UBL3

UBL3 is a prenylated ubiquitin-like protein that functions as a post-translational modification factor and membrane-anchored sorting signal, directing diverse client proteins into multivesicular bodies and small extracellular vesicles (sEVs). UBL3 covalently modifies substrates including Ras, NOTCH2, and α-synuclein, and Ubl3-knockout mice show a ~60% reduction in total sEV protein content, establishing UBL3 as a major determinant of EV cargo loading (PMID:30258067, PMID:39182355). UBL3 also serves as an essential prenylation-dependent adaptor for MARCH1-mediated ubiquitination of MHC class II and CD86 on antigen-presenting cells, and genetic epistasis with CD83 places UBL3 in the MHC II ubiquitination pathway that controls CD4⁺ T cell positive selection in the thymus (PMID:35411049, PMID:41636204). In addition, UBL3 forms a ternary complex with NOTCH2 and ADAM17, promoting ADAM17-mediated NOTCH2 cleavage and sEV-mediated transfer of activated NOTCH2 to recipient cells, thereby driving VEGFR inhibitor resistance in renal cell carcinoma (PMID:41418537).

1. 2018 High

   The fundamental question of UBL3's molecular function was answered: rather than acting as a conventional ubiquitin-like conjugate in protein degradation, UBL3 operates as a PTM factor that modifies client proteins (e.g., Ras) and serves as a sorting signal directing cargo into MVBs and sEVs, with knockout mice showing a dramatic reduction in sEV protein content.

   Evidence Proteomics identifying 1241 interactors, in vitro UBL3 modification assays, and Ubl3-KO mouse sEV quantification

   PMID:30258067

   Open questions at the time

   • The enzymatic machinery (E1/E2/E3) catalyzing UBL3 conjugation to substrates remains unidentified
   • The structural basis of UBL3 attachment to client proteins is undefined
   • Whether UBL3 modification is reversible (deconjugation) is unknown
2. 2022 High

   A genome-wide CRISPR screen revealed that UBL3 has a second, mechanistically distinct function as an essential adaptor for MARCH1-mediated ubiquitination of MHC II and CD86, requiring prenylation-dependent plasma membrane anchoring rather than its PTM/conjugation activity, establishing UBL3 as a critical regulator of antigen-presenting cell surface phenotype.

   Evidence CRISPR KO screen, Ubl3-deficient mice with flow cytometry for MHC II/CD86, ubiquitination assays, prenylation mutant analysis

   PMID:35411049

   Open questions at the time

   • Whether UBL3 directly bridges MARCH1 to its substrates via a defined binding interface is not resolved
   • The relationship between UBL3's adaptor role for MARCH1 and its sEV sorting function is unclear
3. 2023 Medium

   The question of how UBL3 reaches specific subcellular compartments was addressed: PDE6D binds prenylated UBL3 and mediates its trafficking to primary cilia, where UBL3 localizes in a prenylation-dependent manner, revealing a ciliary function and extending UBL3's role beyond the endosomal/plasma membrane compartments.

   Evidence Affinity proteomics, immunofluorescence in photoreceptors, prenylation mutant analysis

   PMID:36672247

   Open questions at the time

   • The functional consequence of UBL3 ciliary localization (e.g., ciliary signaling, cargo sorting) is unknown
   • Whether UBL3 modifies ciliary proteins as a PTM factor has not been tested
   • Single-lab finding awaits independent confirmation
4. 2023 Medium

   UBL3 was linked to neurodegeneration-associated proteins: it interacts with α-synuclein (independent of its C-terminal PTM cysteines), and this interaction is modulated by the mitochondrial toxin MPP+ and regulated by the antioxidant enzyme MGST3, with MGST3 overexpression promoting UBL3-dependent sEV secretion of α-synuclein.

   Evidence Split luciferase complementation, co-IP, RNAi screen, MGST3 overexpression with sEV secretion assays

   PMID:37371780 PMID:37760932 PMID:39000460

   Open questions at the time

   • Whether UBL3-dependent α-synuclein sEV release is pathogenic or protective in vivo is unknown
   • The mechanism by which MGST3 modulates UBL3–α-synuclein interaction is indirect and undefined
   • All studies from single lab
5. 2024 Medium

   The UBL3 brain interactome was expanded in vivo, revealing interactions with RNA-binding proteins (FUS) and metabolic enzymes (HPRT1, LYPLA1), and UBL3 was shown to interact with polyQ-expanded huntingtin fragments and co-localize with mHTT inclusions in Huntington's disease patient brain, suggesting UBL3 participates in intracellular sorting of aggregation-prone proteins.

   Evidence Biotinylated-UBL3 transgenic mouse proteomics with endogenous IP validation; split luciferase, co-IP, and immunohistochemistry on HD postmortem brain

   PMID:39148092 PMID:39449505

   Open questions at the time

   • Whether UBL3 modification of mHTT or FUS alters their aggregation or toxicity in vivo is untested
   • The functional significance of UBL3 interaction with RNA-binding proteins is unknown
   • Findings from single lab require independent validation
6. 2025 High

   UBL3's sEV sorting function was shown to have direct pathological relevance: UBL3 forms a ternary complex with NOTCH2 and ADAM17, accelerates NOTCH2 cleavage, and packages modified NOTCH2 into sEVs that activate NOTCH signaling in recipient cells, driving VEGFR inhibitor resistance in renal cell carcinoma through NOTCH-PTEN-AKT and NOTCH-FOS pathways.

   Evidence Genome-wide CRISPR screen, co-IP of ternary complex, sEV proteomics, RNA-seq, ChIP, in vivo LNP-CRISPR rescue

   PMID:41418537

   Open questions at the time

   • Whether UBL3-NOTCH2 sEV-mediated resistance operates in other cancer types is unknown
   • The precise UBL3 modification site(s) on NOTCH2 are not mapped
7. 2026 Medium

   Genetic epistasis in double-mutant mice established that UBL3 functions in the CD83-dependent MHC II ubiquitination pathway in thymic epithelial cells, with UBL3 deletion rescuing CD4+ T cell positive selection in CD83-deficient animals, placing UBL3 as a physiological regulator of T cell development.

   Evidence Cd83anu/anu Ubl3−/− double-KO mice, flow cytometry of thymocyte subsets and TEC surface MHC II

   PMID:41636204

   Open questions at the time

   • Whether UBL3 acts through MARCH1 or an alternative E3 ligase in thymic epithelial cells is not resolved
   • The broader impact on immune tolerance and autoimmunity is untested

• Key unresolved questions include the identity of the E1/E2/E3 enzymatic cascade for UBL3 conjugation, the structural basis of UBL3 attachment to substrates, whether UBL3 modification is reversible, and the in vivo consequences of UBL3-dependent sEV transfer of neurodegeneration-associated proteins.
• No E1, E2, or E3 enzyme for UBL3 conjugation has been identified
• No crystal or cryo-EM structure of UBL3 in complex with any substrate exists
• Whether UBL3-mediated sEV sorting of α-synuclein/mHTT is protective or pathogenic in vivo remains untested