Cloning of karyopherin-alpha3 from Drosophila through its interaction with the nuclear localization sequence of germ cell-less protein.

The D. melanogaster germ cell-less (gcl) gene has previously been shown to play a key role in the establishment of the germ cell lineage during fly embryogenesis. To identify other molecules that function with Gcl in this process, we have conducted a yeast two-hybrid screen that utilized Gcl protein as bait. A predominant class of Gcl-interacting clones encodes a species of importin-alpha from Drosophila (karyopherin-alpha3; kap-alpha3), a nuclear-localization sequence binding protein previously shown to act in the transport of proteins from the cytoplasm to the nucleus. The expression of kap-alpha3 is widespread both temporally and spatially throughout the embryo during development, as judged by Northern blotting and whole-mount in situ hybridization to Drosophila embryos, suggesting that it functions at multiple stages of development. Studies of the Gcl/Kap-alpha3 interaction have identified a functional nuclear-localization sequence in Gcl protein which is necessary for an in vivo interaction and for nuclear entry of Gcl, making it likely that one role for Kap-alpha3 is to deliver Gcl protein to the nucleus. The identification of Kap-alpha3 and an in vivo substrate will allow for further characterization of the basis for specificity between importin-alpha molecules and their binding substrates.