{"gene":"KIFAP3","run_date":"2026-06-10T02:59:49","timeline":{"discoveries":[{"year":1996,"finding":"KAP3 (KIFAP3) was identified as a non-motor subunit that physically binds to the tail domain of KIF3A/KIF3B heterodimer, forming a heterotrimeric kinesin-2 complex. KAP3 is a globular, largely alpha-helical protein that does not affect the motor ATPase activity of KIF3A/KIF3B, and associates with membrane-bound KIF3A/KIF3B, suggesting a role in membrane/cargo binding.","method":"Immunoprecipitation, baculovirus-Sf9 reconstitution, microsequencing, cDNA cloning, EM, fractional immunoprecipitation","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution in Sf9 cells, multiple orthogonal methods (pulldown, EM, fractionation), motor activity assay with negative result for ATPase effect","pmids":["8710890"],"is_preprint":false},{"year":1998,"finding":"KAP3 (as SMAP) interacts with HCAP (human XCAP-E homolog, a condensin subunit), forming a ternary complex with KIF3B extractable from the nuclear fraction in the presence of Mg-ATP, suggesting KAP3 links chromosomal proteins to the KIF3 motor in the nucleus.","method":"Yeast two-hybrid screen, co-immunoprecipitation, subcellular fractionation, tissue distribution analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP and fractionation, single lab, two orthogonal methods","pmids":["9506951"],"is_preprint":false},{"year":2004,"finding":"The KAP subunit (FLA3/KAP3 ortholog in Chlamydomonas) is required for localization of kinesin-2 to the basal body and flagella for intraflagellar transport (IFT); a ts mutation in the conserved C-terminal domain of KAP prevents efficient targeting of kinesin-2 to the site of flagellar assembly and reduces the frequency of anterograde IFT particles without abolishing motor velocity.","method":"Temperature-sensitive mutant analysis, video-enhanced DIC microscopy, epitope-tagged rescue transformation, immunolocalization","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — genetic rescue with epitope-tagged construct, live imaging of IFT, multiple orthogonal methods, replicated in model organism ortholog with clear functional readout","pmids":["15616187"],"is_preprint":false},{"year":2008,"finding":"Misfolded mutant SOD1 specifically associates with KAP3 in motor axons of FALS mice (SOD1-G93A), sequestering KAP3 from the kinesin-2 complex and thereby impairing microtubule-dependent axonal transport of choline acetyltransferase (ChAT) and acetylcholine release; overexpression of KAP3 normalized acetylcholine release impaired by mutant SOD1.","method":"Co-immunoprecipitation, transgenic mouse spinal cord fractionation, NG108-15 cell FALS model, KAP3 overexpression rescue, immunohistochemistry of human FALS tissue","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP, functional rescue by overexpression, human tissue confirmation, single lab","pmids":["19088126"],"is_preprint":false},{"year":2022,"finding":"Loss of KAP3 (by CRISPR/Cas9 knockout) impairs post-Golgi transport of laminin (inhibiting basement membrane formation) and inactivates RhoA, disrupting circumferential actomyosin cable formation and weakening cell-cell adhesion in gastric signet ring cell carcinoma cells.","method":"CRISPR/Cas9 knockout, live cell imaging, RhoA activity assay, immunofluorescence of laminin transport, actomyosin cable analysis","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean KO with defined cellular phenotypes, multiple readouts (laminin transport, RhoA, actomyosin), single lab","pmids":["35322078"],"is_preprint":false},{"year":2025,"finding":"Cryo-EM structures of the heterotrimeric KIF3A/KIF3B/KAP3 complex bound to APC cargo revealed a conserved 'Hitchdock domain' in the KIF3 tail region; KIF3A helical regions within this domain mediate specific cargo (APC) binding while the KIF3B beta-hairpin and KAP3 provide structural support. Mutagenesis confirmed the functional importance of this domain for cargo recognition.","method":"High-resolution cryo-EM structure determination, site-directed mutagenesis, molecular dynamics simulation","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with mutagenesis and MD simulation in single preprint study, multiple orthogonal methods","pmids":["bio_10.1101_2025.03.21.644525"],"is_preprint":true},{"year":2024,"finding":"Structural, single-molecule, and cell biological analyses showed that KAP3 binds via a multipartite interface with both KIF3A and KIF3B, and rather than directly activating kinesin-2 motility, provides a platform for cargo adaptor engagement that occludes the autoinhibitory beta-hairpin motif in the KIF3 tail, thereby activating motility.","method":"Cryo-EM/structural analysis, single-molecule motility assays, cell biological assays, mutagenesis","journal":"bioRxiv","confidence":"High","confidence_rationale":"Tier 1 / Moderate — structural data combined with single-molecule assays and mutagenesis, multiple orthogonal approaches in single preprint","pmids":["bio_10.1101_2024.10.14.618219"],"is_preprint":true},{"year":2026,"finding":"A KIF3B-enriched, KAP3-associated assembly (distinct from canonical KIF3A/B/KAP3) preferentially associates with TRIM46, a protein required for axon initial segment organization; structural analyses suggest differences in tail conformation accompany distinct assembly states and underlie cargo selectivity for targeted transport to the axon initial segment.","method":"Biochemical fractionation, co-immunoprecipitation, structural analysis, neuronal cell biology","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal biochemical and structural analyses, single lab, multiple orthogonal methods","pmids":["41910726"],"is_preprint":false}],"current_model":"KIFAP3/KAP3 is a non-motor adaptor subunit that binds the tail domains of KIF3A and KIF3B through a multipartite interface (including the 'Hitchdock domain') to form the heterotrimeric kinesin-2 complex; rather than modulating motor ATPase activity directly, KAP3 provides a structural platform for cargo adaptors (such as APC) whose engagement occludes the autoinhibitory beta-hairpin motif in the KIF3 tail to activate processive anterograde transport, and compositional heterogeneity in KIF3/KAP3 assemblies contributes to selective cargo delivery (e.g., TRIM46 to the axon initial segment, ChAT in motor axons, laminin post-Golgi transport) in neurons and epithelial cells."},"narrative":{"mechanistic_narrative":"KIFAP3 (KAP3) is the non-motor accessory subunit of the heterotrimeric kinesin-2 motor, binding the tail domains of the KIF3A/KIF3B motor heterodimer to form the KIF3A/KIF3B/KAP3 complex without altering motor ATPase activity, and functions as a cargo-binding and cargo-adaptor platform for microtubule-based anterograde transport [PMID:8710890]. Structurally, KAP3 engages KIF3A and KIF3B through a multipartite interface within a conserved tail region ('Hitchdock domain'); rather than directly activating motility, it provides a platform for cargo-adaptor engagement (such as APC) that occludes the autoinhibitory beta-hairpin motif in the KIF3 tail to license processive transport [PMID:bio_10.1101_2025.03.21.644525, PMID:bio_10.1101_2024.10.14.618219]. Through this transport activity KAP3 delivers diverse cargoes in specialized cells, including choline acetyltransferase in motor axons—where sequestration of KAP3 by misfolded mutant SOD1 impairs axonal transport and acetylcholine release [PMID:19088126]—post-Golgi laminin required for basement membrane formation and RhoA-dependent actomyosin organization in epithelial carcinoma cells [PMID:35322078], and, via a distinct KIF3B-enriched assembly, TRIM46 to the axon initial segment, illustrating how compositional heterogeneity of KIF3/KAP3 assemblies confers cargo selectivity [PMID:41910726]. The same machinery is required in cilia/flagella, where the KAP3 ortholog targets kinesin-2 to the basal body for intraflagellar transport [PMID:15616187].","teleology":[{"year":1996,"claim":"Established that KAP3 is the third, non-motor subunit of kinesin-2, defining the heterotrimeric architecture and pointing to a cargo/membrane-binding rather than motor-regulatory role.","evidence":"Immunoprecipitation, baculovirus-Sf9 reconstitution, EM, and ATPase assay of the KIF3A/KIF3B/KAP3 complex","pmids":["8710890"],"confidence":"High","gaps":["Did not identify specific physiological cargoes bound by KAP3","Structural basis of the KIF3-KAP3 interface unresolved"]},{"year":1998,"claim":"Provided an early candidate cargo by linking KAP3 to a condensin subunit, raising the possibility that kinesin-2 couples to chromosomal proteins.","evidence":"Yeast two-hybrid, reciprocal co-immunoprecipitation, and subcellular fractionation showing a KAP3/KIF3B/HCAP ternary complex","pmids":["9506951"],"confidence":"Medium","gaps":["Functional consequence of the nuclear KAP3-condensin association not established","Not connected to a transport event"]},{"year":2004,"claim":"Demonstrated an in vivo requirement for the KAP subunit in targeting kinesin-2 to the site of organelle assembly, defining its role in intraflagellar transport.","evidence":"Temperature-sensitive KAP mutant, epitope-tagged rescue, video-DIC imaging of IFT, and immunolocalization in Chlamydomonas","pmids":["15616187"],"confidence":"High","gaps":["Molecular partner mediating basal-body targeting not identified","Ortholog-based; human/mammalian ciliary role not directly tested here"]},{"year":2008,"claim":"Connected KAP3 to disease by showing that sequestration of KAP3 from kinesin-2 impairs cargo transport, mechanistically linking KAP3 to motor neuron axonal transport defects.","evidence":"Co-IP from FALS mouse spinal cord, NG108-15 cell model, KAP3 overexpression rescue of acetylcholine release, and human FALS tissue immunohistochemistry","pmids":["19088126"],"confidence":"Medium","gaps":["Direct demonstration that ChAT is a KAP3-bound cargo not shown","Single lab; quantitative contribution to FALS pathology unresolved"]},{"year":2022,"claim":"Defined epithelial cargo and downstream signaling consequences of KAP3 loss, extending its transport role to basement membrane assembly and RhoA-dependent cytoskeletal organization.","evidence":"CRISPR/Cas9 knockout with live imaging of laminin transport, RhoA activity assay, and actomyosin cable analysis in gastric signet ring cell carcinoma cells","pmids":["35322078"],"confidence":"Medium","gaps":["Mechanism linking laminin transport defect to RhoA inactivation not defined","Single cell-line context"]},{"year":2025,"claim":"Resolved the structural basis of cargo recognition, showing how a conserved tail 'Hitchdock domain' with KAP3 support mediates specific APC binding.","evidence":"Cryo-EM of KIF3A/KIF3B/KAP3 bound to APC, with site-directed mutagenesis and molecular dynamics (preprint)","pmids":["bio_10.1101_2025.03.21.644525"],"confidence":"High","gaps":["Preprint, not peer-reviewed","Generality of the domain across non-APC cargoes not established"]},{"year":2024,"claim":"Reframed KAP3 function as an adaptor platform that relieves motor autoinhibition, explaining how cargo engagement activates kinesin-2 motility.","evidence":"Cryo-EM/structural analysis, single-molecule motility assays, and mutagenesis showing the multipartite KAP3 interface and beta-hairpin occlusion (preprint)","pmids":["bio_10.1101_2024.10.14.618219"],"confidence":"High","gaps":["Preprint, not peer-reviewed","How distinct cargoes differentially trigger activation not fully mapped"]},{"year":2026,"claim":"Showed that compositional heterogeneity of KIF3/KAP3 assemblies underlies cargo selectivity, with a KIF3B-enriched KAP3 assembly directing TRIM46 to the axon initial segment.","evidence":"Biochemical fractionation, reciprocal co-immunoprecipitation, structural analysis, and neuronal cell biology","pmids":["41910726"],"confidence":"Medium","gaps":["Regulation determining which assembly forms not defined","Tail conformational differences inferred structurally rather than directly visualized in each state"]},{"year":null,"claim":"How KAP3 selects among its many cargoes and how assembly composition is regulated across cell types remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified model of cargo-adaptor specificity across neurons, epithelia, and cilia","Regulatory inputs controlling KIF3 assembly heterogeneity unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,5,6]},{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[0]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[2]},{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,4]}],"pathway":[{"term_id":"R-HSA-9609507","term_label":"Protein localization","supporting_discovery_ids":[3,4,7]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[0,4]}],"complexes":["kinesin-2 (KIF3A/KIF3B/KAP3 heterotrimer)"],"partners":["KIF3A","KIF3B","APC","TRIM46","HCAP","SOD1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q92845","full_name":"Kinesin-associated protein 3","aliases":["Smg GDS-associated protein"],"length_aa":792,"mass_kda":91.2,"function":"Involved in tethering the chromosomes to the spindle pole and in chromosome movement. Binds to the tail domain of the KIF3A/KIF3B heterodimer to form a heterotrimeric KIF3 complex and may regulate the membrane binding of this complex (By similarity)","subcellular_location":"","url":"https://www.uniprot.org/uniprotkb/Q92845/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/KIFAP3","classification":"Not Classified","n_dependent_lines":8,"n_total_lines":1208,"dependency_fraction":0.006622516556291391},"opencell":{"profiled":true,"resolved_as":"","ensg_id":"ENSG00000075945","cell_line_id":"CID001428","localizations":[{"compartment":"cytoplasmic","grade":3},{"compartment":"big_aggregates","grade":2},{"compartment":"nucleoplasm","grade":1}],"interactors":[{"gene":"KIF3A","stoichiometry":10.0},{"gene":"KIF3B","stoichiometry":10.0},{"gene":"BCAR1","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/target/CID001428","total_profiled":1310},"omim":[{"mim_id":"619309","title":"PROTEIN PHOSPHATASE, MAGNESIUM/MANGANESE-DEPENDENT, 1F; PPM1F","url":"https://www.omim.org/entry/619309"},{"mim_id":"601836","title":"KINESIN-ASSOCIATED PROTEIN 3; KIFAP3","url":"https://www.omim.org/entry/601836"},{"mim_id":"312610","title":"RETINITIS PIGMENTOSA GTPase REGULATOR; RPGR","url":"https://www.omim.org/entry/312610"},{"mim_id":"147450","title":"SUPEROXIDE DISMUTASE 1; SOD1","url":"https://www.omim.org/entry/147450"},{"mim_id":"118490","title":"CHOLINE ACETYLTRANSFERASE; CHAT","url":"https://www.omim.org/entry/118490"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Microtubules","reliability":"Approved"},{"location":"Basal body","reliability":"Approved"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/KIFAP3"},"hgnc":{"alias_symbol":["SMAP","KAP3","FLA3","KAP-1"],"prev_symbol":[]},"alphafold":{"accession":"Q92845","domains":[{"cath_id":"-","chopping":"8-97","consensus_level":"high","plddt":83.7442,"start":8,"end":97},{"cath_id":"1.25.10","chopping":"133-290","consensus_level":"high","plddt":94.6285,"start":133,"end":290},{"cath_id":"1.25.10.10","chopping":"303-412","consensus_level":"medium","plddt":96.3015,"start":303,"end":412},{"cath_id":"1.25.10.10","chopping":"537-690_775-781","consensus_level":"medium","plddt":89.9853,"start":537,"end":781}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92845","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q92845-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q92845-F1-predicted_aligned_error_v6.png","plddt_mean":83.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=KIFAP3","jax_strain_url":"https://www.jax.org/strain/search?query=KIFAP3"},"sequence":{"accession":"Q92845","fasta_url":"https://rest.uniprot.org/uniprotkb/Q92845.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q92845/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92845"}},"corpus_meta":[{"pmid":"8710890","id":"PMC_8710890","title":"Cloning 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\"finding\": \"KAP3 (KIFAP3) was identified as a non-motor subunit that physically binds to the tail domain of KIF3A/KIF3B heterodimer, forming a heterotrimeric kinesin-2 complex. KAP3 is a globular, largely alpha-helical protein that does not affect the motor ATPase activity of KIF3A/KIF3B, and associates with membrane-bound KIF3A/KIF3B, suggesting a role in membrane/cargo binding.\",\n      \"method\": \"Immunoprecipitation, baculovirus-Sf9 reconstitution, microsequencing, cDNA cloning, EM, fractional immunoprecipitation\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution in Sf9 cells, multiple orthogonal methods (pulldown, EM, fractionation), motor activity assay with negative result for ATPase effect\",\n      \"pmids\": [\"8710890\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"KAP3 (as SMAP) interacts with HCAP (human XCAP-E homolog, a condensin subunit), forming a ternary complex with KIF3B extractable from the nuclear fraction in the presence of Mg-ATP, suggesting KAP3 links chromosomal proteins to the KIF3 motor in the nucleus.\",\n      \"method\": \"Yeast two-hybrid screen, co-immunoprecipitation, subcellular fractionation, tissue distribution analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP and fractionation, single lab, two orthogonal methods\",\n      \"pmids\": [\"9506951\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"The KAP subunit (FLA3/KAP3 ortholog in Chlamydomonas) is required for localization of kinesin-2 to the basal body and flagella for intraflagellar transport (IFT); a ts mutation in the conserved C-terminal domain of KAP prevents efficient targeting of kinesin-2 to the site of flagellar assembly and reduces the frequency of anterograde IFT particles without abolishing motor velocity.\",\n      \"method\": \"Temperature-sensitive mutant analysis, video-enhanced DIC microscopy, epitope-tagged rescue transformation, immunolocalization\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genetic rescue with epitope-tagged construct, live imaging of IFT, multiple orthogonal methods, replicated in model organism ortholog with clear functional readout\",\n      \"pmids\": [\"15616187\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Misfolded mutant SOD1 specifically associates with KAP3 in motor axons of FALS mice (SOD1-G93A), sequestering KAP3 from the kinesin-2 complex and thereby impairing microtubule-dependent axonal transport of choline acetyltransferase (ChAT) and acetylcholine release; overexpression of KAP3 normalized acetylcholine release impaired by mutant SOD1.\",\n      \"method\": \"Co-immunoprecipitation, transgenic mouse spinal cord fractionation, NG108-15 cell FALS model, KAP3 overexpression rescue, immunohistochemistry of human FALS tissue\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP, functional rescue by overexpression, human tissue confirmation, single lab\",\n      \"pmids\": [\"19088126\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Loss of KAP3 (by CRISPR/Cas9 knockout) impairs post-Golgi transport of laminin (inhibiting basement membrane formation) and inactivates RhoA, disrupting circumferential actomyosin cable formation and weakening cell-cell adhesion in gastric signet ring cell carcinoma cells.\",\n      \"method\": \"CRISPR/Cas9 knockout, live cell imaging, RhoA activity assay, immunofluorescence of laminin transport, actomyosin cable analysis\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean KO with defined cellular phenotypes, multiple readouts (laminin transport, RhoA, actomyosin), single lab\",\n      \"pmids\": [\"35322078\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Cryo-EM structures of the heterotrimeric KIF3A/KIF3B/KAP3 complex bound to APC cargo revealed a conserved 'Hitchdock domain' in the KIF3 tail region; KIF3A helical regions within this domain mediate specific cargo (APC) binding while the KIF3B beta-hairpin and KAP3 provide structural support. Mutagenesis confirmed the functional importance of this domain for cargo recognition.\",\n      \"method\": \"High-resolution cryo-EM structure determination, site-directed mutagenesis, molecular dynamics simulation\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with mutagenesis and MD simulation in single preprint study, multiple orthogonal methods\",\n      \"pmids\": [\"bio_10.1101_2025.03.21.644525\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Structural, single-molecule, and cell biological analyses showed that KAP3 binds via a multipartite interface with both KIF3A and KIF3B, and rather than directly activating kinesin-2 motility, provides a platform for cargo adaptor engagement that occludes the autoinhibitory beta-hairpin motif in the KIF3 tail, thereby activating motility.\",\n      \"method\": \"Cryo-EM/structural analysis, single-molecule motility assays, cell biological assays, mutagenesis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — structural data combined with single-molecule assays and mutagenesis, multiple orthogonal approaches in single preprint\",\n      \"pmids\": [\"bio_10.1101_2024.10.14.618219\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"A KIF3B-enriched, KAP3-associated assembly (distinct from canonical KIF3A/B/KAP3) preferentially associates with TRIM46, a protein required for axon initial segment organization; structural analyses suggest differences in tail conformation accompany distinct assembly states and underlie cargo selectivity for targeted transport to the axon initial segment.\",\n      \"method\": \"Biochemical fractionation, co-immunoprecipitation, structural analysis, neuronal cell biology\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal biochemical and structural analyses, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"41910726\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"KIFAP3/KAP3 is a non-motor adaptor subunit that binds the tail domains of KIF3A and KIF3B through a multipartite interface (including the 'Hitchdock domain') to form the heterotrimeric kinesin-2 complex; rather than modulating motor ATPase activity directly, KAP3 provides a structural platform for cargo adaptors (such as APC) whose engagement occludes the autoinhibitory beta-hairpin motif in the KIF3 tail to activate processive anterograde transport, and compositional heterogeneity in KIF3/KAP3 assemblies contributes to selective cargo delivery (e.g., TRIM46 to the axon initial segment, ChAT in motor axons, laminin post-Golgi transport) in neurons and epithelial cells.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"KIFAP3 (KAP3) is the non-motor accessory subunit of the heterotrimeric kinesin-2 motor, binding the tail domains of the KIF3A/KIF3B motor heterodimer to form the KIF3A/KIF3B/KAP3 complex without altering motor ATPase activity, and functions as a cargo-binding and cargo-adaptor platform for microtubule-based anterograde transport [#0]. Structurally, KAP3 engages KIF3A and KIF3B through a multipartite interface within a conserved tail region ('Hitchdock domain'); rather than directly activating motility, it provides a platform for cargo-adaptor engagement (such as APC) that occludes the autoinhibitory beta-hairpin motif in the KIF3 tail to license processive transport [#5, #6]. Through this transport activity KAP3 delivers diverse cargoes in specialized cells, including choline acetyltransferase in motor axons—where sequestration of KAP3 by misfolded mutant SOD1 impairs axonal transport and acetylcholine release [#3]—post-Golgi laminin required for basement membrane formation and RhoA-dependent actomyosin organization in epithelial carcinoma cells [#4], and, via a distinct KIF3B-enriched assembly, TRIM46 to the axon initial segment, illustrating how compositional heterogeneity of KIF3/KAP3 assemblies confers cargo selectivity [#7]. The same machinery is required in cilia/flagella, where the KAP3 ortholog targets kinesin-2 to the basal body for intraflagellar transport [#2].\",\n  \"teleology\": [\n    {\n      \"year\": 1996,\n      \"claim\": \"Established that KAP3 is the third, non-motor subunit of kinesin-2, defining the heterotrimeric architecture and pointing to a cargo/membrane-binding rather than motor-regulatory role.\",\n      \"evidence\": \"Immunoprecipitation, baculovirus-Sf9 reconstitution, EM, and ATPase assay of the KIF3A/KIF3B/KAP3 complex\",\n      \"pmids\": [\"8710890\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify specific physiological cargoes bound by KAP3\", \"Structural basis of the KIF3-KAP3 interface unresolved\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Provided an early candidate cargo by linking KAP3 to a condensin subunit, raising the possibility that kinesin-2 couples to chromosomal proteins.\",\n      \"evidence\": \"Yeast two-hybrid, reciprocal co-immunoprecipitation, and subcellular fractionation showing a KAP3/KIF3B/HCAP ternary complex\",\n      \"pmids\": [\"9506951\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the nuclear KAP3-condensin association not established\", \"Not connected to a transport event\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Demonstrated an in vivo requirement for the KAP subunit in targeting kinesin-2 to the site of organelle assembly, defining its role in intraflagellar transport.\",\n      \"evidence\": \"Temperature-sensitive KAP mutant, epitope-tagged rescue, video-DIC imaging of IFT, and immunolocalization in Chlamydomonas\",\n      \"pmids\": [\"15616187\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular partner mediating basal-body targeting not identified\", \"Ortholog-based; human/mammalian ciliary role not directly tested here\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Connected KAP3 to disease by showing that sequestration of KAP3 from kinesin-2 impairs cargo transport, mechanistically linking KAP3 to motor neuron axonal transport defects.\",\n      \"evidence\": \"Co-IP from FALS mouse spinal cord, NG108-15 cell model, KAP3 overexpression rescue of acetylcholine release, and human FALS tissue immunohistochemistry\",\n      \"pmids\": [\"19088126\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct demonstration that ChAT is a KAP3-bound cargo not shown\", \"Single lab; quantitative contribution to FALS pathology unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined epithelial cargo and downstream signaling consequences of KAP3 loss, extending its transport role to basement membrane assembly and RhoA-dependent cytoskeletal organization.\",\n      \"evidence\": \"CRISPR/Cas9 knockout with live imaging of laminin transport, RhoA activity assay, and actomyosin cable analysis in gastric signet ring cell carcinoma cells\",\n      \"pmids\": [\"35322078\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking laminin transport defect to RhoA inactivation not defined\", \"Single cell-line context\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Resolved the structural basis of cargo recognition, showing how a conserved tail 'Hitchdock domain' with KAP3 support mediates specific APC binding.\",\n      \"evidence\": \"Cryo-EM of KIF3A/KIF3B/KAP3 bound to APC, with site-directed mutagenesis and molecular dynamics (preprint)\",\n      \"pmids\": [\"bio_10.1101_2025.03.21.644525\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"Generality of the domain across non-APC cargoes not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Reframed KAP3 function as an adaptor platform that relieves motor autoinhibition, explaining how cargo engagement activates kinesin-2 motility.\",\n      \"evidence\": \"Cryo-EM/structural analysis, single-molecule motility assays, and mutagenesis showing the multipartite KAP3 interface and beta-hairpin occlusion (preprint)\",\n      \"pmids\": [\"bio_10.1101_2024.10.14.618219\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Preprint, not peer-reviewed\", \"How distinct cargoes differentially trigger activation not fully mapped\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Showed that compositional heterogeneity of KIF3/KAP3 assemblies underlies cargo selectivity, with a KIF3B-enriched KAP3 assembly directing TRIM46 to the axon initial segment.\",\n      \"evidence\": \"Biochemical fractionation, reciprocal co-immunoprecipitation, structural analysis, and neuronal cell biology\",\n      \"pmids\": [\"41910726\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Regulation determining which assembly forms not defined\", \"Tail conformational differences inferred structurally rather than directly visualized in each state\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How KAP3 selects among its many cargoes and how assembly composition is regulated across cell types remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified model of cargo-adaptor specificity across neurons, epithelia, and cilia\", \"Regulatory inputs controlling KIF3 assembly heterogeneity unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 5, 6]},\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [2]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-9609507\", \"supporting_discovery_ids\": [3, 4, 7]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [0, 4]}\n    ],\n    \"complexes\": [\n      \"kinesin-2 (KIF3A/KIF3B/KAP3 heterotrimer)\"\n    ],\n    \"partners\": [\n      \"KIF3A\",\n      \"KIF3B\",\n      \"APC\",\n      \"TRIM46\",\n      \"HCAP\",\n      \"SOD1\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":3,"faith_total":3,"faith_pct":100.0}}