Bi-partite binding of the N-terminus of Skp2 to cyclin A

Skp2 and cyclin A are cell cycle regulators that control the activity of CDK2. Cyclin A acts as an activator and substrate recruitment factor of CDK2, while Skp2 mediates the ubiquitination and subsequent destruction of the CDK inhibitor protein p27. The N-terminus of Skp2 can interact directly with cyclin A but is not required for p27 ubiquitination. To gain insight into this poorly understood interaction, we have solved the 3.2 Å X-ray crystal structure of the N-terminus of Skp2 bound to cyclin A. The structure reveals a bi-partite mode of interaction with two motifs in Skp2 recognizing two discrete surfaces on cyclin A. The uncovered binding mechanism allows for a rationalization of the inhibitory effect of Skp2 on CDK2-cyclin A kinase activity towards RxL motif containing substrates and raises the possibility that other intermolecular regulators and substrates may use similar non-canonical modes of interaction for cyclin targeting. Kelso et al. demonstrate that the Skp2 N-terminus contains two motifs that bind cyclin A but not cyclin E. One resembles the known RxL cyclin binding motif, but in the reverse direction. Binding of the Skp2 N-terminus to cyclin A blocks recruitment of CDK substrates.