GPS1

GPS1 (CSN1) is a core subunit of the eight-subunit COP9 signalosome (CSN), a conserved complex that functions as a deneddylase removing NEDD8 from cullins to regulate cullin-RING ubiquitin ligase activity, thereby controlling protein ubiquitination in pathways including nucleotide excision repair and p53 degradation (PMID:11337588, PMID:12732143, PMID:11285227). GPS1 directly recruits inositol 1,3,4-trisphosphate 5/6-kinase to the CSN, providing an associated kinase activity that phosphorylates IκBα, p53 (at Thr155), and c-Jun (PMID:12324474, PMID:9535219). Independent of CSN deneddylase activity, GPS1 negatively regulates the AP-1/JNK signaling axis by suppressing c-Jun phosphorylation and JNK1 expression, and interacts with the SMC5/6 complex component NSMCE4A to coordinate DNA end resection during homologous recombination repair (PMID:21604193, PMID:32384871). Somatic missense mutations in GPS1 in penile squamous cell carcinoma disrupt miRNA-mediated gene silencing through aberrant AGO2 colocalization, implicating GPS1 as a tumor suppressor (PMID:27325650).

1. 1998 High

   Identification of GPS1 as a subunit of the COP9 signalosome established that a ~450 kDa complex distinct from the 26S proteasome harbors associated kinase activity toward IκBα, p105, and c-Jun, opening the question of what enzymatic activities this complex carries.

   Evidence Biochemical purification from human cells with kinase assays and sequence analysis

   PMID:9535219

   Open questions at the time

   • Identity of the CSN-associated kinase was unknown
   • Physiological substrates of the complex not validated in vivo
   • Stoichiometry and architecture of GPS1 within the complex unresolved
2. 2001 High

   Discovery that the CSN is a deneddylase that cleaves NEDD8 from cullins revealed the primary enzymatic activity of the GPS1-containing complex and explained its role in regulating SCF ubiquitin ligase cycles.

   Evidence In vitro NEDD8 cleavage assay plus analysis of CSN-deficient S. pombe showing accumulation of neddylated proteins

   PMID:11337588

   Open questions at the time

   • Contribution of individual subunits including GPS1 to deneddylation catalysis not dissected
   • Scope of cullin substrates beyond CUL1 not fully mapped
3. 2001 High

   Demonstration that the CSN phosphorylates p53 at Thr155 to promote its ubiquitin-dependent degradation connected the GPS1-containing complex to a major tumor suppressor pathway and distinguished the CSN kinase from known p53 kinases.

   Evidence Phosphorylation site mapping, T155V mutagenesis stabilizing p53, electron microscopy of p53–CSN complex, curcumin inhibition of degradation

   PMID:11285227

   Open questions at the time

   • Molecular identity of the responsible kinase still unresolved at this point
   • In vivo relevance of Thr155 phosphorylation in tumor settings not tested
4. 2002 High

   Identification of inositol 1,3,4-trisphosphate 5/6-kinase as the CSN-associated kinase, recruited through direct binding to GPS1, resolved the long-standing question of the kinase's identity and revealed GPS1 as a scaffold bridging kinase and deneddylase activities.

   Evidence Purification from bovine brain, reciprocal co-IP in HEK293 cells, substrate phosphorylation assays matching CSN kinase specificity, curcumin sensitivity

   PMID:12324474

   Open questions at the time

   • Structural basis of GPS1–5/6-kinase interaction not determined
   • How GPS1 binding inhibits inositol kinase activity mechanistically unclear
5. 2003 High

   Showing that the CSN is incorporated into CUL4A-based DDB2 and CSA ubiquitin ligase complexes and that CSN knockdown impairs nucleotide excision repair extended GPS1's functional reach from SCF regulation to the DNA damage response.

   Evidence Biochemical purification of DDB2/CSA ligase complexes, ubiquitin ligase activity assays, RNAi knockdown with NER phenotype

   PMID:12732143

   Open questions at the time

   • Whether GPS1 subunit specifically contacts CUL4A ligase components not resolved
   • Relative contributions of deneddylation versus kinase activity to NER unclear
6. 2007 Medium

   Discovery that Asb-4 binds GPS1 and reduces its protein levels, thereby attenuating GPS1-enhanced JNK activity and IRS-1 Ser307 phosphorylation, placed GPS1 at an intersection of JNK and insulin signaling.

   Evidence Yeast two-hybrid confirmed by co-IP, kinase assays and co-transfection in HEK293/3T3-L1 cells

   PMID:17276034

   Open questions at the time

   • Mechanism by which Asb-4 reduces GPS1 levels (proteasomal vs. transcriptional) not defined
   • Physiological relevance in metabolic tissues not demonstrated in vivo
7. 2011 Medium

   Establishing GPS1 as a negative regulator of c-Jun phosphorylation and JNK1 expression clarified its role opposing CSN5-mediated AP-1 activation, revealing subunit-specific signaling polarity within the CSN.

   Evidence Phosphorylation and transcriptional reporter assays, JNK1 expression analysis ruling out proteolysis and mRNA instability

   PMID:21604193

   Open questions at the time

   • Mechanism of JNK1 transcriptional/posttranscriptional repression by GPS1 not identified
   • Whether this function requires intact CSN or is a moonlighting role is unclear
8. 2016 Medium

   Identification of somatic GPS1 mutations in penile squamous cell carcinoma that cause AGO2 mislocalization and loss of miRNA silencing expanded GPS1's functional repertoire to include regulation of the RISC pathway.

   Evidence Whole-exome sequencing with targeted resequencing, cell-based expression of mutant GPS1 with AGO2 colocalization and miRNA silencing readouts

   PMID:27325650

   Open questions at the time

   • Direct physical interaction between GPS1 and AGO2/RISC components not demonstrated
   • Whether miRNA silencing defect is CSN-dependent or GPS1-autonomous unknown
   • Single tumor type studied; generality of tumor suppressor role unclear
9. 2020 Medium

   Demonstration that GPS1 physically interacts with NSMCE4A and that GPS1 depletion or CSN inhibition causes excess SMC5/6 accumulation at DNA damage sites linked the CSN to homologous recombination repair through the SMC5/6 genome maintenance complex.

   Evidence Yeast two-hybrid confirmed by co-IP, colocalization imaging across cell cycle, GPS1 depletion with laser-induced damage and CSN inhibition

   PMID:32384871

   Open questions at the time

   • Whether GPS1–NSMCE4A interaction requires intact CSN or occurs independently not resolved
   • Which cullin-RING ligase is the deneddylation target relevant to SMC5/6 recruitment unknown
10. 2020 Medium

    GPS1 knockdown in hepatocellular carcinoma cells inhibited proliferation and migration, with GPS1 promoting cyclin A2 expression independently of ubiquitination, revealing a pro-tumorigenic role in HCC that contrasts with its tumor suppressor function in penile SCC.

    Evidence siRNA knockdown, proliferation/migration assays, cyclin A2 expression analysis, in vivo xenograft validation

    PMID:33200803

    Open questions at the time

    • Mechanism by which GPS1 upregulates cyclin A2 in a ubiquitination-independent manner not defined
    • Context-dependent oncogenic versus tumor-suppressive roles not reconciled

• Key unresolved questions include the structural basis by which GPS1 scaffolds kinase and deneddylase activities within the CSN, whether GPS1's roles in miRNA silencing and AP-1/JNK regulation are CSN-dependent or moonlighting functions, and the mechanism underlying context-dependent oncogenic versus tumor-suppressive activities across cancer types.
• No high-resolution structure of GPS1 within a full CSN complex with bound kinase
• CSN-dependent versus CSN-independent functions of GPS1 not systematically dissected
• In vivo genetic models (knockout mouse) characterizing GPS1 function in development and disease are lacking in the timeline