Abstract
D-type cyclin plays essential roles in the G1/S transition. CycDs form complexes with CDKs (cyclin-dependent kinases) that contribute to the phosphorylation of the retinoblastoma protein (pRb) to control the onset of entry into S-phase. Recently new CycDs have been isolated from Arabidopsis and Tobacco in which these genes expressed preferentially in G2-M phase. We isolated two CycD3 genes (NtcycD3-2a, NtcycD3-2b) whose transcripts accumulated in G2-M phase. To investigate the role of CycD3, we performed a GST pull-down assay with tobacco CDK (Ntcdc2a, Ntcdc2b), tobacco Kip-related protein KIS1a and tobacco Rb and found that both CycD3 can bind to all these factors. Importantly, both CycD3/Cdc2a and CycD3/Cdc2b complexes exhibited kinase activity against histone H1 in vitro. Cdc2b represents a plant-specific CDKs involved in the control of G2/M progression, suggesting that these CycD3 may function as a regulatory partner of Cdc2b in G2/M progression.
