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The mitotic inducer p34cdc2 requires association with a cyclin and phosphorylation on Thr161 for its activity as a protein kinase. CAK, the p34cdc2 activating kinase, was previously identified as an enzyme necessary for this activating phosphorylation. We confirm here that CAK is a protein kinase and describe its purification over 13,000-fold from Xenopus egg extracts. We further show that CAK contains a protein identical or closely related to the previously identified Xenopus MO15 gene: p40MO15 copurifies with CAK, and an antiserum to p40MO15 specifically depletes cAK activity. CAK appears to be the only protein in Xenopus egg extracts that can activate complexes of either p34cdc2 or the closely related protein kinase, p33cdk2, with either cyclin A or cyclin B. The sequence similarity between p40MO15 and p34cdc2, and the approximately 200 kDa size of CAK, suggest that p40MO15 may itself be regulated by subunit association and by protein phosphorylations.
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