{"gene":"B9D1","run_date":"2026-04-28T17:12:37","timeline":{"discoveries":[{"year":2011,"finding":"B9D1 is required for normal Hedgehog (Hh) signaling, ciliogenesis, and ciliary protein localization in mice; B9d1 knockout mice display polydactyly, kidney cysts, ductal plate malformations, and neural tube patterning defects concomitant with compromised ciliogenesis and Hh signal transduction.","method":"Mouse knockout (B9d1-null), immunofluorescence for ciliary protein localization, Hh pathway readouts","journal":"American Journal of Human Genetics","confidence":"High","confidence_rationale":"Tier 2 — clean KO with multiple defined cellular and developmental phenotypes, replicated across multiple assays","pmids":["21763481"],"is_preprint":false},{"year":2011,"finding":"MKS1, B9D1, and B9D2 form a physical protein complex at the ciliary transition zone; coimmunoprecipitation and mass spectrometric analyses demonstrated their interaction, and a disease-causing B9D2 p.Ser101Arg mutation abrogates B9D2 binding to MKS1.","method":"Coimmunoprecipitation, mass spectrometry, zebrafish rescue assay","journal":"American Journal of Human Genetics","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP plus MS, functional validation by zebrafish rescue","pmids":["21763481"],"is_preprint":false},{"year":2008,"finding":"C. elegans B9 proteins XBX-7 (MKS1), TZA-1 (B9D2), and TZA-2 (B9D1) 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 in sensory neurons.","method":"Genetic epistasis (double mutants with nph-1/nph-4), localization by fluorescence microscopy, complex formation by Co-IP","journal":"Molecular Biology of the Cell","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis plus localization plus complex formation, replicated across B9 family members","pmids":["18337471"],"is_preprint":false},{"year":2011,"finding":"B9D1 is a causative Meckel syndrome gene; a splice-donor site mutation in B9D1 causes frameshifting exclusion of exon 4, and patient fibroblasts show significantly reduced ciliation confirming B9D1 is required for ciliogenesis in human cells.","method":"Next-generation sequencing, RT-PCR of patient RNA, array CGH, immunofluorescence ciliogenesis assay in patient cells","journal":"Human Molecular Genetics","confidence":"High","confidence_rationale":"Tier 2 — patient mutation identified with functional ciliogenesis readout in primary cells","pmids":["21493627"],"is_preprint":false},{"year":2020,"finding":"The B9-domain protein complex assembles in the order MKS1–B9D2–B9D1 at the ciliary transition zone; MKS1-KO and B9D2-KO cells lose all three B9D proteins from the TZ, demonstrating interdependent localization; formation of this complex is essential for creating a diffusion barrier for ciliary membrane proteins.","method":"Knockout cell lines (MKS1-KO, B9D2-KO), rescue experiments, immunofluorescence for TZ localization, diffusion barrier assay for membrane proteins","journal":"Molecular Biology of the Cell","confidence":"High","confidence_rationale":"Tier 2 — KO cells with rescue, multiple orthogonal methods demonstrating complex order and barrier function","pmids":["32726168"],"is_preprint":false},{"year":2012,"finding":"The B9 protein complex (MKS1, B9D1, B9D2) plays a critical role in a network of protein interactions at the ciliary transition zone, essential for sonic hedgehog signaling and ciliogenesis; loss or dysfunction of this complex is linked to neural ciliopathies.","method":"Review of genetic and biochemical data from multiple model organisms (mouse KO, C. elegans genetics, human patient mutations)","journal":"Molecular Neurobiology","confidence":"Medium","confidence_rationale":"Tier 3 — review synthesizing prior experimental data without new primary experiments","pmids":["22644387"],"is_preprint":false}],"current_model":"B9D1 is a transition zone (TZ) protein that assembles into an obligate MKS1–B9D2–B9D1 complex at the ciliary base; this complex functions as a diffusion barrier for ciliary membrane proteins, is required for proper ciliogenesis and ciliary protein localization, and is essential for Hedgehog signal transduction—loss of B9D1 in mice or humans causes Meckel syndrome-spectrum ciliopathy with polydactyly, kidney cysts, and neural tube defects."},"narrative":{"teleology":[{"year":2008,"claim":"Establishing that B9 proteins form a complex at the ciliary transition zone and cooperate with nephrocystins resolved the subcellular site of action and revealed genetic redundancy with other ciliopathy modules in cilia maintenance.","evidence":"Genetic epistasis with nph-1/nph-4 double mutants, fluorescence localization, and Co-IP in C. elegans sensory neurons","pmids":["18337471"],"confidence":"High","gaps":["Order of complex assembly not determined","Mammalian relevance not yet demonstrated","Molecular mechanism of cilia maintenance by B9 proteins unknown"]},{"year":2011,"claim":"Demonstrating that B9D1 is required for Hedgehog signaling and ciliogenesis in mice, and that mutations cause Meckel syndrome in humans, established B9D1 as a bona fide ciliopathy gene with a defined developmental role.","evidence":"B9d1 knockout mice with Hh pathway readouts and ciliary protein immunofluorescence; patient splice-site mutation with RT-PCR and ciliogenesis assay in primary fibroblasts","pmids":["21763481","21493627"],"confidence":"High","gaps":["Precise mechanism by which B9D1 loss impairs Hedgehog signaling not resolved","Whether B9D1 has functions independent of MKS1–B9D2 not addressed"]},{"year":2011,"claim":"Reciprocal Co-IP and mass spectrometry confirmed the physical MKS1–B9D1–B9D2 complex in mammalian cells and showed that a disease-causing B9D2 mutation disrupts MKS1 binding, linking complex integrity to pathogenesis.","evidence":"Coimmunoprecipitation, mass spectrometry in mammalian cells, zebrafish rescue assay for B9D2 mutant","pmids":["21763481"],"confidence":"High","gaps":["Structural basis of B9 domain interactions unknown","Whether additional TZ proteins are stoichiometric subunits not determined"]},{"year":2020,"claim":"Defining the hierarchical assembly order (MKS1→B9D2→B9D1) and demonstrating that this complex constitutes a ciliary membrane diffusion barrier resolved how B9D proteins control ciliary composition at a mechanistic level.","evidence":"MKS1-KO and B9D2-KO cell lines with rescue, TZ immunofluorescence, and membrane protein diffusion barrier assays","pmids":["32726168"],"confidence":"High","gaps":["Structural model of the assembled barrier not available","Molecular basis of selective gating (which cargo excluded vs. permitted) unresolved","How the B9 complex interfaces physically with Y-linker or membrane-anchored TZ structures unknown"]},{"year":null,"claim":"The structural basis of B9D1 within the transition zone barrier, how it selectively gates specific ciliary membrane proteins, and whether B9D1 has signaling roles beyond Hedgehog remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No high-resolution structure of the MKS1–B9D2–B9D1 complex","Selectivity rules for the diffusion barrier not defined","Role in signaling pathways beyond Hedgehog not tested systematically"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,4]}],"localization":[{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[0,2,4]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,5]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,2,4]}],"complexes":["MKS1–B9D2–B9D1 complex"],"partners":["MKS1","B9D2","NPH1","NPH4"],"other_free_text":[]},"mechanistic_narrative":"B9D1 is a ciliary transition zone protein that assembles into an obligate MKS1–B9D2–B9D1 complex, forming a diffusion barrier that restricts ciliary membrane protein composition and is essential for ciliogenesis and Hedgehog signal transduction. The complex assembles in a defined order (MKS1→B9D2→B9D1) at the transition zone, and loss of any subunit abolishes localization of all three B9D proteins and disrupts barrier function [PMID:32726168]. In C. elegans, B9D1 orthologs function redundantly with nephrocystins to regulate cilia formation in sensory neurons [PMID:18337471], while B9d1 knockout mice exhibit polydactyly, kidney cysts, and neural tube defects due to compromised Hedgehog signaling [PMID:21763481]. Loss-of-function mutations in B9D1 cause Meckel syndrome in humans, with patient fibroblasts showing severely reduced ciliation [PMID:21493627]."},"prefetch_data":{"uniprot":{"accession":"Q9UPM9","full_name":"B9 domain-containing protein 1","aliases":["MKS1-related protein 1"],"length_aa":204,"mass_kda":22.8,"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. Required for ciliogenesis and sonic hedgehog/SHH signaling (By similarity)","subcellular_location":"Cytoplasm, cytoskeleton, cilium basal body; Cytoplasm, cytoskeleton, cilium axoneme","url":"https://www.uniprot.org/uniprotkb/Q9UPM9/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/B9D1","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/B9D1","total_profiled":1310},"omim":[{"mim_id":"617121","title":"JOUBERT SYNDROME 28; JBTS28","url":"https://www.omim.org/entry/617121"},{"mim_id":"617120","title":"JOUBERT SYNDROME 27; JBTS27","url":"https://www.omim.org/entry/617120"},{"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":"614209","title":"MECKEL SYNDROME, TYPE 9; MKS9","url":"https://www.omim.org/entry/614209"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Uncertain","locations":[{"location":"Nucleoplasm","reliability":"Uncertain"},{"location":"Acrosome","reliability":"Uncertain"},{"location":"Equatorial segment","reliability":"Uncertain"},{"location":"Mid piece","reliability":"Uncertain"},{"location":"Vesicles","reliability":"Additional"},{"location":"Basal body","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"},{"location":"Principal piece","reliability":"Additional"},{"location":"End piece","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/B9D1"},"hgnc":{"alias_symbol":["B9","EPPB9","MKS9","MKSR-1"],"prev_symbol":[]},"alphafold":{"accession":"Q9UPM9","domains":[{"cath_id":"2.60.40.150","chopping":"8-180","consensus_level":"high","plddt":87.0025,"start":8,"end":180}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UPM9","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UPM9-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9UPM9-F1-predicted_aligned_error_v6.png","plddt_mean":82.69},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=B9D1","jax_strain_url":"https://www.jax.org/strain/search?query=B9D1"},"sequence":{"accession":"Q9UPM9","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9UPM9.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9UPM9/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9UPM9"}},"corpus_meta":[{"pmid":"25403254","id":"PMC_25403254","title":"Conditional 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B9d1 knockout mice display polydactyly, kidney cysts, ductal plate malformations, and neural tube patterning defects concomitant with compromised ciliogenesis and Hh signal transduction.\",\n      \"method\": \"Mouse knockout (B9d1-null), immunofluorescence for ciliary protein localization, Hh pathway readouts\",\n      \"journal\": \"American Journal of Human Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with multiple defined cellular and developmental phenotypes, replicated across multiple assays\",\n      \"pmids\": [\"21763481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"MKS1, B9D1, and B9D2 form a physical protein complex at the ciliary transition zone; coimmunoprecipitation and mass spectrometric analyses demonstrated their interaction, and a disease-causing B9D2 p.Ser101Arg mutation abrogates B9D2 binding to MKS1.\",\n      \"method\": \"Coimmunoprecipitation, mass spectrometry, zebrafish rescue assay\",\n      \"journal\": \"American Journal of Human Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP plus MS, functional validation by zebrafish rescue\",\n      \"pmids\": [\"21763481\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"C. elegans B9 proteins XBX-7 (MKS1), TZA-1 (B9D2), and TZA-2 (B9D1) 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 in sensory neurons.\",\n      \"method\": \"Genetic epistasis (double mutants with nph-1/nph-4), localization by fluorescence microscopy, complex formation by Co-IP\",\n      \"journal\": \"Molecular Biology of the Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis plus localization plus complex formation, replicated across B9 family members\",\n      \"pmids\": [\"18337471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"B9D1 is a causative Meckel syndrome gene; a splice-donor site mutation in B9D1 causes frameshifting exclusion of exon 4, and patient fibroblasts show significantly reduced ciliation confirming B9D1 is required for ciliogenesis in human cells.\",\n      \"method\": \"Next-generation sequencing, RT-PCR of patient RNA, array CGH, immunofluorescence ciliogenesis assay in patient cells\",\n      \"journal\": \"Human Molecular Genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — patient mutation identified with functional ciliogenesis readout in primary cells\",\n      \"pmids\": [\"21493627\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The B9-domain protein complex assembles in the order MKS1–B9D2–B9D1 at the ciliary transition zone; MKS1-KO and B9D2-KO cells lose all three B9D proteins from the TZ, demonstrating interdependent localization; formation of this complex is essential for creating a diffusion barrier for ciliary membrane proteins.\",\n      \"method\": \"Knockout cell lines (MKS1-KO, B9D2-KO), rescue experiments, immunofluorescence for TZ localization, diffusion barrier assay for membrane proteins\",\n      \"journal\": \"Molecular Biology of the Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO cells with rescue, multiple orthogonal methods demonstrating complex order and barrier function\",\n      \"pmids\": [\"32726168\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"The B9 protein complex (MKS1, B9D1, B9D2) plays a critical role in a network of protein interactions at the ciliary transition zone, essential for sonic hedgehog signaling and ciliogenesis; loss or dysfunction of this complex is linked to neural ciliopathies.\",\n      \"method\": \"Review of genetic and biochemical data from multiple model organisms (mouse KO, C. elegans genetics, human patient mutations)\",\n      \"journal\": \"Molecular Neurobiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — review synthesizing prior experimental data without new primary experiments\",\n      \"pmids\": [\"22644387\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"B9D1 is a transition zone (TZ) protein that assembles into an obligate MKS1–B9D2–B9D1 complex at the ciliary base; this complex functions as a diffusion barrier for ciliary membrane proteins, is required for proper ciliogenesis and ciliary protein localization, and is essential for Hedgehog signal transduction—loss of B9D1 in mice or humans causes Meckel syndrome-spectrum ciliopathy with polydactyly, kidney cysts, and neural tube defects.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"B9D1 is a ciliary transition zone protein that assembles into an obligate MKS1–B9D2–B9D1 complex, forming a diffusion barrier that restricts ciliary membrane protein composition and is essential for ciliogenesis and Hedgehog signal transduction. The complex assembles in a defined order (MKS1→B9D2→B9D1) at the transition zone, and loss of any subunit abolishes localization of all three B9D proteins and disrupts barrier function [PMID:32726168]. In C. elegans, B9D1 orthologs function redundantly with nephrocystins to regulate cilia formation in sensory neurons [PMID:18337471], while B9d1 knockout mice exhibit polydactyly, kidney cysts, and neural tube defects due to compromised Hedgehog signaling [PMID:21763481]. Loss-of-function mutations in B9D1 cause Meckel syndrome in humans, with patient fibroblasts showing severely reduced ciliation [PMID:21493627].\",\n  \"teleology\": [\n    {\n      \"year\": 2008,\n      \"claim\": \"Establishing that B9 proteins form a complex at the ciliary transition zone and cooperate with nephrocystins resolved the subcellular site of action and revealed genetic redundancy with other ciliopathy modules in cilia maintenance.\",\n      \"evidence\": \"Genetic epistasis with nph-1/nph-4 double mutants, fluorescence localization, and Co-IP in C. elegans sensory neurons\",\n      \"pmids\": [\"18337471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Order of complex assembly not determined\",\n        \"Mammalian relevance not yet demonstrated\",\n        \"Molecular mechanism of cilia maintenance by B9 proteins unknown\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Demonstrating that B9D1 is required for Hedgehog signaling and ciliogenesis in mice, and that mutations cause Meckel syndrome in humans, established B9D1 as a bona fide ciliopathy gene with a defined developmental role.\",\n      \"evidence\": \"B9d1 knockout mice with Hh pathway readouts and ciliary protein immunofluorescence; patient splice-site mutation with RT-PCR and ciliogenesis assay in primary fibroblasts\",\n      \"pmids\": [\"21763481\", \"21493627\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Precise mechanism by which B9D1 loss impairs Hedgehog signaling not resolved\",\n        \"Whether B9D1 has functions independent of MKS1–B9D2 not addressed\"\n      ]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Reciprocal Co-IP and mass spectrometry confirmed the physical MKS1–B9D1–B9D2 complex in mammalian cells and showed that a disease-causing B9D2 mutation disrupts MKS1 binding, linking complex integrity to pathogenesis.\",\n      \"evidence\": \"Coimmunoprecipitation, mass spectrometry in mammalian cells, zebrafish rescue assay for B9D2 mutant\",\n      \"pmids\": [\"21763481\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural basis of B9 domain interactions unknown\",\n        \"Whether additional TZ proteins are stoichiometric subunits not determined\"\n      ]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defining the hierarchical assembly order (MKS1→B9D2→B9D1) and demonstrating that this complex constitutes a ciliary membrane diffusion barrier resolved how B9D proteins control ciliary composition at a mechanistic level.\",\n      \"evidence\": \"MKS1-KO and B9D2-KO cell lines with rescue, TZ immunofluorescence, and membrane protein diffusion barrier assays\",\n      \"pmids\": [\"32726168\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Structural model of the assembled barrier not available\",\n        \"Molecular basis of selective gating (which cargo excluded vs. permitted) unresolved\",\n        \"How the B9 complex interfaces physically with Y-linker or membrane-anchored TZ structures unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of B9D1 within the transition zone barrier, how it selectively gates specific ciliary membrane proteins, and whether B9D1 has signaling roles beyond Hedgehog remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No high-resolution structure of the MKS1–B9D2–B9D1 complex\",\n        \"Selectivity rules for the diffusion barrier not defined\",\n        \"Role in signaling pathways beyond Hedgehog not tested systematically\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [0, 2, 4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 5]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 2, 4]}\n    ],\n    \"complexes\": [\n      \"MKS1–B9D2–B9D1 complex\"\n    ],\n    \"partners\": [\n      \"MKS1\",\n      \"B9D2\",\n      \"NPH1\",\n      \"NPH4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}