{"gene":"CYTH1","run_date":"2026-04-28T17:28:53","timeline":{"discoveries":[{"year":1996,"finding":"Cytohesin-1 specifically interacts with the intracellular portion of the integrin β2 chain (CD18) and overexpression of either full-length cytohesin-1 or its SEC7 domain induces β2 integrin-dependent binding of Jurkat cells to ICAM-1, while expression of the isolated PH domain inhibits T cell receptor-stimulated adhesion.","method":"Co-immunoprecipitation, overexpression/dominant-negative functional assay in Jurkat cells","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — foundational discovery, reciprocal domain dissection with multiple orthogonal functional readouts, highly cited","pmids":["8706128"],"is_preprint":false},{"year":1997,"finding":"Cytohesin-1 functions as a guanine nucleotide-exchange protein (GEP) for ADP-ribosylation factors (ARFs), accelerating GDP-to-GTP exchange on ARF1 and ARF3, and increasing ARF1 binding to a Golgi fraction in a brefeldin A-insensitive manner.","method":"In vitro GEF assay with recombinant cytohesin-1 and purified bovine brain ARF; [35S]GTPγS and [3H]GDP binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstituted GEF assay with purified components, replicated in subsequent studies","pmids":["9050849"],"is_preprint":false},{"year":1998,"finding":"PI 3-kinase activation is sufficient to induce membrane recruitment of cytohesin-1 and activate β2 integrin-dependent adhesion; this is abrogated by overexpression of the isolated cytohesin-1 PH domain, placing PI 3-kinase upstream of cytohesin-1 in the inside-out signaling pathway.","method":"Constitutively active PI 3-kinase overexpression, dominant-negative PH domain constructs, membrane fractionation, adhesion assay in Jurkat cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis via dominant-negative constructs combined with biochemical membrane fractionation, replicated across studies","pmids":["9614087"],"is_preprint":false},{"year":1998,"finding":"Both the PH domain and the adjacent carboxyl-terminal polybasic (c) domain of cytohesin-1 are required cooperatively for high-affinity plasma membrane association (Kd ~100 nM for PtdIns(3,4,5)P3) and for biological inhibition of T cell adhesion; the isolated PH domain has substantially lower affinity (~2–3 µM).","method":"Biosensor (BIAcore) lipid-binding measurements, domain deletion/chimera overexpression, functional adhesion assays in Jurkat cells","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 — in vitro binding measurements with defined affinities combined with functional domain-swap experiments","pmids":["9693361"],"is_preprint":false},{"year":1998,"finding":"The solution structure of the cytohesin-1 Sec7 domain (10 α-helices) was determined by NMR; ARF1 binding occurs through a large surface on the C-terminal subdomain involving hydrophobic and polar residues; structure-based mutagenesis identified residues critical for ARF binding and nucleotide exchange; the Sec7 domain does not interact with the β2 integrin cytoplasmic domain in solution.","method":"NMR spectroscopy, 1H-15N and 1H-13C chemical shift perturbation mapping, structure-based mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — NMR structure with mutagenesis and functional validation","pmids":["9653114"],"is_preprint":false},{"year":1998,"finding":"Cytohesin-1 and its isolated Sec7 domain catalyze guanine nucleotide exchange on ARF1, ARF3, yeast ARF1–3, and ARD1 but not on ARL proteins; the Sec7 domain alone is broader in substrate specificity than full-length cytohesin-1, indicating that regions outside the Sec7 domain impose ARF specificity.","method":"In vitro GEF assay with recombinant proteins and [35S]GTPγS binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro assay with multiple substrate comparisons","pmids":["9756891"],"is_preprint":false},{"year":1999,"finding":"EGF- and NGF-stimulated PI 3-kinase activity drives rapid translocation of cytohesin-1 from cytosol to the plasma membrane in PC12 cells via PH domain binding to PIP3; the PH domain binds the inositol head group of PIP3 (IP4) with >200-fold higher affinity than PIP2 or PI(3,4)P2 in vitro.","method":"GFP-fusion live-cell confocal microscopy, wortmannin/LY294002 PI 3-kinase inhibition, in vitro IP4 binding assay","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro binding combined with live-cell translocation imaging and pharmacological inhibition","pmids":["10341214"],"is_preprint":false},{"year":1999,"finding":"LPS-induced monocyte adherence to ICAM-1 involves CD14, Rho, PI 3-kinase, and cytohesin-1; cytohesin-1 antisense knockdown attenuates LPS-induced adhesion, placing cytohesin-1 downstream of PI 3-kinase in inside-out LFA-1 activation.","method":"Antisense knockdown, PI 3-kinase inhibitors (LY294002, wortmannin), dominant-negative PI 3-kinase transfection, Clostridium difficile toxin B, adhesion assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis established by multiple inhibitors and antisense, single lab","pmids":["9873050"],"is_preprint":false},{"year":1999,"finding":"Multiple structural elements of ARF1 are required for functional interaction with cytohesin-1: a non-specific N-terminal sequence (residues 1–13), residues 28–50 (including Lys-38 in switch 1), and the C-terminal region; the Sec7 domain alone has broader ARF recognition than full-length cytohesin-1.","method":"ARF1/ARL1 chimeric protein GEF assay, site-directed mutagenesis, [35S]GTPγS binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — systematic in vitro mutagenesis with reconstituted GEF assay","pmids":["10212218"],"is_preprint":false},{"year":2000,"finding":"Cytohesin-1 regulates β2 integrin-mediated cell adhesion through two dissectable mechanisms: (1) direct interaction with the LFA-1 β2 cytoplasmic domain to induce LFA-1 activation epitope expression (GEF-independent), and (2) ARF-GEF activity required for cell spreading on ICAM-1; a GEF-dead mutant fails to support spreading.","method":"β2 cytoplasmic domain mutational analysis, activation epitope antibody staining, in vitro GEF assay, cell spreading and adhesion assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1–2 — mutagenesis separating two distinct functions with orthogonal functional readouts","pmids":["10835351"],"is_preprint":false},{"year":2000,"finding":"PtdIns(3,4,5)P3 determines the substrate specificity of cytohesin-1 GEF activity: it suppresses GEF activity toward ARF6 but enhances it toward ARF1, revealing lipid-dependent substrate switching.","method":"In vitro GEF assay with mammalian ARF-Ig chimeras, immunoprecipitation, phosphoinositide supplementation","journal":"European journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstituted assay, single lab","pmids":["10848997"],"is_preprint":false},{"year":2000,"finding":"Cytohesin-1 specifically activates ARD1 (a 64-kDa GTPase with C-terminal ARF domain) as a GEF substrate in a manner requiring residue 30 in the Sec7 domain; ARD1 and cytohesin-1 partially colocalize in transfected COS-7 cells, consistent with a role in vesicular trafficking.","method":"Yeast two-hybrid screen, in vitro GEF assay, site-directed mutagenesis of Sec7 domain, confocal fluorescence microscopy of co-transfected cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — yeast two-hybrid plus in vitro GEF reconstitution and colocalization, single lab","pmids":["10748148"],"is_preprint":false},{"year":2000,"finding":"B2-1 (cytohesin-1) localizes to the Golgi complex rather than the plasma membrane; this localization is disrupted by brefeldin A, indicating dependence on ARF activity at the Golgi; overexpression causes partial Golgi dispersion.","method":"Immunofluorescence, GFP-fusion confocal microscopy, brefeldin A treatment in transfected cells","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 3 — direct localization experiment with functional consequence (BFA sensitivity), single lab","pmids":["10772823"],"is_preprint":false},{"year":2001,"finding":"The kaposin A protein of human herpesvirus 8 directly interacts with cytohesin-1 and recruits it to the membrane; kaposin A stimulates cytohesin-1-dependent GTP loading of myristoylated ARF1 in vitro; a GEF-dead cytohesin-1 mutant (E157K) reverts kaposin A-induced focus formation, stress fiber dissolution, and ERK-1/2 MAP kinase activation.","method":"Direct protein interaction assay, in vitro liposome-reconstituted GEF assay with myristoylated ARF1, dominant-negative mutant rescue, ERK-1/2 activation assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro reconstituted assay combined with dominant-negative rescue of multiple cellular phenotypes","pmids":["11336706"],"is_preprint":false},{"year":2001,"finding":"Protein kinase Cδ phosphorylates a serine/threonine motif in the polybasic c domain of cytohesin-1 in vitro, and phorbol ester stimulation induces the same phosphorylation in vivo; phosphorylated cytohesin-1 associates with the actin cytoskeleton, and this phosphorylation is required for maximal LFA-1-mediated adhesion.","method":"In vitro PKCδ kinase assay, phorbol ester stimulation, actin cytoskeleton co-sedimentation, functional adhesion assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay corroborated by in vivo phosphorylation and functional consequence","pmids":["11438522"],"is_preprint":false},{"year":2001,"finding":"Cytohesin-1 PH domain (GEF-independent) mediates leukocyte arrest on cytokine-activated endothelium triggered by chemokines, whereas both GEF activity and LFA-1/cytohesin-1 interaction are required for cell shape change and transendothelial chemotaxis; ARF6 (but not ARF1) is identified as the downstream GEF target in chemotaxis.","method":"Overexpression of dominant-negative PH domain construct, GEF-dead mutant, β2 mutant interrupting cytohesin-1 interaction, flow-based leukocyte adhesion assay, transendothelial chemotaxis assay","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 — multiple dominant-negative/mutant constructs dissecting distinct functions with orthogonal cellular assays","pmids":["11747824"],"is_preprint":false},{"year":2002,"finding":"Cybr (cytohesin binder and regulator) physically interacts with cytohesin-1 through coiled-coil domain interactions and enhances cytohesin-1-mediated acceleration of GTPγS binding to ARF in vitro.","method":"Co-immunoprecipitation of overexpressed proteins from 293T cells, in vitro GEF assay with Cybr","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP plus in vitro GEF assay, single lab","pmids":["11867758"],"is_preprint":false},{"year":2003,"finding":"LFA-1 activation induces phosphorylation of the β2 integrin chain, release of JAB-1, and ERK1/2 signaling through cytohesin-1; dominant-negative cytohesin-1 inhibits IL-2 production and T helper type 1 differentiation, establishing cytohesin-1 as a mediator of LFA-1 co-stimulatory signaling.","method":"Intracellular phosphoprotein staining with 13-dimensional flow cytometry, dominant-negative overexpression, cytokine production assays","journal":"Nature immunology","confidence":"Medium","confidence_rationale":"Tier 2 — phosphoprotein flow cytometry plus dominant-negative functional assays, single lab","pmids":["14528303"],"is_preprint":false},{"year":2005,"finding":"Cytohesin-1 promotes phagocytosis of M. bovis BCG via a CD14-TLR2-PI3K-cytohesin-1 inside-out signaling axis that activates complement receptor 3 (CR3/Mac-1); cytohesin-1 knockdown abrogates CD14-regulated CR3-dependent BCG internalization and cytohesin-1 physically associates with CR3.","method":"siRNA knockdown, blocking antibodies, PI3K inhibitors, flow cytometry phagocytosis assay, co-immunoprecipitation of CR3 and cytohesin-1","journal":"Journal of immunology","confidence":"Medium","confidence_rationale":"Tier 2 — siRNA knockdown combined with co-IP and pathway inhibition, single lab","pmids":["15778383"],"is_preprint":false},{"year":2009,"finding":"Cytohesin-1 activates Arf6 (but not Arf1) in human neutrophils stimulated with fMLF, linking fMLF receptor signaling to phospholipase D activation, superoxide production, and granule secretion; cytohesin-1 inhibition with SecinH3 or siRNA knockdown selectively suppresses Arf6 activation and downstream oxidative burst.","method":"SecinH3 pharmacological inhibition, cytohesin-1 siRNA knockdown and stable overexpression in PLB-985 cells, Arf6/Arf1 activation assays, PLD activity assay, NADPH oxidase assay, granule marker surface expression","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — knockdown, overexpression, and pharmacological inhibition with multiple orthogonal readouts","pmids":["20018626"],"is_preprint":false},{"year":2011,"finding":"Cytohesin-1 restrains Mac-1 (αMβ2) activation in neutrophils; inhibition or knockdown of cytohesin-1 increases Mac-1 conformational activation, cell adhesion to fibrinogen, phagocytosis of zymosan, and chemotaxis, while overexpression has opposite effects; the mechanism involves crosstalk between FPRL-1 receptor and Mac-1 through cytohesin-1.","method":"SecinH3 inhibition, siRNA knockdown, stable overexpression, Mac-1 activation epitope antibody (CBRM1/5), adhesion assay, phagocytosis assay, transwell chemotaxis assay","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 — multiple complementary loss- and gain-of-function approaches, single lab","pmids":["21233413"],"is_preprint":false},{"year":2012,"finding":"Fyn (Src-family kinase) phosphorylates tyrosine 382 of cytohesin-1 in Schwann cells; this phosphorylation is required for cytohesin-1-mediated Arf6 activation and myelination; transgenic mice expressing phosphorylation-deficient Y382F cytohesin-1 in Schwann cells display delayed and reduced myelination similar to cytohesin-1 knockout mice.","method":"In vitro Fyn kinase assay, cytohesin-1 Y382F knock-in transgenic mice, cytohesin-1 knockout mice, electron microscopy of myelin thickness, Arf6 activation assay","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay corroborated by transgenic and knockout mouse phenotypes with defined molecular mechanism","pmids":["23012656"],"is_preprint":false},{"year":2013,"finding":"Cytohesin-1 and Arf6 are required for migration of primary Schwann cells toward neuronal conditioned medium and individual growth factors; knockdown of cytohesin-1 or Arf6, SecinH3 treatment, or cytohesin-1 knockout inhibits migration; a Tyr-382 phosphorylation-deficient mutant fails to rescue blunted migration.","method":"siRNA knockdown, cytohesin-1 knockout, SecinH3 pharmacological inhibition, rescue with siRNA-resistant WT or Y382F mutant, Schwann cell migration assay","journal":"Cellular signalling","confidence":"High","confidence_rationale":"Tier 2 — genetic KO corroborated by siRNA, chemical inhibition, and rescue experiments","pmids":["23517829"],"is_preprint":false},{"year":2013,"finding":"Schwann cell-specific overexpression of wild-type cytohesin-1 in transgenic mice results in enhanced myelin thickness in PNS nerves and downstream Arf6 activation, confirming that cytohesin-1-Arf6 signaling promotes myelination in vivo.","method":"Schwann cell-specific transgenic mice, electron microscopy measurement of myelin thickness, Arf6 activation assay","journal":"Journal of molecular neuroscience","confidence":"High","confidence_rationale":"Tier 2 — in vivo transgenic overexpression with quantitative structural phenotype and downstream pathway measurement","pmids":["23636892"],"is_preprint":false},{"year":2016,"finding":"CYTH1 knockdown disrupts adhesion of human hematopoietic stem and progenitor cells (HSPCs) to stromal cells, fibronectin, and ICAM-1 by impairing integrin β1 and β2 activation; transplantation of knockdown cells into immunodeficient mice results in reduced bone marrow homing and long-term engraftment, demonstrated by intravital microscopy.","method":"RNA interference screen, integrin activation assays, adhesion assays, HSPC transplantation into immunodeficient mice, intravital microscopy","journal":"Blood","confidence":"High","confidence_rationale":"Tier 2 — RNAi loss-of-function with in vivo engraftment readout and intravital imaging, mechanistic link to integrin activation","pmids":["27899358"],"is_preprint":false},{"year":2018,"finding":"Alternative splicing of a three-nucleotide microexon in the PH domain of cytohesin-1 generates isoforms with differential phosphoinositide binding: the diglycine isoform preferentially binds PIP3 and localizes to the leading edge, while the triglycine isoform binds PI(4,5)P2 and localizes to the plasma membrane; the PI(3,4,5)P3-binding diglycine isoform specifically mediates HGF/Met receptor-induced Arf6 activation and cancer cell migration.","method":"Isoform-specific expression, phosphoinositide-binding assays, live-cell GFP/mCherry localization imaging, Met receptor activation, Arf6 activation assay, migration assay","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1–2 — biochemical binding assays combined with live-cell imaging and functional migration assays linking isoform-specific lipid recognition to subcellular localization and signaling","pmids":["30404949"],"is_preprint":false}],"current_model":"Cytohesin-1 (CYTH1) is a cytosolic ARF-family guanine nucleotide exchange factor (GEF) whose Sec7 domain catalyzes GDP-to-GTP exchange on ARF1 and ARF6, while its PH domain and adjacent polybasic c domain cooperatively bind PI(3,4,5)P3 generated by PI 3-kinase to drive plasma membrane recruitment; it directly binds the β2 integrin (CD18) cytoplasmic tail to promote LFA-1-dependent adhesion independently of GEF activity, while its GEF/Arf6 activity mediates cell spreading, transendothelial migration, Schwann cell myelination (regulated by Fyn-mediated Y382 phosphorylation), and neutrophil oxidative burst; microexon splicing in the PH domain generates isoforms with distinct phosphoinositide selectivity that control subcellular localization and RTK-dependent Arf6 signaling during cell migration."},"narrative":{"teleology":[{"year":1996,"claim":"The discovery that cytohesin-1 directly binds the β2 integrin cytoplasmic tail and promotes LFA-1-dependent adhesion established the first molecular link between a Sec7-domain protein and integrin inside-out signaling.","evidence":"Co-immunoprecipitation and domain-dissection overexpression/dominant-negative assays in Jurkat T cells","pmids":["8706128"],"confidence":"High","gaps":["Mechanism by which cytohesin-1 binding alters integrin conformation was unknown","Whether GEF activity was required for adhesion was untested"]},{"year":1997,"claim":"Demonstration that cytohesin-1 catalyzes GDP-to-GTP exchange on ARF1 and ARF3 in vitro defined its enzymatic function as an ARF-GEF, raising the question of how GEF activity relates to integrin regulation.","evidence":"In vitro GEF assay with recombinant cytohesin-1 and purified ARFs using radiolabeled nucleotides","pmids":["9050849"],"confidence":"High","gaps":["In vivo ARF substrate identity unresolved","Relationship between GEF activity and adhesion function unclear"]},{"year":1998,"claim":"Establishing that PI 3-kinase activation drives cytohesin-1 membrane recruitment through cooperative PH domain + polybasic c domain binding to PIP3 (Kd ~100 nM) placed the lipid second messenger upstream of cytohesin-1 and explained its stimulus-dependent translocation.","evidence":"BIAcore lipid-binding assays, domain deletion/chimera constructs, constitutively active PI 3-kinase overexpression, membrane fractionation in Jurkat cells","pmids":["9614087","9693361"],"confidence":"High","gaps":["Whether other phosphoinositides could substitute in specific cellular contexts","Structural basis of PH–lipid cooperativity with the c domain"]},{"year":1998,"claim":"NMR structure of the Sec7 domain and systematic ARF1 mutagenesis mapped the GEF catalytic interface and showed that the Sec7 domain alone does not bind the β2 integrin tail, establishing that adhesion and GEF functions reside in separable domains.","evidence":"NMR spectroscopy, chemical shift perturbation mapping, structure-based and ARF1 chimeric mutagenesis","pmids":["9653114","9756891","10212218"],"confidence":"High","gaps":["Full-length structure including PH and c domains not determined","How autoinhibition might regulate GEF activity"]},{"year":2000,"claim":"Separation-of-function experiments showed that β2 integrin binding activates LFA-1 epitope expression independently of GEF activity, while ARF-GEF activity is specifically required for post-adhesion cell spreading, resolving two mechanistically distinct outputs of cytohesin-1.","evidence":"GEF-dead mutant, β2 cytoplasmic domain mutants, activation epitope antibodies, spreading and adhesion assays","pmids":["10835351"],"confidence":"High","gaps":["Which ARF substrate mediates spreading was not identified in this study","How GEF-independent integrin activation is transmitted structurally"]},{"year":2001,"claim":"Identification that PKCδ phosphorylates the polybasic c domain and that this modification drives actin cytoskeleton association and maximal LFA-1 adhesion revealed a post-translational regulatory mechanism for cytohesin-1 localization and function.","evidence":"In vitro PKCδ kinase assay, phorbol ester stimulation in vivo, actin co-sedimentation, adhesion assays","pmids":["11438522"],"confidence":"High","gaps":["Precise phosphorylation site(s) not fully mapped","Whether PKCδ phosphorylation affects GEF activity toward ARFs"]},{"year":2001,"claim":"Dissection of leukocyte transendothelial migration showed that PH domain–mediated arrest is GEF-independent, while shape change and chemotaxis require both GEF activity and β2 interaction, with ARF6 (not ARF1) identified as the relevant GEF substrate during diapedesis.","evidence":"Multiple dominant-negative/mutant constructs in flow-based adhesion and transendothelial chemotaxis assays","pmids":["11747824"],"confidence":"High","gaps":["Downstream effectors of Arf6 in diapedesis not identified","In vivo validation in animal models absent"]},{"year":2005,"claim":"The finding that cytohesin-1 associates with CR3 (Mac-1) and mediates CD14-TLR2-PI3K–driven CR3 activation for mycobacterial phagocytosis extended its integrin-activating role beyond LFA-1 to complement receptors in innate immunity.","evidence":"siRNA knockdown, blocking antibodies, PI3K inhibitors, co-immunoprecipitation of CR3 and cytohesin-1, phagocytosis assay","pmids":["15778383"],"confidence":"Medium","gaps":["Direct binding site on CR3 not mapped","Single lab, awaits independent confirmation"]},{"year":2009,"claim":"Pharmacological inhibition and knockdown in neutrophils demonstrated that cytohesin-1 selectively activates Arf6 downstream of fMLF receptor signaling to drive PLD activation, superoxide production, and granule secretion, linking it to the oxidative burst.","evidence":"SecinH3 inhibition, siRNA knockdown and overexpression in PLB-985 neutrophil-like cells, Arf6/Arf1 activation assays, NADPH oxidase assay","pmids":["20018626"],"confidence":"High","gaps":["Specificity of SecinH3 among cytohesin family members","Whether cytohesin-1 is redundant with other cytohesins in neutrophils"]},{"year":2012,"claim":"Discovery that Fyn phosphorylates cytohesin-1 at Y382 and that this event is required for Arf6 activation and Schwann cell myelination—validated in both knockout and phospho-deficient transgenic mice—established a non-immune developmental function for cytohesin-1.","evidence":"In vitro Fyn kinase assay, cytohesin-1 KO mice, Y382F knock-in transgenic mice, electron microscopy of myelin, Arf6 activation assay","pmids":["23012656","23636892"],"confidence":"High","gaps":["Downstream Arf6 effectors in myelination unidentified","Whether Y382 phosphorylation affects integrin-binding function"]},{"year":2016,"claim":"An RNAi screen followed by in vivo transplantation demonstrated that cytohesin-1 is essential for integrin β1 and β2 activation in human hematopoietic stem/progenitor cells and for their bone marrow homing and engraftment, broadening its role to stem cell biology.","evidence":"RNA interference, integrin activation and adhesion assays, HSPC transplantation into immunodeficient mice, intravital microscopy","pmids":["27899358"],"confidence":"High","gaps":["Which cytohesin-1 domain mediates β1 integrin activation is unknown","Redundancy with other cytohesins in HSPCs not assessed"]},{"year":2018,"claim":"Characterization of a PH domain microexon splice variant showed that a single-residue insertion switches phosphoinositide selectivity from PIP3 to PI(4,5)P2, controlling subcellular localization and coupling the PIP3-binding isoform specifically to HGF/Met-induced Arf6 activation and cancer cell migration.","evidence":"Isoform-specific expression, phosphoinositide-binding assays, live-cell imaging, Met receptor and Arf6 activation assays, migration assays","pmids":["30404949"],"confidence":"High","gaps":["Splicing regulators controlling microexon inclusion unknown","In vivo relevance to tumor metastasis not tested","Whether isoform switching also governs immune cell functions"]},{"year":null,"claim":"Key unresolved questions include the full-length autoinhibited structure of cytohesin-1, the identity of Arf6 effectors mediating myelination and cell spreading, redundancy versus specificity among the four cytohesin family members in vivo, and whether isoform-specific lipid selectivity differentially regulates immune versus developmental functions.","evidence":"","pmids":[],"confidence":"Low","gaps":["No full-length structure available","Arf6 effectors downstream of cytohesin-1 in myelination and spreading not identified","Systematic in vivo comparison of cytohesin family members lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[1,5,8,10,11,13,19]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[14]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[3,6,25]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,9,24]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,6,25]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,6]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[12]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[14]}],"pathway":[{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[0,7,15,17,18,19,20]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,6,13,17,21,25]},{"term_id":"R-HSA-1500931","term_label":"Cell-Cell communication","supporting_discovery_ids":[0,9,15,24]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[21,22,23]}],"complexes":[],"partners":["ITGB2","ARF1","ARF6","PRKCQ","FYN","PHLDB1","ITGAM"],"other_free_text":[]},"mechanistic_narrative":"Cytohesin-1 (CYTH1) is an ARF-family guanine nucleotide exchange factor that couples PI 3-kinase–dependent membrane recruitment to integrin inside-out signaling and ARF-dependent cytoskeletal remodeling in leukocytes, Schwann cells, and hematopoietic progenitors. Its Sec7 domain catalyzes GDP-to-GTP exchange on ARF1 and ARF6, while its PH domain and adjacent polybasic c domain cooperatively bind PI(3,4,5)P3 to drive plasma membrane translocation; a three-nucleotide microexon splice variant in the PH domain switches phosphoinositide selectivity between PIP3 and PI(4,5)P2, thereby controlling subcellular localization and RTK-driven Arf6 activation during cell migration [PMID:9050849, PMID:9693361, PMID:30404949]. Independently of GEF activity, cytohesin-1 directly binds the β2 integrin cytoplasmic tail to activate LFA-1 and CR3, promoting leukocyte adhesion, transendothelial migration, phagocytosis, and hematopoietic stem cell bone marrow homing, whereas its Arf6-directed GEF activity is required for cell spreading, neutrophil oxidative burst, and Schwann cell myelination—the latter regulated by Fyn-mediated phosphorylation of Tyr-382 [PMID:8706128, PMID:10835351, PMID:20018626, PMID:23012656, PMID:27899358]. PKCδ phosphorylation of the polybasic c domain promotes cytohesin-1 association with the actin cytoskeleton and maximal LFA-1–dependent adhesion [PMID:11438522]."},"prefetch_data":{"uniprot":{"accession":"Q15438","full_name":"Cytohesin-1","aliases":["PH, SEC7 and coiled-coil domain-containing protein 1","SEC7 homolog B2-1"],"length_aa":398,"mass_kda":46.4,"function":"Promotes guanine-nucleotide exchange on ARF1, ARF5 and ARF6. Promotes the activation of ARF factors through replacement of GDP with GTP. Plays an important role in membrane trafficking, during junctional remodeling and epithelial polarization, through regulation of ARF6 activity","subcellular_location":"Cell membrane; Cytoplasm, cytosol; Cell junction, tight junction; Cell junction, adherens junction","url":"https://www.uniprot.org/uniprotkb/Q15438/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CYTH1","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CYTH1","total_profiled":1310},"omim":[{"mim_id":"618077","title":"INFLAMMATORY BOWEL DISEASE 29; IBD29","url":"https://www.omim.org/entry/618077"},{"mim_id":"618051","title":"INNATE IMMUNITY ACTIVATOR; INAVA","url":"https://www.omim.org/entry/618051"},{"mim_id":"616822","title":"MON2, REGULATOR OF ENDOSOME-TO-GOLGI TRAFFICKING; 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receptor-stimulated adhesion.\",\n      \"method\": \"Co-immunoprecipitation, overexpression/dominant-negative functional assay in Jurkat cells\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — foundational discovery, reciprocal domain dissection with multiple orthogonal functional readouts, highly cited\",\n      \"pmids\": [\"8706128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Cytohesin-1 functions as a guanine nucleotide-exchange protein (GEP) for ADP-ribosylation factors (ARFs), accelerating GDP-to-GTP exchange on ARF1 and ARF3, and increasing ARF1 binding to a Golgi fraction in a brefeldin A-insensitive manner.\",\n      \"method\": \"In vitro GEF assay with recombinant cytohesin-1 and purified bovine brain ARF; [35S]GTPγS and [3H]GDP binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstituted GEF assay with purified components, replicated in subsequent studies\",\n      \"pmids\": [\"9050849\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"PI 3-kinase activation is sufficient to induce membrane recruitment of cytohesin-1 and activate β2 integrin-dependent adhesion; this is abrogated by overexpression of the isolated cytohesin-1 PH domain, placing PI 3-kinase upstream of cytohesin-1 in the inside-out signaling pathway.\",\n      \"method\": \"Constitutively active PI 3-kinase overexpression, dominant-negative PH domain constructs, membrane fractionation, adhesion assay in Jurkat cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis via dominant-negative constructs combined with biochemical membrane fractionation, replicated across studies\",\n      \"pmids\": [\"9614087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Both the PH domain and the adjacent carboxyl-terminal polybasic (c) domain of cytohesin-1 are required cooperatively for high-affinity plasma membrane association (Kd ~100 nM for PtdIns(3,4,5)P3) and for biological inhibition of T cell adhesion; the isolated PH domain has substantially lower affinity (~2–3 µM).\",\n      \"method\": \"Biosensor (BIAcore) lipid-binding measurements, domain deletion/chimera overexpression, functional adhesion assays in Jurkat cells\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro binding measurements with defined affinities combined with functional domain-swap experiments\",\n      \"pmids\": [\"9693361\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"The solution structure of the cytohesin-1 Sec7 domain (10 α-helices) was determined by NMR; ARF1 binding occurs through a large surface on the C-terminal subdomain involving hydrophobic and polar residues; structure-based mutagenesis identified residues critical for ARF binding and nucleotide exchange; the Sec7 domain does not interact with the β2 integrin cytoplasmic domain in solution.\",\n      \"method\": \"NMR spectroscopy, 1H-15N and 1H-13C chemical shift perturbation mapping, structure-based mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure with mutagenesis and functional validation\",\n      \"pmids\": [\"9653114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Cytohesin-1 and its isolated Sec7 domain catalyze guanine nucleotide exchange on ARF1, ARF3, yeast ARF1–3, and ARD1 but not on ARL proteins; the Sec7 domain alone is broader in substrate specificity than full-length cytohesin-1, indicating that regions outside the Sec7 domain impose ARF specificity.\",\n      \"method\": \"In vitro GEF assay with recombinant proteins and [35S]GTPγS binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro assay with multiple substrate comparisons\",\n      \"pmids\": [\"9756891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"EGF- and NGF-stimulated PI 3-kinase activity drives rapid translocation of cytohesin-1 from cytosol to the plasma membrane in PC12 cells via PH domain binding to PIP3; the PH domain binds the inositol head group of PIP3 (IP4) with >200-fold higher affinity than PIP2 or PI(3,4)P2 in vitro.\",\n      \"method\": \"GFP-fusion live-cell confocal microscopy, wortmannin/LY294002 PI 3-kinase inhibition, in vitro IP4 binding assay\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro binding combined with live-cell translocation imaging and pharmacological inhibition\",\n      \"pmids\": [\"10341214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"LPS-induced monocyte adherence to ICAM-1 involves CD14, Rho, PI 3-kinase, and cytohesin-1; cytohesin-1 antisense knockdown attenuates LPS-induced adhesion, placing cytohesin-1 downstream of PI 3-kinase in inside-out LFA-1 activation.\",\n      \"method\": \"Antisense knockdown, PI 3-kinase inhibitors (LY294002, wortmannin), dominant-negative PI 3-kinase transfection, Clostridium difficile toxin B, adhesion assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis established by multiple inhibitors and antisense, single lab\",\n      \"pmids\": [\"9873050\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Multiple structural elements of ARF1 are required for functional interaction with cytohesin-1: a non-specific N-terminal sequence (residues 1–13), residues 28–50 (including Lys-38 in switch 1), and the C-terminal region; the Sec7 domain alone has broader ARF recognition than full-length cytohesin-1.\",\n      \"method\": \"ARF1/ARL1 chimeric protein GEF assay, site-directed mutagenesis, [35S]GTPγS binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic in vitro mutagenesis with reconstituted GEF assay\",\n      \"pmids\": [\"10212218\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cytohesin-1 regulates β2 integrin-mediated cell adhesion through two dissectable mechanisms: (1) direct interaction with the LFA-1 β2 cytoplasmic domain to induce LFA-1 activation epitope expression (GEF-independent), and (2) ARF-GEF activity required for cell spreading on ICAM-1; a GEF-dead mutant fails to support spreading.\",\n      \"method\": \"β2 cytoplasmic domain mutational analysis, activation epitope antibody staining, in vitro GEF assay, cell spreading and adhesion assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — mutagenesis separating two distinct functions with orthogonal functional readouts\",\n      \"pmids\": [\"10835351\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"PtdIns(3,4,5)P3 determines the substrate specificity of cytohesin-1 GEF activity: it suppresses GEF activity toward ARF6 but enhances it toward ARF1, revealing lipid-dependent substrate switching.\",\n      \"method\": \"In vitro GEF assay with mammalian ARF-Ig chimeras, immunoprecipitation, phosphoinositide supplementation\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstituted assay, single lab\",\n      \"pmids\": [\"10848997\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cytohesin-1 specifically activates ARD1 (a 64-kDa GTPase with C-terminal ARF domain) as a GEF substrate in a manner requiring residue 30 in the Sec7 domain; ARD1 and cytohesin-1 partially colocalize in transfected COS-7 cells, consistent with a role in vesicular trafficking.\",\n      \"method\": \"Yeast two-hybrid screen, in vitro GEF assay, site-directed mutagenesis of Sec7 domain, confocal fluorescence microscopy of co-transfected cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — yeast two-hybrid plus in vitro GEF reconstitution and colocalization, single lab\",\n      \"pmids\": [\"10748148\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"B2-1 (cytohesin-1) localizes to the Golgi complex rather than the plasma membrane; this localization is disrupted by brefeldin A, indicating dependence on ARF activity at the Golgi; overexpression causes partial Golgi dispersion.\",\n      \"method\": \"Immunofluorescence, GFP-fusion confocal microscopy, brefeldin A treatment in transfected cells\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — direct localization experiment with functional consequence (BFA sensitivity), single lab\",\n      \"pmids\": [\"10772823\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"The kaposin A protein of human herpesvirus 8 directly interacts with cytohesin-1 and recruits it to the membrane; kaposin A stimulates cytohesin-1-dependent GTP loading of myristoylated ARF1 in vitro; a GEF-dead cytohesin-1 mutant (E157K) reverts kaposin A-induced focus formation, stress fiber dissolution, and ERK-1/2 MAP kinase activation.\",\n      \"method\": \"Direct protein interaction assay, in vitro liposome-reconstituted GEF assay with myristoylated ARF1, dominant-negative mutant rescue, ERK-1/2 activation assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro reconstituted assay combined with dominant-negative rescue of multiple cellular phenotypes\",\n      \"pmids\": [\"11336706\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Protein kinase Cδ phosphorylates a serine/threonine motif in the polybasic c domain of cytohesin-1 in vitro, and phorbol ester stimulation induces the same phosphorylation in vivo; phosphorylated cytohesin-1 associates with the actin cytoskeleton, and this phosphorylation is required for maximal LFA-1-mediated adhesion.\",\n      \"method\": \"In vitro PKCδ kinase assay, phorbol ester stimulation, actin cytoskeleton co-sedimentation, functional adhesion assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay corroborated by in vivo phosphorylation and functional consequence\",\n      \"pmids\": [\"11438522\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Cytohesin-1 PH domain (GEF-independent) mediates leukocyte arrest on cytokine-activated endothelium triggered by chemokines, whereas both GEF activity and LFA-1/cytohesin-1 interaction are required for cell shape change and transendothelial chemotaxis; ARF6 (but not ARF1) is identified as the downstream GEF target in chemotaxis.\",\n      \"method\": \"Overexpression of dominant-negative PH domain construct, GEF-dead mutant, β2 mutant interrupting cytohesin-1 interaction, flow-based leukocyte adhesion assay, transendothelial chemotaxis assay\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple dominant-negative/mutant constructs dissecting distinct functions with orthogonal cellular assays\",\n      \"pmids\": [\"11747824\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Cybr (cytohesin binder and regulator) physically interacts with cytohesin-1 through coiled-coil domain interactions and enhances cytohesin-1-mediated acceleration of GTPγS binding to ARF in vitro.\",\n      \"method\": \"Co-immunoprecipitation of overexpressed proteins from 293T cells, in vitro GEF assay with Cybr\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP plus in vitro GEF assay, single lab\",\n      \"pmids\": [\"11867758\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"LFA-1 activation induces phosphorylation of the β2 integrin chain, release of JAB-1, and ERK1/2 signaling through cytohesin-1; dominant-negative cytohesin-1 inhibits IL-2 production and T helper type 1 differentiation, establishing cytohesin-1 as a mediator of LFA-1 co-stimulatory signaling.\",\n      \"method\": \"Intracellular phosphoprotein staining with 13-dimensional flow cytometry, dominant-negative overexpression, cytokine production assays\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — phosphoprotein flow cytometry plus dominant-negative functional assays, single lab\",\n      \"pmids\": [\"14528303\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cytohesin-1 promotes phagocytosis of M. bovis BCG via a CD14-TLR2-PI3K-cytohesin-1 inside-out signaling axis that activates complement receptor 3 (CR3/Mac-1); cytohesin-1 knockdown abrogates CD14-regulated CR3-dependent BCG internalization and cytohesin-1 physically associates with CR3.\",\n      \"method\": \"siRNA knockdown, blocking antibodies, PI3K inhibitors, flow cytometry phagocytosis assay, co-immunoprecipitation of CR3 and cytohesin-1\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — siRNA knockdown combined with co-IP and pathway inhibition, single lab\",\n      \"pmids\": [\"15778383\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Cytohesin-1 activates Arf6 (but not Arf1) in human neutrophils stimulated with fMLF, linking fMLF receptor signaling to phospholipase D activation, superoxide production, and granule secretion; cytohesin-1 inhibition with SecinH3 or siRNA knockdown selectively suppresses Arf6 activation and downstream oxidative burst.\",\n      \"method\": \"SecinH3 pharmacological inhibition, cytohesin-1 siRNA knockdown and stable overexpression in PLB-985 cells, Arf6/Arf1 activation assays, PLD activity assay, NADPH oxidase assay, granule marker surface expression\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockdown, overexpression, and pharmacological inhibition with multiple orthogonal readouts\",\n      \"pmids\": [\"20018626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Cytohesin-1 restrains Mac-1 (αMβ2) activation in neutrophils; inhibition or knockdown of cytohesin-1 increases Mac-1 conformational activation, cell adhesion to fibrinogen, phagocytosis of zymosan, and chemotaxis, while overexpression has opposite effects; the mechanism involves crosstalk between FPRL-1 receptor and Mac-1 through cytohesin-1.\",\n      \"method\": \"SecinH3 inhibition, siRNA knockdown, stable overexpression, Mac-1 activation epitope antibody (CBRM1/5), adhesion assay, phagocytosis assay, transwell chemotaxis assay\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple complementary loss- and gain-of-function approaches, single lab\",\n      \"pmids\": [\"21233413\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Fyn (Src-family kinase) phosphorylates tyrosine 382 of cytohesin-1 in Schwann cells; this phosphorylation is required for cytohesin-1-mediated Arf6 activation and myelination; transgenic mice expressing phosphorylation-deficient Y382F cytohesin-1 in Schwann cells display delayed and reduced myelination similar to cytohesin-1 knockout mice.\",\n      \"method\": \"In vitro Fyn kinase assay, cytohesin-1 Y382F knock-in transgenic mice, cytohesin-1 knockout mice, electron microscopy of myelin thickness, Arf6 activation assay\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay corroborated by transgenic and knockout mouse phenotypes with defined molecular mechanism\",\n      \"pmids\": [\"23012656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Cytohesin-1 and Arf6 are required for migration of primary Schwann cells toward neuronal conditioned medium and individual growth factors; knockdown of cytohesin-1 or Arf6, SecinH3 treatment, or cytohesin-1 knockout inhibits migration; a Tyr-382 phosphorylation-deficient mutant fails to rescue blunted migration.\",\n      \"method\": \"siRNA knockdown, cytohesin-1 knockout, SecinH3 pharmacological inhibition, rescue with siRNA-resistant WT or Y382F mutant, Schwann cell migration assay\",\n      \"journal\": \"Cellular signalling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO corroborated by siRNA, chemical inhibition, and rescue experiments\",\n      \"pmids\": [\"23517829\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Schwann cell-specific overexpression of wild-type cytohesin-1 in transgenic mice results in enhanced myelin thickness in PNS nerves and downstream Arf6 activation, confirming that cytohesin-1-Arf6 signaling promotes myelination in vivo.\",\n      \"method\": \"Schwann cell-specific transgenic mice, electron microscopy measurement of myelin thickness, Arf6 activation assay\",\n      \"journal\": \"Journal of molecular neuroscience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo transgenic overexpression with quantitative structural phenotype and downstream pathway measurement\",\n      \"pmids\": [\"23636892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"CYTH1 knockdown disrupts adhesion of human hematopoietic stem and progenitor cells (HSPCs) to stromal cells, fibronectin, and ICAM-1 by impairing integrin β1 and β2 activation; transplantation of knockdown cells into immunodeficient mice results in reduced bone marrow homing and long-term engraftment, demonstrated by intravital microscopy.\",\n      \"method\": \"RNA interference screen, integrin activation assays, adhesion assays, HSPC transplantation into immunodeficient mice, intravital microscopy\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — RNAi loss-of-function with in vivo engraftment readout and intravital imaging, mechanistic link to integrin activation\",\n      \"pmids\": [\"27899358\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Alternative splicing of a three-nucleotide microexon in the PH domain of cytohesin-1 generates isoforms with differential phosphoinositide binding: the diglycine isoform preferentially binds PIP3 and localizes to the leading edge, while the triglycine isoform binds PI(4,5)P2 and localizes to the plasma membrane; the PI(3,4,5)P3-binding diglycine isoform specifically mediates HGF/Met receptor-induced Arf6 activation and cancer cell migration.\",\n      \"method\": \"Isoform-specific expression, phosphoinositide-binding assays, live-cell GFP/mCherry localization imaging, Met receptor activation, Arf6 activation assay, migration assay\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — biochemical binding assays combined with live-cell imaging and functional migration assays linking isoform-specific lipid recognition to subcellular localization and signaling\",\n      \"pmids\": [\"30404949\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Cytohesin-1 (CYTH1) is a cytosolic ARF-family guanine nucleotide exchange factor (GEF) whose Sec7 domain catalyzes GDP-to-GTP exchange on ARF1 and ARF6, while its PH domain and adjacent polybasic c domain cooperatively bind PI(3,4,5)P3 generated by PI 3-kinase to drive plasma membrane recruitment; it directly binds the β2 integrin (CD18) cytoplasmic tail to promote LFA-1-dependent adhesion independently of GEF activity, while its GEF/Arf6 activity mediates cell spreading, transendothelial migration, Schwann cell myelination (regulated by Fyn-mediated Y382 phosphorylation), and neutrophil oxidative burst; microexon splicing in the PH domain generates isoforms with distinct phosphoinositide selectivity that control subcellular localization and RTK-dependent Arf6 signaling during cell migration.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"Cytohesin-1 (CYTH1) is an ARF-family guanine nucleotide exchange factor that couples PI 3-kinase–dependent membrane recruitment to integrin inside-out signaling and ARF-dependent cytoskeletal remodeling in leukocytes, Schwann cells, and hematopoietic progenitors. Its Sec7 domain catalyzes GDP-to-GTP exchange on ARF1 and ARF6, while its PH domain and adjacent polybasic c domain cooperatively bind PI(3,4,5)P3 to drive plasma membrane translocation; a three-nucleotide microexon splice variant in the PH domain switches phosphoinositide selectivity between PIP3 and PI(4,5)P2, thereby controlling subcellular localization and RTK-driven Arf6 activation during cell migration [PMID:9050849, PMID:9693361, PMID:30404949]. Independently of GEF activity, cytohesin-1 directly binds the β2 integrin cytoplasmic tail to activate LFA-1 and CR3, promoting leukocyte adhesion, transendothelial migration, phagocytosis, and hematopoietic stem cell bone marrow homing, whereas its Arf6-directed GEF activity is required for cell spreading, neutrophil oxidative burst, and Schwann cell myelination—the latter regulated by Fyn-mediated phosphorylation of Tyr-382 [PMID:8706128, PMID:10835351, PMID:20018626, PMID:23012656, PMID:27899358]. PKCδ phosphorylation of the polybasic c domain promotes cytohesin-1 association with the actin cytoskeleton and maximal LFA-1–dependent adhesion [PMID:11438522].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"The discovery that cytohesin-1 directly binds the β2 integrin cytoplasmic tail and promotes LFA-1-dependent adhesion established the first molecular link between a Sec7-domain protein and integrin inside-out signaling.\",\n      \"evidence\": \"Co-immunoprecipitation and domain-dissection overexpression/dominant-negative assays in Jurkat T cells\",\n      \"pmids\": [\"8706128\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which cytohesin-1 binding alters integrin conformation was unknown\", \"Whether GEF activity was required for adhesion was untested\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstration that cytohesin-1 catalyzes GDP-to-GTP exchange on ARF1 and ARF3 in vitro defined its enzymatic function as an ARF-GEF, raising the question of how GEF activity relates to integrin regulation.\",\n      \"evidence\": \"In vitro GEF assay with recombinant cytohesin-1 and purified ARFs using radiolabeled nucleotides\",\n      \"pmids\": [\"9050849\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo ARF substrate identity unresolved\", \"Relationship between GEF activity and adhesion function unclear\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Establishing that PI 3-kinase activation drives cytohesin-1 membrane recruitment through cooperative PH domain + polybasic c domain binding to PIP3 (Kd ~100 nM) placed the lipid second messenger upstream of cytohesin-1 and explained its stimulus-dependent translocation.\",\n      \"evidence\": \"BIAcore lipid-binding assays, domain deletion/chimera constructs, constitutively active PI 3-kinase overexpression, membrane fractionation in Jurkat cells\",\n      \"pmids\": [\"9614087\", \"9693361\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other phosphoinositides could substitute in specific cellular contexts\", \"Structural basis of PH–lipid cooperativity with the c domain\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"NMR structure of the Sec7 domain and systematic ARF1 mutagenesis mapped the GEF catalytic interface and showed that the Sec7 domain alone does not bind the β2 integrin tail, establishing that adhesion and GEF functions reside in separable domains.\",\n      \"evidence\": \"NMR spectroscopy, chemical shift perturbation mapping, structure-based and ARF1 chimeric mutagenesis\",\n      \"pmids\": [\"9653114\", \"9756891\", \"10212218\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length structure including PH and c domains not determined\", \"How autoinhibition might regulate GEF activity\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Separation-of-function experiments showed that β2 integrin binding activates LFA-1 epitope expression independently of GEF activity, while ARF-GEF activity is specifically required for post-adhesion cell spreading, resolving two mechanistically distinct outputs of cytohesin-1.\",\n      \"evidence\": \"GEF-dead mutant, β2 cytoplasmic domain mutants, activation epitope antibodies, spreading and adhesion assays\",\n      \"pmids\": [\"10835351\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which ARF substrate mediates spreading was not identified in this study\", \"How GEF-independent integrin activation is transmitted structurally\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identification that PKCδ phosphorylates the polybasic c domain and that this modification drives actin cytoskeleton association and maximal LFA-1 adhesion revealed a post-translational regulatory mechanism for cytohesin-1 localization and function.\",\n      \"evidence\": \"In vitro PKCδ kinase assay, phorbol ester stimulation in vivo, actin co-sedimentation, adhesion assays\",\n      \"pmids\": [\"11438522\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise phosphorylation site(s) not fully mapped\", \"Whether PKCδ phosphorylation affects GEF activity toward ARFs\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Dissection of leukocyte transendothelial migration showed that PH domain–mediated arrest is GEF-independent, while shape change and chemotaxis require both GEF activity and β2 interaction, with ARF6 (not ARF1) identified as the relevant GEF substrate during diapedesis.\",\n      \"evidence\": \"Multiple dominant-negative/mutant constructs in flow-based adhesion and transendothelial chemotaxis assays\",\n      \"pmids\": [\"11747824\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream effectors of Arf6 in diapedesis not identified\", \"In vivo validation in animal models absent\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The finding that cytohesin-1 associates with CR3 (Mac-1) and mediates CD14-TLR2-PI3K–driven CR3 activation for mycobacterial phagocytosis extended its integrin-activating role beyond LFA-1 to complement receptors in innate immunity.\",\n      \"evidence\": \"siRNA knockdown, blocking antibodies, PI3K inhibitors, co-immunoprecipitation of CR3 and cytohesin-1, phagocytosis assay\",\n      \"pmids\": [\"15778383\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct binding site on CR3 not mapped\", \"Single lab, awaits independent confirmation\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Pharmacological inhibition and knockdown in neutrophils demonstrated that cytohesin-1 selectively activates Arf6 downstream of fMLF receptor signaling to drive PLD activation, superoxide production, and granule secretion, linking it to the oxidative burst.\",\n      \"evidence\": \"SecinH3 inhibition, siRNA knockdown and overexpression in PLB-985 neutrophil-like cells, Arf6/Arf1 activation assays, NADPH oxidase assay\",\n      \"pmids\": [\"20018626\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specificity of SecinH3 among cytohesin family members\", \"Whether cytohesin-1 is redundant with other cytohesins in neutrophils\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Discovery that Fyn phosphorylates cytohesin-1 at Y382 and that this event is required for Arf6 activation and Schwann cell myelination—validated in both knockout and phospho-deficient transgenic mice—established a non-immune developmental function for cytohesin-1.\",\n      \"evidence\": \"In vitro Fyn kinase assay, cytohesin-1 KO mice, Y382F knock-in transgenic mice, electron microscopy of myelin, Arf6 activation assay\",\n      \"pmids\": [\"23012656\", \"23636892\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Downstream Arf6 effectors in myelination unidentified\", \"Whether Y382 phosphorylation affects integrin-binding function\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"An RNAi screen followed by in vivo transplantation demonstrated that cytohesin-1 is essential for integrin β1 and β2 activation in human hematopoietic stem/progenitor cells and for their bone marrow homing and engraftment, broadening its role to stem cell biology.\",\n      \"evidence\": \"RNA interference, integrin activation and adhesion assays, HSPC transplantation into immunodeficient mice, intravital microscopy\",\n      \"pmids\": [\"27899358\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which cytohesin-1 domain mediates β1 integrin activation is unknown\", \"Redundancy with other cytohesins in HSPCs not assessed\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Characterization of a PH domain microexon splice variant showed that a single-residue insertion switches phosphoinositide selectivity from PIP3 to PI(4,5)P2, controlling subcellular localization and coupling the PIP3-binding isoform specifically to HGF/Met-induced Arf6 activation and cancer cell migration.\",\n      \"evidence\": \"Isoform-specific expression, phosphoinositide-binding assays, live-cell imaging, Met receptor and Arf6 activation assays, migration assays\",\n      \"pmids\": [\"30404949\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Splicing regulators controlling microexon inclusion unknown\", \"In vivo relevance to tumor metastasis not tested\", \"Whether isoform switching also governs immune cell functions\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the full-length autoinhibited structure of cytohesin-1, the identity of Arf6 effectors mediating myelination and cell spreading, redundancy versus specificity among the four cytohesin family members in vivo, and whether isoform-specific lipid selectivity differentially regulates immune versus developmental functions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No full-length structure available\", \"Arf6 effectors downstream of cytohesin-1 in myelination and spreading not identified\", \"Systematic in vivo comparison of cytohesin family members lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1, 5, 8, 10, 11, 13, 19]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [14]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [3, 6, 25]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 9, 24]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 6, 25]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 6]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [12]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [0, 7, 15, 17, 18, 19, 20]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 6, 13, 17, 21, 25]},\n      {\"term_id\": \"R-HSA-1500931\", \"supporting_discovery_ids\": [0, 9, 15, 24]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [21, 22, 23]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"ITGB2\",\n      \"ARF1\",\n      \"ARF6\",\n      \"PRKCQ\",\n      \"FYN\",\n      \"PHLDB1\",\n      \"ITGAM\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}