title: Specific Cooperation Between Imp-α2 and Imp-β/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions.
abstract: The multifunctional factors Imp-α and Imp-β are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-α family exerts distinct tasks during development. In Drosophila melanogaster, the imp-α2 gene is critical during oogenesis for ring canal assembly; specific mutations, which allow oogenesis to proceed normally, were found to block early embryonic mitosis. Here, we show that imp-α2 and imp-β genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-α2(D14) and imp-β(KetRE34) females. Embryonic development is arrested in these embryos but is unaffected in combinations between imp-β(KetRE34) and null mutations in imp-α1 or imp-α3. Furthermore, the imp-α2(D14)/imp-β(KetRE34) interaction could only be rescued by an imp-α2 transgene, albeit not imp-α1 or imp-α3, showing the exclusive imp-α2 function with imp-β. Use of transgenes carrying modifications in the major Imp-α2 domains showed the critical requirement of the nuclear localization signal binding (NLSB) site in this process. In the mutant embryos, we found metaphase-arrested mitoses made of enlarged spindles, suggesting an unrestrained activity of factors promoting spindle assembly. In accordance with this, we found that Imp-β(KetRE34) and Imp-β(KetD) bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-β(KetRE34) phenotype. These data suggest that a fine balance among Imp-α2, Imp-β, RanGTP, and the NLS cargos is critical for mitotic progression during early embryonic development.
authors.forename: Erika
authors.initials: E
authors.lastname: Virágh
authors.forename: Mátyás
authors.initials: M
authors.lastname: Gorjánácz
authors.forename: István
authors.initials: I
authors.lastname: Török
authors.forename: Tolga
authors.initials: T
authors.lastname: Eichhorn
authors.forename: Sowjanya
authors.initials: S
authors.lastname: Kallakuri
authors.forename: Tamás
authors.initials: T
authors.lastname: Szlanka
authors.forename: István
authors.initials: I
authors.lastname: Kiss
authors.forename: Bernard M
authors.initials: BM
authors.lastname: Mechler