Considered the "engines" of cell cycle progression, cyclin-dependent kinases (CDKs) phos-phorylate myriad downstream substrates to promote cell growth, replication and division. Tight control of CDK activity results from, among other mechanisms, the opposing actions of inhibitory kinases (Weel and Miki) and activating phosphatases. Cdc25 phospha-tases are essential components of this process, reversing inhibitory phosphorylation of CDKs during key cell cycle transitions and peaking in their activity as cell division nears its mitotic end
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