Importin-α2 Is Critically Required for the Assembly of Ring Canals during Drosophila Oogenesis

AbstractThe interstitial deletion D14 affecting the importin-α2 gene of Drosophila, or imp-α2D14, causes recessive female sterility characterized by a block of nurse cell–oocyte transport during oogenesis. In wild-type egg chambers, the Imp-α2 protein is uniformly distributed in the nurse cell cytoplasm with a moderate accumulation along the oocyte cortex. Cytochalasin D treatment of wild-type egg chambers disrupts the in vivo association of Imp-α2 with F-actin and results in its release from the oocyte cortex and its transfer into nurse cell nuclei. Binding assay shows that the interaction of Imp-α2 with F-actin, albeit not monomeric actin, requires the occurrence of NLS peptides. Phenotypic analysis of imp-α2D14 ovaries reveals that the block of nurse cell–oocyte transport results from the occlusion of the ring canals that constitute cytoplasmic bridges between the nurse cells and the oocyte. Immunohistochemistry shows that, although the Imp-α2 protein cannot be detected on the ring canals, the Kelch protein, a known ring canal component, fails to bind to ring canals in imp-α2D14 egg chambers. Since loss-of-function mutations of kelch results in a similar dumpless phenotype, we propose that the Imp-α2 protein plays a critical role in Kelch function by regulating its deposition on ring canals during their assembly