Ubiquitinating a phosphorylated Cdk inhibitor on the blades of the Cdc4 beta-propeller.

Substrate binding by the SCFCdc4 ubiquitin ligase is regulated by phosphorylation. In this issue of Cell, Orlicky et al. describe the crystal structure of the Cdc4 subunit bound to a high-affinity substrate phosphopeptide. This structure provides insights into the binding interaction and how a precise mechanism involving multiple regulatory phosphorylations may be mediated by a single binding site.