THE NIMA PROTEIN-KINASE IS HYPERPHOSPHORYLATED AND ACTIVATED DOWNSTREAM OF P34(CDC2)/CYCLIN-B - COORDINATION OF 2 MITOSIS PROMOTING KINASES

Initiation of mitosis in Aspergillus nidulans requires activation of two protein kinases, p34(cdc2)/cyclin B and NIMA. Forced expression of NIMA, even when p34(cdc2) was inactivated, promoted chromatin condensation, NIMA may therefore directly cause mitotic chromosome condensation. However, the mitosis-promoting function of NIMA is normally under control of p34(cdc2)/ cyclin B as the active G(2) form of NIMA is hyperphosphorylated and further activated by p34(cdc2)/cyclin B when cells initiate mitosis. To see the p34(cdc2)/cyclin B dependent activation of NIMA, okadaic acid had to be added to isolation buffers to prevent dephosphorylation of NIMA during isolation. Hyperphosphorylated NIMA contained the MPM-2 epitope and, in vitro, phosphorylation of NIMA by p34(cdc2)/cyclin B generated the MPM-2 epitope, suggesting that NIMA is phosphorylated directly by p33(cdc2)/cyclin B during mitotic initiation. These two kinases, which are both essential for mitotic initiation, are therefore independently activated as protein kinases during G(2). Then, to initiate mitosis, we suggest that each activates the other's mitosis-promoting functions. This ensures that cells coordinately activate p34(cdc2)/cyclin B and NIMA to initiate mitosis only upon completion of all interphase events. Finally, we show that NIMA is regulated through the cell cycle like cyclin B, as it accumulates during G(2) and is degraded only when cells traverse mitosis.