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Cyclin-dependent kinases (CDKs) play essential roles in coordinate control of cell cycle progression. Activation of CDKs requires interaction with specific cyclin partners and phosphorylation of their T-loops by CDK-activating kinases (CAKs). The Arabidopsis thaliana genome encodes four potential CAKs. CAK2At (CDKD;3) and CAK4At (CDKD;2) are closely related to the vertebrate CAK, CDK7/p40^sup MO15^; they interact with cyclin H and phosphorylate CDKs, as well as the C-terminal domain (CTD) of the largest subunit of RNA polymerase II. CAK1At (CDKF;1) shows cyclin H-independent CDK-kinase activity and can activate a heterologous CAK, Mcs6, in fission yeast. In Arabidopsis, CAK1At is a subunit of a protein complex of 130 kD, which phosphorylates the T-loop of CAK2At and CAK4At and activates the CTD-kinase activity of CAK4At in vitro and in root protoplasts. These results suggest that CAK1At is a novel CAK-activating kinase that modulates the activity of CAK2At and CAK4At, thereby controlling CDK activities and basal transcription in Arabidopsis.
INTRODUCTION
The activation of cell division and transitions between different phases of the cell cycle are controlled by a family of cyclindependent Ser/Thr protein kinases (CDKs). The activity and substrate specificity of CDKs depend on their association with specific cyclin partners that act as regulatory subunits (Morgan, 1997). Activation of CDKs also requires phosphorylation of conserved Thr residues within their T-loops, such as residues Thr161 and Thr160 in human Cdc2 and CDK2, respectively. This activating phosphorylation is catalyzed by CDK-activating kinases (CAKs; Kaldis, 1999). Crystal structure analyses of monomeric CDK2 showed that the T-loop region blocks access of the potential substrates to the catalytic site (Morgan and De Bondt, 1994). Upon phosphorylation of Thr160 by CAK, the T-loop moves away and the cyclin A-CDK2 complex becomes fully active (Jeffrey et al., 1995; Russo et al., 1996).
CAK in fission yeast (Schizosacchammyces pombe) phosphorylates Cdc2 and consists of catalytic Mcs6/Mop1 /Crk1 and regulatory cyclin Mcs2 subunits (Damagnez et al., 1995). The Mcs2-Mcs6 CAK kinase is part of the general transcription factor UH (TFIIH). In addition to Cdc2, it phosphorylates the C-terminal domain (CTD) of the largest subunit of RNA polymerase Il thereby controlling both basal transcription and cell cycle progression (Buck et al., 1995; Damagnez et al., 1995; Lee et al., 1999). Recently, a monomeric kinase, Csk1, has been identified that...