{"gene":"IFT74","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":2005,"finding":"IFT74/72 and IFT81 directly interact to form a higher-order oligomer (tetrameric complex (IFT81)2(IFT74/72)2) that serves as a scaffold for IFT complex B assembly; IFT complex B has a 500-kDa core containing IFT88, IFT81, IFT74/72, IFT52, IFT46, and IFT27, with IFT172, IFT80, IFT57, and IFT20 as peripheral subunits.","method":"High-ionic-strength fractionation of Chlamydomonas IFT complex B, chemical cross-linking, yeast two-hybrid and three-hybrid analyses, conservation confirmed with vertebrate homologues","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal biochemical methods (fractionation, crosslinking, Y2H/Y3H) in foundational paper with 154 citations","pmids":["15955805"],"is_preprint":false},{"year":2015,"finding":"IFT74 is required to stabilize IFT-B complex in vivo; aa 197-641 are sufficient for IFT-B stabilization. The N-terminal charged region (aa 1-130) contributes to but is not strictly required for tubulin entry into flagella, whereas aa 131-196 (part of coiled-coil 1) are required for IFT-A/IFT-B association at the ciliary base and for flagellar import of IFT-A.","method":"Chlamydomonas ift74 null mutant rescue experiments with truncated IFT74 constructs, fluorescence microscopy of IFT train assembly and movement","journal":"Current biology : CB","confidence":"High","confidence_rationale":"Tier 2 — domain-mapping by rescue genetics with multiple truncation constructs and direct imaging readout, moderate evidence","pmids":["26051893"],"is_preprint":false},{"year":2022,"finding":"The IFT25-IFT27 dimer binds the C-terminal region of the IFT74-IFT81 dimer; Bardet-Biedl syndrome (BBS) variants of IFT74 delete this IFT25-IFT27-binding region, and BBS variants of IFT27 are impaired in IFT74-IFT81 binding, demonstrating that impaired IFT74-IFT81/IFT25-IFT27 interaction causes BBS-associated ciliary defects.","method":"Co-immunoprecipitation, IFT74-knockout and IFT27-knockout cell rescue assays, ciliary phenotype analysis (BBSome export defects)","journal":"Human molecular genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal binding mapping plus KO rescue with functional ciliary phenotype readout, moderate evidence","pmids":["34888642"],"is_preprint":false},{"year":2023,"finding":"The IFT81-IFT74 complex acts as an unconventional GAP for the small GTPase RabL2: a reconstituted pentameric IFT complex containing IFT81/74 enhances RabL2 GTP hydrolysis, with the activity mapped to a 70-aa coiled-coil region of IFT81/74. This provides a molecular mechanism for RabL2 dissociation from anterograde IFT trains after departure from the ciliary base.","method":"In vitro reconstitution and purification of RabL2-IFT complexes, GTPase activity assays, structural modeling validated in vitro and in cellulo, Chlamydomonas IFT81/74 tested with human RabL2","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro GTPase assay with domain mapping and cross-species validation","pmids":["37606072"],"is_preprint":false},{"year":2023,"finding":"The first 40 amino acids of IFT74 (encoded by exon 2) are dispensable for binding to other IFT-B subunits but are important for tubulin binding; deletion of these residues preferentially impairs motile cilia function (mucociliary clearance) over primary cilia assembly, consistent with higher tubulin transport demands in motile cilia.","method":"In vitro binding assays with truncated IFT74, mouse knock-in alleles (exon 2 deletion and null), human patient fibroblast/cell analysis, electron microscopy of cilia","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro tubulin binding assay combined with mouse genetic models and human patient data, multiple orthogonal methods","pmids":["37315079"],"is_preprint":false},{"year":2021,"finding":"IFT74 variants cause Joubert syndrome; patient-derived fibroblasts with IFT74 variants show attenuated ciliogenesis, altered distribution of IFT proteins, mislocalisation of ciliary membrane proteins ARL13B, INPP5E, and GPR161, and disrupted hedgehog signaling.","method":"Zebrafish ift74 morphant rescue with human variant IFT74 (p.Q179E), patient fibroblast ciliogenesis assays, immunofluorescence for ciliary protein distribution, hedgehog signaling assays","journal":"Genetics in medicine : official journal of the American College of Medical Genetics","confidence":"Medium","confidence_rationale":"Tier 2 — zebrafish morphant rescue plus patient fibroblast mechanistic analysis, single study","pmids":["33531668"],"is_preprint":false},{"year":2021,"finding":"A missense variant in IFT74 (c.256G>A; p.Gly86Ser) affects mRNA splicing and produces mutant IFT74 proteins with abnormal subcellular localization along the sperm flagellum, causing male infertility with multiple morphological abnormalities of the sperm flagellum (MMAF) without syndromic features.","method":"Exome sequencing, RT-PCR/splicing analysis, immunofluorescence localization of mutant IFT74 along sperm flagellum","journal":"Human genetics","confidence":"Medium","confidence_rationale":"Tier 2–3 — splicing analysis plus localization study in patient cells, single lab","pmids":["33689014"],"is_preprint":false},{"year":2023,"finding":"An IFT74 exon-2 deletion (removing the first 40 aa) produces truncated IFT74 that still interacts as part of the IFT-B complex but with reduced interaction levels and not with all usual IFT-B partners, as assessed by affinity purification mass spectrometry.","method":"Affinity purification mass spectrometry of exon-2-deleted IFT74, patient nasal epithelial cell analysis, electron microscopy","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 — AP-MS interactome in patient-derived material, single study","pmids":["37555648"],"is_preprint":false},{"year":2021,"finding":"Loss of Ift74 in zebrafish causes ciliogenesis defects; the connecting cilia of photoreceptors initially form but fail to maintain, resulting in slow opsin transport and gradual photoreceptor degeneration, distinct from the rapid degeneration seen in other ift-b mutants.","method":"Zebrafish ift74 mutant analysis, opsin immunofluorescence, live imaging of cilia","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 — genetic loss-of-function with specific cellular phenotype readout in vertebrate model, single study","pmids":["34502236"],"is_preprint":false}],"current_model":"IFT74 is a core IFT-B subunit that heterodimerizes with IFT81 via coiled-coil domains to form a (IFT81)2(IFT74)2 scaffold essential for IFT-B complex stability and ciliary assembly; the IFT74-IFT81 dimer acts as an unconventional GAP for RabL2 to regulate IFT train initiation, its N-terminal domain mediates tubulin binding and IFT-A/IFT-B association at the ciliary base, and its C-terminal region binds the IFT25-IFT27 dimer to coordinate BBSome-dependent ciliary membrane protein export, with loss-of-function causing ciliopathies including Joubert syndrome, Bardet-Biedl syndrome, skeletal ciliopathy, and male infertility."},"narrative":{"teleology":[{"year":2005,"claim":"Establishing IFT74 as a core IFT-B scaffold subunit resolved how the ~500-kDa IFT-B core is organized and showed that IFT74 and IFT81 form the central oligomeric backbone.","evidence":"High-ionic-strength fractionation, chemical cross-linking, and yeast two-/three-hybrid assays using Chlamydomonas and vertebrate IFT-B subunits","pmids":["15955805"],"confidence":"High","gaps":["The functional significance of IFT74 domains beyond the coiled-coil was unknown","No in vivo loss-of-function data for IFT74 at this stage","Cargo-binding activities of the IFT74–IFT81 dimer were uncharacterized"]},{"year":2015,"claim":"Domain-resolution rescue mapping of IFT74 in a null mutant separated three functional regions: aa 197–641 for IFT-B stabilization, aa 131–196 for IFT-A/IFT-B coupling at the ciliary base, and aa 1–130 for tubulin import into flagella.","evidence":"Chlamydomonas ift74-null rescue with serial truncation constructs and fluorescence imaging of IFT train dynamics","pmids":["26051893"],"confidence":"High","gaps":["Whether tubulin binding by the N-terminus is direct or requires co-factors was not resolved","The mechanism linking the 131–196 region to IFT-A association was unclear"]},{"year":2021,"claim":"Identification of IFT74 variants in Joubert syndrome patients, combined with ciliogenesis and Hedgehog signaling defects in patient fibroblasts, established IFT74 as a Joubert syndrome gene and connected its loss to ciliary membrane protein mislocalization.","evidence":"Zebrafish ift74 morphant rescue with patient variant p.Q179E, patient fibroblast ciliary assays, and immunofluorescence for ARL13B/INPP5E/GPR161","pmids":["33531668"],"confidence":"Medium","gaps":["Only a single missense variant tested in zebrafish rescue","Precise structural basis for how Q179E disrupts IFT-B integrity was not determined"]},{"year":2021,"claim":"Zebrafish ift74 mutants revealed that IFT74 is required not just for ciliogenesis but also for cilium maintenance, as photoreceptor connecting cilia formed but progressively degenerated with slow opsin transport—distinguishing IFT74 from other IFT-B subunits.","evidence":"Zebrafish ift74 mutant analysis with opsin immunofluorescence and live cilia imaging","pmids":["34502236"],"confidence":"Medium","gaps":["Mechanism underlying the milder phenotype compared with other IFT-B subunit mutants was not explained","Whether this reflects partial redundancy with IFT81 or cargo selectivity was untested"]},{"year":2021,"claim":"Discovery of an IFT74 missense variant affecting splicing and flagellar localization in infertile males extended the disease spectrum to non-syndromic male infertility (MMAF), implicating IFT74 in sperm flagellum biogenesis.","evidence":"Exome sequencing, RT-PCR splicing analysis, immunofluorescence in patient sperm","pmids":["33689014"],"confidence":"Medium","gaps":["Single family study; independent replication in additional MMAF cohorts not reported","Quantitative impact on IFT-B assembly was not assessed biochemically"]},{"year":2022,"claim":"Mapping the IFT25–IFT27 binding site to the IFT74–IFT81 C-terminal region, and showing that BBS-causing variants in IFT74 or IFT27 disrupt this interaction, provided a molecular mechanism linking the IFT-B core to BBSome-dependent ciliary membrane protein export.","evidence":"Co-immunoprecipitation, IFT74-KO and IFT27-KO cell rescue, ciliary BBSome export assays","pmids":["34888642"],"confidence":"High","gaps":["Structural details of the IFT74–IFT25/27 interface were not resolved","Whether all BBS-associated ciliary cargo is equally affected was not tested"]},{"year":2023,"claim":"Demonstrating that the IFT81–IFT74 coiled-coil acts as a GAP for RabL2 provided a mechanistic explanation for how IFT trains are released from the ciliary base after assembly, linking IFT-B core architecture to train initiation control.","evidence":"In vitro reconstituted pentameric IFT complex GTPase assay, 70-aa coiled-coil domain mapping, cross-species validation","pmids":["37606072"],"confidence":"High","gaps":["In vivo validation of GAP-dead IFT74/81 mutants on IFT train dynamics is lacking","Structural basis of the unconventional GAP mechanism is unresolved"]},{"year":2023,"claim":"Mouse knock-in and patient studies of an IFT74 exon-2 deletion (Δaa 1–40) showed that the extreme N-terminal tubulin-binding segment is selectively required for motile cilia function over primary cilia, explaining tissue-selective ciliopathy phenotypes.","evidence":"Mouse knock-in models, patient nasal epithelial cells, in vitro tubulin binding, AP-MS of truncated IFT74, electron microscopy","pmids":["37315079","37555648"],"confidence":"High","gaps":["How tubulin binding by 40 aa is mechanistically coupled to axonemal assembly remains unclear","Whether other tubulin-binding IFT subunits partially compensate in primary cilia is untested"]},{"year":null,"claim":"No high-resolution structure of the full-length IFT74–IFT81 heterodimer or its interfaces with IFT25–IFT27 and RabL2 exists, and the in vivo consequences of selectively ablating the GAP activity versus tubulin binding versus IFT25/27 binding have not been dissected in a single genetic system.","evidence":"","pmids":[],"confidence":"High","gaps":["Full-length atomic structure of IFT74–IFT81 is unavailable","Separation-of-function alleles addressing GAP vs. tubulin vs. IFT25/27 binding in one model organism are needed","Whether IFT74 has additional non-ciliary functions has not been addressed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[3]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,1]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[1,4,5,8]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[1,7]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,1,4,5,8]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[2,3]}],"complexes":["IFT-B complex","IFT74-IFT81 heterotetramer"],"partners":["IFT81","IFT25","IFT27","RABL2","IFT88","IFT52","IFT46"],"other_free_text":[]},"mechanistic_narrative":"IFT74 is a core subunit of the intraflagellar transport complex B (IFT-B) that is essential for cilium assembly, maintenance, and cargo trafficking. It heterodimerizes with IFT81 via coiled-coil domains to form a tetrameric (IFT81)₂(IFT74)₂ scaffold that nucleates IFT-B core assembly; the IFT74–IFT81 coiled-coil additionally acts as an unconventional GTPase-activating protein (GAP) for RabL2, controlling IFT train initiation at the ciliary base [PMID:15955805, PMID:37606072]. Distinct IFT74 domains serve separable functions: the N-terminal basic region (aa 1–130) binds tubulin and is preferentially required for motile cilia that have high tubulin transport demands, an internal coiled-coil segment (aa 131–196) mediates IFT-A/IFT-B association at the ciliary base, and the C-terminal region binds the IFT25–IFT27 dimer to coordinate BBSome-dependent export of ciliary membrane proteins [PMID:26051893, PMID:37315079, PMID:34888642]. Loss-of-function and hypomorphic variants in IFT74 cause a spectrum of ciliopathies including Joubert syndrome, Bardet–Biedl syndrome, skeletal ciliopathy with motile cilia dysfunction, and isolated male infertility with sperm flagellar defects [PMID:33531668, PMID:34888642, PMID:33689014]."},"prefetch_data":{"uniprot":{"accession":"Q96LB3","full_name":"Intraflagellar transport protein 74 homolog","aliases":["Capillary morphogenesis gene 1 protein","CMG-1","Coiled-coil domain-containing protein 2"],"length_aa":600,"mass_kda":69.2,"function":"Component of the intraflagellar transport (IFT) complex B: together with IFT81, forms a tubulin-binding module that specifically mediates transport of tubulin within the cilium (PubMed:23990561). Binds beta-tubulin via its basic region (PubMed:23990561). Required for ciliogenesis (PubMed:23990561). Essential for flagellogenesis during spermatogenesis (PubMed:33689014)","subcellular_location":"Cell projection, cilium; Cytoplasmic vesicle; Cell projection, cilium, flagellum; Cytoplasmic vesicle, secretory vesicle, acrosome","url":"https://www.uniprot.org/uniprotkb/Q96LB3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/IFT74","classification":"Not Classified","n_dependent_lines":2,"n_total_lines":1208,"dependency_fraction":0.0016556291390728477},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"HSPB11","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/search/IFT74","total_profiled":1310},"omim":[{"mim_id":"620841","title":"INTRAFLAGELLAR TRANSPORT 25; IFT25","url":"https://www.omim.org/entry/620841"},{"mim_id":"620505","title":"INTRAFLAGELLAR TRANSPORT 22; IFT22","url":"https://www.omim.org/entry/620505"},{"mim_id":"620160","title":"IQ MOTIF-CONTAINING PROTEIN N; IQCN","url":"https://www.omim.org/entry/620160"},{"mim_id":"619585","title":"SPERMATOGENIC FAILURE 58; SPGF58","url":"https://www.omim.org/entry/619585"},{"mim_id":"619582","title":"JOUBERT SYNDROME 40; JBTS40","url":"https://www.omim.org/entry/619582"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Golgi apparatus","reliability":"Supported"},{"location":"Centriolar satellite","reliability":"Supported"},{"location":"Basal body","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/IFT74"},"hgnc":{"alias_symbol":["CMG1","CMG-1","FLJ22621"],"prev_symbol":["CCDC2"]},"alphafold":{"accession":"Q96LB3","domains":[{"cath_id":"-","chopping":"96-224","consensus_level":"medium","plddt":91.7653,"start":96,"end":224}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96LB3","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96LB3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96LB3-F1-predicted_aligned_error_v6.png","plddt_mean":80.0},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=IFT74","jax_strain_url":"https://www.jax.org/strain/search?query=IFT74"},"sequence":{"accession":"Q96LB3","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96LB3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96LB3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96LB3"}},"corpus_meta":[{"pmid":"15955805","id":"PMC_15955805","title":"Characterization of the intraflagellar transport complex B core: direct interaction of the IFT81 and IFT74/72 subunits.","date":"2005","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/15955805","citation_count":154,"is_preprint":false},{"pmid":"17166276","id":"PMC_17166276","title":"Analysis of IFT74 as a candidate gene for chromosome 9p-linked ALS-FTD.","date":"2006","source":"BMC neurology","url":"https://pubmed.ncbi.nlm.nih.gov/17166276","citation_count":67,"is_preprint":false},{"pmid":"10887131","id":"PMC_10887131","title":"Analysis of genes under the downstream control of the t(8;21) fusion protein AML1-MTG8: overexpression of the TIS11b (ERF-1, cMG1) gene induces myeloid cell proliferation in response to G-CSF.","date":"2000","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/10887131","citation_count":66,"is_preprint":false},{"pmid":"26051893","id":"PMC_26051893","title":"Assembly of IFT trains at the ciliary base depends on IFT74.","date":"2015","source":"Current biology : CB","url":"https://pubmed.ncbi.nlm.nih.gov/26051893","citation_count":62,"is_preprint":false},{"pmid":"11157256","id":"PMC_11157256","title":"Expression of cmg1, an exo-beta-1,3-glucanase gene from Coniothyrium minitans, increases during sclerotial parasitism.","date":"2001","source":"Applied and environmental microbiology","url":"https://pubmed.ncbi.nlm.nih.gov/11157256","citation_count":38,"is_preprint":false},{"pmid":"33531668","id":"PMC_33531668","title":"Disrupted intraflagellar transport due to IFT74 variants causes Joubert syndrome.","date":"2021","source":"Genetics in medicine : official journal of the American College of Medical Genetics","url":"https://pubmed.ncbi.nlm.nih.gov/33531668","citation_count":33,"is_preprint":false},{"pmid":"34888642","id":"PMC_34888642","title":"Impaired cooperation between IFT74/BBS22-IFT81 and IFT25-IFT27/BBS19 causes Bardet-Biedl syndrome.","date":"2022","source":"Human molecular genetics","url":"https://pubmed.ncbi.nlm.nih.gov/34888642","citation_count":31,"is_preprint":false},{"pmid":"33689014","id":"PMC_33689014","title":"A missense mutation in IFT74, encoding for an essential component for intraflagellar transport of Tubulin, causes asthenozoospermia and male infertility without clinical signs of Bardet-Biedl syndrome.","date":"2021","source":"Human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/33689014","citation_count":25,"is_preprint":false},{"pmid":"1628738","id":"PMC_1628738","title":"Mitogen-induced expression of the primary response gene cMG1 in a rat intestinal epithelial cell-line (RIE-1).","date":"1992","source":"FEBS letters","url":"https://pubmed.ncbi.nlm.nih.gov/1628738","citation_count":21,"is_preprint":false},{"pmid":"37315079","id":"PMC_37315079","title":"IFT74 variants cause skeletal ciliopathy and motile cilia defects in mice and humans.","date":"2023","source":"PLoS 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sciences","url":"https://pubmed.ncbi.nlm.nih.gov/36865301","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":11540,"output_tokens":2449,"usd":0.035678},"stage2":{"model":"claude-opus-4-6","input_tokens":5742,"output_tokens":2651,"usd":0.142478},"total_usd":0.178156,"stage1_batch_id":"msgbatch_01DzrsgVgYngY1hvM3DU1yF2","stage2_batch_id":"msgbatch_01KA9b7iRzKFdhoBUT7vCvSe","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2005,\n      \"finding\": \"IFT74/72 and IFT81 directly interact to form a higher-order oligomer (tetrameric complex (IFT81)2(IFT74/72)2) that serves as a scaffold for IFT complex B assembly; IFT complex B has a 500-kDa core containing IFT88, IFT81, IFT74/72, IFT52, IFT46, and IFT27, with IFT172, IFT80, IFT57, and IFT20 as peripheral subunits.\",\n      \"method\": \"High-ionic-strength fractionation of Chlamydomonas IFT complex B, chemical cross-linking, yeast two-hybrid and three-hybrid analyses, conservation confirmed with vertebrate homologues\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal biochemical methods (fractionation, crosslinking, Y2H/Y3H) in foundational paper with 154 citations\",\n      \"pmids\": [\"15955805\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"IFT74 is required to stabilize IFT-B complex in vivo; aa 197-641 are sufficient for IFT-B stabilization. The N-terminal charged region (aa 1-130) contributes to but is not strictly required for tubulin entry into flagella, whereas aa 131-196 (part of coiled-coil 1) are required for IFT-A/IFT-B association at the ciliary base and for flagellar import of IFT-A.\",\n      \"method\": \"Chlamydomonas ift74 null mutant rescue experiments with truncated IFT74 constructs, fluorescence microscopy of IFT train assembly and movement\",\n      \"journal\": \"Current biology : CB\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — domain-mapping by rescue genetics with multiple truncation constructs and direct imaging readout, moderate evidence\",\n      \"pmids\": [\"26051893\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The IFT25-IFT27 dimer binds the C-terminal region of the IFT74-IFT81 dimer; Bardet-Biedl syndrome (BBS) variants of IFT74 delete this IFT25-IFT27-binding region, and BBS variants of IFT27 are impaired in IFT74-IFT81 binding, demonstrating that impaired IFT74-IFT81/IFT25-IFT27 interaction causes BBS-associated ciliary defects.\",\n      \"method\": \"Co-immunoprecipitation, IFT74-knockout and IFT27-knockout cell rescue assays, ciliary phenotype analysis (BBSome export defects)\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal binding mapping plus KO rescue with functional ciliary phenotype readout, moderate evidence\",\n      \"pmids\": [\"34888642\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The IFT81-IFT74 complex acts as an unconventional GAP for the small GTPase RabL2: a reconstituted pentameric IFT complex containing IFT81/74 enhances RabL2 GTP hydrolysis, with the activity mapped to a 70-aa coiled-coil region of IFT81/74. This provides a molecular mechanism for RabL2 dissociation from anterograde IFT trains after departure from the ciliary base.\",\n      \"method\": \"In vitro reconstitution and purification of RabL2-IFT complexes, GTPase activity assays, structural modeling validated in vitro and in cellulo, Chlamydomonas IFT81/74 tested with human RabL2\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro GTPase assay with domain mapping and cross-species validation\",\n      \"pmids\": [\"37606072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The first 40 amino acids of IFT74 (encoded by exon 2) are dispensable for binding to other IFT-B subunits but are important for tubulin binding; deletion of these residues preferentially impairs motile cilia function (mucociliary clearance) over primary cilia assembly, consistent with higher tubulin transport demands in motile cilia.\",\n      \"method\": \"In vitro binding assays with truncated IFT74, mouse knock-in alleles (exon 2 deletion and null), human patient fibroblast/cell analysis, electron microscopy of cilia\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro tubulin binding assay combined with mouse genetic models and human patient data, multiple orthogonal methods\",\n      \"pmids\": [\"37315079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"IFT74 variants cause Joubert syndrome; patient-derived fibroblasts with IFT74 variants show attenuated ciliogenesis, altered distribution of IFT proteins, mislocalisation of ciliary membrane proteins ARL13B, INPP5E, and GPR161, and disrupted hedgehog signaling.\",\n      \"method\": \"Zebrafish ift74 morphant rescue with human variant IFT74 (p.Q179E), patient fibroblast ciliogenesis assays, immunofluorescence for ciliary protein distribution, hedgehog signaling assays\",\n      \"journal\": \"Genetics in medicine : official journal of the American College of Medical Genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — zebrafish morphant rescue plus patient fibroblast mechanistic analysis, single study\",\n      \"pmids\": [\"33531668\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A missense variant in IFT74 (c.256G>A; p.Gly86Ser) affects mRNA splicing and produces mutant IFT74 proteins with abnormal subcellular localization along the sperm flagellum, causing male infertility with multiple morphological abnormalities of the sperm flagellum (MMAF) without syndromic features.\",\n      \"method\": \"Exome sequencing, RT-PCR/splicing analysis, immunofluorescence localization of mutant IFT74 along sperm flagellum\",\n      \"journal\": \"Human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — splicing analysis plus localization study in patient cells, single lab\",\n      \"pmids\": [\"33689014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"An IFT74 exon-2 deletion (removing the first 40 aa) produces truncated IFT74 that still interacts as part of the IFT-B complex but with reduced interaction levels and not with all usual IFT-B partners, as assessed by affinity purification mass spectrometry.\",\n      \"method\": \"Affinity purification mass spectrometry of exon-2-deleted IFT74, patient nasal epithelial cell analysis, electron microscopy\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — AP-MS interactome in patient-derived material, single study\",\n      \"pmids\": [\"37555648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Loss of Ift74 in zebrafish causes ciliogenesis defects; the connecting cilia of photoreceptors initially form but fail to maintain, resulting in slow opsin transport and gradual photoreceptor degeneration, distinct from the rapid degeneration seen in other ift-b mutants.\",\n      \"method\": \"Zebrafish ift74 mutant analysis, opsin immunofluorescence, live imaging of cilia\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with specific cellular phenotype readout in vertebrate model, single study\",\n      \"pmids\": [\"34502236\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"IFT74 is a core IFT-B subunit that heterodimerizes with IFT81 via coiled-coil domains to form a (IFT81)2(IFT74)2 scaffold essential for IFT-B complex stability and ciliary assembly; the IFT74-IFT81 dimer acts as an unconventional GAP for RabL2 to regulate IFT train initiation, its N-terminal domain mediates tubulin binding and IFT-A/IFT-B association at the ciliary base, and its C-terminal region binds the IFT25-IFT27 dimer to coordinate BBSome-dependent ciliary membrane protein export, with loss-of-function causing ciliopathies including Joubert syndrome, Bardet-Biedl syndrome, skeletal ciliopathy, and male infertility.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"IFT74 is a core subunit of the intraflagellar transport complex B (IFT-B) that is essential for cilium assembly, maintenance, and cargo trafficking. It heterodimerizes with IFT81 via coiled-coil domains to form a tetrameric (IFT81)₂(IFT74)₂ scaffold that nucleates IFT-B core assembly; the IFT74–IFT81 coiled-coil additionally acts as an unconventional GTPase-activating protein (GAP) for RabL2, controlling IFT train initiation at the ciliary base [PMID:15955805, PMID:37606072]. Distinct IFT74 domains serve separable functions: the N-terminal basic region (aa 1–130) binds tubulin and is preferentially required for motile cilia that have high tubulin transport demands, an internal coiled-coil segment (aa 131–196) mediates IFT-A/IFT-B association at the ciliary base, and the C-terminal region binds the IFT25–IFT27 dimer to coordinate BBSome-dependent export of ciliary membrane proteins [PMID:26051893, PMID:37315079, PMID:34888642]. Loss-of-function and hypomorphic variants in IFT74 cause a spectrum of ciliopathies including Joubert syndrome, Bardet–Biedl syndrome, skeletal ciliopathy with motile cilia dysfunction, and isolated male infertility with sperm flagellar defects [PMID:33531668, PMID:34888642, PMID:33689014].\",\n  \"teleology\": [\n    {\n      \"year\": 2005,\n      \"claim\": \"Establishing IFT74 as a core IFT-B scaffold subunit resolved how the ~500-kDa IFT-B core is organized and showed that IFT74 and IFT81 form the central oligomeric backbone.\",\n      \"evidence\": \"High-ionic-strength fractionation, chemical cross-linking, and yeast two-/three-hybrid assays using Chlamydomonas and vertebrate IFT-B subunits\",\n      \"pmids\": [\"15955805\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"The functional significance of IFT74 domains beyond the coiled-coil was unknown\",\n        \"No in vivo loss-of-function data for IFT74 at this stage\",\n        \"Cargo-binding activities of the IFT74–IFT81 dimer were uncharacterized\"\n      ]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Domain-resolution rescue mapping of IFT74 in a null mutant separated three functional regions: aa 197–641 for IFT-B stabilization, aa 131–196 for IFT-A/IFT-B coupling at the ciliary base, and aa 1–130 for tubulin import into flagella.\",\n      \"evidence\": \"Chlamydomonas ift74-null rescue with serial truncation constructs and fluorescence imaging of IFT train dynamics\",\n      \"pmids\": [\"26051893\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Whether tubulin binding by the N-terminus is direct or requires co-factors was not resolved\",\n        \"The mechanism linking the 131–196 region to IFT-A association was unclear\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of IFT74 variants in Joubert syndrome patients, combined with ciliogenesis and Hedgehog signaling defects in patient fibroblasts, established IFT74 as a Joubert syndrome gene and connected its loss to ciliary membrane protein mislocalization.\",\n      \"evidence\": \"Zebrafish ift74 morphant rescue with patient variant p.Q179E, patient fibroblast ciliary assays, and immunofluorescence for ARL13B/INPP5E/GPR161\",\n      \"pmids\": [\"33531668\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Only a single missense variant tested in zebrafish rescue\",\n        \"Precise structural basis for how Q179E disrupts IFT-B integrity was not determined\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Zebrafish ift74 mutants revealed that IFT74 is required not just for ciliogenesis but also for cilium maintenance, as photoreceptor connecting cilia formed but progressively degenerated with slow opsin transport—distinguishing IFT74 from other IFT-B subunits.\",\n      \"evidence\": \"Zebrafish ift74 mutant analysis with opsin immunofluorescence and live cilia imaging\",\n      \"pmids\": [\"34502236\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism underlying the milder phenotype compared with other IFT-B subunit mutants was not explained\",\n        \"Whether this reflects partial redundancy with IFT81 or cargo selectivity was untested\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Discovery of an IFT74 missense variant affecting splicing and flagellar localization in infertile males extended the disease spectrum to non-syndromic male infertility (MMAF), implicating IFT74 in sperm flagellum biogenesis.\",\n      \"evidence\": \"Exome sequencing, RT-PCR splicing analysis, immunofluorescence in patient sperm\",\n      \"pmids\": [\"33689014\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Single family study; independent replication in additional MMAF cohorts not reported\",\n        \"Quantitative impact on IFT-B assembly was not assessed biochemically\"\n      ]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Mapping the IFT25–IFT27 binding site to the IFT74–IFT81 C-terminal region, and showing that BBS-causing variants in IFT74 or IFT27 disrupt this interaction, provided a molecular mechanism linking the IFT-B core to BBSome-dependent ciliary membrane protein export.\",\n      \"evidence\": \"Co-immunoprecipitation, IFT74-KO and IFT27-KO cell rescue, ciliary BBSome export assays\",\n      \"pmids\": [\"34888642\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural details of the IFT74–IFT25/27 interface were not resolved\",\n        \"Whether all BBS-associated ciliary cargo is equally affected was not tested\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Demonstrating that the IFT81–IFT74 coiled-coil acts as a GAP for RabL2 provided a mechanistic explanation for how IFT trains are released from the ciliary base after assembly, linking IFT-B core architecture to train initiation control.\",\n      \"evidence\": \"In vitro reconstituted pentameric IFT complex GTPase assay, 70-aa coiled-coil domain mapping, cross-species validation\",\n      \"pmids\": [\"37606072\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"In vivo validation of GAP-dead IFT74/81 mutants on IFT train dynamics is lacking\",\n        \"Structural basis of the unconventional GAP mechanism is unresolved\"\n      ]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Mouse knock-in and patient studies of an IFT74 exon-2 deletion (Δaa 1–40) showed that the extreme N-terminal tubulin-binding segment is selectively required for motile cilia function over primary cilia, explaining tissue-selective ciliopathy phenotypes.\",\n      \"evidence\": \"Mouse knock-in models, patient nasal epithelial cells, in vitro tubulin binding, AP-MS of truncated IFT74, electron microscopy\",\n      \"pmids\": [\"37315079\", \"37555648\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"How tubulin binding by 40 aa is mechanistically coupled to axonemal assembly remains unclear\",\n        \"Whether other tubulin-binding IFT subunits partially compensate in primary cilia is untested\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"No high-resolution structure of the full-length IFT74–IFT81 heterodimer or its interfaces with IFT25–IFT27 and RabL2 exists, and the in vivo consequences of selectively ablating the GAP activity versus tubulin binding versus IFT25/27 binding have not been dissected in a single genetic system.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Full-length atomic structure of IFT74–IFT81 is unavailable\",\n        \"Separation-of-function alleles addressing GAP vs. tubulin vs. IFT25/27 binding in one model organism are needed\",\n        \"Whether IFT74 has additional non-ciliary functions has not been addressed\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [3]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [1, 4, 5, 8]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [1, 7]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 1, 4, 5, 8]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"complexes\": [\n      \"IFT-B complex\",\n      \"IFT74-IFT81 heterotetramer\"\n    ],\n    \"partners\": [\n      \"IFT81\",\n      \"IFT25\",\n      \"IFT27\",\n      \"RABL2\",\n      \"IFT88\",\n      \"IFT52\",\n      \"IFT46\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}