CDK-related protein kinases in plants

Cyclin-dependent kinases (CDK) form a conserved superfamily of eukaryotic s erine-threonine protein kinases, which require binding to a cyclin protein for activity. CDK are involved in different aspects of cell biology and not ably in cell cycle regulation. The comparison of nearly 50 plant CDK-relate d cDNAs with a selected set of their animal and yeast counterparts reveals five classes of these genes in plants. These are described here with respec t to their phylogenetic, structural and functional properties. A plant-wide nomenclature of CDK-related genes is proposed, using a system similar to t hat of the plant cyclin genes. The most numerous class, CDKA, includes gene s coding for CDK with the PSTAIRE canonical motif. CDKB makes up a class of plant-specific CDK divided into two groups: CDKB1 and CDKB2. CDKC, CDKD an d CDKE form less numerous classes. The CDKD class includes the plant orthol ogues of metazoan CDK7, which correspond to the CDK-activating kinase (CAK) . At present, no functional information is available in plants for CDKC and CDKE.