SPSB3

SPSB3 is a SPRY domain-containing substrate receptor of the CRL5 (Cullin5-ElonginC/B) ubiquitin ligase complex that selectively targets proteins for ubiquitination and proteasomal degradation. Its best-characterized substrate is nuclear cGAS, where a cryo-EM structure reveals that SPSB3 recognizes a conserved C-terminal Asn-Asn degron motif on nucleosome-bound cGAS to promote its ubiquitylation and degradation, thereby restraining tonic type I interferon signaling and antiviral responses (PMID:38418882). SPSB3 also mediates GSK-3β-phosphorylation-dependent degradation of the EMT transcription factor SNAIL, suppressing epithelial-mesenchymal transition and tumor metastasis (PMID:29059170). SPSB3 exhibits substrate selectivity distinct from its paralogs SPSB1/2/4, lacking the ability to bind Par-4 or degrade RevERBα, and it engages FOG-2 through a non-canonical mechanism that does not lead to degradation (PMID:16369487, PMID:31607207, PMID:41418348).

1. 2005 High

   Structural and binding studies established that SPSB3 has a SPRY domain architecture shared with its paralogs but exhibits distinct substrate selectivity — it binds the MET receptor yet, unlike SPSB1/2/4, cannot recognize Par-4, mapping specificity differences to divergent loop regions.

   Evidence NMR structure of the SSB-2 SPRY domain combined with mutational analysis and binding assays; Co-IP of SSB proteins with MET

   PMID:15713673 PMID:16369487

   Open questions at the time

   • MET interaction shown only by Co-IP without reciprocal validation or functional consequence for SPSB3 specifically
   • Which structural features of SPSB3 loops dictate its unique selectivity profile was not resolved
2. 2017 High

   A genome-wide screen identified SPSB3 as an E3 ligase component that targets phosphorylated SNAIL for polyubiquitination and proteasomal degradation, establishing the first functional substrate and linking SPSB3 to EMT suppression.

   Evidence Luciferase-based siRNA screen; Co-IP, ubiquitination assays, overexpression/knockdown, and in vivo metastasis models

   PMID:29059170

   Open questions at the time

   • Whether SPSB3 recognizes SNAIL through the same degron grammar as other SPSB-family substrates was not determined
   • The specific E3 ligase complex components (Cullin5, ElonginB/C) were not biochemically mapped for SNAIL degradation in this study
3. 2019 Medium

   Functional screens defined SPSB3's substrate boundaries by showing it does not target the circadian regulator RevERBα, unlike SPSB1 and SPSB4, further sharpening the paralog-specific substrate model.

   Evidence Cell-based ubiquitin ligase screen with Co-IP and ubiquitination assays; circadian period assays

   PMID:31607207

   Open questions at the time

   • The molecular basis for why SPSB3 fails to recognize RevERBα was not resolved
   • Comprehensive substrate profiling across all four paralogs has not been performed
4. 2024 High

   Cryo-EM structures and degron mutagenesis revealed the atomic mechanism by which SPSB3 acts as the CRL5 substrate receptor for nuclear cGAS, identifying a conserved C-terminal Asn-Asn motif as the minimal degron, and demonstrating that SPSB3-mediated cGAS degradation tonically suppresses type I interferon signaling.

   Evidence Cryo-EM of nucleosome–cGAS–SPSB3 complex; ubiquitination assays; SPSB3 knockout with interferon and viral infection readouts; degron motif mutagenesis

   PMID:38418882

   Open questions at the time

   • Whether the NN degron is the universal recognition element for all SPSB3 substrates (e.g., SNAIL) is unknown
   • Tissue-specific regulation of SPSB3 expression and how it tunes cGAS turnover in vivo remain uncharacterized
5. 2025 Medium

   SPSB3 was shown to bind FOG-2 through a mechanism independent of the canonical D/E-I/L-N-N-N degron used by other SPSB paralogs, but this binding does not lead to FOG-2 degradation, revealing that SPSB3 substrate engagement does not always result in ubiquitin-dependent turnover.

   Evidence Co-IP, ubiquitination assays, bioinformatic screen, 3T3-L1 preadipocyte differentiation assays

   PMID:41418348

   Open questions at the time

   • The non-canonical binding interface on SPSB3 that engages FOG-2 has not been structurally resolved
   • Whether SPSB3 binding to FOG-2 has a non-degradative regulatory function is untested
6. 2025 Medium

   Spsb3 knockout mice showed no defects in spermatogenesis or male fertility despite high testicular expression, establishing that SPSB3 is dispensable for reproduction and suggesting functional redundancy or context-dependent roles.

   Evidence CRISPR/Cas9 knockout mice; CASA, histology, immunostaining, TUNEL assays

   PMID:40225996

   Open questions at the time

   • Whether paralog compensation masks Spsb3 loss-of-function phenotypes was not tested (e.g., double knockouts)
   • Immune or stress-related phenotypes were not assessed in these knockout mice

• A comprehensive substrate catalogue for SPSB3 and the structural basis for its distinct degron recognition compared to SPSB1/2/4 remain unresolved, as does the in vivo physiological consequence of SPSB3 loss beyond reproduction.
• No unbiased proteomics-based substrate identification for SPSB3 has been reported
• No structure of SPSB3 alone or in complex with SNAIL is available
• In vivo immune phenotypes of Spsb3 knockout animals have not been characterized