Importin-9 regulates chromosome segregation and packaging in Drosophila germ cells

Germ cells undergo distinct nuclear processes as they differentiate into gametes. While these events must be coordinated to ensure proper maturation, the stage-specific transport of proteins in and out of germ cell nuclei remains incompletely understood. Our efforts to genetically characterize Drosophila genes that exhibit enriched expression in germ cells led to the finding that loss of the highly-conserved Importin {beta}/karyopherin family member Importin-9 (Ipo9) results in female and male sterility. Immunofluorescence and fluorescent in situ hybridization (FISH) revealed that Ipo9KO mutants display chromosome condensation and segregation defects during meiosis. In addition, Ipo9KO mutant males form abnormally structured sperm and fail to properly exchange histones for protamines. Ipo9 physically interacts with proteasome proteins and Ipo9 mutant males exhibit loss of nuclear ubiquitination and disruption of the nuclear localization of several proteasome components. Thus, Ipo9 coordinates the nuclear import of functionally related factors necessary for the completion of gametogenesis. HighlightsDrosophila Importin-9 functions in female and male meiosis Loss of Importin-9 disrupts the histone to protamine transition during spermiogenesis Importin-9 physically interacts with components of the proteasome Importin-9 is required for the efficient nuclear transport of proteasome proteins during spermiogenesis