Structural basis of T-loop–independent recognition and activation of CDKs by the CDK-activating kinase

Cyclin-dependent kinases (CDKs) are prototypical regulators of the cell cycle. The CDK-activating kinase (CAK) acts as a master regulator of CDK activity by catalyzing the activating phosphorylation of CDKs on a conserved threonine residue within the regulatory T-loop. However, structural data illuminating the mechanism by which the CAK recognizes and activates CDKs have remained elusive. In this study, we determined high-resolution structures of the CAK in complex with CDK2 and CDK2–cyclin A2 by cryogenic electron microscopy. Our structures reveal a T-loop–independent kinase-kinase interface with contributions from both kinase lobes. Computational analysis and structures of the CAK in complex with CDK1–cyclin B1 and CDK11 indicate that these structures represent the general architecture of CAK-CDK complexes. These results advance our mechanistic understanding of cell cycle regulation and kinase signaling cascades.