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 — including polycystin-2/PKD-2, the TRP channel OSM-9 and the GPCR ODR-10 — 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-4–BBS-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 — 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 |
Loading supporting content…
Download this section (compressed HTML)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-4–BBS-5 interaction is specifically required for receptor degradative sorting, separable from bulk IFT assembly.
Type: genetics
What is not known — 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 — the direct effectors linking the ciliary-base BBSome to the endocytic/ubiquitin machinery — 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 ~3–6-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):
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)