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Updated: Feb 23, 2026
Identification of Cyclin-dependent Kinase 1 Specific Phosphorylation Sites by an In Vitro Kinase Assay
Published on: May 3, 2018
The stability of Fbw7α in M-phase requires its phosphorylation by PKC
Sihem Zitouni1, Francisca Méchali1, Catherine Papin2
1Centre de Recherche de Biologie Cellulaire de Montpellier, CNRS, UMR 5237, Université de Montpellier, Montpellier, France.
Abstract:
Fbw7 is a tumor suppressor often deleted or mutated in human cancers. It serves as the substrate-recruiting subunit of a SCF ubiquitin ligase that targets numerous critical proteins for degradation, including oncoproteins and master transcription factors. Cyclin E was the first identified substrate of the SCFFbw7 ubiquitin ligase. In human cancers bearing FBXW7-gene mutations, deregulation of cyclin E turnover leads to its aberrant expression in mitosis. We investigated Fbw7 regulation in Xenopus eggs, which, although arrested in a mitotic-like phase, naturally express high levels of cyclin E. Here, we report that Fbw7α, the only Fbw7 isoform detected in eggs, is phosphorylated by PKC (protein kinase C) at a key residue (S18) in a manner coincident with Fbw7α inactivation. We show that this PKC-dependent phosphorylation and inactivation of Fbw7α also occurs in mitosis during human somatic cell cycles, and importantly is critical for Fbw7α stabilization itself upon nuclear envelope breakdown. Finally, we provide evidence that S18 phosphorylation, which lies within the intrinsically disordered N-terminal region specific to the α-isoform reduces the capacity of Fbw7α to dimerize and to bind cyclin E. Together, these findings implicate PKC in an evolutionarily-conserved pathway that aims to protect Fbw7α from degradation by keeping it transiently in a resting, inactive state.
Insights
Protein kinase C (PKC) inactivates Fbw7α by phosphorylating it during mitosis. This conserved mechanism stabilizes Fbw7α, preventing its degradation and protecting it from premature inactivation.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- Fbw7 is a crucial tumor suppressor and the substrate-recruiting subunit of SCF ubiquitin ligase.
- Fbw7 targets oncoproteins and transcription factors, including cyclin E, for degradation.
- FBXW7 mutations in cancer lead to aberrant cyclin E expression due to impaired turnover.
Purpose of the Study:
- To investigate the regulation of Fbw7 in Xenopus eggs, which express high cyclin E levels.
- To elucidate the role of protein kinase C (PKC) in Fbw7α regulation during mitosis.
- To understand the evolutionary conservation of Fbw7α inactivation mechanisms.
Main Methods:
- Studied Fbw7 regulation in Xenopus eggs and human somatic cells.
- Investigated the phosphorylation of Fbw7α at serine 18 (S18) by PKC.
- Assessed the impact of S18 phosphorylation on Fbw7α dimerization and cyclin E binding.
Main Results:
- Fbw7α is phosphorylated by PKC at S18 in Xenopus eggs and human mitotic cells, coinciding with Fbw7α inactivation.
- PKC-dependent S18 phosphorylation stabilizes Fbw7α during nuclear envelope breakdown in mitosis.
- S18 phosphorylation, located in the unique N-terminal region of Fbw7α, reduces its dimerization and cyclin E binding capacity.
Conclusions:
- PKC-dependent phosphorylation of Fbw7α at S18 is an evolutionarily conserved mechanism.
- This phosphorylation event transiently inactivates Fbw7α, protecting it from degradation during mitosis.
- The findings reveal a novel regulatory pathway for Fbw7α stability and activity.
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