{"gene":"ANKRD27","run_date":"2026-06-09T22:02:43","timeline":{"discoveries":[{"year":2006,"finding":"VARP (Varp/ANKRD27) functions as a guanine nucleotide exchange factor (GEF) for Rab21, preferentially binding GDP-bound Rab21 and catalyzing nucleotide exchange. Both the VPS9 domain and ankyrin repeats are required for endosomal localization and in vivo GEF activity. Ectopic expression causes enlargement of early endosomes and giant late endosomes.","method":"In vitro GEF assay, RNAi knockdown, subcellular localization by fluorescence microscopy, deletion/domain analysis","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — in vitro GEF assay combined with RNAi knockdown and domain mapping in two orthogonal approaches","pmids":["16525121"],"is_preprint":false},{"year":2008,"finding":"VARP physically interacts with active GTP-bound Rab38 via its ankyrin repeat 1 (ANK1) domain, functioning as a Rab38 effector. VARP is recruited to Rab38-positive organelles in an ANK1-dependent manner.","method":"Yeast two-hybrid screen, co-immunoprecipitation, in vitro pulldown with GTP/GDP-locked Rab38, fluorescence colocalization","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — yeast two-hybrid plus co-IP plus in vitro pulldown, single lab","pmids":["18477474"],"is_preprint":false},{"year":2009,"finding":"VARP is a specific Rab32/38 effector; its first ankyrin repeat (ANKR1) domain functions as a GTP-dependent Rab32/38-binding domain. siRNA-mediated knockdown of Varp in melan-a melanocytes causes dramatic reduction of Tyrp1 (tyrosinase-related protein 1) signals from melanosomes without affecting Pmel17, demonstrating a role in melanogenic enzyme trafficking.","method":"Yeast two-hybrid, siRNA knockdown, deletion/domain analysis, immunofluorescence microscopy","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — yeast two-hybrid identification combined with domain mapping and siRNA knockdown with specific phenotypic readout, replicated across multiple studies","pmids":["19403694"],"is_preprint":false},{"year":2009,"finding":"VARP interacts with TI-VAMP/VAMP7 through a specific interacting domain (ID). VARP, TI-VAMP, and Rab21 co-localize in perinuclear regions and transport vesicles in differentiating hippocampal neurons. Silencing Varp by RNAi, or expressing the ID domain or a Varp form lacking its VPS9 domain, impairs neurite growth, establishing VARP as a positive regulator of neurite growth via both its GEF activity and VAMP7 interaction.","method":"Co-immunoprecipitation, RNAi knockdown, domain expression, fluorescence colocalization in hippocampal neurons","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 2 / Moderate — reciprocal binding, RNAi with specific phenotypic readout, domain dissection in two orthogonal approaches","pmids":["19745841"],"is_preprint":false},{"year":2010,"finding":"Ala-based site-directed mutagenesis identified critical residues for Rab32/38–VARP complex formation: Val-92 in Rab32 switch II (Val-78 in Rab38) is required for VARP binding, and Gln-509 and Tyr-550 in the ANKR1 domain of VARP are required for Rab32/38 binding. VARP point mutants Q509A and Y550A do not support peripheral melanosomal distribution of Tyrp1. The VPS9 domain GEF activity is dispensable for Tyrp1 trafficking, whereas VAMP7-binding ability is required.","method":"Site-directed mutagenesis, co-immunoprecipitation, knockdown-rescue experiments, immunofluorescence","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — mutagenesis of specific residues combined with knockdown-rescue and functional assay in two orthogonal approaches, single lab","pmids":["21187289"],"is_preprint":false},{"year":2011,"finding":"Knockdown-rescue experiments in melanocytes showed that the Rab21-GEF activity of VARP (via its VPS9 domain) is required for forskolin-induced dendrite formation, whereas the Rab32/38 effector function (ANKR1) is not. Conversely, the Rab32/38 effector function is required for Tyrp1 transport to melanosomes, not dendrite formation. VAMP7-binding ability is required for both functions.","method":"siRNA knockdown, rescue with point mutants (VPS9 D310A/Y350A, ANKR1 Q509A/Y550A, VAMP7-binding deficient), morphological readout","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Moderate — systematic knockdown-rescue with multiple domain-specific point mutants dissecting two distinct functional outputs","pmids":["22171327"],"is_preprint":false},{"year":2012,"finding":"Crystal structure of the second ankyrin repeat domain of VARP in complex with the cytosolic portion of VAMP7 reveals that the VAMP7 SNARE motif is trapped between VARP and the VAMP7 longin domain. VARP kinetically inhibits VAMP7 SNARE complex formation, trapping it in a closed, fusogenically inactive conformation. This inhibition is enhanced when VARP simultaneously binds Rab32-GTP on the same membrane.","method":"X-ray crystallography, SNARE complex formation assay (in vitro), fluorescence colocalization, binding assays","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution crystal structure plus in vitro functional assay demonstrating kinetic inhibition of SNARE complex formation","pmids":["23104059"],"is_preprint":false},{"year":2014,"finding":"VARP is recruited to endosomal membranes via direct interaction with VPS29, a subunit of the retromer complex; this recruitment is independent of Rab32 binding. The VARP ankyrin repeat/Rab32:GTP complex structure was determined. Transport of GLUT1 from endosomes to the cell surface requires VARP, VPS29, and VAMP7 and depends on the direct VPS29–VARP interaction. Endocytic cycling of VAMP7 depends on its interaction with VARP and consequently on retromer.","method":"X-ray crystallography (VARP ankyrin repeat/Rab32:GTP), direct pulldown, co-immunoprecipitation, siRNA knockdown of VARP/VPS29/VAMP7 with GLUT1 trafficking readout","journal":"Developmental cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — crystal structure plus direct pulldown plus functional trafficking assay with multiple knockdowns, independently replicates VARP-retromer interaction","pmids":["24856514"],"is_preprint":false},{"year":2015,"finding":"Rab40C, an atypical Rab containing a SOCS box that recruits a ubiquitin ligase complex, binds the ANKR2 domain of VARP and promotes its proteasomal degradation. Overexpression of Rab40C reduces Tyrp1 signals by degrading VARP; knockdown of Rab40C increases VARP levels. This identifies Rab40C as a regulator of Tyrp1 trafficking via controlling VARP protein stability.","method":"Co-immunoprecipitation, domain mapping, overexpression and knockdown, proteasome inhibitor experiments, immunofluorescence","journal":"Biology open","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — co-IP with domain mapping and gain/loss of function experiments with functional readout, single lab","pmids":["25661869"],"is_preprint":false},{"year":2016,"finding":"RACK1 binds the VARP ANKR2 domain and competes with Rab40C for the same binding site, thereby stabilizing VARP protein levels. Knockdown of RACK1 reduces Varp protein level and inhibits dendrite outgrowth in melanocytes; RACK1 overexpression inhibits the Varp–Rab40C interaction and counteracts negative effects of Rab40C on dendrite outgrowth.","method":"Co-immunoprecipitation, siRNA knockdown, overexpression, competitive binding assay, morphological readout","journal":"The Journal of investigative dermatology","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — co-IP competition plus knockdown/overexpression with functional phenotype, single lab","pmids":["27066885"],"is_preprint":false},{"year":2018,"finding":"VARP interacts with VAMP7 and kinesin 1, controls the peripheral pool of VAMP7-containing secretory lysosomes, and regulates cellular response to substrate rigidity. LRRK1 and VARP interact with VAMP7 in a competitive manner; LRRK1 negatively regulates VAMP7-mediated exocytosis while VARP promotes it, constituting a tug-of-war mechanism governing biomechanical control of lysosomal secretion.","method":"Co-immunoprecipitation, siRNA knockdown, atomic force microscopy, fluorescence microscopy of vesicle pools","journal":"iScience","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — co-IP competition plus functional secretion assay with biomechanical readout, single lab","pmids":["30240735"],"is_preprint":false},{"year":2020,"finding":"NMR/X-ray structural determination of the complex between retromer subunit VPS29 and a 12-residue, four-cysteine/Zn++ microdomain (termed a Zn-fingernail) present in VARP. Mutations abolishing VPS29–VARP binding inhibit trafficking from endosomes to the cell surface. VARP and TBC1D5 bind the same site on VPS29 and compete for VPS29 binding in vivo. Structural analysis indicates VARP preferentially binds assembled retromer coats by simultaneously engaging two VPS29 subunits.","method":"NMR spectroscopy, X-ray crystallography, mutagenesis, co-immunoprecipitation competition assay, trafficking assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution NMR/X-ray structure plus mutagenesis plus functional trafficking assay plus competitive co-IP, multiple orthogonal methods in one study","pmids":["33024112"],"is_preprint":false},{"year":2020,"finding":"VARP and Rab9 are dispensable for Rab32-mediated killing of Salmonella Typhi in macrophages (BRAM pathway). shRNA knockdown of VARP in macrophages did not affect Rab32 recruitment to Salmonella-containing vacuoles (SCV) or bacterial killing.","method":"shRNA knockdown, immunofluorescence of SCV, bacterial survival assay","journal":"Frontiers in cellular and infection microbiology","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — negative result established by knockdown with specific readout, single lab, single study","pmids":["33392103"],"is_preprint":false},{"year":2025,"finding":"VARP directly interacts with SNX27 (identified biochemically and by AlphaFold modeling). In a fully reconstituted system with purified proteins and membranes, VARP is required to assemble a proposed endosomal supercomplex comprising SNX27, ESCPE-1 (SNX2/SNX6), and Retromer in vitro. VARP co-immunoprecipitates all coat components in cells. This places VARP as a scaffold for metazoan endosomal coat supercomplex assembly.","method":"Biochemical reconstitution with purified proteins, liposome tubulation assay, AlphaFold structural modeling, co-immunoprecipitation","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 1-2 / Moderate — full biochemical reconstitution with purified proteins plus co-IP in cells, multiple orthogonal methods in one study","pmids":["39937906"],"is_preprint":false},{"year":2025,"finding":"Knockout of VARP inhibits starvation-induced autophagic ATP secretion (amphisome pathway). RAB21 overexpression rescues ATP secretion in RAB21 KO cells but not in VARP KO cells, placing VARP downstream or in parallel to RAB21 in this secretory pathway. VARP partially colocalizes with LC3 upon starvation.","method":"CRISPR/KO of VAMP7/RAB21/VARP, ATP release assay, rescue overexpression, fluorescence colocalization","journal":"Autophagy reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO with functional ATP secretion readout plus epistasis rescue experiment, single lab","pmids":["40395984"],"is_preprint":false}],"current_model":"VARP/ANKRD27 is a multifunctional endosomal scaffold protein that acts as a GEF for Rab21 (via its VPS9 domain), as an effector for GTP-bound Rab32/38 (via its ANKR1 domain), as a kinetic inhibitor of VAMP7/TI-VAMP SNARE complex formation (via its ANKR2 domain, trapping VAMP7 in a closed conformation), and as a direct binding partner of retromer subunit VPS29 (via Zn-fingernail motifs) that recruits VARP to endosomal membranes and enables assembly of an SNX27/ESCPE-1/Retromer supercomplex; through these interactions VARP coordinately regulates endosome-to-plasma membrane trafficking of cargo such as GLUT1, melanogenic enzyme (Tyrp1) trafficking to melanosomes, neurite and dendrite outgrowth, lysosomal secretion, and autophagic ATP exocytosis, while its own protein stability is regulated by Rab40C-mediated proteasomal degradation and RACK1-mediated stabilization."},"narrative":{"mechanistic_narrative":"ANKRD27 (VARP) is a multidomain endosomal scaffold that integrates Rab GTPase signaling, SNARE regulation, and retromer-based coat assembly to direct endosome-to-surface and lysosomal/melanosomal trafficking [PMID:16525121, PMID:19403694, PMID:24856514]. Through its VPS9 domain it acts as a guanine nucleotide exchange factor for Rab21, while its first ankyrin repeat (ANKR1) binds GTP-loaded Rab32/38 as an effector, with these two activities driving distinct outputs—Rab21-GEF activity supporting dendrite/neurite outgrowth and Rab32/38 effector function supporting Tyrp1 delivery to melanosomes [PMID:16525121, PMID:19403694, PMID:21187289, PMID:22171327]. Its second ankyrin repeat (ANKR2) binds the VAMP7/TI-VAMP SNARE motif and, as resolved by crystallography, traps VAMP7 in a closed, fusogenically inactive conformation, kinetically inhibiting SNARE complex assembly; this VAMP7 interaction is required for both melanosomal and neurite functions [PMID:22171327, PMID:23104059]. VARP is recruited to endosomes by direct binding to the retromer subunit VPS29 through Zn-fingernail microdomains that engage assembled retromer coats and compete with TBC1D5, and this interaction enables GLUT1 surface delivery and VAMP7 endocytic cycling; VARP further scaffolds assembly of an SNX27/ESCPE-1/Retromer supercomplex [PMID:24856514, PMID:33024112, PMID:39937906]. VARP also controls the peripheral pool of VAMP7-secretory lysosomes via kinesin-1 in a competitive tug-of-war with LRRK1, and is required for starvation-induced autophagic ATP secretion downstream of RAB21 [PMID:30240735, PMID:40395984]. VARP protein stability is set by competition at ANKR2 between Rab40C, which targets it for proteasomal degradation, and RACK1, which stabilizes it [PMID:25661869, PMID:27066885].","teleology":[{"year":2006,"claim":"Established VARP's first molecular activity by showing it is a GEF for Rab21, defining an enzymatic role in early/late endosome regulation.","evidence":"In vitro GEF assay, RNAi knockdown, and domain deletion with endosomal localization readout","pmids":["16525121"],"confidence":"High","gaps":["Did not connect GEF activity to specific cargo","Physiological substrates of Rab21 downstream unresolved"]},{"year":2009,"claim":"Defined VARP as a GTP-dependent Rab32/38 effector via its ANKR1 domain and linked this to melanogenic enzyme delivery, distinguishing effector from GEF function.","evidence":"Yeast two-hybrid, siRNA knockdown in melanocytes with Tyrp1 phenotypic readout, domain mapping","pmids":["18477474","19403694"],"confidence":"High","gaps":["Mechanism linking Rab32/38 binding to Tyrp1 vesicle delivery not fully resolved","Effector output beyond melanosomes unclear at this stage"]},{"year":2009,"claim":"Identified VAMP7 as a VARP partner and showed VARP positively regulates neurite outgrowth through both GEF activity and VAMP7 binding, extending its role to neuronal trafficking.","evidence":"Co-IP, RNAi knockdown, and domain expression in differentiating hippocampal neurons","pmids":["19745841"],"confidence":"High","gaps":["Structural basis of VAMP7 interaction unknown at this stage","How VAMP7 binding feeds into membrane fusion not defined"]},{"year":2010,"claim":"Mapped the precise interface residues for Rab32/38 binding and dissected which domains drive Tyrp1 trafficking, showing GEF activity is dispensable but VAMP7 binding is required.","evidence":"Site-directed mutagenesis, co-IP, and knockdown-rescue with immunofluorescence","pmids":["21187289"],"confidence":"High","gaps":["Did not resolve how VAMP7 binding mechanistically enables Tyrp1 delivery"]},{"year":2011,"claim":"Cleanly separated VARP's two effector/GEF outputs by knockdown-rescue, assigning Rab21-GEF to dendrite formation and Rab32/38-effector to Tyrp1 transport with VAMP7-binding required for both.","evidence":"siRNA knockdown with domain-specific point mutant rescue and morphological readout in melanocytes","pmids":["22171327"],"confidence":"High","gaps":["Why VAMP7 binding is shared by both pathways not mechanistically explained"]},{"year":2012,"claim":"Provided the structural mechanism of VARP–VAMP7 regulation, showing VARP traps VAMP7 in a closed conformation and kinetically inhibits SNARE assembly, enhanced by co-bound Rab32-GTP.","evidence":"X-ray crystallography of ANKR2–VAMP7 complex plus in vitro SNARE assembly assay","pmids":["23104059"],"confidence":"High","gaps":["How inhibition is relieved to permit fusion in vivo not defined","In vivo timing of VAMP7 release unresolved"]},{"year":2014,"claim":"Identified VPS29/retromer as the endosomal recruitment factor for VARP and showed VARP, VPS29, and VAMP7 are jointly required for GLUT1 surface delivery and VAMP7 cycling.","evidence":"X-ray crystallography of ANKR1/Rab32:GTP, direct pulldown, and siRNA knockdown with GLUT1 trafficking readout","pmids":["24856514"],"confidence":"High","gaps":["Atomic detail of VPS29–VARP interface not yet resolved here","Range of cargo using this route incomplete"]},{"year":2015,"claim":"Revealed that VARP levels are post-translationally controlled, with Rab40C binding ANKR2 to drive proteasomal degradation and thereby tune Tyrp1 trafficking.","evidence":"Co-IP, domain mapping, gain/loss-of-function, and proteasome inhibitor experiments","pmids":["25661869"],"confidence":"Medium","gaps":["Single-lab evidence","Identity of the ubiquitin ligase substrate-recognition is inferred from the SOCS box, not directly shown for VARP"]},{"year":2016,"claim":"Showed RACK1 stabilizes VARP by competing with Rab40C for the same ANKR2 site, establishing a balance that controls dendrite outgrowth.","evidence":"Co-IP competition, siRNA knockdown/overexpression with morphological readout","pmids":["27066885"],"confidence":"Medium","gaps":["Single-lab competition assay","Signals that shift the Rab40C/RACK1 balance not identified"]},{"year":2018,"claim":"Placed VARP in biomechanical control of lysosomal secretion via a competitive tug-of-war with LRRK1 over VAMP7 and a kinesin-1 link to the peripheral vesicle pool.","evidence":"Co-IP competition, siRNA knockdown, atomic force microscopy, vesicle-pool imaging","pmids":["30240735"],"confidence":"Medium","gaps":["Single-lab evidence","Direct kinesin-1 binding site on VARP not mapped"]},{"year":2020,"claim":"Defined the atomic VPS29–VARP interface as a Zn-fingernail microdomain that engages assembled retromer and competes with TBC1D5, explaining preferential binding to retromer coats.","evidence":"NMR/X-ray structure, mutagenesis, competitive co-IP, and trafficking assay","pmids":["33024112"],"confidence":"High","gaps":["How TBC1D5/VARP competition is regulated in cells unresolved"]},{"year":2020,"claim":"Delimited VARP's role by showing it is dispensable for Rab32-mediated antibacterial killing of Salmonella, separating the BRAM pathway from VARP-dependent functions.","evidence":"shRNA knockdown in macrophages with SCV imaging and bacterial survival assay","pmids":["33392103"],"confidence":"Medium","gaps":["Negative result from single lab","Does not address other Rab32-dependent functions of VARP"]},{"year":2025,"claim":"Established VARP as a scaffold for metazoan endosomal coat supercomplex assembly by reconstituting SNX27/ESCPE-1/Retromer assembly with purified components.","evidence":"Biochemical reconstitution with purified proteins, liposome tubulation, AlphaFold modeling, and cellular co-IP","pmids":["39937906"],"confidence":"High","gaps":["In vivo requirement for supercomplex assembly across cargo not fully established","SNX27 interaction interface partly model-based"]},{"year":2025,"claim":"Extended VARP function to autophagic secretion, showing it is required for starvation-induced ATP release downstream of or parallel to RAB21 via the amphisome pathway.","evidence":"CRISPR knockout, ATP release assay, epistasis rescue, and LC3 colocalization","pmids":["40395984"],"confidence":"Medium","gaps":["Single-lab evidence","Molecular step VARP performs in amphisome-mediated ATP exocytosis undefined"]},{"year":null,"claim":"How VARP's multiple competing interactions (Rab40C vs RACK1, VARP vs LRRK1, VARP vs TBC1D5) are coordinated in space and time to switch between its trafficking outputs remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No integrated model of how the competing interactions are temporally regulated","Upstream signals controlling VARP function in each pathway unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[6,7,11,13]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,8,9]}],"localization":[{"term_id":"GO:0005768","term_label":"endosome","supporting_discovery_ids":[0,7,11]},{"term_id":"GO:0005764","term_label":"lysosome","supporting_discovery_ids":[10]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[7,11,13]},{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[7,11]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[14]}],"complexes":["SNX27/ESCPE-1/Retromer supercomplex"],"partners":["RAB21","RAB32","RAB38","VAMP7","VPS29","RAB40C","RACK1","SNX27"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96NW4","full_name":"Ankyrin repeat domain-containing protein 27","aliases":["VPS9 domain-containing protein"],"length_aa":1050,"mass_kda":117.0,"function":"May be a guanine exchange factor (GEF) for Rab21, Rab32 and Rab38 and regulate endosome dynamics (PubMed:16525121, PubMed:18477474). May regulate the participation of VAMP7 in membrane fusion events; in vitro inhibits VAMP7-mediated SNARE complex formation by trapping VAMP7 in a closed, fusogenically inactive conformation (PubMed:23104059). Involved in peripheral melanosomal distribution of TYRP1 in melanocytes; the function, which probably is implicating vesicle-trafficking, includes cooperation with Rab32, Rab38 and VAMP7 (By similarity). Involved in the regulation of neurite growth; the function seems to require its GEF activity, probably towards Rab21, and VAMP7 but not Rab32/38 (By similarity). Proposed to be involved in Golgi sorting of VAMP7 and transport of VAMP7 vesicles to the cell surface; the function seems to implicate kinesin heavy chain isoform 5 proteins, GOLGA4, RAB21 and MACF1 (PubMed:22705394). Required for the colocalization of VAMP7 and Rab21, probably on TGN sites (PubMed:19745841). Involved in GLUT1 endosome-to-plasma membrane trafficking; the function is dependent of association with VPS29 (PubMed:24856514). Regulates the proper trafficking of melanogenic enzymes TYR, TYRP1 and DCT/TYRP2 to melanosomes in melanocytes (By similarity)","subcellular_location":"Early endosome; Late endosome; Cytoplasmic vesicle membrane; Lysosome; Cell membrane; Melanosome","url":"https://www.uniprot.org/uniprotkb/Q96NW4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ANKRD27","classification":"Not Classified","n_dependent_lines":7,"n_total_lines":1208,"dependency_fraction":0.005794701986754967},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000105186","cell_line_id":"CID001399","localizations":[{"compartment":"vesicles","grade":3}],"interactors":[{"gene":"SRSF1","stoichiometry":0.2},{"gene":"TBC1D4","stoichiometry":0.2},{"gene":"NCOA1","stoichiometry":0.2},{"gene":"TRIP6","stoichiometry":0.2},{"gene":"VPS29","stoichiometry":0.2},{"gene":"VPS35","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001399","total_profiled":1310},"omim":[{"mim_id":"618957","title":"ANKYRIN REPEAT DOMAIN-CONTAINING PROTEIN 27; ANKRD27","url":"https://www.omim.org/entry/618957"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Vesicles","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ANKRD27"},"hgnc":{"alias_symbol":["FLJ00040","DKFZp434L0718","VARP"],"prev_symbol":[]},"alphafold":{"accession":"Q96NW4","domains":[{"cath_id":"-","chopping":"9-114","consensus_level":"high","plddt":76.4913,"start":9,"end":114},{"cath_id":"1.20.1050.80","chopping":"139-368_384-395","consensus_level":"medium","plddt":76.7187,"start":139,"end":395},{"cath_id":"1.25.40.20","chopping":"665-692_735-803","consensus_level":"medium","plddt":85.1269,"start":665,"end":803},{"cath_id":"1.25.40.20","chopping":"814-901","consensus_level":"medium","plddt":85.2609,"start":814,"end":901}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96NW4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96NW4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96NW4-F1-predicted_aligned_error_v6.png","plddt_mean":68.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ANKRD27","jax_strain_url":"https://www.jax.org/strain/search?query=ANKRD27"},"sequence":{"accession":"Q96NW4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96NW4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96NW4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96NW4"}},"corpus_meta":[{"pmid":"24856514","id":"PMC_24856514","title":"VARP is recruited on to endosomes by direct interaction with retromer, where together they function in export to the cell surface.","date":"2014","source":"Developmental cell","url":"https://pubmed.ncbi.nlm.nih.gov/24856514","citation_count":106,"is_preprint":false},{"pmid":"19403694","id":"PMC_19403694","title":"Varp is a novel Rab32/38-binding protein that regulates Tyrp1 trafficking in melanocytes.","date":"2009","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/19403694","citation_count":96,"is_preprint":false},{"pmid":"19745841","id":"PMC_19745841","title":"Role of Varp, a Rab21 exchange factor and TI-VAMP/VAMP7 partner, in neurite growth.","date":"2009","source":"EMBO reports","url":"https://pubmed.ncbi.nlm.nih.gov/19745841","citation_count":76,"is_preprint":false},{"pmid":"16525121","id":"PMC_16525121","title":"Varp is a Rab21 guanine nucleotide exchange factor and regulates endosome dynamics.","date":"2006","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/16525121","citation_count":76,"is_preprint":false},{"pmid":"23104059","id":"PMC_23104059","title":"The binding of Varp to VAMP7 traps VAMP7 in a closed, fusogenically inactive conformation.","date":"2012","source":"Nature structural & molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/23104059","citation_count":62,"is_preprint":false},{"pmid":"21187289","id":"PMC_21187289","title":"Structure-function analysis of VPS9-ankyrin-repeat protein (Varp) in the trafficking of tyrosinase-related protein 1 in melanocytes.","date":"2010","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21187289","citation_count":55,"is_preprint":false},{"pmid":"22171327","id":"PMC_22171327","title":"The Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for dendrite formation in melanocytes.","date":"2011","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/22171327","citation_count":30,"is_preprint":false},{"pmid":"18477474","id":"PMC_18477474","title":"Varp interacts with Rab38 and functions as its potential effector.","date":"2008","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/18477474","citation_count":29,"is_preprint":false},{"pmid":"30240735","id":"PMC_30240735","title":"Biomechanical Control of Lysosomal Secretion Via the VAMP7 Hub: A Tug-of-War between VARP and LRRK1.","date":"2018","source":"iScience","url":"https://pubmed.ncbi.nlm.nih.gov/30240735","citation_count":28,"is_preprint":false},{"pmid":"27103185","id":"PMC_27103185","title":"Multiple Roles of VARP in Endosomal Trafficking: Rabs, Retromer Components and R-SNARE VAMP7 Meet on VARP.","date":"2016","source":"Traffic (Copenhagen, Denmark)","url":"https://pubmed.ncbi.nlm.nih.gov/27103185","citation_count":27,"is_preprint":false},{"pmid":"25661869","id":"PMC_25661869","title":"Rab40C is a novel Varp-binding protein that promotes proteasomal degradation of Varp in melanocytes.","date":"2015","source":"Biology open","url":"https://pubmed.ncbi.nlm.nih.gov/25661869","citation_count":24,"is_preprint":false},{"pmid":"33024112","id":"PMC_33024112","title":"Mechanism and evolution of the Zn-fingernail required for interaction of VARP with VPS29.","date":"2020","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33024112","citation_count":18,"is_preprint":false},{"pmid":"27066885","id":"PMC_27066885","title":"A Varp-Binding Protein, RACK1, Regulates Dendrite Outgrowth through Stabilization of Varp Protein in Mouse Melanocytes.","date":"2016","source":"The Journal of investigative dermatology","url":"https://pubmed.ncbi.nlm.nih.gov/27066885","citation_count":11,"is_preprint":false},{"pmid":"33392103","id":"PMC_33392103","title":"VARP and Rab9 Are Dispensable for the Rab32/BLOC-3 Dependent Salmonella Killing.","date":"2020","source":"Frontiers in cellular and infection microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/33392103","citation_count":7,"is_preprint":false},{"pmid":"39937906","id":"PMC_39937906","title":"VARP binds SNX27 to promote endosomal supercomplex formation on membranes.","date":"2025","source":"Science advances","url":"https://pubmed.ncbi.nlm.nih.gov/39937906","citation_count":4,"is_preprint":false},{"pmid":"40395984","id":"PMC_40395984","title":"A new role of RAB21 and VARP in autophagy and autophagic exocytosis of ATP.","date":"2025","source":"Autophagy reports","url":"https://pubmed.ncbi.nlm.nih.gov/40395984","citation_count":2,"is_preprint":false},{"pmid":"18318119","id":"PMC_18318119","title":"[Multiadaptor 4.1 and RanBP9 protein family members as putative interaction partners for VARP, a Rab21 GTPase guanine nucleotide exchange factor].","date":"2007","source":"Molekuliarnaia biologiia","url":"https://pubmed.ncbi.nlm.nih.gov/18318119","citation_count":1,"is_preprint":false},{"pmid":"34027682","id":"PMC_34027682","title":"Prophylactic Anticoagulation With Intermediate-Dose Certoparin in Vascular-Risk Pregnancies-The PACER-VARP Registry.","date":"2021","source":"Clinical and applied thrombosis/hemostasis : official journal of the International Academy of Clinical and Applied Thrombosis/Hemostasis","url":"https://pubmed.ncbi.nlm.nih.gov/34027682","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":10122,"output_tokens":4171,"usd":0.046465,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":11947,"output_tokens":4304,"usd":0.083668,"stage2_stop_reason":"end_turn"},"total_usd":0.130133,"stage1_batch_id":"msgbatch_0179Z2pxRYBsXi2xADneafRw","stage2_batch_id":"msgbatch_01LaY9ZaVdgjWArcZo9eqQE5","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2006,\n      \"finding\": \"VARP (Varp/ANKRD27) functions as a guanine nucleotide exchange factor (GEF) for Rab21, preferentially binding GDP-bound Rab21 and catalyzing nucleotide exchange. Both the VPS9 domain and ankyrin repeats are required for endosomal localization and in vivo GEF activity. Ectopic expression causes enlargement of early endosomes and giant late endosomes.\",\n      \"method\": \"In vitro GEF assay, RNAi knockdown, subcellular localization by fluorescence microscopy, deletion/domain analysis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — in vitro GEF assay combined with RNAi knockdown and domain mapping in two orthogonal approaches\",\n      \"pmids\": [\"16525121\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"VARP physically interacts with active GTP-bound Rab38 via its ankyrin repeat 1 (ANK1) domain, functioning as a Rab38 effector. VARP is recruited to Rab38-positive organelles in an ANK1-dependent manner.\",\n      \"method\": \"Yeast two-hybrid screen, co-immunoprecipitation, in vitro pulldown with GTP/GDP-locked Rab38, fluorescence colocalization\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — yeast two-hybrid plus co-IP plus in vitro pulldown, single lab\",\n      \"pmids\": [\"18477474\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"VARP is a specific Rab32/38 effector; its first ankyrin repeat (ANKR1) domain functions as a GTP-dependent Rab32/38-binding domain. siRNA-mediated knockdown of Varp in melan-a melanocytes causes dramatic reduction of Tyrp1 (tyrosinase-related protein 1) signals from melanosomes without affecting Pmel17, demonstrating a role in melanogenic enzyme trafficking.\",\n      \"method\": \"Yeast two-hybrid, siRNA knockdown, deletion/domain analysis, immunofluorescence microscopy\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — yeast two-hybrid identification combined with domain mapping and siRNA knockdown with specific phenotypic readout, replicated across multiple studies\",\n      \"pmids\": [\"19403694\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"VARP interacts with TI-VAMP/VAMP7 through a specific interacting domain (ID). VARP, TI-VAMP, and Rab21 co-localize in perinuclear regions and transport vesicles in differentiating hippocampal neurons. Silencing Varp by RNAi, or expressing the ID domain or a Varp form lacking its VPS9 domain, impairs neurite growth, establishing VARP as a positive regulator of neurite growth via both its GEF activity and VAMP7 interaction.\",\n      \"method\": \"Co-immunoprecipitation, RNAi knockdown, domain expression, fluorescence colocalization in hippocampal neurons\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal binding, RNAi with specific phenotypic readout, domain dissection in two orthogonal approaches\",\n      \"pmids\": [\"19745841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Ala-based site-directed mutagenesis identified critical residues for Rab32/38–VARP complex formation: Val-92 in Rab32 switch II (Val-78 in Rab38) is required for VARP binding, and Gln-509 and Tyr-550 in the ANKR1 domain of VARP are required for Rab32/38 binding. VARP point mutants Q509A and Y550A do not support peripheral melanosomal distribution of Tyrp1. The VPS9 domain GEF activity is dispensable for Tyrp1 trafficking, whereas VAMP7-binding ability is required.\",\n      \"method\": \"Site-directed mutagenesis, co-immunoprecipitation, knockdown-rescue experiments, immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — mutagenesis of specific residues combined with knockdown-rescue and functional assay in two orthogonal approaches, single lab\",\n      \"pmids\": [\"21187289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Knockdown-rescue experiments in melanocytes showed that the Rab21-GEF activity of VARP (via its VPS9 domain) is required for forskolin-induced dendrite formation, whereas the Rab32/38 effector function (ANKR1) is not. Conversely, the Rab32/38 effector function is required for Tyrp1 transport to melanosomes, not dendrite formation. VAMP7-binding ability is required for both functions.\",\n      \"method\": \"siRNA knockdown, rescue with point mutants (VPS9 D310A/Y350A, ANKR1 Q509A/Y550A, VAMP7-binding deficient), morphological readout\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic knockdown-rescue with multiple domain-specific point mutants dissecting two distinct functional outputs\",\n      \"pmids\": [\"22171327\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"Crystal structure of the second ankyrin repeat domain of VARP in complex with the cytosolic portion of VAMP7 reveals that the VAMP7 SNARE motif is trapped between VARP and the VAMP7 longin domain. VARP kinetically inhibits VAMP7 SNARE complex formation, trapping it in a closed, fusogenically inactive conformation. This inhibition is enhanced when VARP simultaneously binds Rab32-GTP on the same membrane.\",\n      \"method\": \"X-ray crystallography, SNARE complex formation assay (in vitro), fluorescence colocalization, binding assays\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution crystal structure plus in vitro functional assay demonstrating kinetic inhibition of SNARE complex formation\",\n      \"pmids\": [\"23104059\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"VARP is recruited to endosomal membranes via direct interaction with VPS29, a subunit of the retromer complex; this recruitment is independent of Rab32 binding. The VARP ankyrin repeat/Rab32:GTP complex structure was determined. Transport of GLUT1 from endosomes to the cell surface requires VARP, VPS29, and VAMP7 and depends on the direct VPS29–VARP interaction. Endocytic cycling of VAMP7 depends on its interaction with VARP and consequently on retromer.\",\n      \"method\": \"X-ray crystallography (VARP ankyrin repeat/Rab32:GTP), direct pulldown, co-immunoprecipitation, siRNA knockdown of VARP/VPS29/VAMP7 with GLUT1 trafficking readout\",\n      \"journal\": \"Developmental cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — crystal structure plus direct pulldown plus functional trafficking assay with multiple knockdowns, independently replicates VARP-retromer interaction\",\n      \"pmids\": [\"24856514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Rab40C, an atypical Rab containing a SOCS box that recruits a ubiquitin ligase complex, binds the ANKR2 domain of VARP and promotes its proteasomal degradation. Overexpression of Rab40C reduces Tyrp1 signals by degrading VARP; knockdown of Rab40C increases VARP levels. This identifies Rab40C as a regulator of Tyrp1 trafficking via controlling VARP protein stability.\",\n      \"method\": \"Co-immunoprecipitation, domain mapping, overexpression and knockdown, proteasome inhibitor experiments, immunofluorescence\",\n      \"journal\": \"Biology open\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — co-IP with domain mapping and gain/loss of function experiments with functional readout, single lab\",\n      \"pmids\": [\"25661869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"RACK1 binds the VARP ANKR2 domain and competes with Rab40C for the same binding site, thereby stabilizing VARP protein levels. Knockdown of RACK1 reduces Varp protein level and inhibits dendrite outgrowth in melanocytes; RACK1 overexpression inhibits the Varp–Rab40C interaction and counteracts negative effects of Rab40C on dendrite outgrowth.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, overexpression, competitive binding assay, morphological readout\",\n      \"journal\": \"The Journal of investigative dermatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — co-IP competition plus knockdown/overexpression with functional phenotype, single lab\",\n      \"pmids\": [\"27066885\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"VARP interacts with VAMP7 and kinesin 1, controls the peripheral pool of VAMP7-containing secretory lysosomes, and regulates cellular response to substrate rigidity. LRRK1 and VARP interact with VAMP7 in a competitive manner; LRRK1 negatively regulates VAMP7-mediated exocytosis while VARP promotes it, constituting a tug-of-war mechanism governing biomechanical control of lysosomal secretion.\",\n      \"method\": \"Co-immunoprecipitation, siRNA knockdown, atomic force microscopy, fluorescence microscopy of vesicle pools\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — co-IP competition plus functional secretion assay with biomechanical readout, single lab\",\n      \"pmids\": [\"30240735\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NMR/X-ray structural determination of the complex between retromer subunit VPS29 and a 12-residue, four-cysteine/Zn++ microdomain (termed a Zn-fingernail) present in VARP. Mutations abolishing VPS29–VARP binding inhibit trafficking from endosomes to the cell surface. VARP and TBC1D5 bind the same site on VPS29 and compete for VPS29 binding in vivo. Structural analysis indicates VARP preferentially binds assembled retromer coats by simultaneously engaging two VPS29 subunits.\",\n      \"method\": \"NMR spectroscopy, X-ray crystallography, mutagenesis, co-immunoprecipitation competition assay, trafficking assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution NMR/X-ray structure plus mutagenesis plus functional trafficking assay plus competitive co-IP, multiple orthogonal methods in one study\",\n      \"pmids\": [\"33024112\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"VARP and Rab9 are dispensable for Rab32-mediated killing of Salmonella Typhi in macrophages (BRAM pathway). shRNA knockdown of VARP in macrophages did not affect Rab32 recruitment to Salmonella-containing vacuoles (SCV) or bacterial killing.\",\n      \"method\": \"shRNA knockdown, immunofluorescence of SCV, bacterial survival assay\",\n      \"journal\": \"Frontiers in cellular and infection microbiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — negative result established by knockdown with specific readout, single lab, single study\",\n      \"pmids\": [\"33392103\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"VARP directly interacts with SNX27 (identified biochemically and by AlphaFold modeling). In a fully reconstituted system with purified proteins and membranes, VARP is required to assemble a proposed endosomal supercomplex comprising SNX27, ESCPE-1 (SNX2/SNX6), and Retromer in vitro. VARP co-immunoprecipitates all coat components in cells. This places VARP as a scaffold for metazoan endosomal coat supercomplex assembly.\",\n      \"method\": \"Biochemical reconstitution with purified proteins, liposome tubulation assay, AlphaFold structural modeling, co-immunoprecipitation\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — full biochemical reconstitution with purified proteins plus co-IP in cells, multiple orthogonal methods in one study\",\n      \"pmids\": [\"39937906\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Knockout of VARP inhibits starvation-induced autophagic ATP secretion (amphisome pathway). RAB21 overexpression rescues ATP secretion in RAB21 KO cells but not in VARP KO cells, placing VARP downstream or in parallel to RAB21 in this secretory pathway. VARP partially colocalizes with LC3 upon starvation.\",\n      \"method\": \"CRISPR/KO of VAMP7/RAB21/VARP, ATP release assay, rescue overexpression, fluorescence colocalization\",\n      \"journal\": \"Autophagy reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO with functional ATP secretion readout plus epistasis rescue experiment, single lab\",\n      \"pmids\": [\"40395984\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"VARP/ANKRD27 is a multifunctional endosomal scaffold protein that acts as a GEF for Rab21 (via its VPS9 domain), as an effector for GTP-bound Rab32/38 (via its ANKR1 domain), as a kinetic inhibitor of VAMP7/TI-VAMP SNARE complex formation (via its ANKR2 domain, trapping VAMP7 in a closed conformation), and as a direct binding partner of retromer subunit VPS29 (via Zn-fingernail motifs) that recruits VARP to endosomal membranes and enables assembly of an SNX27/ESCPE-1/Retromer supercomplex; through these interactions VARP coordinately regulates endosome-to-plasma membrane trafficking of cargo such as GLUT1, melanogenic enzyme (Tyrp1) trafficking to melanosomes, neurite and dendrite outgrowth, lysosomal secretion, and autophagic ATP exocytosis, while its own protein stability is regulated by Rab40C-mediated proteasomal degradation and RACK1-mediated stabilization.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ANKRD27 (VARP) is a multidomain endosomal scaffold that integrates Rab GTPase signaling, SNARE regulation, and retromer-based coat assembly to direct endosome-to-surface and lysosomal/melanosomal trafficking [#0, #2, #7]. Through its VPS9 domain it acts as a guanine nucleotide exchange factor for Rab21, while its first ankyrin repeat (ANKR1) binds GTP-loaded Rab32/38 as an effector, with these two activities driving distinct outputs—Rab21-GEF activity supporting dendrite/neurite outgrowth and Rab32/38 effector function supporting Tyrp1 delivery to melanosomes [#0, #2, #4, #5]. Its second ankyrin repeat (ANKR2) binds the VAMP7/TI-VAMP SNARE motif and, as resolved by crystallography, traps VAMP7 in a closed, fusogenically inactive conformation, kinetically inhibiting SNARE complex assembly; this VAMP7 interaction is required for both melanosomal and neurite functions [#5, #6]. VARP is recruited to endosomes by direct binding to the retromer subunit VPS29 through Zn-fingernail microdomains that engage assembled retromer coats and compete with TBC1D5, and this interaction enables GLUT1 surface delivery and VAMP7 endocytic cycling; VARP further scaffolds assembly of an SNX27/ESCPE-1/Retromer supercomplex [#7, #11, #13]. VARP also controls the peripheral pool of VAMP7-secretory lysosomes via kinesin-1 in a competitive tug-of-war with LRRK1, and is required for starvation-induced autophagic ATP secretion downstream of RAB21 [#10, #14]. VARP protein stability is set by competition at ANKR2 between Rab40C, which targets it for proteasomal degradation, and RACK1, which stabilizes it [#8, #9].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Established VARP's first molecular activity by showing it is a GEF for Rab21, defining an enzymatic role in early/late endosome regulation.\",\n      \"evidence\": \"In vitro GEF assay, RNAi knockdown, and domain deletion with endosomal localization readout\",\n      \"pmids\": [\"16525121\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not connect GEF activity to specific cargo\", \"Physiological substrates of Rab21 downstream unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Defined VARP as a GTP-dependent Rab32/38 effector via its ANKR1 domain and linked this to melanogenic enzyme delivery, distinguishing effector from GEF function.\",\n      \"evidence\": \"Yeast two-hybrid, siRNA knockdown in melanocytes with Tyrp1 phenotypic readout, domain mapping\",\n      \"pmids\": [\"18477474\", \"19403694\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism linking Rab32/38 binding to Tyrp1 vesicle delivery not fully resolved\", \"Effector output beyond melanosomes unclear at this stage\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identified VAMP7 as a VARP partner and showed VARP positively regulates neurite outgrowth through both GEF activity and VAMP7 binding, extending its role to neuronal trafficking.\",\n      \"evidence\": \"Co-IP, RNAi knockdown, and domain expression in differentiating hippocampal neurons\",\n      \"pmids\": [\"19745841\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of VAMP7 interaction unknown at this stage\", \"How VAMP7 binding feeds into membrane fusion not defined\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Mapped the precise interface residues for Rab32/38 binding and dissected which domains drive Tyrp1 trafficking, showing GEF activity is dispensable but VAMP7 binding is required.\",\n      \"evidence\": \"Site-directed mutagenesis, co-IP, and knockdown-rescue with immunofluorescence\",\n      \"pmids\": [\"21187289\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve how VAMP7 binding mechanistically enables Tyrp1 delivery\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Cleanly separated VARP's two effector/GEF outputs by knockdown-rescue, assigning Rab21-GEF to dendrite formation and Rab32/38-effector to Tyrp1 transport with VAMP7-binding required for both.\",\n      \"evidence\": \"siRNA knockdown with domain-specific point mutant rescue and morphological readout in melanocytes\",\n      \"pmids\": [\"22171327\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why VAMP7 binding is shared by both pathways not mechanistically explained\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Provided the structural mechanism of VARP–VAMP7 regulation, showing VARP traps VAMP7 in a closed conformation and kinetically inhibits SNARE assembly, enhanced by co-bound Rab32-GTP.\",\n      \"evidence\": \"X-ray crystallography of ANKR2–VAMP7 complex plus in vitro SNARE assembly assay\",\n      \"pmids\": [\"23104059\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How inhibition is relieved to permit fusion in vivo not defined\", \"In vivo timing of VAMP7 release unresolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identified VPS29/retromer as the endosomal recruitment factor for VARP and showed VARP, VPS29, and VAMP7 are jointly required for GLUT1 surface delivery and VAMP7 cycling.\",\n      \"evidence\": \"X-ray crystallography of ANKR1/Rab32:GTP, direct pulldown, and siRNA knockdown with GLUT1 trafficking readout\",\n      \"pmids\": [\"24856514\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic detail of VPS29–VARP interface not yet resolved here\", \"Range of cargo using this route incomplete\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Revealed that VARP levels are post-translationally controlled, with Rab40C binding ANKR2 to drive proteasomal degradation and thereby tune Tyrp1 trafficking.\",\n      \"evidence\": \"Co-IP, domain mapping, gain/loss-of-function, and proteasome inhibitor experiments\",\n      \"pmids\": [\"25661869\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab evidence\", \"Identity of the ubiquitin ligase substrate-recognition is inferred from the SOCS box, not directly shown for VARP\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed RACK1 stabilizes VARP by competing with Rab40C for the same ANKR2 site, establishing a balance that controls dendrite outgrowth.\",\n      \"evidence\": \"Co-IP competition, siRNA knockdown/overexpression with morphological readout\",\n      \"pmids\": [\"27066885\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab competition assay\", \"Signals that shift the Rab40C/RACK1 balance not identified\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Placed VARP in biomechanical control of lysosomal secretion via a competitive tug-of-war with LRRK1 over VAMP7 and a kinesin-1 link to the peripheral vesicle pool.\",\n      \"evidence\": \"Co-IP competition, siRNA knockdown, atomic force microscopy, vesicle-pool imaging\",\n      \"pmids\": [\"30240735\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab evidence\", \"Direct kinesin-1 binding site on VARP not mapped\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined the atomic VPS29–VARP interface as a Zn-fingernail microdomain that engages assembled retromer and competes with TBC1D5, explaining preferential binding to retromer coats.\",\n      \"evidence\": \"NMR/X-ray structure, mutagenesis, competitive co-IP, and trafficking assay\",\n      \"pmids\": [\"33024112\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How TBC1D5/VARP competition is regulated in cells unresolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Delimited VARP's role by showing it is dispensable for Rab32-mediated antibacterial killing of Salmonella, separating the BRAM pathway from VARP-dependent functions.\",\n      \"evidence\": \"shRNA knockdown in macrophages with SCV imaging and bacterial survival assay\",\n      \"pmids\": [\"33392103\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Negative result from single lab\", \"Does not address other Rab32-dependent functions of VARP\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established VARP as a scaffold for metazoan endosomal coat supercomplex assembly by reconstituting SNX27/ESCPE-1/Retromer assembly with purified components.\",\n      \"evidence\": \"Biochemical reconstitution with purified proteins, liposome tubulation, AlphaFold modeling, and cellular co-IP\",\n      \"pmids\": [\"39937906\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo requirement for supercomplex assembly across cargo not fully established\", \"SNX27 interaction interface partly model-based\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Extended VARP function to autophagic secretion, showing it is required for starvation-induced ATP release downstream of or parallel to RAB21 via the amphisome pathway.\",\n      \"evidence\": \"CRISPR knockout, ATP release assay, epistasis rescue, and LC3 colocalization\",\n      \"pmids\": [\"40395984\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab evidence\", \"Molecular step VARP performs in amphisome-mediated ATP exocytosis undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How VARP's multiple competing interactions (Rab40C vs RACK1, VARP vs LRRK1, VARP vs TBC1D5) are coordinated in space and time to switch between its trafficking outputs remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No integrated model of how the competing interactions are temporally regulated\", \"Upstream signals controlling VARP function in each pathway unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005085\", \"supporting_discovery_ids\": [0]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [6, 7, 11, 13]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 8, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005768\", \"supporting_discovery_ids\": [0, 7, 11]},\n      {\"term_id\": \"GO:0005764\", \"supporting_discovery_ids\": [10]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [7, 11, 13]},\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [7, 11]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [14]}\n    ],\n    \"complexes\": [\n      \"SNX27/ESCPE-1/Retromer supercomplex\"\n    ],\n    \"partners\": [\n      \"RAB21\",\n      \"RAB32\",\n      \"RAB38\",\n      \"VAMP7\",\n      \"VPS29\",\n      \"RAB40C\",\n      \"RACK1\",\n      \"SNX27\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}