# FAF2 (UBXD8/ETEA) review notes

UniProt: Q96CS3 (FAF2_HUMAN), 445 aa. Synonyms: ETEA, UBXD8, UBXN3B, KIAA0887.
Domain architecture: N-terminal UBA domain (12-48), UAS domain (thioredoxin-like, CDD cd02991 UAS_ETEA),
coiled-coil (275-350), C-terminal UBX domain (357-439). The UBX domain binds p97/VCP; the UBA domain binds ubiquitin.

## Core biology
FAF2 is a membrane-anchored UBX-domain p97/VCP cofactor. It recruits the VCP segregase to the ER membrane
and to lipid droplets, where it functions both in ERAD substrate extraction/retrotranslocation and in
lipid-droplet / fatty-acid metabolism.

### p97/VCP cofactor + UBX/UBA
[PMID:18775313 "p97 assembles with all of the 13 mammalian UBX-domain proteins. The UBX proteins that bind ubiquitin conjugates also interact with dozens of E3 ubiquitin ligases"]
- UBXD8/FAF2 is one of the 13 UBX-domain p97 cofactors; UBX binds p97, UBA binds ubiquitin conjugates.
- UniProt IntAct: VCP NbExp=16; UBAC2 NbExp=8. ComplexPortal CPX-8104 "VCP-NPL4-UFD1-FAF2 AAA ATPase complex".

### ERAD
[PMID:18711132 "We identified AUP1, UBXD8, UBC6e, and OS9 as functionally important components of this degradation complex"]
- FAF2/UBXD8 is part of the SEL1L/HRD1 ERAD dislocation complex (with SEL1L, OS9, AUP1, UBE2J1).
[PMID:24215460 "UBXD8 is an endoplasmic membrane protein involved in the translocation of ubiquitinated ERAD substrates, UBQLN2 likely cooperates with UBXD8 to transport defective proteins from the endoplasmic reticulum to the cytosol"]
- Implicated in retrotranslocation/dislocation of ERAD substrates (e.g. NHK alpha-1-antitrypsin, PMID:25660456).

### Lipid droplet / ATGL regulation
[PMID:23297223 "UBXD8-mediated recruitment of p97/VCP to LDs increases LD size by inhibiting the activity of adipose triglyceride lipase (ATGL), the rate-limiting enzyme in triacylglycerol hydrolysis. Our findings show that UBXD8 binds directly to ATGL and promotes dissociation of its endogenous coactivator, CGI-58"]
[PMID:23297223 "association of UBXD8 with the ER-resident rhomboid pseudoprotease UBAC2 specifically restricts trafficking of UBXD8 to LDs"]
- FAF2 binds PNPLA2/ATGL (lipase binding), inhibits it (lipase inhibitor activity), promotes CGI-58/ABHD5 dissociation -> inhibits lipolysis, increases LD size.
- UBAC2 interaction restricts ER->LD trafficking of FAF2.
- LD localization independently confirmed by lipid-droplet proteomics [PMID:14741744] (HuH7 LD fraction).

### Stress granule disassembly
[PMID:34739333 "ubiquitinated G3BP1 interacted with the endoplasmic reticulum–associated protein FAF2, which engaged the ubiquitin-dependent segregase p97/VCP"]
- On heat shock, FAF2 binds ubiquitinated G3BP1 and recruits VCP to extract G3BP1 -> stress granule disassembly. FAF2 here acts as a protein-macromolecule adaptor bridging ubiquitinated substrate to VCP.

## Annotation assessment summary
- Core MF: ubiquitin binding (GO:0043130, UBA), ubiquitin protein ligase binding (GO:0031625), protein-macromolecule adaptor activity (GO:0030674) — these capture the informative p97-cofactor/adaptor function.
- Core CC: ER (GO:0005783), lipid droplet (GO:0005811), VCP-NPL4-UFD1 complex (GO:0034098).
- Core BP: ERAD pathway (GO:0036503), retrograde protein transport ER to cytosol (GO:0030970), lipid droplet organization (GO:0034389), stress granule disassembly (GO:0035617).
- lipase binding (GO:0035473) / lipase inhibitor activity (GO:0055102): specific, experimentally supported (PMID:23297223) — keep.
- Reactome extracellular region / azurophil granule lumen (neutrophil degranulation, R-HSA-6798751): bulk proteomic neutrophil-granule annotations; not the core ER/LD function — KEEP_AS_NON_CORE.
- The many bare "protein binding" (GO:0005515) IPI annotations: KEEP_AS_NON_CORE (uninformative term), though several map to informative interactions captured by specific terms.
- ATPase complex (GO:1904949) NAS / proteasomal protein catabolic process (GO:0010498) IEA: generic parents; keep non-core.
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## Falcon deep-research findings (incorporated 2026-06)

- Systematic VCP-UBXD adaptor interactome (Raman 2015) classifies FAF2/UBXD8 as a membrane-tethered p97 adaptor in ERAD/lipid-droplet homeostasis, recovering interactors AMFR, DERLIN2, AUP1, BAG6, UBAC2 plus the shared UFD1L/NPLOC4 module [PMID:26389662 "a network of over 195 interacting proteins, implicating VCP in diverse cellular pathways"]. Added as a MEDIUM-relevance reference (PMID:26389662; DOI:10.1038/ncb3238) and id-only to the ERAD-pathway supported_by.
- All other major 2022-2025 Falcon papers were ALREADY incorporated in the completed review: Fujisawa 2022 eLife ubiquitin-threshold (PMID:35920641), Ganji 2023 ER-mito contact/lipid-saturation (PMID:36746962), Koyano 2024 basal pexophagy (PMID:39472561), Huo 2025 UBH-UBX module (PMID:41258083), and Olzmann 2013 ATGL/lipid-droplet (PMID:23297223). No new additions needed for these.
- Falcon reiterates FAF2's UBH (ubiquitin-binding helix) adjacent to UBX amplifies p97-UFD1L-NPLOC4 unfolding ~2-fold and is sufficient (with UFD1/NPLOC4) to extract membrane proteins from ER and mitochondria; FAF2-KO stabilized CD4 ~10-fold (already captured via PMID:41258083) [DOI:10.1038/s41467-025-65166-4].
- Falcon confirms FAF2/p97 suppresses basal pexophagy by extracting ubiquitylated PMP70/PEX16, preventing OPTN recruitment (already captured via PMID:39472561) [DOI:10.1038/s41467-024-53558-x].
- No new disease/substrate/localization finding for FAF2 beyond what the completed review already documents; the review was already comprehensive for the recent literature.