Cks is an evolutionarily conserved protein that regulates cyclin-dependent kinase (Cdk) activity. Clarifying the underlying mechanisms and cellular contexts of Cks function is critical, as Cks is essential for proper cell growth, and its overexpression has been linked to cancer. We observe that budding yeast Cks associates with select phosphorylated sequences in cell cycle regulatory proteins. We characterize the molecular interactions responsible for this specificity and demonstrate that Cks enhances Cdk activity in response to specific priming phosphosites. Identification of the binding consensus sequence allows us to identify putative Cks-directed Cdk substrates and binding partners. We characterize novel Cks binding sites in the mitotic regulator Wee1 and discover a novel role for Cks in regulating Cdk activity at mitotic entry. Together our results portray Cks as a multifunctional phosphoadaptor that serves as a specificity factor for Cdk activity.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.