CDKN3

CDKN3 (CDI1/KAP) is a dual-specificity protein phosphatase that regulates cell-cycle progression by directly binding and dephosphorylating cyclin-dependent kinases (PMID:8242750). It forms stable complexes with CDK2 and interacts with CDC2/CDK1 and CDK3 but not CDK4, and its phosphatase activity is strictly required for its cell-cycle function, since an active-site C→S mutant both abolishes catalysis and abolishes the cell-cycle delay caused by overexpression (PMID:8242750). During mitotic exit CDKN3 dephosphorylates Thr-161 of CDC2/CDK1, with its substrate concentrated at kinetochores and centrosomes in early mitosis, and it acts through downstream phosphotargets that control the spindle checkpoint (PMID:23775190). At centrosomes CDKN3 directly binds Mps1 kinase, by which it is recruited to the centrosome, and limits centrosomal Mps1 levels through proteasome-mediated degradation in a self-regulated feedback loop that restrains centrosome overduplication (PMID:26586430). In cancer contexts CDKN3 catalytic activity underlies a tumor-suppressive role in Bcr-Abl leukemogenesis, where wild-type but not phosphatase-dead CDKN3 dephosphorylates CDK2 to lower CDK2-dependent XIAP expression and delay G1/S transition (PMID:25360622); in other tumor settings CDKN3 supports proliferation and chemoresistance through downstream effectors including RAD51 in DNA-damage repair (PMID:31114363), LDHA-dependent aerobic glycolysis (PMID:35388272), and CCNB2-driven G2/M progression (PMID:39865508). CDKN3 expression is transcriptionally driven by N-Myc in neuroblastoma (PMID:38356706) and by BACH1 in cardiomyocytes (PMID:39060883), placed under an ER/E2F1 axis in UV-exposed epidermis (PMID:41876816), and its protein level is stabilized by PSMD12 and GID2 through reduced ubiquitination (PMID:37037907, PMID:36828188).

1. 1993 High

   Established the founding biochemical identity of CDKN3 as a CDK-binding dual-specificity phosphatase whose catalytic activity is essential for cell-cycle control, answering what kind of molecule it is and how it acts.

   Evidence Yeast two-hybrid, in vitro phosphatase assay, active-site C→S mutagenesis, and overexpression with cell-cycle readout in HeLa cells

   PMID:8242750

   Open questions at the time

   • Did not define the physiological CDK phospho-site dephosphorylated in vivo
   • Did not resolve which CDK partner is the dominant cellular substrate
2. 2013 High

   Identified the in vivo substrate and timing of CDKN3 action, showing it removes the Thr-161 phosphate from CDC2/CDK1 at mitotic exit and feeds into spindle-checkpoint control.

   Evidence Genome-wide siRNA phosphatase screen, phospho-specific immunofluorescence across the cell cycle, and phosphokinome-wide mass spectrometry

   PMID:23775190

   Open questions at the time

   • Mechanism coupling CDKN3 relocalization to substrate access not fully resolved
   • Direct enzyme-substrate kinetics for Thr-161 not reconstituted
3. 2015 High

   Defined a centrosomal function distinct from CDK dephosphorylation, showing CDKN3 binds Mps1 to control its proteasomal turnover and prevent centrosome overduplication.

   Evidence Reciprocal co-immunoprecipitation, siRNA knockdown with centrosome-number readout, immunofluorescence localization, and proteasome inhibitor experiments

   PMID:26586430

   Open questions at the time

   • Whether phosphatase activity is required for Mps1 regulation not established
   • E3 ligase mediating Mps1 degradation not identified
4. 2014 Medium

   Connected catalytic activity to tumor suppression, showing CDKN3 dephosphorylates CDK2 to suppress XIAP and delay G1/S in leukemic cells.

   Evidence C140S phosphatase-dead mutant, overexpression/knockdown in K562 cells, xenograft model, and Western blot for CDK2 phosphorylation and XIAP

   PMID:25360622

   Open questions at the time

   • Single lab
   • Mechanism linking CDK2 activity to XIAP expression not detailed
5. 2019 Medium

   Placed CDKN3 upstream of DNA-damage repair, identifying RAD51 as a downstream mediator of CDKN3-driven cisplatin resistance.

   Evidence siRNA knockdown, RAD51 overexpression rescue, xenograft assays, and γH2AX DNA-damage readout in esophageal cancer cells

   PMID:31114363

   Open questions at the time

   • No direct biochemical interaction between CDKN3 and RAD51 shown
   • Mechanism by which CDKN3 controls RAD51 expression unknown
6. 2022 Medium

   Linked CDKN3 to tumor metabolism, showing it supports aerobic glycolysis and chemoresistance through LDHA.

   Evidence siRNA knockdown, LDHA overexpression rescue, glucose-uptake/lactate measurements, and viability assays in bladder cancer cells

   PMID:35388272

   Open questions at the time

   • Direct enzymatic connection to LDHA not established
   • Single lab
7. 2023 Medium

   Revealed post-translational control of CDKN3 abundance, identifying PSMD12 and GID2 as physical partners that stabilize CDKN3 by reducing its ubiquitination.

   Evidence Co-immunoprecipitation, ubiquitination assays, and CDKN3-knockdown rescue of overexpression phenotypes with xenograft validation

   PMID:36828188 PMID:37037907

   Open questions at the time

   • Ubiquitin ligase/deubiquitinase mechanism for CDKN3 not fully defined
   • Single lab per partner
8. 2024 Medium

   Defined transcriptional regulators of CDKN3, showing N-Myc and BACH1 directly drive its expression in neuroblastoma and cardiomyocytes respectively, with phenotypic consequences for differentiation and stress responses.

   Evidence ChIP and luciferase reporter assays, siRNA knockdown, and differentiation/proliferation or apoptosis/ferroptosis readouts

   PMID:38356706 PMID:39060883

   Open questions at the time

   • Downstream AMPK link in cardiomyocytes not biochemically traced to CDKN3 activity
   • CDKN3/CDC6/CDK4 network not reconstituted in vitro
9. 2026 Medium

   Positioned CDKN3 within a sex-specific tumor-protective circuit, showing estrogen receptors downregulate E2F1 and its target CDKN3 to limit UV-induced squamous cell carcinoma.

   Evidence UV-induced SCC mouse model with sex comparison, transcriptional profiling, ER perturbation, and CDKN3 siRNA depletion with S-phase and xenograft readouts

   PMID:41876816

   Open questions at the time

   • Direct ER occupancy at the CDKN3 locus not shown
   • Single lab

• How CDKN3's defined phosphatase activity mechanistically connects to the diverse downstream effectors (RAD51, LDHA, CCNB2, HSP90) reported in tumor models remains unresolved.
• No direct enzyme-substrate link between CDKN3 and these effectors established
• Whether catalytic activity is required for the metabolic and chemoresistance phenotypes untested in most contexts