Cloning and characterization of rat importin 9: implication for its neuronal function

Importin-9 regulates chromosome segregation and packaging in Drosophila germ cells

Germ cells undergo distinct nuclear processes as they differentiate into gametes. Although these events must be coordinated to ensure proper maturation, the stage-specific transport of proteins in and out of germ cell nuclei remains incompletely understood. Our efforts to genetically characterize Drosophila genes that exhibit enriched expression in germ cells led to the finding that loss of the highly conserved Importin β/karyopherin family member Importin-9 (Ipo9, herein referring to Ranbp9) results in female and male sterility. Immunofluorescence and fluorescent in situ hybridization revealed that Ipo9KO mutants display chromosome condensation and segregation defects during meiosis. In addition, Ipo9KO mutant males form abnormally structured sperm and fail to properly exchange histones for protamines. Ipo9 physically interacts with proteasome proteins, and Ipo9 mutant males exhibit disruption of the nuclear localization of several proteasome components. Thus, Ipo9 coordinates the nuclear import of functionally related factors necessary for the completion of gametogenesis. This article has an associated First Person interview with the first author of the paper.

Astrocytic exportin-7 responds to ischemia through mediating LKB1 translocation from the nucleus to the cytoplasm

The superfamily of importin-β-related proteins is the largest class of nuclear transport receptors and can be generally divided into importins and exportins according to their transport directions. Eleven importins and seven exportins have been identified, and the expression patterns of both classes are important for their functions in nucleocytoplasmic transport activities. This study demonstrates that all of the importins (importin-β; transportin-1, -2, and -3; and importin-4, -5, -7, -8, -9, -11, and -13) and all the exportins (exportin-1, -2, -4, -5, -6, -7, and -t) are differentially expressed in the cerebral cortex, cerebellum, hippocampus, and brainstem and in primary cultures of cerebral cortical astrocytes and neurons. For astrocytes, we observed that different importins and exportins displayed different expression changes during 0-6 hr of ischemia treatment, especially an increase of both the mRNA and the protein of exportin-7. Immunostaining showed that exportin-7 accumulated inside the nucleus and around the nuclear envelope. In addition, we noticed an increased cytoplasmic distribution of one of the cargo proteins of exportin-7, LKB1, an important element in maintaining energy homeostasis. This increased cytoplasmic distribution was accompanied by an increased expression of exportin-7 under ischemia in astrocytes. We demonstrate that exportin-7 responds to ischemia in astrocytes and that this response involves translocation of LKB1, a protein that plays important roles during metabolic stress, from the nucleus to the cytoplasm.

Biological significance of the importin-β family-dependent nucleocytoplasmic transport pathways

Importin-β family proteins (Imp-βs) are nucleocytoplasmic transport receptors (NTRs) that import and export proteins and RNAs through the nuclear pores. The family consists of 14-20 members depending on the biological species, and each member transports a specific group of cargoes. Thus, the Imp-βs mediate multiple, parallel transport pathways that can be regulated separately. In fact, the spatiotemporally differential expressions and the functional regulations of Imp-βs have been reported. Additionally, the biological significance of each pathway has been characterized by linking the function of a member of Imp-βs to a cellular consequence. Connecting these concepts, the regulation of the transport pathways conceivably induces alterations in the cellular physiological states. However, few studies have linked the regulation of an importin-β family NTR to an induced cellular response and the corresponding cargoes, despite the significance of this linkage in comprehending the biological relevance of the transport pathways. This review of recent reports on the regulation and biological functions of the Imp-βs highlights the significance of the transport pathways in physiological contexts and points out the possibility that the identification of yet unknown specific cargoes will reinforce the importance of transport regulation.