The Ketel Gene Encodes a Drosophila Homologue of Importin-β

Abstract

Abstract The Drosophila melanogaster Ketel gene was identified via the KetelD dominant female sterile mutations and their ketelr revertant alleles that are recessive zygotic lethals. The maternally acting KetelD mutations inhibit cleavage nuclei formation. We cloned the Ketel gene on the basis of a common breakpoint in 38E1.2-3 in four ketelr alleles. The Ketel+ transgenes rescue ketelr-associated zygotic lethality and slightly reduce KetelD-associated dominant female sterility. Ketel is a single copy gene. It is transcribed to a single 3.6-kb mRNA, predicted to encode the 97-kD Ketel protein. The 884-amino-acid sequence of Ketel is 60% identical and 78% similar to that of human importin-β, the nuclear import receptor for proteins with a classical NLS. Indeed, Ketel supports import of appropriately designed substrates into nuclei of digitonin-permeabilized HeLa cells. As shown by a polyclonal anti-Ketel antibody, nurse cells synthesize and transfer Ketel protein into the oocyte cytoplasm from stage 11 of oogenesis. In cleavage embryos the Ketel protein is cytoplasmic. The Ketel gene appears to be ubiquitously expressed in embryonic cells. Western blot analysis revealed that the Ketel gene is not expressed in several larval cell types of late third instar larvae.