Abstract
Progression of the G1/S transition in plants is mainly regulated by A-type CDK (cyclin-dependent kinase). We have investigated the regulation of CDKA-associated kinase activity in partially synchronized tobacco BY-2 cells, which exit from stationary phase and re-enter the cell cycle. Kinase activity of p13suc1-bound proteins, containing cyclin/CDKA complexes significantly increased at G1/S phase. Western analysis revealed that CDKA protein gradually increased during the cell cycle re-entry, indicating that changes in levels were, at least in part, responsible for its kinase activity. To investigate whether post-translational modification of CDKA was also related to kinase activity, immunoblots were conducted with the phosphorylation-specific Tyr-15 and Thr-161 antibodies against animal Cdc2, homolog of CDKA. Although phosphorylation of Thr-161 rapidly increased during the cell cycle re-entry, no phosphorylation of Tyr-15 is observed, suggesting that CAK (CDK-activating kinase) is most likely involved in activating CDKA kinase activity through phosphorylation of Thr-161 at G1/S phase.
