SDE2

SDE2 is a multifunctional protein synthesized as an inactive precursor whose N-terminal ubiquitin-like (UBL) domain is cleaved at a diglycine motif by deubiquitinating activity to release an active C-terminal product that governs the cellular response to replication stress and pre-mRNA splicing (PMID:27906959, PMID:28947618). Cleavage generates an SDE2 C-terminal fragment bearing an N-terminal lysine that is intrinsically short-lived: it is degraded both by the CRL4CDT2 E3 ligase in a PCNA-interaction-, cell-cycle-, and DNA-damage-dependent manner and by the Arg/N-end rule pathway via UBR1/UBR2, with the VCP/p97-UFD1-NPL4 segregase promoting its chromatin-associated, damage-induced turnover (PMID:27906959, PMID:30698750). This regulated degradation is functionally required, as a degradation-refractory mutant fails to support RPA phosphorylation, ssDNA formation, and PCNA-dependent damage bypass (PMID:30698750). At replication forks, the C-terminal domain uses a SAP domain that binds ssDNA—an interaction extended and augmented by conserved C-terminal lysines—to stabilize the fork protection complex component TIMELESS, aiding its fork localization, supporting CHK1 activation, and protecting reversed forks from MRE11-dependent degradation (PMID:33127907, PMID:35850305). Independently of its fork role, SDE2 acts in RNA metabolism: it facilitates spliceosomal incorporation of Cactin/Cay1 to drive excision of a specific subset of introns, particularly those with branchpoint-distant 3' splice sites, and functions as an RNA-binding adaptor essential for spliceosome and ribosome biogenesis and for cell viability (PMID:28947618, PMID:34365507, PMID:36095128). SDE2 additionally promotes K48-linked ubiquitination and proteasomal degradation of ATG5, thereby suppressing autophagy and ferroptosis (PMID:41666676).

1. 2016 High

   Established that human SDE2 is not a constitutive protein but a self-processing UBL-fusion whose cleaved product controls the replication-stress response and is itself a regulated, degradation-controlled factor.

   Evidence Biochemical cleavage assays, PCNA interaction studies, and genetic knockdown with S phase/survival readouts, plus CRL4CDT2 E3 ligase assays in human cells

   PMID:27906959

   Open questions at the time

   • Did not identify the protease performing UBL cleavage
   • Mechanism by which the cleaved fragment regulates PCNA monoubiquitination not resolved
2. 2017 High

   Resolved the activation logic across yeast and human—cleavage after the diglycine motif yields a short-lived activated fragment—and revealed an unanticipated splicing function via spliceosomal recruitment of Cactin.

   Evidence Genetic screens, splicing reporter assays, and biochemical processing assays in S. pombe with human validation

   PMID:28947618

   Open questions at the time

   • Identity of the human protease(s) responsible for cleavage not established
   • How splicing and replication-stress functions are partitioned unclear
3. 2019 High

   Defined the degradation route of the activated fragment, showing its N-terminal lysine makes it an Arg/N-end rule substrate cleared with help from the VCP/p97 segregase, and that this turnover is required for damage bypass.

   Evidence Site-directed mutagenesis (K78V), biochemical degradation assays, and RPA phosphorylation/ssDNA formation readouts after UVC damage

   PMID:30698750

   Open questions at the time

   • Relationship between Arg/N-end rule and CRL4CDT2 routes not fully reconciled
   • Phosphorylation site(s) triggering degradation not mapped
4. 2020 High

   Identified the direct fork-protection effector of SDE2 by showing it binds and stabilizes TIMELESS to maintain the fork protection complex and guard reversed forks.

   Evidence Reciprocal Co-IP, protein stability assays, DNA fiber fork progression, CHK1 phosphorylation analysis, and MRE11 epistasis in human cells

   PMID:33127907

   Open questions at the time

   • Structural basis of the SDE2-TIM interaction not defined here
   • Whether TIM stabilization requires SDE2 cleavage not addressed
5. 2020 Medium

   Showed SDE2 abundance is environmentally tuned, as hypoxia drives its polyubiquitination and degradation through a route distinct from the Arg/N-end rule and CDT2, linking oxygen status to PCNA monoubiquitination control.

   Evidence SILAC quantitative proteomics, ubiquitination assays, and knockdown/overexpression with PCNA monoubiquitination readouts

   PMID:32743553

   Open questions at the time

   • The E3 ligase mediating hypoxic degradation not identified
   • Single-lab biochemistry without orthogonal genetic validation
6. 2021 Medium

   Broadened SDE2 from a splicing factor to an essential dual RNA-binding/adaptor protein required for both spliceosome and ribosome assembly and for viability in mammalian cells.

   Evidence RNA-binding assays, RNA-seq splicing analysis, ribosome biogenesis assays, and knockdown viability readouts

   PMID:34365507

   Open questions at the time

   • Direct RNA targets and binding motif not defined
   • How the same protein coordinates splicing and ribosome biogenesis mechanistically unclear
7. 2022 High

   Provided the structural and biochemical basis for fork function by solving the SAP domain and showing ssDNA binding, augmented by C-terminal lysines, is required for TIM localization and fork progression.

   Evidence NMR solution structure, mutagenesis, ssDNA binding assays, immunofluorescence TIM localization, and DNA fiber assays

   PMID:35850305

   Open questions at the time

   • Whether ssDNA and TIM binding are competitive or cooperative not resolved
   • No full-length structure of the cleaved C-terminal domain
8. 2022 Medium

   Refined the splicing specificity, demonstrating that activated Sde2 with Cactin/Cay1 and Tls1 is selectively needed for introns with branchpoint-distant 3' splice sites.

   Evidence ura4 splicing reporter assays and engineered intron variants with extended BP-3'ss spacing in S. pombe

   PMID:36095128

   Open questions at the time

   • Direct demonstration of RNA folding between BP and 3'ss lacking
   • Conservation of this intron-class specificity in human cells not shown
9. 2025 Medium

   Filled the long-standing gap of the activating protease, identifying USP5 as the human DUB that cleaves the SDE2 UBL via ubiquitin substrate mimicry to release the functional C-terminal domain.

   Evidence In vitro cleavage assays, mass spectrometry, an SDE2-UBL activity-based probe, cellular reporter, and biophysical binding analysis (preprint)

   PMID:bio_10.1101_2025.05.23.655772

   Open questions at the time

   • Preprint not yet peer-reviewed
   • Whether USP5 accounts for all cellular SDE2 processing not established
10. 2026 Medium

    Extended SDE2 into proteostasis control of cell death by showing it binds ATG5 and drives its K48-linked ubiquitination and degradation to suppress autophagy and ferroptosis.

    Evidence Reciprocal Co-IP, ubiquitination assays, and knockdown/overexpression in multiple myeloma cell lines and xenografts

    PMID:41666676

    Open questions at the time

    • The E3 ligase SDE2 recruits to ATG5 not identified
    • Whether this function requires SDE2 cleavage or its RNA/fork roles unclear

• How SDE2's distinct activities—replication-fork protection, intron-specific splicing, ribosome biogenesis, and ATG5/autophagy control—are coordinated, and whether they share a common requirement for UBL cleavage and regulated degradation, remains unresolved.
• No unifying model linking the RNA-metabolism and DNA-replication functions
• Cross-talk between the multiple degradation pathways (CRL4CDT2, Arg/N-end rule, hypoxic) not integrated