Abstract
D-type cyclins (CycDs) act as the mediators linking extracellular and developmental signals to the cell division cycle. CycDs associate with CDKs (cyclin-dependent kinases) to phosphorylate the retinoblastoma protein (pRb) that plays a pivotal role in regulating the G1/S transition. We have isolated two cDNA clones of tobacco CycD3 (CycD3;1a, CycD3;1b) in which these genes expressed predominantly during G2 to M phase. We previously showed that these CycD3s bind to both of A-type and B1-type CDKs, and CycD3/CDKA purified from insect cells exhibited histone H1 kinase activity. In this study, we prepared the peptide antibodies against CycD3;1a and CycD3;1b. Using the antibodies, immunoprecipitations were conducted to analyze their kinase activity. Although CycD3;1a and CycD3;1b associated kinases phosphorylate both histone H1 and the tobacco Rb, histone H1 kinase activity remained through the cell cycle and Rb kinase activity increased at middle of G1 phase and declined rapidly during S phase.
