{"gene":"CYTH3","run_date":"2026-06-09T22:57:19","timeline":{"discoveries":[{"year":1998,"finding":"ARNO3/CYTH3 (GRP1) Sec7 domain catalyzes guanine nucleotide exchange on ARF1 in vitro, and overexpression of ARNO3 in mammalian cells causes Golgi fragmentation, redistribution of Golgi resident proteins and beta-COP, and inhibition of secretory transport (SEAP assay), establishing a role for CYTH3 in Golgi structure and function through ARF1 activation.","method":"In vitro GEF assay; overexpression in mammalian cells with Golgi morphology readout and secretion assay","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — in vitro GEF assay plus multiple cellular readouts (Golgi morphology, cargo redistribution, secretion inhibition) in a single rigorous study","pmids":["9707577"],"is_preprint":false},{"year":1998,"finding":"GRP1/CYTH3 Sec7 domain catalyzes guanine nucleotide exchange on ARF1 and ARF5 but not ARF6 in vitro; PtdIns(3,4,5)P3 (but not PtdIns(4,5)P2) markedly enhances ARF exchange activity; the PH domain binds PtdIns(3,4,5)P3 with Kd ~0.5 µM, ~100-fold higher affinity than PtdIns(4,5)P2; and this activation is selectively blocked by inositol 1,3,4,5-tetrakisphosphate.","method":"In vitro ARF GEF assay with dioctanoyl lipids; radiolabeled nucleotide exchange; lipid binding assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted in vitro GEF assay with lipid titrations and mutagenic lipid analogs, multiple orthogonal binding measurements","pmids":["9442017"],"is_preprint":false},{"year":1998,"finding":"GRP1/CYTH3 PH domain binds the inositol head group of PtdIns(3,4,5)P3 with high affinity (Kd ~32 nM for Ins(1,3,4,5)P4) and translocates from cytosol to plasma membrane upon NGF or EGF stimulation in PC12 cells in a PI3K-dependent and PH-domain-dependent manner, establishing PtdIns(3,4,5)P3 as the in vivo recruitment signal.","method":"GFP-fusion live-cell confocal microscopy; radiolabeled inositol phosphate binding assay; wortmannin/LY294002/dominant-negative p85 inhibition","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 / Moderate — biochemical binding assay combined with live-cell imaging and pharmacological/genetic inhibition of PI3K","pmids":["9742223"],"is_preprint":false},{"year":1999,"finding":"GRP1/CYTH3 catalyzes GTP/GDP exchange on ARF6 in a cell-free system and co-localizes with endogenous ARF6 at insulin/EGF-induced plasma membrane ruffles; co-expression of GRP1 elevates GTP-loaded ARF6 in intact cells, establishing ARF6 as a physiological substrate of GRP1.","method":"In vitro GEF assay with recombinant proteins; immunofluorescence co-localization; HA-tagged ARF GTP-loading assay in intact cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — cell-free reconstitution plus orthogonal intact-cell GTP-loading assay and co-localization","pmids":["10480924"],"is_preprint":false},{"year":1999,"finding":"The GRP1/CYTH3 PH domain selectively binds PtdIns(3,4,5)P3 and translocates to the plasma membrane in response to insulin (HEK 293 cells) or PDGF (Swiss 3T3 cells); under oxidative stress conditions that generate only PtdIns(3,4)P2, the GRP1 PH domain does not translocate, confirming its strict PtdIns(3,4,5)P3 selectivity in vivo.","method":"GFP-PH domain fusion live-cell confocal microscopy; radioligand displacement lipid assay","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 2 / Moderate — live-cell imaging with orthogonal biochemical lipid measurement, demonstrated with two cell lines and two stimuli","pmids":["10585883"],"is_preprint":false},{"year":2000,"finding":"The unique diglycine motif (Gly274-Gly275) in the GRP1/CYTH3 PH domain is the structural determinant of its ~650-fold selectivity for PtdIns(3,4,5)P3 over PtdIns(4,5)P2; adding a glycine to this motif increases PtdIns(4,5)P2 affinity without affecting PtdIns(3,4,5)P3 binding, whereas deleting a glycine from the ARNO triglycine motif produces the opposite effect.","method":"Mutagenesis of PH domain glycine motif; in vitro lipid binding assay; in-cell HA-PH domain translocation assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — systematic mutagenesis with in vitro binding quantification replicated across multiple protein variants and confirmed in cells","pmids":["10913124"],"is_preprint":false},{"year":2001,"finding":"GRP1/CYTH3 interacts with GRSP1 (a FERM domain-containing protein) via coiled-coil domains in both proteins; virtually all endogenous GRSP1 in lung tissue co-precipitates with GRP1; upon insulin stimulation both proteins co-translocate to plasma membrane ruffles.","method":"(32)P-labeled GRP1 probe screening of cDNA library; immunodepletion; co-expression with immunofluorescence in CHO-T cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — endogenous co-immunodepletion plus domain mapping and co-localization, single lab","pmids":["11445584"],"is_preprint":false},{"year":2004,"finding":"Crystal structures of dual-specificity splice variants of the Grp1/CYTH3 PH domain bound to PtdIns(4,5)P2 and PtdIns(3,4,5)P3 headgroups reveal that a glycine insertion in the beta1/beta2 loop alleviates unfavorable contacts and enables PtdIns(4,5)P2 binding via a novel binding mode, while reducing PtdIns(3,4,5)P3 affinity through loss of beta1/beta2 loop contacts; systematic mutagenesis validates these structural determinants.","method":"X-ray crystallography; systematic mutagenesis; in vitro binding assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structures with functional mutagenesis, multiple constructs and ligands","pmids":["15359279"],"is_preprint":false},{"year":2004,"finding":"GRP1/CYTH3 PH domain binds membrane-embedded PIP3 with Kd ~50 nM; background anionic lipids (PS, PI) facilitate PIP3 docking by increasing the on-rate via a two-step electrostatic search mechanism, without changing the off-rate from the specific PIP3 site.","method":"Protein-to-membrane FRET equilibrium and stopped-flow kinetics; competitive binding assay with defined lipid bilayers","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — reconstituted bilayer system with quantitative kinetic and equilibrium FRET measurements, rigorous mechanistic dissection","pmids":["15610010"],"is_preprint":false},{"year":2008,"finding":"GRP1/CYTH3 PH domain membrane anchoring is multivalent: specific PtdIns(3,4,5)P3 recognition triggers insertion into the membrane; acidic pH enhances binding ~22-fold partly through protonation of His355; phosphatidylserine/PI further amplifies affinity ~6-fold via electrostatics.","method":"NMR; surface plasmon resonance; monolayer surface tension experiments; site-directed mutagenesis (H355 mutant)","journal":"Journal of lipid research","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR plus biophysical assays plus mutagenesis, three orthogonal methods in one study","pmids":["18469301"],"is_preprint":false},{"year":2011,"finding":"Hydrophobic residues in loops flanking the PIP3-binding site of the GRP1/CYTH3 PH domain contribute to membrane penetration; mutations converting these hydrophobic residues to polar residues reduce membrane insertion, supporting a dual-recognition model involving both specific PIP3 contacts and nonspecific loop-bilayer interactions.","method":"Molecular dynamics simulations; NMR chemical shift perturbation; monolayer penetration experiments; mutagenesis","journal":"Structure (London, England : 1993)","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR and biophysical experiments with mutagenesis validated by MD simulations, multiple orthogonal methods","pmids":["21893292"],"is_preprint":false},{"year":2011,"finding":"Residue E345 in the GRP1/CYTH3 PH domain acts as a sentry glutamate that excludes PtdIns(4,5)P2; the E345K charge-reversal mutation increases PI(4,5)P2 affinity 8-fold and causes constitutive plasma membrane targeting in cells; hydrolysis of PI(4,5)P2 releases the E345K mutant, with efficiency increased when Arf6 binding is also disrupted.","method":"Site-directed mutagenesis; in vitro lipid binding assay; live-cell GFP translocation imaging; PI(4,5)P2 hydrolysis experiment","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis with quantitative binding plus live-cell localization, mechanistically rigorous","pmids":["21932773"],"is_preprint":false},{"year":2012,"finding":"EPR site-directed spin labeling defines the membrane docking geometry of GRP1/CYTH3 PH domain bound to bilayer-embedded PIP3: the domain engulfs the PIP3 headgroup with minimal bilayer penetration, representing the shallowest membrane docking geometry yet described for a lipid-binding domain.","method":"EPR site-directed spin labeling and relaxation; 18 spin-labeled positions; comparison with crystal structure","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — systematic EPR spin-labeling across 18 positions with structural modeling, rigorous biophysical determination","pmids":["22479423"],"is_preprint":false},{"year":2012,"finding":"Grp1/CYTH3 acts as a GEF for ARF6 to promote GLUT4 vesicle formation and subsequent recycling steps; insulin signaling regulates Grp1 through Akt-mediated phosphorylation; phosphomimetic mutations of Grp1 can bypass upstream insulin signaling to induce GLUT4 recycling.","method":"siRNA knockdown; phosphomimetic mutagenesis; GLUT4 recycling assay; co-IP; in vitro kinase assay","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — GEF activity assay, genetic rescue with phosphomimetic mutant, and functional GLUT4 trafficking readout, multiple orthogonal methods","pmids":["22609160"],"is_preprint":false},{"year":2013,"finding":"Background anionic PS lipids recruit GRP1/CYTH3 PH domain to the membrane via nonspecific electrostatic interactions enabling a two-dimensional 'hopping' search mechanism for the rare PIP3 target lipid prior to specific docking, as revealed by combining MD simulations with EPR and FRET kinetics.","method":"All-atom molecular dynamics simulations; coarse-grained simulations; EPR membrane docking geometry; FRET kinetic studies","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — computational modeling validated by orthogonal experimental methods (EPR, FRET)","pmids":["23747485"],"is_preprint":false},{"year":2014,"finding":"In C. elegans, GRP-1 (the sole cytohesin ortholog) controls asymmetric neuroblast divisions that produce apoptotic daughters; loss of grp-1 results in more symmetric cell sizes and conversion of the apoptotic daughter to its sister fate (extra neurons); GRP-1's GEF activity (Sec7 domain) is necessary, and genetic interactions place GRP-1 in a pathway with ARF GAP CNT-2 and ARF GEFs EFA-6 and BRIS-1; GRP-1 acts at the plasma membrane/cytokinetic furrow.","method":"C. elegans genetics; loss-of-function mutants; rescue with Sec7-domain constructs; GFP localization; genetic epistasis","journal":"Genetics","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with multiple alleles, domain-specific rescue, and localization, multiple orthogonal approaches in single lab","pmids":["25053664"],"is_preprint":false},{"year":2010,"finding":"Grp1/CYTH3 forms homodimers at low micromolar concentrations via N-terminal heptad repeats and spontaneously re-equilibrates with Grsp1 to form heterodimers in an antiparallel orientation; formation of Grsp1-Grp1 heterodimers does not substantially alter Grp1 binding to PtdIns(3,4,5)P3 or PtdIns(4,5)P2 headgroups or liposome partitioning.","method":"Analytical ultracentrifugation; FRET; liposome binding assay","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — analytical ultracentrifugation and FRET for oligomeric state, functional lipid binding measured in parallel, single lab","pmids":["20527794"],"is_preprint":false},{"year":2012,"finding":"GRASP (Grp1-associated scaffold protein) interacts with Grp1/CYTH3 and regulates ARF6-dependent endocytic recycling; co-expression of GRASP and Grp1 promotes membrane ruffling (hallmark of ARF6 activation); overexpressed GRASP blocks MHC-I recycling via the Arf6-dependent pathway but does not affect clathrin-dependent transferrin receptor recycling.","method":"Co-expression; immunofluorescence co-localization; MHC-I and transferrin receptor recycling assays","journal":"Cell biology international","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — co-localization and functional recycling assays, single lab, no direct biochemical interaction measurement reported in abstract","pmids":["22931251"],"is_preprint":false},{"year":2019,"finding":"Cytohesin-3/CYTH3-deficient mice show significantly reduced insulin receptor-dependent signaling (reduced phosphorylation downstream of insulin receptor) in liver and adipose tissue; cyth3-deficient mice on high-fat diet display reduced weight gain, reduced body fat, and increased lipid excretion with reduced bile acid synthesis gene expression, establishing CYTH3 as required for full insulin receptor signaling in mammals.","method":"Genetic knockout mouse model; insulin injection and signaling readout by Western blot; metabolic phenotyping; fecal lipid analysis","journal":"Scientific reports","confidence":"High","confidence_rationale":"Tier 2 / Moderate — clean genetic KO with defined molecular signaling phenotype plus multiple metabolic readouts, rigorous in vivo study","pmids":["30837656"],"is_preprint":false},{"year":2020,"finding":"Phosphorylation of Grp1/CYTH3 switches its PH domain specificity from PtdIns(3,4,5)P3 (plasma membrane) to phosphatidylinositol 4-phosphate (PI4P; recycling endosome); phosphorylation also releases an autoinhibitory mechanism allowing the coiled-coil domain to engage two peripheral membrane proteins of the recycling endosome, redirecting Grp1 recruitment from plasma membrane to recycling endosome.","method":"Phosphomimetic and phosphodeficient mutants; lipid binding assays; subcellular fractionation/localization; co-IP of recycling endosome peripheral membrane proteins","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mechanistic dissection with phosphomimetic mutations, quantitative lipid binding, and localization readouts showing both specificity switch and autoinhibition release","pmids":["33026967"],"is_preprint":false},{"year":1998,"finding":"CYTH3/ARNO3 chromosomal locus maps to human chromosome 7p21 by radiation hybrid mapping.","method":"PCR of radiation hybrid panel and somatic cell hybrid panel","journal":"Annals of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — straightforward chromosomal mapping by radiation hybrid, single study","pmids":["10363132"],"is_preprint":false}],"current_model":"CYTH3 (GRP1/ARNO3/cytohesin-3) is a PtdIns(3,4,5)P3-activated ARF guanine nucleotide exchange factor whose PH domain selectively binds PI(3,4,5)P3 at the plasma membrane inner leaflet via a diglycine motif and a sentry glutamate (E345), recruiting the protein from the cytosol to membrane ruffles in response to growth factor and insulin signaling; once membrane-localized, its Sec7 domain catalyzes GTP loading on ARF1, ARF5, and ARF6 to regulate Golgi structure, vesicle trafficking, and GLUT4 recycling; Akt-mediated phosphorylation of CYTH3 switches its PH domain specificity from PI(3,4,5)P3 to PI4P and releases an autoinhibitory coiled-coil interaction, redirecting the protein to the recycling endosome; CYTH3 is required for full insulin receptor signaling in mammals and interacts with scaffold proteins GRSP1/GRASP through coiled-coil domains to form signaling complexes."},"narrative":{"mechanistic_narrative":"CYTH3 (GRP1/ARNO3/cytohesin-3) is a phosphoinositide-regulated guanine nucleotide exchange factor that couples PI3K signaling to ARF GTPase activation at membranes, governing Golgi structure, vesicle trafficking, and growth-factor/insulin-responsive cargo recycling [PMID:9707577, PMID:22609160]. Its Sec7 domain catalyzes GTP loading on ARF1 and ARF5 in vitro and on ARF6 both in cell-free systems and in intact cells, where overexpression fragments the Golgi and inhibits secretion via ARF1, while ARF6 activation drives plasma membrane ruffling [PMID:9707577, PMID:9442017, PMID:10480924]. Membrane recruitment is directed by a C-terminal PH domain that binds PtdIns(3,4,5)P3 with high affinity and ~650-fold selectivity over PtdIns(4,5)P2; this selectivity is set by a unique diglycine motif and a sentry glutamate (E345) that excludes PI(4,5)P2, and PIP3 binding both recruits the protein and markedly enhances its ARF exchange activity [PMID:9442017, PMID:9742223, PMID:10913124, PMID:21932773]. Consequently, growth factor and insulin stimulation triggers PI3K-dependent translocation of CYTH3 from cytosol to plasma membrane ruffles [PMID:9742223, PMID:10585883]. Akt-mediated phosphorylation reprograms this behavior: it switches PH-domain specificity from PI(3,4,5)P3 to PI4P and releases an autoinhibitory coiled-coil interaction, redirecting the protein to the recycling endosome to promote ARF6-dependent GLUT4 vesicle formation and recycling [PMID:22609160, PMID:33026967]. CYTH3 forms homodimers and antiparallel heterodimers with the scaffold protein GRSP1 through N-terminal coiled-coil heptad repeats, and engages the related scaffold GRASP to organize ARF6-dependent endocytic recycling [PMID:11445584, PMID:20527794, PMID:22931251]. In vivo, CYTH3 is required for full insulin receptor signaling in mouse liver and adipose tissue and influences body fat and lipid handling, and its sole C. elegans ortholog controls asymmetric, apoptosis-generating neuroblast divisions in a Sec7-dependent manner [PMID:30837656, PMID:25053664].","teleology":[{"year":1998,"claim":"Established that CYTH3 is a catalytically active ARF GEF and linked its activity to Golgi architecture and secretion, defining its core enzymatic and cellular function.","evidence":"In vitro Sec7-domain GEF assay on ARF1 plus overexpression with Golgi morphology and SEAP secretion readouts in mammalian cells","pmids":["9707577"],"confidence":"High","gaps":["Did not establish the physiological recruitment signal","ARF6 not yet tested as substrate"]},{"year":1998,"claim":"Defined the PIP3-dependence of GEF activity and the PH-domain lipid ligand, revealing that 3-phosphoinositides both recruit and activate the enzyme.","evidence":"Reconstituted in vitro GEF assays with dioctanoyl lipids, radiolabeled nucleotide exchange, and lipid binding with Ins(1,3,4,5)P4 competition; GFP live-cell imaging with PI3K inhibition in PC12 cells","pmids":["9442017","9742223"],"confidence":"High","gaps":["Structural basis of PIP3 selectivity unknown","Coupling between lipid binding and catalytic enhancement not resolved"]},{"year":1999,"claim":"Identified ARF6 as a physiological substrate and placed CYTH3 at growth-factor-induced plasma membrane ruffles, broadening its substrate range beyond Golgi ARFs.","evidence":"Cell-free GEF assay on ARF6, intact-cell HA-ARF6 GTP-loading, and immunofluorescence co-localization at insulin/EGF-induced ruffles","pmids":["10480924","10585883"],"confidence":"High","gaps":["In vitro and in-cell ARF6 results required reconciliation with earlier ARF6-negative data","Effectors downstream of ARF6 at ruffles not defined"]},{"year":2000,"claim":"Mapped the structural determinant of phosphoinositide selectivity to a diglycine motif, explaining strict PIP3 preference among cytohesin PH domains.","evidence":"Glycine-insertion/deletion mutagenesis of the PH-domain loop with in vitro binding and in-cell translocation assays","pmids":["10913124"],"confidence":"High","gaps":["Atomic structure of headgroup recognition not yet solved"]},{"year":2001,"claim":"Identified GRSP1 as a coiled-coil binding partner that co-translocates with CYTH3 upon insulin, suggesting assembly of an insulin-responsive signaling complex.","evidence":"32P-labeled GRP1 probe cDNA screen, endogenous co-immunodepletion, and co-localization in CHO-T cells","pmids":["11445584"],"confidence":"Medium","gaps":["Functional consequence of GRSP1 binding for GEF activity not established","Single lab, no reciprocal biochemical validation of stoichiometry in vivo"]},{"year":2004,"claim":"Solved crystal structures of PH-domain splice variants bound to PIP2 and PIP3 headgroups, providing the atomic explanation for how loop length tunes lipid specificity.","evidence":"X-ray crystallography of dual-specificity variants with systematic mutagenesis and in vitro binding","pmids":["15359279"],"confidence":"High","gaps":["Headgroup-only structures did not capture membrane-bilayer docking geometry"]},{"year":2004,"claim":"Quantified membrane-embedded PIP3 binding and revealed that background anionic lipids accelerate docking, refining the recruitment mechanism beyond simple headgroup affinity.","evidence":"Protein-to-membrane FRET equilibrium and stopped-flow kinetics with defined bilayers and competitive binding","pmids":["15610010"],"confidence":"High","gaps":["In vivo relevance of the two-step electrostatic search not directly tested"]},{"year":2008,"claim":"Showed PH-domain anchoring is multivalent, integrating PIP3 recognition with pH and anionic-lipid electrostatics through residue H355.","evidence":"NMR, surface plasmon resonance, monolayer surface tension, and H355 mutagenesis","pmids":["18469301"],"confidence":"High","gaps":["Physiological pH-dependence in a cellular context not demonstrated"]},{"year":2010,"claim":"Defined the oligomeric basis of CYTH3 scaffolding, showing homodimerization and antiparallel heterodimerization with GRSP1 that do not perturb lipid binding.","evidence":"Analytical ultracentrifugation, FRET, and liposome binding assays","pmids":["20527794"],"confidence":"Medium","gaps":["Functional output of dimerization on GEF signaling not resolved","Single-lab biophysical study"]},{"year":2011,"claim":"Identified the sentry glutamate E345 and hydrophobic flanking-loop residues as determinants of PIP2 exclusion and membrane penetration, explaining how the domain enforces specificity and inserts into the bilayer.","evidence":"Site-directed mutagenesis (E345K, hydrophobic-to-polar), in vitro binding, live-cell GFP imaging, PI(4,5)P2 hydrolysis, NMR, MD, and monolayer penetration","pmids":["21932773","21893292"],"confidence":"High","gaps":["Whether endogenous regulation exploits E345 charge state was not addressed"]},{"year":2012,"claim":"Determined the shallow membrane docking geometry and demonstrated that CYTH3 acts as an ARF6 GEF driving GLUT4 vesicle formation and recycling under insulin/Akt control.","evidence":"EPR spin-labeling across 18 positions; siRNA knockdown, phosphomimetic mutagenesis, GLUT4 recycling assays, co-IP, and in vitro kinase assay","pmids":["22479423","22609160"],"confidence":"High","gaps":["Identity of the Akt phosphosites and their structural consequence not fully resolved at this stage","Direct in vivo demonstration of insulin-dependent recycling not yet shown"]},{"year":2012,"claim":"Linked CYTH3 to the GRASP scaffold in directing ARF6-dependent (but not clathrin-dependent) endocytic recycling, extending its recycling role to MHC-I cargo.","evidence":"Co-expression, co-localization, and MHC-I versus transferrin receptor recycling assays","pmids":["22931251"],"confidence":"Medium","gaps":["No direct biochemical interaction measurement reported","Single-lab functional assay"]},{"year":2013,"claim":"Provided a mechanistic model for membrane targeting in which nonspecific PS-driven electrostatics enable a 2D hopping search for rare PIP3 before specific docking.","evidence":"All-atom and coarse-grained MD simulations validated by EPR docking geometry and FRET kinetics","pmids":["23747485"],"confidence":"High","gaps":["Search mechanism inferred largely computationally; in-cell visualization of hopping not achieved"]},{"year":2014,"claim":"Demonstrated conserved physiological function of the cytohesin in C. elegans, where Sec7-dependent GEF activity controls asymmetric divisions producing apoptotic daughters.","evidence":"Loss-of-function genetics, domain-specific rescue, GFP localization, and epistasis with ARF GAP CNT-2 and ARF GEFs EFA-6/BRIS-1","pmids":["25053664"],"confidence":"High","gaps":["Direct ARF substrate engaged in this division pathway not biochemically defined","Mammalian counterpart of this developmental role not tested"]},{"year":2019,"claim":"Established the in vivo requirement of CYTH3 for full insulin receptor signaling and its impact on systemic lipid metabolism using a knockout mouse.","evidence":"Cyth3 knockout mice with insulin-stimulation Western blots, high-fat-diet metabolic phenotyping, and fecal lipid analysis","pmids":["30837656"],"confidence":"High","gaps":["Tissue-specific GEF substrate driving the insulin signaling defect not pinpointed","Link between insulin signaling defect and altered bile acid gene expression not mechanistically resolved"]},{"year":2020,"claim":"Unified the regulatory logic by showing phosphorylation simultaneously switches PH-domain lipid specificity (PIP3 to PI4P) and releases autoinhibition, redirecting CYTH3 from plasma membrane to recycling endosome.","evidence":"Phosphomimetic/phosphodeficient mutants with lipid binding assays, subcellular localization, and co-IP of recycling endosome peripheral proteins","pmids":["33026967"],"confidence":"High","gaps":["Identity of the two recycling-endosome coiled-coil partners not fully characterized","Kinase-substrate dynamics in vivo not temporally resolved"]},{"year":null,"claim":"How CYTH3's distinct GEF outputs (ARF1 at Golgi versus ARF6 at plasma membrane/recycling endosome) are selectively deployed by specific stimuli and scaffolds in different tissues remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No integrated model linking stimulus, phosphorylation state, scaffold choice, and ARF isoform selection","Disease relevance in humans not established by direct genetic evidence in the corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,1,3,13]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[1,2,4,5,8,9,11]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[2,4,19]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[6,16,17]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[2,3,4,6,11,15]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,19]},{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[13,17,19]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,13,18,19]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,13,17]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[13]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[15]}],"complexes":[],"partners":["ARF1","ARF5","ARF6","GRSP1","GRASP"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O43739","full_name":"Cytohesin-3","aliases":["ARF nucleotide-binding site opener 3","Protein ARNO3","General receptor of phosphoinositides 1","Grp1","PH, SEC7 and coiled-coil domain-containing protein 3"],"length_aa":400,"mass_kda":46.3,"function":"Promotes guanine-nucleotide exchange on ARF1 and ARF6. Promotes the activation of ARF factors through replacement of GDP with GTP. Plays a role in the epithelial polarization (By similarity)","subcellular_location":"Cytoplasm, cytosol; Cell membrane; Cell junction, adherens junction; Cell junction, tight junction","url":"https://www.uniprot.org/uniprotkb/O43739/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CYTH3","classification":"Not Classified","n_dependent_lines":9,"n_total_lines":1208,"dependency_fraction":0.0074503311258278145},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CYTH3","total_profiled":1310},"omim":[{"mim_id":"617467","title":"FERM DOMAIN-CONTAINING PROTEIN 4B; FRMD4B","url":"https://www.omim.org/entry/617467"},{"mim_id":"605081","title":"CYTOHESIN 3; CYTH3","url":"https://www.omim.org/entry/605081"},{"mim_id":"182115","title":"CYTOHESIN 1; CYTH1","url":"https://www.omim.org/entry/182115"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CYTH3"},"hgnc":{"alias_symbol":["GRP1","ARNO3","cytohesin-3"],"prev_symbol":["PSCD3"]},"alphafold":{"accession":"O43739","domains":[{"cath_id":"1.10.220.20","chopping":"59-137","consensus_level":"medium","plddt":91.6909,"start":59,"end":137},{"cath_id":"1.10.1000.11","chopping":"142-259","consensus_level":"high","plddt":89.806,"start":142,"end":259},{"cath_id":"2.30.29.30","chopping":"266-381","consensus_level":"high","plddt":88.1064,"start":266,"end":381},{"cath_id":"1.10.287","chopping":"24-56","consensus_level":"medium","plddt":75.5958,"start":24,"end":56}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O43739","model_url":"https://alphafold.ebi.ac.uk/files/AF-O43739-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O43739-F1-predicted_aligned_error_v6.png","plddt_mean":84.19},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CYTH3","jax_strain_url":"https://www.jax.org/strain/search?query=CYTH3"},"sequence":{"accession":"O43739","fasta_url":"https://rest.uniprot.org/uniprotkb/O43739.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O43739/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O43739"}},"corpus_meta":[{"pmid":"11048651","id":"PMC_11048651","title":"The 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overexpression in mammalian cells with Golgi morphology readout and secretion assay\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro GEF assay plus multiple cellular readouts (Golgi morphology, cargo redistribution, secretion inhibition) in a single rigorous study\",\n      \"pmids\": [\"9707577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"GRP1/CYTH3 Sec7 domain catalyzes guanine nucleotide exchange on ARF1 and ARF5 but not ARF6 in vitro; PtdIns(3,4,5)P3 (but not PtdIns(4,5)P2) markedly enhances ARF exchange activity; the PH domain binds PtdIns(3,4,5)P3 with Kd ~0.5 µM, ~100-fold higher affinity than PtdIns(4,5)P2; and this activation is selectively blocked by inositol 1,3,4,5-tetrakisphosphate.\",\n      \"method\": \"In vitro ARF GEF assay with dioctanoyl lipids; radiolabeled nucleotide exchange; lipid binding assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted in vitro GEF assay with lipid titrations and mutagenic lipid analogs, multiple orthogonal binding measurements\",\n      \"pmids\": [\"9442017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"GRP1/CYTH3 PH domain binds the inositol head group of PtdIns(3,4,5)P3 with high affinity (Kd ~32 nM for Ins(1,3,4,5)P4) and translocates from cytosol to plasma membrane upon NGF or EGF stimulation in PC12 cells in a PI3K-dependent and PH-domain-dependent manner, establishing PtdIns(3,4,5)P3 as the in vivo recruitment signal.\",\n      \"method\": \"GFP-fusion live-cell confocal microscopy; radiolabeled inositol phosphate binding assay; wortmannin/LY294002/dominant-negative p85 inhibition\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical binding assay combined with live-cell imaging and pharmacological/genetic inhibition of PI3K\",\n      \"pmids\": [\"9742223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"GRP1/CYTH3 catalyzes GTP/GDP exchange on ARF6 in a cell-free system and co-localizes with endogenous ARF6 at insulin/EGF-induced plasma membrane ruffles; co-expression of GRP1 elevates GTP-loaded ARF6 in intact cells, establishing ARF6 as a physiological substrate of GRP1.\",\n      \"method\": \"In vitro GEF assay with recombinant proteins; immunofluorescence co-localization; HA-tagged ARF GTP-loading assay in intact cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — cell-free reconstitution plus orthogonal intact-cell GTP-loading assay and co-localization\",\n      \"pmids\": [\"10480924\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The GRP1/CYTH3 PH domain selectively binds PtdIns(3,4,5)P3 and translocates to the plasma membrane in response to insulin (HEK 293 cells) or PDGF (Swiss 3T3 cells); under oxidative stress conditions that generate only PtdIns(3,4)P2, the GRP1 PH domain does not translocate, confirming its strict PtdIns(3,4,5)P3 selectivity in vivo.\",\n      \"method\": \"GFP-PH domain fusion live-cell confocal microscopy; radioligand displacement lipid assay\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — live-cell imaging with orthogonal biochemical lipid measurement, demonstrated with two cell lines and two stimuli\",\n      \"pmids\": [\"10585883\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The unique diglycine motif (Gly274-Gly275) in the GRP1/CYTH3 PH domain is the structural determinant of its ~650-fold selectivity for PtdIns(3,4,5)P3 over PtdIns(4,5)P2; adding a glycine to this motif increases PtdIns(4,5)P2 affinity without affecting PtdIns(3,4,5)P3 binding, whereas deleting a glycine from the ARNO triglycine motif produces the opposite effect.\",\n      \"method\": \"Mutagenesis of PH domain glycine motif; in vitro lipid binding assay; in-cell HA-PH domain translocation assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — systematic mutagenesis with in vitro binding quantification replicated across multiple protein variants and confirmed in cells\",\n      \"pmids\": [\"10913124\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"GRP1/CYTH3 interacts with GRSP1 (a FERM domain-containing protein) via coiled-coil domains in both proteins; virtually all endogenous GRSP1 in lung tissue co-precipitates with GRP1; upon insulin stimulation both proteins co-translocate to plasma membrane ruffles.\",\n      \"method\": \"(32)P-labeled GRP1 probe screening of cDNA library; immunodepletion; co-expression with immunofluorescence in CHO-T cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — endogenous co-immunodepletion plus domain mapping and co-localization, single lab\",\n      \"pmids\": [\"11445584\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"Crystal structures of dual-specificity splice variants of the Grp1/CYTH3 PH domain bound to PtdIns(4,5)P2 and PtdIns(3,4,5)P3 headgroups reveal that a glycine insertion in the beta1/beta2 loop alleviates unfavorable contacts and enables PtdIns(4,5)P2 binding via a novel binding mode, while reducing PtdIns(3,4,5)P3 affinity through loss of beta1/beta2 loop contacts; systematic mutagenesis validates these structural determinants.\",\n      \"method\": \"X-ray crystallography; systematic mutagenesis; in vitro binding assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structures with functional mutagenesis, multiple constructs and ligands\",\n      \"pmids\": [\"15359279\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"GRP1/CYTH3 PH domain binds membrane-embedded PIP3 with Kd ~50 nM; background anionic lipids (PS, PI) facilitate PIP3 docking by increasing the on-rate via a two-step electrostatic search mechanism, without changing the off-rate from the specific PIP3 site.\",\n      \"method\": \"Protein-to-membrane FRET equilibrium and stopped-flow kinetics; competitive binding assay with defined lipid bilayers\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — reconstituted bilayer system with quantitative kinetic and equilibrium FRET measurements, rigorous mechanistic dissection\",\n      \"pmids\": [\"15610010\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"GRP1/CYTH3 PH domain membrane anchoring is multivalent: specific PtdIns(3,4,5)P3 recognition triggers insertion into the membrane; acidic pH enhances binding ~22-fold partly through protonation of His355; phosphatidylserine/PI further amplifies affinity ~6-fold via electrostatics.\",\n      \"method\": \"NMR; surface plasmon resonance; monolayer surface tension experiments; site-directed mutagenesis (H355 mutant)\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR plus biophysical assays plus mutagenesis, three orthogonal methods in one study\",\n      \"pmids\": [\"18469301\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Hydrophobic residues in loops flanking the PIP3-binding site of the GRP1/CYTH3 PH domain contribute to membrane penetration; mutations converting these hydrophobic residues to polar residues reduce membrane insertion, supporting a dual-recognition model involving both specific PIP3 contacts and nonspecific loop-bilayer interactions.\",\n      \"method\": \"Molecular dynamics simulations; NMR chemical shift perturbation; monolayer penetration experiments; mutagenesis\",\n      \"journal\": \"Structure (London, England : 1993)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR and biophysical experiments with mutagenesis validated by MD simulations, multiple orthogonal methods\",\n      \"pmids\": [\"21893292\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Residue E345 in the GRP1/CYTH3 PH domain acts as a sentry glutamate that excludes PtdIns(4,5)P2; the E345K charge-reversal mutation increases PI(4,5)P2 affinity 8-fold and causes constitutive plasma membrane targeting in cells; hydrolysis of PI(4,5)P2 releases the E345K mutant, with efficiency increased when Arf6 binding is also disrupted.\",\n      \"method\": \"Site-directed mutagenesis; in vitro lipid binding assay; live-cell GFP translocation imaging; PI(4,5)P2 hydrolysis experiment\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis with quantitative binding plus live-cell localization, mechanistically rigorous\",\n      \"pmids\": [\"21932773\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"EPR site-directed spin labeling defines the membrane docking geometry of GRP1/CYTH3 PH domain bound to bilayer-embedded PIP3: the domain engulfs the PIP3 headgroup with minimal bilayer penetration, representing the shallowest membrane docking geometry yet described for a lipid-binding domain.\",\n      \"method\": \"EPR site-directed spin labeling and relaxation; 18 spin-labeled positions; comparison with crystal structure\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — systematic EPR spin-labeling across 18 positions with structural modeling, rigorous biophysical determination\",\n      \"pmids\": [\"22479423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Grp1/CYTH3 acts as a GEF for ARF6 to promote GLUT4 vesicle formation and subsequent recycling steps; insulin signaling regulates Grp1 through Akt-mediated phosphorylation; phosphomimetic mutations of Grp1 can bypass upstream insulin signaling to induce GLUT4 recycling.\",\n      \"method\": \"siRNA knockdown; phosphomimetic mutagenesis; GLUT4 recycling assay; co-IP; in vitro kinase assay\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — GEF activity assay, genetic rescue with phosphomimetic mutant, and functional GLUT4 trafficking readout, multiple orthogonal methods\",\n      \"pmids\": [\"22609160\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Background anionic PS lipids recruit GRP1/CYTH3 PH domain to the membrane via nonspecific electrostatic interactions enabling a two-dimensional 'hopping' search mechanism for the rare PIP3 target lipid prior to specific docking, as revealed by combining MD simulations with EPR and FRET kinetics.\",\n      \"method\": \"All-atom molecular dynamics simulations; coarse-grained simulations; EPR membrane docking geometry; FRET kinetic studies\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — computational modeling validated by orthogonal experimental methods (EPR, FRET)\",\n      \"pmids\": [\"23747485\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In C. elegans, GRP-1 (the sole cytohesin ortholog) controls asymmetric neuroblast divisions that produce apoptotic daughters; loss of grp-1 results in more symmetric cell sizes and conversion of the apoptotic daughter to its sister fate (extra neurons); GRP-1's GEF activity (Sec7 domain) is necessary, and genetic interactions place GRP-1 in a pathway with ARF GAP CNT-2 and ARF GEFs EFA-6 and BRIS-1; GRP-1 acts at the plasma membrane/cytokinetic furrow.\",\n      \"method\": \"C. elegans genetics; loss-of-function mutants; rescue with Sec7-domain constructs; GFP localization; genetic epistasis\",\n      \"journal\": \"Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with multiple alleles, domain-specific rescue, and localization, multiple orthogonal approaches in single lab\",\n      \"pmids\": [\"25053664\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Grp1/CYTH3 forms homodimers at low micromolar concentrations via N-terminal heptad repeats and spontaneously re-equilibrates with Grsp1 to form heterodimers in an antiparallel orientation; formation of Grsp1-Grp1 heterodimers does not substantially alter Grp1 binding to PtdIns(3,4,5)P3 or PtdIns(4,5)P2 headgroups or liposome partitioning.\",\n      \"method\": \"Analytical ultracentrifugation; FRET; liposome binding assay\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — analytical ultracentrifugation and FRET for oligomeric state, functional lipid binding measured in parallel, single lab\",\n      \"pmids\": [\"20527794\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"GRASP (Grp1-associated scaffold protein) interacts with Grp1/CYTH3 and regulates ARF6-dependent endocytic recycling; co-expression of GRASP and Grp1 promotes membrane ruffling (hallmark of ARF6 activation); overexpressed GRASP blocks MHC-I recycling via the Arf6-dependent pathway but does not affect clathrin-dependent transferrin receptor recycling.\",\n      \"method\": \"Co-expression; immunofluorescence co-localization; MHC-I and transferrin receptor recycling assays\",\n      \"journal\": \"Cell biology international\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — co-localization and functional recycling assays, single lab, no direct biochemical interaction measurement reported in abstract\",\n      \"pmids\": [\"22931251\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Cytohesin-3/CYTH3-deficient mice show significantly reduced insulin receptor-dependent signaling (reduced phosphorylation downstream of insulin receptor) in liver and adipose tissue; cyth3-deficient mice on high-fat diet display reduced weight gain, reduced body fat, and increased lipid excretion with reduced bile acid synthesis gene expression, establishing CYTH3 as required for full insulin receptor signaling in mammals.\",\n      \"method\": \"Genetic knockout mouse model; insulin injection and signaling readout by Western blot; metabolic phenotyping; fecal lipid analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean genetic KO with defined molecular signaling phenotype plus multiple metabolic readouts, rigorous in vivo study\",\n      \"pmids\": [\"30837656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Phosphorylation of Grp1/CYTH3 switches its PH domain specificity from PtdIns(3,4,5)P3 (plasma membrane) to phosphatidylinositol 4-phosphate (PI4P; recycling endosome); phosphorylation also releases an autoinhibitory mechanism allowing the coiled-coil domain to engage two peripheral membrane proteins of the recycling endosome, redirecting Grp1 recruitment from plasma membrane to recycling endosome.\",\n      \"method\": \"Phosphomimetic and phosphodeficient mutants; lipid binding assays; subcellular fractionation/localization; co-IP of recycling endosome peripheral membrane proteins\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mechanistic dissection with phosphomimetic mutations, quantitative lipid binding, and localization readouts showing both specificity switch and autoinhibition release\",\n      \"pmids\": [\"33026967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"CYTH3/ARNO3 chromosomal locus maps to human chromosome 7p21 by radiation hybrid mapping.\",\n      \"method\": \"PCR of radiation hybrid panel and somatic cell hybrid panel\",\n      \"journal\": \"Annals of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — straightforward chromosomal mapping by radiation hybrid, single study\",\n      \"pmids\": [\"10363132\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CYTH3 (GRP1/ARNO3/cytohesin-3) is a PtdIns(3,4,5)P3-activated ARF guanine nucleotide exchange factor whose PH domain selectively binds PI(3,4,5)P3 at the plasma membrane inner leaflet via a diglycine motif and a sentry glutamate (E345), recruiting the protein from the cytosol to membrane ruffles in response to growth factor and insulin signaling; once membrane-localized, its Sec7 domain catalyzes GTP loading on ARF1, ARF5, and ARF6 to regulate Golgi structure, vesicle trafficking, and GLUT4 recycling; Akt-mediated phosphorylation of CYTH3 switches its PH domain specificity from PI(3,4,5)P3 to PI4P and releases an autoinhibitory coiled-coil interaction, redirecting the protein to the recycling endosome; CYTH3 is required for full insulin receptor signaling in mammals and interacts with scaffold proteins GRSP1/GRASP through coiled-coil domains to form signaling complexes.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"CYTH3 (GRP1/ARNO3/cytohesin-3) is a phosphoinositide-regulated guanine nucleotide exchange factor that couples PI3K signaling to ARF GTPase activation at membranes, governing Golgi structure, vesicle trafficking, and growth-factor/insulin-responsive cargo recycling [#0, #13]. Its Sec7 domain catalyzes GTP loading on ARF1 and ARF5 in vitro and on ARF6 both in cell-free systems and in intact cells, where overexpression fragments the Golgi and inhibits secretion via ARF1, while ARF6 activation drives plasma membrane ruffling [#0, #1, #3]. Membrane recruitment is directed by a C-terminal PH domain that binds PtdIns(3,4,5)P3 with high affinity and ~650-fold selectivity over PtdIns(4,5)P2; this selectivity is set by a unique diglycine motif and a sentry glutamate (E345) that excludes PI(4,5)P2, and PIP3 binding both recruits the protein and markedly enhances its ARF exchange activity [#1, #2, #5, #11]. Consequently, growth factor and insulin stimulation triggers PI3K-dependent translocation of CYTH3 from cytosol to plasma membrane ruffles [#2, #4]. Akt-mediated phosphorylation reprograms this behavior: it switches PH-domain specificity from PI(3,4,5)P3 to PI4P and releases an autoinhibitory coiled-coil interaction, redirecting the protein to the recycling endosome to promote ARF6-dependent GLUT4 vesicle formation and recycling [#13, #19]. CYTH3 forms homodimers and antiparallel heterodimers with the scaffold protein GRSP1 through N-terminal coiled-coil heptad repeats, and engages the related scaffold GRASP to organize ARF6-dependent endocytic recycling [#6, #16, #17]. In vivo, CYTH3 is required for full insulin receptor signaling in mouse liver and adipose tissue and influences body fat and lipid handling, and its sole C. elegans ortholog controls asymmetric, apoptosis-generating neuroblast divisions in a Sec7-dependent manner [#18, #15].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established that CYTH3 is a catalytically active ARF GEF and linked its activity to Golgi architecture and secretion, defining its core enzymatic and cellular function.\",\n      \"evidence\": \"In vitro Sec7-domain GEF assay on ARF1 plus overexpression with Golgi morphology and SEAP secretion readouts in mammalian cells\",\n      \"pmids\": [\"9707577\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish the physiological recruitment signal\", \"ARF6 not yet tested as substrate\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Defined the PIP3-dependence of GEF activity and the PH-domain lipid ligand, revealing that 3-phosphoinositides both recruit and activate the enzyme.\",\n      \"evidence\": \"Reconstituted in vitro GEF assays with dioctanoyl lipids, radiolabeled nucleotide exchange, and lipid binding with Ins(1,3,4,5)P4 competition; GFP live-cell imaging with PI3K inhibition in PC12 cells\",\n      \"pmids\": [\"9442017\", \"9742223\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of PIP3 selectivity unknown\", \"Coupling between lipid binding and catalytic enhancement not resolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identified ARF6 as a physiological substrate and placed CYTH3 at growth-factor-induced plasma membrane ruffles, broadening its substrate range beyond Golgi ARFs.\",\n      \"evidence\": \"Cell-free GEF assay on ARF6, intact-cell HA-ARF6 GTP-loading, and immunofluorescence co-localization at insulin/EGF-induced ruffles\",\n      \"pmids\": [\"10480924\", \"10585883\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vitro and in-cell ARF6 results required reconciliation with earlier ARF6-negative data\", \"Effectors downstream of ARF6 at ruffles not defined\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Mapped the structural determinant of phosphoinositide selectivity to a diglycine motif, explaining strict PIP3 preference among cytohesin PH domains.\",\n      \"evidence\": \"Glycine-insertion/deletion mutagenesis of the PH-domain loop with in vitro binding and in-cell translocation assays\",\n      \"pmids\": [\"10913124\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic structure of headgroup recognition not yet solved\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified GRSP1 as a coiled-coil binding partner that co-translocates with CYTH3 upon insulin, suggesting assembly of an insulin-responsive signaling complex.\",\n      \"evidence\": \"32P-labeled GRP1 probe cDNA screen, endogenous co-immunodepletion, and co-localization in CHO-T cells\",\n      \"pmids\": [\"11445584\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of GRSP1 binding for GEF activity not established\", \"Single lab, no reciprocal biochemical validation of stoichiometry in vivo\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Solved crystal structures of PH-domain splice variants bound to PIP2 and PIP3 headgroups, providing the atomic explanation for how loop length tunes lipid specificity.\",\n      \"evidence\": \"X-ray crystallography of dual-specificity variants with systematic mutagenesis and in vitro binding\",\n      \"pmids\": [\"15359279\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Headgroup-only structures did not capture membrane-bilayer docking geometry\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Quantified membrane-embedded PIP3 binding and revealed that background anionic lipids accelerate docking, refining the recruitment mechanism beyond simple headgroup affinity.\",\n      \"evidence\": \"Protein-to-membrane FRET equilibrium and stopped-flow kinetics with defined bilayers and competitive binding\",\n      \"pmids\": [\"15610010\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of the two-step electrostatic search not directly tested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Showed PH-domain anchoring is multivalent, integrating PIP3 recognition with pH and anionic-lipid electrostatics through residue H355.\",\n      \"evidence\": \"NMR, surface plasmon resonance, monolayer surface tension, and H355 mutagenesis\",\n      \"pmids\": [\"18469301\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological pH-dependence in a cellular context not demonstrated\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Defined the oligomeric basis of CYTH3 scaffolding, showing homodimerization and antiparallel heterodimerization with GRSP1 that do not perturb lipid binding.\",\n      \"evidence\": \"Analytical ultracentrifugation, FRET, and liposome binding assays\",\n      \"pmids\": [\"20527794\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional output of dimerization on GEF signaling not resolved\", \"Single-lab biophysical study\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identified the sentry glutamate E345 and hydrophobic flanking-loop residues as determinants of PIP2 exclusion and membrane penetration, explaining how the domain enforces specificity and inserts into the bilayer.\",\n      \"evidence\": \"Site-directed mutagenesis (E345K, hydrophobic-to-polar), in vitro binding, live-cell GFP imaging, PI(4,5)P2 hydrolysis, NMR, MD, and monolayer penetration\",\n      \"pmids\": [\"21932773\", \"21893292\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether endogenous regulation exploits E345 charge state was not addressed\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Determined the shallow membrane docking geometry and demonstrated that CYTH3 acts as an ARF6 GEF driving GLUT4 vesicle formation and recycling under insulin/Akt control.\",\n      \"evidence\": \"EPR spin-labeling across 18 positions; siRNA knockdown, phosphomimetic mutagenesis, GLUT4 recycling assays, co-IP, and in vitro kinase assay\",\n      \"pmids\": [\"22479423\", \"22609160\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the Akt phosphosites and their structural consequence not fully resolved at this stage\", \"Direct in vivo demonstration of insulin-dependent recycling not yet shown\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Linked CYTH3 to the GRASP scaffold in directing ARF6-dependent (but not clathrin-dependent) endocytic recycling, extending its recycling role to MHC-I cargo.\",\n      \"evidence\": \"Co-expression, co-localization, and MHC-I versus transferrin receptor recycling assays\",\n      \"pmids\": [\"22931251\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No direct biochemical interaction measurement reported\", \"Single-lab functional assay\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Provided a mechanistic model for membrane targeting in which nonspecific PS-driven electrostatics enable a 2D hopping search for rare PIP3 before specific docking.\",\n      \"evidence\": \"All-atom and coarse-grained MD simulations validated by EPR docking geometry and FRET kinetics\",\n      \"pmids\": [\"23747485\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Search mechanism inferred largely computationally; in-cell visualization of hopping not achieved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrated conserved physiological function of the cytohesin in C. elegans, where Sec7-dependent GEF activity controls asymmetric divisions producing apoptotic daughters.\",\n      \"evidence\": \"Loss-of-function genetics, domain-specific rescue, GFP localization, and epistasis with ARF GAP CNT-2 and ARF GEFs EFA-6/BRIS-1\",\n      \"pmids\": [\"25053664\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct ARF substrate engaged in this division pathway not biochemically defined\", \"Mammalian counterpart of this developmental role not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Established the in vivo requirement of CYTH3 for full insulin receptor signaling and its impact on systemic lipid metabolism using a knockout mouse.\",\n      \"evidence\": \"Cyth3 knockout mice with insulin-stimulation Western blots, high-fat-diet metabolic phenotyping, and fecal lipid analysis\",\n      \"pmids\": [\"30837656\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific GEF substrate driving the insulin signaling defect not pinpointed\", \"Link between insulin signaling defect and altered bile acid gene expression not mechanistically resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Unified the regulatory logic by showing phosphorylation simultaneously switches PH-domain lipid specificity (PIP3 to PI4P) and releases autoinhibition, redirecting CYTH3 from plasma membrane to recycling endosome.\",\n      \"evidence\": \"Phosphomimetic/phosphodeficient mutants with lipid binding assays, subcellular localization, and co-IP of recycling endosome peripheral proteins\",\n      \"pmids\": [\"33026967\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the two recycling-endosome coiled-coil partners not fully characterized\", \"Kinase-substrate dynamics in vivo not temporally resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How CYTH3's distinct GEF outputs (ARF1 at Golgi versus ARF6 at plasma membrane/recycling endosome) are selectively deployed by specific stimuli and scaffolds in different tissues remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No integrated model linking stimulus, phosphorylation state, scaffold choice, and ARF isoform selection\", \"Disease relevance in humans not established by direct genetic evidence in the corpus\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005086\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 1, 3, 13]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 2, 4, 5, 8, 9, 11]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [2, 4, 19]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [6, 16, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [2, 3, 4, 6, 11, 15]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 19]},\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [13, 17, 19]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 13, 18, 19]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 13, 17]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [13]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [15]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ARF1\", \"ARF5\", \"ARF6\", \"GRSP1\", \"GRASP\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":7,"faith_total":7,"faith_pct":100.0}}