Forms a complex with CYCB2-1 or CYCB2-2 that activates CDK kinase in tobacco BY2 cells during G2/M (mitosis) phases. May be involved in the regulation of the cell cycle at the G2/M transition. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] [DNA-directed RNA polymerase] + ATP = ADP + H(+) + phospho-[DNA-directed RNA polymerase] Interacts with CYCB2-1 and CYCB2-2. Binding to CYCB2-1 or CYCB2-2 activates CDK kinase. Associated to mitotic spindle during metaphase, and progressively localized the phragmoplast during telophase. Expressed in the dividing region of the root apex and the intercalary meristem of internodes. Expressed in the G2/M phases. By cytokinin, gibberellin (GA3) and submergence. Down-regulated by the replication blocking agent hydroxyurea. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. CDC2/CDKX subfamily.