β-importins Tnpo-SR and Cadmus and the small GTPase Ran promote ovarian cyst formation in Drosophila

Abstract

Germ cells undergo mitotic expansion via incomplete cytokinesis, forming cysts of undifferentiated cells that remain interconnected prior to meiotic initiation, through mechanisms that are not well-defined. In somatic cells, Ras-related nuclear protein (Ran) spatiotemporally regulates mitotic spindle assembly, cleavage furrow formation and abscission. Here, we identify Ran and {beta}-importins as critical regulators of cyst development in the Drosophila ovary. Depletion of Ran or the {beta}-importins Tnpo-SR and cadmus disrupts oocyte selection and results in egg chambers with variable numbers of germ cells, suggesting abnormal cyst development and cyst fragmentation. We demonstrate that Ran, Tnpo-SR, and Cadmus regulate key cellular processes during cyst formation, including cell cycle dynamics, fusome biogenesis, and ring canal stability, yet do so independently of mitotic spindle assembly. Further, Tnpo-SR and Cadmus control cyclin accumulation and suppress cytokinesis independent of Ran-GTP, suggesting that {beta}-importins sequester protein cargos that normally promote the mitotic-to-meiotic transition. Our data demonstrates that Ran and {beta}-importins are critical for germ cell cyst formation, a role that is likely conserved in other organisms. SUMMARY STATEMENTRan and two {beta}-importins function coordinately to promote oocyte selection and cyst development in the Drosophila ovary.