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 SCFFbw7 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.
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