Structural basis of divergent cyclin‐dependent kinase activation by Spy1/ RINGO proteins

Abstract

Cyclin‐dependent kinases (Cdks) are principal drivers of cell division and are an important therapeutic target to inhibit aberrant proliferation. Cdk enzymatic activity is tightly controlled through cyclin interactions, posttranslational modifications, and binding of inhibitors such as the p27 tumor suppressor protein. Spy1/ RINGO (Spy1) proteins bind and activate Cdk but are resistant to canonical regulatory mechanisms that establish cell‐cycle checkpoints. Cancer cells exploit Spy1 to stimulate proliferation through inappropriate activation of Cdks, yet the mechanism is unknown. We have determined crystal structures of the Cdk2‐Spy1 and p27‐Cdk2‐Spy1 complexes that reveal how Spy1 activates Cdk. We find that Spy1 confers structural changes to Cdk2 that obviate the requirement of Cdk activation loop phosphorylation. Spy1 lacks the cyclin‐binding site that mediates p27 and substrate affinity, explaining why Cdk‐Spy1 is poorly inhibited by p27 and lacks specificity for substrates with cyclin‐docking sites. We identify mutations in Spy1 that ablate its ability to activate Cdk2 and to proliferate cells. Our structural description of Spy1 provides important mechanistic insights that may be utilized for targeting upregulated Spy1 in cancer. image The structure of the Spy1/ RINGO protein in complex with cyclin‐dependent kinase 2 (Cdk2) reveals how this cancer‐associated non‐cyclin activator induces an active Cdk conformation that is insensitive to canonical regulatory signals. Spy1 adopts a similar fold as cyclin A despite low sequence homology to cyclins. Like cyclin A, Spy1 binds Cdk2 to properly organize the kinase active site. Activating T‐loop phosphorylation is dispensable for repositioning the T‐loop away from the Cdk substrate‐binding site when bound to Spy1. A cleft that in cyclins binds Cdk inhibitors such as p27 and mediates substrate specificity is absent in Spy1.