Existing Annotations Review

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.

Core Functions

GTP-dependent molecular switch (Arf-family small GTPase) that, in its GTP-bound state, associates with the ciliary membrane and recruits the BBSome coat.

Molecular Function:

GTP binding

Supporting Evidence:

PMID:20603001
The BBSome is the major effector of the Arf-like GTPase Arl6/BBS3, and the BBSome and GTP-bound Arl6 colocalize at ciliary punctae in an interdependent manner.

Hydrolyzable small GTPase; nucleotide cycling between GTP- and GDP-bound states governs membrane association and BBSome recruitment.

Molecular Function:

GTPase activity

Supporting Evidence:

PMID:20207729
By determining the structure of GTP-bound ARL6/BBS3, coupled with functional assays, we provide a mechanistic explanation for such pathogenic alterations, namely altered nucleotide binding.

Recruits and nucleates polymerization of the BBSome coat on the ciliary membrane to sort membrane proteins into and out of primary cilia (protein localization to cilium).

Molecular Function:

GTP binding

Directly Involved In:

protein localization to cilium

Supporting Evidence:

PMID:20603001
Arl6(GTP)-mediated recruitment of the BBSome to synthetic liposomes produces distinct patches of polymerized coat apposed onto the lipid bilayer.
PMID:22139371
Loss of Bbs3 does not affect BBSome formation but disrupts normal localization of melanin concentrating hormone receptor 1 to ciliary membranes and affects retrograde transport of Smoothened inside cilia.
file:human/ARL6/ARL6-deep-research-falcon.md
triggering a conformational change that activates the BBSome for membrane protein trafficking

References

GO_REF:0000002

Gene Ontology annotation through association of InterPro records with GO terms

file:human/ARL6/ARL6-deep-research-falcon.md

Falcon deep research report for ARL6

Falcon deep research corroborates ARL6/BBS3 as the GTP-dependent membrane-targeting switch that recruits the BBSome coat to cilia, and refines the interface to a composite BBS1/BBS7 site exposed by a BBSome conformational change (Singh et al. 2020 cryo-EM); upstream ARL6 GEF/GAP regulators remain undefined.

"triggering a conformational change that activates the BBSome for membrane protein trafficking"

GO_REF:0000024

Manual transfer of experimentally-verified manual GO annotation data to orthologs by curator judgment of sequence similarity

GO_REF:0000033

Annotation inferences using phylogenetic trees

GO_REF:0000052

Gene Ontology annotation based on curation of immunofluorescence data

GO_REF:0000107

Automatic transfer of experimentally verified manual GO annotation data to orthologs using Ensembl Compara

GO_REF:0000117

Electronic Gene Ontology annotations created by ARBA machine learning models

PMID:17646400

Functional dissection of Rab GTPases involved in primary cilium formation.

PMID:19056867

Large-scale proteomics and phosphoproteomics of urinary exosomes.

PMID:20207729

Bardet-Biedl syndrome-associated small GTPase ARL6 (BBS3) functions at or near the ciliary gate and modulates Wnt signaling.

PMID:20603001

The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia.

PMID:22072986

A novel protein LZTFL1 regulates ciliary trafficking of the BBSome and Smoothened.

PMID:22139371

Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals common BBS-associated phenotypes and Bbs3 unique phenotypes.

PMID:25443296

The intraflagellar transport protein IFT27 promotes BBSome exit from cilia through the GTPase ARL6/BBS3.

Reactome:R-HSA-5624126

ARL6:GTP and the BBSome bind ciliary cargo

Reactome:R-HSA-5624127

ARL6:GTP and the BBSome target cargo to the primary cilium

Suggested Experiments

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.

Deep Research

Falcon

(ARL6-deep-research-falcon.md)

Comprehensive Research Report: ARL6 (BBS3) Gene Function and Annotation Falcon Edison Scientific Literature 23 citations 1 artifacts 2026-06-21T06:27:28.573495

Edison artifact artifact-00 (text/markdown)

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.

Comprehensive Research Report: ARL6 (BBS3) Gene Function and Annotation

Gene Identity Verification

ARL6 (UniProt: Q9H0F7) encodes ADP-ribosylation factor-like protein 6, also known as Bardet-Biedl syndrome 3 protein (BBS3), in Homo sapiens (singh2020structureandactivation pages 1-2, wingfield2018traffickingofciliary pages 1-2). This protein belongs to the small GTPase superfamily, specifically the ARF family, confirming alignment with the provided UniProt annotation (fisher2020arffamilygtpases pages 1-5). The gene symbol ARL6 is consistently used in the literature and corresponds to the correct human protein involved in ciliary function and Bardet-Biedl syndrome.

Molecular Function and Enzymatic Activity

GTPase Function

ARL6 functions as a small GTP-binding protein with intrinsic GTPase activity, operating as a molecular switch that cycles between GDP-bound (inactive) and GTP-bound (active) states (singh2020structureandactivation pages 2-3, fisher2020arffamilygtpases pages 1-5). The protein's primary biochemical activity is not the catalysis of metabolic substrates, but rather the binding and hydrolysis of GTP to regulate protein-protein interactions and membrane recruitment (fisher2020arffamilygtpases pages 1-5, chiuso2023ubiquitylationofbbsome pages 1-2).

The GTP-bound form of ARL6 represents the functionally active state. Structural studies using electron cryomicroscopy (cryo-EM) at 3.5 Å resolution demonstrated that the BBSome interacts exclusively with the GTP-bound form of ARL6, not the GDP-bound form (singh2020structureandactivation pages 2-3). This nucleotide-dependent specificity is essential for ARL6's regulatory function in ciliary trafficking.

BBSome Recruitment Mechanism

The primary function of ARL6 is to recruit the BBSome, an octameric protein complex, to ciliary membranes (singh2020structureandactivation pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, chou2019themoleculararchitecture pages 1-3). This recruitment is mediated by ARL6:GTP binding to a composite interface formed by the β-propeller domains of BBS1 and BBS7 within the BBSome (singh2020structureandactivation pages 1-2, singh2020structureandactivation pages 2-3, singh2020structureandactivation pages 3-5).

High-resolution structural analyses revealed that BBSome activation requires an unexpected conformational change. In solution, the BBSome exists in an autoinhibited closed state where the ARL6-binding site is occluded (singh2020structureandactivation pages 1-2, chou2019themoleculararchitecture pages 1-3). Activation requires swiveling of the β-propeller domain of BBS1, which widens a central cavity of the BBSome and exposes the composite binding site for ARL6 (singh2020structureandactivation pages 1-2, singh2020structureandactivation pages 3-5). This structural rearrangement is critical for ARL6 to bind and recruit the BBSome to membranes.

The ARL6-BBS1 interaction is reinforced by BBS9, another BBSome subunit, which strengthens the binding interface within the intact BBSome core (nozaki2018bbs1isinvolved pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2). Studies in BBS1-knockout cells demonstrated that loss of BBS1 prevents ciliary entry of other BBSome subunits and ARL6, confirming the integral nature of this interaction for complex assembly and function (nozaki2018bbs1isinvolved pages 1-2).

Membrane Association

As a member of the ARF family, ARL6 possesses an amphipathic N-terminal helix that associates with membranes in a GTP-dependent manner (singh2020structureandactivation pages 1-2). This property allows ARL6:GTP to anchor the BBSome to ciliary membranes, enabling the BBSome to function as a coat-like adaptor for membrane protein trafficking (chou2019themoleculararchitecture pages 1-3, wingfield2018traffickingofciliary pages 1-2).

Subcellular Localization

ARL6 localizes specifically to the primary cilium, ciliary membrane, and basal body (singh2020structureandactivation pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, tian2023organizationfunctionsand pages 1-2, melluso2023bardetbiedlsyndromecurrent pages 1-3). It is described as a cilium-specific member of the ARF family GTPases (singh2020structureandactivation pages 1-2). The protein concentrates at the ciliary base and within ciliary trafficking routes, where it performs its function in recruiting the BBSome to facilitate protein transport across the transition zone—a diffusion barrier that separates the ciliary from the plasma membrane (chou2019themoleculararchitecture pages 1-3, wingfield2018traffickingofciliary pages 1-2).

Studies using immunofluorescence and cell biological approaches consistently demonstrate ARL6's enrichment in ciliary compartments, particularly at the basal body where BBSome recruitment initiates (tian2023organizationfunctionsand pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2). The ciliary localization is essential for ARL6's role in coordinating membrane protein trafficking into and out of the cilium.

Biochemical Pathways and Signaling Functions

Ciliary Membrane Protein Trafficking Pathway

ARL6 functions as a central regulator of ciliary membrane protein trafficking (chou2019themoleculararchitecture pages 1-3, wingfield2018traffickingofciliary pages 1-2, singh2020structureandactivation pages 1-2). The protein operates within a sophisticated pathway that controls which membrane proteins enter and exit the cilium, thereby maintaining the unique composition of the ciliary membrane.

The mechanistic pathway proceeds as follows: ARL6:GTP recruits the BBSome to the ciliary base and ciliary membranes (chiuso2023ubiquitylationofbbsome pages 1-2, chou2019themoleculararchitecture pages 1-3). The BBSome then functions as an adaptor complex that recognizes specific cargo proteins, including G-protein-coupled receptors (GPCRs) and other transmembrane proteins (wingfield2018traffickingofciliary pages 1-2, chou2019themoleculararchitecture pages 1-3). The BBSome-cargo complex associates with intraflagellar transport (IFT) trains—comprised of IFT-A and IFT-B complexes along with molecular motors—which mediate bidirectional transport along the ciliary microtubule axoneme (chiuso2023ubiquitylationofbbsome pages 1-2, wingfield2018traffickingofciliary pages 1-2, chou2019themoleculararchitecture pages 1-3).

Recent evidence indicates that the BBSome primarily functions in removing proteins from cilia rather than importing them (singh2020structureandactivation pages 1-2, chou2019themoleculararchitecture pages 1-3). The BBSome promotes retrieval and export of specific transmembrane proteins from the cilium, with the IFT-A complex mediating entry (singh2020structureandactivation pages 1-2). This export function is critical for the signal-dependent exit of ciliary GPCRs and for preventing accumulation of proteins not normally destined for cilia (singh2020structureandactivation pages 1-2).

GPCR Trafficking and Signaling Regulation

ARL6-dependent BBSome function is essential for proper trafficking of multiple GPCRs and signaling receptors in cilia. Specific cargoes identified include Smoothened (SMO), GPR161, and somatostatin receptor 3 (SSTR3) (nozaki2018bbs1isinvolved pages 1-2, shinde2020ubiquitinchainsearmark pages 1-1, singh2020structureandactivation pages 1-2). In cells lacking functional BBS1 (a key ARL6-binding partner), these GPCRs show defects in ciliary retrograde trafficking and export (nozaki2018bbs1isinvolved pages 1-2).

Recent work from 2020 demonstrated that upon GPCR activation, receptors become tagged with K63-linked ubiquitin chains in a β-arrestin-dependent manner, and this ubiquitination marks the GPCRs for BBSome-mediated removal from cilia (shinde2020ubiquitinchainsearmark pages 1-1). The BBSome/ARL6 system is required for this signal-dependent exit mechanism (shinde2020ubiquitinchainsearmark pages 1-1).

Hedgehog Signaling Pathway

Through its control of GPCR trafficking, ARL6 indirectly regulates the Hedgehog (Hh) signaling pathway, which relies on dynamic changes in ciliary receptor localization (tian2023organizationfunctionsand pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, wingfield2018traffickingofciliary pages 1-2). During Hh signaling, GPR161 must exit cilia while SMO accumulates within cilia (chou2019themoleculararchitecture pages 1-3). Disruption of ARL6 or BBSome function leads to ciliary mislocalization of these receptors, causing abnormal Hedgehog signal transduction (tian2023organizationfunctionsand pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2).

Studies using BBS1 mutants lacking BBS9-binding ability demonstrated impaired trafficking of SMO and GPR161, resulting in defects in Shh-dependent gene transcription (nozaki2018bbs1isinvolved pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2). This establishes that the intact BBSome, properly recruited by ARL6, is required for Hedgehog pathway regulation.

Post-translational Regulation

Recent research from 2023 identified a novel regulatory mechanism involving ubiquitylation. The E3 ubiquitin ligase PJA2 localizes to the ciliary compartment and ubiquitylates BBSome subunits upon GPCR-cAMP stimulation (chiuso2023ubiquitylationofbbsome pages 1-2). Specifically, ubiquitylation of BBS1 at lysine 143 increases BBSome stability and promotes its binding to BBS3/ARL6, thereby enhancing trafficking to the ciliary membrane (chiuso2023ubiquitylationofbbsome pages 1-2). Expression of a ubiquitylation-defective BBS1 mutant (BBS1K143R) impairs GPCR trafficking and Shh-dependent gene transcription, demonstrating functional significance (chiuso2023ubiquitylationofbbsome pages 1-2).

Regulation and Upstream Factors

Despite extensive characterization of ARL6 function, the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that regulate ARL6 remain incompletely defined (fisher2020arffamilygtpases pages 1-5). Reviews of ARF family GTPases emphasize that identification of the GEFs and GAPs for ARL6 represents a key knowledge gap and an important area for future research (fisher2020arffamilygtpases pages 1-5). Understanding these regulators would provide deeper insight into how ARL6 activation is controlled spatially and temporally within cilia.

Disease Associations: Bardet-Biedl Syndrome

Clinical Manifestations

Mutations in ARL6 cause Bardet-Biedl syndrome type 3 (BBS3), an autosomal recessive ciliopathy characterized by multisystem manifestations (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, wingfield2018traffickingofciliary pages 1-2). The core clinical features of BBS include:

Retinal degeneration: Early-onset progressive retinal dystrophy leading to visual impairment and blindness (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, delvallee2023retinaldegenerationanimal pages 1-2)
Obesity: Early onset truncal obesity with associated metabolic complications (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, tian2023organizationfunctionsand pages 1-2)
Polydactyly: Postaxial polydactyly, often one of the earliest signs detectable prenatally (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2)
Renal and genitourinary anomalies: Structural kidney defects and functional abnormalities that can progress to chronic kidney disease (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2)
Learning disabilities and neurodevelopmental abnormalities: Cognitive impairment, developmental delay, and behavioral disturbances (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, tian2023organizationfunctionsand pages 1-2)
Hypogonadism: Genital abnormalities and reproductive dysfunction (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2)

Genotype-Phenotype Correlations

Importantly, BBS3/ARL6 deficiency shows genotype-phenotype variability. Recent consensus statements from 2024 note that patients with mutations in BBS3/ARL6 have a significantly lower syndromic score and lower penetrance of kidney anomalies compared to patients with mutations in other BBS genes (dollfus2024bardetbiedlsyndromeimproved pages 1-2, melluso2023bardetbiedlsyndromecurrent pages 1-3). This suggests that ARL6 dysfunction may produce a somewhat milder or more variable phenotype than mutations affecting BBSome structural subunits or chaperonins.

Pathogenic Mechanisms

The pathogenesis of BBS features results from impaired ciliary trafficking and consequent ciliary dysfunction (tian2023organizationfunctionsand pages 1-2, melluso2023bardetbiedlsyndromecurrent pages 1-3). Structural mapping of disease-causing mutations suggests that pathogenesis predominantly results from disruption of autoinhibition and activation of the BBSome, folding defects, or altered ARL6-BBSome interactions (singh2020structureandactivation pages 1-2).

In retinal photoreceptors, which possess highly specialized cilia called outer segments, ARL6/BBSome dysfunction causes rhodopsin localization defects and photoreceptor apoptosis, contributing to retinal degeneration (tian2023organizationfunctionsand pages 1-2, delvallee2023retinaldegenerationanimal pages 1-2). Obesity in BBS can be explained by mislocalization of receptors such as neuropeptide Y receptor, serotonin receptor, and leptin receptor in hypothalamic neuronal cilia, leading to hyperphagia (tian2023organizationfunctionsand pages 1-2). Polydactyly results from loss of Hedgehog signaling components from cilia, causing embryonic developmental defects (tian2023organizationfunctionsand pages 1-2).

Recent Clinical Developments

In 2024, four European Reference Networks (ERN-EYE, ERKNet, Endo-ERN, ERN-ITHACA) published revised diagnostic criteria for BBS that incorporate molecular diagnosis alongside clinical features (dollfus2024bardetbiedlsyndromeimproved pages 1-2). The updated criteria take into account the age of the patient and emphasize that genetic testing has progressively improved, prompting revision of purely clinical diagnostic approaches (dollfus2024bardetbiedlsyndromeimproved pages 1-2). These guidelines provide a framework for initial diagnosis, lifelong monitoring, and symptomatic management of BBS patients (dollfus2024bardetbiedlsyndromeimproved pages 1-2).

Current management remains supportive, as there is no cure for BBS (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2). Patients benefit from multidisciplinary intervention including visual services, physical therapy, endocrinological management of obesity and metabolic complications, and renal monitoring to detect and manage chronic kidney disease (melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2). Recent advances include better evaluation of eating behavior problems, improved lifestyle programs, and novel pharmacological therapies for obesity management in BBS (dollfus2024bardetbiedlsyndromeimproved pages 1-2).

Summary Table of Key Findings

Category	ARL6-specific finding	Key details	Evidence
Protein identity	ARL6 encodes ADP-ribosylation factor-like protein 6 and is also called BBS3	ARL6/BBS3 is a member of the ARF/ARL small GTPase family and is part of the conserved core BBS machinery linked to cilia and Bardet-Biedl syndrome	(singh2020structureandactivation pages 1-2, wingfield2018traffickingofciliary pages 1-2, fisher2020arffamilygtpases pages 1-5)
Molecular function as a GTPase	Small ARF-like GTP-binding protein with intrinsic GTPase activity	Functions as a nucleotide-dependent molecular switch; the GTP-bound form is the functionally relevant state for BBSome recruitment, and ARL6 has been described as having intrinsic GTPase activity in recent mechanistic work	(singh2020structureandactivation pages 2-3, chiuso2023ubiquitylationofbbsome pages 1-2, fisher2020arffamilygtpases pages 1-5)
Primary biochemical role	Recruits the BBSome to membranes/cilia	ARL6:GTP binds the BBSome and promotes its targeting to the basal body/ciliary membrane, a prerequisite for BBSome-dependent trafficking across the transition zone	(singh2020structureandactivation pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, chou2019themoleculararchitecture pages 1-3)
Structural mechanism	ARL6 recognizes a composite BBSome binding site	Cryo-EM work showed that ARL6 binds an active BBSome conformation, contacting a site formed by BBS1 and BBS7; BBS1 β-propeller movement is required to expose this site	(singh2020structureandactivation pages 1-2, singh2020structureandactivation pages 2-3, singh2020structureandactivation pages 3-5)
Substrate/nucleotide specificity	Prefers/acts through GTP-bound state rather than GDP-bound state	The BBSome interacts with only the GTP-bound form of ARL6 in the structural purification/reconstitution studies, indicating nucleotide-state specificity for effector engagement	(singh2020structureandactivation pages 2-3)
Enzymatic interpretation	Not a metabolic enzyme; its key “substrate” is guanine nucleotide cycling	For this protein, the relevant biochemical activity is binding and hydrolysis of GTP to regulate effector recruitment rather than catalysis of a small-molecule transformation	(fisher2020arffamilygtpases pages 1-5, chiuso2023ubiquitylationofbbsome pages 1-2)
Subcellular localization	Primary cilium, ciliary membrane, and basal body/ciliary base	Reviews and primary studies place ARL6 at the ciliary compartment, where it helps recruit the BBSome; BBS-related proteins are concentrated at the ciliary base and within ciliary trafficking routes	(singh2020structureandactivation pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, tian2023organizationfunctionsand pages 1-2, melluso2023bardetbiedlsyndromecurrent pages 1-3)
Membrane association	Associates with membranes in a GTP-dependent manner	As an Arf-family GTPase with an amphipathic N-terminus, ARL6 is described as associating with membranes when GTP-bound, consistent with its role in docking trafficking machinery to ciliary membranes	(singh2020structureandactivation pages 1-2)
Principal binding partners/effectors	BBSome, especially BBS1 and BBS7; BBS1 interaction reinforced by BBS9	ARL6 directly binds the BBSome through BBS1-containing interfaces; cell and biochemical work indicates that BBS9 strengthens the ARL6–BBS1 interaction within the intact BBSome core	(singh2020structureandactivation pages 1-2, nozaki2018bbs1isinvolved pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2)
Functional relationship to IFT	Works with IFT/BBSome trafficking machinery	ARL6 enables BBSome loading/recruitment so the BBSome can function as an adaptor linking membrane cargoes to intraflagellar transport for ciliary trafficking and exit	(chiuso2023ubiquitylationofbbsome pages 1-2, wingfield2018traffickingofciliary pages 1-2, chou2019themoleculararchitecture pages 1-3)
Role in pathway	Central regulator of ciliary membrane protein trafficking	ARL6 is required for proper movement of signaling receptors and other membrane proteins through the ciliary compartment, especially at the transition zone and during cargo export/removal	(chou2019themoleculararchitecture pages 1-3, wingfield2018traffickingofciliary pages 1-2, singh2020structureandactivation pages 1-2)
Cargo/processes influenced	Supports trafficking of GPCRs and other ciliary membrane proteins	BBSome/ARL6 function is linked to the trafficking or removal of receptors such as Smoothened, GPR161, SSTR3, and other ciliary membrane proteins; defects cause receptor mislocalization	(nozaki2018bbs1isinvolved pages 1-2, shinde2020ubiquitinchainsearmark pages 1-1, singh2020structureandactivation pages 1-2)
Signaling pathways impacted	Modulates Hedgehog and broader cilium-dependent signaling	Because ARL6 drives BBSome-dependent receptor trafficking, it indirectly controls signaling outputs that depend on receptor localization in cilia, including Shh/Hedgehog and GPCR signaling	(tian2023organizationfunctionsand pages 1-2, chiuso2023ubiquitylationofbbsome pages 1-2, wingfield2018traffickingofciliary pages 1-2)
Regulation noted in recent work	BBSome–ARL6 interaction can be enhanced by BBS1 ubiquitylation	A 2023 study reported that ubiquitylation of BBS1 K143 increases BBSome stability and promotes binding to BBS3/ARL6, linking post-translational regulation to ARL6-dependent trafficking	(chiuso2023ubiquitylationofbbsome pages 1-2)
Current unknowns	Upstream GEF/GAP regulators for ARL6 remain incompletely defined	Reviews emphasize that, unlike some other ciliary ARLs, the dedicated regulators that activate/inactivate ARL6 are still not well established and remain a key knowledge gap	(fisher2020arffamilygtpases pages 1-5)
Disease association	Bardet-Biedl syndrome type 3 (BBS3)	ARL6 mutations cause a ciliopathy with multisystem manifestations; ARL6/BBS3 is one of the established BBS genes and belongs to the non-motile ciliopathy spectrum	(melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, wingfield2018traffickingofciliary pages 1-2)
Disease features linked to ARL6 dysfunction	Retinal degeneration, obesity, polydactyly, renal/genitourinary anomalies, learning/neurodevelopmental issues	These are core BBS features described across recent reviews and consensus guidance; ARL6 defects impair ciliary trafficking, providing the mechanistic basis for the phenotype spectrum	(melluso2023bardetbiedlsyndromecurrent pages 1-3, dollfus2024bardetbiedlsyndromeimproved pages 1-2, tian2023organizationfunctionsand pages 1-2)
Clinical nuance	ARL6 deficiency may show lower penetrance of some features, including kidney anomalies, than some other BBS genotypes	Recent reviews/consensus statements note genotype-phenotype variability and specifically mention comparatively lower penetrance of renal manifestations in BBS3/ARL6 deficiency	(dollfus2024bardetbiedlsyndromeimproved pages 1-2, melluso2023bardetbiedlsyndromecurrent pages 1-3)

Table: This table summarizes the verified identity, molecular function, localization, interacting partners, pathway role, and disease relevance of human ARL6/BBS3. It is useful as a compact evidence map for understanding ARL6 as a ciliary ARF-like GTPase that recruits the BBSome and whose disruption causes Bardet-Biedl syndrome.

Conclusions

ARL6 (BBS3) is a cilium-specific small GTPase that functions as the master regulator for recruitment of the BBSome to ciliary membranes. In its GTP-bound active state, ARL6 binds a composite interface on the BBSome formed by BBS1 and BBS7, triggering a conformational change that activates the BBSome for membrane protein trafficking. The ARL6-BBSome system controls the ciliary trafficking of GPCRs and other membrane proteins, particularly mediating their export from cilia. This function is essential for proper Hedgehog signaling and other cilium-dependent signaling pathways.

Mutations in ARL6 cause Bardet-Biedl syndrome type 3, a multisystem ciliopathy characterized by retinal degeneration, obesity, polydactyly, renal defects, and neurodevelopmental abnormalities. The disease arises from defective ciliary protein trafficking leading to receptor mislocalization and impaired ciliary signaling. Recent clinical developments include updated diagnostic criteria and management guidelines published in 2023-2024.

Future research priorities include identification of the GEF and GAP regulators of ARL6, further characterization of post-translational modifications regulating the ARL6-BBSome interaction, and development of therapeutic strategies targeting the ciliary trafficking pathway for treatment of BBS and related ciliopathies.

References

(singh2020structureandactivation pages 1-2): Sandeep K Singh, Miao Gui, Fujiet Koh, Matthew CJ Yip, and Alan Brown. Structure and activation mechanism of the bbsome membrane protein trafficking complex. Jan 2020. URL: https://doi.org/10.7554/elife.53322, doi:10.7554/elife.53322. This article has 105 citations and is from a domain leading peer-reviewed journal.
(wingfield2018traffickingofciliary pages 1-2): Jenna L. Wingfield, Karl-Ferdinand Lechtreck, and Esben Lorentzen. Trafficking of ciliary membrane proteins by the intraflagellar transport/bbsome machinery. Essays in biochemistry, 62 6:753-763, Oct 2018. URL: https://doi.org/10.1042/ebc20180030, doi:10.1042/ebc20180030. This article has 183 citations and is from a peer-reviewed journal.
(fisher2020arffamilygtpases pages 1-5): Skylar Fisher, Damian Kuna, Tamara Caspary, Richard A. Kahn, and Elizabeth Sztul. Arf family gtpases with links to cilia. American Journal of Physiology-Cell Physiology, 319:C404-C418, Aug 2020. URL: https://doi.org/10.1152/ajpcell.00188.2020, doi:10.1152/ajpcell.00188.2020. This article has 44 citations.
(singh2020structureandactivation pages 2-3): Sandeep K Singh, Miao Gui, Fujiet Koh, Matthew CJ Yip, and Alan Brown. Structure and activation mechanism of the bbsome membrane protein trafficking complex. Jan 2020. URL: https://doi.org/10.7554/elife.53322, doi:10.7554/elife.53322. This article has 105 citations and is from a domain leading peer-reviewed journal.
(chiuso2023ubiquitylationofbbsome pages 1-2): Francesco Chiuso, Rossella delle Donne, Giuliana Giamundo, Laura Rinaldi, Domenica Borzacchiello, Federica Moraca, Daniela Intartaglia, Rosa Iannucci, Emanuela Senatore, Luca Lignitto, Corrado Garbi, Paolo Conflitti, Bruno Catalanotti, Ivan Conte, and Antonio Feliciello. Ubiquitylation of bbsome is required for ciliary assembly and signaling. EMBO Reports, Feb 2023. URL: https://doi.org/10.15252/embr.202255571, doi:10.15252/embr.202255571. This article has 19 citations and is from a highest quality peer-reviewed journal.
(chou2019themoleculararchitecture pages 1-3): Hui-Ting Chou, Luise Apelt, Daniel P. Farrell, Susan Roehl White, Jonathan Woodsmith, Vladimir Svetlov, Jaclyn S. Goldstein, Andrew R. Nager, Zixuan Li, Jean Muller, Hélène Dollfus, Evgeny Nudler, Ulrich Stelzl, Frank DiMaio, Maxence V. Nachury, and Thomas Walz. The molecular architecture of native bbsome obtained by an integrated structural approach. Structure, 27:1384-1394.e4, Sep 2019. URL: https://doi.org/10.1016/j.str.2019.06.006, doi:10.1016/j.str.2019.06.006. This article has 73 citations and is from a domain leading peer-reviewed journal.
(singh2020structureandactivation pages 3-5): Sandeep K Singh, Miao Gui, Fujiet Koh, Matthew CJ Yip, and Alan Brown. Structure and activation mechanism of the bbsome membrane protein trafficking complex. Jan 2020. URL: https://doi.org/10.7554/elife.53322, doi:10.7554/elife.53322. This article has 105 citations and is from a domain leading peer-reviewed journal.
(nozaki2018bbs1isinvolved pages 1-2): Shohei Nozaki, Yohei Katoh, Takuya Kobayashi, and Kazuhisa Nakayama. Bbs1 is involved in retrograde trafficking of ciliary gpcrs in the context of the bbsome complex. PLoS ONE, 13:e0195005, Mar 2018. URL: https://doi.org/10.1371/journal.pone.0195005, doi:10.1371/journal.pone.0195005. This article has 75 citations and is from a peer-reviewed journal.
(tian2023organizationfunctionsand pages 1-2): Xiaoyu Tian, Huijie Zhao, and Jun Zhou. Organization, functions, and mechanisms of the bbsome in development, ciliopathies, and beyond. eLife, Jul 2023. URL: https://doi.org/10.7554/elife.87623, doi:10.7554/elife.87623. This article has 84 citations and is from a domain leading peer-reviewed journal.
(melluso2023bardetbiedlsyndromecurrent pages 1-3): Andrea Melluso, Floriana Secondulfo, Giovanna Capolongo, Giovambattista Capasso, and Miriam Zacchia. Bardet-biedl syndrome: current perspectives and clinical outlook. Therapeutics and Clinical Risk Management, 19:115-132, Jan 2023. URL: https://doi.org/10.2147/tcrm.s338653, doi:10.2147/tcrm.s338653. This article has 107 citations and is from a peer-reviewed journal.
(shinde2020ubiquitinchainsearmark pages 1-1): Swapnil Rohidas Shinde, Andrew R. Nager, and Maxence V. Nachury. Ubiquitin chains earmark gpcrs for bbsome-mediated removal from cilia. The Journal of Cell Biology, Nov 2020. URL: https://doi.org/10.1083/jcb.202003020, doi:10.1083/jcb.202003020. This article has 100 citations.
(dollfus2024bardetbiedlsyndromeimproved pages 1-2): Hélène Dollfus, Marc R. Lilien, Pietro Maffei, Alain Verloes, Jean Muller, Giacomo M. Bacci, Metin Cetiner, Erica L. T. van den Akker, Monika Grudzinska Pechhacker, Francesco Testa, Didier Lacombe, Marijn F. Stokman, Francesca Simonelli, Aurélie Gouronc, Amélie Gavard, Mieke M. van Haelst, Jens Koenig, Sylvie Rossignol, Carsten Bergmann, Miriam Zacchia, Bart P. Leroy, Héléna Mosbah, Albertien M. Van Eerde, Djalila Mekahli, Aude Servais, Christine Poitou, and Diana Valverde. Bardet-biedl syndrome improved diagnosis criteria and management: inter european reference networks consensus statement and recommendations. European Journal of Human Genetics, 32:1347-1360, Jul 2024. URL: https://doi.org/10.1038/s41431-024-01634-7, doi:10.1038/s41431-024-01634-7. This article has 79 citations and is from a domain leading peer-reviewed journal.
(delvallee2023retinaldegenerationanimal pages 1-2): Clarisse Delvallée and Hélène Dollfus. Retinal degeneration animal models in bardet-biedl syndrome and related ciliopathies. Cold Spring Harbor perspectives in medicine, 13 1:a041303, Jan 2023. URL: https://doi.org/10.1101/cshperspect.a041303, doi:10.1101/cshperspect.a041303. This article has 15 citations and is from a peer-reviewed journal.

Artifacts

Edison artifact artifact-00

Citations

fisher2020arffamilygtpases pages 1-5
singh2020structureandactivation pages 2-3
singh2020structureandactivation pages 1-2
shinde2020ubiquitinchainsearmark pages 1-1
chou2019themoleculararchitecture pages 1-3
chiuso2023ubiquitylationofbbsome pages 1-2
tian2023organizationfunctionsand pages 1-2
dollfus2024bardetbiedlsyndromeimproved pages 1-2
wingfield2018traffickingofciliary pages 1-2
singh2020structureandactivation pages 3-5
melluso2023bardetbiedlsyndromecurrent pages 1-3
delvallee2023retinaldegenerationanimal pages 1-2
https://doi.org/10.7554/elife.53322,
https://doi.org/10.1042/ebc20180030,
https://doi.org/10.1152/ajpcell.00188.2020,
https://doi.org/10.15252/embr.202255571,
https://doi.org/10.1016/j.str.2019.06.006,
https://doi.org/10.1371/journal.pone.0195005,
https://doi.org/10.7554/elife.87623,
https://doi.org/10.2147/tcrm.s338653,
https://doi.org/10.1083/jcb.202003020,
https://doi.org/10.1038/s41431-024-01634-7,
https://doi.org/10.1101/cshperspect.a041303,

📚 Additional Documentation

Notes

(ARL6-notes.md)

ARL6 (BBS3) review notes

UniProt: Q9H0F7. ADP-ribosylation factor-like protein 6 / Bardet-Biedl syndrome 3 protein.
186 aa, small GTPase, Arf family (Ras superfamily). Gene on chr 3. HGNC:13210.

Core identity

Arf-like small GTPase. Belongs to small GTPase superfamily, Arf family (UniProt SIMILARITY).
GTP/GDP nucleotide switch. Crystal structure of GTP-bound ARL6 (PDB 2H57) solved
PMID:20207729.
P-loop / nucleotide binding residues mapped (UniProt BINDING 24-31, 50, 69-73, 130-133, 164;
Mg2+ at 31, 50). BBS variants T31M/T31R, G169A, L170W abrogate or alter GTP binding and
increase proteasomal degradation [PMID:19236846; PMID:20207729; PMID:15314642].
Core MF: GTP binding (GO:0005525) and GTPase activity (GO:0003924). Both are core for this gene.

Central function: membrane-targeting switch for the BBSome

ARL6/BBS3 is NOT a structural subunit of the BBSome
PMID:20603001
PMID:22139371.
The BBSome is the major effector of GTP-bound Arl6
PMID:20603001.
GTP-bound Arl6 recruits the BBSome to ciliary membrane; Arl6 and BBSome colocalize at ciliary
punctae interdependently PMID:20603001. Arl6 binds the N-terminus of BBS1
PMID:20603001.
Arl6 GTP loading nucleates BBSome polymerization into a coat on membranes
PMID:20603001. Reconstituted from purified
components: only GTP-bound Arl6 needed to recruit BBSome to liposomes.
Arl6 binds membranes via an N-terminal amphipathic helix exposed upon GTP binding (Arf/Sar1-like)
PMID:20603001; multi-phosphorylated PIPs (e.g. PI(3,4)P2) and acidic
phospholipids synergize for BBSome recruitment (basis for phospholipid binding annotation).
Note: in this study Arl6 was reported NOT myristoylated for the recombinant prep, but UniProt LIPID
feature annotates N-myristoyl glycine at residue 2 (ECO:0000255, by similarity).
Targeting requires GTP binding but not GTP hydrolysis: T31R (GTP-deficient) absent from cilia;
Q73L (hydrolysis-deficient) recruits MORE BBSome PMID:20603001.
BBSome recognizes ciliary targeting signal (CTS) of cargo e.g. SSTR3; ARL6 depletion removes SSTR3
from cilia PMID:20603001.

Localization

Cilium / cilium membrane (peripheral membrane protein, cytoplasmic side). Endogenous Arl6 stains
cilia; lost on siRNA PMID:20603001. IDA cilium also from MGI PMID:22139371 and HPA.
Ring-like localization at distal end of basal bodies / ciliary gate
PMID:20207729. Supports basal body / ciliary gate localization.
Punctae flanking the axoneme (membrane-associated patches) PMID:20603001. Basis for axoneme-area
annotations, though Arl6 is membrane-associated, not a structural axoneme/axonemal-microtubule component.
NOT cilium (GO:0005929) IDA, PMID:17646400: this is a Rab-GTPase overexpression screen (Yoshimura
et al.) where only Rab8a was enriched at cilia; ARL6 was tested and not enriched in that particular
assay. This negative result is in a specific overexpression context and is contradicted by multiple
direct endogenous-localization studies. Keep the NOT as recorded by the curator (assay-specific) but
it does not reflect the consensus biology.

Biological processes

Protein localization to cilium / BBSome-mediated cargo trafficking to cilia (core downstream role).
IMP in mouse Bbs3-/- : disrupts ciliary localization of MCHR1 and Smoothened retrograde transport
PMID:22139371.
Cilium assembly / length: GDP- or GTP-locked ARL6 over-expression influences cilium length and
abundance PMID:20207729. ARL6 is a trafficking switch rather than a core
ciliogenesis/axoneme-assembly factor; cilia still form in its absence (BBSome cargo defects dominate).
Wnt signaling: ARL6/BBS3 modulates Wnt; signaling function lost in BBS point mutants
PMID:20207729. IMP-supported but pleiotropic / non-core.
Hedgehog (Smoothened) trafficking: with BBSome and LZTFL1 controls SMO ciliary trafficking
PMID:22072986; affects SMO retrograde transport PMID:22139371. (No SHH GO term currently annotated.)

IFT27 / BBSome exit

IFT27/RABL4 binds nucleotide-free ARL6 and prevents its aggregation; promotes ARL6 activation,
BBSome coat assembly and BBSome+cargo EXIT from cilia
PMID:25443296. Basis for the GOA IPI protein-binding annotation (WITH IFT27, Q9BW83).

Disease

Bardet-Biedl syndrome 3 (BBS3, MIM 600151), autosomal recessive [PMID:15258860; PMID:15314642].
Retinitis pigmentosa 55 (RP55, MIM 613575), variant A89V PMID:19956407.
Isoform 2 (BBS3L) is vision-specific / required for proper retinal function (UniProt; PMID:20333246).

GOA IPI WITH/FROM cross-checks (citation correctness)

PMID:20603001 IPI WITH BBS1 (Q8NFJ9): correct — Arl6GTP binds BBS1 N-terminus. VERIFIED.
PMID:22139371 IPI WITH BBS1 (Q8NFJ9): correct — endogenous BBS3-BBSome physical interaction. VERIFIED.
PMID:25443296 IPI WITH IFT27 (Q9BW83): correct — ARL6 binds nucleotide-free IFT27. VERIFIED.

Term-by-term reasoning summary

GTP binding (GO:0005525), GTPase activity (GO:0003924): ACCEPT, core MF.
protein binding (GO:0005515) IPI x3: uninformative bare term; KEEP_AS_NON_CORE (real interactions,
but "protein binding" not informative; effector/BBSome-recruitment captured elsewhere).
protein localization to cilium (GO:0061512): core BP. ACCEPT the experimental IMP (PMID:22139371);
IBA/IEA duplicates ACCEPT/KEEP_AS_NON_CORE.
cilium (GO:0005929) IDA x2: ACCEPT. ciliary tip (GO:0097542) IDA: ACCEPT (HPA).
cilium membrane is the most accurate compartment but not currently in GOA list.
axoneme (GO:0005930) / axonemal microtubule (GO:0005879): ARL6 is membrane-patch associated near the
axoneme, not a structural axoneme component. KEEP_AS_NON_CORE / MARK_AS_OVER_ANNOTATED.
membrane coat (GO:0030117) part_of, ISS: the coat is the BBSome; ARL6 recruits it but is not itself a
structural coat subunit. MARK_AS_OVER_ANNOTATED.
protein polymerization (GO:0051258) ISS: ARL6 nucleates BBSome coat polymerization (it does not itself
polymerize). KEEP_AS_NON_CORE (defensible as "involved_in" the polymerization of the coat).
phospholipid binding (GO:0005543) ISS: supported — GTP-Arl6 + acidic PIPs synergize. KEEP_AS_NON_CORE.
protein targeting to membrane (GO:0006612) ISS: GO:0006612 is SRP-dependent targeting to ER membrane;
WRONG specific term for ARL6's role (it recruits BBSome to ciliary membrane). MARK_AS_OVER_ANNOTATED.
cilium assembly (GO:0060271): pleiotropic/modulatory, not core ciliogenesis. KEEP_AS_NON_CORE.
Wnt signaling (GO:0016055) IMP: pleiotropic, KEEP_AS_NON_CORE.
intracellular protein transport (GO:0006886) IBA, vesicle-mediated transport: general but consistent;
KEEP_AS_NON_CORE.
determination of left/right symmetry (GO:0007368) ISS, melanosome transport (GO:0032402) ISS,
fat cell differentiation (GO:0045444) IEA, protein localization to non-motile cilium (GO:0097499) IEA:
weak electronic/ISS transfers; KEEP_AS_NON_CORE or MARK_AS_OVER_ANNOTATED (melanosome transport is a
questionable ortholog transfer; left/right symmetry plausible ciliopathy phenotype but indirect).
cytoplasm/cytosol/membrane (GO:0005737/0005829/0016020): ACCEPT/KEEP_AS_NON_CORE (ARL6 cycles between
cytosol and membrane).
extracellular exosome (GO:0070062) HDA: high-throughput proteomics catch-all; MARK_AS_OVER_ANNOTATED.
plasma membrane (GO:0005886) TAS Reactome x2: ARL6 acts at ciliary/plasma membrane interface; the
GTP-bound form is on membranes. KEEP_AS_NON_CORE.
NOT cilium (GO:0005929) PMID:17646400: keep negated annotation (assay-specific); ACCEPT as recorded.

Falcon deep research integration (2026-06-21)

Falcon (Edison) deep research generated in ARL6-deep-research-falcon.md (23 citations). It is
strongly consistent with the existing COMPLETE review; no annotation actions need changing. New or
refined points worth recording (citations are from the falcon report's primary sources; not yet
independently verified against full text, so not marked VERIFIED):

Composite BBS1+BBS7 binding interface. Cryo-EM of the BBSome (Singh et al. 2020, eLife
10.7554/elife.53322) shows ARL6:GTP binds a composite site formed by the β-propellers of BBS1
and BBS7, not BBS1 alone, and that the BBSome is autoinhibited (closed) in solution; activation
requires the BBS1 β-propeller to swivel and expose the ARL6 site. This refines the existing
notes/description, which cite only the BBS1 N-terminus (PMID:20603001, Jin 2010). Both are
compatible — the older work mapped the minimal interaction; the cryo-EM adds the BBS7 contribution
and the autoinhibition→activation mechanism. Consider citing Singh 2020 in the description if
verified.
BBSome is primarily an export/retrieval coat, not an import coat. Falcon emphasizes the now-
prevailing view that the ARL6/BBSome system mainly removes signaling GPCRs from cilia (with IFT-A
mediating entry). This is consistent with the existing "cargo trafficking into and out of cilia"
framing but sharpens the directionality (export-dominant).
Signal-dependent, ubiquitin-marked GPCR exit. Shinde, Nager & Nachury 2020 (JCB
10.1083/jcb.202003020): activated ciliary GPCRs are tagged with K63-linked ubiquitin chains in a
β-arrestin-dependent manner, earmarking them for BBSome/ARL6-mediated removal (GPR161, SSTR3, SMO
as cargo). Adds mechanistic detail to the existing SMO/MCHR1/SSTR3 trafficking annotations.
Post-translational control of the ARL6–BBSome interaction. Chiuso et al. 2023 (EMBO Rep
10.15252/embr.202255571): the E3 ligase PJA2 ubiquitylates BBS1 at K143 upon GPCR-cAMP stimulation,
increasing BBSome stability and its binding to BBS3/ARL6; a BBS1-K143R mutant impairs GPCR
trafficking and Shh-dependent transcription. New regulatory axis not in current notes.
BBS9 reinforces the ARL6–BBS1 interaction within the intact BBSome core (Nozaki et al. 2018,
PLoS ONE 10.1371/journal.pone.0195005); BBS1 knockout blocks ciliary entry of ARL6 and other
subunits.
Knowledge gap: the GEF(s) and GAP(s) that activate/inactivate ARL6 remain undefined (Fisher
et al. 2020, Am J Physiol Cell Physiol 10.1152/ajpcell.00188.2020). Good candidate for
suggested_experiments.
Clinical nuance: BBS3/ARL6 genotypes show lower penetrance of renal anomalies and a lower
syndromic score than other BBS genes (Dollfus et al. 2024 ERN consensus; Melluso et al. 2023).
Background only; does not affect GO annotations.

📄 View Raw YAML

id: Q9H0F7
gene_symbol: ARL6
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: 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.
alternative_products:
- name: 1 (BBS3)
  id: Q9H0F7-1
- name: 2 (BBS3L)
  id: Q9H0F7-2
  sequence_note: VSP_040511
existing_annotations:
- term:
    id: GO:0006886
    label: intracellular protein transport
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
- term:
    id: GO:0005525
    label: GTP binding
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    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.
    action: ACCEPT
    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.
- term:
    id: GO:0005930
    label: axoneme
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
- term:
    id: GO:0060271
    label: cilium assembly
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
- term:
    id: GO:0061512
    label: protein localization to cilium
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: ARL6 recruits the BBSome to traffic membrane proteins to/from cilia; protein
      localization to cilium is a core downstream biological process for ARL6.
    action: ACCEPT
    reason: Well supported (PMID:20603001, PMID:22139371). This is the central biological
      process ARL6 enables via BBSome recruitment.
- term:
    id: GO:0003924
    label: GTPase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: ARL6 is a small GTPase; nucleotide cycling (GTP binding and hydrolysis)
      underlies its switch function. GTPase activity is a core molecular function.
    action: ACCEPT
    reason: Consistent with the Arf-family GTPase identity (UniProt; PDB 2H57). The
      Q73L hydrolysis-deficient variant studies confirm a hydrolyzable GTPase. Core MF.
- term:
    id: GO:0005525
    label: GTP binding
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: InterPro-based electronic annotation duplicating the experimentally supported
      GTP binding function.
    action: ACCEPT
    reason: Core molecular function, independently supported by structure and biochemistry.
- term:
    id: GO:0061512
    label: protein localization to cilium
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: ARBA electronic annotation duplicating the experimentally supported protein
      localization to cilium process.
    action: ACCEPT
    reason: Core biological process, independently supported by experimental evidence.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:20603001
  qualifier: enables
  review:
    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.
    action: KEEP_AS_NON_CORE
    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.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:22139371
  qualifier: enables
  review:
    summary: IPI with BBS1 (Q8NFJ9). Endogenous BBS3 and the BBSome physically interact
      and depend on each other for ciliary localization.
    action: KEEP_AS_NON_CORE
    reason: Verified interaction supporting BBSome recruitment, but protein binding
      is uninformative as a standalone MF.
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: ARL6 cycles between a soluble cytosolic GDP-bound form and a membrane-bound
      GTP-bound form, so cytoplasmic localization is expected.
    action: KEEP_AS_NON_CORE
    reason: Consistent with a peripheral membrane GTPase that has a cytosolic pool;
      not the functionally salient compartment (cilium/cilium membrane is).
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: Cytosolic (GDP-bound) pool of ARL6 consistent with its nucleotide-dependent
      membrane cycling.
    action: KEEP_AS_NON_CORE
    reason: Reasonable for the inactive soluble pool; not the core functional location.
- term:
    id: GO:0016020
    label: membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: located_in
  review:
    summary: GTP-bound ARL6 is a peripheral membrane protein that inserts an N-terminal
      amphipathic helix into the (ciliary) membrane.
    action: KEEP_AS_NON_CORE
    reason: Correct but high-level; the specific cilium/cilium membrane localization
      is more informative.
- term:
    id: GO:0045444
    label: fat cell differentiation
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    reason: Reflects a systemic ciliopathy phenotype rather than a demonstrated molecular/cellular
      role of ARL6 in fat cell differentiation; weak electronic ortholog transfer.
- term:
    id: GO:0097499
    label: protein localization to non-motile cilium
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: ARL6 traffics membrane proteins to primary (non-motile) cilia, so this
      specific child of protein localization to cilium is appropriate.
    action: KEEP_AS_NON_CORE
    reason: Consistent with ARL6/BBSome trafficking in primary cilia (PMID:20603001);
      redundant with the more frequently supported protein localization to cilium.
- term:
    id: GO:0005929
    label: cilium
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence localization of ARL6 to the cilium (HPA). Endogenous
      ARL6 stains cilia and staining is lost on depletion.
    action: ACCEPT
    reason: Well supported by multiple direct localization studies (PMID:20603001,
      PMID:22139371) plus HPA.
- term:
    id: GO:0097542
    label: ciliary tip
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: HPA direct immunofluorescence places ARL6 at the ciliary tip, consistent
      with BBSome turnaround/exit at the tip and IFT-coupled trafficking.
    action: ACCEPT
    reason: Direct experimental localization; consistent with BBSome trafficking dynamics
      along the cilium.
- term:
    id: GO:0005929
    label: cilium
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5624127
  qualifier: located_in
  review:
    summary: Reactome traceable assertion of ciliary localization, consistent with experimental
      data.
    action: ACCEPT
    reason: Consistent with direct localization evidence.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25443296
  qualifier: enables
  review:
    summary: IPI with IFT27/RABL4 (Q9BW83). IFT27 directly binds nucleotide-free ARL6,
      preventing its aggregation and promoting its activation and BBSome exit from cilia.
    action: KEEP_AS_NON_CORE
    reason: Verified, biologically important interaction (PMID:25443296), but the bare
      protein binding term is uninformative as a molecular function.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5624126
  qualifier: located_in
  review:
    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.
    action: KEEP_AS_NON_CORE
    reason: Defensible given membrane association at the ciliary base/gate; the ciliary
      membrane is the more precise compartment.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-5624127
  qualifier: located_in
  review:
    summary: Duplicate Reactome plasma membrane assertion.
    action: KEEP_AS_NON_CORE
    reason: As above; cilium membrane is the more precise location.
- term:
    id: GO:0005929
    label: cilium
  evidence_type: IDA
  original_reference_id: PMID:22139371
  qualifier: located_in
  review:
    summary: MGI direct localization of ARL6 to cilium (mouse Bbs3 study). Endogenous
      BBS3 localizes to cilia and depends on the BBSome for this localization.
    action: ACCEPT
    reason: Direct experimental evidence supporting ciliary localization.
- term:
    id: GO:0061512
    label: protein localization to cilium
  evidence_type: IMP
  original_reference_id: PMID:22139371
  qualifier: acts_upstream_of_or_within
  review:
    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.
    action: ACCEPT
    reason: Experimental IMP directly supporting the core biological process of ARL6.
- term:
    id: GO:0005929
    label: cilium
  evidence_type: IDA
  original_reference_id: PMID:17646400
  qualifier: located_in
  negated: true
  review:
    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.
    action: ACCEPT
    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.
    supported_by:
    - reference_id: PMID:17646400
      supporting_text: 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.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19056867
  qualifier: located_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    reason: High-throughput exosome proteomics frequently captures cytosolic/membrane
      proteins nonspecifically; not a functionally meaningful localization for ARL6.
- term:
    id: GO:0005879
    label: axonemal microtubule
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ARL6 is a peripheral membrane GTPase associated with patches near the axoneme;
      it is not a structural component of axonemal microtubules.
    action: MARK_AS_OVER_ANNOTATED
    reason: Over-specific structural localization by ortholog ISS transfer; ARL6 acts
      at the ciliary membrane, not as an axonemal microtubule component.
- term:
    id: GO:0016055
    label: Wnt signaling pathway
  evidence_type: IMP
  original_reference_id: PMID:20207729
  qualifier: involved_in
  review:
    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.
    action: KEEP_AS_NON_CORE
    reason: Experimentally supported (PMID:20207729) but pleiotropic/downstream of the
      core BBSome-trafficking activity; not a core function.
- term:
    id: GO:0030117
    label: membrane coat
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: part_of
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    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).
- term:
    id: GO:0060271
    label: cilium assembly
  evidence_type: IMP
  original_reference_id: PMID:20207729
  qualifier: involved_in
  review:
    summary: Overproduction of GDP-/GTP-locked ARL6 influences cilium length and abundance,
      supporting a modulatory role in ciliogenesis.
    action: KEEP_AS_NON_CORE
    reason: Experimentally supported modulation of cilium length/number (PMID:20207729),
      but secondary to the core BBSome-cargo trafficking switch role.
- term:
    id: GO:0005543
    label: phospholipid binding
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: GTP-bound ARL6 binds membranes via an N-terminal amphipathic helix and
      synergizes with acidic phospholipids/multi-phosphorylated PIPs to recruit the BBSome.
    action: KEEP_AS_NON_CORE
    reason: Supported by liposome reconstitution showing acidic phospholipid/PIP dependence
      (PMID:20603001); a real but ancillary molecular property supporting membrane targeting.
- term:
    id: GO:0005930
    label: axoneme
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ARL6 localizes to membrane patches flanking the axoneme; acceptable as
      a broad localization but not a structural axonemal role.
    action: KEEP_AS_NON_CORE
    reason: Consistent with punctae flanking the axoneme (PMID:20603001); cilium/cilium
      membrane is the more accurate compartment.
- term:
    id: GO:0006612
    label: protein targeting to membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: ARL6 directs membrane cargo proteins to the ciliary membrane via BBSome
      recruitment, consistent with the general process of protein targeting to membrane.
    action: KEEP_AS_NON_CORE
    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.
- term:
    id: GO:0051258
    label: protein polymerization
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: GTP-bound ARL6 nucleates polymerization of the BBSome into a membrane-apposed
      coat. ARL6 drives the polymerization although it does not itself polymerize.
    action: KEEP_AS_NON_CORE
    reason: Defensible as involved_in BBSome coat polymerization (PMID:20603001 shows
      Arl6GTP-mediated formation of polymerized coat patches); not a core function term.
- term:
    id: GO:0007368
    label: determination of left/right symmetry
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    reason: Indirect, phenotype-level inference via ortholog transfer; not supported
      by direct experimental evidence for human ARL6.
- term:
    id: GO:0032402
    label: melanosome transport
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    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.
    action: MARK_AS_OVER_ANNOTATED
    reason: Questionable ortholog-based transfer with no supporting evidence for ARL6;
      not part of its characterized BBSome/ciliary function.
- term:
    id: GO:0060271
    label: cilium assembly
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: involved_in
  review:
    summary: ISS duplicate of the cilium assembly annotation; modulatory role in cilium
      length/abundance.
    action: KEEP_AS_NON_CORE
    reason: Consistent with a modulatory ciliogenesis role; non-core.
- term:
    id: GO:0005737
    label: cytoplasm
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ISS duplicate of cytoplasmic localization; consistent with the soluble GDP-bound
      pool.
    action: KEEP_AS_NON_CORE
    reason: Reasonable for the cytosolic pool; not the core functional location.
- term:
    id: GO:0016020
    label: membrane
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ISS duplicate of membrane localization; GTP-bound ARL6 is a peripheral membrane
      protein.
    action: KEEP_AS_NON_CORE
    reason: Correct but high-level; cilium membrane is more informative.
core_functions:
- description: GTP-dependent molecular switch (Arf-family small GTPase) that, in its
    GTP-bound state, associates with the ciliary membrane and recruits the BBSome coat.
  supported_by:
  - reference_id: PMID:20603001
    supporting_text: The BBSome is the major effector of the Arf-like GTPase Arl6/BBS3,
      and the BBSome and GTP-bound Arl6 colocalize at ciliary punctae in an interdependent
      manner.
  molecular_function:
    id: GO:0005525
    label: GTP binding
- description: Hydrolyzable small GTPase; nucleotide cycling between GTP- and GDP-bound
    states governs membrane association and BBSome recruitment.
  supported_by:
  - reference_id: PMID:20207729
    supporting_text: By determining the structure of GTP-bound ARL6/BBS3, coupled with
      functional assays, we provide a mechanistic explanation for such pathogenic alterations,
      namely altered nucleotide binding.
  molecular_function:
    id: GO:0003924
    label: GTPase activity
- description: Recruits and nucleates polymerization of the BBSome coat on the ciliary
    membrane to sort membrane proteins into and out of primary cilia (protein localization
    to cilium).
  supported_by:
  - reference_id: PMID:20603001
    supporting_text: Arl6(GTP)-mediated recruitment of the BBSome to synthetic liposomes
      produces distinct patches of polymerized coat apposed onto the lipid bilayer.
  - reference_id: PMID:22139371
    supporting_text: Loss of Bbs3 does not affect BBSome formation but disrupts normal
      localization of melanin concentrating hormone receptor 1 to ciliary membranes
      and affects retrograde transport of Smoothened inside cilia.
  - reference_id: file:human/ARL6/ARL6-deep-research-falcon.md
    supporting_text: triggering a conformational change that activates the BBSome for
      membrane protein trafficking
  molecular_function:
    id: GO:0005525
    label: GTP binding
  directly_involved_in:
  - id: GO:0061512
    label: protein localization to cilium
proposed_new_terms: []
suggested_questions:
- question: 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?
- question: 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?
- question: To what extent are the Wnt- and Hedgehog-signaling phenotypes direct ARL6
    functions versus indirect consequences of disrupted BBSome cargo trafficking?
suggested_experiments:
- description: 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.
- description: 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.
- description: 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.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
  findings: []
- id: file:human/ARL6/ARL6-deep-research-falcon.md
  title: Falcon deep research report for ARL6
  findings:
  - statement: Falcon deep research corroborates ARL6/BBS3 as the GTP-dependent membrane-targeting
      switch that recruits the BBSome coat to cilia, and refines the interface to a
      composite BBS1/BBS7 site exposed by a BBSome conformational change (Singh et al.
      2020 cryo-EM); upstream ARL6 GEF/GAP regulators remain undefined.
    supporting_text: triggering a conformational change that activates the BBSome for
      membrane protein trafficking
- 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:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000117
  title: Electronic Gene Ontology annotations created by ARBA machine learning models
  findings: []
- id: PMID:17646400
  title: Functional dissection of Rab GTPases involved in primary cilium formation.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Rab/Arf GTPase overexpression screen in which only Rab8a was enriched
      at cilia; ARL6 was a negative result in this specific assay. Basis for the curator's
      NOT located_in cilium annotation. Does not reflect endogenous ARL6 biology.
- id: PMID:19056867
  title: Large-scale proteomics and phosphoproteomics of urinary exosomes.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput urinary exosome proteomics; basis for extracellular
      exosome annotation. Not functionally informative for ARL6.
- id: PMID:20207729
  title: Bardet-Biedl syndrome-associated small GTPase ARL6 (BBS3) functions at or near
    the ciliary gate and modulates Wnt signaling.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Crystal structure of GTP-bound ARL6 (PDB 2H57); ring-like localization
      at distal basal body/ciliary gate; modulates Wnt and cilium length; BBS point
      mutants characterized. Directly establishes core MF and localization.
- id: PMID:20603001
  title: The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics
    membrane proteins to cilia.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Defines ARL6 as the membrane switch whose GTP-bound form recruits the
      BBSome (binding BBS1 N-terminus) and nucleates coat polymerization; ARL6 explicitly
      not part of the BBSome. Central reference for ARL6 function.
- id: PMID:22072986
  title: A novel protein LZTFL1 regulates ciliary trafficking of the BBSome and Smoothened.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Shows BBS3/ARL6 (with the BBSome and LZTFL1) is required for BBSome
      ciliary entry and SMO trafficking; supports ciliary trafficking/Hedgehog role.
- id: PMID:22139371
  title: Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals common BBS-associated
    phenotypes and Bbs3 unique phenotypes.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Mouse Bbs3 knockout; BBS3-BBSome interdependent ciliary localization;
      loss disrupts MCHR1 ciliary localization and SMO retrograde transport. Supports
      IDA cilium and IMP protein localization to cilium.
- id: PMID:25443296
  title: The intraflagellar transport protein IFT27 promotes BBSome exit from cilia
    through the GTPase ARL6/BBS3.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: IFT27/RABL4 binds nucleotide-free ARL6 (WITH Q9BW83), preventing aggregation
      and promoting ARL6 activation, BBSome coat assembly and ciliary exit. Verifies
      the IPI interaction.
- id: Reactome:R-HSA-5624126
  title: ARL6:GTP and the BBSome bind ciliary cargo
  findings: []
- id: Reactome:R-HSA-5624127
  title: ARL6:GTP and the BBSome target cargo to the primary cilium
  findings: []

ARL6

Gene Description

Existing Annotations Review

Core Functions

GTP-dependent molecular switch (Arf-family small GTPase) that, in its GTP-bound state, associates with the ciliary membrane and recruits the BBSome coat.

Hydrolyzable small GTPase; nucleotide cycling between GTP- and GDP-bound states governs membrane association and BBSome recruitment.

Recruits and nucleates polymerization of the BBSome coat on the ciliary membrane to sort membrane proteins into and out of primary cilia (protein localization to cilium).

References

Suggested Questions for Experts

Suggested Experiments

Deep Research

Falcon

Comprehensive Research Report: ARL6 (BBS3) Gene Function and Annotation

Gene Identity Verification

Molecular Function and Enzymatic Activity

GTPase Function

BBSome Recruitment Mechanism

Membrane Association

Subcellular Localization

Biochemical Pathways and Signaling Functions

Ciliary Membrane Protein Trafficking Pathway

GPCR Trafficking and Signaling Regulation

Hedgehog Signaling Pathway

Post-translational Regulation

Regulation and Upstream Factors

Disease Associations: Bardet-Biedl Syndrome

Clinical Manifestations

Genotype-Phenotype Correlations

Pathogenic Mechanisms

Recent Clinical Developments

Summary Table of Key Findings

Conclusions

Artifacts

Citations

📚 Additional Documentation

Notes

ARL6 (BBS3) review notes

Core identity

Central function: membrane-targeting switch for the BBSome

Localization

Biological processes

IFT27 / BBSome exit

Disease

GOA IPI WITH/FROM cross-checks (citation correctness)

Term-by-term reasoning summary

Falcon deep research integration (2026-06-21)

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