ATXN3 (Ataxin-3) — Gene Review Notes

UniProt: P54252 | HGNC: ATXN3 | Gene aliases: MJD, MJD1, SCA3, ATX3
Organism: Homo sapiens (NCBITaxon:9606)

Note on naming: the review was requested as "SCA3". SCA3 (spinocerebellar
ataxia type 3 / Machado-Joseph disease) is the disease; the HGNC gene
symbol is ATXN3. Per project convention (human = HGNC symbols) the
review is filed under ATXN3.

Provenance note: automated deep-research providers were unavailable in this
session (falcon returned HTTP 402 Payment Required; perplexity/asta/openscientist
not wired into the CLI provider enum). These notes are synthesized from the
cached UniProt record, the 18 cached publications, and Reactome. No
-deep-research-<provider>.md file was fabricated, per CLAUDE.md.

Summary

Ataxin-3 is a deubiquitinating enzyme (DUB) of the Josephin (Machado-Joseph
domain, MJD) cysteine-protease family. It is the protein whose gene, when its
CAG/polyglutamine tract expands beyond ~52 repeats, causes spinocerebellar
ataxia type 3 (SCA3) / Machado-Joseph disease, the most common dominantly
inherited ataxia worldwide.

Domain architecture:
- Josephin domain (aa 1–180): catalytic papain-like cysteine-protease
DUB domain. Catalytic triad Cys14–His119–Asn134 (UniProt ACT_SITE 14,
119, 134; MUTAGENESIS of Cys-14 abolishes activity, PMID:23625928,
PMID:16118278, PMID:33157014).
- Ubiquitin-interacting motifs (UIMs): UIM1 (224–243), UIM2 (244–263),
and (isoform-dependent) UIM3 (331–349). UIMs bind ubiquitin and are
required to limit the length of ubiquitin chains.
- PolyQ tract (~aa 292 onward): normal 12–40 Gln; pathogenic when
expanded (~>52). The poly-Gln region also mediates interaction with BECN1.
- Multiple splice isoforms (P54252-1..-5); isoform composition affects the
C-terminal UIM3 and the tail (see PMID:30455355).

Molecular function — DUB activity and linkage specificity

• Ataxin-3 is an active DUB: Josephin-domain proteins from many species are
  active de-ubiquitinating enzymes PMID:17696782.
• It is a chain-trimming / editing DUB, not a broad rescue DUB: it binds
  long polyubiquitin chains and trims them, with weak/no activity on chains of
  4 or fewer ubiquitins (UniProt FUNCTION, from PMID:17696782).
• Linkage specificity: cleaves both K48-linked and K63-linked
  chains; VCP/p97 stimulates activity PMID:22970133. This paper is the
  IDA source for the K48-linked (GO:1990380) and K63-linked (GO:0061578)
  deubiquitinase-activity terms and their process terms (GO:0070536,
  GO:0071108).
• The core MF term cysteine-type deubiquitinase activity (GO:0004843) is
  well supported by direct assay (EXP PMID:17696782; IDA PMID:33157014).

Biological processes

1. ERAD / retrotranslocation (endoplasmic-reticulum-associated degradation)
   Ataxin-3 is a p97/VCP-associated DUB that regulates retrotranslocation of
   ERAD substrates PMID:17000876. A later study shows it
   negatively regulates retro-translocation of nonubiquitinated substrates
   PMID:24068323. Supports GO:1904294 (positive regulation of ERAD pathway,
   IMP PMID:17000876) and ER-membrane colocalization (GO:0005789).

2. Protein quality control / proteasomal degradation
   Interacts with STUB1/CHIP and the proteasome shuttle factors RAD23A/RAD23B
   (HHR23A/B) PMID:10915768. Restricts
   ubiquitin-chain length on CHIP substrates. Supports GO:0006515, GO:0043161.

3. mTORC1 signaling (amino-acid starvation)
   Lysosome-localized ataxin-3 deubiquitinates RHEB; amino acids block this
   activity, so RHEB stays polyubiquitinated and mTORC1 is activated
   PMID:33157014.
   Supports GO:1904262 (negative regulation of TORC1 signaling),
   GO:0034198 (cellular response to amino acid starvation), and the
   lysosomal-membrane localization (GO:0005765).

4. Autophagy
   Deubiquitinates BECN1 at Lys-402, stabilizing it and promoting
   starvation-induced autophagy; poly-Gln domain mediates the BECN1
   interaction (UniProt FUNCTION, PMID:28445460 — not in GOA set).

5. Transcription regulation
   Histone-binding protein that represses transcription (UniProt FUNCTION,
   PMID:12297501). Acts with FOXO4 to regulate the SOD2 (MnSOD) promoter
   (Reactome R-HSA-9617832, R-HSA-9617927; Reactome pathway
   R-HSA-9615017 "FOXO-mediated transcription of oxidative stress ...").

6. Cytoskeleton regulation
   Loss of ataxin-3 causes cytoskeletal disorganization and increased cell
   death PMID:20637808. Source for the four
   cytoskeleton acts_upstream_of_or_within IMP terms (GO:0000226,
   GO:0030036, GO:0045104, GO:0010810). These are downstream/pleiotropic
   phenotypes of a knockdown, best kept as non-core.

Localization

Predominantly cytoplasmic but shuttles to the nucleus and associates with the
nuclear matrix [PMID:9580663 "Ataxin-3 is transported into the nucleus and
associates with the nuclear matrix"; PMID:9124802 "Machado-Joseph disease gene
product is a cytoplasmic protein widely expressed in brain"]. Both cytosol
(GO:0005829) and nucleus/nucleoplasm (GO:0005634, GO:0005654) are supported.
Lysosomal membrane (GO:0005765) is supported in the mTORC1 context
(PMID:33157014). Nuclear inclusion bodies (GO:0042405) are the pathological
polyQ-aggregate hallmark.

Interactions of note (IPI protein-binding annotations)

• RAD23A/RAD23B (HHR23A/B) — proteasome shuttle / NER factors (PMID:10915768)
• VCP/p97 — DUB activator, ERAD (PMID:16525503, PMID:22970133, PMID:17000876)
• PRKN/parkin — ataxin-3 is a multivalent ligand for the parkin Ubl domain
  (PMID:24063750); deubiquitinates parkin (Reactome R-HSA-5688837)
• STUB1/CHIP — ubiquitin ligase partner (GO:0031625 sources incl. PMID:24068323)
• HSJ1a (DNAJB2) co-chaperone — regulates ataxin-3 proteasomal degradation (PMID:21625540)
• NOD2/TLR2 innate immune signaling in myeloid cells (PMID:31379806)
• Interactome / aggregation network (PMID:32814053), ataxia PPI network (PMID:16713569)

Curation considerations for the annotation review

• protein binding (GO:0005515, IPI): multiple instances. Per project
  guidance this uninformative term is not a core function; where a more
  specific MF exists (ubiquitin ligase binding GO:0031625, ATPase binding
  GO:0051117) that is preferable. Keep IPI protein-binding as accepted
  evidence-of-interaction but non-core.
• DUB activity (GO:0004843) and linkage-specific DUB (GO:1990380,
  GO:0061578): these are the core molecular function — accept.
• ERAD, PQC, mTORC1, autophagy, transcription: genuine biological roles —
  accept; mTORC1/ERAD/PQC are strong. Some are non-core relative to the
  central DUB activity.
• Cytoskeleton IMP terms (PMID:20637808): downstream/indirect phenotypes
  from ataxin-3 depletion — keep as non-core (KEEP_AS_NON_CORE), not core MF.
• Mitochondrial ISS terms (GO:0005759, GO:0031966, GO_REF:0000024): transfer
  by sequence similarity to mouse Q9CVD2; ataxin-3 is not primarily a
  mitochondrial-matrix protein. Treat with caution — over-annotation risk.
• nucleotide-excision repair (GO:0006289, TAS PMID:10915768): the paper
  shows interaction with RAD23 (a NER/shuttle factor), not that ataxin-3
  itself does NER. Likely over-annotation of an interaction into a process.
• chemical synaptic transmission (GO:0007268, TAS PMID:7655453) and
  nervous system development (GO:0007399, TAS PMID:9124802): the cited
  papers are the disease-gene cloning / expression papers; these processes are
  not demonstrated molecular roles of ataxin-3 — over-annotation candidates.
• synapse (GO:0045202, IEA GO_REF:0000108): inter-ontology inferred; weak.

Update: Affinage deep-research integration (full-text verified)

Ran the endorsed Affinage integration (projects/AFFINAGE_EVALUATION/affinage_deep_research.py)
producing ATXN3-deep-research-affinage.md (gates passed; human; UniProt P54252
matches; self-eval pairwise=win; 24 citation-anchored findings). Fetched and
read the full text of the strongest normal-function findings and added two NEW
annotations. The DSB-repair role has since been promoted to a core
function (nuclear DUB activity on stalled RNAP II); the chromatin role is
kept non-core (catalytic-independent):

• double-strand break repair via nonhomologous end joining (GO:0006303) —
  WT ATXN3 associates with RNAP II, PNKP and LIG4 and is required for error-free,
  PNKP-mediated transcription-coupled classical NHEJ of transcribed genes; ATXN3
  deubiquitinates stalled RNAP II [PMID:32205441 "ATXN3 depletion significantly
  decreased global transcription, repair of transcribed genes, and error-free
  double-strand break repair"; PMID:30994454 TCR complex]. This is the real
  DNA-repair mechanism — distinct from the legacy NER TAS term (GO:0006289,
  RAD23-interaction-derived) that I flagged as over-annotated.
• chromatin organization (GO:0006325) — catalytic (DUB)-independent control
  of chromatin organization, DNA replication timing and HDAC3 chromatin
  recruitment PMID:36971114.

Other Affinage substrate-specific DUB findings (HDAC3/PMID:29802126, KLF4, YAP,
JunB/IRF1/STAT3/HIF-2α→PD-L1, EIF5A2; D2 retrotranslocation/PMID:24196352) are
instances of the already-annotated general deubiquitinase activity or are
cancer/disease-context roles; not added as separate process terms to avoid
substrate-level over-annotation. PMID:17764659 (Atxn3-KO mice accumulate
ubiquitinated proteins) corroborates the in-vivo DUB/PQC role. New full-text
references cached: PMID:32205441, PMID:30994454, PMID:36971114, PMID:24196352
(full text) and PMID:29802126, PMID:17764659 (abstract only).