Protein phosphatase V ensures timely cell cycle remodeling during the mid-blastula transition in Drosophila

Cell cycle remodeling from fast nuclear cycles to a generic cell cycle mode is a major feature of the mid-blastula transition (MBT) in Drosophila. Remodeling occurs when Twine/Cdc25 falls below a critical threshold. Timing is based on Twine destabilization induced by zygotic transcription. It is conceivable that appropriate starting levels are also important for timely reaching the threshold. Mechanisms for controlling Twine levels at the onset of MBT are unknown. Here we identify a function of the protein phosphatase V in this mechanism. Twine was increased in P p V mutants, whereas the decay rate was comparable to wildtype. P p V mutants frequently underwent an extra nuclear division. We detected P p V-dependent phosphosites in Twine. Phosphosite mutants contain higher Twine levels and frequently underwent an extra nuclear division, comparable to P p V mutants. Our data support a model that the cell cycle remodeling is controlled by induced destabilization and P p V-d ependent control of Twine levels.