ASB1

ASB1 is an ankyrin repeat and SOCS box-containing protein that functions as the substrate-recognition subunit of a Cullin5-Rbx2-Elongin B/C (CRL5) E3 ubiquitin ligase complex, targeting substrates including SQOR and CHCHD3 for K48-linked polyubiquitination and proteasomal degradation (PMID:16325183, PMID:39733518, PMID:39113857). In the testis, ASB1-mediated degradation of SQOR maintains hydrogen sulfide homeostasis and redox balance during spermiogenesis, and Asb1 knockout mice exhibit diminished spermatogenesis (PMID:11509662, PMID:39733518). ASB1 also possesses a non-canonical, E3 ligase-independent function in which it binds TAB2 and inhibits its K48-linked ubiquitination, stabilizing TAB2 to promote NF-κB and MAPK inflammatory signaling downstream of cytokine and LPS stimulation (PMID:33431678). In prostate cancer cells, ASB1-directed ubiquitination and degradation of the mitochondrial protein CHCHD3 suppresses proliferation and migration through modulation of reactive oxygen species (PMID:39113857).

1. 1998 Medium

   ASB1 was classified within a new protein family defined by ankyrin repeats and a C-terminal SOCS box, establishing it as part of the SOCS box superfamily and predicting a role in protein-protein interaction and possible Elongin-mediated degradation pathways.

   Evidence Database homology search and structural domain classification

   PMID:9419338

   Open questions at the time

   • No direct biochemical activity demonstrated for ASB1
   • Substrates unknown
   • Expression pattern only partially characterized
2. 2000 Medium

   Molecular cloning of ASB1 confirmed its domain architecture and suggested its SOCS box couples binding partners to Elongin B/C for potential degradation, providing a mechanistic framework before direct biochemical evidence existed.

   Evidence Molecular cloning, expression analysis, and sequence characterization

   PMID:11111040

   Open questions at the time

   • Elongin B/C interaction not directly demonstrated for ASB1
   • No functional assay performed
3. 2001 High

   Genetic loss-of-function established that ASB1 is dispensable for hematopoiesis and general development but specifically required for normal spermatogenesis, identifying its principal physiological context.

   Evidence Asb1 knockout and overexpression mouse models with histological analysis of testes and hematopoietic compartments

   PMID:11509662

   Open questions at the time

   • Molecular substrate(s) in the testis not identified
   • Mechanism linking ASB1 to spermatogenic defect unknown
4. 2005 High

   Biochemical demonstration that ASB1 assembles with Cullin5 and Rbx2 via its BC box and Cul5 box to form a functional E3 ubiquitin ligase complex resolved the catalytic mechanism predicted by domain homology.

   Evidence Co-immunoprecipitation, BC box and Cul5 box mutagenesis, and in vitro ubiquitin ligase activity assay

   PMID:16325183

   Open questions at the time

   • No physiological substrate identified for the ASB1-CRL5 complex
   • Ubiquitin chain linkage type not determined
5. 2013 High

   Structural determination of the homologous ASB9-Elongin B/C-Cul5 ternary complex revealed the architecture by which ASB ankyrin repeat domains present substrates toward the E2-ubiquitin active site, providing a generalizable structural model for ASB1.

   Evidence X-ray crystallography of ASB9-ElonginB/C complex with structural modeling

   PMID:23806657

   Open questions at the time

   • No ASB1-specific structure solved
   • Substrate docking geometry for ASB1 ankyrin repeats not determined
6. 2021 High

   Discovery that ASB1 stabilizes TAB2 by inhibiting its K48-linked ubiquitination — rather than promoting degradation — revealed a non-canonical anti-degradative function that activates NF-κB and MAPK inflammatory signaling in vivo.

   Evidence Reciprocal Co-IP; K48-linkage-specific ubiquitination assays; Asb1 KO mice challenged with Salmonella typhimurium, LPS, and DSS colitis model; cycloheximide chase

   PMID:33431678

   Open questions at the time

   • Mechanism by which ASB1 blocks TAB2 ubiquitination (competitive, allosteric, or E3 ligase recruitment inhibition) not defined
   • Identity of the E3 ligase that ubiquitinates TAB2 in the absence of ASB1 unknown
7. 2024 High

   Identification of SQOR as a direct ASB1-CRL5 substrate ubiquitinated at K207 and K344 linked the spermatogenesis defect of Asb1 KO mice to accumulation of SQOR, disrupted H₂S homeostasis, and oxidative stress during spermiogenesis.

   Evidence Co-IP of ASB1-ELOB; K48-linked ubiquitination assay with K207/K344 mutagenesis; Asb1 KO mouse with NaHS rescue; H₂S and oxidative stress measurements

   PMID:39733518

   Open questions at the time

   • Whether SQOR is the sole spermatogenesis-relevant substrate is unknown
   • Contribution of Cul5/Rbx2 vs. other E3 components not re-examined in testis
8. 2024 High

   Identification of CHCHD3 as a second direct ASB1 substrate extended the CRL5-dependent degradation mechanism to a mitochondrial target whose accumulation drives ROS-dependent prostate cancer cell proliferation.

   Evidence Quantitative MS interactome; Co-IP; cycloheximide chase; ubiquitination assay; knockdown/overexpression rescue in prostate cancer cells

   PMID:39113857

   Open questions at the time

   • Specific ubiquitination sites on CHCHD3 not mapped
   • In vivo tumor suppression by ASB1 not tested
   • Whether CHCHD3 degradation is relevant outside prostate cancer unknown

• Key unresolved questions include how ASB1 switches between its canonical E3 ligase substrate-degradation mode and its non-canonical TAB2-stabilizing function, whether additional substrates exist, and how ASB1 activity is itself regulated.
• No structural model of ASB1 itself or ASB1-substrate complexes
• Regulatory inputs controlling ASB1 expression or activity (post-translational modification, transcriptional regulation beyond initial cloning data) poorly defined
• Mechanism distinguishing degradative vs. stabilizing modes unresolved