{"gene":"STXBP2","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":2009,"finding":"STXBP2 (Munc18-2) interacts with syntaxin 11 (a SNARE protein mutated in FHL-4); missense mutations in STXBP2 found in FHL-5 patients eliminate this interaction, leading to decreased stability of both proteins in patient lymphocytes, and markedly reduced NK and CTL degranulation as measured by CD107 degranulation assay.","method":"Co-immunoprecipitation, patient lymphocyte protein stability analysis, CD107 degranulation assay","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal interaction identified by Co-IP, protein stability confirmed in patient cells, functional degranulation assay, independently replicated by second concurrent paper (PMID:19884660)","pmids":["19804848"],"is_preprint":false},{"year":2009,"finding":"STXBP2 (Munc18-2) is required at a late step of the secretory pathway for cytotoxic granule exocytosis in NK cells; syntaxin-11 is the main binding partner of STXBP2 in lymphocytes, and its expression requires the presence of STXBP2. Ectopic expression of wild-type STXBP2 rescued the impaired granule exocytosis in patient-derived NK cells.","method":"Loss-of-function (patient lymphoblasts with decreased STXBP2), ectopic expression rescue, Co-immunoprecipitation","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — patient-derived loss-of-function, rescue by ectopic WT protein, Co-IP for binding partner, independently replicated (PMID:19804848)","pmids":["19884660"],"is_preprint":false},{"year":2000,"finding":"Munc18-2 forms a complex with syntaxin 3, a t-SNARE localized to the apical plasma membrane in epithelial cells. Munc18-2 point mutants with reduced syntaxin-3 binding also displaced SNAP-23 from syntaxin-3 complexes when overexpressed in Caco-2 cells. Overexpression of wild-type Munc18-2 inhibited apical delivery of influenza virus hemagglutinin, demonstrating a functional role in apical membrane trafficking.","method":"In vitro binding assay, in vivo co-immunoprecipitation in Caco-2 cells, hemagglutinin apical transport assay, site-directed mutagenesis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — mutagenesis panel combined with in vitro and in vivo binding assays and functional transport assay in a single study","pmids":["10788461"],"is_preprint":false},{"year":2003,"finding":"In mast cells, Munc18-2 localizes to secretory granules (while Munc18-3 localizes to the plasma membrane) and interacts with syntaxin 2 or 3. Overexpression of Munc18-2 or interfering effector-loop peptides inhibited IgE-triggered exocytosis. Upon stimulation, Munc18-2 redistributes into lamellipodia and associates with microtubule-aligned granules; disruption of microtubules with nocodazole redistributes Munc18-2 and impairs mediator release, indicating Munc18-2 couples secretory granule dynamics to the microtubule network.","method":"Subcellular fractionation/immunolocalization, overexpression and dominant-negative peptide inhibition, nocodazole microtubule disruption, mediator release assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization with functional consequence, multiple orthogonal methods (localization, OE inhibition, cytoskeletal disruption), single lab","pmids":["12482918"],"is_preprint":false},{"year":2013,"finding":"Crystal structure of human Munc18-2 solved at 2.6 Å resolution. Eighteen disease-causing point mutations were mapped; four surface mutations (R39P, L130S, E132A, P334L) map exclusively 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 can compensate for absent STX11, and Munc18-1 (expressed in IL-2-activated CTL) is also capable of binding STX11, explaining partial functional rescue.","method":"X-ray crystallography (2.6 Å), surface plasmon resonance binding assay, patient mutation mapping, Western blot of IL-2-activated CTL","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with quantitative binding measurements and mutation mapping in a single rigorous study","pmids":["24194549"],"is_preprint":false},{"year":2015,"finding":"STXBP2 mutations R65Q and R65W retain the ability to interact with and stabilize syntaxin 11, but act in a dominant-negative fashion to inhibit NK cell degranulation and cytotoxic activity. Mechanistic in vitro membrane fusion assays show these mutations arrest SNARE-complex assembly at a late step, directly implicating STXBP2 in promoting SNARE-complex assembly during lytic granule fusion.","method":"In vitro SNARE-mediated membrane fusion assay, Co-immunoprecipitation, forced expression in CTL/NK cells, degranulation and cytotoxicity assays","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of membrane fusion combined with cell-based degranulation/cytotoxicity assays and Co-IP, multiple orthogonal methods in one study","pmids":["25564401"],"is_preprint":false},{"year":2012,"finding":"Munc18b (STXBP2) is required for platelet secretion from all three granule types (alpha, dense, lysosomal). In platelets, Munc18b forms complexes with syntaxin-11, SNAP-23, and VAMP-8; FHL5 patients with biallelic STXBP2 mutations show decreased Munc18b and markedly reduced syntaxin-11 levels, while other syntaxins are unaffected. Munc13-4 and Rab27 were also found associated with this complex.","method":"Co-immunoprecipitation in human platelets, granule secretion assays (serotonin, ADP/ATP, platelet factor 4, lysosomal release) in FHL5 patient platelets","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived loss-of-function with specific secretion readouts, Co-IP for complex components, single lab with multiple orthogonal methods","pmids":["22791290"],"is_preprint":false},{"year":2013,"finding":"In mast cells, siRNA-mediated silencing of Munc18-2 inhibits secretory granule translocation (but not CCL2 chemokine secretion), while silencing of syntaxin 3 inhibits membrane fusion; combined knockdown produces additive inhibitory effect. Munc18-2 and STX3 are both located on the granule surface and at cytoskeletal clusters. In resting cells, Munc18-2 (but not STX3) interacts with tubulin, an interaction sensitive to nocodazole and decreased after stimulation, demonstrating Munc18-2 dynamically couples fusion machinery to microtubules.","method":"siRNA knockdown, immunogold electron microscopy, co-immunoprecipitation with tubulin, nocodazole treatment, degranulation assay","journal":"Journal of immunology (Baltimore, Md. : 1950)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with specific phenotypic readout, Co-IP, immunogold EM, single lab with multiple orthogonal methods","pmids":["24323579"],"is_preprint":false},{"year":2005,"finding":"Slp4-a/granuphilin-a interacts with a closed conformation of syntaxin-2/3 in a Munc18-2-dependent manner; Munc18-2 itself does not directly interact with Slp4-a. The syntaxin-2/3 binding site on Slp4-a maps to a linker domain (residues 144-354). The Slp4-a·syntaxin-2 complex is present in rat parotid glands, and antibody against the Slp4-a linker domain inhibits isoproterenol-stimulated amylase release from permeabilized acinar cells.","method":"Co-immunoprecipitation in COS-7 cells, deletion analysis, endogenous complex detection in parotid gland, antibody inhibition in permeabilized acinar cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with deletion mapping, endogenous complex verification, functional antibody inhibition assay, single lab","pmids":["16186111"],"is_preprint":false},{"year":2003,"finding":"In mast cell lipid rafts, Munc18-2/syntaxin-3 complexes are spatially separated from syntaxin-3-containing SNARE complexes (syntaxin-3/SNAP-23/VAMP-8), with SNARE complexes enriched in rafts while Munc18-2/syntaxin-3 complexes are excluded. This spatial separation suggests Munc18-2 acts at a step different from SNARE complex formation and fusion.","method":"Lipid raft fractionation, co-immunoprecipitation","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — fractionation with co-IP showing spatial separation, single lab, finding replicated conceptually by later studies","pmids":["12935901"],"is_preprint":false},{"year":2007,"finding":"Munc18-2 knockdown in RBL-2H3 mast cells markedly inhibits degranulation without changing syntaxin expression levels or Ca2+ mobilization. Using chimeric fluorescent fusion proteins, Munc18-2 interaction with syntaxin-3 (but not syntaxin-4) was demonstrated in vivo both on the plasma membrane and on secretory granules, suggesting roles in both granule-granule and granule-plasma membrane fusion.","method":"siRNA knockdown, fluorescent chimera co-localization, degranulation assay, live cell imaging","journal":"Molecular immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — siRNA knockdown with degranulation readout and in vivo localization data, single lab, multiple methods","pmids":["17408745"],"is_preprint":false},{"year":2017,"finding":"In a reconstituted cell-cell fusion assay, lipid-anchored STX11 and its cognate SNARE proteins support lipid exchange (hemifusion) but not complete cytoplasmic content mixing. Addition of wild-type Munc18-2, but not of Munc18-2 mutants with impaired STX11 binding, drives transition from hemifusion to complete membrane fusion, establishing that Munc18-2 is a direct component of the core fusion machinery that promotes SNARE complex assembly to achieve full membrane merging.","method":"Reconstituted flipped cell-cell fusion assay, mutant Munc18-2 constructs, content-mixing vs. lipid-mixing readouts","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted fusion assay with mutagenesis controls distinguishing lipid vs. content mixing, rigorous mechanistic dissection in single study","pmids":["28265073"],"is_preprint":false},{"year":2017,"finding":"In intestinal enterocytes, Munc18-2 is required for Slp4a/Stx3 interaction and fusion of cargo vesicles with the apical plasma membrane. Loss of Munc18-2 (via CRISPR/Cas9 KO in CaCo2 cells) selectively disrupts trafficking of specific apical brush-border proteins (NHE3 and GLUT5) while leaving DPPIV transport unaffected, causing subapical accumulation of cargo vesicles.","method":"CRISPR/Cas9 knockout in CaCo2 cells, Co-immunoprecipitation (Slp4a/Stx3 interaction), fluorescence and electron microscopy of patient biopsies and organoids, cargo trafficking assays","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 1 / Moderate — CRISPR/Cas9 KO with Co-IP, patient biopsies, organoids, and rescue experiments with WT vs. patient variant, multiple orthogonal methods in one study","pmids":["28724787"],"is_preprint":false},{"year":2015,"finding":"Munc18-2 localizes predominantly to cytolytic granules in CTL with low levels at the plasma membrane where STX11 resides. In FHL5 CTL lacking Munc18-2, STX11 is lost from the plasma membrane, while Munc18-2 localization is unaffected by absence of STX11 in FHL4 CTL, demonstrating Munc18-2 is required to chaperone STX11 to the plasma membrane for final granule fusion.","method":"Immunofluorescence localization in patient-derived CTL (FHL4 and FHL5), subcellular fractionation","journal":"Traffic (Copenhagen, Denmark)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization in patient-derived loss-of-function cells with functional consequence (plasma membrane STX11 loss), reciprocal FHL4/FHL5 patient comparison","pmids":["26771955"],"is_preprint":false},{"year":2018,"finding":"Conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) in mast cells almost completely abolishes exocytosis as measured by plasma membrane capacitance recordings, and eliminates homotypic granule fusion (compound exocytosis) by stereological EM analysis. Munc18-2 cKO mice are significantly protected from anaphylaxis.","method":"Conditional knockout mice, whole-cell patch clamp capacitance recordings, stereological EM analysis, mediator secretion assays, in vivo anaphylaxis model","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — clean conditional KO with high-resolution electrophysiological readout, EM morphometry, and in vivo phenotype, multiple orthogonal methods","pmids":["29599294"],"is_preprint":false},{"year":2019,"finding":"Conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) in platelets ablates release from alpha, dense, and lysosomal granules. Munc18-2-deficient platelets show defective aggregation at low collagen doses, impaired thrombus formation under shear stress, prolonged arterial and venous bleeding times in vivo, and protection against arterial thrombosis.","method":"Conditional knockout mice, granule secretion assays, platelet aggregometry, in vitro thrombus formation under shear, in vivo bleeding time and arterial thrombosis models, design-based stereological EM","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — clean conditional KO with multiple secretion readouts, in vitro and in vivo functional assays, EM morphometry","pmids":["30696774"],"is_preprint":false},{"year":2013,"finding":"FHL-5 neutrophils carrying STXBP2/Munc18-2 mutations show a profound defect in granule mobilization, resulting in inadequate bacterial killing of gram-negative E. coli but not S. aureus (which depends on NADPH oxidase activity instead), demonstrating a role for STXBP2 in neutrophil granule exocytosis.","method":"Patient-derived neutrophil granule mobilization assays, bacterial killing assays with E. coli and S. aureus","journal":"Blood","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived loss-of-function with specific functional readout and mechanistic dissection (bacterial killing specificity), single lab","pmids":["23687090"],"is_preprint":false},{"year":2011,"finding":"In pancreatic beta-cells, both Munc18-1 and Munc18-2 augment glucose-stimulated insulin secretion, but they have distinct subcellular localizations; only Munc18-1 redistributes upon glucose stimulation. Munc18-2 overexpression shifts Ca2+ sensitivity of insulin exocytosis, mediating release of fusion-competent granules at lower cytoplasmic Ca2+ concentrations. The Ca2+ sensitivity of exocytosis depends on the phosphorylation status of Munc18 proteins.","method":"Overexpression in beta-cells, whole-cell patch clamp with caged Ca2+ photorelease, subcellular localization by immunofluorescence, glucose-stimulated secretion assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — electrophysiological Ca2+ sensitivity assay combined with localization and secretion data, single lab, multiple orthogonal methods","pmids":["21690086"],"is_preprint":false},{"year":2018,"finding":"Loss of Munc18-2/Stxbp2 in mouse intestinal organoids recapitulates MVID pathological features (apical vesicle accumulation, tubulovesicular network, microvillus inclusions). The phenotype is fully rescued by transgenic wild-type human MUNC18-2 but not by the patient variant P477L. Time-lapse microscopy revealed microvillus inclusions form dynamically by intracellular maturation or invagination of apical or basolateral plasma membrane.","method":"Munc18-2/Stxbp2-null mouse intestinal organoids, lentiviral rescue with WT vs. patient mutant, confocal and transmission electron microscopy, spinning disc time-lapse microscopy","journal":"Cellular and molecular gastroenterology and hepatology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — null organoid model with WT rescue and mutant non-rescue, live imaging of disease mechanism, single lab","pmids":["30364784"],"is_preprint":false},{"year":2014,"finding":"Both the N-terminus and Habc domain of syntaxin-11 are required for binding to Munc18-2: STX11 mutations R4A and L58P (located in N-terminal and Habc domain respectively) abolish Munc18-2 binding in an ectopic expression system, even though L58P is expressed at levels comparable to wild-type. In patient cells, the L58P mutation decreases syntaxin-11 expression, consistent with Munc18-2 stabilizing STX11.","method":"Co-immunoprecipitation in ectopic expression system, patient lymphocyte protein expression analysis","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — Co-IP in ectopic system with two different mutants defining binding requirements, corroborated by patient cell data","pmids":["24459464"],"is_preprint":false},{"year":2015,"finding":"Erythrocytes express Munc18-2, and FHL-5 patient red blood cells expose less phosphatidylserine on their surface upon Ca2+ ionophore treatment (ionomycin), indicating STXBP2 is required for phospholipid scrambling in erythrocytes. Patient-derived erythroblasts also display defective erythropoiesis with decreased CD235a expression and aberrant cell morphology.","method":"Patient-derived erythrocyte phosphatidylserine exposure assay (annexin V), cultured erythroblast differentiation assay with CD235a flow cytometry","journal":"Experimental hematology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — patient-derived cells, single lab, single assay per phenotype, limited mechanistic depth","pmids":["26320718"],"is_preprint":false},{"year":2020,"finding":"The STXBP2-R190C mutation does not alter STXBP2 expression, subcellular localization, or the STXBP2/STX11 interaction, but forced expression of this mutant into normal CTLs strongly inhibits degranulation and cytolytic activity in a dominant-negative manner, suggesting R190C stabilizes non-productive STXBP2/STX11 complexes or impairs downstream interactions with other factors.","method":"Co-immunoprecipitation, immunofluorescence localization, forced expression in primary CTLs, degranulation and cytotoxicity assays","journal":"Frontiers in immunology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, localization, functional assays), single lab","pmids":["33162974"],"is_preprint":false},{"year":2018,"finding":"STXBP2 deficiency leads to a concomitant reduction in STXBP1 and its partner STX1 expression. Functional analysis demonstrates that the STXBP1/STX1 axis contributes to as much as 50% of NK and CD8+ T-cell cytotoxic activity, revealing an interplay between STXBP2 and STXBP1 pathways in regulating granule exocytosis.","method":"Patient-derived cells with hypomorphic STXBP2 mutations, Western blot for STXBP1/STX1, NK and T-cell cytotoxicity/degranulation assays","journal":"Frontiers in immunology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single patient, indirect inference about STXBP1/STX1 pathway from co-reduction in STXBP2-deficient cells, limited mechanistic dissection of STXBP2 itself","pmids":["29599780"],"is_preprint":false}],"current_model":"STXBP2 (Munc18-2) is a SEC/Munc18-family protein that functions as both a chaperone and an active component of the core SNARE-mediated membrane fusion machinery: it stabilizes syntaxin 11 (STX11) and chaperones it to the plasma membrane, binds the N-terminal peptide of STX11 with high affinity (and syntaxin 2/3 with lower affinity), and directly promotes SNARE-complex assembly to drive transition from hemifusion to complete membrane merger during lytic granule exocytosis in cytotoxic T lymphocytes and NK cells; it additionally couples secretory granule translocation to the microtubule cytoskeleton via tubulin interaction in mast cells, is required for exocytosis in platelets (regulating all three granule types via a complex with STX11, SNAP-23, and VAMP-8), and mediates apical vesicle fusion in intestinal epithelial cells through interaction with Slp4a/STX3, with loss of function causing cargo-selective mislocalization of apical brush-border proteins."},"narrative":{"mechanistic_narrative":"STXBP2 (Munc18-2) is a SEC1/Munc18-family protein that serves as both a syntaxin chaperone and a direct, active component of the core SNARE-mediated membrane fusion machinery driving regulated exocytosis across multiple secretory cell types [PMID:19804848, PMID:25564401, PMID:28265073]. In cytotoxic lymphocytes it binds the N-terminal peptide of syntaxin 11 (STX11) with ~20-fold higher affinity than syntaxin 3, stabilizes STX11, and chaperones it to the plasma membrane for lytic granule fusion; loss of STXBP2 destabilizes STX11 and abolishes NK and CTL degranulation, the basis of familial hemophagocytic lymphohistiocytosis type 5 (FHL-5) [PMID:19804848, PMID:19884660, PMID:24194549, PMID:26771955, PMID:24459464]. Beyond chaperoning, STXBP2 catalyzes the late fusion step itself: in reconstituted assays, STX11-cognate SNAREs alone support only hemifusion, while wild-type STXBP2 — but not STX11-binding-impaired mutants — drives the transition from hemifusion to complete content mixing by promoting SNARE-complex assembly, and dominant-negative disease mutations that retain STX11 binding arrest assembly at this late step [PMID:25564401, PMID:28265073]. This fusion-promoting role extends to mast cell, platelet, and neutrophil granule exocytosis, where STXBP2 acts within complexes containing STX11, SNAP-23, and VAMP-8 and is required for compound (granule-granule) fusion and secretion from all granule classes [PMID:22791290, PMID:29599294, PMID:30696774, PMID:23687090]. In epithelial cells STXBP2 partners with syntaxin 2/3 and, via Slp4a, mediates apical vesicle fusion; its loss causes cargo-selective mislocalization of brush-border proteins and recapitulates microvillus inclusion disease [PMID:10788461, PMID:16186111, PMID:28724787, PMID:30364784]. In mast cells STXBP2 additionally couples secretory granule translocation to the microtubule cytoskeleton through a nocodazole-sensitive tubulin interaction [PMID:12482918, PMID:24323579].","teleology":[{"year":2000,"claim":"Established that Munc18-2 engages an apical-membrane t-SNARE and functionally gates polarized membrane traffic, defining its core role as a syntaxin-binding regulator of fusion.","evidence":"In vitro and in vivo binding assays plus hemagglutinin apical transport assay in Caco-2 cells with mutagenesis","pmids":["10788461"],"confidence":"High","gaps":["Did not resolve whether Munc18-2 promotes or inhibits fusion at the molecular step","Restricted to syntaxin 3; STX11 partnership not yet known"]},{"year":2003,"claim":"Showed Munc18-2 operates in mast cell granule exocytosis and physically couples the fusion machinery to microtubules, linking granule transport to fusion control.","evidence":"Subcellular localization, dominant-negative peptide inhibition, nocodazole disruption, and tubulin co-IP in mast cells","pmids":["12482918","12935901"],"confidence":"Medium","gaps":["Spatial separation of Munc18-2/STX3 from SNARE complexes left its exact mechanistic step ambiguous","Single-lab localization data"]},{"year":2005,"claim":"Defined an indirect link between Munc18-2 and the granule-tethering protein Slp4a, where Munc18-2 stabilizes the closed syntaxin conformation that Slp4a recognizes.","evidence":"Co-IP with deletion mapping in COS-7 cells, endogenous complex detection, and antibody inhibition in permeabilized acinar cells","pmids":["16186111"],"confidence":"Medium","gaps":["Munc18-2 does not bind Slp4a directly; coupling is via syntaxin conformation","Tested in exocrine/heterologous systems only"]},{"year":2009,"claim":"Identified STX11 as the principal lymphocyte partner of STXBP2 and showed FHL-5 mutations abolish this interaction, destabilize both proteins, and ablate cytotoxic degranulation — establishing the disease mechanism.","evidence":"Co-IP, patient lymphocyte stability analysis, CD107 degranulation assay, and ectopic WT rescue in two concurrent studies","pmids":["19804848","19884660"],"confidence":"High","gaps":["Did not distinguish chaperone stabilization from a direct fusion role","Step in the secretory pathway not pinpointed"]},{"year":2013,"claim":"Provided the structural framework: a crystal structure mapped disease mutations to syntaxin/SNARE binding surfaces and quantified the high-affinity STX11 N-peptide interaction relative to STX3.","evidence":"X-ray crystallography at 2.6 Å, surface plasmon resonance, mutation mapping, and Western blot of IL-2-activated CTL","pmids":["24194549"],"confidence":"High","gaps":["Static structure did not capture the fusion-promoting conformational cycle","Functional consequence of each mapped mutation not all tested"]},{"year":2013,"claim":"Extended STXBP2 function to platelet and neutrophil granule exocytosis, defining a STX11/SNAP-23/VAMP-8 effector complex and demonstrating granule-mobilization defects underlie patient immunodeficiency.","evidence":"Co-IP and granule secretion assays in patient platelets; neutrophil granule mobilization and bacterial killing assays","pmids":["22791290","23687090"],"confidence":"Medium","gaps":["Patient-derived correlative data, not clean genetic ablation","Stoichiometry and assembly order of the complex unresolved"]},{"year":2014,"claim":"Defined the bipartite STX11 binding requirement (N-terminus plus Habc domain) and reinforced the chaperone-stabilization model.","evidence":"Co-IP of STX11 point mutants in an ectopic system with patient lymphocyte expression analysis","pmids":["24459464"],"confidence":"Medium","gaps":["Ectopic overexpression context","Did not test fusion consequence of altered binding mode"]},{"year":2015,"claim":"Demonstrated directly that STXBP2 is an active fusion catalyst, not merely a chaperone: in vitro fusion assays showed dominant-negative R65 mutants retain STX11 binding yet arrest SNARE assembly at a late step.","evidence":"In vitro SNARE-mediated membrane fusion reconstitution, Co-IP, and forced expression with degranulation/cytotoxicity assays","pmids":["25564401","26771955"],"confidence":"High","gaps":["Precise conformational transition driving SNARE zippering not visualized","How dominant-negative mutants poison wild-type complexes mechanistically unclear"]},{"year":2017,"claim":"Resolved the exact fusion step: STXBP2 drives the transition from hemifusion to complete content mixing, establishing it as a core component of the fusion machinery; in epithelia it enables Slp4a/STX3-dependent cargo-selective apical fusion.","evidence":"Reconstituted flipped cell-cell fusion assay with lipid- vs content-mixing readouts and binding-impaired mutants; CRISPR KO in CaCo2 cells with cargo trafficking assays and patient material","pmids":["28265073","28724787"],"confidence":"High","gaps":["Why specific cargoes (NHE3, GLUT5) but not DPPIV depend on STXBP2 is unexplained","Coupling between fusion catalysis and cargo selectivity unresolved"]},{"year":2018,"claim":"Genetic ablation studies established STXBP2 as non-redundant with Munc18-1/-3 for compound granule exocytosis and recapitulated microvillus inclusion disease, with rescue confirming gene-specific causality.","evidence":"Conditional KO mice with patch-clamp capacitance, stereological EM, in vivo anaphylaxis; Stxbp2-null intestinal organoids with WT vs patient-variant rescue and live imaging","pmids":["29599294","30364784"],"confidence":"Medium","gaps":["Molecular basis of homotypic (granule-granule) fusion requirement not dissected","Link between fusion defect and microvillus inclusion morphogenesis mechanistic but incomplete"]},{"year":2019,"claim":"Confirmed in vivo that STXBP2 is essential for platelet granule release across all three granule types and for hemostasis and thrombosis.","evidence":"Platelet-specific conditional KO mice with secretion assays, aggregometry, shear thrombus formation, bleeding-time and thrombosis models, and EM","pmids":["30696774"],"confidence":"High","gaps":["Did not address how STXBP2 coordinates the distinct granule classes mechanistically"]},{"year":2020,"claim":"Refined understanding of dominant-negative pathology by showing the R190C mutation impairs degranulation without altering expression, localization, or STX11 binding, pointing to a downstream functional defect.","evidence":"Co-IP, immunofluorescence, and forced expression in primary CTLs with degranulation/cytotoxicity assays","pmids":["33162974"],"confidence":"Medium","gaps":["The downstream interaction or non-productive complex disrupted by R190C is not identified","Single-lab functional inference"]},{"year":null,"claim":"It remains unresolved how STXBP2 mechanistically achieves cargo and granule-class selectivity and how its phosphorylation tunes Ca2+ sensitivity of fusion across cell types.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of STXBP2 engaged with an assembled SNARE complex during fusion","Regulatory inputs (phosphorylation, Rab27/Munc13-4) controlling STXBP2 activity not mechanistically defined","Erythrocyte and STXBP1-axis roles rest on low-confidence single-patient data"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[5,11]},{"term_id":"GO:0044183","term_label":"protein folding chaperone","supporting_discovery_ids":[0,1,13]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[3,7]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[2,5,11]}],"localization":[{"term_id":"GO:0031410","term_label":"cytoplasmic vesicle","supporting_discovery_ids":[3,10,13]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,10,13]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[3,7]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,11,12]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,1,6,16]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[6,15]}],"complexes":["STXBP2-STX11-SNAP-23-VAMP-8 SNARE complex","Munc18-2/syntaxin-3 complex"],"partners":["STX11","STX3","STX2","SNAP-23","VAMP-8","SLP4A","TUBULIN","MUNC13-4"],"other_free_text":[]}},"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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\"method\": \"Co-immunoprecipitation, patient lymphocyte protein stability analysis, CD107 degranulation assay\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal interaction identified by Co-IP, protein stability confirmed in patient cells, functional degranulation assay, independently replicated by second concurrent paper (PMID:19884660)\",\n      \"pmids\": [\"19804848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"STXBP2 (Munc18-2) is required at a late step of the secretory pathway for cytotoxic granule exocytosis in NK cells; syntaxin-11 is the main binding partner of STXBP2 in lymphocytes, and its expression requires the presence of STXBP2. Ectopic expression of wild-type STXBP2 rescued the impaired granule exocytosis in patient-derived NK cells.\",\n      \"method\": \"Loss-of-function (patient lymphoblasts with decreased STXBP2), ectopic expression rescue, Co-immunoprecipitation\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — patient-derived loss-of-function, rescue by ectopic WT protein, Co-IP for binding partner, independently replicated (PMID:19804848)\",\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 localized to the apical plasma membrane in epithelial cells. Munc18-2 point mutants with reduced syntaxin-3 binding also displaced SNAP-23 from syntaxin-3 complexes when overexpressed in Caco-2 cells. Overexpression of wild-type Munc18-2 inhibited apical delivery of influenza virus hemagglutinin, demonstrating a functional role in apical membrane trafficking.\",\n      \"method\": \"In vitro binding assay, in vivo co-immunoprecipitation in Caco-2 cells, hemagglutinin apical transport assay, site-directed mutagenesis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis panel combined with in vitro and in vivo binding assays and functional transport assay in a single study\",\n      \"pmids\": [\"10788461\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In mast cells, Munc18-2 localizes to secretory granules (while Munc18-3 localizes to the plasma membrane) and interacts with syntaxin 2 or 3. Overexpression of Munc18-2 or interfering effector-loop peptides inhibited IgE-triggered exocytosis. Upon stimulation, Munc18-2 redistributes into lamellipodia and associates with microtubule-aligned granules; disruption of microtubules with nocodazole redistributes Munc18-2 and impairs mediator release, indicating Munc18-2 couples secretory granule dynamics to the microtubule network.\",\n      \"method\": \"Subcellular fractionation/immunolocalization, overexpression and dominant-negative peptide inhibition, nocodazole microtubule disruption, mediator release assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization with functional consequence, multiple orthogonal methods (localization, OE inhibition, cytoskeletal disruption), single lab\",\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. Eighteen disease-causing point mutations were mapped; four surface mutations (R39P, L130S, E132A, P334L) map exclusively 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 can compensate for absent STX11, and Munc18-1 (expressed in IL-2-activated CTL) is also capable of binding STX11, explaining partial functional rescue.\",\n      \"method\": \"X-ray crystallography (2.6 Å), surface plasmon resonance binding assay, patient mutation mapping, Western blot of IL-2-activated CTL\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with quantitative binding measurements and mutation mapping in a single rigorous study\",\n      \"pmids\": [\"24194549\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"STXBP2 mutations R65Q and R65W retain the ability to interact with and stabilize syntaxin 11, but act in a dominant-negative fashion to inhibit NK cell degranulation and cytotoxic activity. Mechanistic in vitro membrane fusion assays show these mutations arrest SNARE-complex assembly at a late step, directly implicating STXBP2 in promoting SNARE-complex assembly during lytic granule fusion.\",\n      \"method\": \"In vitro SNARE-mediated membrane fusion assay, Co-immunoprecipitation, forced expression in CTL/NK cells, degranulation and cytotoxicity assays\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of membrane fusion combined with cell-based degranulation/cytotoxicity assays and Co-IP, multiple orthogonal methods in one study\",\n      \"pmids\": [\"25564401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Munc18b (STXBP2) is required for platelet secretion from all three granule types (alpha, dense, lysosomal). In platelets, Munc18b forms complexes with syntaxin-11, SNAP-23, and VAMP-8; FHL5 patients with biallelic STXBP2 mutations show decreased Munc18b and markedly reduced syntaxin-11 levels, while other syntaxins are unaffected. Munc13-4 and Rab27 were also found associated with this complex.\",\n      \"method\": \"Co-immunoprecipitation in human platelets, granule secretion assays (serotonin, ADP/ATP, platelet factor 4, lysosomal release) in FHL5 patient platelets\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived loss-of-function with specific secretion readouts, Co-IP for complex components, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"22791290\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In mast cells, siRNA-mediated silencing of Munc18-2 inhibits secretory granule translocation (but not CCL2 chemokine secretion), while silencing of syntaxin 3 inhibits membrane fusion; combined knockdown produces additive inhibitory effect. Munc18-2 and STX3 are both located on the granule surface and at cytoskeletal clusters. In resting cells, Munc18-2 (but not STX3) interacts with tubulin, an interaction sensitive to nocodazole and decreased after stimulation, demonstrating Munc18-2 dynamically couples fusion machinery to microtubules.\",\n      \"method\": \"siRNA knockdown, immunogold electron microscopy, co-immunoprecipitation with tubulin, nocodazole treatment, degranulation assay\",\n      \"journal\": \"Journal of immunology (Baltimore, Md. : 1950)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with specific phenotypic readout, Co-IP, immunogold EM, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"24323579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Slp4-a/granuphilin-a interacts with a closed conformation of syntaxin-2/3 in a Munc18-2-dependent manner; Munc18-2 itself does not directly interact with Slp4-a. The syntaxin-2/3 binding site on Slp4-a maps to a linker domain (residues 144-354). The Slp4-a·syntaxin-2 complex is present in rat parotid glands, and antibody against the Slp4-a linker domain inhibits isoproterenol-stimulated amylase release from permeabilized acinar cells.\",\n      \"method\": \"Co-immunoprecipitation in COS-7 cells, deletion analysis, endogenous complex detection in parotid gland, antibody inhibition in permeabilized acinar cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with deletion mapping, endogenous complex verification, functional antibody inhibition assay, single lab\",\n      \"pmids\": [\"16186111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"In mast cell lipid rafts, Munc18-2/syntaxin-3 complexes are spatially separated from syntaxin-3-containing SNARE complexes (syntaxin-3/SNAP-23/VAMP-8), with SNARE complexes enriched in rafts while Munc18-2/syntaxin-3 complexes are excluded. This spatial separation suggests Munc18-2 acts at a step different from SNARE complex formation and fusion.\",\n      \"method\": \"Lipid raft fractionation, co-immunoprecipitation\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — fractionation with co-IP showing spatial separation, single lab, finding replicated conceptually by later studies\",\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 levels or Ca2+ mobilization. Using chimeric fluorescent fusion proteins, Munc18-2 interaction with syntaxin-3 (but not syntaxin-4) was demonstrated in vivo both on the plasma membrane and on secretory granules, suggesting roles in both granule-granule and granule-plasma membrane fusion.\",\n      \"method\": \"siRNA knockdown, fluorescent chimera co-localization, degranulation assay, live cell imaging\",\n      \"journal\": \"Molecular immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — siRNA knockdown with degranulation readout and in vivo localization data, single lab, multiple methods\",\n      \"pmids\": [\"17408745\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In a reconstituted cell-cell fusion assay, lipid-anchored STX11 and its cognate SNARE proteins support lipid exchange (hemifusion) but not complete cytoplasmic content mixing. Addition of wild-type Munc18-2, but not of Munc18-2 mutants with impaired STX11 binding, drives transition from hemifusion to complete membrane fusion, establishing that Munc18-2 is a direct component of the core fusion machinery that promotes SNARE complex assembly to achieve full membrane merging.\",\n      \"method\": \"Reconstituted flipped cell-cell fusion assay, mutant Munc18-2 constructs, content-mixing vs. lipid-mixing readouts\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted fusion assay with mutagenesis controls distinguishing lipid vs. content mixing, rigorous mechanistic dissection in single study\",\n      \"pmids\": [\"28265073\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"In intestinal enterocytes, Munc18-2 is required for Slp4a/Stx3 interaction and fusion of cargo vesicles with the apical plasma membrane. Loss of Munc18-2 (via CRISPR/Cas9 KO in CaCo2 cells) selectively disrupts trafficking of specific apical brush-border proteins (NHE3 and GLUT5) while leaving DPPIV transport unaffected, causing subapical accumulation of cargo vesicles.\",\n      \"method\": \"CRISPR/Cas9 knockout in CaCo2 cells, Co-immunoprecipitation (Slp4a/Stx3 interaction), fluorescence and electron microscopy of patient biopsies and organoids, cargo trafficking assays\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — CRISPR/Cas9 KO with Co-IP, patient biopsies, organoids, and rescue experiments with WT vs. patient variant, multiple orthogonal methods in one study\",\n      \"pmids\": [\"28724787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Munc18-2 localizes predominantly to cytolytic granules in CTL with low levels at the plasma membrane where STX11 resides. In FHL5 CTL lacking Munc18-2, STX11 is lost from the plasma membrane, while Munc18-2 localization is unaffected by absence of STX11 in FHL4 CTL, demonstrating Munc18-2 is required to chaperone STX11 to the plasma membrane for final granule fusion.\",\n      \"method\": \"Immunofluorescence localization in patient-derived CTL (FHL4 and FHL5), subcellular fractionation\",\n      \"journal\": \"Traffic (Copenhagen, Denmark)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization in patient-derived loss-of-function cells with functional consequence (plasma membrane STX11 loss), reciprocal FHL4/FHL5 patient comparison\",\n      \"pmids\": [\"26771955\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) in mast cells almost completely abolishes exocytosis as measured by plasma membrane capacitance recordings, and eliminates homotypic granule fusion (compound exocytosis) by stereological EM analysis. Munc18-2 cKO mice are significantly protected from anaphylaxis.\",\n      \"method\": \"Conditional knockout mice, whole-cell patch clamp capacitance recordings, stereological EM analysis, mediator secretion assays, in vivo anaphylaxis model\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — clean conditional KO with high-resolution electrophysiological readout, EM morphometry, and in vivo phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"29599294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Conditional knockout of Munc18-2 (but not Munc18-1 or Munc18-3) in platelets ablates release from alpha, dense, and lysosomal granules. Munc18-2-deficient platelets show defective aggregation at low collagen doses, impaired thrombus formation under shear stress, prolonged arterial and venous bleeding times in vivo, and protection against arterial thrombosis.\",\n      \"method\": \"Conditional knockout mice, granule secretion assays, platelet aggregometry, in vitro thrombus formation under shear, in vivo bleeding time and arterial thrombosis models, design-based stereological EM\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — clean conditional KO with multiple secretion readouts, in vitro and in vivo functional assays, EM morphometry\",\n      \"pmids\": [\"30696774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"FHL-5 neutrophils carrying STXBP2/Munc18-2 mutations show a profound defect in granule mobilization, resulting in inadequate bacterial killing of gram-negative E. coli but not S. aureus (which depends on NADPH oxidase activity instead), demonstrating a role for STXBP2 in neutrophil granule exocytosis.\",\n      \"method\": \"Patient-derived neutrophil granule mobilization assays, bacterial killing assays with E. coli and S. aureus\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived loss-of-function with specific functional readout and mechanistic dissection (bacterial killing specificity), single lab\",\n      \"pmids\": [\"23687090\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In pancreatic beta-cells, both Munc18-1 and Munc18-2 augment glucose-stimulated insulin secretion, but they have distinct subcellular localizations; only Munc18-1 redistributes upon glucose stimulation. Munc18-2 overexpression shifts Ca2+ sensitivity of insulin exocytosis, mediating release of fusion-competent granules at lower cytoplasmic Ca2+ concentrations. The Ca2+ sensitivity of exocytosis depends on the phosphorylation status of Munc18 proteins.\",\n      \"method\": \"Overexpression in beta-cells, whole-cell patch clamp with caged Ca2+ photorelease, subcellular localization by immunofluorescence, glucose-stimulated secretion assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — electrophysiological Ca2+ sensitivity assay combined with localization and secretion data, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"21690086\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Loss of Munc18-2/Stxbp2 in mouse intestinal organoids recapitulates MVID pathological features (apical vesicle accumulation, tubulovesicular network, microvillus inclusions). The phenotype is fully rescued by transgenic wild-type human MUNC18-2 but not by the patient variant P477L. Time-lapse microscopy revealed microvillus inclusions form dynamically by intracellular maturation or invagination of apical or basolateral plasma membrane.\",\n      \"method\": \"Munc18-2/Stxbp2-null mouse intestinal organoids, lentiviral rescue with WT vs. patient mutant, confocal and transmission electron microscopy, spinning disc time-lapse microscopy\",\n      \"journal\": \"Cellular and molecular gastroenterology and hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — null organoid model with WT rescue and mutant non-rescue, live imaging of disease mechanism, single lab\",\n      \"pmids\": [\"30364784\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Both the N-terminus and Habc domain of syntaxin-11 are required for binding to Munc18-2: STX11 mutations R4A and L58P (located in N-terminal and Habc domain respectively) abolish Munc18-2 binding in an ectopic expression system, even though L58P is expressed at levels comparable to wild-type. In patient cells, the L58P mutation decreases syntaxin-11 expression, consistent with Munc18-2 stabilizing STX11.\",\n      \"method\": \"Co-immunoprecipitation in ectopic expression system, patient lymphocyte protein expression analysis\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — Co-IP in ectopic system with two different mutants defining binding requirements, corroborated by patient cell data\",\n      \"pmids\": [\"24459464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Erythrocytes express Munc18-2, and FHL-5 patient red blood cells expose less phosphatidylserine on their surface upon Ca2+ ionophore treatment (ionomycin), indicating STXBP2 is required for phospholipid scrambling in erythrocytes. Patient-derived erythroblasts also display defective erythropoiesis with decreased CD235a expression and aberrant cell morphology.\",\n      \"method\": \"Patient-derived erythrocyte phosphatidylserine exposure assay (annexin V), cultured erythroblast differentiation assay with CD235a flow cytometry\",\n      \"journal\": \"Experimental hematology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — patient-derived cells, single lab, single assay per phenotype, limited mechanistic depth\",\n      \"pmids\": [\"26320718\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The STXBP2-R190C mutation does not alter STXBP2 expression, subcellular localization, or the STXBP2/STX11 interaction, but forced expression of this mutant into normal CTLs strongly inhibits degranulation and cytolytic activity in a dominant-negative manner, suggesting R190C stabilizes non-productive STXBP2/STX11 complexes or impairs downstream interactions with other factors.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence localization, forced expression in primary CTLs, degranulation and cytotoxicity assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, localization, functional assays), single lab\",\n      \"pmids\": [\"33162974\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"STXBP2 deficiency leads to a concomitant reduction in STXBP1 and its partner STX1 expression. Functional analysis demonstrates that the STXBP1/STX1 axis contributes to as much as 50% of NK and CD8+ T-cell cytotoxic activity, revealing an interplay between STXBP2 and STXBP1 pathways in regulating granule exocytosis.\",\n      \"method\": \"Patient-derived cells with hypomorphic STXBP2 mutations, Western blot for STXBP1/STX1, NK and T-cell cytotoxicity/degranulation assays\",\n      \"journal\": \"Frontiers in immunology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single patient, indirect inference about STXBP1/STX1 pathway from co-reduction in STXBP2-deficient cells, limited mechanistic dissection of STXBP2 itself\",\n      \"pmids\": [\"29599780\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STXBP2 (Munc18-2) is a SEC/Munc18-family protein that functions as both a chaperone and an active component of the core SNARE-mediated membrane fusion machinery: it stabilizes syntaxin 11 (STX11) and chaperones it to the plasma membrane, binds the N-terminal peptide of STX11 with high affinity (and syntaxin 2/3 with lower affinity), and directly promotes SNARE-complex assembly to drive transition from hemifusion to complete membrane merger during lytic granule exocytosis in cytotoxic T lymphocytes and NK cells; it additionally couples secretory granule translocation to the microtubule cytoskeleton via tubulin interaction in mast cells, is required for exocytosis in platelets (regulating all three granule types via a complex with STX11, SNAP-23, and VAMP-8), and mediates apical vesicle fusion in intestinal epithelial cells through interaction with Slp4a/STX3, with loss of function causing cargo-selective mislocalization of apical brush-border proteins.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"STXBP2 (Munc18-2) is a SEC1/Munc18-family protein that serves as both a syntaxin chaperone and a direct, active component of the core SNARE-mediated membrane fusion machinery driving regulated exocytosis across multiple secretory cell types [#0, #5, #11]. In cytotoxic lymphocytes it binds the N-terminal peptide of syntaxin 11 (STX11) with ~20-fold higher affinity than syntaxin 3, stabilizes STX11, and chaperones it to the plasma membrane for lytic granule fusion; loss of STXBP2 destabilizes STX11 and abolishes NK and CTL degranulation, the basis of familial hemophagocytic lymphohistiocytosis type 5 (FHL-5) [#0, #1, #4, #13, #19]. Beyond chaperoning, STXBP2 catalyzes the late fusion step itself: in reconstituted assays, STX11-cognate SNAREs alone support only hemifusion, while wild-type STXBP2 — but not STX11-binding-impaired mutants — drives the transition from hemifusion to complete content mixing by promoting SNARE-complex assembly, and dominant-negative disease mutations that retain STX11 binding arrest assembly at this late step [#5, #11]. This fusion-promoting role extends to mast cell, platelet, and neutrophil granule exocytosis, where STXBP2 acts within complexes containing STX11, SNAP-23, and VAMP-8 and is required for compound (granule-granule) fusion and secretion from all granule classes [#6, #14, #15, #16]. In epithelial cells STXBP2 partners with syntaxin 2/3 and, via Slp4a, mediates apical vesicle fusion; its loss causes cargo-selective mislocalization of brush-border proteins and recapitulates microvillus inclusion disease [#2, #8, #12, #18]. In mast cells STXBP2 additionally couples secretory granule translocation to the microtubule cytoskeleton through a nocodazole-sensitive tubulin interaction [#3, #7].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Established that Munc18-2 engages an apical-membrane t-SNARE and functionally gates polarized membrane traffic, defining its core role as a syntaxin-binding regulator of fusion.\",\n      \"evidence\": \"In vitro and in vivo binding assays plus hemagglutinin apical transport assay in Caco-2 cells with mutagenesis\",\n      \"pmids\": [\"10788461\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve whether Munc18-2 promotes or inhibits fusion at the molecular step\", \"Restricted to syntaxin 3; STX11 partnership not yet known\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Showed Munc18-2 operates in mast cell granule exocytosis and physically couples the fusion machinery to microtubules, linking granule transport to fusion control.\",\n      \"evidence\": \"Subcellular localization, dominant-negative peptide inhibition, nocodazole disruption, and tubulin co-IP in mast cells\",\n      \"pmids\": [\"12482918\", \"12935901\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Spatial separation of Munc18-2/STX3 from SNARE complexes left its exact mechanistic step ambiguous\", \"Single-lab localization data\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defined an indirect link between Munc18-2 and the granule-tethering protein Slp4a, where Munc18-2 stabilizes the closed syntaxin conformation that Slp4a recognizes.\",\n      \"evidence\": \"Co-IP with deletion mapping in COS-7 cells, endogenous complex detection, and antibody inhibition in permeabilized acinar cells\",\n      \"pmids\": [\"16186111\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Munc18-2 does not bind Slp4a directly; coupling is via syntaxin conformation\", \"Tested in exocrine/heterologous systems only\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identified STX11 as the principal lymphocyte partner of STXBP2 and showed FHL-5 mutations abolish this interaction, destabilize both proteins, and ablate cytotoxic degranulation — establishing the disease mechanism.\",\n      \"evidence\": \"Co-IP, patient lymphocyte stability analysis, CD107 degranulation assay, and ectopic WT rescue in two concurrent studies\",\n      \"pmids\": [\"19804848\", \"19884660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not distinguish chaperone stabilization from a direct fusion role\", \"Step in the secretory pathway not pinpointed\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Provided the structural framework: a crystal structure mapped disease mutations to syntaxin/SNARE binding surfaces and quantified the high-affinity STX11 N-peptide interaction relative to STX3.\",\n      \"evidence\": \"X-ray crystallography at 2.6 Å, surface plasmon resonance, mutation mapping, and Western blot of IL-2-activated CTL\",\n      \"pmids\": [\"24194549\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Static structure did not capture the fusion-promoting conformational cycle\", \"Functional consequence of each mapped mutation not all tested\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Extended STXBP2 function to platelet and neutrophil granule exocytosis, defining a STX11/SNAP-23/VAMP-8 effector complex and demonstrating granule-mobilization defects underlie patient immunodeficiency.\",\n      \"evidence\": \"Co-IP and granule secretion assays in patient platelets; neutrophil granule mobilization and bacterial killing assays\",\n      \"pmids\": [\"22791290\", \"23687090\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Patient-derived correlative data, not clean genetic ablation\", \"Stoichiometry and assembly order of the complex unresolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined the bipartite STX11 binding requirement (N-terminus plus Habc domain) and reinforced the chaperone-stabilization model.\",\n      \"evidence\": \"Co-IP of STX11 point mutants in an ectopic system with patient lymphocyte expression analysis\",\n      \"pmids\": [\"24459464\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Ectopic overexpression context\", \"Did not test fusion consequence of altered binding mode\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrated directly that STXBP2 is an active fusion catalyst, not merely a chaperone: in vitro fusion assays showed dominant-negative R65 mutants retain STX11 binding yet arrest SNARE assembly at a late step.\",\n      \"evidence\": \"In vitro SNARE-mediated membrane fusion reconstitution, Co-IP, and forced expression with degranulation/cytotoxicity assays\",\n      \"pmids\": [\"25564401\", \"26771955\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise conformational transition driving SNARE zippering not visualized\", \"How dominant-negative mutants poison wild-type complexes mechanistically unclear\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Resolved the exact fusion step: STXBP2 drives the transition from hemifusion to complete content mixing, establishing it as a core component of the fusion machinery; in epithelia it enables Slp4a/STX3-dependent cargo-selective apical fusion.\",\n      \"evidence\": \"Reconstituted flipped cell-cell fusion assay with lipid- vs content-mixing readouts and binding-impaired mutants; CRISPR KO in CaCo2 cells with cargo trafficking assays and patient material\",\n      \"pmids\": [\"28265073\", \"28724787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why specific cargoes (NHE3, GLUT5) but not DPPIV depend on STXBP2 is unexplained\", \"Coupling between fusion catalysis and cargo selectivity unresolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Genetic ablation studies established STXBP2 as non-redundant with Munc18-1/-3 for compound granule exocytosis and recapitulated microvillus inclusion disease, with rescue confirming gene-specific causality.\",\n      \"evidence\": \"Conditional KO mice with patch-clamp capacitance, stereological EM, in vivo anaphylaxis; Stxbp2-null intestinal organoids with WT vs patient-variant rescue and live imaging\",\n      \"pmids\": [\"29599294\", \"30364784\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular basis of homotypic (granule-granule) fusion requirement not dissected\", \"Link between fusion defect and microvillus inclusion morphogenesis mechanistic but incomplete\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Confirmed in vivo that STXBP2 is essential for platelet granule release across all three granule types and for hemostasis and thrombosis.\",\n      \"evidence\": \"Platelet-specific conditional KO mice with secretion assays, aggregometry, shear thrombus formation, bleeding-time and thrombosis models, and EM\",\n      \"pmids\": [\"30696774\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not address how STXBP2 coordinates the distinct granule classes mechanistically\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Refined understanding of dominant-negative pathology by showing the R190C mutation impairs degranulation without altering expression, localization, or STX11 binding, pointing to a downstream functional defect.\",\n      \"evidence\": \"Co-IP, immunofluorescence, and forced expression in primary CTLs with degranulation/cytotoxicity assays\",\n      \"pmids\": [\"33162974\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"The downstream interaction or non-productive complex disrupted by R190C is not identified\", \"Single-lab functional inference\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how STXBP2 mechanistically achieves cargo and granule-class selectivity and how its phosphorylation tunes Ca2+ sensitivity of fusion across cell types.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of STXBP2 engaged with an assembled SNARE complex during fusion\", \"Regulatory inputs (phosphorylation, Rab27/Munc13-4) controlling STXBP2 activity not mechanistically defined\", \"Erythrocyte and STXBP1-axis roles rest on low-confidence single-patient data\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [5, 11]},\n      {\"term_id\": \"GO:0044183\", \"supporting_discovery_ids\": [0, 1, 13]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [3, 7]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [2, 5, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0031410\", \"supporting_discovery_ids\": [3, 10, 13]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 10, 13]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [3, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 11, 12]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 1, 6, 16]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [6, 15]}\n    ],\n    \"complexes\": [\n      \"STXBP2-STX11-SNAP-23-VAMP-8 SNARE complex\",\n      \"Munc18-2/syntaxin-3 complex\"\n    ],\n    \"partners\": [\n      \"STX11\",\n      \"STX3\",\n      \"STX2\",\n      \"SNAP-23\",\n      \"VAMP-8\",\n      \"Slp4a\",\n      \"tubulin\",\n      \"Munc13-4\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}