The first 7 syncytial mitoses of the Drosophila embryo occur without fluctuation in cyclin levels or CDK1 kinase activity. Cyclin levels and CDK1 kinase levels begin to fluctuate in cycle 8, and the cycles begin to slow down. We are interested in understanding how the cell cycle machinery drives early syncytial divisions. Microinjection of stable versions of cyclins A or B, or injections of an inhibitor of cyclin destruction, arrests the early mitoses near the point of injection. These results implicate APC dependent proteolysis in exit from mitosis in these early cycles, despite lack of bulk cyclin oscillation.
Additional results suggest that protein degradation plays a role at several steps of the cell cycle.
The expression of different stabilized cyclins and the N'-termCyc B inhibits passage through several steps of the cell cycle. These observations suggest that the time course of destruction of different proteins might guide progression through the complex events of mitosis.