{"gene":"STXBP5","run_date":"2026-06-10T10:51:54","timeline":{"discoveries":[{"year":1998,"finding":"Tomosyn (STXBP5) was identified as a syntaxin-1-binding protein that displaces Munc18 from syntaxin-1 and forms a novel 10S complex with syntaxin-1, SNAP-25, and synaptotagmin. High-level expression of tomosyn in PC12 cells specifically reduced Ca2+-dependent exocytosis.","method":"Co-immunoprecipitation, biochemical fractionation, PC12 cell overexpression assay","journal":"Neuron","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, biochemical complex characterization, functional overexpression assay; foundational paper replicated extensively","pmids":["9620695"],"is_preprint":false},{"year":2003,"finding":"The C-terminal R-SNARE motif of tomosyn forms genuine four-helical bundle SNARE core complexes with syntaxin-1 and SNAP-25, competing with synaptobrevin for binding to endogenous syntaxin/SNAP-25 on plasma membranes. Tomosyn-SNARE complexes are disassembled by NSF/α-SNAP ATPase activity. Overexpression in PC12 cells massively reduces exocytosis without altering individual fusion event parameters.","method":"In vitro reconstitution with recombinant proteins, CD spectroscopy, inside-out plasma membrane sheets competition assay, NSF disassembly assay, PC12 cell overexpression capacitance measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with multiple orthogonal biochemical methods plus functional cell assay; independently replicated","pmids":["12782620"],"is_preprint":false},{"year":2004,"finding":"Crystal structure of the tomosyn R-SNARE core complex with syntaxin-1a and SNAP-25 resolved at 2.0 Å. The complex forms a four-helical bundle highly similar to the synaptobrevin-containing SNARE complex. Synaptobrevin cannot displace the tomosyn helix from the assembled complex (and vice versa), indicating both are thermodynamic end products. Complexin binding to the tomosyn complex is impaired due to surface differences.","method":"X-ray crystallography (2.0 Å), CD spectroscopy kinetics, displacement assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure with functional validation by kinetic and displacement assays","pmids":["15316007"],"is_preprint":false},{"year":2004,"finding":"Tomosyn inhibits the priming step of large dense-core vesicle exocytosis in adrenal chromaffin cells, reducing the number of fusion-competent vesicles by ~50% without affecting docked vesicle number or individual fusion kinetics. This inhibition is partially relieved at elevated calcium concentrations, indicating a calcium-dependent shift in release threshold.","method":"Capacitance measurements, amperometry, morphological analysis (electron microscopy), calcium ramp experiments in chromaffin cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal electrophysiological and morphological methods in a single rigorous study","pmids":["14983051"],"is_preprint":false},{"year":2004,"finding":"Tomosyn localizes at growth cone palms via binding to ROCK-phosphorylated syntaxin-1 (Rho/ROCK phosphorylates syntaxin-1, increasing its affinity for tomosyn), thereby inhibiting SNARE-mediated vesicle fusion at palm regions and promoting vesicle transport to leading edges to regulate neurite extension and retraction.","method":"Immunolocalization, co-immunoprecipitation, kinase assay (ROCK phosphorylation of syntaxin-1), neuronal overexpression/dominant-negative experiments","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP and kinase assay with functional localization data, single lab","pmids":["15240567"],"is_preprint":false},{"year":2005,"finding":"Tomosyn is directly phosphorylated by PKA; this phosphorylation reduces its interaction with syntaxin-1, enhances SNARE complex formation, increases the readily releasable pool of synaptic vesicles, and thereby enhances neurotransmitter release. This mechanism underlies PACAP-induced facilitation of neurotransmitter release in SCG neurons.","method":"In vitro kinase assay, co-immunoprecipitation, electrophysiology in SCG neurons, pharmacological manipulation with PKA activators","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — in vitro kinase assay plus electrophysiological validation and pharmacological epistasis in neurons","pmids":["16186257"],"is_preprint":false},{"year":2003,"finding":"Tomosyn interacts with the t-SNAREs syntaxin-4 and SNAP-23 (adipocyte SNARE complex), forming a high-affinity ternary complex competitively inhibited by VAMP-2. The VAMP-like domain of tomosyn mediates the interaction with syntaxin-4. Overexpression of tomosyn in 3T3-L1 adipocytes inhibits insulin-stimulated GFP-GLUT4 translocation to the plasma membrane. Munc18c interacts with both syntaxin-4/tomosyn complexes and syntaxin-4-containing SNARE complexes.","method":"Yeast two-hybrid, in vitro binding assay, co-immunoprecipitation, GFP-GLUT4 translocation assay in adipocytes","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Y2H, in vitro binding, co-IP, live-cell imaging) in a single study","pmids":["12832401"],"is_preprint":false},{"year":2006,"finding":"C. elegans TOM-1 (tomosyn ortholog) negatively regulates synaptic vesicle priming in vivo. tom-1 mutants show increased primed vesicle numbers at the plasma membrane, enhanced evoked responses, and enhanced hyperosmotic responses. Epistasis with unc-13 priming-defective mutants shows TOM-1 acts as an endogenous inhibitor of the primed vesicle pool.","method":"Electrophysiology (evoked postsynaptic currents, hyperosmotic responses), electron microscopy ultrastructure, genetic epistasis (tom-1; unc-13 double mutants), neuronal rescue expression","journal":"PLoS biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple orthogonal in vivo methods; independently replicated in subsequent studies","pmids":["16895441"],"is_preprint":false},{"year":2006,"finding":"Tomosyn is expressed in pancreatic beta-cells; syntaxin-1 co-immunoprecipitates with tomosyn. Overexpression of m-tomosyn in mouse beta-cells significantly decreased exocytosis, while siRNA knockdown of tomosyn increased exocytosis, demonstrating that tomosyn negatively regulates insulin exocytosis.","method":"Co-immunoprecipitation, tomosyn overexpression in primary beta-cells, siRNA knockdown, exocytosis measurements","journal":"Diabetes","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus bidirectional functional manipulation (OE and KD) in a single lab","pmids":["16505218"],"is_preprint":false},{"year":2006,"finding":"Tomosyn-1 localizes to compartments enriched in insulin granules in beta-cells. The SNARE-like domain of tomosyn-1 forms a complex with syntaxin-1 and SNAP25 with weaker binding forces than VAMP2 (237 vs. 279 pN by AFM). siRNA silencing of tomosyn-1 reduces stimulus-induced exocytosis without affecting the number of docked granules, indicating tomosyn-1 facilitates a post-docking event required for exocytosis.","method":"Atomic force microscopy binding force measurements, siRNA knockdown, electron microscopy (docked granule counting), stimulus-secretion assay","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel AFM method plus KD with defined morphological and secretory readouts, single lab","pmids":["16787939"],"is_preprint":false},{"year":2007,"finding":"C. elegans TOM-1 negatively regulates dense-core vesicle (DCV) exocytosis. tom-1 mutants show 50% reduction in presynaptic DCVs corresponding to enhanced neuropeptide release; TOM-1 overexpression causes DCV accumulation. Genetic epistasis shows TOM-1 antagonizes UNC-31 (CAPS)-dependent DCV release; loss of TOM-1 suppresses unc-31 behavioral, electrophysiological, and ultrastructural phenotypes.","method":"Electron microscopy ultrastructure, electrophysiology, genetic epistasis (tom-1;unc-31 double mutants), neuropeptide release assays, behavioral analysis","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple orthogonal in vivo readouts, replicated findings","pmids":["17881523"],"is_preprint":false},{"year":2007,"finding":"The N-terminal WD40 repeat domain of tomosyn is required for its inhibitory activity and is sufficient to inhibit neurotransmitter release by catalyzing oligomerization of SNARE complexes. The C-terminal VAMP-like domain (VLD) inhibits SNARE complex formation by sequestering syntaxin-1. Together, these represent a dual inhibitory mechanism. Microinjection of the isolated N-terminal WD40 domain into neurons prevented stimulated acetylcholine release.","method":"Microinjection of domain-truncation constructs into neurons, co-immunoprecipitation to assess SNARE complex oligomerization, tomosyn-KO mouse analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 / Moderate — direct microinjection functional assay with domain dissection and biochemical validation; combined mechanistic approach in single paper","pmids":["18936251"],"is_preprint":false},{"year":2007,"finding":"Secretagogue stimulation causes rapid translocation of tomosyn from cytosol to plasma membrane regions in chromaffin cells, associated with increased tomosyn–syntaxin-1A interaction and increased cycling of tomosyn into SNARE complexes. This translocation is strongly reduced by ROCK inhibition, consistent with RhoA-mediated regulation. LPA (a RhoA activator) mimics secretagogue-induced tomosyn–syntaxin interaction.","method":"Live-cell optical imaging (FRET/translocation assay), pharmacological ROCK inhibition, LPA stimulation, co-immunoprecipitation, PC12 secretory assays","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — live imaging plus biochemical and pharmacological validation, single lab","pmids":["17545156"],"is_preprint":false},{"year":2007,"finding":"Tomosyn's inhibition of exocytosis requires the integrity of its N-terminal WD40 domain; a tomosyn mutant lacking the entire SNARE domain inhibits vesicle priming as potently as full-length protein, while the isolated SNARE domain fails to inhibit exocytosis. An N-terminally truncated mutant that retains SNARE-domain syntaxin binding does not inhibit exocytosis. Both the WD40 repeats and linker are required for inhibitory function.","method":"Domain-deletion mutant overexpression in chromaffin cells, capacitance measurements, co-immunoprecipitation","journal":"Journal of neurochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain deletion mapping with electrophysiological secretion readout, single lab","pmids":["17666050"],"is_preprint":false},{"year":2011,"finding":"Positional cloning and functional characterization identified tomosyn-2 (STXBP5L) as a negative regulator of insulin secretion. Tomosyn-2 binds syntaxin-1A and syntaxin-4 in vitro; overexpression in INS1 cells inhibits insulin secretion. The BTBR allele of tomosyn-2 resists proteasomal degradation compared to the B6 allele, establishing a functional consequence of a coding SNP.","method":"In vitro binding assay with recombinant proteins, INS1 cell overexpression secretion assay, proteasomal degradation assay, sub-congenic mouse islet secretion phenotyping","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 2 / Moderate — in vitro binding plus cell secretion assay plus in vivo mouse islet phenotyping with allelic degradation analysis; multiple orthogonal methods","pmids":["21998599"],"is_preprint":false},{"year":2011,"finding":"Structural analysis by homology modeling (based on yeast Sro7 crystal structure) reveals that deletion of loops 1 and 3 from the β-propeller core of tomosyn eliminates its inhibitory activity on secretion without altering SNARE pairing with syntaxin-1A. Deletion of loop 2 (hypervariable splice region) does not reduce inhibition but affects protein accumulation of tomosyn-2 isoforms. m-Tomosyn-1 is a substrate for SUMO-2/3 conjugation at K730; mutation of this site enhances secretion inhibition without altering syntaxin-1A binding.","method":"Homology modeling, deletion mutagenesis, PC12 cell secretion assay, co-immunoprecipitation, SUMO conjugation assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — structure-guided mutagenesis with functional secretion assay and biochemical validation, single lab","pmids":["21330375"],"is_preprint":false},{"year":2013,"finding":"In C. elegans, Tomosyn inhibits slow neurotransmitter release while UNC-13L mediates fast release. Tomosyn's spatial position (diffuse/distal from dense projection) correlates with its control over slow release. Genetic analysis places tomosyn as a negative regulator in a molecular code (UNC-13L, UNC-13S, and Tomosyn) that dictates the timing of neurotransmitter release.","method":"Genetic epistasis (multiple mutant combinations), electrophysiology, fluorescence imaging of protein localization","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic epistasis with multiple alleles plus electrophysiology plus localization, replicated across experiments","pmids":["23951547"],"is_preprint":false},{"year":2014,"finding":"STXBP5 is expressed in human endothelial cells and colocalizes with and binds syntaxin-4. Knockdown of STXBP5 in endothelial cells increases exocytosis of vWF and P-selectin. Stxbp5 KO mice have higher plasma vWF, increased P-selectin translocation, and more platelet-endothelial interactions, demonstrating that STXBP5 inhibits endothelial exocytosis. Stxbp5 KO mice also have defects in platelet secretion and hemostasis (prolonged bleeding times, impaired thrombosis).","method":"Co-immunoprecipitation, siRNA knockdown (endothelial cells), Stxbp5 KO mouse model, plasma vWF ELISA, P-selectin translocation FACS, tail bleeding and mesenteric/carotid thrombosis assays","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP plus bidirectional loss-of-function (siRNA + KO mouse) with multiple orthogonal in vivo and in vitro readouts","pmids":["25244095"],"is_preprint":false},{"year":2014,"finding":"STXBP5 was identified by mass spectrometry from SNARE-containing affinity purifications from human platelet extracts. STXBP5 interacts with syntaxin-11/SNAP23 heterodimers by co-immunoprecipitation and also associates with the platelet cytoskeleton. Stxbp5 KO platelets have markedly defective stimulation-dependent secretion from all three granule types (dense, alpha, lysosomal) and altered granule cargo levels despite normal granule numbers and morphology.","method":"Mass spectrometry, co-immunoprecipitation, fractionation, Stxbp5 KO mouse model, lumi-aggregometry, FACS (P-selectin/LAMP-1), bone marrow transplantation","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — MS identification plus reciprocal co-IP plus KO mouse with multiple platelet function readouts and BM transplant epistasis","pmids":["25244094"],"is_preprint":false},{"year":2014,"finding":"The C-terminal domain (CTD) of tomosyn, containing the R-SNARE-like motif, mediates inhibition of SNARE-dependent membrane fusion by recognizing the t-SNARE complex and preventing pairing with the v-SNARE, arresting the fusion reaction at a pre-docking stage. The N-terminal domain (NTD) is critical (but not sufficient) for tomosyn recruitment to fusion sites via syntaxin monomer binding. Tomosyn inhibitory activity is dominant over the stimulatory Sec1/Munc18 protein in fusion.","method":"In vitro reconstituted SNARE-dependent liposome fusion assay with purified full-length tomosyn and domain truncations, co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified full-length protein and domain truncations, multiple defined fusion assays; single lab","pmids":["25063806"],"is_preprint":false},{"year":2014,"finding":"In vitro, tomosyn-2 is phosphorylated in response to glucose, phorbol esters, and cAMP analogs; 11 phosphorylation sites were identified by mass spectrometry. Phosphomimetic (Ser→Asp) tomosyn-2 shows enhanced proteasomal turnover and reduced ability to inhibit insulin secretion. Tomosyn-2 is ubiquitinated by the E3 ligase Hrd-1; Hrd-1 knockdown increases tomosyn-2 abundance, identifying a phosphorylation-dependent proteasomal degradation mechanism for de-repression of insulin secretion.","method":"32P labeling, mass spectrometry phosphosite identification, site-directed mutagenesis, proteasome inhibitor experiments, proteomic screen for binding partners, co-immunoprecipitation, shRNA knockdown, ubiquitination assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — mass spectrometry plus mutagenesis plus E3 ligase identification with functional secretion readout; multiple orthogonal methods in single study","pmids":["25002582"],"is_preprint":false},{"year":2014,"finding":"By dSTORM super-resolution imaging, tomosyn is organized in small clusters adjacent to syntaxin clusters on the plasma membrane. Tomosyn forms both binary (tomosyn–syntaxin) and ternary (tomosyn–syntaxin–SNAP25) complexes at the PM. Deletion of β-propeller core residues 537–578 or 897–917 reduces SNAP25 binding and PM cluster residence time, shifting equilibrium toward binary tomosyn–syntaxin complexes and reducing inhibition of exocytosis, indicating tomosyn inhibits exocytosis via the ternary complex.","method":"dSTORM super-resolution microscopy, deletion mutagenesis, co-immunoprecipitation, fluorescence recovery assays, exocytosis assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — novel super-resolution imaging with domain mutagenesis and functional secretion readout, single lab","pmids":["24782308"],"is_preprint":false},{"year":2014,"finding":"STXBP5 knockdown in vascular endothelial cells decreased tPA release, functionally linking STXBP5 to regulation of tPA exocytosis from endothelium.","method":"siRNA knockdown in vascular endothelial cells, tPA release assay","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single siRNA knockdown experiment with secretion assay, single lab, but consistent with STXBP5's established role in endothelial exocytosis","pmids":["24578379"],"is_preprint":false},{"year":2015,"finding":"Tomosyn is phosphorylated at Ser-783 by both Akt1 and Akt2; this phosphorylation inhibits tomosyn's interaction with syntaxin-4, as shown by in vitro pull-down. Expression of phosphorylation-deficient (S783A) tomosyn attenuates insulin-stimulated GLUT4 surface expression, suggesting that Akt-mediated phosphorylation of tomosyn relieves its inhibition of GLUT4 exocytosis.","method":"In vitro kinase assay (Akt1/Akt2), in vitro pulldown assay, intact cell 32P labeling with PI3K inhibitor, GLUT4 cell-surface assay with mutant overexpression","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro kinase assay plus cell functional assay with phospho-mutant, single lab","pmids":["25725259"],"is_preprint":false},{"year":2015,"finding":"Tomosyn knockdown at hippocampal mossy fiber–CA3 synapses (via combined lentiviral KD and optogenetic activation) impairs synaptic facilitation, PKA-dependent long-term potentiation, and PKA-induced potentiation. This establishes tomosyn as a key regulator of mossy fiber presynaptic plasticity.","method":"Combined KD-optogenetic strategy in hippocampal slices, electrophysiology (facilitation, LTP, PKA-induced potentiation)","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — selective presynaptic KD plus optogenetic activation with multiple electrophysiological plasticity readouts, single lab","pmids":["26166572"],"is_preprint":false},{"year":2016,"finding":"Tomo1 regulates synaptic vesicle pool partitioning at rat hippocampal synapses in an activity-dependent manner. Tomo1 knockdown facilitates release from the Readily Releasable Pool (RRP) and alters Total Recycling Pool and Resting Pool distribution. These effects are regulated by Cdk5-dependent phosphorylation of Tomo1. Tomo1 interacts with GTP-bound Rab3A and, via Rab3A, with Synapsin 1a/b.","method":"VGlut1-pHluorin fluorescence SV pool assay, KD and rescue, co-immunoprecipitation (Tomo1–Rab3A-GTP, Tomo1–Synapsin), Cdk5 phosphorylation assay, chronic activity manipulation","journal":"The Journal of neuroscience","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple co-IP interactions plus functional SV pool assay with kinase regulation, single lab","pmids":["27807164"],"is_preprint":false},{"year":2017,"finding":"Tomosyn negatively regulates SNARE complex formation in a manner requiring its SUMOylation at K298. Glucose-dependent de-SUMOylation of tomosyn-1 releases syntaxin-1A. Tomosyn-1 interacts with secretagogin, a Ca2+-binding protein that dissociates from tomosyn-1 in response to Ca2+-raising stimuli and is required for insulin granule trafficking and exocytosis. Together, SUMOylation and Ca2+-dependent secretagogin release coordinate amplification of insulin secretion.","method":"Co-immunoprecipitation, site-directed mutagenesis (K298), SUMO modification assay, secretagogin binding assay, human beta-cell exocytosis measurements","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis of SUMO site plus co-IP plus functional exocytosis assay; single lab, multiple orthogonal methods","pmids":["28325894"],"is_preprint":false},{"year":2017,"finding":"UNC-18(P334A) gain-of-function in C. elegans partially bypasses UNC-13 requirement for synaptic vesicle fusion and shows synergistic suppression with tom-1 null mutation, placing tomosyn/TOM-1 and Munc18/UNC-18 as antagonistic regulators of SNARE complex assembly downstream of Munc13/UNC-13. Biochemically, Munc18-1(P335A) shows enhanced SNARE complex formation and partial bypass of Munc13-1 requirement in liposome fusion assays.","method":"Genetic epistasis (unc-18; tom-1; unc-13 multiple mutants), electrophysiology, liposome fusion assay, co-immunoprecipitation (SNARE complex formation)","journal":"The Journal of neuroscience","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo genetic epistasis plus in vitro biochemical/fusion assay, multiple orthogonal methods","pmids":["28821673"],"is_preprint":false},{"year":2017,"finding":"Tomosyn-1 is ubiquitinated in hippocampal neurons and undergoes HRD1-dependent proteasomal degradation. Immunoprecipitation of Tomo-1 from neurons co-precipitates HRD1, and in vitro reactions show direct, HRD1 concentration-dependent Tomo-1 ubiquitination. HRD1 knockdown increases Tomo-1 levels and dendritic spine density; Tomo-1 co-knockdown reverses this effect, establishing a direct HRD1→Tomo-1 effector relationship.","method":"Immunoprecipitation (ubiquitin/HRD1), in vitro ubiquitination assay, proteasome inhibitor treatment, shRNA KD (HRD1 and Tomo-1), dendritic spine density quantification","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro ubiquitination assay plus co-IP plus bidirectional genetic manipulation with structural readout, single lab","pmids":["29269412"],"is_preprint":false},{"year":2018,"finding":"Tomosyn-1 (STXBP5) acts as an inhibitory fusion clamp in mast cell degranulation downstream of FcεRI. After activation, tomosyn-1 is phosphorylated on serine and threonine residues, dissociates from syntaxin-4 (STX4), and re-associates with syntaxin-3 (STX3). PKCδ is the major kinase required for tomosyn-1 threonine phosphorylation and for regulating the switch between STX partners.","method":"Co-immunoprecipitation (tomosyn-STX4/STX3 interactions), phosphorylation assays, PKCδ inhibitor/knockout studies, FcεRI-stimulated degranulation assay, patient basophil analysis","journal":"Science signaling","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP plus kinase identification with pharmacological and genetic manipulation; single lab","pmids":["29970602"],"is_preprint":false},{"year":2018,"finding":"NSF/α-SNAP disassembles the tomosyn-SNARE arrest complex, allowing syntaxin-1 to enter the Munc18-1/syntaxin-1 complex. Munc13-1 then catalyzes transfer of syntaxin-1 from the Munc18-1/syntaxin-1 complex into the SNARE complex in a manner specific to synaptobrevin-2 but resistant to tomosyn. This establishes a sequential pathway: tomosyn arrest → NSF/α-SNAP release → Munc18-1 scaffolding → Munc13-1 SNARE assembly.","method":"In vitro biochemical reconstitution with purified proteins (NSF/α-SNAP disassembly assay, Munc18-1/Munc13-1 SNARE assembly assay), co-immunoprecipitation","journal":"FEBS letters","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with purified proteins identifying sequential mechanism, multiple biochemical assays","pmids":["29485200"],"is_preprint":false},{"year":2020,"finding":"CRISPR-Cas9 double knockout of both tomosyn-encoding genes in adipocytes markedly elevated both basal and insulin-stimulated GLUT4 exocytosis without affecting adipocyte differentiation or insulin signaling. In reconstituted liposome fusion, tomosyn inhibited all SNARE complexes underlying GLUT4 exocytosis, and this inhibition was relieved by NSF/α-SNAP, which removes tomosyn from GLUT4 exocytic SNAREs.","method":"CRISPR-Cas9 double KO in adipocytes, GLUT4 exocytosis assay, in vitro liposome fusion reconstitution with purified tomosyn, NSF/α-SNAP disassembly assay","journal":"Traffic","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — CRISPR KO genetic evidence plus in vitro reconstitution with mechanistic NSF/α-SNAP rescue, multiple orthogonal methods","pmids":["32851733"],"is_preprint":false},{"year":2020,"finding":"Tomosyn knockdown in mouse primary neurons increases RhoA GTPase activity, leading to compromised dendritic arborization, loss of dendritic spines, decreased surface AMPA receptor expression, and reduced mEPSC frequency. Inhibiting RhoA signaling rescues the morphological and receptor surface expression defects. The N-terminal WD40 domain mediates tomosyn's suppression of RhoA activity. Two ASD-associated missense variants in the WD40 domain show loss-of-function for these postsynaptic phenotypes.","method":"shRNA knockdown in primary neurons, RhoA GTPase activity assay, dendritic spine/arborization morphometry, AMPA receptor surface expression (immunostaining/biotinylation), mEPSC recordings, domain truncation and ASD variant rescue experiments","journal":"Journal of neuroscience research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KD with multiple readouts plus domain and variant rescue, single lab; novel non-SNARE mechanism","pmids":["32133675"],"is_preprint":false},{"year":2021,"finding":"In Drosophila, Tomosyn acts as a decoy SNARE that sets release probability (Pr) and tonic vs. phasic release properties. Tomosyn is differentially expressed between tonic (Ib, high Tomosyn, low Pr) and phasic (Is, low Tomosyn, high Pr) motoneurons. Loss of Tomosyn impairs synaptic facilitation, LTP, and presynaptic homeostatic potentiation specifically at tonic synapses.","method":"Genetic loss-of-function in Drosophila motoneurons, electrophysiology (evoked responses, facilitation, homeostatic plasticity), immunofluorescence protein quantification","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic LOF with multiple electrophysiological plasticity readouts in Drosophila, single lab","pmids":["34713802"],"is_preprint":false},{"year":2023,"finding":"In tomosyn/tomosyn-2 conditional double-knockout mouse neurons, DCV exocytosis frequency was not significantly affected; however, intracellular levels of DCV cargos (NPY, BDNF) were strongly reduced. BDNF levels were restored by re-expression of tomosyn but not by lysosomal protease inhibition, and tomosyn's SNARE domain was dispensable for rescue. Trans-Golgi network and DCV size were decreased, and DCV cargo flux through Golgi was accelerated in KO neurons, suggesting tomosyns function in DCV biogenesis/cargo packaging at the Golgi rather than DCV fusion.","method":"Conditional double-KO mouse neurons, pHluorin-based single-vesicle DCV exocytosis assay, cargo level quantification (NPY, BDNF), tomosyn re-expression rescue (full-length and SNARE-domain-deleted), trans-Golgi network morphometry, DCV size analysis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 / Strong — conditional DKO with single-vesicle resolution assay plus domain rescue experiments and multiple orthogonal morphological readouts; challenges prior model, rigorous controls","pmids":["37695731"],"is_preprint":false},{"year":2023,"finding":"C. elegans TOM-1 short isoform acts downstream of the UNC-5 netrin receptor to inhibit growth cone protrusion, and this mechanism requires syntaxin/UNC-64, consistent with TOM-1 inhibiting vesicle fusion needed for membrane addition during protrusion. The long TOM-1 isoform has a pro-protrusive role.","method":"Genetic epistasis in C. elegans (tom-1; unc-5; unc-64 mutant analysis), growth cone protrusion quantification, isoform-specific rescue experiments","journal":"Development","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with isoform-specific rescue in defined cellular context, single lab","pmids":["37014062"],"is_preprint":false},{"year":2014,"finding":"Tomosyn-1 interacts with the SUMO E3 ligase PIASγ (PIAS4). The interaction involves the C-terminus of tomosyn-1 and the N-terminus of PIASγ, confirmed by yeast two-hybrid and bidirectional immunoprecipitation in HEK293T cells. Tomosyn-1 is preferentially modified by SUMO-2/3.","method":"Yeast two-hybrid, bidirectional co-immunoprecipitation in HEK293T cells, SUMO modification assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP plus Y2H with SUMO modification assay, single lab","pmids":["24614299"],"is_preprint":false},{"year":2023,"finding":"Synaptotagmin-9 (Syt9) colocalizes and binds tomosyn-1 and syntaxin-1A (Stx1A) in pancreatic beta-cells; the Syt9–tomosyn-1–Stx1A complex is inhibitory for insulin secretion. Syt9 knockdown reduces tomosyn-1 protein abundance via proteasomal degradation, decreases tomosyn-1/Stx1A interaction, and increases SNARE complex formation and insulin secretion. Rescuing tomosyn-1 blocks the Syt9-knockdown-mediated increases in insulin secretion, establishing that Syt9's inhibitory effects on insulin secretion are mediated through tomosyn-1.","method":"Co-localization, co-immunoprecipitation, Syt9 KO/KD (mouse and cell), tomosyn-1 rescue expression, SNARE complex assay, insulin secretion assay","journal":"FASEB journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus bidirectional genetic manipulation (KO mouse + KD) with epistatic rescue, single lab","pmids":["37432648"],"is_preprint":false},{"year":2016,"finding":"STXBP5/tomosyn-1 KO mice show increased KCl-evoked glutamate release in the hippocampal dentate gyrus and accelerated kindling progression (fewer stimuli required to reach fully kindled state), establishing STXBP5 as an endogenous brake on glutamate release that opposes epileptiform activity.","method":"Glutamate-selective microelectrode array (MEA) in vivo measurements, amygdala kindling stimulation, STXBP5 KO mouse model","journal":"Brain and behavior","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO mouse with in vivo microelectrode glutamate measurements and behavioral kindling assay, single lab","pmids":["28948088"],"is_preprint":false},{"year":2025,"finding":"Tomosyn-2 interacts with syntaxin-1A to inhibit insulin granule exocytosis by limiting SNARE complex formation in pancreatic beta-cells. Tomosyn-2 KO mice show improved glucose clearance and enhanced biphasic insulin secretion. Loss of tomosyn-2 also reduces beta-cell proliferation via downregulation of Akt1 signaling and cell-cycle mediators, identifying a dual role in insulin secretion and beta-cell maturation.","method":"Co-immunoprecipitation (tomosyn-2/Stx1A), tomosyn-2 KO mouse model, glucose tolerance tests, insulin secretion assay from isolated islets, transcriptomic analysis, Akt1/cyclinD1 signaling assay","journal":"Diabetes","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus KO mouse with multiple secretion and signaling readouts; single lab, not yet independently replicated","pmids":["42008692"],"is_preprint":false}],"current_model":"STXBP5 (tomosyn-1) is a soluble decoy R-SNARE protein that inhibits exocytosis primarily by: (1) sequestering syntaxin-1 (and other t-SNAREs) in a non-fusogenic four-helical bundle SNARE complex via its C-terminal R-SNARE domain, competing with synaptobrevin/VAMP and arresting membrane fusion; (2) catalyzing oligomerization of SNARE complexes via its N-terminal WD40 β-propeller domain; and (3) regulating synaptic vesicle pool partitioning through Rab3A-GTP and synapsin interactions; this inhibitory activity is dynamically regulated by PKA-, Akt-, Cdk5-, and PKCδ-dependent phosphorylation and by SUMO-2/3 modification at K730 (and K298 in beta-cells), all of which relieve syntaxin binding and promote secretion, while HRD1-dependent ubiquitin-proteasomal degradation controls tomosyn abundance; in addition to its canonical presynaptic exocytic role, tomosyn suppresses RhoA activity via its WD40 domain to maintain dendritic spine stability and AMPA receptor surface expression, and may also function in DCV cargo packaging at the Golgi independently of its SNARE domain."},"narrative":{"mechanistic_narrative":"STXBP5 (tomosyn-1) is a soluble inhibitory regulator of SNARE-mediated membrane fusion that acts as a presynaptic and secretory brake across neurons, neuroendocrine cells, beta-cells, platelets, endothelium, and immune cells [PMID:9620695, PMID:12782620, PMID:16895441]. It was first identified as a syntaxin-1-binding protein that displaces Munc18 and assembles a non-fusogenic complex with syntaxin-1 and SNAP-25, reducing Ca2+-dependent exocytosis [PMID:9620695]. Its C-terminal R-SNARE (VAMP-like) motif forms a genuine four-helical bundle SNARE core complex with syntaxin-1 and SNAP-25 that is structurally homologous to the synaptobrevin complex, competing with synaptobrevin/VAMP for t-SNARE binding to arrest fusion as a thermodynamic end product [PMID:12782620, PMID:15316007, PMID:25063806]. A parallel and dominant inhibitory mechanism resides in the N-terminal WD40 β-propeller domain, which is required and sufficient to inhibit release by catalyzing oligomerization of SNARE complexes, and acts through assembly of the ternary tomosyn–syntaxin–SNAP25 complex at the plasma membrane [PMID:18936251, PMID:17666050, PMID:24782308]. Functionally, tomosyn inhibits the priming/post-docking step rather than vesicle docking, limiting the readily releasable and primed vesicle pools without altering individual fusion kinetics [PMID:14983051, PMID:16895441, PMID:17881523]. The tomosyn-SNARE arrest is reversed by NSF/α-SNAP disassembly, after which Munc18-1 scaffolding and Munc13-1 catalysis assemble fusogenic synaptobrevin-containing complexes, defining tomosyn and Munc18/Munc13 as antagonistic regulators of SNARE assembly [PMID:28821673, PMID:29485200]. This inhibitory activity is dynamically de-repressed by phosphorylation via PKA, Akt, Cdk5, and PKCδ and by SUMO-2/3 modification, each of which relieves syntaxin binding to promote secretion, while phosphorylation-dependent HRD1-mediated ubiquitin-proteasomal degradation controls tomosyn abundance [PMID:16186257, PMID:25725259, PMID:27807164, PMID:28325894, PMID:29970602, PMID:25002582, PMID:29269412]. Tomosyn engages tissue-specific t-SNAREs—syntaxin-4/SNAP-23 in adipocytes to inhibit insulin-stimulated GLUT4 translocation, syntaxin-11/SNAP-23 in platelet granule secretion, and syntaxin-4 in endothelial Weibel-Palade body exocytosis—and Stxbp5 knockout impairs platelet secretion, hemostasis, and thrombosis [PMID:12832401, PMID:25244094, PMID:25244095, PMID:32851733]. Beyond its canonical exocytic role, tomosyn suppresses RhoA GTPase activity via its WD40 domain to maintain dendritic arborization, spine stability, and surface AMPA receptor expression, with ASD-associated WD40 variants showing loss of this function [PMID:32133675], and its SNARE-independent function in dense-core vesicle cargo packaging at the Golgi controls intracellular DCV cargo levels [PMID:37695731].","teleology":[{"year":1998,"claim":"Established tomosyn as a syntaxin-1-binding protein, answering how an exocytic inhibitor could intercept the core fusion machinery by displacing Munc18 and forming an alternative syntaxin complex.","evidence":"Co-immunoprecipitation, biochemical fractionation, and overexpression-based exocytosis assay in PC12 cells","pmids":["9620695"],"confidence":"High","gaps":["Did not resolve which tomosyn domain mediates inhibition","Mechanism of fusion arrest at the molecular level unresolved"]},{"year":2003,"claim":"Defined the molecular basis of inhibition by showing the C-terminal R-SNARE motif forms a bona fide four-helical SNARE complex that competes with synaptobrevin and is reversible by NSF/α-SNAP.","evidence":"In vitro reconstitution, CD spectroscopy, plasma membrane sheet competition, NSF disassembly assay, and PC12 capacitance measurements","pmids":["12782620"],"confidence":"High","gaps":["Did not establish the contribution of the N-terminal domain","Step in the secretory pathway inhibited not yet mapped"]},{"year":2004,"claim":"Resolved the atomic structure of the tomosyn SNARE complex and showed both tomosyn and synaptobrevin complexes are mutually exclusive thermodynamic end products with impaired complexin binding.","evidence":"2.0 Å X-ray crystallography with CD kinetics and displacement assays","pmids":["15316007"],"confidence":"High","gaps":["Structure of the WD40 domain not determined","Did not address how the arrested complex is recycled in vivo"]},{"year":2004,"claim":"Localized tomosyn's action to the vesicle priming step, showing it reduces fusion-competent vesicles in a calcium-tunable manner without affecting docking.","evidence":"Capacitance, amperometry, EM, and calcium ramp experiments in chromaffin cells","pmids":["14983051"],"confidence":"High","gaps":["Molecular mechanism of calcium-dependent relief of inhibition unresolved"]},{"year":2003,"claim":"Extended tomosyn's t-SNARE repertoire beyond neurons by showing it binds syntaxin-4/SNAP-23 and inhibits insulin-stimulated GLUT4 translocation in adipocytes.","evidence":"Yeast two-hybrid, in vitro binding, co-IP, and GFP-GLUT4 translocation imaging in 3T3-L1 adipocytes","pmids":["12832401"],"confidence":"High","gaps":["Endogenous loss-of-function effect not tested in this study","How Munc18c integrates with the tomosyn complex unresolved"]},{"year":2006,"claim":"Provided in vivo genetic proof that tomosyn (C. elegans TOM-1) is an endogenous negative regulator of synaptic vesicle priming, complementing prior overexpression data.","evidence":"Electrophysiology, EM ultrastructure, and tom-1;unc-13 genetic epistasis in C. elegans","pmids":["16895441"],"confidence":"High","gaps":["Domain requirements not dissected genetically here"]},{"year":2005,"claim":"Identified PKA phosphorylation as a physiological switch that relieves syntaxin binding to enhance the releasable pool and neurotransmitter release, linking tomosyn to PACAP-mediated facilitation.","evidence":"In vitro kinase assay, co-IP, electrophysiology, and PKA pharmacology in SCG neurons","pmids":["16186257"],"confidence":"High","gaps":["Phosphorylation sites not mapped in this study"]},{"year":2007,"claim":"Resolved that tomosyn uses a dual inhibitory mechanism: the WD40 domain catalyzes SNARE complex oligomerization while the VAMP-like domain sequesters syntaxin, with the WD40 domain alone sufficient to inhibit release.","evidence":"Neuronal microinjection of domain truncations, co-IP oligomerization assay, and tomosyn-KO mouse analysis; corroborated by chromaffin cell domain-deletion capacitance mapping","pmids":["18936251","17666050"],"confidence":"High","gaps":["Structural basis of WD40-driven oligomerization unresolved","How the two mechanisms are coordinated temporally unknown"]},{"year":2007,"claim":"Showed tomosyn inhibition extends to dense-core vesicle/neuropeptide release and is dynamically recruited to the membrane by RhoA/ROCK-dependent translocation upon stimulation.","evidence":"tom-1;unc-31 genetic epistasis with EM and neuropeptide assays; FRET/translocation imaging with ROCK inhibition and LPA in chromaffin/PC12 cells","pmids":["17881523","17545156"],"confidence":"High","gaps":["Direct ROCK substrate site on syntaxin/tomosyn not fully defined"]},{"year":2006,"claim":"Demonstrated tomosyn negatively regulates insulin granule exocytosis at a post-docking step in beta-cells, generalizing the brake function to endocrine secretion.","evidence":"Co-IP, bidirectional overexpression/siRNA, AFM binding-force measurement, and EM docked-granule counting in beta-cells","pmids":["16505218","16787939"],"confidence":"Medium","gaps":["Single-lab functional data","Physiological regulation of beta-cell tomosyn not addressed here"]},{"year":2011,"claim":"Identified the paralog tomosyn-2 (STXBP5L) as a genetically validated negative regulator of insulin secretion whose abundance is set by allele-specific proteasomal degradation.","evidence":"Positional cloning, in vitro binding, INS1 secretion assay, proteasomal degradation assay, and sub-congenic mouse islet phenotyping","pmids":["21998599"],"confidence":"High","gaps":["Degradation machinery not identified in this study"]},{"year":2011,"claim":"Mapped β-propeller loops required for inhibition independent of syntaxin pairing and identified SUMO-2/3 conjugation at K730 as a modification that tunes inhibitory strength.","evidence":"Homology modeling, deletion mutagenesis, PC12 secretion assay, and SUMO conjugation assay","pmids":["21330375"],"confidence":"Medium","gaps":["No experimental structure of the propeller","SUMO E3 ligase not identified here"]},{"year":2013,"claim":"Positioned tomosyn within a molecular code that dictates the timing of neurotransmitter release, inhibiting slow release in a position-dependent manner relative to UNC-13 isoforms.","evidence":"Multi-allele genetic epistasis, electrophysiology, and localization imaging in C. elegans","pmids":["23951547"],"confidence":"High","gaps":["Molecular determinant of spatial positioning unknown"]},{"year":2014,"claim":"Demonstrated a physiological role in vascular and hemostatic exocytosis, showing STXBP5 brakes endothelial vWF/P-selectin/tPA release and platelet granule secretion via syntaxin-4 and syntaxin-11/SNAP-23.","evidence":"Co-IP, siRNA, Stxbp5 KO mice, plasma vWF/P-selectin/tPA assays, bleeding/thrombosis assays, and platelet MS/aggregometry","pmids":["25244095","25244094","24578379"],"confidence":"High","gaps":["Why platelet KO impairs rather than enhances secretion (vs. endothelium) not fully reconciled","Cytoskeletal association role unresolved"]},{"year":2014,"claim":"Reconstituted full-length tomosyn inhibition of liposome fusion and resolved the spatial nanoscale organization of tomosyn–syntaxin–SNAP25 ternary complexes that mediate inhibition.","evidence":"In vitro liposome fusion with purified protein and truncations; dSTORM super-resolution with deletion mutagenesis and exocytosis assays","pmids":["25063806","24782308"],"confidence":"High","gaps":["Relative in vivo contribution of binary vs. ternary complexes uncertain"]},{"year":2014,"claim":"Defined the phosphorylation-dependent degradation circuit for tomosyn-2, identifying HRD1 as the E3 ligase and Akt as a kinase that de-represses secretion by abrogating syntaxin-4 binding.","evidence":"32P labeling, MS phosphosite mapping, mutagenesis, ubiquitination/proteasome assays, and GLUT4/insulin secretion readouts","pmids":["25002582","25725259"],"confidence":"High","gaps":["Whether neuronal tomosyn-1 uses the same HRD1 pathway not shown here"]},{"year":2014,"claim":"Identified the SUMO E3 ligase PIASγ (PIAS4) as a direct tomosyn-1 partner mediating SUMO-2/3 modification, providing the enzymatic basis for SUMO regulation.","evidence":"Yeast two-hybrid, bidirectional co-IP in HEK293T, and SUMO modification assay","pmids":["24614299"],"confidence":"Medium","gaps":["Functional consequence on secretion not tested in this study"]},{"year":2015,"claim":"Established tomosyn as a regulator of presynaptic plasticity and synaptic vesicle pool partitioning, acting through Cdk5 phosphorylation and Rab3A-GTP/synapsin interactions.","evidence":"KD-optogenetics in hippocampal slices with plasticity electrophysiology; VGlut1-pHluorin pool assay with co-IP and Cdk5 phosphorylation assay","pmids":["26166572","27807164"],"confidence":"Medium","gaps":["Single-lab studies","Direct link between pool partitioning and SNARE arrest not fully defined"]},{"year":2016,"claim":"Showed STXBP5 is an endogenous brake on hippocampal glutamate release that opposes epileptogenesis in vivo.","evidence":"In vivo glutamate microelectrode arrays and amygdala kindling in STXBP5 KO mice","pmids":["28948088"],"confidence":"Medium","gaps":["Mechanistic link to seizure circuits beyond glutamate release not established"]},{"year":2017,"claim":"Resolved the sequential SNARE-assembly pathway placing tomosyn arrest upstream of NSF/α-SNAP release, Munc18-1 scaffolding, and Munc13-1 catalysis, and genetically defined tomosyn and Munc18 as antagonists.","evidence":"In vitro reconstitution with purified NSF/α-SNAP, Munc18-1/Munc13-1; unc-18;tom-1;unc-13 genetic epistasis with electrophysiology and liposome fusion","pmids":["29485200","28821673"],"confidence":"High","gaps":["Kinetics of in vivo handoff between tomosyn release and fusogenic assembly not quantified"]},{"year":2017,"claim":"Defined glucose-regulated SUMOylation at K298 and a secretagogin/Ca2+ axis as coordinated inputs that release syntaxin-1A to amplify insulin secretion.","evidence":"K298 mutagenesis, SUMO and secretagogin binding assays, and human beta-cell exocytosis measurements","pmids":["28325894"],"confidence":"Medium","gaps":["Single-lab data","Interplay between K298 and K730 SUMO sites unresolved"]},{"year":2017,"claim":"Demonstrated HRD1-dependent ubiquitin-proteasomal degradation of neuronal tomosyn-1 controls its abundance and downstream dendritic spine density.","evidence":"Ubiquitin/HRD1 co-IP, in vitro ubiquitination, proteasome inhibition, and bidirectional shRNA with spine quantification in hippocampal neurons","pmids":["29269412"],"confidence":"Medium","gaps":["Single-lab study","Signal triggering HRD1-tomosyn engagement in neurons unknown"]},{"year":2018,"claim":"Extended the inhibitory clamp model to immune secretion, showing PKCδ-driven phosphorylation switches tomosyn-1 between syntaxin-4 and syntaxin-3 to permit FcεRI-triggered mast cell degranulation.","evidence":"Reciprocal co-IP, phosphorylation assays, PKCδ inhibitor/knockout, degranulation assay, and patient basophil analysis","pmids":["29970602"],"confidence":"Medium","gaps":["Single-lab study","Phosphosites mediating the syntaxin partner switch not mapped"]},{"year":2020,"claim":"Provided genetic and biochemical confirmation that tomosyn restrains GLUT4 exocytosis by inhibiting all relevant SNARE complexes, with NSF/α-SNAP relieving the arrest.","evidence":"CRISPR-Cas9 double KO in adipocytes with GLUT4 exocytosis and in vitro liposome fusion plus NSF/α-SNAP disassembly","pmids":["32851733"],"confidence":"High","gaps":["Physiological trigger for NSF/α-SNAP-mediated tomosyn removal in adipocytes not defined"]},{"year":2020,"claim":"Uncovered a SNARE-independent postsynaptic function: tomosyn suppresses RhoA via its WD40 domain to maintain dendrites, spines, and surface AMPA receptors, with ASD variants causing loss of function.","evidence":"shRNA KD in primary neurons, RhoA activity assay, morphometry, AMPAR surface assays, mEPSC recordings, and domain/variant rescue","pmids":["32133675"],"confidence":"Medium","gaps":["Single-lab study","Direct biochemical link between WD40 domain and RhoA regulators not established"]},{"year":2021,"claim":"Showed tomosyn sets release probability and tonic vs. phasic release identity, acting as a decoy SNARE that enables facilitation and homeostatic plasticity at tonic synapses.","evidence":"Genetic loss-of-function in Drosophila motoneurons with plasticity electrophysiology and immunofluorescence quantification","pmids":["34713802"],"confidence":"Medium","gaps":["Single-lab study","Mechanism setting differential tomosyn expression between motoneuron types unknown"]},{"year":2023,"claim":"Revised the model for dense-core vesicles by showing tomosyns function in DCV cargo packaging/biogenesis at the Golgi—independently of the SNARE domain—rather than DCV fusion.","evidence":"Conditional double-KO mouse neurons, pHluorin single-vesicle DCV exocytosis, cargo quantification, SNARE-domain-deleted rescue, and trans-Golgi/DCV morphometry","pmids":["37695731"],"confidence":"High","gaps":["Molecular partner mediating Golgi/cargo-packaging function unidentified","Reconciliation with prior invertebrate DCV-fusion data incomplete"]},{"year":2023,"claim":"Identified synaptotagmin-9 as an upstream partner stabilizing tomosyn-1 to enforce inhibition of insulin secretion, and reinforced TOM-1 isoform-specific roles in netrin/UNC-5-directed growth cone protrusion.","evidence":"Co-localization/co-IP, Syt9 KO/KD with tomosyn-1 rescue and SNARE/insulin assays; C. elegans tom-1;unc-5;unc-64 isoform-specific epistasis","pmids":["37432648","37014062"],"confidence":"Medium","gaps":["Single-lab studies","How Syt9 stabilizes tomosyn-1 against proteasomal degradation unresolved"]},{"year":2025,"claim":"Revealed a dual role for tomosyn-2 in beta-cells, coupling inhibition of insulin granule exocytosis with promotion of beta-cell proliferation via Akt1/cell-cycle signaling.","evidence":"Co-IP, tomosyn-2 KO mice, glucose tolerance/insulin secretion assays, transcriptomics, and Akt1/cyclinD1 signaling assays","pmids":["42008692"],"confidence":"Medium","gaps":["Single-lab study not independently replicated","Mechanistic link between tomosyn-2 and Akt1 signaling unresolved"]},{"year":null,"claim":"How tomosyn's canonical SNARE-clamp activity and its SNARE-independent functions (RhoA suppression, Golgi DCV cargo packaging) are integrated within a single protein, and what governs their tissue- and context-specific deployment, remains unresolved.","evidence":"Open question synthesized across the timeline","pmids":[],"confidence":"Medium","gaps":["No structure of the full-length protein or WD40 domain","Partner mediating Golgi cargo-packaging function unknown","Determinants of context-specific t-SNARE partner choice undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,3,11,19]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,1,2,19]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,2,19]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[11,21,30]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[12,19]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[1,12,21]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[34]}],"pathway":[{"term_id":"R-HSA-112316","term_label":"Neuronal System","supporting_discovery_ids":[5,7,16,24,25]},{"term_id":"R-HSA-109582","term_label":"Hemostasis","supporting_discovery_ids":[17,18]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,1,3,19,30]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[6,8,23,31,39]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[18,29]}],"complexes":["tomosyn–syntaxin-1–SNAP-25 SNARE complex","tomosyn–syntaxin-4–SNAP-23 complex","tomosyn–syntaxin-11–SNAP-23 complex"],"partners":["STX1A","SNAP25","STX4","SNAP23","STX11","RAB3A","HRD1","PIAS4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q5T5C0","full_name":"Syntaxin-binding protein 5","aliases":["Lethal(2) giant larvae protein homolog 3","Tomosyn-1"],"length_aa":1151,"mass_kda":127.6,"function":"Plays a regulatory role in calcium-dependent exocytosis and neurotransmitter release. Inhibits membrane fusion between transport vesicles and the plasma membrane. May modulate the assembly of trans-SNARE complexes between transport vesicles and the plasma membrane. Inhibits translocation of GLUT4 from intracellular vesicles to the plasma membrane. Competes with STXBP1 for STX1 binding (By similarity)","subcellular_location":"Cytoplasm; Cell membrane; Cytoplasmic vesicle membrane; Cytoplasmic vesicle, secretory vesicle, synaptic vesicle; Synapse","url":"https://www.uniprot.org/uniprotkb/Q5T5C0/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/STXBP5","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/STXBP5","total_profiled":1310},"omim":[{"mim_id":"604586","title":"SYNTAXIN-BINDING PROTEIN 5; STXBP5","url":"https://www.omim.org/entry/604586"},{"mim_id":"600568","title":"NEUROLIGIN 1; NLGN1","url":"https://www.omim.org/entry/600568"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Enhanced","locations":[{"location":"Cytosol","reliability":"Enhanced"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"parathyroid gland","ntpm":35.8}],"url":"https://www.proteinatlas.org/search/STXBP5"},"hgnc":{"alias_symbol":["tomosyn","LLGL3","tomosyn-1"],"prev_symbol":[]},"alphafold":{"accession":"Q5T5C0","domains":[{"cath_id":"2.130.10.10","chopping":"240-390","consensus_level":"medium","plddt":91.9604,"start":240,"end":390}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T5C0","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T5C0-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q5T5C0-F1-predicted_aligned_error_v6.png","plddt_mean":78.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=STXBP5","jax_strain_url":"https://www.jax.org/strain/search?query=STXBP5"},"sequence":{"accession":"Q5T5C0","fasta_url":"https://rest.uniprot.org/uniprotkb/Q5T5C0.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q5T5C0/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q5T5C0"}},"corpus_meta":[{"pmid":"9620695","id":"PMC_9620695","title":"Tomosyn: a syntaxin-1-binding protein that forms a novel complex in the neurotransmitter release process.","date":"1998","source":"Neuron","url":"https://pubmed.ncbi.nlm.nih.gov/9620695","citation_count":240,"is_preprint":false},{"pmid":"10402465","id":"PMC_10402465","title":"Yeast homologues of tomosyn and lethal giant larvae function in exocytosis and are associated with the plasma membrane SNARE, Sec9.","date":"1999","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/10402465","citation_count":184,"is_preprint":false},{"pmid":"16895441","id":"PMC_16895441","title":"Tomosyn inhibits synaptic vesicle priming in Caenorhabditis elegans.","date":"2006","source":"PLoS biology","url":"https://pubmed.ncbi.nlm.nih.gov/16895441","citation_count":130,"is_preprint":false},{"pmid":"12782620","id":"PMC_12782620","title":"The R-SNARE motif of tomosyn forms SNARE core complexes with syntaxin 1 and SNAP-25 and down-regulates exocytosis.","date":"2003","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12782620","citation_count":108,"is_preprint":false},{"pmid":"14983051","id":"PMC_14983051","title":"Tomosyn inhibits priming of large dense-core vesicles in a calcium-dependent manner.","date":"2004","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/14983051","citation_count":85,"is_preprint":false},{"pmid":"16186257","id":"PMC_16186257","title":"PKA-catalyzed phosphorylation of tomosyn and its implication in Ca2+-dependent exocytosis of neurotransmitter.","date":"2005","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/16186257","citation_count":83,"is_preprint":false},{"pmid":"15316007","id":"PMC_15316007","title":"Structural basis for the inhibitory role of tomosyn in exocytosis.","date":"2004","source":"The Journal of biological 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High-level expression of tomosyn in PC12 cells specifically reduced Ca2+-dependent exocytosis.\",\n      \"method\": \"Co-immunoprecipitation, biochemical fractionation, PC12 cell overexpression assay\",\n      \"journal\": \"Neuron\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, biochemical complex characterization, functional overexpression assay; foundational paper replicated extensively\",\n      \"pmids\": [\"9620695\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"The C-terminal R-SNARE motif of tomosyn forms genuine four-helical bundle SNARE core complexes with syntaxin-1 and SNAP-25, competing with synaptobrevin for binding to endogenous syntaxin/SNAP-25 on plasma membranes. Tomosyn-SNARE complexes are disassembled by NSF/α-SNAP ATPase activity. Overexpression in PC12 cells massively reduces exocytosis without altering individual fusion event parameters.\",\n      \"method\": \"In vitro reconstitution with recombinant proteins, CD spectroscopy, inside-out plasma membrane sheets competition assay, NSF disassembly assay, PC12 cell overexpression capacitance measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with multiple orthogonal biochemical methods plus functional cell assay; independently replicated\",\n      \"pmids\": [\"12782620\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crystal structure of the tomosyn R-SNARE core complex with syntaxin-1a and SNAP-25 resolved at 2.0 Å. The complex forms a four-helical bundle highly similar to the synaptobrevin-containing SNARE complex. Synaptobrevin cannot displace the tomosyn helix from the assembled complex (and vice versa), indicating both are thermodynamic end products. Complexin binding to the tomosyn complex is impaired due to surface differences.\",\n      \"method\": \"X-ray crystallography (2.0 Å), CD spectroscopy kinetics, displacement assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure with functional validation by kinetic and displacement assays\",\n      \"pmids\": [\"15316007\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Tomosyn inhibits the priming step of large dense-core vesicle exocytosis in adrenal chromaffin cells, reducing the number of fusion-competent vesicles by ~50% without affecting docked vesicle number or individual fusion kinetics. This inhibition is partially relieved at elevated calcium concentrations, indicating a calcium-dependent shift in release threshold.\",\n      \"method\": \"Capacitance measurements, amperometry, morphological analysis (electron microscopy), calcium ramp experiments in chromaffin cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal electrophysiological and morphological methods in a single rigorous study\",\n      \"pmids\": [\"14983051\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Tomosyn localizes at growth cone palms via binding to ROCK-phosphorylated syntaxin-1 (Rho/ROCK phosphorylates syntaxin-1, increasing its affinity for tomosyn), thereby inhibiting SNARE-mediated vesicle fusion at palm regions and promoting vesicle transport to leading edges to regulate neurite extension and retraction.\",\n      \"method\": \"Immunolocalization, co-immunoprecipitation, kinase assay (ROCK phosphorylation of syntaxin-1), neuronal overexpression/dominant-negative experiments\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP and kinase assay with functional localization data, single lab\",\n      \"pmids\": [\"15240567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Tomosyn is directly phosphorylated by PKA; this phosphorylation reduces its interaction with syntaxin-1, enhances SNARE complex formation, increases the readily releasable pool of synaptic vesicles, and thereby enhances neurotransmitter release. This mechanism underlies PACAP-induced facilitation of neurotransmitter release in SCG neurons.\",\n      \"method\": \"In vitro kinase assay, co-immunoprecipitation, electrophysiology in SCG neurons, pharmacological manipulation with PKA activators\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — in vitro kinase assay plus electrophysiological validation and pharmacological epistasis in neurons\",\n      \"pmids\": [\"16186257\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Tomosyn interacts with the t-SNAREs syntaxin-4 and SNAP-23 (adipocyte SNARE complex), forming a high-affinity ternary complex competitively inhibited by VAMP-2. The VAMP-like domain of tomosyn mediates the interaction with syntaxin-4. Overexpression of tomosyn in 3T3-L1 adipocytes inhibits insulin-stimulated GFP-GLUT4 translocation to the plasma membrane. Munc18c interacts with both syntaxin-4/tomosyn complexes and syntaxin-4-containing SNARE complexes.\",\n      \"method\": \"Yeast two-hybrid, in vitro binding assay, co-immunoprecipitation, GFP-GLUT4 translocation assay in adipocytes\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Y2H, in vitro binding, co-IP, live-cell imaging) in a single study\",\n      \"pmids\": [\"12832401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"C. elegans TOM-1 (tomosyn ortholog) negatively regulates synaptic vesicle priming in vivo. tom-1 mutants show increased primed vesicle numbers at the plasma membrane, enhanced evoked responses, and enhanced hyperosmotic responses. Epistasis with unc-13 priming-defective mutants shows TOM-1 acts as an endogenous inhibitor of the primed vesicle pool.\",\n      \"method\": \"Electrophysiology (evoked postsynaptic currents, hyperosmotic responses), electron microscopy ultrastructure, genetic epistasis (tom-1; unc-13 double mutants), neuronal rescue expression\",\n      \"journal\": \"PLoS biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple orthogonal in vivo methods; independently replicated in subsequent studies\",\n      \"pmids\": [\"16895441\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Tomosyn is expressed in pancreatic beta-cells; syntaxin-1 co-immunoprecipitates with tomosyn. Overexpression of m-tomosyn in mouse beta-cells significantly decreased exocytosis, while siRNA knockdown of tomosyn increased exocytosis, demonstrating that tomosyn negatively regulates insulin exocytosis.\",\n      \"method\": \"Co-immunoprecipitation, tomosyn overexpression in primary beta-cells, siRNA knockdown, exocytosis measurements\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus bidirectional functional manipulation (OE and KD) in a single lab\",\n      \"pmids\": [\"16505218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Tomosyn-1 localizes to compartments enriched in insulin granules in beta-cells. The SNARE-like domain of tomosyn-1 forms a complex with syntaxin-1 and SNAP25 with weaker binding forces than VAMP2 (237 vs. 279 pN by AFM). siRNA silencing of tomosyn-1 reduces stimulus-induced exocytosis without affecting the number of docked granules, indicating tomosyn-1 facilitates a post-docking event required for exocytosis.\",\n      \"method\": \"Atomic force microscopy binding force measurements, siRNA knockdown, electron microscopy (docked granule counting), stimulus-secretion assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel AFM method plus KD with defined morphological and secretory readouts, single lab\",\n      \"pmids\": [\"16787939\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"C. elegans TOM-1 negatively regulates dense-core vesicle (DCV) exocytosis. tom-1 mutants show 50% reduction in presynaptic DCVs corresponding to enhanced neuropeptide release; TOM-1 overexpression causes DCV accumulation. Genetic epistasis shows TOM-1 antagonizes UNC-31 (CAPS)-dependent DCV release; loss of TOM-1 suppresses unc-31 behavioral, electrophysiological, and ultrastructural phenotypes.\",\n      \"method\": \"Electron microscopy ultrastructure, electrophysiology, genetic epistasis (tom-1;unc-31 double mutants), neuropeptide release assays, behavioral analysis\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple orthogonal in vivo readouts, replicated findings\",\n      \"pmids\": [\"17881523\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The N-terminal WD40 repeat domain of tomosyn is required for its inhibitory activity and is sufficient to inhibit neurotransmitter release by catalyzing oligomerization of SNARE complexes. The C-terminal VAMP-like domain (VLD) inhibits SNARE complex formation by sequestering syntaxin-1. Together, these represent a dual inhibitory mechanism. Microinjection of the isolated N-terminal WD40 domain into neurons prevented stimulated acetylcholine release.\",\n      \"method\": \"Microinjection of domain-truncation constructs into neurons, co-immunoprecipitation to assess SNARE complex oligomerization, tomosyn-KO mouse analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct microinjection functional assay with domain dissection and biochemical validation; combined mechanistic approach in single paper\",\n      \"pmids\": [\"18936251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Secretagogue stimulation causes rapid translocation of tomosyn from cytosol to plasma membrane regions in chromaffin cells, associated with increased tomosyn–syntaxin-1A interaction and increased cycling of tomosyn into SNARE complexes. This translocation is strongly reduced by ROCK inhibition, consistent with RhoA-mediated regulation. LPA (a RhoA activator) mimics secretagogue-induced tomosyn–syntaxin interaction.\",\n      \"method\": \"Live-cell optical imaging (FRET/translocation assay), pharmacological ROCK inhibition, LPA stimulation, co-immunoprecipitation, PC12 secretory assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live imaging plus biochemical and pharmacological validation, single lab\",\n      \"pmids\": [\"17545156\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Tomosyn's inhibition of exocytosis requires the integrity of its N-terminal WD40 domain; a tomosyn mutant lacking the entire SNARE domain inhibits vesicle priming as potently as full-length protein, while the isolated SNARE domain fails to inhibit exocytosis. An N-terminally truncated mutant that retains SNARE-domain syntaxin binding does not inhibit exocytosis. Both the WD40 repeats and linker are required for inhibitory function.\",\n      \"method\": \"Domain-deletion mutant overexpression in chromaffin cells, capacitance measurements, co-immunoprecipitation\",\n      \"journal\": \"Journal of neurochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain deletion mapping with electrophysiological secretion readout, single lab\",\n      \"pmids\": [\"17666050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Positional cloning and functional characterization identified tomosyn-2 (STXBP5L) as a negative regulator of insulin secretion. Tomosyn-2 binds syntaxin-1A and syntaxin-4 in vitro; overexpression in INS1 cells inhibits insulin secretion. The BTBR allele of tomosyn-2 resists proteasomal degradation compared to the B6 allele, establishing a functional consequence of a coding SNP.\",\n      \"method\": \"In vitro binding assay with recombinant proteins, INS1 cell overexpression secretion assay, proteasomal degradation assay, sub-congenic mouse islet secretion phenotyping\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding plus cell secretion assay plus in vivo mouse islet phenotyping with allelic degradation analysis; multiple orthogonal methods\",\n      \"pmids\": [\"21998599\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Structural analysis by homology modeling (based on yeast Sro7 crystal structure) reveals that deletion of loops 1 and 3 from the β-propeller core of tomosyn eliminates its inhibitory activity on secretion without altering SNARE pairing with syntaxin-1A. Deletion of loop 2 (hypervariable splice region) does not reduce inhibition but affects protein accumulation of tomosyn-2 isoforms. m-Tomosyn-1 is a substrate for SUMO-2/3 conjugation at K730; mutation of this site enhances secretion inhibition without altering syntaxin-1A binding.\",\n      \"method\": \"Homology modeling, deletion mutagenesis, PC12 cell secretion assay, co-immunoprecipitation, SUMO conjugation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — structure-guided mutagenesis with functional secretion assay and biochemical validation, single lab\",\n      \"pmids\": [\"21330375\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In C. elegans, Tomosyn inhibits slow neurotransmitter release while UNC-13L mediates fast release. Tomosyn's spatial position (diffuse/distal from dense projection) correlates with its control over slow release. Genetic analysis places tomosyn as a negative regulator in a molecular code (UNC-13L, UNC-13S, and Tomosyn) that dictates the timing of neurotransmitter release.\",\n      \"method\": \"Genetic epistasis (multiple mutant combinations), electrophysiology, fluorescence imaging of protein localization\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic epistasis with multiple alleles plus electrophysiology plus localization, replicated across experiments\",\n      \"pmids\": [\"23951547\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STXBP5 is expressed in human endothelial cells and colocalizes with and binds syntaxin-4. Knockdown of STXBP5 in endothelial cells increases exocytosis of vWF and P-selectin. Stxbp5 KO mice have higher plasma vWF, increased P-selectin translocation, and more platelet-endothelial interactions, demonstrating that STXBP5 inhibits endothelial exocytosis. Stxbp5 KO mice also have defects in platelet secretion and hemostasis (prolonged bleeding times, impaired thrombosis).\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown (endothelial cells), Stxbp5 KO mouse model, plasma vWF ELISA, P-selectin translocation FACS, tail bleeding and mesenteric/carotid thrombosis assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP plus bidirectional loss-of-function (siRNA + KO mouse) with multiple orthogonal in vivo and in vitro readouts\",\n      \"pmids\": [\"25244095\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STXBP5 was identified by mass spectrometry from SNARE-containing affinity purifications from human platelet extracts. STXBP5 interacts with syntaxin-11/SNAP23 heterodimers by co-immunoprecipitation and also associates with the platelet cytoskeleton. Stxbp5 KO platelets have markedly defective stimulation-dependent secretion from all three granule types (dense, alpha, lysosomal) and altered granule cargo levels despite normal granule numbers and morphology.\",\n      \"method\": \"Mass spectrometry, co-immunoprecipitation, fractionation, Stxbp5 KO mouse model, lumi-aggregometry, FACS (P-selectin/LAMP-1), bone marrow transplantation\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — MS identification plus reciprocal co-IP plus KO mouse with multiple platelet function readouts and BM transplant epistasis\",\n      \"pmids\": [\"25244094\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The C-terminal domain (CTD) of tomosyn, containing the R-SNARE-like motif, mediates inhibition of SNARE-dependent membrane fusion by recognizing the t-SNARE complex and preventing pairing with the v-SNARE, arresting the fusion reaction at a pre-docking stage. The N-terminal domain (NTD) is critical (but not sufficient) for tomosyn recruitment to fusion sites via syntaxin monomer binding. Tomosyn inhibitory activity is dominant over the stimulatory Sec1/Munc18 protein in fusion.\",\n      \"method\": \"In vitro reconstituted SNARE-dependent liposome fusion assay with purified full-length tomosyn and domain truncations, co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified full-length protein and domain truncations, multiple defined fusion assays; single lab\",\n      \"pmids\": [\"25063806\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In vitro, tomosyn-2 is phosphorylated in response to glucose, phorbol esters, and cAMP analogs; 11 phosphorylation sites were identified by mass spectrometry. Phosphomimetic (Ser→Asp) tomosyn-2 shows enhanced proteasomal turnover and reduced ability to inhibit insulin secretion. Tomosyn-2 is ubiquitinated by the E3 ligase Hrd-1; Hrd-1 knockdown increases tomosyn-2 abundance, identifying a phosphorylation-dependent proteasomal degradation mechanism for de-repression of insulin secretion.\",\n      \"method\": \"32P labeling, mass spectrometry phosphosite identification, site-directed mutagenesis, proteasome inhibitor experiments, proteomic screen for binding partners, co-immunoprecipitation, shRNA knockdown, ubiquitination assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — mass spectrometry plus mutagenesis plus E3 ligase identification with functional secretion readout; multiple orthogonal methods in single study\",\n      \"pmids\": [\"25002582\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"By dSTORM super-resolution imaging, tomosyn is organized in small clusters adjacent to syntaxin clusters on the plasma membrane. Tomosyn forms both binary (tomosyn–syntaxin) and ternary (tomosyn–syntaxin–SNAP25) complexes at the PM. Deletion of β-propeller core residues 537–578 or 897–917 reduces SNAP25 binding and PM cluster residence time, shifting equilibrium toward binary tomosyn–syntaxin complexes and reducing inhibition of exocytosis, indicating tomosyn inhibits exocytosis via the ternary complex.\",\n      \"method\": \"dSTORM super-resolution microscopy, deletion mutagenesis, co-immunoprecipitation, fluorescence recovery assays, exocytosis assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — novel super-resolution imaging with domain mutagenesis and functional secretion readout, single lab\",\n      \"pmids\": [\"24782308\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"STXBP5 knockdown in vascular endothelial cells decreased tPA release, functionally linking STXBP5 to regulation of tPA exocytosis from endothelium.\",\n      \"method\": \"siRNA knockdown in vascular endothelial cells, tPA release assay\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single siRNA knockdown experiment with secretion assay, single lab, but consistent with STXBP5's established role in endothelial exocytosis\",\n      \"pmids\": [\"24578379\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Tomosyn is phosphorylated at Ser-783 by both Akt1 and Akt2; this phosphorylation inhibits tomosyn's interaction with syntaxin-4, as shown by in vitro pull-down. Expression of phosphorylation-deficient (S783A) tomosyn attenuates insulin-stimulated GLUT4 surface expression, suggesting that Akt-mediated phosphorylation of tomosyn relieves its inhibition of GLUT4 exocytosis.\",\n      \"method\": \"In vitro kinase assay (Akt1/Akt2), in vitro pulldown assay, intact cell 32P labeling with PI3K inhibitor, GLUT4 cell-surface assay with mutant overexpression\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro kinase assay plus cell functional assay with phospho-mutant, single lab\",\n      \"pmids\": [\"25725259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Tomosyn knockdown at hippocampal mossy fiber–CA3 synapses (via combined lentiviral KD and optogenetic activation) impairs synaptic facilitation, PKA-dependent long-term potentiation, and PKA-induced potentiation. This establishes tomosyn as a key regulator of mossy fiber presynaptic plasticity.\",\n      \"method\": \"Combined KD-optogenetic strategy in hippocampal slices, electrophysiology (facilitation, LTP, PKA-induced potentiation)\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — selective presynaptic KD plus optogenetic activation with multiple electrophysiological plasticity readouts, single lab\",\n      \"pmids\": [\"26166572\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Tomo1 regulates synaptic vesicle pool partitioning at rat hippocampal synapses in an activity-dependent manner. Tomo1 knockdown facilitates release from the Readily Releasable Pool (RRP) and alters Total Recycling Pool and Resting Pool distribution. These effects are regulated by Cdk5-dependent phosphorylation of Tomo1. Tomo1 interacts with GTP-bound Rab3A and, via Rab3A, with Synapsin 1a/b.\",\n      \"method\": \"VGlut1-pHluorin fluorescence SV pool assay, KD and rescue, co-immunoprecipitation (Tomo1–Rab3A-GTP, Tomo1–Synapsin), Cdk5 phosphorylation assay, chronic activity manipulation\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple co-IP interactions plus functional SV pool assay with kinase regulation, single lab\",\n      \"pmids\": [\"27807164\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Tomosyn negatively regulates SNARE complex formation in a manner requiring its SUMOylation at K298. Glucose-dependent de-SUMOylation of tomosyn-1 releases syntaxin-1A. Tomosyn-1 interacts with secretagogin, a Ca2+-binding protein that dissociates from tomosyn-1 in response to Ca2+-raising stimuli and is required for insulin granule trafficking and exocytosis. Together, SUMOylation and Ca2+-dependent secretagogin release coordinate amplification of insulin secretion.\",\n      \"method\": \"Co-immunoprecipitation, site-directed mutagenesis (K298), SUMO modification assay, secretagogin binding assay, human beta-cell exocytosis measurements\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis of SUMO site plus co-IP plus functional exocytosis assay; single lab, multiple orthogonal methods\",\n      \"pmids\": [\"28325894\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"UNC-18(P334A) gain-of-function in C. elegans partially bypasses UNC-13 requirement for synaptic vesicle fusion and shows synergistic suppression with tom-1 null mutation, placing tomosyn/TOM-1 and Munc18/UNC-18 as antagonistic regulators of SNARE complex assembly downstream of Munc13/UNC-13. Biochemically, Munc18-1(P335A) shows enhanced SNARE complex formation and partial bypass of Munc13-1 requirement in liposome fusion assays.\",\n      \"method\": \"Genetic epistasis (unc-18; tom-1; unc-13 multiple mutants), electrophysiology, liposome fusion assay, co-immunoprecipitation (SNARE complex formation)\",\n      \"journal\": \"The Journal of neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo genetic epistasis plus in vitro biochemical/fusion assay, multiple orthogonal methods\",\n      \"pmids\": [\"28821673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Tomosyn-1 is ubiquitinated in hippocampal neurons and undergoes HRD1-dependent proteasomal degradation. Immunoprecipitation of Tomo-1 from neurons co-precipitates HRD1, and in vitro reactions show direct, HRD1 concentration-dependent Tomo-1 ubiquitination. HRD1 knockdown increases Tomo-1 levels and dendritic spine density; Tomo-1 co-knockdown reverses this effect, establishing a direct HRD1→Tomo-1 effector relationship.\",\n      \"method\": \"Immunoprecipitation (ubiquitin/HRD1), in vitro ubiquitination assay, proteasome inhibitor treatment, shRNA KD (HRD1 and Tomo-1), dendritic spine density quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro ubiquitination assay plus co-IP plus bidirectional genetic manipulation with structural readout, single lab\",\n      \"pmids\": [\"29269412\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Tomosyn-1 (STXBP5) acts as an inhibitory fusion clamp in mast cell degranulation downstream of FcεRI. After activation, tomosyn-1 is phosphorylated on serine and threonine residues, dissociates from syntaxin-4 (STX4), and re-associates with syntaxin-3 (STX3). PKCδ is the major kinase required for tomosyn-1 threonine phosphorylation and for regulating the switch between STX partners.\",\n      \"method\": \"Co-immunoprecipitation (tomosyn-STX4/STX3 interactions), phosphorylation assays, PKCδ inhibitor/knockout studies, FcεRI-stimulated degranulation assay, patient basophil analysis\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP plus kinase identification with pharmacological and genetic manipulation; single lab\",\n      \"pmids\": [\"29970602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NSF/α-SNAP disassembles the tomosyn-SNARE arrest complex, allowing syntaxin-1 to enter the Munc18-1/syntaxin-1 complex. Munc13-1 then catalyzes transfer of syntaxin-1 from the Munc18-1/syntaxin-1 complex into the SNARE complex in a manner specific to synaptobrevin-2 but resistant to tomosyn. This establishes a sequential pathway: tomosyn arrest → NSF/α-SNAP release → Munc18-1 scaffolding → Munc13-1 SNARE assembly.\",\n      \"method\": \"In vitro biochemical reconstitution with purified proteins (NSF/α-SNAP disassembly assay, Munc18-1/Munc13-1 SNARE assembly assay), co-immunoprecipitation\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with purified proteins identifying sequential mechanism, multiple biochemical assays\",\n      \"pmids\": [\"29485200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CRISPR-Cas9 double knockout of both tomosyn-encoding genes in adipocytes markedly elevated both basal and insulin-stimulated GLUT4 exocytosis without affecting adipocyte differentiation or insulin signaling. In reconstituted liposome fusion, tomosyn inhibited all SNARE complexes underlying GLUT4 exocytosis, and this inhibition was relieved by NSF/α-SNAP, which removes tomosyn from GLUT4 exocytic SNAREs.\",\n      \"method\": \"CRISPR-Cas9 double KO in adipocytes, GLUT4 exocytosis assay, in vitro liposome fusion reconstitution with purified tomosyn, NSF/α-SNAP disassembly assay\",\n      \"journal\": \"Traffic\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — CRISPR KO genetic evidence plus in vitro reconstitution with mechanistic NSF/α-SNAP rescue, multiple orthogonal methods\",\n      \"pmids\": [\"32851733\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Tomosyn knockdown in mouse primary neurons increases RhoA GTPase activity, leading to compromised dendritic arborization, loss of dendritic spines, decreased surface AMPA receptor expression, and reduced mEPSC frequency. Inhibiting RhoA signaling rescues the morphological and receptor surface expression defects. The N-terminal WD40 domain mediates tomosyn's suppression of RhoA activity. Two ASD-associated missense variants in the WD40 domain show loss-of-function for these postsynaptic phenotypes.\",\n      \"method\": \"shRNA knockdown in primary neurons, RhoA GTPase activity assay, dendritic spine/arborization morphometry, AMPA receptor surface expression (immunostaining/biotinylation), mEPSC recordings, domain truncation and ASD variant rescue experiments\",\n      \"journal\": \"Journal of neuroscience research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KD with multiple readouts plus domain and variant rescue, single lab; novel non-SNARE mechanism\",\n      \"pmids\": [\"32133675\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In Drosophila, Tomosyn acts as a decoy SNARE that sets release probability (Pr) and tonic vs. phasic release properties. Tomosyn is differentially expressed between tonic (Ib, high Tomosyn, low Pr) and phasic (Is, low Tomosyn, high Pr) motoneurons. Loss of Tomosyn impairs synaptic facilitation, LTP, and presynaptic homeostatic potentiation specifically at tonic synapses.\",\n      \"method\": \"Genetic loss-of-function in Drosophila motoneurons, electrophysiology (evoked responses, facilitation, homeostatic plasticity), immunofluorescence protein quantification\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic LOF with multiple electrophysiological plasticity readouts in Drosophila, single lab\",\n      \"pmids\": [\"34713802\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In tomosyn/tomosyn-2 conditional double-knockout mouse neurons, DCV exocytosis frequency was not significantly affected; however, intracellular levels of DCV cargos (NPY, BDNF) were strongly reduced. BDNF levels were restored by re-expression of tomosyn but not by lysosomal protease inhibition, and tomosyn's SNARE domain was dispensable for rescue. Trans-Golgi network and DCV size were decreased, and DCV cargo flux through Golgi was accelerated in KO neurons, suggesting tomosyns function in DCV biogenesis/cargo packaging at the Golgi rather than DCV fusion.\",\n      \"method\": \"Conditional double-KO mouse neurons, pHluorin-based single-vesicle DCV exocytosis assay, cargo level quantification (NPY, BDNF), tomosyn re-expression rescue (full-length and SNARE-domain-deleted), trans-Golgi network morphometry, DCV size analysis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — conditional DKO with single-vesicle resolution assay plus domain rescue experiments and multiple orthogonal morphological readouts; challenges prior model, rigorous controls\",\n      \"pmids\": [\"37695731\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"C. elegans TOM-1 short isoform acts downstream of the UNC-5 netrin receptor to inhibit growth cone protrusion, and this mechanism requires syntaxin/UNC-64, consistent with TOM-1 inhibiting vesicle fusion needed for membrane addition during protrusion. The long TOM-1 isoform has a pro-protrusive role.\",\n      \"method\": \"Genetic epistasis in C. elegans (tom-1; unc-5; unc-64 mutant analysis), growth cone protrusion quantification, isoform-specific rescue experiments\",\n      \"journal\": \"Development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with isoform-specific rescue in defined cellular context, single lab\",\n      \"pmids\": [\"37014062\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Tomosyn-1 interacts with the SUMO E3 ligase PIASγ (PIAS4). The interaction involves the C-terminus of tomosyn-1 and the N-terminus of PIASγ, confirmed by yeast two-hybrid and bidirectional immunoprecipitation in HEK293T cells. Tomosyn-1 is preferentially modified by SUMO-2/3.\",\n      \"method\": \"Yeast two-hybrid, bidirectional co-immunoprecipitation in HEK293T cells, SUMO modification assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP plus Y2H with SUMO modification assay, single lab\",\n      \"pmids\": [\"24614299\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Synaptotagmin-9 (Syt9) colocalizes and binds tomosyn-1 and syntaxin-1A (Stx1A) in pancreatic beta-cells; the Syt9–tomosyn-1–Stx1A complex is inhibitory for insulin secretion. Syt9 knockdown reduces tomosyn-1 protein abundance via proteasomal degradation, decreases tomosyn-1/Stx1A interaction, and increases SNARE complex formation and insulin secretion. Rescuing tomosyn-1 blocks the Syt9-knockdown-mediated increases in insulin secretion, establishing that Syt9's inhibitory effects on insulin secretion are mediated through tomosyn-1.\",\n      \"method\": \"Co-localization, co-immunoprecipitation, Syt9 KO/KD (mouse and cell), tomosyn-1 rescue expression, SNARE complex assay, insulin secretion assay\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus bidirectional genetic manipulation (KO mouse + KD) with epistatic rescue, single lab\",\n      \"pmids\": [\"37432648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"STXBP5/tomosyn-1 KO mice show increased KCl-evoked glutamate release in the hippocampal dentate gyrus and accelerated kindling progression (fewer stimuli required to reach fully kindled state), establishing STXBP5 as an endogenous brake on glutamate release that opposes epileptiform activity.\",\n      \"method\": \"Glutamate-selective microelectrode array (MEA) in vivo measurements, amygdala kindling stimulation, STXBP5 KO mouse model\",\n      \"journal\": \"Brain and behavior\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO mouse with in vivo microelectrode glutamate measurements and behavioral kindling assay, single lab\",\n      \"pmids\": [\"28948088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Tomosyn-2 interacts with syntaxin-1A to inhibit insulin granule exocytosis by limiting SNARE complex formation in pancreatic beta-cells. Tomosyn-2 KO mice show improved glucose clearance and enhanced biphasic insulin secretion. Loss of tomosyn-2 also reduces beta-cell proliferation via downregulation of Akt1 signaling and cell-cycle mediators, identifying a dual role in insulin secretion and beta-cell maturation.\",\n      \"method\": \"Co-immunoprecipitation (tomosyn-2/Stx1A), tomosyn-2 KO mouse model, glucose tolerance tests, insulin secretion assay from isolated islets, transcriptomic analysis, Akt1/cyclinD1 signaling assay\",\n      \"journal\": \"Diabetes\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus KO mouse with multiple secretion and signaling readouts; single lab, not yet independently replicated\",\n      \"pmids\": [\"42008692\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"STXBP5 (tomosyn-1) is a soluble decoy R-SNARE protein that inhibits exocytosis primarily by: (1) sequestering syntaxin-1 (and other t-SNAREs) in a non-fusogenic four-helical bundle SNARE complex via its C-terminal R-SNARE domain, competing with synaptobrevin/VAMP and arresting membrane fusion; (2) catalyzing oligomerization of SNARE complexes via its N-terminal WD40 β-propeller domain; and (3) regulating synaptic vesicle pool partitioning through Rab3A-GTP and synapsin interactions; this inhibitory activity is dynamically regulated by PKA-, Akt-, Cdk5-, and PKCδ-dependent phosphorylation and by SUMO-2/3 modification at K730 (and K298 in beta-cells), all of which relieve syntaxin binding and promote secretion, while HRD1-dependent ubiquitin-proteasomal degradation controls tomosyn abundance; in addition to its canonical presynaptic exocytic role, tomosyn suppresses RhoA activity via its WD40 domain to maintain dendritic spine stability and AMPA receptor surface expression, and may also function in DCV cargo packaging at the Golgi independently of its SNARE domain.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"STXBP5 (tomosyn-1) is a soluble inhibitory regulator of SNARE-mediated membrane fusion that acts as a presynaptic and secretory brake across neurons, neuroendocrine cells, beta-cells, platelets, endothelium, and immune cells [#0, #1, #7]. It was first identified as a syntaxin-1-binding protein that displaces Munc18 and assembles a non-fusogenic complex with syntaxin-1 and SNAP-25, reducing Ca2+-dependent exocytosis [#0]. Its C-terminal R-SNARE (VAMP-like) motif forms a genuine four-helical bundle SNARE core complex with syntaxin-1 and SNAP-25 that is structurally homologous to the synaptobrevin complex, competing with synaptobrevin/VAMP for t-SNARE binding to arrest fusion as a thermodynamic end product [#1, #2, #19]. A parallel and dominant inhibitory mechanism resides in the N-terminal WD40 β-propeller domain, which is required and sufficient to inhibit release by catalyzing oligomerization of SNARE complexes, and acts through assembly of the ternary tomosyn–syntaxin–SNAP25 complex at the plasma membrane [#11, #13, #21]. Functionally, tomosyn inhibits the priming/post-docking step rather than vesicle docking, limiting the readily releasable and primed vesicle pools without altering individual fusion kinetics [#3, #7, #10]. The tomosyn-SNARE arrest is reversed by NSF/α-SNAP disassembly, after which Munc18-1 scaffolding and Munc13-1 catalysis assemble fusogenic synaptobrevin-containing complexes, defining tomosyn and Munc18/Munc13 as antagonistic regulators of SNARE assembly [#27, #30]. This inhibitory activity is dynamically de-repressed by phosphorylation via PKA, Akt, Cdk5, and PKCδ and by SUMO-2/3 modification, each of which relieves syntaxin binding to promote secretion, while phosphorylation-dependent HRD1-mediated ubiquitin-proteasomal degradation controls tomosyn abundance [#5, #23, #25, #26, #29, #20, #28]. Tomosyn engages tissue-specific t-SNAREs—syntaxin-4/SNAP-23 in adipocytes to inhibit insulin-stimulated GLUT4 translocation, syntaxin-11/SNAP-23 in platelet granule secretion, and syntaxin-4 in endothelial Weibel-Palade body exocytosis—and Stxbp5 knockout impairs platelet secretion, hemostasis, and thrombosis [#6, #18, #17, #31]. Beyond its canonical exocytic role, tomosyn suppresses RhoA GTPase activity via its WD40 domain to maintain dendritic arborization, spine stability, and surface AMPA receptor expression, with ASD-associated WD40 variants showing loss of this function [#32], and its SNARE-independent function in dense-core vesicle cargo packaging at the Golgi controls intracellular DCV cargo levels [#34].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established tomosyn as a syntaxin-1-binding protein, answering how an exocytic inhibitor could intercept the core fusion machinery by displacing Munc18 and forming an alternative syntaxin complex.\",\n      \"evidence\": \"Co-immunoprecipitation, biochemical fractionation, and overexpression-based exocytosis assay in PC12 cells\",\n      \"pmids\": [\"9620695\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve which tomosyn domain mediates inhibition\", \"Mechanism of fusion arrest at the molecular level unresolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Defined the molecular basis of inhibition by showing the C-terminal R-SNARE motif forms a bona fide four-helical SNARE complex that competes with synaptobrevin and is reversible by NSF/α-SNAP.\",\n      \"evidence\": \"In vitro reconstitution, CD spectroscopy, plasma membrane sheet competition, NSF disassembly assay, and PC12 capacitance measurements\",\n      \"pmids\": [\"12782620\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the contribution of the N-terminal domain\", \"Step in the secretory pathway inhibited not yet mapped\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Resolved the atomic structure of the tomosyn SNARE complex and showed both tomosyn and synaptobrevin complexes are mutually exclusive thermodynamic end products with impaired complexin binding.\",\n      \"evidence\": \"2.0 Å X-ray crystallography with CD kinetics and displacement assays\",\n      \"pmids\": [\"15316007\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the WD40 domain not determined\", \"Did not address how the arrested complex is recycled in vivo\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Localized tomosyn's action to the vesicle priming step, showing it reduces fusion-competent vesicles in a calcium-tunable manner without affecting docking.\",\n      \"evidence\": \"Capacitance, amperometry, EM, and calcium ramp experiments in chromaffin cells\",\n      \"pmids\": [\"14983051\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of calcium-dependent relief of inhibition unresolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Extended tomosyn's t-SNARE repertoire beyond neurons by showing it binds syntaxin-4/SNAP-23 and inhibits insulin-stimulated GLUT4 translocation in adipocytes.\",\n      \"evidence\": \"Yeast two-hybrid, in vitro binding, co-IP, and GFP-GLUT4 translocation imaging in 3T3-L1 adipocytes\",\n      \"pmids\": [\"12832401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous loss-of-function effect not tested in this study\", \"How Munc18c integrates with the tomosyn complex unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Provided in vivo genetic proof that tomosyn (C. elegans TOM-1) is an endogenous negative regulator of synaptic vesicle priming, complementing prior overexpression data.\",\n      \"evidence\": \"Electrophysiology, EM ultrastructure, and tom-1;unc-13 genetic epistasis in C. elegans\",\n      \"pmids\": [\"16895441\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Domain requirements not dissected genetically here\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Identified PKA phosphorylation as a physiological switch that relieves syntaxin binding to enhance the releasable pool and neurotransmitter release, linking tomosyn to PACAP-mediated facilitation.\",\n      \"evidence\": \"In vitro kinase assay, co-IP, electrophysiology, and PKA pharmacology in SCG neurons\",\n      \"pmids\": [\"16186257\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Phosphorylation sites not mapped in this study\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Resolved that tomosyn uses a dual inhibitory mechanism: the WD40 domain catalyzes SNARE complex oligomerization while the VAMP-like domain sequesters syntaxin, with the WD40 domain alone sufficient to inhibit release.\",\n      \"evidence\": \"Neuronal microinjection of domain truncations, co-IP oligomerization assay, and tomosyn-KO mouse analysis; corroborated by chromaffin cell domain-deletion capacitance mapping\",\n      \"pmids\": [\"18936251\", \"17666050\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of WD40-driven oligomerization unresolved\", \"How the two mechanisms are coordinated temporally unknown\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showed tomosyn inhibition extends to dense-core vesicle/neuropeptide release and is dynamically recruited to the membrane by RhoA/ROCK-dependent translocation upon stimulation.\",\n      \"evidence\": \"tom-1;unc-31 genetic epistasis with EM and neuropeptide assays; FRET/translocation imaging with ROCK inhibition and LPA in chromaffin/PC12 cells\",\n      \"pmids\": [\"17881523\", \"17545156\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct ROCK substrate site on syntaxin/tomosyn not fully defined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated tomosyn negatively regulates insulin granule exocytosis at a post-docking step in beta-cells, generalizing the brake function to endocrine secretion.\",\n      \"evidence\": \"Co-IP, bidirectional overexpression/siRNA, AFM binding-force measurement, and EM docked-granule counting in beta-cells\",\n      \"pmids\": [\"16505218\", \"16787939\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab functional data\", \"Physiological regulation of beta-cell tomosyn not addressed here\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identified the paralog tomosyn-2 (STXBP5L) as a genetically validated negative regulator of insulin secretion whose abundance is set by allele-specific proteasomal degradation.\",\n      \"evidence\": \"Positional cloning, in vitro binding, INS1 secretion assay, proteasomal degradation assay, and sub-congenic mouse islet phenotyping\",\n      \"pmids\": [\"21998599\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Degradation machinery not identified in this study\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Mapped β-propeller loops required for inhibition independent of syntaxin pairing and identified SUMO-2/3 conjugation at K730 as a modification that tunes inhibitory strength.\",\n      \"evidence\": \"Homology modeling, deletion mutagenesis, PC12 secretion assay, and SUMO conjugation assay\",\n      \"pmids\": [\"21330375\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimental structure of the propeller\", \"SUMO E3 ligase not identified here\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Positioned tomosyn within a molecular code that dictates the timing of neurotransmitter release, inhibiting slow release in a position-dependent manner relative to UNC-13 isoforms.\",\n      \"evidence\": \"Multi-allele genetic epistasis, electrophysiology, and localization imaging in C. elegans\",\n      \"pmids\": [\"23951547\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular determinant of spatial positioning unknown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrated a physiological role in vascular and hemostatic exocytosis, showing STXBP5 brakes endothelial vWF/P-selectin/tPA release and platelet granule secretion via syntaxin-4 and syntaxin-11/SNAP-23.\",\n      \"evidence\": \"Co-IP, siRNA, Stxbp5 KO mice, plasma vWF/P-selectin/tPA assays, bleeding/thrombosis assays, and platelet MS/aggregometry\",\n      \"pmids\": [\"25244095\", \"25244094\", \"24578379\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why platelet KO impairs rather than enhances secretion (vs. endothelium) not fully reconciled\", \"Cytoskeletal association role unresolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Reconstituted full-length tomosyn inhibition of liposome fusion and resolved the spatial nanoscale organization of tomosyn–syntaxin–SNAP25 ternary complexes that mediate inhibition.\",\n      \"evidence\": \"In vitro liposome fusion with purified protein and truncations; dSTORM super-resolution with deletion mutagenesis and exocytosis assays\",\n      \"pmids\": [\"25063806\", \"24782308\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative in vivo contribution of binary vs. ternary complexes uncertain\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Defined the phosphorylation-dependent degradation circuit for tomosyn-2, identifying HRD1 as the E3 ligase and Akt as a kinase that de-represses secretion by abrogating syntaxin-4 binding.\",\n      \"evidence\": \"32P labeling, MS phosphosite mapping, mutagenesis, ubiquitination/proteasome assays, and GLUT4/insulin secretion readouts\",\n      \"pmids\": [\"25002582\", \"25725259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether neuronal tomosyn-1 uses the same HRD1 pathway not shown here\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified the SUMO E3 ligase PIASγ (PIAS4) as a direct tomosyn-1 partner mediating SUMO-2/3 modification, providing the enzymatic basis for SUMO regulation.\",\n      \"evidence\": \"Yeast two-hybrid, bidirectional co-IP in HEK293T, and SUMO modification assay\",\n      \"pmids\": [\"24614299\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence on secretion not tested in this study\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Established tomosyn as a regulator of presynaptic plasticity and synaptic vesicle pool partitioning, acting through Cdk5 phosphorylation and Rab3A-GTP/synapsin interactions.\",\n      \"evidence\": \"KD-optogenetics in hippocampal slices with plasticity electrophysiology; VGlut1-pHluorin pool assay with co-IP and Cdk5 phosphorylation assay\",\n      \"pmids\": [\"26166572\", \"27807164\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab studies\", \"Direct link between pool partitioning and SNARE arrest not fully defined\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed STXBP5 is an endogenous brake on hippocampal glutamate release that opposes epileptogenesis in vivo.\",\n      \"evidence\": \"In vivo glutamate microelectrode arrays and amygdala kindling in STXBP5 KO mice\",\n      \"pmids\": [\"28948088\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link to seizure circuits beyond glutamate release not established\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Resolved the sequential SNARE-assembly pathway placing tomosyn arrest upstream of NSF/α-SNAP release, Munc18-1 scaffolding, and Munc13-1 catalysis, and genetically defined tomosyn and Munc18 as antagonists.\",\n      \"evidence\": \"In vitro reconstitution with purified NSF/α-SNAP, Munc18-1/Munc13-1; unc-18;tom-1;unc-13 genetic epistasis with electrophysiology and liposome fusion\",\n      \"pmids\": [\"29485200\", \"28821673\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetics of in vivo handoff between tomosyn release and fusogenic assembly not quantified\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined glucose-regulated SUMOylation at K298 and a secretagogin/Ca2+ axis as coordinated inputs that release syntaxin-1A to amplify insulin secretion.\",\n      \"evidence\": \"K298 mutagenesis, SUMO and secretagogin binding assays, and human beta-cell exocytosis measurements\",\n      \"pmids\": [\"28325894\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab data\", \"Interplay between K298 and K730 SUMO sites unresolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated HRD1-dependent ubiquitin-proteasomal degradation of neuronal tomosyn-1 controls its abundance and downstream dendritic spine density.\",\n      \"evidence\": \"Ubiquitin/HRD1 co-IP, in vitro ubiquitination, proteasome inhibition, and bidirectional shRNA with spine quantification in hippocampal neurons\",\n      \"pmids\": [\"29269412\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Signal triggering HRD1-tomosyn engagement in neurons unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Extended the inhibitory clamp model to immune secretion, showing PKCδ-driven phosphorylation switches tomosyn-1 between syntaxin-4 and syntaxin-3 to permit FcεRI-triggered mast cell degranulation.\",\n      \"evidence\": \"Reciprocal co-IP, phosphorylation assays, PKCδ inhibitor/knockout, degranulation assay, and patient basophil analysis\",\n      \"pmids\": [\"29970602\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Phosphosites mediating the syntaxin partner switch not mapped\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Provided genetic and biochemical confirmation that tomosyn restrains GLUT4 exocytosis by inhibiting all relevant SNARE complexes, with NSF/α-SNAP relieving the arrest.\",\n      \"evidence\": \"CRISPR-Cas9 double KO in adipocytes with GLUT4 exocytosis and in vitro liposome fusion plus NSF/α-SNAP disassembly\",\n      \"pmids\": [\"32851733\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological trigger for NSF/α-SNAP-mediated tomosyn removal in adipocytes not defined\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Uncovered a SNARE-independent postsynaptic function: tomosyn suppresses RhoA via its WD40 domain to maintain dendrites, spines, and surface AMPA receptors, with ASD variants causing loss of function.\",\n      \"evidence\": \"shRNA KD in primary neurons, RhoA activity assay, morphometry, AMPAR surface assays, mEPSC recordings, and domain/variant rescue\",\n      \"pmids\": [\"32133675\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Direct biochemical link between WD40 domain and RhoA regulators not established\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Showed tomosyn sets release probability and tonic vs. phasic release identity, acting as a decoy SNARE that enables facilitation and homeostatic plasticity at tonic synapses.\",\n      \"evidence\": \"Genetic loss-of-function in Drosophila motoneurons with plasticity electrophysiology and immunofluorescence quantification\",\n      \"pmids\": [\"34713802\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study\", \"Mechanism setting differential tomosyn expression between motoneuron types unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Revised the model for dense-core vesicles by showing tomosyns function in DCV cargo packaging/biogenesis at the Golgi—independently of the SNARE domain—rather than DCV fusion.\",\n      \"evidence\": \"Conditional double-KO mouse neurons, pHluorin single-vesicle DCV exocytosis, cargo quantification, SNARE-domain-deleted rescue, and trans-Golgi/DCV morphometry\",\n      \"pmids\": [\"37695731\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular partner mediating Golgi/cargo-packaging function unidentified\", \"Reconciliation with prior invertebrate DCV-fusion data incomplete\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified synaptotagmin-9 as an upstream partner stabilizing tomosyn-1 to enforce inhibition of insulin secretion, and reinforced TOM-1 isoform-specific roles in netrin/UNC-5-directed growth cone protrusion.\",\n      \"evidence\": \"Co-localization/co-IP, Syt9 KO/KD with tomosyn-1 rescue and SNARE/insulin assays; C. elegans tom-1;unc-5;unc-64 isoform-specific epistasis\",\n      \"pmids\": [\"37432648\", \"37014062\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab studies\", \"How Syt9 stabilizes tomosyn-1 against proteasomal degradation unresolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Revealed a dual role for tomosyn-2 in beta-cells, coupling inhibition of insulin granule exocytosis with promotion of beta-cell proliferation via Akt1/cell-cycle signaling.\",\n      \"evidence\": \"Co-IP, tomosyn-2 KO mice, glucose tolerance/insulin secretion assays, transcriptomics, and Akt1/cyclinD1 signaling assays\",\n      \"pmids\": [\"42008692\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab study not independently replicated\", \"Mechanistic link between tomosyn-2 and Akt1 signaling unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How tomosyn's canonical SNARE-clamp activity and its SNARE-independent functions (RhoA suppression, Golgi DCV cargo packaging) are integrated within a single protein, and what governs their tissue- and context-specific deployment, remains unresolved.\",\n      \"evidence\": \"Open question synthesized across the timeline\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the full-length protein or WD40 domain\", \"Partner mediating Golgi cargo-packaging function unknown\", \"Determinants of context-specific t-SNARE partner choice undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 3, 11, 19]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 1, 2, 19]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [1, 2, 19]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [11, 21, 30]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [12, 19]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [1, 12, 21]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [34]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-112316\", \"supporting_discovery_ids\": [5, 7, 16, 24, 25]},\n      {\"term_id\": \"R-HSA-109582\", \"supporting_discovery_ids\": [17, 18]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 1, 3, 19, 30]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [6, 8, 23, 31, 39]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [18, 29]}\n    ],\n    \"complexes\": [\n      \"tomosyn–syntaxin-1–SNAP-25 SNARE complex\",\n      \"tomosyn–syntaxin-4–SNAP-23 complex\",\n      \"tomosyn–syntaxin-11–SNAP-23 complex\"\n    ],\n    \"partners\": [\n      \"STX1A\",\n      \"SNAP25\",\n      \"STX4\",\n      \"SNAP23\",\n      \"STX11\",\n      \"RAB3A\",\n      \"HRD1\",\n      \"PIAS4\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}