Translational regulation at the oocyte to embryo transition in Drosophila. Iva Kronja¹, Bingbing Yuan¹, Kristina Dzeyk², Joanna Kirkpatrick², Jeroen Krijgsveld², Terry Orr-Weaver¹. 1) Whitehead Institute, MIT, Cambridge, MA; 2) EMBL, Heidelberg, Germany.

   Drosophila oogenesis is an excellent system to study the contribution of translational regulation to cell cycle progression. It is thought that two bursts of renewed protein synthesis are correlated with progression through meiosis, one at oocyte maturation and the other at egg activation. Identifying proteins whose levels increase at maturation will reveal candidates required for meiotic progression, and proteins upregulated at egg activation may lead to the players needed for completion of meiosis and the onset of embryogenesis. To identify these candidates, we applied two complementary genome-wide approaches: polysome profiling followed by mRNA sequencing and in vitro dimethyl labeling combined with quantitative mass spectrometry. Our proteomic approach showed that levels of only a limited set of proteins increase at oocyte maturation or egg activation. Surprisingly, we observed that a more drastic aspect of proteome remodeling is a decrease in protein levels. To understand if the differences in protein levels stem from changes in translation or protein stability, we performed genome-wide polysome profiling. This approach provided information on the translational status at egg activation for all mRNAs. Although the polysome profile of mature oocytes suggests slower translational initiation than in activated eggs, these two samples overall have comparable translational activity. Despite this similarity of the polysome profiles, at egg activation several hundred mRNAs are loaded onto polysomes while over a thousand mRNAs are released from the polysomes. Importantly, we observed a significant overlap among candidates identified by the proteome- and the translation-based methods, confirming the importance of translational regulation in proteome remodeling at egg activation. The agreement of the two methods also provides validation that the identified candidates may indeed be potentially important regulators of meiosis and early embryogenesis.