Mechanism of Cell Cycle. Entry Mediated by the Intrinsically Disordered Protein p27Kip1

p27(Kip1) (p27), a prototypical intrinsically disordered protein (IDP), regulates eukaryotic cell division through interactions with cyclin-dependent kinase (Cdk)/cyclin complexes. The activity, stability and subcellular localization of p27 are regulated by phosphorylation. We illustrate how p27 integrates regulatory signals from several non-receptor tyrosin kinases (NRTKs) to activate Cdk4 and initiate cell cycle entry. Unmodified p27 potently inhibits Cdk/cyclin complexes, including Cdk4/cyclin D (IC50 1 nm). Some NRTKs (e.g., Abl) phosphorylate p27 on Tyr 88, which facilitates a second modification on Tyr 74 by another NRTK (e.g., Src). Importantly, this second modification causes partial reactivation of Cdk4 within ternary complexes containing doubly Tyr phosphorylated p27. Partial activation of Cdk4 initiates entry into the cell division cycle. Therefore, p27s disordered features enable NRTKs to sequentially promote a phosphorylation cascade that controls cell fate. Beyond cell cycle control these results illustrate general concepts regarding why IDPs are well-suited for roles in signaling and regulation in biological systems.