Characterization of the Beta-Karyopherin Protein Ipo9 During Germ Cell Differentiation

Germ cells participate in the most fascinating process in nature, the fusion of two cells that ultimately give rise to an entire organism. Errors in germ cell development directly affect the fertility of the parent organism and/or the health of their offspring. A myriad of molecules such as transcription factors, signaling pathway effectors, chromatin modifiers and meiotic related proteins play fundamental roles during germ cell development. In order for these proteins to work they need to access the nucleus, through nuclear trafficking machinery. The karyopherin family of proteins is responsible for nuclear import and export. The contribution of nuclear trafficking during gametogenesis is not well understood. Here we demonstrated that the well conserved β-karyopherin Importin-9 (Ipo9) is essential in the germline for proper gametogenesis in female and male flies. We generated a molecular null allele of Ipo9 and showed that Ipo9 is required in females for chromosome segregation during meiosis and the accumulation of nuclear actin during egg chamber development. Additionally, Ipo9 is essential during spermatogenesis for spermatid elongation, proper elimination of histones during the transition from histone-based to protamine-based chromatin packaging, import of proteasome components and chromosome segregation during meiosis. Lastly, at the molecular level, we showed that the N-terminal domain, which is critical for nuclear import, is required for Ipo9 function during gametogenesis. Overall, this work advances our understanding of how nuclear trafficking regulates germ cell development.