# ARL8A review notes

## Deep research provider status

Falcon deep research was attempted for the PN batch review, but the provider timed out after 600 seconds. The configured perplexity-lite fallback was then attempted and failed with a 401 quota error, so no `ARL8A-deep-research-falcon.md` or fallback provider output was produced. This review therefore uses the fetched UniProt/GOA records, cached PMID texts, Reactome cache, and PN projection/audit reports.

## Evidence summary

ARL8A encodes one of the two mammalian ARL8 small GTPases. The clearest direct experimental result is that ARL8A and ARL8B localize to lysosomes and affect lysosome motility: Hofmann and Munro report that "Arl8a and Arl8b ... localise to lysosomes in mammalian cells" and that overexpression redistributes lysosomes toward the cell periphery in a microtubule-dependent manner [PMID:16537643 "Arl8a and Arl8b ... localise to lysosomes in mammalian cells"]. The original GIE/ARL8 paper also supports GTP binding, tubulin association, spindle-midzone localization, and a chromosome-segregation phenotype, but this is a secondary mitotic context for the PN review rather than the proteostasis-centered function [PMID:15331635 "Gie protein has ability to bind to tubulin and localizes with microtubules on the spindle mid-zone in late mitosis"].

The BORC paper places ARL8 in lysosome positioning: BORC recruits Arl8 to lysosomes and initiates kinesin-dependent movement toward microtubule plus ends [PMID:25898167 "BORC associates peripherally with the lysosomal membrane, where it functions to recruit the small GTPase Arl8"]. PLEKHM1/HOPS and RUFY3/RUFY4 papers expand this into a broader endolysosomal transport/fusion network. PLEKHM1 binds Arl8b and promotes delivery/degradation of endocytic and autophagic cargo in lysosomes [PMID:28325809 "Arl8b binding to PLEKHM1 is required for its function in delivery and, therefore, degradation of endocytic and autophagic cargo in lysosomes"]. RUFY3/RUFY4 are ARL8 effectors that couple endolysosomes to dynein-dynactin for retrograde microtubule transport [PMID:35314674 "RUFY3 and RUFY4 are ARL8 effectors that promote coupling of endolysosomes to dynein-dynactin"].

## PN projection decision

The PN projection file proposes ARL8A as a candidate new annotation to GO:0061906 autophagosome localization from the Autophagy-Lysosome Pathway / Localization of the autophagosome / Movement of autophagosomes along microtubules / HOPS-BORC complex bridging path. The mapping audit marks this projection family as requiring manual gene-level review before changing a gene review. For ARL8A, the strongest direct evidence supports lysosome/endolysosome localization and transport. The available ARL8 autophagy evidence is mostly about lysosome positioning, HOPS recruitment, and delivery or degradation of autophagic cargo in lysosomes, not direct ARL8A-dependent positioning of autophagosomes themselves. Therefore this review does not add GO:0061906 for ARL8A; it records the projection as an expert question/experimental follow-up.

## Falcon deep research findings (2026-06-07)

A Falcon (Edison) deep research report was generated and is now available (`ARL8A-deep-research-falcon.md`); it supersedes the earlier "provider timed out" status above. The report adds several primary references absent from the prior review. PMIDs below were resolved via PubMed. Most ARL8A-specific conclusions remain paralog-inferred from ARL8B / shared ARL8-family or double-KD/double-KO experiments, which I label explicitly.

- CONFIRMS (with new direct ARL8A evidence): BORC functions upstream of ARL8 to drive kinesin-dependent peripheral lysosome dispersal, coupling to both kinesin-1 (KIF5B) and kinesin-3 (KIF1A/KIF1Bbeta) on distinct microtubule tracks (KIF5B on central acetylated tracks; KIF1A/KIF1Bbeta on peripheral tyrosinated tracks). Notably, in an ARL8B-knockout background, siRNA against ARL8A removes the residual ability of kinesin constructs to disperse lysosomes — direct evidence that ARL8A itself contributes to the transport program, not just ARL8B [PMID:27851960 Guardia 2016 "BORC Functions Upstream of Kinesins 1 and 3..."]. This strengthens the existing lysosome-localization / anterograde-transport annotations.

- NEW (pathway / process): The BORC-ARL8-HOPS ensemble is required for lysosomal free-cholesterol egress via NPC2; depletion of BORC, ARL8, or HOPS causes lysosomal free-cholesterol accumulation, reduced NPC2 retention with increased NPC2 secretion, and increased lysosomal degradation of CI-MPR [PMID:35653304 Anderson 2022 "BORC-ARL8-HOPS ensemble is required for lysosomal cholesterol egress through NPC2"]. Pathway-level (ARL8 family), not ARL8A-specific.

- NEW (process), with strong ARL8A relevance: Disruption of BORC-ARL8-HOPS-dependent endolysosome fusion increases exosome secretion (multivesicular endosomes fail to fuse with lysosomes, so intraluminal vesicles are released extracellularly). This study used HeLa ARL8A/ARL8B double-knockout models, making it one of the clearer settings where ARL8A is explicitly perturbed alongside ARL8B [PMID:37213076 Shelke 2023 "Inhibition of endolysosome fusion increases exosome secretion"]. This is mechanistically relevant context for the existing extracellular-exosome annotation (which remains MARK_AS_OVER_ANNOTATED as a steady-state location; the new data concern a functional fusion role, not ARL8A residing in exosomes).

- NEW (interactions/retrograde), ARL8B-centered: DENND6A is an ARL8B effector that activates Rab34, recruiting a RILP/dynein-dynactin complex to drive retrograde (juxtanuclear) lysosome transport and supporting autophagic flux; double knockdown of ARL8A and ARL8B reduces DENND6A localization phenotypes (ARL8-family requirement) [PMID:38296963 Kumar 2024 "DENND6A links Arl8b to a Rab34/RILP/dynein complex..."]. Complements the existing RUFY3/RUFY4 retrograde-coupling annotation (PMID:35314674).

- NEW (disease, pathway-level/translational): Biallelic BORCS8 (a BORC subunit upstream of ARL8) variants cause a severe early-infantile neurodegenerative disorder in five children from three families; patient alleles impair BORC assembly and the ability to restore peripheral lysosome distribution, and zebrafish borcs8 knockout recapitulates brain/eye and locomotor phenotypes [PMID:38128568 De Pace 2024 "Biallelic BORCS8 variants cause an infantile-onset neurodegenerative disorder..."]. This is upstream of ARL8A, not a monogenic ARL8A disease, but validates the BORC->ARL8->kinesin axis physiologically in humans.

- PROVISIONAL / low-confidence (do NOT use to change annotations): In prostate cancer cohorts, ARL8A expression showed only a non-significant survival association (log-rank P=0.1471, HR=1.927, 95% CI 0.6692-5.547) [PMID:39217195 Nturubika 2024 "Altered expression of vesicular trafficking machinery in prostate cancer..."]. Non-significant; recorded for completeness only.

- CONFIRMS (localization mechanism): ARL8 proteins lack canonical Arf N-myristoylation; membrane targeting to lysosomes relies on an N-terminal amphipathic helix and N-terminal acetylation (NatC), consistent with the existing PMID:16537643-based localization (the original paper title itself is "An N-terminally acetylated Arf-like GTPase..."). Acetylation data are best established at the ARL8-family/ARL8B level [Khatter 2015 review doi:10.1080/21592799.2015.1086501; PMID:16537643].

Decision: I will add the six newly-resolved primary references (Guardia 2016, Anderson 2022, Shelke 2023, Kumar 2024, De Pace 2024, Nturubika 2024) to the review `references:` as statement-only findings (no supporting_text, since none of these are cached in /publications). I will not change any existing annotation `action`: none of the new evidence contradicts prior calls; rather it reinforces lysosome localization, anterograde/retrograde transport, and fusion-linked roles, and adds new pathway context (cholesterol egress, exosome secretion, BORC-disease axis). I add a couple of suggested questions/experiments for the cholesterol-egress and exosome roles.