The PP2A-B56 Phosphatase Opposes Cyclin E Autocatalytic Degradation via Site-Specific Dephosphorylation

ABSTRACT

Cyclin E, in conjunction with its catalytic partner cyclin-dependent kinase 2 (CDK2), regulates cell cycle progression as cells exit quiescence and enter S-phase. Multiple mechanisms control cyclin E periodicity during the cell cycle, including phosphorylation-dependent cyclin E ubiquitylation by the SCF^Fbw7 ubiquitin ligase. Serine 384 (S384) is the critical cyclin E phosphorylation site that stimulates Fbw7 binding and cyclin E ubiquitylation and degradation. Because S384 is autophosphorylated by bound CDK2, this presents a paradox as to how cyclin E can evade autocatalytically induced degradation in order to phosphorylate its other substrates. We found that S384 phosphorylation is dynamically regulated in cells and that cyclin E is specifically dephosphorylated at S384 by the PP2A-B56 phosphatase, thereby uncoupling cyclin E degradation from cyclin E-CDK2 activity. Furthermore, the rate of S384 dephosphorylation is high in interphase but low in mitosis. This provides a mechanism whereby interphase cells can oppose autocatalytic cyclin E degradation and maintain cyclin E-CDK2 activity while also enabling cyclin E destruction in mitosis, when inappropriate cyclin E expression is genotoxic.

KEYWORDS:

• cell cycle
• cyclin-dependent kinases
• cyclins
• serine/threonine phosphatases
• ubiquitination

View publisher note:

Articles of Significant Interest Selected from This Issue by the Editors

Supplemental material for this article may be found at https://doi.org/10.1128/MCB.00657-16.

ACKNOWLEDGMENTS

We thank members of the Clurman Lab for helpful discussions and feedback on the work.

This work was supported by NIH grants R01 CA193808-02 and R21 CA187324-02 (B.E.C.) and by the National Science Foundation Graduate Research Fellowship Program under grant numbers DGE-0718124 and DGE-1256082 (R.J.D.).

The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

We declare that we have no conflicts of interest regarding the contents of this article.

R.J.D. contributed to research direction, designed and performed most of the experiments, and wrote the manuscript. J.S. performed experiments and contributed to the manuscript. B.T.H. performed experiments and contributed to the research direction. B.E.C. led the research, designed experiments, and contributed to the manuscript.