PHOSPHORYLATION CHANGES DURING EGG ACTIVATION AND ON REGULATORS OF THE FIRST EMBRYONIC CELL CYCLE IN DROSOPHILA
At the transition from oocyte to embryo, a quiescent mature oocyte is transformed in a short span of time to a rapidly dividing embryo. In the first two hours of Drosophila melanogaster embryogenesis there is very little transcription of the zygotic genome, so regulation of translation of maternally-loaded mRNAs and post-translational modifications are the primary mechanisms of controlling the events of the oocyte-to-embryo transition. Changes in protein phosphorylation occur during egg activation, and I have investigated several aspects of these phosphorylation changes by testing the activity/or and roles of MAP kinases, and PP1 and MKP3 phosphatases during this developmental transition. I also examined the relationship between these kinases/phosphatases, four genes known to be necessary for egg activation (sarah, cortex, prage, and wispy), and two proteins (YA and GNU) that are dephosphorylated upon egg activation and act to initiate or modulate embryonic mitosis. I further defined the cell-cycle arrest point of one of these latter phosphoproteins, YA. Changes in phosphorylation state are likely to be important for regulating multiple aspects of the oocyte to embryo transition. My studies have shown that MAP kinases are dephosphorylated upon egg activation. In addition, the egg activation genes cort, sra, and prg are upstream of YA dephosphorylation upon egg activation, and cort and sra are also upstream of GNU dephosphorylation. I identified the phosphorylation sites on YA from oocytes and embryos by mass spectrometry. During oocyte maturation, dmos, Ya and gnu mRNA polyadenylation and protein levels are regulated by wispy, which also regulates ERK and JNK phosphorylation in oocytes, probably through dmos. Testing the roles of phosphatases, I have shown that PP1s may have a role in egg activation, but MKP3 is primarily needed after egg activation, prior to the first mitosis. I also clarified the time of the cell cycle when YA is necessary; YA acts after egg activation to regulate the entry into mitosis after the first S phase.
mitosis; phosphorylation; egg activation; Drosophila