USE1

USE1 is a bifunctional protein that operates both as the dedicated E2 enzyme of the UBA6-driven ubiquitin-like conjugation system and as a Q-SNARE in early secretory-pathway membrane trafficking (PMID:20975683, PMID:16354670). In its conjugation role, USE1 receives activated FAT10 from the E1 enzyme UBA6 and serves as the primary E2 of the FAT10ylation cascade, with USE1 itself being the first identified FAT10 substrate via cis-autoFAT10ylation (PMID:20975683); the same UBA6–USE1 axis also catalyzes ubiquitylation of substrates including the E3 ligase Ube3a (E6-AP), targeting them for proteasomal turnover (PMID:23499007). This pathway is gated upstream: LMO2 binds the UBA6 ubiquitin-fold domain and competitively displaces USE1, dampening cellular FAT10ylation (PMID:27569286), and under TNF-driven inflammation FAT10 conjugation becomes USE1-independent through alternative FAT10-charged E2 enzymes (PMID:37604583). The UBA6–USE1 axis is required in vivo for neuronal patterning and dendritic spine density, where loss of upstream Uba6 elevates Ube3a and Shank3 (PMID:23499007), and for erythroblast expansion, where USE1 translation is driven by the RNA-binding protein Grsf1 acting on G-repeats in the Use1 5'UTR (PMID:25184340). Independently, USE1 forms a tight ER/ERGIC-localized SNARE complex with syntaxin 18 and Sec22b, associates with VAMP7, and is needed for post-Golgi cathepsin D maturation and lysosomal degradative function (PMID:16354670), and supports maturation, glycosylation, and propagation of mumps virus fusion protein (PMID:36480520). USE1 protein level is controlled by APC/C through a conserved D-box, and stabilizing missense mutations or overexpression promote lung cancer cell proliferation, migration, and invasion (PMID:28376205, PMID:31791842).

1. 2005 High

   Before its role in ubiquitin-like conjugation was known, the question was what membrane-trafficking machinery USE1 belongs to; this established USE1 (D12) as a bona fide Q-SNARE of the early secretory pathway with a functional consequence for lysosomal maturation.

   Evidence Reciprocal co-IP defining a syntaxin 18/Sec22b/α-SNAP complex, immunofluorescence/fractionation localization, and siRNA knockdown with cathepsin D maturation and lipofuscin/apoptosis readouts

   PMID:16354670

   Open questions at the time

   • Does not resolve the structural arrangement of the SNARE bundle
   • Does not connect the SNARE function to the later-discovered E2 conjugation role
2. 2010 High

   The question of how FAT10 is transferred from its E1 to substrates was answered by placing USE1 as the dedicated E2 of the cascade and revealing it as the first FAT10 substrate.

   Evidence In vitro FAT10 transfer assay from UBA6 to USE1, co-IP from cells, and siRNA knockdown with FAT10 conjugate readout; cis-versus-trans auto-FAT10ylation tested

   PMID:20975683

   Open questions at the time

   • Does not identify the broader set of FAT10 substrates downstream of USE1
   • Functional consequence of USE1 auto-FAT10ylation unresolved
3. 2013 High

   It was unclear whether the UBA6–USE1 axis had physiological substrates and in vivo importance; this showed it ubiquitylates Ube3a for degradation and is required for neuronal architecture.

   Evidence In vitro ubiquitylation reconstitution, neuronal conditional Uba6 knockout mouse with Ube3a/Shank3/Arc western blots, confocal spine-density analysis, and behavioral assays

   PMID:23499007

   Open questions at the time

   • Establishes Uba6 loss-of-function phenotypes but does not isolate USE1's specific contribution genetically
   • Does not distinguish ubiquitin versus FAT10 conjugation in the neuronal phenotypes
4. 2014 Medium

   How USE1 abundance is set translationally and what tissue depends on it was addressed by identifying Grsf1-driven translation required for erythroid expansion.

   Evidence RNA band-shift mapping of G-repeats in the Use1 5'UTR, Grsf1 dose-dependent reporter translation assay, and siRNA knockdown of Grsf1/Use1 with erythroblast expansion readout

   PMID:25184340

   Open questions at the time

   • Does not determine whether the erythroid requirement reflects USE1's SNARE or E2 function
   • Mechanism of Grsf1-stimulated translation initiation not defined
5. 2016 Medium

   The question of how the FAT10ylation cascade is regulated upstream of USE1 was addressed by showing LMO2 competitively blocks the UBA6–USE1 interaction.

   Evidence Co-IP, domain mapping to the UBA6 UFD, and cellular FAT10ylation assays with LMO2 overexpression measuring p62 degradation

   PMID:27569286

   Open questions at the time

   • Single lab without reciprocal in vivo validation
   • Does not establish physiological contexts where LMO2 levels gate USE1 charging
6. 2017 Medium

   How USE1 protein stability is controlled and whether it contributes to cancer was answered by identifying APC/C-mediated turnover via a D-box and oncogenic stabilizing mutations.

   Evidence Proteomics of interactors, D-box mutation half-life analysis, and stable overexpression/knockdown in lung cancer lines plus xenografts measuring proliferation, migration, invasion

   PMID:28376205

   Open questions at the time

   • Does not define which USE1 enzymatic activity drives the oncogenic phenotype
   • Single lab; D-box ubiquitylation not reconstituted with purified APC/C
7. 2019 Medium

   Whether USE1 is required for tumor cell survival was tested, showing its knockdown triggers cell cycle arrest and apoptosis in lung cancer models.

   Evidence siRNA knockdown with viability, cell cycle, and apoptosis assays in lung cancer cells and A549 xenografts

   PMID:31791842

   Open questions at the time

   • Does not identify the molecular mechanism linking USE1 loss to arrest/apoptosis
   • Off-target effects of knockdown not fully excluded
8. 2022 Medium

   A pathogen-relevant role was established by showing USE1 supports mumps virus fusion protein maturation and propagation.

   Evidence Proximity labeling, reciprocal co-IP with MuV F protein, and siRNA knockdown with F glycosylation and viral propagation readouts

   PMID:36480520

   Open questions at the time

   • Does not determine whether the SNARE complex or USE1's E2 activity mediates F protein support
   • Mechanism of glycosylation defect upon USE1 loss unresolved
9. 2023 Medium

   Whether USE1 is the obligate E2 for FAT10 under all conditions was answered: USE1 is required basally but dispensable under TNF, where alternative FAT10-charged E2s operate.

   Evidence USE1 knockout cells with FAT10 conjugation assays under basal versus TNF stimulation and active-site cysteine charging assays for alternative E2 enzymes

   PMID:37604583

   Open questions at the time

   • Identities and substrate specificities of the alternative E2 enzymes not fully resolved
   • Mechanism by which TNF redirects FAT10 conjugation away from USE1 unknown

• It remains unresolved how USE1's two distinct functions—Q-SNARE in ER/ERGIC trafficking and E2 in FAT10/ubiquitin conjugation—are coordinated, regulated, and partitioned across the tissue-specific phenotypes attributed to it.
• No study dissects which activity drives the erythroid, neuronal, viral, or oncogenic phenotypes
• No structural basis for dual functionality reported