargo-selective sorting signals downstream of Munc18-2/Stx3 undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [2, 5, 6, 15, 16]},\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 1, 10]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3, 10, 15]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 10, 13]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [3, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 3, 6, 9, 15, 16, 18]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 14]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [7, 16]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [9, 10, 18]}\n    ],\n    \"complexes\": [\n      \"Munc18-2/STX11 binary complex\",\n      \"Munc18-2/STX3 binary complex\",\n      \"STX11/SNAP-23/VAMP-8/Munc18-2 SNARE complex\"\n    ],\n    \"partners\": [\n      \"STX11\",\n      \"STX3\",\n      \"STX2\",\n      \"SNAP23\",\n      \"VAMP8\",\n      \"SYTL4\",\n      \"UNC13D\",\n      \"TUBA1A\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}