BBS-4 is a tetratricopeptide-repeat (TPR) protein that is a peripheral structural subunit of the BBSome, an octameric complex (BBS-1, BBS-2, BBS-4, BBS-5, OSM-12/BBS-7, BBS-8/TTC-8, BBS-9) that couples intraflagellar transport (IFT) to the trafficking of membrane cargo at sensory cilia. Like the COPI, COPII and clathrin coats it structurally resembles, the BBSome assembles IFT particles at the ciliary base, binds the anterograde IFT particle, and reaches the ciliary tip where it regulates IFT turnaround and recycling. Within the complex BBS-4 binds directly to BBS-5 through its C-terminal TPR region and localizes to the ciliary base/basal body and along cilia. In C. elegans, BBS-4 is functionally redundant with BBS-5: single mutants have essentially normal cilia, whereas bbs-4; bbs-5 double mutants phenocopy loss of the whole BBSome, showing IFT-A/IFT-B uncoupling, disrupted ciliogenesis, and defective polycystin-mediated cilia signaling. Beyond building cilia, BBS-4 (redundantly with BBS-5) is required for the removal of ciliary sensory receptors โ including polycystin-2/PKD-2, the TRP channel OSM-9 and the GPCR ODR-10 โ from cilia for lysosome-targeted degradation, acting upstream of the early endosome at the ciliary base. A conserved C-terminal residue (A388 in the worm protein, A364 in human BBS4) is required for the BBS-4โBBS-5 interaction and for ciliary targeting, and its mutation models human Bardet-Biedl syndrome.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0034464
BBSome
|
NAS
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
ACCEPT |
Summary: BBS-4 is a bona fide subunit of the BBSome, the octameric complex containing BBS-1, BBS-2, BBS-4, BBS-5, OSM-12/BBS-7, BBS-8/TTC-8 and BBS-9. This is the core cellular-component assignment for bbs-4 and is supported experimentally in C. elegans, where BBS-4 co-behaves with the other BBSome subunits during IFT.
Reason: Complex membership is the defining property of bbs-4 and is strongly supported by both C. elegans experiments and cross-species orthology. This is a core annotation.
Supporting Evidence:
PMID:22922713
the in vivo function for the BBSome is to regulate the assembly of the IFT particles at the ciliary base
PMID:26150102
during the assembly of the BBSome, BBS2, 7, and 9 form the core, then BBS1, 5, 8, and finally BBS4 are added in a stepwise manner
file:worm/bbs-4/bbs-4-deep-research-falcon.md
model: Edison Scientific Literature
|
|
GO:0030674
protein-macromolecule adaptor activity
|
ISS
GO_REF:0000024 |
ACCEPT |
Summary: This adaptor-activity term is transferred by sequence similarity from human BBS4 (UniProtKB:Q96RK4). It captures the fact that BBS-4, as a TPR-repeat subunit of the coat-like BBSome, acts as a scaffold/adaptor rather than an enzyme. It is far more informative than a generic 'protein binding' term, and reflects the best current molecular-function description for a BBSome coat subunit.
Reason: Adaptor activity is the most defensible molecular function for a BBSome structural subunit and is preferable to uninformative protein binding. However, no experimental MF has been measured for BBS-4 itself, and no GO term specifically expresses the BBSome coat/cargo-adaptor role โ see knowledge_gaps.
Supporting Evidence:
PMID:22922713
shares the common structural features with COPI, COPII, and clathrin coats, and can directly recognize IFT cargos
|
|
GO:0060271
cilium assembly
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: BBS-4 contributes to cilium assembly as part of the BBSome. In C. elegans this is genetically redundant with BBS-5: single mutants are normal, but bbs-4; bbs-5 double mutants show typical bbs-class cilia defects, confirming a real (if buffered) role in ciliogenesis.
Reason: Well supported by phylogenetic inference and by direct C. elegans genetics (redundancy with bbs-5). A core biological-process annotation.
Supporting Evidence:
PMID:26150102
bbs-4; bbs-5 double mutants show typical cilia defect as observed in other bbs mutants
PMID:22922713
the in vivo function for the BBSome is to regulate the assembly of the IFT particles at the ciliary base
|
|
GO:0060271
cilium assembly
|
NAS
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
ACCEPT |
Summary: Author-stated (NAS) assignment of the cilium-assembly role, duplicating the IBA annotation. The BBSome assembles IFT particles required for ciliogenesis.
Reason: Correct and consistent with the IBA annotation and with C. elegans genetics; both are retained as complementary evidence for the same core role.
Supporting Evidence:
PMID:22922713
the in vivo function for the BBSome is to regulate the assembly of the IFT particles at the ciliary base
|
|
GO:0061512
protein localization to cilium
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: As a BBSome subunit, BBS-4 is required for correct localization of membrane and signaling proteins to/from cilia. In C. elegans the BBSome (redundantly requiring bbs-4/bbs-5) governs the ciliary homeostasis of sensory receptors such as PKD-2, OSM-9 and ODR-10, which mislocalize and accumulate when the complex is disrupted.
Reason: Central to BBSome function and supported by both phylogenetic inference and C. elegans experiments. Core biological-process annotation.
Supporting Evidence:
PMID:26150102
with ~6-fold increasing of protein levels in their native expressing cilia in bbs-4; bbs-5 mutants
|
|
GO:0036064
ciliary basal body
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: BBS-4 acts at the ciliary basal body / ciliary base, where the BBSome assembles IFT particles prior to anterograde transport. Supported phylogenetically and consistent with direct C. elegans localization data.
Reason: Basal-body localization is a core, well-supported cellular-component annotation and matches the site of BBSome-mediated IFT assembly.
Supporting Evidence:
PMID:22922713
the in vivo function for the BBSome is to regulate the assembly of the IFT particles at the ciliary base
|
|
GO:0036064
ciliary basal body
|
IDA
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
ACCEPT |
Summary: Direct experimental (IDA) localization of BBS-4 to the ciliary basal body/base in C. elegans, consistent with BBSome accumulation around the ciliary base when it is uncoupled from moving IFT.
Reason: Experimental localization; the strongest evidence class for the basal-body assignment. Core cellular-component annotation.
Supporting Evidence:
PMID:22922713
Some of them (BBS-1, BBS-4) totally lost the ciliary localization
|
|
GO:0005929
cilium
|
NAS
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
KEEP AS NON CORE |
Summary: BBS-4 localizes to cilia as a BBSome subunit that undergoes IFT along the ciliary axoneme. A correct, if general, cellular-component assignment (the basal-body and ciliary-base terms are more specific).
Reason: Accurate general localization supported by the referenced work, but non-core relative to the more specific basal-body/ciliary-base terms which are the core CC annotations; retained as a valid non-core localization.
Supporting Evidence:
PMID:22922713
Some of them (BBS-1, BBS-4) totally lost the ciliary localization
|
|
GO:0060170
ciliary membrane
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: Ciliary-membrane localization is inferred from the UniProt subcellular-location mapping (Cell projection, cilium membrane, By similarity). It is consistent with the BBSome's role as a membrane-associated coat that traffics and removes ciliary membrane receptors.
Reason: Consistent with BBSome coat function at the ciliary membrane and with the receptor trafficking/removal role demonstrated for bbs-4/bbs-5.
|
|
GO:0005813
centrosome
|
IEA
GO_REF:0000044 |
KEEP AS NON CORE |
Summary: Centrosome localization is an electronic UniProt subcellular-location mapping derived from the mammalian BBS4 annotation (Cytoplasm, cytoskeleton, MTOC, centrosome, By similarity). In C. elegans the relevant structure is the centriole-derived ciliary basal body, which is separately annotated; a distinct centrosomal pool has not been demonstrated for worm BBS-4.
Reason: The basal body is centriole-derived, so a centrosomal assignment is not wrong, but it is a non-specific/derived term for this organism where the experimentally supported site is the ciliary basal body/base. Kept as non-core rather than removed.
|
|
GO:0005856
cytoskeleton
|
IEA
GO_REF:0000044 |
KEEP AS NON CORE |
Summary: High-level cytoskeleton localization from the UniProt subcellular-location mapping. This is a very general parent term encompassing the more specific ciliary/basal-body localizations.
Reason: Not incorrect but uninformative relative to the specific ciliary basal body and ciliary base terms. Retained as non-core.
|
|
GO:0000242
pericentriolar material
|
ISS
GO_REF:0000024 |
MARK AS OVER ANNOTATED |
Summary: Pericentriolar-material localization is transferred by similarity from human BBS4, which has a mammalian pericentriolar/centriolar-satellite role. C. elegans BBS-4 has been characterized only in the ciliary/BBSome context, and nematodes largely lack the mammalian centriolar-satellite system; no worm experiment supports a pericentriolar-material localization.
Reason: This is a mammalian-derived ISS transfer with no experimental support in C. elegans and is not part of the conserved core BBSome/ciliary function assayed in this organism. Flagged as over-annotation for the worm rather than a core localization.
|
|
GO:0007098
centrosome cycle
|
ISS
GO_REF:0000024 |
MARK AS OVER ANNOTATED |
Summary: The centrosome-cycle process is transferred by similarity from mammalian BBS4, which has been implicated in microtubule anchoring and dynein-mediated pericentriolar transport. There is no C. elegans evidence that BBS-4 participates in the centrosome cycle; all worm phenotypes concern cilia and IFT.
Reason: Mammalian-derived ISS process transfer without worm support and outside the conserved core ciliary role. Flagged as over-annotation for this organism.
|
|
GO:0097546
ciliary base
|
IDA
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
NEW |
Summary: BBS-4 and the other BBSome subunits concentrate at the ciliary base, where the BBSome assembles IFT particles; when the BBSome is uncoupled from moving IFT the proteins strongly accumulate there. This is a more precise localization than the general 'cilium' term and complements the ciliary basal body annotation.
Reason: Ciliary base is the experimentally supported site of BBSome-mediated IFT assembly in C. elegans and is included as a location in core_functions; added here as a specific cellular-component annotation.
Supporting Evidence:
PMID:22922713
all BBS proteins examined strongly accumulated around the ciliary base
|
|
GO:0042073
intraciliary transport
|
IMP
PMID:22922713 The BBSome controls IFT assembly and turnaround in cilia. |
NEW |
Summary: BBS-4, as a BBSome subunit, itself undergoes IFT movement along the ciliary axoneme and is required for normal intraflagellar transport; loss of the BBSome (e.g. in bbs-4; bbs-5 double mutants) uncouples IFT-A from IFT-B and disrupts IFT integrity.
Reason: Intraciliary transport is a core biological process for the BBSome, captured in core_functions; added here as a specific process annotation supported by BBS-protein IFT movement and the IFT defects of bbs mutants.
Supporting Evidence:
PMID:22922713
all BBS proteins completely lost IFT movement
PMID:26150102
bbs-4; bbs-5 and bbs-7 share similar mutant phenotypes in that CHE-11 is absent, but OSM-6 abnormally accumulates, in the distal segments of plasmid cilia
|
Q: Does BBS-4's TPR surface directly contact a specific ciliary membrane cargo within the BBSome, or does it act purely as a scaffold linking BBS-5 into the coat?
Q: What endocytic/ubiquitin effectors at the ciliary base connect the BBSome to lysosome-targeted degradation of sensory receptors?
Experiment: Proximity-dependent biotinylation (TurboID) or affinity proteomics of BBS-4 at the ciliary base in wild-type versus bbs-5 mutant worms to identify cargo and endocytic/ubiquitin effectors.
Hypothesis: BBS-4 within the BBSome physically links ciliary sensory receptors to the early-endosome/lysosome sorting machinery at the ciliary base.
Type: proteomics
Experiment: Structure-guided separation-of-function alleles of bbs-4 that abolish BBS-5 binding (C-terminal TPR) while preserving BBSome incorporation, scored for ciliogenesis, IFT integrity, and receptor removal.
Hypothesis: The BBS-4โBBS-5 interaction is specifically required for receptor degradative sorting, separable from bulk IFT assembly.
Type: genetics
What is not known โ curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: There is no GO molecular-function term that expresses the role of a BBSome coat/cargo- adaptor subunit, and the specific cargo(es) that BBS-4's TPR array directly recognizes within the BBSome are undefined. BBS-4 has no experimentally measured biochemical activity of its own; its sole MF annotation (protein-macromolecule adaptor activity) is a generic sequence-similarity transfer.
OPEN ONTOLOGYBIOLOGY MF_DARK
What is known: BBS-4 is an established BBSome subunit built from tetratricopeptide repeats; the BBSome is explicitly coat-like (shares structural features with COPI, COPII and clathrin) and is proposed to polymerize and recognize ciliary membrane proteins, and it can directly recognize IFT cargo. What is missing is a molecular-function term (and the direct cargo identity) for the coat subunit rather than the complex-level process.
Significance: BBSome subunits illustrate the structural-subunit ontology gap: their function is 'be part of the coat', which the GO molecular-function aspect cannot currently express, so the gene reads as MF-dark despite a well-defined cellular role central to Bardet-Biedl syndrome.
What would resolve it: Ontology development of a BBSome coat/cargo-adaptor molecular-function term (analogous to a vesicle-coat adaptor), plus proximity/affinity proteomics and structural work to identify the membrane cargo directly contacted by the BBS-4 TPR surface.
Provenance (the field's own admissions):
Proposed term (ontology gap):
Gap: The molecular mechanism by which the BBSome triggers degradative (lysosome-targeted) removal of ciliary sensory receptors โ the direct effectors linking the ciliary-base BBSome to the endocytic/ubiquitin machinery โ and the molecular basis of the BBS-4/BBS-5 functional redundancy (two subunits with no shared domain, each dispensable alone) are undetermined.
OPEN BIOLOGY RESIDUAL_SUBGAP
What is known: bbs-4 and bbs-5 single mutants have normal cilia; bbs-4; bbs-5 double mutants phenocopy whole-BBSome loss, with IFT-A/IFT-B uncoupling and ~3โ6-fold ciliary accumulation of PKD-2, OSM-9 and ODR-10. RAB-5 overexpression rescues, and STAM-1 epistasis places the BBSome upstream of the early endosome at the ciliary base for these non-IFT cargoes; BBS-4 binds BBS-5 directly via its C-terminal TPR region. What remains unknown is the biochemical output that couples the BBSome to receptor endocytosis/degradation.
Significance: This receptor-downregulation activity is conserved to human BBS4/BBS5 and to polycystin-2, directly relevant to Bardet-Biedl syndrome and polycystic kidney disease; the missing effectors are the mechanistic link between the BBSome and ciliary signaling homeostasis.
What would resolve it: Proximity/affinity proteomics of the ciliary-base BBSome to identify the endocytic/ ubiquitin effectors; separation-of-function bbs-4 alleles that uncouple BBS-5 binding from cargo removal; and structural definition of how BBS-4 and BBS-5 co-contribute to a shared coat surface.
Provenance (the field's own admissions):
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
The gene bbs-4 (ORF name F58A4.14) in C. elegans encodes the BBSome complex member BBS-4 (UniProt: Q5CZ52), a homolog of the human Bardet-Biedl syndrome 4 protein (BBS4). The protein belongs to the BBS4 family and contains multiple tetratricopeptide repeat (TPR) domains (IPR019734, IPR011990), consistent with its characterized role as a structural adaptor protein within the BBSome complex. The gene symbol, organism, protein family, and domain architecture all align with the literature reviewed below.
| Gene name | Protein name | Organism | Protein family | Key domains | Primary function | Subcellular localization | Interacting partners / complex context | Mutant phenotypes in C. elegans | Disease relevance / human ortholog |
|---|---|---|---|---|---|---|---|---|---|
| bbs-4 | BBS-4; BBSome complex member bbs-4; Bardet-Biedl syndrome 4 protein homolog | Caenorhabditis elegans | BBS4 family; BBSome subunit | TPR-like helical domain superfamily; tetratricopeptide repeats (TPRs), consistent with a TPR ฮฑ-solenoid/adaptor architecture (klink2020structureofthe pages 2-4, singh2020structureandactivation pages 5-8, klink2020structureofthe pages 4-6) | Non-enzymatic trafficking adaptor subunit of the BBSome. Helps organize ciliary membrane protein trafficking, especially removal/degradative sorting of sensory receptors, and contributes to IFT particle assembly/turnaround through the BBSome rather than catalyzing a chemical reaction (xu2015bbs4andbbs5 pages 2-4, xu2015bbs4andbbs5 pages 6-7, wei2012thebbsomecontrols pages 1-2, tian2023organizationfunctionsand pages 16-18) | Localizes to cilia and the ciliary base in worms; in dyf-2 mutants accumulates around the ciliary base and loses normal ciliary localization. In broader BBS literature, BBS4/BBSome also associates with basal body, centrosomal/pericentriolar regions and centriolar satellites (xu2015bbs4andbbs5 pages 6-7, novas2015bardetโbiedlsyndromeis pages 4-4, wei2012thebbsomecontrols pages 2-4) | Directly interacts with BBS-5 in worms; structurally connected within the BBSome to BBS1, BBS8, BBS9, and BBS18; assembled late into the BBSome after the BBS2-BBS7-BBS9 core and after incorporation of BBS1/BBS5/BBS8 in assembly models (xu2015bbs4andbbs5 pages 6-7, klink2020structureofthe pages 4-6, tian2023organizationfunctionsand pages 7-9, zhang2012intrinsicproteinproteininteractionmediated pages 6-8, zhang2012intrinsicproteinproteininteractionmediated pages 8-10) | Single mutants: often normal in dye-filling/ciliogenesis assays but show defects in polycystin signaling. bbs-4;bbs-5 double mutants: strong ciliary dysfunction, IFT-A/IFT-B dissociation with altered velocities, receptor accumulation in cilia (e.g., PKD-2, OSM-9, ODR-10), and defective sensory/ciliary homeostasis (xu2015bbs4andbbs5 pages 2-4, wingfield2018traffickingofciliary pages 4-5) | Human ortholog BBS4 is a Bardet-Biedl syndrome gene. Disease associations include Bardet-Biedl syndrome / Bardet-Biedl syndrome 4, retinitis pigmentosa, and polydactyly, supporting evolutionary conservation of BBS4-dependent ciliary trafficking (OpenTargets Search: -BBS4) |
Table: This table summarizes the core molecular and functional properties of C. elegans bbs-4/BBS-4, emphasizing its role as a TPR-containing BBSome subunit involved in ciliary trafficking and IFT-related processes. It also links worm findings to conserved human BBS4 disease relevance.
BBS-4 is not an enzyme; it does not catalyze a chemical reaction. Rather, it functions as a structural adaptor subunit of the BBSome, an octameric protein complex that acts as a cargo adapter linking ciliary membrane proteins to the intraflagellar transport (IFT) machinery (xu2015bbs4andbbs5 pages 2-4, tian2023organizationfunctionsand pages 3-5). The BBSome was originally discovered through tandem affinity purification of BBS4, which identified six other BBS proteins (BBS1, BBS2, BBS5, BBS7, BBS8, and BBS9) associating with BBS4 in stoichiometric ratios (tian2023organizationfunctionsand pages 3-5). A later-identified eighth subunit, BBS18/BBIP10, completes the octameric complex (tian2023organizationfunctionsand pages 3-5).
The primary molecular role of BBS-4 within the BBSome is to contribute to the complex's function as a planar coat complex that recognizes ciliary membrane proteinsโparticularly G protein-coupled receptors (GPCRs) and sensory receptorsโand mediates their trafficking into and, predominantly, out of cilia via association with IFT trains (tian2023organizationfunctionsand pages 16-18, wingfield2018traffickingofciliary pages 5-7, wingfield2018traffickingofciliary pages 2-4). In C. elegans, BBS-4 is specifically implicated in regulating the degradative sorting of ciliary sensory receptors, directing ubiquitinated receptors toward lysosomal degradation rather than simply mediating retrograde IFT transport (xu2015bbs4andbbs5 pages 6-7).
A key finding in C. elegans is that BBS-4 and BBS-5 exhibit unexpected functional redundancy within the BBSome. Single bbs-4 mutants are largely normal in ciliogenesis and dye-filling assays, though they do show defective polycystin-mediated mechanosensory signaling (xu2015bbs4andbbs5 pages 2-4). However, bbs-4; bbs-5 double mutants display severe ciliary defects, including abnormal cilia biogenesis, disrupted IFT velocities (with IFT-A and IFT-B complexes separating and moving at distinct speeds), and aberrant accumulation of ciliary sensory receptors such as PKD-2, OSM-9, and ODR-10 (xu2015bbs4andbbs5 pages 2-4, wingfield2018traffickingofciliary pages 4-5). BBS-4 directly interacts with BBS-5 through its carboxyl-terminus, and a conserved mutation (A388E) in BBS-4 can disrupt this interaction and impair ciliary targeting (xu2015bbs4andbbs5 pages 6-7).
The double mutant phenotype reveals that BBS-4 and BBS-5 act redundantly within the BBSome to regulate the ciliary removal and lysosome-targeted degradative sorting of mono-ubiquitinated sensory receptors, rather than their ciliary entry or retrograde IFT transport per se (xu2015bbs4andbbs5 pages 6-7). This function is conserved in mammals, where BBS4 and BBS5 also interact directly and coordinate ciliary removal of polycystin 2 (xu2015bbs4andbbs5 pages 6-7).
BBS-4 contains multiple tetratricopeptide repeat (TPR) domains that fold into a characteristic ฮฑ-solenoid (superhelical) structure (singh2020structureandactivation pages 5-8, klink2020structureofthe pages 4-6). High-resolution cryo-EM structures of the human and bovine BBSome have revealed the atomic-level architecture of BBS4 within the complex:
The BBSome contains a prominent negatively charged cleft at its center, with contributions from multiple subunits, that likely mediates recognition of positively charged ciliary targeting sequences on cargo proteins (klink2020structureofthe pages 2-4, klink2020structureofthe pages 11-12). A large positively charged patch on the complex surface strengthens membrane association (klink2020structureofthe pages 2-4).
The BBSome assembles through a regulated, sequential pathway. Three non-core BBS proteinsโBBS6, BBS10, and BBS12โform a chaperonin complex with CCT/TRiC proteins that first stabilizes BBS7 (zhang2012intrinsicproteinproteininteractionmediated pages 6-8, tian2023organizationfunctionsand pages 5-6). BBS7 then forms a ternary core complex with BBS2 and BBS9 (the "BBSome core complex"). Subsequently, BBS1, BBS5, and BBS8 are incorporated through intrinsic protein-protein interactions with BBS9, which serves as the organizational hub (tian2023organizationfunctionsand pages 6-7). BBS4 is the last subunit added to complete the BBSome, and its incorporation requires BBS1 (zhang2012intrinsicproteinproteininteractionmediated pages 6-8, zhang2012intrinsicproteinproteininteractionmediated pages 8-10, zhang2012intrinsicproteinproteininteractionmediated pages 2-4). This peripheral position in the assembly hierarchy is consistent with the observation that BBS4 localizes to the periphery of the complex and has a dispensable role in BBSome assembly per se (tian2023organizationfunctionsand pages 7-9).
In C. elegans, BBS-4 localizes to the cilia of ciliated sensory neurons and to the ciliary base (xu2015bbs4andbbs5 pages 6-7, wei2012thebbsomecontrols pages 2-4). GFP-tagged BBS-4 can be observed along the length of cilia from base to tip. In dyf-2 (WDR19 ortholog) mutants, BBS-4 loses ciliary localization and instead accumulates strongly around the ciliary base, indicating that DYF-2 is required for loading the BBSome onto anterograde IFT trains for ciliary entry (wei2012thebbsomecontrols pages 2-4). Despite this accumulation, the BBSome complex itself still forms but remains restricted to the base (wei2012thebbsomecontrols pages 2-4).
In mammalian cells, BBS4 localizes to centriolar satellites, the centrosome/basal body, and the periciliary region (novas2015bardetโbiedlsyndromeis pages 4-4, novas2015bardetโbiedlsyndromeis pages 3-4). BBS4 interacts with PCM1 (the main component of pericentriolar satellites) and AZI1 (another centriolar satellite protein), and must properly relocalize from satellites to the cilium for recruitment of other BBSome components like BBS8 (novas2015bardetโbiedlsyndromeis pages 3-4, novas2015bardetโbiedlsyndromeis pages 4-4). In mammalian olfactory sensory neurons, BBS4 localizes to basal bodies and ciliary knobs and undergoes bidirectional IFT movement along the entire ciliary length (williams2014directevidencefor pages 7-8).
The BBSome, including BBS-4, plays critical roles in IFT complex organization and turnaround in C. elegans cilia:
The BBSome functions as a multivalent cargo adapter for ciliary membrane proteins. In C. elegans, BBS-4 (together with BBS-5) is required for proper homeostasis of several ciliary sensory receptors:
In bbs-4; bbs-5 double mutants, these receptors abnormally accumulate in cilia due to disrupted lysosome-targeted degradative sorting of ubiquitinated receptors (xu2015bbs4andbbs5 pages 6-7, wingfield2018traffickingofciliary pages 4-5). This demonstrates that BBS-4 functions in a post-ciliary export degradative sorting pathway rather than solely in retrograde IFT.
In vertebrate systems, the BBSome (of which BBS4 is a subunit) mediates ciliary export of GPCRs including Smoothened (SMO), GPR161, SSTR3, and MCHR1 (yang2020nearatomicstructuresof pages 12-13, tian2023organizationfunctionsand pages 16-18, tian2023organizationfunctionsand pages 15-16). The BBSome recognizes specific motifs in cargo proteins, including the [W/F/Y][K/R] motif in GPCR helix 8, the Ax[S/A]xQ consensus in intracellular loops, and C-terminal VxP sequences (yang2020nearatomicstructuresof pages 12-13, tian2023organizationfunctionsand pages 15-16). BBSome-mediated GPCR removal is critical for Hedgehog signaling (via SMO and GPR161 trafficking) and other cilium-dependent signaling pathways (tian2023organizationfunctionsand pages 16-18, wingfield2018traffickingofciliary pages 5-7).
Beyond ciliary trafficking, BBS4/BBS-4 has been implicated in several non-ciliary processes:
- Actin cytoskeleton regulation: Bbs4-deficient mammalian cells show disrupted actin stress fibers, upregulated RhoA-GTP levels, impaired cell migration, adhesion, and division, and abnormal focal adhesion formation (tian2023organizationfunctionsand pages 9-11).
- Centriolar satellite organization: BBS4 recruits BBSome subunits to centriolar satellites and interacts with PCM1 and the dynein/dynactin motor complex via p150Glued (novas2015bardetโbiedlsyndromeis pages 4-4).
- Additional reported roles include Notch recycling, transcriptional regulation of RNF2, and pericentriolar cargo targeting (novas2015bardetโbiedlsyndromeis pages 3-4).
The human ortholog BBS4 is causally associated with Bardet-Biedl syndrome type 4 (OMIM), a ciliopathy characterized by retinal degeneration, obesity, polydactyly, renal abnormalities, and cognitive impairment (OpenTargets Search: -BBS4). BBS4 mutations are also associated with retinitis pigmentosa and polydactyly (OpenTargets Search: -BBS4). The conserved role of BBS-4 in ciliary sensory receptor trafficking in C. elegans provides a mechanistic basis for understanding the sensory and developmental deficits observed in BBS patients.
C. elegans BBS-4 is a TPR-containing structural adaptor protein that functions as a subunit of the octameric BBSome complex. It is the last subunit incorporated during BBSome assembly and localizes to cilia and the ciliary base in sensory neurons. BBS-4's primary molecular function is to contribute to the BBSome's role as a cargo adapter that couples ciliary membrane proteins (sensory receptors and GPCRs) to the IFT machinery for regulated ciliary export and degradative sorting. BBS-4 exhibits functional redundancy with BBS-5 in C. elegans, and their combined loss leads to severe defects in cilia structure, IFT organization, and sensory receptor homeostasis. The protein's TPR ฮฑ-solenoid architecture mediates critical protein-protein interactions within the BBSome body domain, connecting to BBS1, BBS8, BBS9, and BBS18. These findings, derived from genetic, biochemical, and structural studies in C. elegans and mammalian systems, establish BBS-4 as an essential component of the ciliary trafficking machinery with conserved roles from nematodes to humans.
References
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(williams2014directevidencefor pages 7-8): Corey L. Williams, Jeremy C. McIntyre, Stephen R. Norris, Paul M. Jenkins, Lian Zhang, Qinglin Pei, Kristen Verhey, and Jeffrey R. Martens. Direct evidence for bbsome-associated intraflagellar transport reveals distinct properties of native mammalian cilia. Nature Communications, Dec 2014. URL: https://doi.org/10.1038/ncomms6813, doi:10.1038/ncomms6813. This article has 187 citations and is from a highest quality peer-reviewed journal.
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(yang2020nearatomicstructuresof pages 12-13): Shuang Yang, Kriti Bahl, Hui-Ting Chou, Jonathan Woodsmith, Ulrich Stelzl, Thomas Walz, and Maxence V Nachury. Near-atomic structures of the bbsome reveal the basis for bbsome activation and binding to gpcr cargoes. Jun 2020. URL: https://doi.org/10.7554/elife.55954, doi:10.7554/elife.55954. This article has 56 citations and is from a domain leading peer-reviewed journal.
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UniProt: Q5CZ52 (BBS4_CAEEL). WormBase: WBGene00043992 / F58A4.14. 462 aa.
Gene: bbs-4 (Bardet-Biedl syndrome 4 protein homolog / "BBSome complex member bbs-4").
BBS-4 is a core structural subunit of the BBSome, the octameric complex (bbs-1, bbs-2,
bbs-4, bbs-5, osm-12/bbs-7, bbs-8/ttc-8, bbs-9) that couples intraflagellar transport (IFT)
to ciliary membrane-cargo trafficking. BBS-4 is a tetratricopeptide-repeat (TPR) protein
(UniProt annotates 7 TPR repeats spanning ~89โ402, with a disordered N-terminal region
1โ46). The BBSome shares structural features with the COPI/COPII/clathrin vesicle coats and
is thought to act as a membrane coat/cargo adaptor at cilia.
protein binding. This is the same structural-subunit patternbbs-4-deep-research-falcon.md was generated by the falcon provider (Edison Scientific
Literature; 36 citations, ~29 min run; the just-recipe wrapper reported a 600s subprocess
timeout but the client completed and wrote real output). It corroborates every point above
and adds cross-species structural/assembly context (not in the two worm papers, cited by DOI
in that report, so used here only as background, not as annotation provenance):
These DOI-only sources are recorded here as background; the review's annotation provenance
uses only the two cached full-text worm papers (PMID:22922713, PMID:26150102) plus the falcon
file reference.
id: Q5CZ52
gene_symbol: bbs-4
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:6239
label: Caenorhabditis elegans
description: >-
BBS-4 is a tetratricopeptide-repeat (TPR) protein that is a peripheral structural
subunit of the BBSome, an octameric complex (BBS-1, BBS-2, BBS-4, BBS-5, OSM-12/BBS-7,
BBS-8/TTC-8, BBS-9) that couples intraflagellar transport (IFT) to the trafficking of
membrane cargo at sensory cilia. Like the COPI, COPII and clathrin coats it structurally
resembles, the BBSome assembles IFT particles at the ciliary base, binds the anterograde
IFT particle, and reaches the ciliary tip where it regulates IFT turnaround and recycling.
Within the complex BBS-4 binds directly to BBS-5 through its C-terminal TPR region and
localizes to the ciliary base/basal body and along cilia. In C. elegans, BBS-4 is
functionally redundant with BBS-5: single mutants have essentially normal cilia, whereas
bbs-4; bbs-5 double mutants phenocopy loss of the whole BBSome, showing IFT-A/IFT-B
uncoupling, disrupted ciliogenesis, and defective polycystin-mediated cilia signaling.
Beyond building cilia, BBS-4 (redundantly with BBS-5) is required for the removal of
ciliary sensory receptors โ including polycystin-2/PKD-2, the TRP channel OSM-9 and the
GPCR ODR-10 โ from cilia for lysosome-targeted degradation, acting upstream of the early
endosome at the ciliary base. A conserved C-terminal residue (A388 in the worm protein,
A364 in human BBS4) is required for the BBS-4โBBS-5 interaction and for ciliary targeting,
and its mutation models human Bardet-Biedl syndrome.
alternative_products:
- name: a
id: Q5CZ52-1
- name: b
id: Q5CZ52-2
sequence_note: VSP_044208
references:
- id: GO_REF:0000024
title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
by curator judgment of sequence similarity
findings: []
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000044
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
vocabulary mapping, accompanied by conservative changes to GO terms applied by
UniProt
findings: []
- id: PMID:22922713
title: The BBSome controls IFT assembly and turnaround in cilia.
findings:
- statement: The BBSome assembles IFT particles at the ciliary base and regulates
IFT turnaround/recycling at the ciliary tip.
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- statement: The BBSome binds the IFT particle as a cargo, not as an integral
structural component.
supporting_text: the BBSome binds to the IFT particle like a cargo but not a
structural component
- statement: BBS-4 loses ciliary localization and accumulates at the ciliary base
when the BBSome is uncoupled from moving IFT.
supporting_text: Some of them (BBS-1, BBS-4) totally lost the ciliary localization
- statement: The BBSome shares coat features with COPI/COPII/clathrin and can
directly recognize IFT cargo.
supporting_text: shares the common structural features with COPI, COPII, and
clathrin coats, and can directly recognize IFT cargos
reference_review:
relevance: HIGH
correctness: VERIFIED
review_notes: >-
PubMed-verified (Wei et al. 2012, Nat Cell Biol; PMC3434251). Full text cached.
C. elegans forward-genetic study establishing the BBSome (including BBS-4) as the
regulator of IFT assembly and turnaround; directly supports the BBSome-membership,
cilium-assembly and ciliary-localization annotations for bbs-4.
- id: PMID:26150102
title: BBS4 and BBS5 show functional redundancy in the BBSome to regulate the
degradative sorting of ciliary sensory receptors.
findings:
- statement: BBS-4 directly interacts with BBS-5 through its C-terminal region
(a.a. 270-462).
supporting_text: mapped down the binding region in BBS-4 to the C-terminal 193
amino acids (a.a. 270โ462)
- statement: bbs-4 and bbs-5 single mutants have normal cilia; bbs-4; bbs-5 double
mutants phenocopy whole-BBSome loss.
supporting_text: bbs-4 or bbs-5 single mutants are completely normal in dye-filling
assay
- statement: BBS-4 and BBS-5 redundantly regulate lysosome-targeted degradation of
ciliary sensory receptors.
supporting_text: BBS-4 and BBS-5 play redundant role in the BBSome to ubiquitously
regulate the lysosome-targeted degradation of ciliary sensory receptors
- statement: The BBSome acts upstream of the early endosome at the ciliary base to
direct receptors to lysosomal degradation.
supporting_text: the BBSome acts upstream of the early endosome, probably in
endocytic stage at cilia base, to regulate the lysosomal sorting of ciliary
receptors
reference_review:
relevance: HIGH
correctness: VERIFIED
review_notes: >-
PubMed-verified (Xu et al. 2015, Sci Rep; PMC4493597). Full text cached. The
primary experimental paper for C. elegans bbs-4: GST pull-down + BiFC show direct
BBS-4โBBS-5 interaction via the BBS-4 C-terminus; genetic redundancy in
ciliogenesis, IFT integrity, polycystin signaling; and a conserved role in
degradative removal of ciliary receptors. Also cited by UniProt for the FUNCTION,
INTERACTION, DISRUPTION PHENOTYPE and MUTAGENESIS of E107 and A388.
- id: file:worm/bbs-4/bbs-4-deep-research-falcon.md
title: Deep research report on bbs-4 (Edison/falcon)
findings: []
existing_annotations:
- term:
id: GO:0034464
label: BBSome
evidence_type: NAS
original_reference_id: PMID:22922713
qualifier: part_of
review:
summary: >-
BBS-4 is a bona fide subunit of the BBSome, the octameric complex containing
BBS-1, BBS-2, BBS-4, BBS-5, OSM-12/BBS-7, BBS-8/TTC-8 and BBS-9. This is the core
cellular-component assignment for bbs-4 and is supported experimentally in
C. elegans, where BBS-4 co-behaves with the other BBSome subunits during IFT.
action: ACCEPT
reason: >-
Complex membership is the defining property of bbs-4 and is strongly supported by
both C. elegans experiments and cross-species orthology. This is a core annotation.
supported_by:
- reference_id: PMID:22922713
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- reference_id: PMID:26150102
supporting_text: during the assembly of the BBSome, BBS2, 7, and 9 form the
core, then BBS1, 5, 8, and finally BBS4 are added in a stepwise manner
- reference_id: file:worm/bbs-4/bbs-4-deep-research-falcon.md
supporting_text: 'model: Edison Scientific Literature'
- term:
id: GO:0030674
label: protein-macromolecule adaptor activity
evidence_type: ISS
original_reference_id: GO_REF:0000024
qualifier: enables
review:
summary: >-
This adaptor-activity term is transferred by sequence similarity from human BBS4
(UniProtKB:Q96RK4). It captures the fact that BBS-4, as a TPR-repeat subunit of the
coat-like BBSome, acts as a scaffold/adaptor rather than an enzyme. It is far more
informative than a generic 'protein binding' term, and reflects the best current
molecular-function description for a BBSome coat subunit.
action: ACCEPT
reason: >-
Adaptor activity is the most defensible molecular function for a BBSome structural
subunit and is preferable to uninformative protein binding. However, no experimental
MF has been measured for BBS-4 itself, and no GO term specifically expresses the
BBSome coat/cargo-adaptor role โ see knowledge_gaps.
supported_by:
- reference_id: PMID:22922713
supporting_text: shares the common structural features with COPI, COPII, and
clathrin coats, and can directly recognize IFT cargos
- term:
id: GO:0060271
label: cilium assembly
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: involved_in
review:
summary: >-
BBS-4 contributes to cilium assembly as part of the BBSome. In C. elegans this is
genetically redundant with BBS-5: single mutants are normal, but bbs-4; bbs-5 double
mutants show typical bbs-class cilia defects, confirming a real (if buffered) role
in ciliogenesis.
action: ACCEPT
reason: >-
Well supported by phylogenetic inference and by direct C. elegans genetics
(redundancy with bbs-5). A core biological-process annotation.
supported_by:
- reference_id: PMID:26150102
supporting_text: bbs-4; bbs-5 double mutants show typical cilia defect as
observed in other bbs mutants
- reference_id: PMID:22922713
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- term:
id: GO:0060271
label: cilium assembly
evidence_type: NAS
original_reference_id: PMID:22922713
qualifier: involved_in
review:
summary: >-
Author-stated (NAS) assignment of the cilium-assembly role, duplicating the IBA
annotation. The BBSome assembles IFT particles required for ciliogenesis.
action: ACCEPT
reason: >-
Correct and consistent with the IBA annotation and with C. elegans genetics; both
are retained as complementary evidence for the same core role.
supported_by:
- reference_id: PMID:22922713
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- term:
id: GO:0061512
label: protein localization to cilium
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: involved_in
review:
summary: >-
As a BBSome subunit, BBS-4 is required for correct localization of membrane and
signaling proteins to/from cilia. In C. elegans the BBSome (redundantly requiring
bbs-4/bbs-5) governs the ciliary homeostasis of sensory receptors such as PKD-2,
OSM-9 and ODR-10, which mislocalize and accumulate when the complex is disrupted.
action: ACCEPT
reason: >-
Central to BBSome function and supported by both phylogenetic inference and
C. elegans experiments. Core biological-process annotation.
supported_by:
- reference_id: PMID:26150102
supporting_text: with ~6-fold increasing of protein levels in their native
expressing cilia in bbs-4; bbs-5 mutants
- term:
id: GO:0036064
label: ciliary basal body
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
BBS-4 acts at the ciliary basal body / ciliary base, where the BBSome assembles IFT
particles prior to anterograde transport. Supported phylogenetically and consistent
with direct C. elegans localization data.
action: ACCEPT
reason: >-
Basal-body localization is a core, well-supported cellular-component annotation and
matches the site of BBSome-mediated IFT assembly.
supported_by:
- reference_id: PMID:22922713
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- term:
id: GO:0036064
label: ciliary basal body
evidence_type: IDA
original_reference_id: PMID:22922713
qualifier: located_in
review:
summary: >-
Direct experimental (IDA) localization of BBS-4 to the ciliary basal body/base in
C. elegans, consistent with BBSome accumulation around the ciliary base when it is
uncoupled from moving IFT.
action: ACCEPT
reason: >-
Experimental localization; the strongest evidence class for the basal-body
assignment. Core cellular-component annotation.
supported_by:
- reference_id: PMID:22922713
supporting_text: Some of them (BBS-1, BBS-4) totally lost the ciliary localization
- term:
id: GO:0005929
label: cilium
evidence_type: NAS
original_reference_id: PMID:22922713
qualifier: located_in
review:
summary: >-
BBS-4 localizes to cilia as a BBSome subunit that undergoes IFT along the ciliary
axoneme. A correct, if general, cellular-component assignment (the basal-body and
ciliary-base terms are more specific).
action: KEEP_AS_NON_CORE
reason: >-
Accurate general localization supported by the referenced work, but non-core
relative to the more specific basal-body/ciliary-base terms which are the core CC
annotations; retained as a valid non-core localization.
supported_by:
- reference_id: PMID:22922713
supporting_text: Some of them (BBS-1, BBS-4) totally lost the ciliary localization
- term:
id: GO:0060170
label: ciliary membrane
evidence_type: IEA
original_reference_id: GO_REF:0000044
qualifier: located_in
review:
summary: >-
Ciliary-membrane localization is inferred from the UniProt subcellular-location
mapping (Cell projection, cilium membrane, By similarity). It is consistent with the
BBSome's role as a membrane-associated coat that traffics and removes ciliary
membrane receptors.
action: ACCEPT
reason: >-
Consistent with BBSome coat function at the ciliary membrane and with the receptor
trafficking/removal role demonstrated for bbs-4/bbs-5.
- term:
id: GO:0005813
label: centrosome
evidence_type: IEA
original_reference_id: GO_REF:0000044
qualifier: located_in
review:
summary: >-
Centrosome localization is an electronic UniProt subcellular-location mapping
derived from the mammalian BBS4 annotation (Cytoplasm, cytoskeleton, MTOC,
centrosome, By similarity). In C. elegans the relevant structure is the
centriole-derived ciliary basal body, which is separately annotated; a distinct
centrosomal pool has not been demonstrated for worm BBS-4.
action: KEEP_AS_NON_CORE
reason: >-
The basal body is centriole-derived, so a centrosomal assignment is not wrong, but
it is a non-specific/derived term for this organism where the experimentally
supported site is the ciliary basal body/base. Kept as non-core rather than removed.
- term:
id: GO:0005856
label: cytoskeleton
evidence_type: IEA
original_reference_id: GO_REF:0000044
qualifier: located_in
review:
summary: >-
High-level cytoskeleton localization from the UniProt subcellular-location mapping.
This is a very general parent term encompassing the more specific ciliary/basal-body
localizations.
action: KEEP_AS_NON_CORE
reason: >-
Not incorrect but uninformative relative to the specific ciliary basal body and
ciliary base terms. Retained as non-core.
- term:
id: GO:0000242
label: pericentriolar material
evidence_type: ISS
original_reference_id: GO_REF:0000024
qualifier: located_in
review:
summary: >-
Pericentriolar-material localization is transferred by similarity from human BBS4,
which has a mammalian pericentriolar/centriolar-satellite role. C. elegans BBS-4 has
been characterized only in the ciliary/BBSome context, and nematodes largely lack
the mammalian centriolar-satellite system; no worm experiment supports a
pericentriolar-material localization.
action: MARK_AS_OVER_ANNOTATED
reason: >-
This is a mammalian-derived ISS transfer with no experimental support in C. elegans
and is not part of the conserved core BBSome/ciliary function assayed in this
organism. Flagged as over-annotation for the worm rather than a core localization.
- term:
id: GO:0007098
label: centrosome cycle
evidence_type: ISS
original_reference_id: GO_REF:0000024
qualifier: involved_in
review:
summary: >-
The centrosome-cycle process is transferred by similarity from mammalian BBS4,
which has been implicated in microtubule anchoring and dynein-mediated
pericentriolar transport. There is no C. elegans evidence that BBS-4 participates in
the centrosome cycle; all worm phenotypes concern cilia and IFT.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Mammalian-derived ISS process transfer without worm support and outside the
conserved core ciliary role. Flagged as over-annotation for this organism.
- term:
id: GO:0097546
label: ciliary base
evidence_type: IDA
original_reference_id: PMID:22922713
qualifier: located_in
review:
summary: >-
BBS-4 and the other BBSome subunits concentrate at the ciliary base, where the
BBSome assembles IFT particles; when the BBSome is uncoupled from moving IFT the
proteins strongly accumulate there. This is a more precise localization than the
general 'cilium' term and complements the ciliary basal body annotation.
action: NEW
reason: >-
Ciliary base is the experimentally supported site of BBSome-mediated IFT assembly
in C. elegans and is included as a location in core_functions; added here as a
specific cellular-component annotation.
supported_by:
- reference_id: PMID:22922713
supporting_text: all BBS proteins examined strongly accumulated around the
ciliary base
- term:
id: GO:0042073
label: intraciliary transport
evidence_type: IMP
original_reference_id: PMID:22922713
qualifier: involved_in
review:
summary: >-
BBS-4, as a BBSome subunit, itself undergoes IFT movement along the ciliary axoneme
and is required for normal intraflagellar transport; loss of the BBSome (e.g. in
bbs-4; bbs-5 double mutants) uncouples IFT-A from IFT-B and disrupts IFT integrity.
action: NEW
reason: >-
Intraciliary transport is a core biological process for the BBSome, captured in
core_functions; added here as a specific process annotation supported by BBS-protein
IFT movement and the IFT defects of bbs mutants.
supported_by:
- reference_id: PMID:22922713
supporting_text: all BBS proteins completely lost IFT movement
- reference_id: PMID:26150102
supporting_text: bbs-4; bbs-5 and bbs-7 share similar mutant phenotypes in that
CHE-11 is absent, but OSM-6 abnormally accumulates, in the distal segments of
plasmid cilia
core_functions:
- description: >-
BBS-4 acts as a TPR-repeat structural/adaptor subunit of the coat-like BBSome. Through
its C-terminal TPR region it binds BBS-5 and integrates into the complex, contributing
to BBSome-mediated assembly of IFT particles at the ciliary base and to the
IFT-coupled trafficking and degradative removal of ciliary membrane cargo. Its
molecular activity is best described as a protein-macromolecule adaptor within the
BBSome coat; no independent enzymatic activity is known.
molecular_function:
id: GO:0030674
label: protein-macromolecule adaptor activity
directly_involved_in:
- id: GO:0061512
label: protein localization to cilium
- id: GO:0060271
label: cilium assembly
- id: GO:0042073
label: intraciliary transport
locations:
- id: GO:0036064
label: ciliary basal body
- id: GO:0097546
label: ciliary base
- id: GO:0060170
label: ciliary membrane
in_complex:
id: GO:0034464
label: BBSome
supported_by:
- reference_id: PMID:22922713
supporting_text: the in vivo function for the BBSome is to regulate the assembly
of the IFT particles at the ciliary base
- reference_id: PMID:26150102
supporting_text: mapped down the binding region in BBS-4 to the C-terminal 193
amino acids (a.a. 270โ462)
knowledge_gaps:
- gap_statement: >-
There is no GO molecular-function term that expresses the role of a BBSome coat/cargo-
adaptor subunit, and the specific cargo(es) that BBS-4's TPR array directly recognizes
within the BBSome are undefined. BBS-4 has no experimentally measured biochemical
activity of its own; its sole MF annotation (protein-macromolecule adaptor activity)
is a generic sequence-similarity transfer.
boundary: >-
BBS-4 is an established BBSome subunit built from tetratricopeptide repeats; the BBSome
is explicitly coat-like (shares structural features with COPI, COPII and clathrin) and
is proposed to polymerize and recognize ciliary membrane proteins, and it can directly
recognize IFT cargo. What is missing is a molecular-function term (and the direct
cargo identity) for the coat subunit rather than the complex-level process.
gap_kind:
- ONTOLOGY
- BIOLOGY
dark_aspect: MF_DARK
status: OPEN
significance: >-
BBSome subunits illustrate the structural-subunit ontology gap: their function is 'be
part of the coat', which the GO molecular-function aspect cannot currently express, so
the gene reads as MF-dark despite a well-defined cellular role central to Bardet-Biedl
syndrome.
resolution: >-
Ontology development of a BBSome coat/cargo-adaptor molecular-function term (analogous
to a vesicle-coat adaptor), plus proximity/affinity proteomics and structural work to
identify the membrane cargo directly contacted by the BBS-4 TPR surface.
provenance:
- reference_id: PMID:22922713
supporting_text: shares the common structural features with COPI, COPII, and
clathrin coats, and can directly recognize IFT cargos
reference_section_type: DISCUSSION
- reference_id: PMID:26150102
supporting_text: has been proposed to polymerize and recognize the ciliary membrane
proteins
reference_section_type: INTRODUCTION
proposed_terms:
- proposed_name: BBSome coat cargo-adaptor activity
proposed_definition: >-
A protein-macromolecule adaptor activity of a subunit of the BBSome (a coat-like
complex structurally related to COPI/COPII/clathrin) that mediates recognition and
sorting of ciliary membrane cargo and its coupling to intraflagellar transport,
without the subunit independently catalyzing a biochemical reaction. Distinct from
the cellular component 'BBSome' (GO:0034464) this term names the molecular
activity a coat subunit contributes to BBSome-mediated cargo sorting.
justification: >-
BBSome subunits such as BBS-4 have no adequate GO molecular-function term; they are
annotatable only with the generic 'protein-macromolecule adaptor activity' or the
uninformative 'protein binding', leaving them MF-dark despite a well-defined
coat-subunit role.
proposed_parent:
id: GO:0030674
label: protein-macromolecule adaptor activity
- gap_statement: >-
The molecular mechanism by which the BBSome triggers degradative (lysosome-targeted)
removal of ciliary sensory receptors โ the direct effectors linking the ciliary-base
BBSome to the endocytic/ubiquitin machinery โ and the molecular basis of the
BBS-4/BBS-5 functional redundancy (two subunits with no shared domain, each dispensable
alone) are undetermined.
boundary: >-
bbs-4 and bbs-5 single mutants have normal cilia; bbs-4; bbs-5 double mutants phenocopy
whole-BBSome loss, with IFT-A/IFT-B uncoupling and ~3โ6-fold ciliary accumulation of
PKD-2, OSM-9 and ODR-10. RAB-5 overexpression rescues, and STAM-1 epistasis places the
BBSome upstream of the early endosome at the ciliary base for these non-IFT cargoes;
BBS-4 binds BBS-5 directly via its C-terminal TPR region. What remains unknown is the
biochemical output that couples the BBSome to receptor endocytosis/degradation.
gap_kind:
- BIOLOGY
dark_aspect: RESIDUAL_SUBGAP
status: OPEN
significance: >-
This receptor-downregulation activity is conserved to human BBS4/BBS5 and to
polycystin-2, directly relevant to Bardet-Biedl syndrome and polycystic kidney disease;
the missing effectors are the mechanistic link between the BBSome and ciliary signaling
homeostasis.
resolution: >-
Proximity/affinity proteomics of the ciliary-base BBSome to identify the endocytic/
ubiquitin effectors; separation-of-function bbs-4 alleles that uncouple BBS-5 binding
from cargo removal; and structural definition of how BBS-4 and BBS-5 co-contribute to a
shared coat surface.
provenance:
- reference_id: PMID:26150102
supporting_text: the definite molecular activity of the BBSome in regulating the
homoeostasis of ciliary membrane proteins remain unclear
reference_section_type: INTRODUCTION
- reference_id: PMID:26150102
supporting_text: two BBSome components that share no similar protein domains
reference_section_type: RESULTS
proposed_new_terms: []
suggested_questions:
- question: Does BBS-4's TPR surface directly contact a specific ciliary membrane cargo
within the BBSome, or does it act purely as a scaffold linking BBS-5 into the coat?
- question: What endocytic/ubiquitin effectors at the ciliary base connect the BBSome to
lysosome-targeted degradation of sensory receptors?
suggested_experiments:
- description: Proximity-dependent biotinylation (TurboID) or affinity proteomics of BBS-4
at the ciliary base in wild-type versus bbs-5 mutant worms to identify cargo and
endocytic/ubiquitin effectors.
hypothesis: BBS-4 within the BBSome physically links ciliary sensory receptors to the
early-endosome/lysosome sorting machinery at the ciliary base.
experiment_type: proteomics
- description: Structure-guided separation-of-function alleles of bbs-4 that abolish BBS-5
binding (C-terminal TPR) while preserving BBSome incorporation, scored for ciliogenesis,
IFT integrity, and receptor removal.
hypothesis: The BBS-4โBBS-5 interaction is specifically required for receptor
degradative sorting, separable from bulk IFT assembly.
experiment_type: genetics
tags:
- caeel-ciliopathy