Analysis of the role of XGeF in early meiotic maturation and a requirement for MAPK in activation of CPEB

Meiotic maturation in Xenopus oocytes is the biological process that transforms an immature oocyte into an egg competent for fertilization. During early meiosis, Xenopus Guanine Exchange Factor (XGef) mediates the activation of Cytoplasmic Polyadenylation Element Binding Protein (CPEB), which is required for polyadenylation-induced translation of a subset of mRNA.; The focus of this thesis is twofold: (a) the examination of a role for XGef in early CPEB phosphorylation and (b) an analysis of upstream signaling events in early meiosis required for early CPEB phosphorylation. We demonstrate that Aurora A kinase (Aur-A), contrary to previous reports, is likely not required for CPEB activation during early meiosis. Our data demonstrates that activation of CPEB requires MAPK and identifies residues in CPEB targeted by MAPK. In addition, we detect activation of MAPK in oocytes prior to Mos synthesis, providing additional support for a revised model of early signaling events. Furthermore, we find that XGef and MAPK reside in a signaling complex that phosphorylates CPEB, implicating a mechanism for how XGef mediates early CPEB phosphorylation.; In addition, we report the characterization of a novel CPEB interacting protein, XPIAS, which functions as an E3 ligase for sumoylation (a ubiquitin like post-translational modification). Our data suggests an intriguing link between sumoylation and CPEB and provides insight into the potential role of sumoylation during meiotic maturation in Xenopus oocytes. Finally, this thesis also contains an appendix related to an additional project that contributes to our understanding of an enzyme required for translation, although unrelated to CPEB function. Appendix I describes the identification of a CCA adding enzyme in Xenopus (XCCA) and characterization of differential processing in XCCA mRNA.