Arabidopsis D-Type Cyclin CYCD4;1 Is a Novel Cyclin Partner of B2-Type Cyclin-Dependent Kinase

B-type cyclin-dependent kinases (CDKs) are unique to plants and are assumed to be involved in the control of the G2-to-M phase progression and mitotic events. However, little is known about their cyclin partners. In Arabidopsis, we isolated cDNA encoding the D-type cyclin CYCD4;1 by a yeast (Saccharomyces cerevisiae) two-hybrid screening using CDKB2;1 as bait. In vitro pull-down assay showed that CYCD4;1 bound to CDKB2;1 and CDKA;1. Protein complexes of CYCD4;1-CDKA;1 and CYCD4;1-CDKB2;1 in insect cells exhibited histone H1-kinase activity. Promoter analysis using the luciferase reporter gene showed that CDKB2;1 was expressed from early G2 to M phase, whereas CYCD4;1 was expressed throughout the cell cycle. In situ hybridization of plant tissues revealed that both CDKB2;1 and CYCD4;1 transcripts accumulated in the shoot apical meristem, leaf primordia, vasculature of leaves, and tapetal cells in anthers. Our results suggest that CDKB2;1 and CYCD4;1 may form an active kinase complex during G2/M phase and control the development of particular tissues.