SAE1

SAE1 (AOS1) is the regulatory subunit of the heterodimeric SUMO E1 activating enzyme, which initiates the SUMOylation cascade (PMID:10217437). Together with UBA2 it forms a thioester-linked conjugate selectively with SUMO/sentrin family members but not ubiquitin or NEDD8, establishing the heterodimer as the dedicated SUMO E1 (PMID:10217437); both subunits together constitute the catalytically active enzyme (PMID:23832333). With the E2 Ubc9, the SAE1/UBA2 enzyme catalyzes formation of polymeric SUMO-2 and SUMO-3 chains via their internal consensus motifs, whereas SUMO-1 lacks this motif and cannot form chains under the same conditions (PMID:11451954). Charging of UBA2 with SUMO1 in the presence of ATP stabilizes the heterodimer [PMID:bio_10.1101_2025.07.30.667747]. Through this activating activity, SAE1 supplies the SUMOylation of a broad set of substrates with downstream effects on cell proliferation, oncogenic signaling, glycolysis, EMT, DNA repair, and protein stability: it promotes AKT SUMOylation and Ser473 phosphorylation with cell-cycle and survival consequences (PMID:31345225), mTOR SUMOylation driving hepatocellular carcinoma progression (PMID:36748193), PKM2 SUMOylation that lowers pyruvate kinase activity and promotes nuclear translocation and glycolysis (PMID:32938830), N-cadherin SUMOylation/stabilization driving EMT and metastasis (PMID:40643801), and SUMO-2/3 modification of RhoGDI1 and SUMOylation of ZFHX3 that protect both from ubiquitin-mediated degradation (PMID:32249212, PMID:33891248). SAE1-mediated SUMOylation also governs nuclear localization, including XRCC4 nuclear retention required for genome integrity (PMID:31933971), LLPS-driven nuclear export of p27 (PMID:40486843), and suppression of p53 nuclear retention (PMID:41024125). SAE1 is a direct transcriptional target of Myc, which binds E-Box sequences near its transcription start site (PMID:22679563). A crystal structure of the SAE1 monomer has been determined, and colchicine binds SAE1 (PMID:40486843).

1. 1999 High

   Established the molecular identity of the SUMO activating enzyme by showing SAE1 partners with UBA2 to form a SUMO-specific E1, answering which enzyme initiates SUMO conjugation.

   Evidence Molecular cloning, co-expression, and beta-mercaptoethanol-sensitive thioester conjugation assay

   PMID:10217437

   Open questions at the time

   • Did not resolve the catalytic contribution of each subunit
   • Substrate repertoire and downstream signaling not addressed
2. 2001 High

   Defined a unique catalytic capability of the SAE1/UBA2 E1, showing it supports SUMO-2/3 chain polymerization but not SUMO-1 chains, explaining isoform-specific SUMO topology.

   Evidence In vitro reconstitution with purified SAE1/SAE2 and Ubc9 plus in vivo immunoblot

   PMID:11451954

   Open questions at the time

   • Physiological consequences of chain polymerization not addressed
   • In vivo substrates of poly-SUMO chains not identified
3. 2012 Medium

   Linked SAE1 expression to oncogenic transcriptional control by identifying it as a direct Myc target, placing SUMOylation capacity downstream of Myc.

   Evidence ChIP and reporter/transcriptional activation assays

   PMID:22679563

   Open questions at the time

   • Single-lab ChIP/reporter evidence
   • Functional consequence of Myc-driven SAE1 upregulation not directly tested here
4. 2013 Medium

   Confirmed that both subunits jointly constitute the active enzyme using a recombinant Aos1-Uba2 fusion that retains SUMO E1 activity.

   Evidence Recombinant fusion protein expression and in vitro SUMOylation assay

   PMID:23832333

   Open questions at the time

   • Single in vitro study
   • Did not address subunit-specific catalytic roles
5. 2019 Medium

   Connected SAE1-dependent SUMOylation to oncogenic signaling and cell-cycle control by showing it drives AKT SUMOylation and phosphorylation, with knockdown causing G2 arrest, apoptosis, and reduced tumor growth.

   Evidence Anti-SUMO1 enrichment, siRNA knockdown, flow cytometry, nude mouse xenograft in glioma

   PMID:31345225

   Open questions at the time

   • Single-lab co-IP/enrichment evidence
   • SUMO acceptor site on AKT not mapped
   • Mechanistic link between AKT SUMOylation and phosphorylation not resolved
6. 2019 Medium

   Linked SAE1 to genome integrity by showing it is required for XRCC4 SUMOylation and nuclear localization, with loss increasing DNA damage.

   Evidence 3'UTR reporter, Western blot for SUMO-XRCC4, immunofluorescence, DNA damage assay in gastric cancer cells

   PMID:31933971

   Open questions at the time

   • Single-lab study
   • XRCC4 SUMO site not mapped
   • Direct effect on NHEJ repair efficiency not quantified
7. 2020 Medium

   Established SAE1-dependent SUMOylation as a regulator of metabolism and substrate stability, modifying PKM2 to shift glycolysis and SUMOylating ZFHX3 at Lys-2806 to block its proteasomal degradation.

   Evidence siRNA knockdown, co-IP, nuclear fractionation, PK activity assay, Lys-2806 mutagenesis, proteasome inhibition, xenograft models

   PMID:32249212 PMID:32938830

   Open questions at the time

   • Single-lab studies for each substrate
   • E3 specificity beyond PIAS2 for ZFHX3 not generalized
8. 2021 Medium

   Demonstrated isoform-selective protection of a substrate, showing SAE1 is required for SUMO-2/3 (not SUMO-1) modification of RhoGDI1 that prevents its ubiquitination and degradation.

   Evidence siRNA knockdown of Aos1/Uba2, reciprocal co-IP, EdU proliferation assay in vascular smooth muscle cells

   PMID:33891248

   Open questions at the time

   • Single-lab study
   • RhoGDI1 SUMO acceptor sites not mapped
9. 2023 Medium

   Extended the substrate range to mTOR, showing SAE1-dependent mTOR SUMOylation drives hepatocellular carcinoma progression.

   Evidence siRNA knockdown, co-IP for mTOR SUMOylation, xenograft in vivo

   PMID:36748193

   Open questions at the time

   • Single-lab co-IP evidence
   • mTOR SUMO site and effect on kinase activity not defined
10. 2025 Medium

    Resolved high-resolution structural and mechanistic features of SAE1 while expanding its substrate logic into phase separation, transcriptional control, and tumor suppressor regulation, and biophysically characterized E1 heterodimer assembly dynamics.

    Evidence Crystal structure of SAE1 monomer, proteome microarray, LLPS assays, fractionation for p27/p53; co-IP and AlphaFold3 for YY1; protein profiling for N-cadherin; qFRET and SPR for SAE1-UBA2 binding (one preprint)

    PMID:40486843 PMID:40513659 PMID:40643801 PMID:41024125 PMID:bio_10.1101_2025.07.30.667747

    Open questions at the time

    • Most substrate findings from single labs
    • Heterodimer kinetics from an unreviewed preprint
    • Generality of LLPS mechanism across substrates not established

• How SAE1 substrate selectivity is determined in vivo and how distinct SUMOylation outcomes (degradation protection, localization change, LLPS) are routed for different substrates remains unresolved.
• No unifying model for substrate-specific E3 pairing
• Limited structural data on the charged SAE1/UBA2-substrate interface
• Most disease links are correlative oncology studies without germline causation