The Drosophila nuclear lamina protein YA is essential for the transition from female meiosis to embryo mitosis. Its localization and, hence, function is under developmental and cell cycle controls. YA protein is hyperphosphorylated and cytoplasmic in ovaries. Upon egg activation, YA is partially dephosphorylated and acquires the ability to enter nuclei. Its function is first detected at this time. To investigate the cytoplasmic retention machinery that keeps YA from entering nuclei, we used affinity chromatography and blot overlay assays to identify cytoplasmic proteins that associate with YA. Drosophila P0/AP3, a ribosomal protein that is also an apurinic/apyrimidinic endonuclease, binds to YA in ovary and embryo cytoplasms. P0 and YA bind specifically and directly in vitro and are present in a 20S complex in the cytoplasmic extracts. YA protein can be phosphorylated by MAPK, but not by p34(Cdc2) kinase, in vitro. This phosphorylation increases YA's binding to P0. We propose that the P0-containing 20S cytoplasmic complex retains hyperphosphorylated ovarian YA in the cytoplasm. In response to egg activation, YA is partially dephosphorylated and its binding to the 20S complex is reduced. Hence, some YA dissociates from the complex and enters nuclei. Consistent with this model, decreasing P0 levels partially suppress a hypomorphic Ya mutant allele.

The nuclear lamina provides an architectural framework for the nuclear envelope and an attachment site for interphase chromatin. In Drosophila eggs and early embryos its major constituent, lamin Dm0, interacts with a lamina protein called YA. When the lamin-interaction region of YA is deleted, YA still enters nuclei but fails to localize to nuclear envelopes, suggesting that lamin interaction targets YA to the nuclear envelope. Here, we show that C-terminal lamin-interacting region of YA is sufficient to target the heterologous soluble protein GFP-NLS to the nuclear periphery in Drosophila tissue culture cells. Yeast two-hybrid analysis and transient transfection assays further defined this domain: residues 556-696 of YA are sufficient for both lamin Dm0interaction and the targeting of GFP-NLS to the nuclear periphery. This region of YA is hydrophilic and lacks any transmembrane domain or known membrane-targeting motifs. We propose that the localization of YA to the nuclear lamina involves interaction with polymerized lamin Dm0mediated by the lamin-targeting domain of YA. This hydrophilic YA domain might provide a useful molecular tool for targeting heterologous non-membrane-associated proteins to the nuclear envelope.