Exit from mitosis in Drosophila syncytial embryos requires proteolysis and cyclin degradation, and is associated with localized dephosphorylation.

The cyclin proteolysis that accompanies the exit from mitosis in diverse systems appears to be essential for restoration of interphase. The early syncytial divisions of Drosophila embryos, however, occur without detectable oscillations in the total cyclin level or Cdk1 activity. Nonetheless, we found that injection of an established inhibitor of cyclin proteolysis, a cyclin B amino-terminal peptide, prevents exit from mitosis in syncytial embryos. Similarly, injection of a version of Drosophila cyclin B that is refractory to proteolysis results in mitotic arrest. We infer that proteolysis of cyclins is required for exit from syncytial mitoses. This inference can be reconciled with the failure to observe oscillations in total cyclin levels if only a small pool of cyclins is destroyed in each cycle. We find that antibody detection of histone H3 phosphorylation (PH3) acts as a reporter for Cdk1 activity. A gradient of PH3 along anaphase chromosomes suggests local Cdk1 inactivation near the spindle poles in syncytial embryos. This pattern of Cdk1 inactivation would be consistent with local cyclin destruction at centrosomes or kinetochores. The local loss of PH3 during anaphase is specific to the syncytial divisions and is not observed after cellularization. We suggest that exit from mitosis in syncytial cycles is modified to allow nuclear autonomy within a common cytoplasm.