Specific Cooperation Between Imp-alpha 2 and Imp-beta/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions
Abstract
The multifunctional factors Imp-alpha and Imp-beta are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-alpha family exerts distinct tasks during development.
In Drosophila melanogaster, the imp-alpha 2 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-alpha 2 and imp-beta genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-alpha 2(D14) and imp-beta(KetRE34) females.
Embryonic development is arrested in these embryos but is unaffected in combinations between imp-beta(KetRE34) and null mutations in imp-alpha 1 or imp-alpha 3. Furthermore, the imp-alpha 2(D14)/imp-beta(KetRE34) interaction could only be rescued by an imp-alpha 2 transgene, albeit not imp-alpha 1 or imp-alpha 3, showing the exclusive imp-alpha 2 function with imp-beta.
Use of transgenes carrying modifications in the major Imp-alpha 2 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-beta(KetRE34) and Imp-beta(KetD) bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-beta(KetRE34) phenotype. These data suggest that a fine balance among Imp-alpha 2, Imp-beta, RanGTP, and the NLS cargos is critical for mitotic progression during early embryonic development.