ARL6 (BBS3) is a 186-residue ADP-ribosylation factor-like small GTPase of the Arf family within the Ras superfamily. Like canonical Arf/Sar1 GTPases it cycles between an inactive GDP-bound and an active GTP-bound state; GTP loading exposes an N-terminal amphipathic helix that inserts into membranes, so the GTP-bound form is peripherally associated with the cytoplasmic face of the ciliary membrane. The defining activity of ARL6 is to act as the membrane-targeting switch for the BBSome, an octameric Bardet-Biedl syndrome protein complex that functions as a coat sorting membrane proteins into and out of primary cilia. ARL6 is not itself a structural subunit of the BBSome; rather, GTP-bound ARL6 binds the BBS1 beta-propeller and recruits the BBSome onto the ciliary membrane, where it nucleates polymerization of the BBSome coat. Through this activity ARL6 controls ciliary trafficking of signaling receptors (e.g. SSTR3, MCHR1, Smoothened) and contributes to Hedgehog and Wnt signaling. ARL6 localizes to the primary cilium, concentrating in a ring at the distal basal body near the ciliary gate and in membrane-associated patches flanking the axoneme. The intraflagellar transport protein IFT27/RABL4 binds nucleotide-free ARL6 to promote its activation and BBSome exit from cilia. Loss-of-function mutations that impair GTP binding cause Bardet-Biedl syndrome type 3 and retinitis pigmentosa 55.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0006886 intracellular protein transport | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: General phylogenetic annotation. ARL6 functions in ciliary membrane protein trafficking, so intracellular protein transport is correct but high-level and not the most informative description of its role. Reason: Consistent with ARL6's role recruiting the BBSome coat to traffic membrane proteins to cilia, but too general to represent the core function. The specific child term protein localization to cilium is the more informative process. |
| GO:0005525 GTP binding | IBA GO_REF:0000033 | ACCEPT | Summary: ARL6 is an Arf-family small GTPase whose GTP-bound state is the active, membrane-binding, BBSome-recruiting form. GTP binding is a core molecular function. Reason: Strongly supported experimentally (crystal structure of GTP-bound ARL6, PDB 2H57; BBS variants abrogate GTP binding) and is a core molecular function of this GTPase. |
| GO:0005930 axoneme | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: ARL6 appears in punctae/membrane patches flanking the microtubule axoneme and in the ciliary membrane, but it is a peripheral membrane GTPase, not a structural axonemal component. Reason: ARL6 is detected near the axoneme as membrane-associated patches (PMID:20603001), but its activity is at the ciliary membrane; axoneme is acceptable as a localization but not core and risks over-specifying a structural axonemal role. |
| GO:0060271 cilium assembly | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Overproduction of GDP- or GTP-locked ARL6 alters cilium length and abundance, and loss causes ciliary trafficking defects, but cilia still form without ARL6; its role is modulatory rather than core ciliogenesis. Reason: Supported as a modulator of ciliary length/abundance (PMID:20207729) but ARL6 is a membrane-cargo trafficking switch, not a core axoneme-assembly factor. |
| GO:0061512 protein localization to cilium | IBA GO_REF:0000033 | ACCEPT | Summary: ARL6 recruits the BBSome to traffic membrane proteins to/from cilia; protein localization to cilium is a core downstream biological process for ARL6. Reason: Well supported (PMID:20603001, PMID:22139371). This is the central biological process ARL6 enables via BBSome recruitment. |
| GO:0003924 GTPase activity | IEA GO_REF:0000002 | ACCEPT | Summary: ARL6 is a small GTPase; nucleotide cycling (GTP binding and hydrolysis) underlies its switch function. GTPase activity is a core molecular function. Reason: Consistent with the Arf-family GTPase identity (UniProt; PDB 2H57). The Q73L hydrolysis-deficient variant studies confirm a hydrolyzable GTPase. Core MF. |
| GO:0005525 GTP binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based electronic annotation duplicating the experimentally supported GTP binding function. Reason: Core molecular function, independently supported by structure and biochemistry. |
| GO:0061512 protein localization to cilium | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA electronic annotation duplicating the experimentally supported protein localization to cilium process. Reason: Core biological process, independently supported by experimental evidence. |
| GO:0005515 protein binding | IPI PMID:20603001 The conserved Bardet-Biedl syndrome proteins assemble a coat... | KEEP AS NON CORE | Summary: IPI with BBS1 (Q8NFJ9). GTP-bound ARL6 binds the N-terminus of BBS1, recruiting the BBSome. The interaction is real and central, but the bare term protein binding is uninformative. Reason: Verified interaction with BBS1, but per curation guidance protein binding is not informative as a molecular function; the BBSome-recruitment role is captured by the trafficking process terms. A more informative MF (e.g. molecular adaptor/effector of the BBSome) is not currently in the ontology annotation set. |
| GO:0005515 protein binding | IPI PMID:22139371 Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals ... | KEEP AS NON CORE | Summary: IPI with BBS1 (Q8NFJ9). Endogenous BBS3 and the BBSome physically interact and depend on each other for ciliary localization. Reason: Verified interaction supporting BBSome recruitment, but protein binding is uninformative as a standalone MF. |
| GO:0005737 cytoplasm | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: ARL6 cycles between a soluble cytosolic GDP-bound form and a membrane-bound GTP-bound form, so cytoplasmic localization is expected. Reason: Consistent with a peripheral membrane GTPase that has a cytosolic pool; not the functionally salient compartment (cilium/cilium membrane is). |
| GO:0005829 cytosol | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: Cytosolic (GDP-bound) pool of ARL6 consistent with its nucleotide-dependent membrane cycling. Reason: Reasonable for the inactive soluble pool; not the core functional location. |
| GO:0016020 membrane | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: GTP-bound ARL6 is a peripheral membrane protein that inserts an N-terminal amphipathic helix into the (ciliary) membrane. Reason: Correct but high-level; the specific cilium/cilium membrane localization is more informative. |
| GO:0045444 fat cell differentiation | IEA GO_REF:0000107 | MARK AS OVER ANNOTATED | Summary: Electronic transfer from mouse ortholog. Obesity/adiposity is a BBS phenotype, but a direct role of ARL6 in adipocyte differentiation is indirect and unestablished. Reason: Reflects a systemic ciliopathy phenotype rather than a demonstrated molecular/cellular role of ARL6 in fat cell differentiation; weak electronic ortholog transfer. |
| GO:0097499 protein localization to non-motile cilium | IEA GO_REF:0000107 | KEEP AS NON CORE | Summary: ARL6 traffics membrane proteins to primary (non-motile) cilia, so this specific child of protein localization to cilium is appropriate. Reason: Consistent with ARL6/BBSome trafficking in primary cilia (PMID:20603001); redundant with the more frequently supported protein localization to cilium. |
| GO:0005929 cilium | IDA GO_REF:0000052 | ACCEPT | Summary: Direct immunofluorescence localization of ARL6 to the cilium (HPA). Endogenous ARL6 stains cilia and staining is lost on depletion. Reason: Well supported by multiple direct localization studies (PMID:20603001, PMID:22139371) plus HPA. |
| GO:0097542 ciliary tip | IDA GO_REF:0000052 | ACCEPT | Summary: HPA direct immunofluorescence places ARL6 at the ciliary tip, consistent with BBSome turnaround/exit at the tip and IFT-coupled trafficking. Reason: Direct experimental localization; consistent with BBSome trafficking dynamics along the cilium. |
| GO:0005929 cilium | TAS Reactome:R-HSA-5624127 | ACCEPT | Summary: Reactome traceable assertion of ciliary localization, consistent with experimental data. Reason: Consistent with direct localization evidence. |
| GO:0005515 protein binding | IPI PMID:25443296 The intraflagellar transport protein IFT27 promotes BBSome e... | KEEP AS NON CORE | Summary: IPI with IFT27/RABL4 (Q9BW83). IFT27 directly binds nucleotide-free ARL6, preventing its aggregation and promoting its activation and BBSome exit from cilia. Reason: Verified, biologically important interaction (PMID:25443296), but the bare protein binding term is uninformative as a molecular function. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-5624126 | KEEP AS NON CORE | Summary: Reactome assertion. ARL6 acts at the plasma/ciliary membrane interface; GTP-bound ARL6 binds the (ciliary) membrane where it recruits the BBSome and cargo. Reason: Defensible given membrane association at the ciliary base/gate; the ciliary membrane is the more precise compartment. |
| GO:0005886 plasma membrane | TAS Reactome:R-HSA-5624127 | KEEP AS NON CORE | Summary: Duplicate Reactome plasma membrane assertion. Reason: As above; cilium membrane is the more precise location. |
| GO:0005929 cilium | IDA PMID:22139371 Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals ... | ACCEPT | Summary: MGI direct localization of ARL6 to cilium (mouse Bbs3 study). Endogenous BBS3 localizes to cilia and depends on the BBSome for this localization. Reason: Direct experimental evidence supporting ciliary localization. |
| GO:0061512 protein localization to cilium | IMP PMID:22139371 Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals ... | ACCEPT | Summary: In Bbs3-/- mice, ARL6 loss disrupts ciliary localization of MCHR1 and affects Smoothened retrograde transport, demonstrating ARL6 is required for protein localization to cilium. Reason: Experimental IMP directly supporting the core biological process of ARL6. |
| GO:0005929 cilium | IDA NOT PMID:17646400 Functional dissection of Rab GTPases involved in primary cil... | ACCEPT | Summary: NOT located_in cilium from a Rab/Arf GTPase overexpression screen (Yoshimura et al.) in which only Rab8a was enriched at cilia; ARL6 was not enriched in that specific assay. This negative result is assay-context-specific and is contradicted by multiple direct endogenous localization studies showing ARL6 in cilia. Reason: Retain the curator-recorded negated annotation as a faithful record of that overexpression screen; it does not overturn the consensus that endogenous ARL6 localizes to cilia. Supporting Evidence: PMID:17646400 We show that the Rab GTPase membrane trafficking regulators Rab8a, -17, and -23, and their cognate GTPase-activating proteins (GAPs), XM_037557, TBC1D7, and EVI5like, are involved in primary cilia formation. However, other human Rabs and GAPs are not. |
| GO:0070062 extracellular exosome | HDA PMID:19056867 Large-scale proteomics and phosphoproteomics of urinary exos... | MARK AS OVER ANNOTATED | Summary: Detected in a large-scale urinary exosome proteomics study. This is a high-throughput proteomic catch-all not reflecting a defined biological localization of ARL6. Reason: High-throughput exosome proteomics frequently captures cytosolic/membrane proteins nonspecifically; not a functionally meaningful localization for ARL6. |
| GO:0005879 axonemal microtubule | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: ARL6 is a peripheral membrane GTPase associated with patches near the axoneme; it is not a structural component of axonemal microtubules. Reason: Over-specific structural localization by ortholog ISS transfer; ARL6 acts at the ciliary membrane, not as an axonemal microtubule component. |
| GO:0016055 Wnt signaling pathway | IMP PMID:20207729 Bardet-Biedl syndrome-associated small GTPase ARL6 (BBS3) fu... | KEEP AS NON CORE | Summary: ARL6/BBS3 modulates Wnt signaling and this function is lost in BBS-associated point mutants. A pleiotropic, cilium-dependent signaling role rather than a core molecular function. Reason: Experimentally supported (PMID:20207729) but pleiotropic/downstream of the core BBSome-trafficking activity; not a core function. |
| GO:0030117 membrane coat | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: The membrane coat assembled at the ciliary membrane is the polymerized BBSome. ARL6 recruits and nucleates this coat but is not itself a structural subunit of the coat. Reason: ARL6 is the recruiting GTPase, not a structural coat component; part_of membrane coat mischaracterizes its role (PMID:20603001 explicitly states Arl6 is not part of the BBSome). |
| GO:0060271 cilium assembly | IMP PMID:20207729 Bardet-Biedl syndrome-associated small GTPase ARL6 (BBS3) fu... | KEEP AS NON CORE | Summary: Overproduction of GDP-/GTP-locked ARL6 influences cilium length and abundance, supporting a modulatory role in ciliogenesis. Reason: Experimentally supported modulation of cilium length/number (PMID:20207729), but secondary to the core BBSome-cargo trafficking switch role. |
| GO:0005543 phospholipid binding | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: GTP-bound ARL6 binds membranes via an N-terminal amphipathic helix and synergizes with acidic phospholipids/multi-phosphorylated PIPs to recruit the BBSome. Reason: Supported by liposome reconstitution showing acidic phospholipid/PIP dependence (PMID:20603001); a real but ancillary molecular property supporting membrane targeting. |
| GO:0005930 axoneme | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ARL6 localizes to membrane patches flanking the axoneme; acceptable as a broad localization but not a structural axonemal role. Reason: Consistent with punctae flanking the axoneme (PMID:20603001); cilium/cilium membrane is the more accurate compartment. |
| GO:0006612 protein targeting to membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ARL6 directs membrane cargo proteins to the ciliary membrane via BBSome recruitment, consistent with the general process of protein targeting to membrane. Reason: General but consistent with ARL6's role in targeting membrane proteins to the cilium; protein localization to cilium is the more specific and informative term. |
| GO:0051258 protein polymerization | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: GTP-bound ARL6 nucleates polymerization of the BBSome into a membrane-apposed coat. ARL6 drives the polymerization although it does not itself polymerize. Reason: Defensible as involved_in BBSome coat polymerization (PMID:20603001 shows Arl6GTP-mediated formation of polymerized coat patches); not a core function term. |
| GO:0007368 determination of left/right symmetry | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: A plausible ciliopathy-associated developmental phenotype but no direct evidence that ARL6 itself acts in left/right axis determination; transferred by ISS from an ortholog. Reason: Indirect, phenotype-level inference via ortholog transfer; not supported by direct experimental evidence for human ARL6. |
| GO:0032402 melanosome transport | ISS GO_REF:0000024 | MARK AS OVER ANNOTATED | Summary: ISS transfer suggesting a melanosome transport role; no experimental support links human ARL6 to melanosome transport, and this is outside its established ciliary trafficking function. Reason: Questionable ortholog-based transfer with no supporting evidence for ARL6; not part of its characterized BBSome/ciliary function. |
| GO:0060271 cilium assembly | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS duplicate of the cilium assembly annotation; modulatory role in cilium length/abundance. Reason: Consistent with a modulatory ciliogenesis role; non-core. |
| GO:0005737 cytoplasm | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS duplicate of cytoplasmic localization; consistent with the soluble GDP-bound pool. Reason: Reasonable for the cytosolic pool; not the core functional location. |
| GO:0016020 membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: ISS duplicate of membrane localization; GTP-bound ARL6 is a peripheral membrane protein. Reason: Correct but high-level; cilium membrane is more informative. |
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Download this section (compressed HTML)Q: Does ARL6 have a dedicated guanine nucleotide exchange factor (GEF) at the ciliary gate, and is IFT27 acting as part of (or upstream of) that activation step?
Q: Is human ARL6 N-myristoylated in vivo (UniProt annotates N-myristoyl glycine by similarity), and how does this reconcile with reports that recombinant ARL6 binds membranes via an amphipathic helix independently of myristoylation?
Q: To what extent are the Wnt- and Hedgehog-signaling phenotypes direct ARL6 functions versus indirect consequences of disrupted BBSome cargo trafficking?
Experiment: Structure-function dissection of the ARL6 nucleotide cycle in cilia using live-cell imaging of GTP-/GDP-locked and myristoylation-deficient variants to quantify BBSome recruitment kinetics and cargo entry/exit.
Experiment: Proximity-labeling (BioID/TurboID) from GTP-locked vs nucleotide-free ARL6 in ciliated cells to define the full effector/regulator interactome (beyond BBS1 and IFT27) and identify a candidate ARL6 GEF/GAP.
Experiment: Reconstitute ARL6-driven BBSome coat polymerization on supported lipid bilayers with cryo-EM to determine the molecular architecture of the ARL6-nucleated BBSome coat and how BBS variants disrupt it.
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