CDK15

CDK15 is a cyclin-dependent kinase that phosphorylates specific substrates to promote cell survival and proliferation. It directly binds and phosphorylates survivin at Thr34, stabilizing survivin expression and suppressing TRAIL-induced apoptosis by reducing activation of caspases-3, -8, and -9 and PARP cleavage (PMID:24866247). CDK15 also binds and phosphorylates PAK4 at Ser291, thereby activating β-catenin/c-Myc and MEK/ERK signaling to drive colorectal cancer cell proliferation and anchorage-independent growth (PMID:34262144). CDK15 protein levels are negatively regulated by the PA28α/β immunoproteasome activator complex, and loss of CDK15 is required for PA28α/β-mediated breast cancer cell invasion and metastasis (PMID:31824858).

1. 2014 Medium

   Identifying CDK15 as a kinase for survivin established its first known substrate and linked it to apoptosis resistance, answering the question of how TRAIL-resistant cancer cells stabilize survivin.

   Evidence Overexpression and siRNA knockdown of CDK15 in TRAIL-sensitive and -resistant cancer cell lines with co-immunoprecipitation, phospho-survivin immunoblotting, and apoptosis assays

   PMID:24866247

   Open questions at the time

   • Binding and phosphorylation demonstrated only in overexpression systems without in vitro kinase assay using purified components
   • No cyclin partner identified for CDK15 in this context
   • Phosphorylation stoichiometry and kinetics not determined
2. 2019 Medium

   Showing that PA28α/β promotes CDK15 protein degradation revealed a proteostatic control mechanism for CDK15 and connected immunoproteasome activity to CDK15-dependent suppression of invasion.

   Evidence siRNA knockdown of PA28α/β and β5i subunit with immunoblotting for CDK15 and functional invasion/migration assays in breast cancer cells

   PMID:31824858

   Open questions at the time

   • Molecular mechanism of CDK15 degradation (direct ubiquitination, proteasomal targeting) not resolved
   • Whether CDK15 is a direct substrate of the immunoproteasome or is regulated indirectly remains unclear
   • Findings from a single cell system without in vivo validation
3. 2021 High

   Identifying PAK4 Ser291 as a second CDK15 substrate and linking it to β-catenin/c-Myc and MEK/ERK signaling broadened CDK15's role from anti-apoptotic kinase to a driver of proliferative oncogenic signaling.

   Evidence Reciprocal co-immunoprecipitation, phospho-site mutagenesis (S291A), CDK15 knockout in AOM/DSS, CDX, and PDX mouse models, and PAK4 inhibitor rescue experiments in colorectal cancer cells

   PMID:34262144

   Open questions at the time

   • No activating cyclin partner for CDK15 has been identified
   • Whether CDK15 phosphorylation of PAK4 is direct (in vitro kinase assay with purified proteins) was not explicitly shown
   • Structural basis for CDK15 substrate selectivity is unknown

• The activating cyclin partner of CDK15, its full substrate repertoire, and the structural basis of its substrate specificity remain uncharacterized.
• No cyclin partner identified for CDK15
• No crystal structure or cryo-EM structure available
• Physiological roles outside oncogenic contexts are unexplored