The Drosophila melanogaster importin (alpha)3 Locus Encodes an Essential Gene Required for the Development of Both Larval and Adult Tissues

The nuclear transport of classical nuclear localization signal (cNLS)-containing proteins is mediated by the cNLS receptor importin (alpha). The conventional importin (alpha) gene family in metazoan animals is composed of three clades that are conserved between flies and mammals and are referred to here as (alpha)1, (alpha)2, and (alpha)3. In contrast, plants and fungi contain only (alpha)1 genes. In this study we report that Drosophila importin (alpha)3 is required for the development of both larval and adult tissues. Importin (alpha)3 mutant flies die around the transition from first to second instar larvae, and homozygous importin (alpha)3 mutant eyes are defective. The transition to second instar larvae was rescued with importin (alpha)1, (alpha)2, or (alpha)3 transgenes, indicating that Importin (alpha)3 is normally required at this stage for an activity shared by all three importin (alpha)ʹs. In contrast, an (alpha)3specific biochemical activity(s) of Importin (alpha)3 is probably required for development to adults and photoreceptor cell development, since only an importin (alpha)3 transgene rescued these processes. These results are consistent with the view that the importin (alpha)ʹs have both overlapping and distinct functions and that their role in animal development involves the spatial and temporal control of their expression.