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.