Regulation of p27 Phosphorylation during Cell Cycle Progression
p27/Kip1 regulates the cell cycle by inhibiting the checkpoint kinase cdk2/cyclin E and blocking cell cycle progression through the G1-S transition. Cancer cells in some cases have reduced levels of p27, supporting the importance of p27 in cell cycle regulation. The activity of p27 is regulated by phosphorylation, synthesis and degradation. Phosphorylation of p27 at threonine-187 by cdk2 causes p27 to associate with an SCF complex that targets p27 for proteolytic degradation. The F box protein Skp2 binds specifically to threonine-187 phosphorylated p27, recruiting it to the SCF complex for degradation. Other components of the p27 ubiquitin ligase complex include Skp1, Cul1, and Roc1/Rbx-1. Cks-1 has also been identified in a reconstituted system as an essential component for recruitment of p27 for degradation by the SCF complex. Signaling by cytokines may modulate cell survival, proliferation, or apoptosis through modulation of p27 expression. Cytokine and AKT signaling pathways activate forkhead transcription factors that induce p27 expression at the transcriptional level.
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