Importin-α2 is critically required for the assembly of ring canals during Drosophila oogenesis

Abstract

The interstitial deletion D14 affecting the importin-α2 gene of Drosophila, or imp-α32D14, 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. © 2002 Elsevier Science (USA).