id: MODULE:bbsome title: BBSome ciliary trafficking complex module description: >- The BBSome is a conserved octameric protein complex that acts as a coat-like adaptor for ciliary membrane-protein trafficking. It is built from eight core subunits (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8/TTC8, BBS9, and BBIP1/BBS18), assembled with the help of a dedicated chaperonin-like module (BBS6/MKKS, BBS10, BBS12 acting with the CCT/TRiC chaperonin). Once assembled, the BBSome is recruited to the ciliary membrane by the GTP-bound Arf-like GTPase ARL6/BBS3, where it polymerizes into a coat that recognizes signaling-receptor cargo (ciliary GPCRs and Hedgehog-pathway components) and couples them to the intraflagellar transport (IFT) machinery, mediating ciliary import and export of receptors out of the cilium. LZTFL1/BBS17 regulates BBSome ciliary trafficking, and CCDC28B is a BBSome-associated accessory factor. Loss of BBSome function causes Bardet-Biedl syndrome. This module models the BBSome as a cellular component / protein complex grounded in GO:0034464, capturing its composition, assembly, membrane recruitment, cargo trafficking, and regulation. status: COMPLETE evidence: - source_id: PMID:17574030 title: "A core complex of BBS proteins cooperates with the GTPase Rab8 to promote ciliary membrane biogenesis" statement: >- Identifies the BBSome as a stable complex of seven BBS proteins plus BBIP10 that associates with the ciliary membrane and promotes ciliary membrane trafficking. - source_id: PMID:19081074 title: "A BBSome subunit links ciliogenesis, microtubule stability, and acetylation" statement: >- Establishes BBIP10 (BBIP1) as an integral eighth BBSome subunit required for complex integrity and links the BBSome to microtubule stability and acetylation. - source_id: PMID:20603001 title: "The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia" statement: >- Shows the BBSome forms a planar coat on membranes whose assembly is nucleated by ARL6/BBS3-GTP, supporting the coat-adaptor model for ciliary cargo trafficking. - source_id: PMID:25402481 title: "Structural basis for membrane targeting of the BBSome by ARL6" statement: >- Provides the structural basis for GTP-dependent recruitment of the BBSome to membranes by ARL6/BBS3 via the BBS1 beta-propeller. - source_id: PMID:20080638 title: "BBS6, BBS10, and BBS12 form a complex with CCT/TRiC family chaperonins and mediate BBSome assembly" statement: >- Demonstrates that the chaperonin-like BBS proteins (MKKS/BBS6, BBS10, BBS12) act with the CCT/TRiC chaperonin to assemble the BBSome core, supporting a dedicated assembly module distinct from the mature complex. - source_id: PMID:22072986 title: "A novel protein LZTFL1 regulates ciliary trafficking of the BBSome and Smoothened" statement: >- Shows LZTFL1/BBS17 regulates ciliary entry and retrieval of the BBSome and of Smoothened, supporting a trafficking-regulation interface to the complex. - source_id: PMID:25443296 title: "The intraflagellar transport protein IFT27 promotes BBSome exit from cilia through the GTPase ARL6/BBS3." statement: >- Shows that IFT27 acts through nucleotide-free ARL6 to drive ciliary exit of the BBSome; loss of IFT27 slows BBSome exit and causes ciliary accumulation of ARL6, the BBSome, and the Hedgehog-pathway GPCR GPR161. - source_id: PMID:22922713 title: "The BBSome controls IFT assembly and turnaround in cilia." statement: >- Shows in C. elegans that the BBSome is required for IFT assembly and turnaround at the ciliary tip, coupling it to retrograde IFT. - source_id: PMID:22139371 title: "Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals common BBS-associated phenotypes and Bbs3 unique phenotypes." statement: >- Loss of ARL6/BBS3 leaves BBSome formation intact but disrupts localization of MCHR1 to ciliary membranes and retrograde transport of Smoothened inside cilia. - source_id: PMID:31939736 title: "Structure and activation mechanism of the BBSome membrane protein trafficking complex." statement: >- Cryo-EM structures of the mammalian BBSome alone and bound to ARL6-GTP, defining subunit architecture and the ARL6-induced conformational change. - source_id: PMID:31951201 title: "Structure of the human BBSome core complex." statement: >- Cryo-EM structure of the human BBSome core, placing the beta-propeller, GAE, platform, and TPR-superhelix domains and the BBS5 PH domains. - source_id: PMID:16327777 title: "Dissection of epistasis in oligogenic Bardet-Biedl syndrome." statement: >- Identifies CCDC28B (MGC1203) as a pericentriolar protein that interacts and colocalizes with BBS proteins and acts as a BBS modifier. notes: >- This module is intentionally BBSome-centric: it represents the eight-subunit core complex, its dedicated chaperonin-like assembly module, its ARL6/BBS3 membrane recruiter, and its LZTFL1/CCDC28B trafficking regulators. Broader ciliary machinery that the BBSome cooperates with (the IFT-A/IFT-B trains, the transition-zone MKS/NPHP modules, and Rab8/Rab23 membrane-trafficking regulators) is treated as an interface rather than part of the complex. Several proteins carry "BBS" disease-locus numbers but belong primarily to other ciliary modules (e.g. IFT27/BBS19, IFT172/BBS20, MKS1/BBS13, CEP290/BBS14, SDCCAG8/BBS16, WDPCP/BBS15) and are out of scope here. Subunit architecture follows the cryo-EM structures of the mammalian BBSome (PMID:31939736, PMID:31951201) and the biochemical definition of the complex (PMID:17574030, PMID:19081074); where a precise GO molecular function is uncertain, only a preferred_term and description are given. module: id: bbsome label: BBSome ciliary trafficking complex module_type: PROTEIN_COMPLEX concepts: - preferred_term: BBSome term: id: GO:0034464 label: BBSome description: >- Octameric ciliary coat/adaptor complex (BBS1, BBS2, BBS4, BBS5, BBS7, BBS8/TTC8, BBS9, BBIP1) that traffics membrane proteins to and from cilia. - preferred_term: intraciliary transport term: id: GO:0042073 label: intraciliary transport description: >- Process the BBSome couples cargo to; the complex acts as an IFT adaptor for selected membrane proteins. context: taxa: - preferred_term: ciliated eukaryotes term: id: NCBITaxon:2759 label: Eukaryota description: >- The BBSome is conserved across ciliated eukaryotes and is lost in lineages that have lost cilia. cellular_components: - preferred_term: cilium term: id: GO:0005929 label: cilium - preferred_term: ciliary membrane term: id: GO:0060170 label: ciliary membrane - preferred_term: ciliary basal body term: id: GO:0036064 label: ciliary basal body - preferred_term: ciliary tip term: id: GO:0097542 label: ciliary tip parts: - order: 1 role: assembly of the BBSome core by chaperonin-like factors node: id: bbsome_assembly label: Chaperonin-mediated BBSome assembly module_type: PROTEIN_COMPLEX description: >- The chaperonin-like BBS proteins MKKS/BBS6, BBS10, and BBS12 form a complex with the CCT/TRiC chaperonin that folds BBS7 and nucleates assembly of the eight-subunit BBSome core. These factors are required to build the complex but are not stable subunits of the mature BBSome. concepts: - preferred_term: chaperone-mediated protein complex assembly term: id: GO:0051131 label: chaperone-mediated protein complex assembly annotons: - id: bbs_chaperonin_assembly_module label: BBS chaperonin-like assembly module (BBS6/BBS10/BBS12 + CCT) participant: selector_type: PROTEIN_COMPLEX protein_complex: preferred_term: BBS chaperonin-like assembly complex with CCT/TRiC description: >- Chaperonin-like BBS proteins acting with the cytosolic CCT/TRiC chaperonin to assemble the BBSome core. active_units: - id: mkks_bbs6_unit label: MKKS/BBS6 chaperonin-like unit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: MKKS (BBS6) term: id: UniProtKB:Q9NPJ1 label: McKusick-Kaufman/Bardet-Biedl syndromes putative chaperonin role: chaperonin-like assembly factor function: preferred_term: protein folding chaperone term: id: GO:0044183 label: protein folding chaperone - id: bbs10_unit label: BBS10 chaperonin-like unit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS10 term: id: UniProtKB:Q8TAM1 label: Bardet-Biedl syndrome 10 protein role: chaperonin-like assembly factor function: preferred_term: protein folding chaperone term: id: GO:0044183 label: protein folding chaperone - id: bbs12_unit label: BBS12 chaperonin-like unit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS12 term: id: UniProtKB:Q6ZW61 label: Bardet-Biedl syndrome 12 protein role: chaperonin-like assembly factor function: preferred_term: protein folding chaperone term: id: GO:0044183 label: protein folding chaperone - id: cct_tric_unit label: CCT/TRiC chaperonin participant: selector_type: PROTEIN_COMPLEX protein_complex: preferred_term: CCT/TRiC chaperonin description: Cytosolic chaperonin containing TCP-1 (TRiC) that cooperates with the BBS chaperonins. role: cytosolic chaperonin that folds BBSome subunits function: preferred_term: chaperonin-like BBSome core assembly factor description: >- Assembly-factor role for the BBS6/BBS10/BBS12-CCT module; no exact GO molecular-function term is asserted here. targets: - preferred_term: BBSome core subunits processes: - preferred_term: chaperone-mediated protein complex assembly term: id: GO:0051131 label: chaperone-mediated protein complex assembly evidence: - source_id: PMID:20080638 statement: BBS6/BBS10/BBS12 form a complex with CCT/TRiC chaperonins and mediate BBSome assembly. - order: 2 role: mature BBSome core complex node: id: bbsome_core_octamer label: BBSome core octamer module_type: PROTEIN_COMPLEX description: >- The mature eight-subunit BBSome. BBS2, BBS7, and BBS9 form an interlocking beta-propeller/GAE/platform scaffold; BBS1 is the principal cargo- and ARL6/BBS3-interaction subunit; BBS4 and BBS8/TTC8 are TPR-superhelix adaptors; BBS5 contributes phosphoinositide-binding pleckstrin-homology domains for membrane association; and the small BBIP1/BBS18 subunit is required for complex integrity. concepts: - preferred_term: BBSome term: id: GO:0034464 label: BBSome annotons: - id: bbsome_octamer_units label: BBSome octamer subunits participant: selector_type: PROTEIN_COMPLEX protein_complex: preferred_term: BBSome octamer term: id: GO:0034464 label: BBSome active_units: - id: bbs1_unit label: BBS1 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS1 term: id: UniProtKB:Q8NFJ9 label: Bardet-Biedl syndrome 1 protein role: beta-propeller subunit; principal cargo recognition and ARL6/BBS3-GTP binding interface stoichiometry: "1" - id: bbs2_unit label: BBS2 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS2 term: id: UniProtKB:Q9BXC9 label: Bardet-Biedl syndrome 2 protein role: beta-propeller/GAE/platform scaffold subunit stoichiometry: "1" - id: bbs4_unit label: BBS4 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS4 term: id: UniProtKB:Q96RK4 label: Bardet-Biedl syndrome 4 protein role: TPR-superhelix adaptor subunit stoichiometry: "1" - id: bbs5_unit label: BBS5 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS5 term: id: UniProtKB:Q8N3I7 label: Bardet-Biedl syndrome 5 protein role: pleckstrin-homology subunit; phosphoinositide-dependent membrane association stoichiometry: "1" function: preferred_term: phosphatidylinositol-3-phosphate binding term: id: GO:0032266 label: phosphatidylinositol-3-phosphate binding - id: bbs7_unit label: BBS7 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS7 term: id: UniProtKB:Q8IWZ6 label: Bardet-Biedl syndrome 7 protein role: beta-propeller/GAE/platform scaffold subunit; CCT/chaperonin client during assembly stoichiometry: "1" - id: ttc8_bbs8_unit label: BBS8/TTC8 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: TTC8 (BBS8) term: id: UniProtKB:Q8TAM2 label: Tetratricopeptide repeat protein 8 role: TPR-superhelix adaptor subunit stoichiometry: "1" - id: bbs9_unit label: BBS9 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBS9 term: id: UniProtKB:Q3SYG4 label: Protein PTHB1 role: central beta-propeller/GAE/platform/alpha-helical scaffold subunit stoichiometry: "1" - id: bbip1_unit label: BBIP1/BBS18 subunit participant: selector_type: GENE_PRODUCT gene_product: preferred_term: BBIP1 (BBIP10, BBS18) term: id: UniProtKB:A8MTZ0 label: BBSome-interacting protein 1 role: small subunit required for BBSome integrity; links to microtubule stability/acetylation stoichiometry: "1" function: preferred_term: ciliary membrane-protein cargo adaptor description: >- The assembled octamer functions as a coat-like adaptor that selects membrane-protein cargo for intraflagellar transport. locations: - preferred_term: ciliary membrane term: id: GO:0060170 label: ciliary membrane - preferred_term: ciliary basal body term: id: GO:0036064 label: ciliary basal body evidence: - source_id: PMID:17574030 statement: Defines the seven-subunit BBSome plus BBIP10 associated with the ciliary membrane. - source_id: PMID:19081074 statement: Establishes BBIP10/BBIP1 as the integral eighth subunit required for complex integrity. - source_id: PMID:31939736 statement: Cryo-EM structure of the mammalian BBSome defining subunit domain architecture. - source_id: PMID:31951201 statement: Cryo-EM structure of the human BBSome core placing the beta-propeller, GAE/platform, TPR, and PH domains. - order: 3 role: GTP-dependent membrane recruitment node: id: arl6_membrane_recruitment label: ARL6/BBS3 membrane recruitment of the BBSome module_type: REGULATORY_STEP description: >- The Arf-like GTPase ARL6/BBS3, in its GTP-bound state, binds the BBS1 beta-propeller and recruits the BBSome to the ciliary membrane, where BBSome coat polymerization is nucleated. This is the membrane-targeting switch for BBSome function. annotons: - id: arl6_gtpase_recruitment label: ARL6/BBS3 GTP-dependent BBSome recruitment participant: selector_type: GENE_PRODUCT gene_product: preferred_term: ARL6 (BBS3) term: id: UniProtKB:Q9H0F7 label: ADP-ribosylation factor-like protein 6 function: preferred_term: GTP-dependent recruitment of the BBSome to the ciliary membrane description: >- GTP-bound ARL6 binds the BBS1 beta-propeller of the BBSome and recruits it to the membrane; GTP hydrolysis (GTPase activity, GO:0003924) and IFT27-assisted nucleotide release gate the cycle. term: id: GO:0005525 label: GTP binding locations: - preferred_term: ciliary membrane term: id: GO:0060170 label: ciliary membrane evidence: - source_id: PMID:20603001 statement: ARL6/BBS3-GTP nucleates BBSome coat assembly on membranes. - source_id: PMID:25402481 statement: Structural basis for ARL6-GTP recruitment of the BBSome via the BBS1 beta-propeller. - source_id: PMID:25443296 statement: IFT27 stabilizes nucleotide-free ARL6 to promote BBSome exit from cilia. - order: 4 role: cargo recognition and ciliary trafficking node: id: bbsome_cargo_trafficking label: BBSome cargo recognition and ciliary import/retrieval module_type: TRANSPORT_STEP description: >- The membrane-bound BBSome recognizes signaling-receptor cargo (ciliary GPCRs such as SSTR3 and MCHR1, and Hedgehog-pathway components such as Smoothened and GPR161) and couples them to the IFT trains. The BBSome is required for ciliary exit of the BBSome and associated receptor cargo such as GPR161, driven through ARL6 and IFT27, and for IFT turnaround at the ciliary tip. concepts: - preferred_term: protein localization to cilium term: id: GO:0061512 label: protein localization to cilium annotons: - id: bbsome_cargo_adaptor label: BBSome ciliary cargo adaptor activity participant: selector_type: PROTEIN_COMPLEX protein_complex: preferred_term: membrane-bound BBSome coat term: id: GO:0034464 label: BBSome function: preferred_term: ciliary membrane-protein cargo adaptor coupling to intraflagellar transport description: >- Cargo adaptor role coupling ciliary membrane proteins to IFT; no exact GO molecular-function term is asserted here. cargo: - preferred_term: ciliary GPCR cargo (e.g. SSTR3, MCHR1) - preferred_term: Hedgehog signaling components (Smoothened, GPR161) processes: - preferred_term: intraciliary transport term: id: GO:0042073 label: intraciliary transport locations: - preferred_term: ciliary membrane term: id: GO:0060170 label: ciliary membrane - preferred_term: ciliary tip term: id: GO:0097542 label: ciliary tip evidence: - source_id: PMID:20603001 statement: The BBSome coat traffics membrane proteins to and within cilia (SSTR3, Smoothened). - source_id: PMID:25443296 statement: IFT27 and ARL6 drive ciliary exit of the BBSome and the GPCR GPR161. - source_id: PMID:22922713 statement: The BBSome controls IFT assembly and turnaround at the ciliary tip. - source_id: PMID:22139371 statement: Loss of ARL6/BBS3 disrupts MCHR1 ciliary membrane localization and retrograde Smoothened transport. - order: 5 role: regulation of BBSome ciliary trafficking optional: true node: id: bbsome_trafficking_regulation label: Regulation of BBSome ciliary trafficking module_type: REGULATORY_STEP description: >- LZTFL1/BBS17 negatively regulates BBSome ciliary entry. LZTFL1 associates with the BBSome in the cytoplasm, and its depletion increases ciliary trafficking of the BBSome and Smoothened. annotons: - id: lztfl1_regulation label: LZTFL1/BBS17 regulation of BBSome ciliary trafficking participant: selector_type: GENE_PRODUCT gene_product: preferred_term: LZTFL1 (BBS17) term: id: UniProtKB:Q9NQ48 label: Leucine zipper transcription factor-like protein 1 function: preferred_term: BBSome binding (protein-containing complex binding) description: >- Binds the BBSome and restrains its ciliary entry, thereby controlling BBSome and Smoothened ciliary localization. term: id: GO:0044877 label: protein-containing complex binding targets: - preferred_term: BBSome term: id: GO:0034464 label: BBSome - preferred_term: Smoothened processes: - preferred_term: negative regulation of protein localization to cilium term: id: GO:1903565 label: negative regulation of protein localization to cilium evidence: - source_id: PMID:22072986 statement: LZTFL1 regulates ciliary trafficking of the BBSome and Smoothened. - order: 6 role: BBSome-associated accessory factor optional: true node: id: ccdc28b_accessory label: CCDC28B BBSome-associated accessory factor module_type: BIOLOGICAL_PROCESS description: >- CCDC28B is a pericentriolar BBS-interacting protein and genetic modifier of Bardet-Biedl syndrome that is required for normal ciliogenesis. It associates with BBSome subunits but is not a core subunit, and its effect on BBSome trafficking is not directional enough to model as positive or negative regulation. annotons: - id: ccdc28b_modifier label: CCDC28B accessory modifier of BBSome ciliary localization participant: selector_type: GENE_PRODUCT gene_product: preferred_term: CCDC28B term: id: UniProtKB:Q9BUN5 label: Coiled-coil domain-containing protein 28B function: preferred_term: modifier of BBSome ciliary localization description: >- Accessory factor that influences BBSome ciliary localization; acts as a genetic modifier of Bardet-Biedl syndrome rather than a core subunit. No precise GO molecular function is asserted here. role_description: Accessory modifier of BBSome ciliary trafficking. evidence: - source_id: PMID:16327777 statement: CCDC28B (MGC1203) is a pericentriolar protein that interacts and colocalizes with BBS proteins. connections: - source: bbsome_assembly target: bbsome_core_octamer connection_type: PRECEDES description: Chaperonin-mediated assembly produces the mature BBSome octamer. - source: bbsome_core_octamer target: arl6_membrane_recruitment connection_type: PROVIDES_INPUT_FOR description: The assembled octamer is the substrate recruited to the ciliary membrane by ARL6/BBS3. - source: arl6_membrane_recruitment target: bbsome_cargo_trafficking connection_type: PROVIDES_INPUT_FOR description: Membrane recruitment positions the BBSome coat to capture cargo and couple it to IFT. - source: bbsome_trafficking_regulation target: bbsome_cargo_trafficking connection_type: NEGATIVELY_REGULATES description: LZTFL1 restrains BBSome ciliary entry and thereby BBSome-dependent cargo trafficking.