The N-terminal peptide of the Kaposi's sarcoma-associated herpesvirus (KSHV)-cyclin determines substrate specificity

Cyclin- dependent kinases ( Cdks) are activated by cyclin binding and phosphorylation by the Cdk- activating kinase ( CAK). Activation of Cdk6 by the D- type cyclins requires phosphorylation of Cdk6 by CAK on threonine 177. In contrast, Cdk6 is activated by the Kaposi's sarcomaassociated herpesvirus ( KSHV)- cyclin in the absence and presence of CAK phosphorylation. The activity of Cdk6 (.) KSHV- cyclin complexes was investigated here by analyzing mutants of the KSHV- cyclin and Cdk6 in vitro as well as in U2OS cells. Deletion of the N terminus of the KSHV- cyclin affects the substrate specificity indicating that the N terminus is required for phosphorylation of histone H1 but not for other substrates. Mutation of residues in the region 180 - 200 of the KSHVcyclin decreases the binding affinity to Cdk6 in U2OS cells but increases the activity of Cdk6 (.) KSHV- cyclin complexes in vitro indicating that low affinity binding of cyclins to the Cdk subunit might favor increased on- or off- rates of Cdk substrates. Expression of high levels of p16(INK4a) in cells leads to the formation of a heterotrimeric complex composed of Cdk6 (.) KSHV- cyclin, and p16(INK4a). Some of the Cdk6 (.) KSHV- cyclin (.) p16 complexes were found to be active indicating that there might be different modes of p16 binding to Cdk6 (.) cyclin complexes

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