{"gene":"B9D2","run_date":"2026-04-28T17:12:38","timeline":{"discoveries":[{"year":2011,"finding":"B9D2 (MKSR-2) forms a physical complex with MKS1 and B9D1; a pathogenic MKS-associated p.Ser101Arg mutation in B9D2 abrogates its interaction with MKS1, demonstrating that complex integrity is required for B9D2 function in ciliogenesis and Hedgehog signaling.","method":"Co-immunoprecipitation and mass spectrometry; zebrafish rescue assay; mouse knockout phenotypic analysis","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (Co-IP/MS, in vivo rescue, mouse KO) in a single study","pmids":["21763481"],"is_preprint":false},{"year":2008,"finding":"C. elegans B9 proteins (TZA-2/MKSR-2/B9D2 ortholog, TZA-1/B9D1 ortholog, XBX-7/MKS1 ortholog) form a complex that localizes to the base of cilia (transition zone), and function redundantly with nephrocystins (NPH-1, NPH-4) to regulate cilia formation and maintenance.","method":"Genetic analysis of double mutants (epistasis); fluorescence localization of tagged proteins in C. elegans","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with direct localization, replicated across labs","pmids":["18337471"],"is_preprint":false},{"year":2009,"finding":"C. elegans MKSR-2 (B9D2 ortholog) and MKSR-1 (B9D1 ortholog) localize to ciliary transition zone/basal bodies in a co-dependent manner with MKS-1; disruption of human MKSR2 causes ciliogenesis defects, and genetic interactions among all three C. elegans mks/mksr proteins affect insulin-IGF-I signaling.","method":"Fluorescence microscopy of GFP-tagged proteins; RNAi knockdown; genetic epistasis; ciliogenesis assays","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — reciprocal localization interdependence plus functional epistasis, replicated in worm and human cells","pmids":["19208769"],"is_preprint":false},{"year":2011,"finding":"Zebrafish B9d2 binds IFT particle components (Fleer/IFT88) and contributes to ciliary localization of Inversin (Nephrocystin-2); B9d2, Inversin, and Nephrocystin-5 collectively support transport of Opsin but not Peripherin into photoreceptor cilia, revealing a selective cargo-transport mechanism.","method":"Co-immunoprecipitation; zebrafish morpholino knockdown with ciliary cargo localization assays; planar cell polarity assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — binding partner identified by Co-IP plus functional cargo-specificity assay in vivo","pmids":["21602787"],"is_preprint":false},{"year":2011,"finding":"In C. elegans, MKSR-2 (B9D2 ortholog) genetically interacts with JBTS-14/TMEM237, MKS-2/TMEM216, and MKSR-1/B9D1 at the transition zone, and TMEM237/JBTS-14 requires RPGRIP1L/MKS5 for correct TZ localization.","method":"C. elegans genetic interaction (double mutants); fluorescence localization; ciliogenesis assays","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis in C. elegans, single lab study","pmids":["22152675"],"is_preprint":false},{"year":2012,"finding":"C. elegans mksr-2 (B9D2 ortholog) genetically interacts with nphp-2/inversin and other MKS-module genes (mks-1, mks-3, mks-6, mksr-1) in a sensilla-dependent manner to control cilia formation and placement.","method":"C. elegans genetic analysis; double/triple mutant phenotypic assays; fluorescence localization","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 — genetic epistasis, single lab, C. elegans ortholog","pmids":["22393243"],"is_preprint":false},{"year":2020,"finding":"The B9 domain proteins MKS1, B9D2, and B9D1 interact in a defined order (MKS1–B9D2–B9D1) and show interdependent localization to the ciliary transition zone; B9D2-knockout cells display impaired diffusion barrier for ciliary membrane proteins, and rescue requires formation of the intact three-protein complex.","method":"Co-immunoprecipitation; CRISPR/Cas9 knockout; fluorescence-based diffusion barrier assay; rescue experiments","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1–2 — KO + Co-IP + functional rescue with multiple orthogonal readouts","pmids":["32726168"],"is_preprint":false},{"year":2021,"finding":"Two Joubert syndrome-associated B9D2 missense variants (P74S and G155S) are pathogenic in C. elegans: G155S disrupts endogenous MKSR-2 organization at the transition zone and reveals a close functional association between the B9 complex and MKS-2/TMEM216.","method":"CRISPR/Cas9 knock-in of patient variants in C. elegans; quantitative TZ structure/function assays; fluorescence imaging of endogenous tagged proteins","journal":"Disease models & mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 — gene editing with multiple functional assays, single lab","pmids":["33234550"],"is_preprint":false},{"year":2020,"finding":"The B9 domain of MKS1 is essential for its interaction with B9D2 and for localization of MKS1 to the ciliary transition zone; a c.1058delG MKS1 mutation disrupting the B9 domain attenuates MKS1–B9D2 interaction and impairs ciliary TZ localization.","method":"Co-immunoprecipitation; fluorescence localization; functional mutation analysis","journal":"Frontiers in genetics","confidence":"Medium","confidence_rationale":"Tier 2–3 — Co-IP plus localization, single lab","pmids":["33193692"],"is_preprint":false},{"year":2022,"finding":"MKS1 mutations disrupting the B9-C2 domain attenuate interaction with B9D2, confirming B9D2 as an essential binding partner for MKS1 at the ciliary transition zone.","method":"Co-immunoprecipitation; RT-PCR; functional mutation analysis","journal":"Frontiers in genetics","confidence":"Low","confidence_rationale":"Tier 3 — single Co-IP, single lab, primarily a clinical genetics study","pmids":["35360848"],"is_preprint":false},{"year":2024,"finding":"Before ciliogenesis, B9D2 localizes at tight junctions in biliary epithelial cells and is required for maturation and maintenance of tight junctions, epithelial barrier tightness, and proper biliary lumen formation—an extraciliary function distinct from its TZ role.","method":"Immunofluorescence localization; tight junction permeability assay; RNAi/loss-of-function in biliary epithelial cell models; lumen formation assay","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2–3 — direct localization with functional knockdown phenotype, single lab","pmids":["39455645"],"is_preprint":false},{"year":2025,"finding":"The B9D1–B9D2–MKS1 complex (i) anchors TMEM67 to the TZ membrane, and disruption of this complex reduces posttranslational modifications (e.g., acetylation, glutamylation) of axonemal microtubules by deregulating tubulin-modifying enzymes within cilia; (ii) B9 proteins localize to centrioles before ciliogenesis and facilitate initiation of ciliogenesis. Joubert syndrome-associated B9D2 variants primarily affect microtubule modifications without blocking ciliogenesis, whereas the MKS-associated B9D2 variant disrupts both.","method":"Co-immunoprecipitation; CRISPR/Cas9 knockout; immunofluorescence; Western blot for PTMs; patient cohort variant analysis","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 1–2 — multiple orthogonal methods (Co-IP, KO, PTM assay, patient variants) establishing two distinct mechanisms","pmids":["41165761"],"is_preprint":false}],"current_model":"B9D2 is a transition-zone protein that forms a defined MKS1–B9D2–B9D1 complex at the ciliary base, where it acts as a diffusion barrier for ciliary membrane proteins, anchors TMEM67 to the TZ membrane to regulate axonemal microtubule post-translational modifications, facilitates the initiation of ciliogenesis from centrioles, supports selective IFT-dependent cargo transport (e.g., Opsin via interaction with IFT88/Fleer and Inversin), and—prior to ciliogenesis—maintains tight junction integrity and epithelial polarity in biliary epithelia; different B9D2 pathogenic variants differentially impair these functions, explaining why some cause Joubert syndrome (primarily affecting microtubule modifications) while others cause the more severe Meckel syndrome (disrupting both ciliogenesis and microtubule modifications)."},"narrative":{"teleology":[{"year":2008,"claim":"Establishing that B9D2 is a ciliary transition-zone protein that acts in a module with B9D1 and MKS1 and functions redundantly with nephrocystins to maintain cilia resolved where and with whom B9D2 operates.","evidence":"Genetic epistasis and fluorescence localization of tagged C. elegans orthologs","pmids":["18337471"],"confidence":"High","gaps":["Biochemical nature of the B9-protein complex not yet defined","Mammalian relevance not directly tested","Mechanism of redundancy with nephrocystins unclear"]},{"year":2009,"claim":"Demonstrating co-dependent TZ localization of MKSR-2/B9D2 with MKS-1 and MKSR-1/B9D1 and showing that human B9D2 disruption causes ciliogenesis defects established functional conservation and mutual dependence within the complex.","evidence":"GFP-tagged protein localization, RNAi knockdown, and genetic epistasis in C. elegans and human cells","pmids":["19208769"],"confidence":"High","gaps":["Order of assembly within the complex unknown","Whether B9D2 has functions beyond ciliogenesis not addressed"]},{"year":2011,"claim":"Identifying the MKS1–B9D2–B9D1 physical complex by Co-IP/MS and showing that the MKS-associated S101R mutation in B9D2 disrupts MKS1 binding linked human disease directly to complex integrity.","evidence":"Co-immunoprecipitation/mass spectrometry, zebrafish rescue, and mouse knockout phenotyping","pmids":["21763481"],"confidence":"High","gaps":["Structural basis for complex assembly not resolved","Whether other TZ proteins depend on this complex for localization unknown"]},{"year":2011,"claim":"Revealing that B9D2 binds IFT88/Fleer and supports selective transport of Opsin (but not Peripherin) into photoreceptor cilia demonstrated a cargo-selective transport function beyond simple barrier activity.","evidence":"Co-immunoprecipitation and morpholino knockdown with cargo localization assays in zebrafish","pmids":["21602787"],"confidence":"High","gaps":["How B9D2–IFT88 interaction confers cargo selectivity unknown","Whether this transport role extends to non-photoreceptor cilia untested"]},{"year":2011,"claim":"Placing B9D2 in a broader transition-zone genetic interaction network with TMEM237, TMEM216, and RPGRIP1L mapped the module hierarchy at the TZ.","evidence":"C. elegans double-mutant genetic analysis and fluorescence localization","pmids":["22152675"],"confidence":"Medium","gaps":["Direct physical interactions between B9D2 and TMEM237/TMEM216 not confirmed biochemically","Hierarchy may differ in mammalian systems"]},{"year":2020,"claim":"Establishing the ordered MKS1–B9D2–B9D1 assembly and showing that B9D2 knockout specifically impairs the ciliary membrane diffusion barrier defined B9D2's core gatekeeping function at the TZ.","evidence":"CRISPR/Cas9 knockout, Co-IP, fluorescence diffusion barrier assay, and rescue experiments in mammalian cells","pmids":["32726168"],"confidence":"High","gaps":["Structural determinants of the diffusion barrier not resolved","Whether barrier impairment alone explains ciliopathy phenotypes unclear"]},{"year":2021,"claim":"CRISPR knock-in of Joubert syndrome patient variants P74S and G155S in C. elegans showed that G155S disrupts MKSR-2 TZ organization and revealed a close functional link between the B9 complex and TMEM216, connecting genotype to TZ structural integrity.","evidence":"CRISPR/Cas9 knock-in in C. elegans with quantitative TZ assays and fluorescence imaging","pmids":["33234550"],"confidence":"Medium","gaps":["Mammalian validation of variant-specific effects needed","Whether P74S acts through a distinct mechanism remains unclear"]},{"year":2024,"claim":"Discovery that B9D2 localizes to tight junctions in biliary epithelial cells before ciliogenesis and is required for junction maturation and lumen formation revealed an extraciliary function independent of its TZ role.","evidence":"Immunofluorescence, tight junction permeability assay, and RNAi knockdown in biliary epithelial models","pmids":["39455645"],"confidence":"Medium","gaps":["Molecular mechanism of B9D2 at tight junctions unknown","Whether this extraciliary function involves the same B9 complex partners untested","Relevance to biliary disease in patients not established"]},{"year":2025,"claim":"Demonstrating that the B9D1–B9D2–MKS1 complex anchors TMEM67 to the TZ, regulates axonemal microtubule post-translational modifications, and that Joubert variants selectively impair PTMs while MKS variants additionally block ciliogenesis provided a unified genotype–mechanism–phenotype framework.","evidence":"CRISPR/Cas9 knockout, Co-IP, immunofluorescence, Western blot for tubulin PTMs, patient variant analysis","pmids":["41165761"],"confidence":"High","gaps":["Which tubulin-modifying enzymes are directly regulated by B9D2/TMEM67 axis unknown","Structural basis of variant-specific differential effects not resolved"]},{"year":null,"claim":"The structural basis of B9D2 complex assembly, the mechanism by which B9D2 confers cargo selectivity to IFT, and how B9D2 functions at tight junctions remain unresolved.","evidence":"","pmids":[],"confidence":"High","gaps":["No high-resolution structure of the MKS1–B9D2–B9D1 complex","Molecular basis of cargo selectivity (Opsin vs. Peripherin) through IFT88 interaction unknown","Tight junction mechanism of B9D2 entirely uncharacterized at the molecular level"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,3,6]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[1,2,6,11]},{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[11]}],"pathway":[{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[1,2,6,11]},{"term_id":"R-HSA-5653656","term_label":"Vesicle-mediated transport","supporting_discovery_ids":[3,6]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[2,3]}],"complexes":["MKS1–B9D2–B9D1 complex"],"partners":["MKS1","B9D1","IFT88","TMEM67","TMEM216","INVS"],"other_free_text":[]},"mechanistic_narrative":"B9D2 is a transition-zone protein that functions as a core component of the ciliary diffusion barrier and a regulator of ciliogenesis initiation, ciliary cargo transport, and axonemal microtubule post-translational modifications. B9D2 assembles into a defined MKS1–B9D2–B9D1 complex at the ciliary transition zone in an interdependent manner, and integrity of this complex is required to restrict diffusion of membrane proteins into the cilium, anchor TMEM67 to the TZ membrane, and regulate tubulin acetylation and glutamylation via tubulin-modifying enzymes [PMID:32726168, PMID:41165761]. B9D2 also interacts with IFT components (IFT88/Fleer) and supports selective intraflagellar transport of specific cargoes such as Opsin into photoreceptor cilia [PMID:21602787]. Pathogenic B9D2 variants cause Joubert syndrome when they primarily impair microtubule modifications or Meckel syndrome when they additionally disrupt ciliogenesis, and B9D2 also has an extraciliary role at tight junctions in biliary epithelial cells where it maintains epithelial barrier integrity before ciliogenesis [PMID:41165761, PMID:39455645]."},"prefetch_data":{"uniprot":{"accession":"Q9BPU9","full_name":"B9 domain-containing protein 2","aliases":["MKS1-related protein 2"],"length_aa":175,"mass_kda":19.3,"function":"Component of the tectonic-like complex, a complex localized at the transition zone of primary cilia and acting as a barrier that prevents diffusion of transmembrane proteins between the cilia and plasma membranes","subcellular_location":"Cytoplasm, cytoskeleton, cilium basal body; Cytoplasm, cytoskeleton, cilium axoneme; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9BPU9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/B9D2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/B9D2","total_profiled":1310},"omim":[{"mim_id":"614950","title":"TRANSMEMBRANE PROTEIN 17; TMEM17","url":"https://www.omim.org/entry/614950"},{"mim_id":"614949","title":"TRANSMEMBRANE PROTEIN 231; TMEM231","url":"https://www.omim.org/entry/614949"},{"mim_id":"614289","title":"SUPPRESSOR OF LIN12-LIKE 2; SEL1L2","url":"https://www.omim.org/entry/614289"},{"mim_id":"614175","title":"MECKEL SYNDROME, TYPE 10; MKS10","url":"https://www.omim.org/entry/614175"},{"mim_id":"614144","title":"B9 DOMAIN-CONTAINING PROTEIN 1; B9D1","url":"https://www.omim.org/entry/614144"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Nucleoli","reliability":"Uncertain"},{"location":"Golgi apparatus","reliability":"Uncertain"},{"location":"Vesicles","reliability":"Additional"},{"location":"Centrosome","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/B9D2"},"hgnc":{"alias_symbol":["MGC4093","MKS10","MKSR-2"],"prev_symbol":[]},"alphafold":{"accession":"Q9BPU9","domains":[{"cath_id":"2.60.40.150","chopping":"3-113_156-172","consensus_level":"high","plddt":93.7292,"start":3,"end":172}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BPU9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BPU9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9BPU9-F1-predicted_aligned_error_v6.png","plddt_mean":89.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=B9D2","jax_strain_url":"https://www.jax.org/strain/search?query=B9D2"},"sequence":{"accession":"Q9BPU9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9BPU9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9BPU9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9BPU9"}},"corpus_meta":[{"pmid":"26092869","id":"PMC_26092869","title":"Joubert syndrome: a model for untangling recessive disorders with extreme genetic heterogeneity.","date":"2015","source":"Journal of medical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/26092869","citation_count":231,"is_preprint":false},{"pmid":"22152675","id":"PMC_22152675","title":"TMEM237 is mutated in individuals with a Joubert syndrome related disorder and expands the role of the TMEM family at the ciliary transition zone.","date":"2011","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/22152675","citation_count":165,"is_preprint":false},{"pmid":"21763481","id":"PMC_21763481","title":"Disruption of a ciliary B9 protein complex causes Meckel syndrome.","date":"2011","source":"American journal of human genetics","url":"https://pubmed.ncbi.nlm.nih.gov/21763481","citation_count":120,"is_preprint":false},{"pmid":"18337471","id":"PMC_18337471","title":"Functional redundancy of the B9 proteins and nephrocystins in Caenorhabditis elegans ciliogenesis.","date":"2008","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/18337471","citation_count":88,"is_preprint":false},{"pmid":"21602787","id":"PMC_21602787","title":"Nephrocystins and MKS proteins interact with IFT particle and facilitate transport of selected ciliary cargos.","date":"2011","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/21602787","citation_count":81,"is_preprint":false},{"pmid":"19208769","id":"PMC_19208769","title":"Functional interactions between the ciliopathy-associated Meckel syndrome 1 (MKS1) protein and two novel MKS1-related (MKSR) proteins.","date":"2009","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/19208769","citation_count":66,"is_preprint":false},{"pmid":"22393243","id":"PMC_22393243","title":"Ciliogenesis in Caenorhabditis elegans requires genetic interactions between ciliary middle segment localized NPHP-2 (inversin) and transition zone-associated proteins.","date":"2012","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/22393243","citation_count":42,"is_preprint":false},{"pmid":"33234550","id":"PMC_33234550","title":"Interpreting the pathogenicity of Joubert syndrome missense variants in Caenorhabditis elegans.","date":"2021","source":"Disease models & mechanisms","url":"https://pubmed.ncbi.nlm.nih.gov/33234550","citation_count":27,"is_preprint":false},{"pmid":"32726168","id":"PMC_32726168","title":"Formation of the B9-domain protein complex MKS1-B9D2-B9D1 is essential as a diffusion barrier for ciliary membrane proteins.","date":"2020","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/32726168","citation_count":26,"is_preprint":false},{"pmid":"31411728","id":"PMC_31411728","title":"Meckel syndrome: Clinical and mutation profile in six fetuses.","date":"2019","source":"Clinical genetics","url":"https://pubmed.ncbi.nlm.nih.gov/31411728","citation_count":21,"is_preprint":false},{"pmid":"36533556","id":"PMC_36533556","title":"Variable phenotypes and penetrance between and within different zebrafish ciliary transition zone mutants.","date":"2022","source":"Disease models & mechanisms","url":"https://pubmed.ncbi.nlm.nih.gov/36533556","citation_count":17,"is_preprint":false},{"pmid":"26459857","id":"PMC_26459857","title":"A functional genomics screen identifies an Importin-α homolog as a regulator of stem cell function and tissue patterning during planarian regeneration.","date":"2015","source":"BMC genomics","url":"https://pubmed.ncbi.nlm.nih.gov/26459857","citation_count":15,"is_preprint":false},{"pmid":"34110414","id":"PMC_34110414","title":"Identification of new semen trait-related candidate genes in Duroc boars through genome-wide association and weighted gene co-expression network analyses.","date":"2021","source":"Journal of animal 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research","url":"https://pubmed.ncbi.nlm.nih.gov/31494809","citation_count":6,"is_preprint":false},{"pmid":"33193692","id":"PMC_33193692","title":"Novel Compound Heterozygous Variants in MKS1 Leading to Joubert Syndrome.","date":"2020","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/33193692","citation_count":6,"is_preprint":false},{"pmid":"35360848","id":"PMC_35360848","title":"Case Report: Preimplantation Genetic Testing for Meckel Syndrome Induced by Novel Compound Heterozygous Mutations of MKS1.","date":"2022","source":"Frontiers in genetics","url":"https://pubmed.ncbi.nlm.nih.gov/35360848","citation_count":4,"is_preprint":false},{"pmid":"21731048","id":"PMC_21731048","title":"Understanding cargo specificity in intraflagellar transport.","date":"2011","source":"The EMBO journal","url":"https://pubmed.ncbi.nlm.nih.gov/21731048","citation_count":3,"is_preprint":false},{"pmid":"39455645","id":"PMC_39455645","title":"New functions of B9D2 in tight junctions and epithelial polarity.","date":"2024","source":"Scientific reports","url":"https://pubmed.ncbi.nlm.nih.gov/39455645","citation_count":1,"is_preprint":false},{"pmid":"41165761","id":"PMC_41165761","title":"Ciliopathy-related B9 protein complex regulates ciliary axonemal microtubule posttranslational modifications and initiation of ciliogenesis.","date":"2025","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/41165761","citation_count":1,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":11293,"output_tokens":2851,"usd":0.038322},"stage2":{"model":"claude-opus-4-6","input_tokens":6234,"output_tokens":2381,"usd":0.136043},"total_usd":0.174365,"stage1_batch_id":"msgbatch_01TqNM6p6FsoMBT2jzQTwKBr","stage2_batch_id":"msgbatch_01Ya473eco8sTHbvRymQaU8o","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2011,\n      \"finding\": \"B9D2 (MKSR-2) forms a physical complex with MKS1 and B9D1; a pathogenic MKS-associated p.Ser101Arg mutation in B9D2 abrogates its interaction with MKS1, demonstrating that complex integrity is required for B9D2 function in ciliogenesis and Hedgehog signaling.\",\n      \"method\": \"Co-immunoprecipitation and mass spectrometry; zebrafish rescue assay; mouse knockout phenotypic analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (Co-IP/MS, in vivo rescue, mouse KO) in a single study\",\n      \"pmids\": [\"21763481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"C. elegans B9 proteins (TZA-2/MKSR-2/B9D2 ortholog, TZA-1/B9D1 ortholog, XBX-7/MKS1 ortholog) form a complex that localizes to the base of cilia (transition zone), and function redundantly with nephrocystins (NPH-1, NPH-4) to regulate cilia formation and maintenance.\",\n      \"method\": \"Genetic analysis of double mutants (epistasis); fluorescence localization of tagged proteins in C. elegans\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with direct localization, replicated across labs\",\n      \"pmids\": [\"18337471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"C. elegans MKSR-2 (B9D2 ortholog) and MKSR-1 (B9D1 ortholog) localize to ciliary transition zone/basal bodies in a co-dependent manner with MKS-1; disruption of human MKSR2 causes ciliogenesis defects, and genetic interactions among all three C. elegans mks/mksr proteins affect insulin-IGF-I signaling.\",\n      \"method\": \"Fluorescence microscopy of GFP-tagged proteins; RNAi knockdown; genetic epistasis; ciliogenesis assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal localization interdependence plus functional epistasis, replicated in worm and human cells\",\n      \"pmids\": [\"19208769\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Zebrafish B9d2 binds IFT particle components (Fleer/IFT88) and contributes to ciliary localization of Inversin (Nephrocystin-2); B9d2, Inversin, and Nephrocystin-5 collectively support transport of Opsin but not Peripherin into photoreceptor cilia, revealing a selective cargo-transport mechanism.\",\n      \"method\": \"Co-immunoprecipitation; zebrafish morpholino knockdown with ciliary cargo localization assays; planar cell polarity assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — binding partner identified by Co-IP plus functional cargo-specificity assay in vivo\",\n      \"pmids\": [\"21602787\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In C. elegans, MKSR-2 (B9D2 ortholog) genetically interacts with JBTS-14/TMEM237, MKS-2/TMEM216, and MKSR-1/B9D1 at the transition zone, and TMEM237/JBTS-14 requires RPGRIP1L/MKS5 for correct TZ localization.\",\n      \"method\": \"C. elegans genetic interaction (double mutants); fluorescence localization; ciliogenesis assays\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis in C. elegans, single lab study\",\n      \"pmids\": [\"22152675\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"C. elegans mksr-2 (B9D2 ortholog) genetically interacts with nphp-2/inversin and other MKS-module genes (mks-1, mks-3, mks-6, mksr-1) in a sensilla-dependent manner to control cilia formation and placement.\",\n      \"method\": \"C. elegans genetic analysis; double/triple mutant phenotypic assays; fluorescence localization\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis, single lab, C. elegans ortholog\",\n      \"pmids\": [\"22393243\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The B9 domain proteins MKS1, B9D2, and B9D1 interact in a defined order (MKS1–B9D2–B9D1) and show interdependent localization to the ciliary transition zone; B9D2-knockout cells display impaired diffusion barrier for ciliary membrane proteins, and rescue requires formation of the intact three-protein complex.\",\n      \"method\": \"Co-immunoprecipitation; CRISPR/Cas9 knockout; fluorescence-based diffusion barrier assay; rescue experiments\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — KO + Co-IP + functional rescue with multiple orthogonal readouts\",\n      \"pmids\": [\"32726168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Two Joubert syndrome-associated B9D2 missense variants (P74S and G155S) are pathogenic in C. elegans: G155S disrupts endogenous MKSR-2 organization at the transition zone and reveals a close functional association between the B9 complex and MKS-2/TMEM216.\",\n      \"method\": \"CRISPR/Cas9 knock-in of patient variants in C. elegans; quantitative TZ structure/function assays; fluorescence imaging of endogenous tagged proteins\",\n      \"journal\": \"Disease models & mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — gene editing with multiple functional assays, single lab\",\n      \"pmids\": [\"33234550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The B9 domain of MKS1 is essential for its interaction with B9D2 and for localization of MKS1 to the ciliary transition zone; a c.1058delG MKS1 mutation disrupting the B9 domain attenuates MKS1–B9D2 interaction and impairs ciliary TZ localization.\",\n      \"method\": \"Co-immunoprecipitation; fluorescence localization; functional mutation analysis\",\n      \"journal\": \"Frontiers in genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — Co-IP plus localization, single lab\",\n      \"pmids\": [\"33193692\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MKS1 mutations disrupting the B9-C2 domain attenuate interaction with B9D2, confirming B9D2 as an essential binding partner for MKS1 at the ciliary transition zone.\",\n      \"method\": \"Co-immunoprecipitation; RT-PCR; functional mutation analysis\",\n      \"journal\": \"Frontiers in genetics\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP, single lab, primarily a clinical genetics study\",\n      \"pmids\": [\"35360848\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Before ciliogenesis, B9D2 localizes at tight junctions in biliary epithelial cells and is required for maturation and maintenance of tight junctions, epithelial barrier tightness, and proper biliary lumen formation—an extraciliary function distinct from its TZ role.\",\n      \"method\": \"Immunofluorescence localization; tight junction permeability assay; RNAi/loss-of-function in biliary epithelial cell models; lumen formation assay\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — direct localization with functional knockdown phenotype, single lab\",\n      \"pmids\": [\"39455645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"The B9D1–B9D2–MKS1 complex (i) anchors TMEM67 to the TZ membrane, and disruption of this complex reduces posttranslational modifications (e.g., acetylation, glutamylation) of axonemal microtubules by deregulating tubulin-modifying enzymes within cilia; (ii) B9 proteins localize to centrioles before ciliogenesis and facilitate initiation of ciliogenesis. Joubert syndrome-associated B9D2 variants primarily affect microtubule modifications without blocking ciliogenesis, whereas the MKS-associated B9D2 variant disrupts both.\",\n      \"method\": \"Co-immunoprecipitation; CRISPR/Cas9 knockout; immunofluorescence; Western blot for PTMs; patient cohort variant analysis\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — multiple orthogonal methods (Co-IP, KO, PTM assay, patient variants) establishing two distinct mechanisms\",\n      \"pmids\": [\"41165761\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"B9D2 is a transition-zone protein that forms a defined MKS1–B9D2–B9D1 complex at the ciliary base, where it acts as a diffusion barrier for ciliary membrane proteins, anchors TMEM67 to the TZ membrane to regulate axonemal microtubule post-translational modifications, facilitates the initiation of ciliogenesis from centrioles, supports selective IFT-dependent cargo transport (e.g., Opsin via interaction with IFT88/Fleer and Inversin), and—prior to ciliogenesis—maintains tight junction integrity and epithelial polarity in biliary epithelia; different B9D2 pathogenic variants differentially impair these functions, explaining why some cause Joubert syndrome (primarily affecting microtubule modifications) while others cause the more severe Meckel syndrome (disrupting both ciliogenesis and microtubule modifications).\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"B9D2 is a transition-zone protein that functions as a core component of the ciliary diffusion barrier and a regulator of ciliogenesis initiation, ciliary cargo transport, and axonemal microtubule post-translational modifications. B9D2 assembles into a defined MKS1–B9D2–B9D1 complex at the ciliary transition zone in an interdependent manner, and integrity of this complex is required to restrict diffusion of membrane proteins into the cilium, anchor TMEM67 to the TZ membrane, and regulate tubulin acetylation and glutamylation via tubulin-modifying enzymes [PMID:32726168, PMID:41165761]. B9D2 also interacts with IFT components (IFT88/Fleer) and supports selective intraflagellar transport of specific cargoes such as Opsin into photoreceptor cilia [PMID:21602787]. Pathogenic B9D2 variants cause Joubert syndrome when they primarily impair microtubule modifications or Meckel syndrome when they additionally disrupt ciliogenesis, and B9D2 also has an extraciliary role at tight junctions in biliary epithelial cells where it maintains epithelial barrier integrity before ciliogenesis [PMID:41165761, PMID:39455645].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Establishing that B9D2 is a ciliary transition-zone protein that acts in a module with B9D1 and MKS1 and functions redundantly with nephrocystins to maintain cilia resolved where and with whom B9D2 operates.\",\n      \"evidence\": \"Genetic epistasis and fluorescence localization of tagged C. elegans orthologs\",\n      \"pmids\": [\"18337471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Biochemical nature of the B9-protein complex not yet defined\", \"Mammalian relevance not directly tested\", \"Mechanism of redundancy with nephrocystins unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrating co-dependent TZ localization of MKSR-2/B9D2 with MKS-1 and MKSR-1/B9D1 and showing that human B9D2 disruption causes ciliogenesis defects established functional conservation and mutual dependence within the complex.\",\n      \"evidence\": \"GFP-tagged protein localization, RNAi knockdown, and genetic epistasis in C. elegans and human cells\",\n      \"pmids\": [\"19208769\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Order of assembly within the complex unknown\", \"Whether B9D2 has functions beyond ciliogenesis not addressed\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Identifying the MKS1–B9D2–B9D1 physical complex by Co-IP/MS and showing that the MKS-associated S101R mutation in B9D2 disrupts MKS1 binding linked human disease directly to complex integrity.\",\n      \"evidence\": \"Co-immunoprecipitation/mass spectrometry, zebrafish rescue, and mouse knockout phenotyping\",\n      \"pmids\": [\"21763481\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for complex assembly not resolved\", \"Whether other TZ proteins depend on this complex for localization unknown\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Revealing that B9D2 binds IFT88/Fleer and supports selective transport of Opsin (but not Peripherin) into photoreceptor cilia demonstrated a cargo-selective transport function beyond simple barrier activity.\",\n      \"evidence\": \"Co-immunoprecipitation and morpholino knockdown with cargo localization assays in zebrafish\",\n      \"pmids\": [\"21602787\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How B9D2–IFT88 interaction confers cargo selectivity unknown\", \"Whether this transport role extends to non-photoreceptor cilia untested\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Placing B9D2 in a broader transition-zone genetic interaction network with TMEM237, TMEM216, and RPGRIP1L mapped the module hierarchy at the TZ.\",\n      \"evidence\": \"C. elegans double-mutant genetic analysis and fluorescence localization\",\n      \"pmids\": [\"22152675\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct physical interactions between B9D2 and TMEM237/TMEM216 not confirmed biochemically\", \"Hierarchy may differ in mammalian systems\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Establishing the ordered MKS1–B9D2–B9D1 assembly and showing that B9D2 knockout specifically impairs the ciliary membrane diffusion barrier defined B9D2's core gatekeeping function at the TZ.\",\n      \"evidence\": \"CRISPR/Cas9 knockout, Co-IP, fluorescence diffusion barrier assay, and rescue experiments in mammalian cells\",\n      \"pmids\": [\"32726168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural determinants of the diffusion barrier not resolved\", \"Whether barrier impairment alone explains ciliopathy phenotypes unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"CRISPR knock-in of Joubert syndrome patient variants P74S and G155S in C. elegans showed that G155S disrupts MKSR-2 TZ organization and revealed a close functional link between the B9 complex and TMEM216, connecting genotype to TZ structural integrity.\",\n      \"evidence\": \"CRISPR/Cas9 knock-in in C. elegans with quantitative TZ assays and fluorescence imaging\",\n      \"pmids\": [\"33234550\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mammalian validation of variant-specific effects needed\", \"Whether P74S acts through a distinct mechanism remains unclear\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Discovery that B9D2 localizes to tight junctions in biliary epithelial cells before ciliogenesis and is required for junction maturation and lumen formation revealed an extraciliary function independent of its TZ role.\",\n      \"evidence\": \"Immunofluorescence, tight junction permeability assay, and RNAi knockdown in biliary epithelial models\",\n      \"pmids\": [\"39455645\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular mechanism of B9D2 at tight junctions unknown\", \"Whether this extraciliary function involves the same B9 complex partners untested\", \"Relevance to biliary disease in patients not established\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Demonstrating that the B9D1–B9D2–MKS1 complex anchors TMEM67 to the TZ, regulates axonemal microtubule post-translational modifications, and that Joubert variants selectively impair PTMs while MKS variants additionally block ciliogenesis provided a unified genotype–mechanism–phenotype framework.\",\n      \"evidence\": \"CRISPR/Cas9 knockout, Co-IP, immunofluorescence, Western blot for tubulin PTMs, patient variant analysis\",\n      \"pmids\": [\"41165761\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which tubulin-modifying enzymes are directly regulated by B9D2/TMEM67 axis unknown\", \"Structural basis of variant-specific differential effects not resolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of B9D2 complex assembly, the mechanism by which B9D2 confers cargo selectivity to IFT, and how B9D2 functions at tight junctions remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No high-resolution structure of the MKS1–B9D2–B9D1 complex\", \"Molecular basis of cargo selectivity (Opsin vs. Peripherin) through IFT88 interaction unknown\", \"Tight junction mechanism of B9D2 entirely uncharacterized at the molecular level\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 3, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [1, 2, 6, 11]},\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [11]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [1, 2, 6, 11]},\n      {\"term_id\": \"R-HSA-5653656\", \"supporting_discovery_ids\": [3, 6]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 3]}\n    ],\n    \"complexes\": [\n      \"MKS1–B9D2–B9D1 complex\"\n    ],\n    \"partners\": [\n      \"MKS1\",\n      \"B9D1\",\n      \"IFT88\",\n      \"TMEM67\",\n      \"TMEM216\",\n      \"INVS\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}