KCTD2

KCTD2 is a BTB-domain-containing substrate adaptor for Cullin3-RING E3 ubiquitin ligase complexes that directs ubiquitination of diverse substrates to regulate cell proliferation, GPCR signaling, and neuronal function. It recruits c-Myc for polyubiquitination and proteasomal degradation, thereby suppressing glioma stem cell self-renewal and aerobic glycolysis (PMID:28060381). KCTD2 also forms hetero-oligomers with KCTD5 to recruit Gβγ subunits in an agonist-dependent manner, promoting monoubiquitination of Gβ1/2 at K23 and attenuating Gβγ-mediated sensitization of adenylyl cyclase 5, thus shaping cAMP signaling downstream of GPCRs (PMID:34342229, PMID:36736897). KCTD2 localizes to neuronal synapses, functionally substitutes for the Drosophila sleep regulator Insomniac in vivo, and acts redundantly with KCTD5 and KCTD17 to control cell growth and Gβ1 protein levels (PMID:28558011, PMID:38732215).

1. 2016 Medium

   Structural analysis of the Drosophila ortholog Insomniac established the biophysical framework for understanding how KCTD2-family proteins simultaneously engage Cul3 and substrates such as dGRASP through a two-domain architecture.

   Evidence SAXS, recombinant protein binding assays with quantified affinities for Cul3 and dGRASP using purified Drosophila Inc protein

   PMID:27678190

   Open questions at the time

   • Data are from Drosophila ortholog; direct structural characterization of mammalian KCTD2 is lacking
   • dGRASP ubiquitination was not directly demonstrated
   • No high-resolution structure of the Inc–Cul3 complex
2. 2017 High

   Identification of KCTD2 as a CUL3 adaptor that targets c-Myc for ubiquitin-dependent degradation established its first defined substrate relationship and linked it to glioma stem cell biology.

   Evidence Yeast two-hybrid, reciprocal co-IP, ubiquitination assay, shRNA knockdown phenotypes, and intracranial xenograft tumor growth in mouse models

   PMID:28060381

   Open questions at the time

   • The specific lysine residues on c-Myc targeted for ubiquitination were not mapped
   • Whether KCTD2-mediated c-Myc degradation operates outside glioma is unknown
3. 2017 High

   Cross-species rescue experiments demonstrated that mammalian KCTD2 is a functional ortholog of the Drosophila sleep regulator Insomniac, establishing an evolutionarily conserved role for KCTD2–Cul3 complexes at synapses.

   Evidence In vivo rescue of inc mutant Drosophila sleep phenotype by mouse KCTD2 expression, neuronal synaptic localization imaging, co-association with Cul3

   PMID:28558011

   Open questions at the time

   • The neuronal substrates ubiquitinated by KCTD2 at synapses remain unidentified
   • Whether KCTD2 regulates sleep in mammals has not been tested
4. 2021 High

   Discovery of the KCTD2–KCTD5 hetero-oligomer and its role in monoubiquitinating Gβ1/2 at K23 revealed a second major substrate axis and showed that KCTD2 functions not only as a homo-oligomer but also in mixed assemblies.

   Evidence Mass spectrometry interactome, reciprocal co-IP, in vitro ubiquitination with site-specific K23 mapping, siRNA depletion with downstream cAMP signaling readouts in HEK-293 cells

   PMID:34342229

   Open questions at the time

   • Stoichiometry and structure of the KCTD2–KCTD5 hetero-oligomer are unresolved
   • In vivo consequences of Gβ1/2 K23 monoubiquitination are not established
5. 2023 High

   Domain-mapping and live-cell BRET studies showed that KCTD2 engages Gβγ through both its BTB domain and C-terminal region in an agonist-induced manner, and functionally blunts Gβγ-mediated AC5 sensitization, clarifying how KCTD2 shapes GPCR-cAMP signaling dynamics.

   Evidence Live-cell BRET, co-IP, C-terminal deletion/mutation analysis, cAMP functional assay

   PMID:36736897

   Open questions at the time

   • Whether monoubiquitination is required for the signaling attenuation or whether KCTD2 binding alone suffices is unclear
   • The role of specific C-terminal residues in substrate discrimination has not been fully defined
6. 2024 Medium

   Sequential CRISPR knockout of KCTD2, KCTD5, and KCTD17 demonstrated functional redundancy among these three paralogs in controlling cell growth and Gβ1 protein levels, explaining why single-gene loss-of-function studies show mild phenotypes.

   Evidence CRISPR/Cas9 sequential KO in HEK293 cells, proliferation assays, transcriptomic profiling

   PMID:38732215

   Open questions at the time

   • Individual contributions of KCTD2 versus KCTD5 versus KCTD17 to specific substrates are not deconvoluted
   • Findings are from a single cell line; in vivo redundancy has not been tested

• The full repertoire of KCTD2 substrates, the structural basis of hetero-oligomer assembly with KCTD5, and whether KCTD2 regulates sleep or synaptic function in mammals remain open questions.
• No high-resolution structure of mammalian KCTD2 alone or in complex with CUL3/substrates
• Neuronal substrates at synapses are unidentified
• Mammalian sleep regulation by KCTD2 has not been directly tested