Cell division requires that cyclin-dependent kinases (Cdks) be activated by phosphorylation. In mitotic cells, this is accomplished by the Cdk-activating-kinase (CAK), which is a complex of cyclin H and Cdk7. There are currently no data on the role of CAK in meiotic cells. Previously, we have shown that cyclin A1 is meiosis-specific and forms an active kinase with Cdk2. Because cyclin A1 is required for meiosis, and its associated kinase must be phosphorylated (activated), we propose that cyclin H/Cdk7 function to activate cyclin A1/Cdk2 in meiotic cells. Here, we show that cyclin H and Cdk7 are present during meiosis. Using reverse transcription-polymerase chain reaction and in situ hybridization, we show that the mRNAs encoding cyclin H and Cdk7 are abundant in spermatocytes. Immunohistochemistry localized cyclin H and Cdk7 to the nucleus of spermatocytes in stages IV to XII of the spermatogenic cycle, overlapping the same stages that express cyclin A1-associated kinases. Finally, immunoprecipitation and histone H1-kinase assays of cyclin H and Cdk7 from testicular extracts show that these proteins interact to form an active kinase. We conclude that cyclin H/Cdk7 complexes are present and during meiosis, form active complexes in testicular cells and are strong candidates for the activating kinase for cyclin A1-associated kinase.
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