THE CDC2MS KINASE IS DIFFERENTLY REGULATED IN THE CYTOPLASM AND IN THE NUCLEUS

To study a cyclin-dependent kinase (CDK) from alfalfa (Medicago sativa L.), an antibody was raised against the C-terminal 16 amino acids of the protein cdc2aMs. The cdc2Ms protein was immunopurified with this a ntibody and its histone kinase activity was measured. The cdc2Ms kinas e is activated at the G1/S transition when phosphate-starved cells fro m the CO phase re-enter the cell cycle and remain active as cells tran sit the S, G2, and M phases, indicating that the same CDK regulates al l of these phases in alfalfa. In contrast, when cdc2Ms kinase was puri fied by binding to p13(suc1), it was active only in the G2 and M phase s. In immunoblots the C-terminal antibody detected an equal amount of the cdc2Ms protein in the cytoplasm and in the nucleus. By indirect im munofluorescence, however, the cytoplasmic form of cdc2Ms could not be found in the S phase of the cells, indicating that the epitope for th e cdc2 antibody is not accessible. Binding of putative inhibitor prote ins to cdc2 was shown by inactivation of purified plant CDK when cell extracts were added. Furthermore, purified CDK inhibitors, such as the mouse p27(kip1) and the yeast p40(sic1), blocked the purified plant C DK activity.