Specificity of Cdk activation in vivo by the two Caks Mcs6 and Csk1 in fission yeast

Activating phosphorylation of cyclin-dependent kinases (Cdks) is mediated b y at least two structurally distinct types of Cdk-activating kinases (Caks) : the trimeric Cdk7-cyclin H-Mat1 complex in metazoans and the single-subun it Cak1 in budding yeast. Fission yeast has both Cak types: Mcs6 is a Cdk7 ortholog and Csk1 a single-subunit kinase, Both phosphorylate Cdks in vitro and rescue a thermosensitive budding yeast CAK1 strain. However, this appa rent redundancy is not observed in fission yeast in vivo. We have identifie d mutants that exhibit phenotypes attributable to defects in either Mcs6-ac tivating phosphorylation or in Cdc2-activating phosphorylation, Mcs6, human Cdk7 and budding yeast Cak1 were all active as Caks for Cdc2 when expresse d in fission yeast. Although Csk1 could activate Mcs6, it was unable to act ivate Cdc2, Biochemical experiments supported these genetic results: buddin g yeast Cak1 could bind and phosphorylate Cdc2 from fission yeast lysates, whereas fission yeast Csk1 could not. These results indicate that Mcs6 is t he direct activator of Cdc2, and Csk1 only activates Mcs6, This demonstrate s in vivo specificity in Cdk activation by Caks.