Nuclear entry of the Drosophila melanogaster nuclear lamina protein YA correlates with developmentally regulated changes in its phosphorylation state.

The Drosophila melanogaster YA protein is a maternally provided nuclear lamina component that is essential during the transition from meiosis to mitosis at the beginning of embryogenesis. Localization of YA to the nuclear envelope is required for its function; this localization is cell cycle-dependent during embryogenesis. Here we show that the ability of YA to enter nuclei is modulated during development. In developing egg chambers, YA protein is made but excluded from nuclei of nurse cells and oocytes; upon egg activation, YA acquires the ability to enter nuclei and becomes incorporated into the nuclear lamina in unfertilized eggs and embryos. This localization switch correlates with changes in the phosphorylation state of YA. YA in ovaries is hyperphosphorylated relative to YA in unfertilized eggs and embryos. Through site-directed mutagenesis, we identified 443T, a potential phosphorylation site for both cyclin-dependent protein kinase and mitogen-activated-protein kinase, as one of the sites likely involved in this developmental control. Our results suggest that phosphorylation plays a role in modulating the localization of YA during development. A model for developmental regulation of the nuclear entry of YA is proposed and implications for understanding Drosophila egg activation are discussed.