Specific Cooperation Between Imp-a2 and Imp-b/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions

The multifunctional factors Imp-a and Imp-b are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-a family exerts distinct tasksduringdevelopment. InDrosophilamelanogaster, the imp-a2gene is criticalduringoogenesis for ring canal assembly; specific mutations, which allow oogenesis to proceed normally, were found to block early embryonic mitosis. Here, we show that imp-a2 and imp-b genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-a2D14 and impbKetRE34 females. Embryonic development is arrested in these embryos but is unaffected in combinations between imp-bKetRE34 and null mutations in imp-a1 or imp-a3. Furthermore, the imp-a2D14/imp-bKetRE34 interaction could only be rescued by an imp-a2 transgene, albeit not imp-a1 or imp-a3, showing the exclusive imp-a2 functionwith imp-b.Useof transgenescarryingmodifications in themajor Imp-a2domainsshowedthecritical requirementof the nuclear localization signal binding (NLSB) site in this process. In themutant embryos,we foundmetaphase-arrested mitoses made of enlarged spindles, suggesting an unrestrained activity of factors promoting spindle assembly. In accordance with this, we found that Imp-bKetRE34 and Imp-bKetD bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-bKetRE34 phenotype. These data suggest that a fine balance among Imp-a2, Imp-b, RanGTP, and theNLScargos is critical formitoticprogressionduringearlyembryonicdevelopment.