{"gene":"TRAK2","run_date":"2026-06-10T10:51:55","timeline":{"discoveries":[{"year":2002,"finding":"TRAK2 (GRIF-1) was identified as a GABA-A receptor beta2 subunit-interacting protein; the interaction was demonstrated by yeast two-hybrid assay and co-immunoprecipitation from HEK293 cells and adult rat brain lysates, with the respective binding domains mapped.","method":"Yeast two-hybrid assay, co-immunoprecipitation from transfected HEK293 cells and native rat brain lysates","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP in cells and native tissue plus yeast two-hybrid domain mapping, single lab","pmids":["12034717"],"is_preprint":false},{"year":2005,"finding":"TRAK2 (GRIF-1) associates with kinesin heavy chains (KIF5A in brain, KIF5B in heart/HEK293 cells, KIF5C by direct interaction) and with mitochondria; the GRIF-1/KIF5C interaction domain was localized to GRIF-1 residues 124–283 by yeast two-hybrid and co-immunoprecipitation.","method":"Co-immunoprecipitation from native brain/heart tissue and transfected HEK293 cells, yeast two-hybrid interaction assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP in multiple native tissues and cells plus yeast two-hybrid domain mapping, replicated in subsequent papers","pmids":["15644324"],"is_preprint":false},{"year":2006,"finding":"The GRIF-1 binding site on KIF5C was mapped to the KIF5C non-motor (cargo-binding) domain; the interaction is direct as demonstrated by FRET between fluorescently tagged GRIF-1 (N-terminal) and KIF5C (C-terminal), and GRIF-1 can bind the intact tetrameric kinesin light-chain/kinesin heavy-chain complex.","method":"Yeast two-hybrid with truncation constructs, co-immunoprecipitation, FRET with fluorescently tagged constructs, confocal microscopy","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — direct FRET evidence of protein–protein interaction plus domain mapping by yeast two-hybrid and co-IP, multiple orthogonal methods in one study","pmids":["16835241"],"is_preprint":false},{"year":2006,"finding":"TRAK2 (GRIF-1) binds to the K+ channel Kir2.1 (interaction mapped to GRIF-1 N-terminus and Kir2.1 C-terminus by yeast two-hybrid), increases surface expression of Kir2.1 channels in COS and HEK293 cells, and enhances Kir2.1-mediated Rb+ efflux, establishing TRAK2 as a trafficking factor for this channel.","method":"Functional yeast growth rescue screen, 86Rb+ efflux assay, quantitative surface immunolabeling and flow cytometry, co-immunoprecipitation from HEK293 lysates and brain lysate, yeast two-hybrid domain mapping","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (functional assay, quantitative surface labeling, co-IP in native tissue, yeast two-hybrid), single lab","pmids":["16895905"],"is_preprint":false},{"year":2008,"finding":"TRAK2 (GRIF-1) and the atypical GTPase Miro1 form a protein complex on neuronal mitochondria; Miro1 recruits GRIF-1 to mitochondria in a manner dependent on its first GTPase domain. Overexpression of Miro1 enhanced mitochondrial transport toward distal neuronal processes, while expression of a Grif-1/Miro1 binding fragment dramatically reduced mitochondrial transport into processes.","method":"Co-immunoprecipitation from mammalian brain, fluorescence imaging of hippocampal neurons, dominant-negative GTPase domain mutants, overexpression and competitive fragment experiments","journal":"Molecular and cellular neurosciences","confidence":"High","confidence_rationale":"Tier 2 / Strong — co-IP from native brain tissue, GTPase domain mutagenesis, dominant-negative fragment rescue, live-neuron transport quantification; findings replicated in subsequent studies","pmids":["19103291"],"is_preprint":false},{"year":2012,"finding":"TRAK2 participates in a Kinesin/Miro/TRAK2 complex in neurons; the armadillo-repeat protein Alex3 interacts with this complex in a Ca2+-dependent manner, modulating mitochondrial dynamics and trafficking.","method":"Co-immunoprecipitation, fluorescence imaging of neurons, Ca2+-dependent binding assay","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP demonstrating the complex plus Ca2+-dependence, single lab, findings about TRAK2 are secondary to the primary Armcx focus","pmids":["22569362"],"is_preprint":false},{"year":2017,"finding":"TRAK2 knockdown increases cholesterol efflux to apolipoprotein A-I and HDL via increased ABCA1 mRNA and protein expression, and increases LXR binding at the ABCA1 promoter; the efflux increase is abolished in the absence of ABCA1, placing TRAK2 as a regulator of LXR-mediated ABCA1 transcription and an ABCA1-dependent cholesterol efflux pathway.","method":"siRNA knockdown in THP-1 macrophages and HepG2 liver cells, cholesterol efflux assay, RT-PCR and western blot for ABCA1, chromatin immunoprecipitation for LXR at ABCA1 promoter, ABCA1 rescue/abolishment experiment","journal":"European heart journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean siRNA KD with specific phenotypic readout and epistasis (ABCA1 KO abolishes effect), multiple orthogonal methods, single lab","pmids":["28655204"],"is_preprint":false},{"year":2020,"finding":"Caspase-3-cleaved tau significantly decreases TRAK2 protein expression in hippocampal and cortical neurons and increases TRAK2 association with mitochondria (without affecting RhoT1/T2, syntaphilin, KIF5, or dynein expression/localization), correlating with reduced mitochondrial transport and bioenergetic deficits; this identifies TRAK2 downregulation/mislocalization as a mechanism linking tau pathology to mitochondrial transport failure.","method":"GFP-tau overexpression in primary hippocampal neurons and immortalized cortical neurons, western blot, co-immunoprecipitation of TRAK2 with mitochondria, live-cell mitochondrial tracking","journal":"Frontiers in cellular neuroscience","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — single lab, overexpression model, specific co-IP of TRAK2-mitochondria plus quantified transport, but mechanism is correlative without direct rescue","pmids":["32848607"],"is_preprint":false},{"year":2021,"finding":"TRAK2 is a dual motor activating adaptor: it activates kinesin-1 for plus-end-directed microtubule transport AND functions as a dynein activating adaptor via a conserved coiled-coil motif for minus-end transport (LIS1-dependent). TRAK2 simultaneously binds both kinesin-1 and dynein-dynactin, forming an interdependent motor complex in which knockdown of either motor reduces transport initiation toward both microtubule ends.","method":"Single-molecule imaging of cell lysates on microtubules, co-immunoprecipitation, co-localization, siRNA knockdown of kinesin-1 and dynein-dynactin, coiled-coil domain mutagenesis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — single-molecule in vitro reconstitution assay with mutagenesis, reciprocal co-IP, and genetic epistasis (dual knockdowns), multiple orthogonal methods in one rigorous study","pmids":["34321481"],"is_preprint":false},{"year":2025,"finding":"A 29 bp 3'UTR motif in TRAK2 mRNA promotes cell-size-dependent polarized targeting of TRAK2 mRNA to distal cell protrusions, which scales mitochondria distribution by defining the site of TRAK2-MIRO1 retrograde transport complex assembly; excision of this motif perturbs size-regulated transport and causes distal mitochondrial accumulation.","method":"3'UTR motif deletion (CRISPR/genome editing implied), live mRNA imaging, mitochondrial distribution quantification, TRAK2-MIRO1 complex localization assay","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — preprint, single lab, motif excision with defined mitochondrial phenotype and complex localization readout, but not yet peer-reviewed","pmids":["bio_10.1101_2025.05.05.652164"],"is_preprint":true}],"current_model":"TRAK2 is a mitochondrial adaptor protein that simultaneously activates kinesin-1 (for anterograde, microtubule plus-end transport) and dynein-dynactin (for retrograde, minus-end transport) through distinct domains, forms a complex with the atypical GTPase Miro1 on the mitochondrial outer membrane in a GTPase-dependent manner, and also functions as a trafficking adaptor for GABA-A receptor beta2 subunits and the K+ channel Kir2.1; additionally, TRAK2 regulates LXR-mediated ABCA1 expression and cholesterol efflux, and its subcellular deployment is governed by polarized 3'UTR-mediated mRNA targeting that scales mitochondrial distribution to cell size."},"narrative":{"mechanistic_narrative":"TRAK2 (GRIF-1) is a mitochondrial trafficking adaptor that couples organelles to the microtubule motor machinery and directs the surface delivery of select membrane proteins [PMID:15644324, PMID:16895905]. On the mitochondrial outer membrane it forms a complex with the atypical GTPase Miro1, which recruits TRAK2 in a manner dependent on its first GTPase domain and thereby controls mitochondrial movement into neuronal processes [PMID:19103291]. TRAK2 functions as a dual motor-activating adaptor: through distinct elements it engages kinesin-1 to drive plus-end-directed transport while a conserved coiled-coil motif activates dynein-dynactin for LIS1-dependent minus-end transport, and the two motors are bound simultaneously in an interdependent complex such that loss of either impairs transport toward both microtubule ends [PMID:15644324, PMID:16835241, PMID:34321481]. Beyond mitochondria, TRAK2 binds the GABA-A receptor beta2 subunit and the inward-rectifier K+ channel Kir2.1, increasing Kir2.1 surface expression and current, establishing a broader role as a trafficking factor for membrane proteins [PMID:12034717, PMID:16895905]. TRAK2 additionally regulates LXR-mediated ABCA1 transcription and ABCA1-dependent cholesterol efflux [PMID:28655204]. Its spatial deployment is set by a 3'UTR motif that targets TRAK2 mRNA to distal protrusions and scales mitochondrial distribution to cell size [PMID:bio_10.1101_2025.05.05.652164].","teleology":[{"year":2002,"claim":"Established TRAK2's first molecular partner, identifying it as a candidate adaptor linking the GABA-A receptor beta2 subunit to intracellular machinery.","evidence":"Yeast two-hybrid and reciprocal co-IP from HEK293 cells and rat brain with domain mapping","pmids":["12034717"],"confidence":"Medium","gaps":["Functional consequence of the interaction for receptor trafficking not established","Single lab; no in vivo validation"]},{"year":2005,"claim":"Connected TRAK2 to the microtubule motor kinesin and to mitochondria, framing it as a motor adaptor.","evidence":"Co-IP from native brain/heart and HEK293 cells plus yeast two-hybrid mapping of the KIF5C-binding region to residues 124–283","pmids":["15644324"],"confidence":"High","gaps":["Directionality and motor activation not addressed","Whether binding is direct vs complex-mediated unresolved at this stage"]},{"year":2006,"claim":"Demonstrated the kinesin interaction is direct and maps to the kinesin cargo-binding domain, and that TRAK2 also traffics a K+ channel.","evidence":"FRET between tagged GRIF-1 and KIF5C, truncation yeast two-hybrid, and surface-expression/Rb+ efflux assays for Kir2.1","pmids":["16835241","16895905"],"confidence":"High","gaps":["How TRAK2 selects between cargoes not defined","Regulation of channel trafficking in neurons not tested"]},{"year":2008,"claim":"Identified Miro1 as the mitochondrial receptor for TRAK2 and showed the complex governs mitochondrial transport into processes.","evidence":"Co-IP from brain, GTPase-domain mutants, dominant-negative fragment competition, and live-neuron transport imaging","pmids":["19103291"],"confidence":"High","gaps":["Coupling between Miro1-bound TRAK2 and motor activation not mechanistically resolved","Ca2+ regulation of the complex not addressed here"]},{"year":2012,"claim":"Placed TRAK2 within a Ca2+-sensitive Kinesin/Miro/TRAK2 complex modulated by Alex3, linking calcium signaling to mitochondrial dynamics.","evidence":"Co-IP, neuronal imaging, and Ca2+-dependent binding assays","pmids":["22569362"],"confidence":"Medium","gaps":["TRAK2 findings secondary to Armcx focus","Direct vs indirect Alex3-TRAK2 contact not defined"]},{"year":2017,"claim":"Extended TRAK2 function beyond transport to transcriptional control of cholesterol efflux via LXR/ABCA1.","evidence":"siRNA knockdown in THP-1 and HepG2, efflux assays, ABCA1 RT-PCR/western, LXR ChIP, and ABCA1-null epistasis","pmids":["28655204"],"confidence":"Medium","gaps":["Molecular mechanism by which TRAK2 represses LXR activity unknown","Connection to its mitochondrial adaptor role unclear"]},{"year":2020,"claim":"Linked tau pathology to TRAK2 loss and mislocalization as a route to mitochondrial transport failure.","evidence":"GFP-tau overexpression in neurons with western blot, TRAK2-mitochondria co-IP, and live mitochondrial tracking","pmids":["32848607"],"confidence":"Medium","gaps":["Correlative; no rescue of transport by restoring TRAK2","Overexpression model only"]},{"year":2021,"claim":"Resolved the core mechanism: TRAK2 is a dual activating adaptor that simultaneously activates kinesin-1 and dynein-dynactin to achieve bidirectional transport.","evidence":"Single-molecule imaging of lysates on microtubules, coiled-coil mutagenesis, reciprocal co-IP, and dual motor knockdown epistasis","pmids":["34321481"],"confidence":"High","gaps":["How directional bias is switched in cells not defined","Regulation by Miro1/Ca2+ within the activated complex not reconstituted"]},{"year":2025,"claim":"Showed that polarized 3'UTR-mediated mRNA targeting sets where the TRAK2-MIRO1 complex assembles, scaling mitochondrial distribution to cell size.","evidence":"3'UTR motif excision, live mRNA imaging, mitochondrial distribution quantification, and complex localization (preprint)","pmids":["bio_10.1101_2025.05.05.652164"],"confidence":"Medium","gaps":["Preprint; not peer-reviewed","RNA-binding factors recognizing the 29 bp motif unidentified"]},{"year":null,"claim":"How TRAK2's directional motor output is coordinated with Miro1/Ca2+ signaling and cargo identity to produce regulated, position-specific mitochondrial distribution remains unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No integrated model linking dual-motor activation to Miro1 GTPase state","Switch governing anterograde vs retrograde bias unknown","Relationship between transport role and LXR/ABCA1 regulation undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[1,2,8]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[2,8]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3,8]}],"localization":[{"term_id":"GO:0005739","term_label":"mitochondrion","supporting_discovery_ids":[1,4,7]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[8]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[4,8]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[8]}],"complexes":["Kinesin-1/Miro1/TRAK2 mitochondrial transport complex","TRAK2-dynein-dynactin complex"],"partners":["MIRO1","KIF5A","KIF5B","KIF5C","GABRB2","KCNJ2","ARMCX3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O60296","full_name":"Trafficking kinesin-binding protein 2","aliases":["Amyotrophic lateral sclerosis 2 chromosomal region candidate gene 3 protein"],"length_aa":914,"mass_kda":101.4,"function":"May regulate endosome-to-lysosome trafficking of membrane cargo, including EGFR","subcellular_location":"Cytoplasm; Early endosome; Mitochondrion","url":"https://www.uniprot.org/uniprotkb/O60296/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/TRAK2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/TRAK2","total_profiled":1310},"omim":[{"mim_id":"617235","title":"MYOCLONUS, INTRACTABLE, NEONATAL; NEIMY","url":"https://www.omim.org/entry/617235"},{"mim_id":"608112","title":"TRAFFICKING PROTEIN, KINESIN-BINDING 1; TRAK1","url":"https://www.omim.org/entry/608112"},{"mim_id":"607334","title":"TRAFFICKING PROTEIN, KINESIN-BINDING 2; TRAK2","url":"https://www.omim.org/entry/607334"},{"mim_id":"602821","title":"KINESIN FAMILY MEMBER 5A; KIF5A","url":"https://www.omim.org/entry/602821"},{"mim_id":"301048","title":"ARMADILLO REPEAT-CONTAINING PROTEIN, X-LINKED 6; ARMCX6","url":"https://www.omim.org/entry/301048"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Nucleoplasm","reliability":"Approved"},{"location":"Nuclear bodies","reliability":"Approved"},{"location":"Vesicles","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"brain","ntpm":195.8}],"url":"https://www.proteinatlas.org/search/TRAK2"},"hgnc":{"alias_symbol":["CALS-C","KIAA0549","GRIF-1","OIP98","MILT2"],"prev_symbol":["ALS2CR3"]},"alphafold":{"accession":"O60296","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O60296","model_url":"https://alphafold.ebi.ac.uk/files/AF-O60296-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O60296-F1-predicted_aligned_error_v6.png","plddt_mean":60.03},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=TRAK2","jax_strain_url":"https://www.jax.org/strain/search?query=TRAK2"},"sequence":{"accession":"O60296","fasta_url":"https://rest.uniprot.org/uniprotkb/O60296.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O60296/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O60296"}},"corpus_meta":[{"pmid":"19103291","id":"PMC_19103291","title":"GTPase dependent recruitment of Grif-1 by Miro1 regulates mitochondrial trafficking in hippocampal neurons.","date":"2008","source":"Molecular and cellular neurosciences","url":"https://pubmed.ncbi.nlm.nih.gov/19103291","citation_count":140,"is_preprint":false},{"pmid":"15644324","id":"PMC_15644324","title":"GRIF-1 and OIP106, members of a novel gene family of coiled-coil domain proteins: association in vivo and in vitro with kinesin.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15644324","citation_count":137,"is_preprint":false},{"pmid":"34321481","id":"PMC_34321481","title":"Mitochondrial adaptor TRAK2 activates and functionally links opposing kinesin and dynein motors.","date":"2021","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/34321481","citation_count":98,"is_preprint":false},{"pmid":"12034717","id":"PMC_12034717","title":"Identification, molecular cloning, and characterization of a novel GABAA receptor-associated protein, GRIF-1.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/12034717","citation_count":92,"is_preprint":false},{"pmid":"22569362","id":"PMC_22569362","title":"The Eutherian Armcx genes regulate mitochondrial trafficking in neurons and interact with Miro and Trak2.","date":"2012","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/22569362","citation_count":89,"is_preprint":false},{"pmid":"16835241","id":"PMC_16835241","title":"Mapping the GRIF-1 binding domain of the kinesin, KIF5C, substantiates a role for GRIF-1 as an adaptor protein in the anterograde trafficking of cargoes.","date":"2006","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16835241","citation_count":69,"is_preprint":false},{"pmid":"32848607","id":"PMC_32848607","title":"Truncated Tau Induces Mitochondrial Transport Failure Through the Impairment of TRAK2 Protein and Bioenergetics Decline in Neuronal Cells.","date":"2020","source":"Frontiers in cellular neuroscience","url":"https://pubmed.ncbi.nlm.nih.gov/32848607","citation_count":42,"is_preprint":false},{"pmid":"16895905","id":"PMC_16895905","title":"Identification of gamma-aminobutyric acid receptor-interacting factor 1 (TRAK2) as a trafficking factor for the K+ channel Kir2.1.","date":"2006","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/16895905","citation_count":38,"is_preprint":false},{"pmid":"28655204","id":"PMC_28655204","title":"TRAK2, a novel regulator of ABCA1 expression, cholesterol efflux and HDL biogenesis.","date":"2017","source":"European heart journal","url":"https://pubmed.ncbi.nlm.nih.gov/28655204","citation_count":30,"is_preprint":false},{"pmid":"16417480","id":"PMC_16417480","title":"GRIF-1-kinesin-1 interactions: a confocal microscopy study.","date":"2006","source":"Biochemical Society transactions","url":"https://pubmed.ncbi.nlm.nih.gov/16417480","citation_count":10,"is_preprint":false},{"pmid":"40448098","id":"PMC_40448098","title":"CircSLC22A3 inhibits the invasion and metastasis of ESCC via the miR-19b-3p/TRAK2 axis and by reducing the stability of m6A-modified ACSBG1 mRNA.","date":"2025","source":"BMC cancer","url":"https://pubmed.ncbi.nlm.nih.gov/40448098","citation_count":0,"is_preprint":false},{"pmid":null,"id":"bio_10.1101_2025.05.05.652164","title":"mRNA trafficking directs cell-size-scaling of mitochondria distribution and function","date":"2025-05-06","source":"bioRxiv","url":"https://doi.org/10.1101/2025.05.05.652164","citation_count":0,"is_preprint":true},{"pmid":null,"id":"bio_10.1101_2025.07.17.660811","title":"Genome-wide association study of cocaine self-administration behavior in Heterogeneous Stock rats","date":"2025-07-18","source":"bioRxiv","url":"https://doi.org/10.1101/2025.07.17.660811","citation_count":0,"is_preprint":true}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9570,"output_tokens":2845,"usd":0.035693,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10186,"output_tokens":3090,"usd":0.06409,"stage2_stop_reason":"end_turn"},"total_usd":0.099783,"stage1_batch_id":"msgbatch_017qqVz33Ga63SCDwzb35XPb","stage2_batch_id":"msgbatch_01Scr2Ei9x4HJMiRGxwrmvLK","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"TRAK2 (GRIF-1) was identified as a GABA-A receptor beta2 subunit-interacting protein; the interaction was demonstrated by yeast two-hybrid assay and co-immunoprecipitation from HEK293 cells and adult rat brain lysates, with the respective binding domains mapped.\",\n      \"method\": \"Yeast two-hybrid assay, co-immunoprecipitation from transfected HEK293 cells and native rat brain lysates\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP in cells and native tissue plus yeast two-hybrid domain mapping, single lab\",\n      \"pmids\": [\"12034717\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"TRAK2 (GRIF-1) associates with kinesin heavy chains (KIF5A in brain, KIF5B in heart/HEK293 cells, KIF5C by direct interaction) and with mitochondria; the GRIF-1/KIF5C interaction domain was localized to GRIF-1 residues 124–283 by yeast two-hybrid and co-immunoprecipitation.\",\n      \"method\": \"Co-immunoprecipitation from native brain/heart tissue and transfected HEK293 cells, yeast two-hybrid interaction assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP in multiple native tissues and cells plus yeast two-hybrid domain mapping, replicated in subsequent papers\",\n      \"pmids\": [\"15644324\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The GRIF-1 binding site on KIF5C was mapped to the KIF5C non-motor (cargo-binding) domain; the interaction is direct as demonstrated by FRET between fluorescently tagged GRIF-1 (N-terminal) and KIF5C (C-terminal), and GRIF-1 can bind the intact tetrameric kinesin light-chain/kinesin heavy-chain complex.\",\n      \"method\": \"Yeast two-hybrid with truncation constructs, co-immunoprecipitation, FRET with fluorescently tagged constructs, confocal microscopy\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — direct FRET evidence of protein–protein interaction plus domain mapping by yeast two-hybrid and co-IP, multiple orthogonal methods in one study\",\n      \"pmids\": [\"16835241\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"TRAK2 (GRIF-1) binds to the K+ channel Kir2.1 (interaction mapped to GRIF-1 N-terminus and Kir2.1 C-terminus by yeast two-hybrid), increases surface expression of Kir2.1 channels in COS and HEK293 cells, and enhances Kir2.1-mediated Rb+ efflux, establishing TRAK2 as a trafficking factor for this channel.\",\n      \"method\": \"Functional yeast growth rescue screen, 86Rb+ efflux assay, quantitative surface immunolabeling and flow cytometry, co-immunoprecipitation from HEK293 lysates and brain lysate, yeast two-hybrid domain mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (functional assay, quantitative surface labeling, co-IP in native tissue, yeast two-hybrid), single lab\",\n      \"pmids\": [\"16895905\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"TRAK2 (GRIF-1) and the atypical GTPase Miro1 form a protein complex on neuronal mitochondria; Miro1 recruits GRIF-1 to mitochondria in a manner dependent on its first GTPase domain. Overexpression of Miro1 enhanced mitochondrial transport toward distal neuronal processes, while expression of a Grif-1/Miro1 binding fragment dramatically reduced mitochondrial transport into processes.\",\n      \"method\": \"Co-immunoprecipitation from mammalian brain, fluorescence imaging of hippocampal neurons, dominant-negative GTPase domain mutants, overexpression and competitive fragment experiments\",\n      \"journal\": \"Molecular and cellular neurosciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — co-IP from native brain tissue, GTPase domain mutagenesis, dominant-negative fragment rescue, live-neuron transport quantification; findings replicated in subsequent studies\",\n      \"pmids\": [\"19103291\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"TRAK2 participates in a Kinesin/Miro/TRAK2 complex in neurons; the armadillo-repeat protein Alex3 interacts with this complex in a Ca2+-dependent manner, modulating mitochondrial dynamics and trafficking.\",\n      \"method\": \"Co-immunoprecipitation, fluorescence imaging of neurons, Ca2+-dependent binding assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP demonstrating the complex plus Ca2+-dependence, single lab, findings about TRAK2 are secondary to the primary Armcx focus\",\n      \"pmids\": [\"22569362\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"TRAK2 knockdown increases cholesterol efflux to apolipoprotein A-I and HDL via increased ABCA1 mRNA and protein expression, and increases LXR binding at the ABCA1 promoter; the efflux increase is abolished in the absence of ABCA1, placing TRAK2 as a regulator of LXR-mediated ABCA1 transcription and an ABCA1-dependent cholesterol efflux pathway.\",\n      \"method\": \"siRNA knockdown in THP-1 macrophages and HepG2 liver cells, cholesterol efflux assay, RT-PCR and western blot for ABCA1, chromatin immunoprecipitation for LXR at ABCA1 promoter, ABCA1 rescue/abolishment experiment\",\n      \"journal\": \"European heart journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean siRNA KD with specific phenotypic readout and epistasis (ABCA1 KO abolishes effect), multiple orthogonal methods, single lab\",\n      \"pmids\": [\"28655204\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Caspase-3-cleaved tau significantly decreases TRAK2 protein expression in hippocampal and cortical neurons and increases TRAK2 association with mitochondria (without affecting RhoT1/T2, syntaphilin, KIF5, or dynein expression/localization), correlating with reduced mitochondrial transport and bioenergetic deficits; this identifies TRAK2 downregulation/mislocalization as a mechanism linking tau pathology to mitochondrial transport failure.\",\n      \"method\": \"GFP-tau overexpression in primary hippocampal neurons and immortalized cortical neurons, western blot, co-immunoprecipitation of TRAK2 with mitochondria, live-cell mitochondrial tracking\",\n      \"journal\": \"Frontiers in cellular neuroscience\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — single lab, overexpression model, specific co-IP of TRAK2-mitochondria plus quantified transport, but mechanism is correlative without direct rescue\",\n      \"pmids\": [\"32848607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"TRAK2 is a dual motor activating adaptor: it activates kinesin-1 for plus-end-directed microtubule transport AND functions as a dynein activating adaptor via a conserved coiled-coil motif for minus-end transport (LIS1-dependent). TRAK2 simultaneously binds both kinesin-1 and dynein-dynactin, forming an interdependent motor complex in which knockdown of either motor reduces transport initiation toward both microtubule ends.\",\n      \"method\": \"Single-molecule imaging of cell lysates on microtubules, co-immunoprecipitation, co-localization, siRNA knockdown of kinesin-1 and dynein-dynactin, coiled-coil domain mutagenesis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — single-molecule in vitro reconstitution assay with mutagenesis, reciprocal co-IP, and genetic epistasis (dual knockdowns), multiple orthogonal methods in one rigorous study\",\n      \"pmids\": [\"34321481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"A 29 bp 3'UTR motif in TRAK2 mRNA promotes cell-size-dependent polarized targeting of TRAK2 mRNA to distal cell protrusions, which scales mitochondria distribution by defining the site of TRAK2-MIRO1 retrograde transport complex assembly; excision of this motif perturbs size-regulated transport and causes distal mitochondrial accumulation.\",\n      \"method\": \"3'UTR motif deletion (CRISPR/genome editing implied), live mRNA imaging, mitochondrial distribution quantification, TRAK2-MIRO1 complex localization assay\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — preprint, single lab, motif excision with defined mitochondrial phenotype and complex localization readout, but not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.05.05.652164\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"TRAK2 is a mitochondrial adaptor protein that simultaneously activates kinesin-1 (for anterograde, microtubule plus-end transport) and dynein-dynactin (for retrograde, minus-end transport) through distinct domains, forms a complex with the atypical GTPase Miro1 on the mitochondrial outer membrane in a GTPase-dependent manner, and also functions as a trafficking adaptor for GABA-A receptor beta2 subunits and the K+ channel Kir2.1; additionally, TRAK2 regulates LXR-mediated ABCA1 expression and cholesterol efflux, and its subcellular deployment is governed by polarized 3'UTR-mediated mRNA targeting that scales mitochondrial distribution to cell size.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"TRAK2 (GRIF-1) is a mitochondrial trafficking adaptor that couples organelles to the microtubule motor machinery and directs the surface delivery of select membrane proteins [#1, #3]. On the mitochondrial outer membrane it forms a complex with the atypical GTPase Miro1, which recruits TRAK2 in a manner dependent on its first GTPase domain and thereby controls mitochondrial movement into neuronal processes [#4]. TRAK2 functions as a dual motor-activating adaptor: through distinct elements it engages kinesin-1 to drive plus-end-directed transport while a conserved coiled-coil motif activates dynein-dynactin for LIS1-dependent minus-end transport, and the two motors are bound simultaneously in an interdependent complex such that loss of either impairs transport toward both microtubule ends [#1, #2, #8]. Beyond mitochondria, TRAK2 binds the GABA-A receptor beta2 subunit and the inward-rectifier K+ channel Kir2.1, increasing Kir2.1 surface expression and current, establishing a broader role as a trafficking factor for membrane proteins [#0, #3]. TRAK2 additionally regulates LXR-mediated ABCA1 transcription and ABCA1-dependent cholesterol efflux [#6]. Its spatial deployment is set by a 3'UTR motif that targets TRAK2 mRNA to distal protrusions and scales mitochondrial distribution to cell size [#9].\",\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established TRAK2's first molecular partner, identifying it as a candidate adaptor linking the GABA-A receptor beta2 subunit to intracellular machinery.\",\n      \"evidence\": \"Yeast two-hybrid and reciprocal co-IP from HEK293 cells and rat brain with domain mapping\",\n      \"pmids\": [\"12034717\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the interaction for receptor trafficking not established\", \"Single lab; no in vivo validation\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Connected TRAK2 to the microtubule motor kinesin and to mitochondria, framing it as a motor adaptor.\",\n      \"evidence\": \"Co-IP from native brain/heart and HEK293 cells plus yeast two-hybrid mapping of the KIF5C-binding region to residues 124–283\",\n      \"pmids\": [\"15644324\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Directionality and motor activation not addressed\", \"Whether binding is direct vs complex-mediated unresolved at this stage\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstrated the kinesin interaction is direct and maps to the kinesin cargo-binding domain, and that TRAK2 also traffics a K+ channel.\",\n      \"evidence\": \"FRET between tagged GRIF-1 and KIF5C, truncation yeast two-hybrid, and surface-expression/Rb+ efflux assays for Kir2.1\",\n      \"pmids\": [\"16835241\", \"16895905\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How TRAK2 selects between cargoes not defined\", \"Regulation of channel trafficking in neurons not tested\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identified Miro1 as the mitochondrial receptor for TRAK2 and showed the complex governs mitochondrial transport into processes.\",\n      \"evidence\": \"Co-IP from brain, GTPase-domain mutants, dominant-negative fragment competition, and live-neuron transport imaging\",\n      \"pmids\": [\"19103291\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Coupling between Miro1-bound TRAK2 and motor activation not mechanistically resolved\", \"Ca2+ regulation of the complex not addressed here\"]\n    },\n    {\n      \"year\": 2012,\n      \"claim\": \"Placed TRAK2 within a Ca2+-sensitive Kinesin/Miro/TRAK2 complex modulated by Alex3, linking calcium signaling to mitochondrial dynamics.\",\n      \"evidence\": \"Co-IP, neuronal imaging, and Ca2+-dependent binding assays\",\n      \"pmids\": [\"22569362\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"TRAK2 findings secondary to Armcx focus\", \"Direct vs indirect Alex3-TRAK2 contact not defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Extended TRAK2 function beyond transport to transcriptional control of cholesterol efflux via LXR/ABCA1.\",\n      \"evidence\": \"siRNA knockdown in THP-1 and HepG2, efflux assays, ABCA1 RT-PCR/western, LXR ChIP, and ABCA1-null epistasis\",\n      \"pmids\": [\"28655204\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular mechanism by which TRAK2 represses LXR activity unknown\", \"Connection to its mitochondrial adaptor role unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Linked tau pathology to TRAK2 loss and mislocalization as a route to mitochondrial transport failure.\",\n      \"evidence\": \"GFP-tau overexpression in neurons with western blot, TRAK2-mitochondria co-IP, and live mitochondrial tracking\",\n      \"pmids\": [\"32848607\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Correlative; no rescue of transport by restoring TRAK2\", \"Overexpression model only\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Resolved the core mechanism: TRAK2 is a dual activating adaptor that simultaneously activates kinesin-1 and dynein-dynactin to achieve bidirectional transport.\",\n      \"evidence\": \"Single-molecule imaging of lysates on microtubules, coiled-coil mutagenesis, reciprocal co-IP, and dual motor knockdown epistasis\",\n      \"pmids\": [\"34321481\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How directional bias is switched in cells not defined\", \"Regulation by Miro1/Ca2+ within the activated complex not reconstituted\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Showed that polarized 3'UTR-mediated mRNA targeting sets where the TRAK2-MIRO1 complex assembles, scaling mitochondrial distribution to cell size.\",\n      \"evidence\": \"3'UTR motif excision, live mRNA imaging, mitochondrial distribution quantification, and complex localization (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.05.05.652164\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Preprint; not peer-reviewed\", \"RNA-binding factors recognizing the 29 bp motif unidentified\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How TRAK2's directional motor output is coordinated with Miro1/Ca2+ signaling and cargo identity to produce regulated, position-specific mitochondrial distribution remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No integrated model linking dual-motor activation to Miro1 GTPase state\", \"Switch governing anterograde vs retrograde bias unknown\", \"Relationship between transport role and LXR/ABCA1 regulation undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [1, 2, 8]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [2, 8]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005739\", \"supporting_discovery_ids\": [1, 4, 7]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [4, 8]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"complexes\": [\n      \"Kinesin-1/Miro1/TRAK2 mitochondrial transport complex\",\n      \"TRAK2-dynein-dynactin complex\"\n    ],\n    \"partners\": [\n      \"MIRO1\",\n      \"KIF5A\",\n      \"KIF5B\",\n      \"KIF5C\",\n      \"GABRB2\",\n      \"KCNJ2\",\n      \"ARMCX3\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}