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 catalysing 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 recognises and activates CDKs have remained elusive. Here, we determine high-resolution structures of the CAK in complex with CDK2 and CDK2-cyclin A 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 CAK in complex with CDK1-cyclin B 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 signalling cascades.