Activation of CDK‐activating kinase is dependent on interaction with H‐type cyclins in plants

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Abstract
Summary: cDNAs encoding cyclin H homologs were isolated from poplar (Populus tremulax tremuloides) and rice (Oryza sativa) plants, and were designated Pt;cycH;1 and Os;cycH;1, respectively. The deduced amino‐acid sequences showed 40–60% similarity to human cyclin H and Schizosaccharomyces pombe Mcs2, with higher similarity in the cyclin box region. While Pt;cycH;1 and Os;cycH;1 were expressed in all tissues examined, the transcripts accumulated abundantly in dividing cells. Expression of Os;cycH;1 was abundant in the S‐phase in partially synchronized suspension cells, and was induced by submergence in internodes of deepwater rice. A yeast two‐hybrid assay demonstrated that both Pt;CycH;1 and Os;CycH;1 were able to interact with rice R2 kinase, which is structurally and functionally similar to cyclin‐dependent kinase (CDK)‐activating kinase (CAK) of vertebrates. Moreover, an in vitro pull‐down assay showed that Os;CycH;1 specifically bound to R2 but not to other rice CDKs. When R2 was expressed in budding yeast CAK mutant, the suppression activity in terms of temperature‐sensitivity was enhanced by co‐expression with Os;cycH;1. Furthermore, in vitro kinase assay indicated that the kinase activities of R2 on CDKs and the carboxy‐terminal domain (CTD) of the largest subunit of RNA polymerase II were markedly elevated by binding to Os;CycH;1. Our results suggest that cyclin H is a regulatory subunit of CAK, which positively controls CDK‐ and CTD‐kinase activities in plant cells.