A cell-cycle-regulated kinase activity phosphorylates plant retinoblastomaprotein and contains, in Arabidopsis, a CDKA/cyclin D complex

The activity of cyclin-dependent kinases (CDK) is crucial for cell-cycle tr ansitions. Here, we report the identification of a CDK activity that phosph orylates the retinoblastoma-related (RBR) protein. A CDK/ cyclin complex th at binds to and phosphorylates RBR may be isolated from various plant sourc es, e.g. wheat, maize, Arabidopsis thaliana and tobacco, and from cells gro wing under various conditions, The presence of an RBR-associated CDK activi ty correlates with the proliferative activity, suggesting that phosphorylat ion of RBR is a major event in actively proliferating tissues. In A. thalia na, this activity comprises a PSTAIRE CDKA and at least cyclin D2. Furtherm ore, this CDK activity is cell-cycle-regulated, as revealed by studies with highly synchronized tobacco BY-2 cells where it is maximal in late G1 and early S phase cells and progressively decreases until G2 phase. Aphidicolin -arrested but not roscovitine-arrested cells contain a PSTAIRE-type CDK tha t binds to and phosphorylates RBR. Thus, association with a D-type cyclin i s a likely mechanism leading to CDK activation late in G1. Our studies cons titute the first report measuring the activity of CDK/cyclin complexes form ed in vivo on RBR, an activity that fluctuates in a cell-cycle-dependent ma nner. This work provides the basis for further studies on the impact of pho sphorylation of RBR on its function during the cell cycle and development.