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Nucleocytoplasmic Transport Machinery and Pathways: Cell Cycle Regulation and Stress Response

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 Major interest of our group is to understand function and structure of nuclear pore complexes (NPCs). NPC is a large protein assembly embedded in the nuclear envelope, which provides a sole pathway for molecular trafficking between the nucleus and the cytoplasm. In the present talk, I will provide two topics of our recent results: one on the regulation of interphase NPC assembly, and second on the novel transport pathway that operates during heat-shock stress response.  NPCs are assembled twice during the cell cycle in metazoa: end of mitosis (post-mitotic NPC formation), and during interphase. Post-mitotic NPC assembly takes place on mitotic chromosomes, whereas interphase NPC assembly takes place on the double lipid bilayer of the nuclear envelope. Because it is more difficult to analyze, less is known about interphase NPC assembly compared to post mitotic NPC assembly. We have developed a system to visualize interphase NPC assembly based on cell-fusion methods and photobleaching. Combining these visualization techniques with siRNA has allowed us to identify cyclin-dependent kinases (Cdks) as an important regulator of interphase NPC assembly. Interestingly, activity of Cdks is not required for the post-mitotic NPC assembly. The issue of target(s) of Cdks, which may explain the different mechanisms underlying between interphase and post-mitotic NPC assembly will be discussed. During the heat-shock stress response, conventional nucleocytoplasmic transport is down-regulated, possibly due to defects in small GTPase Ran system, while the nuclear import of molecular chaperones such as hsp70s are up-regulated. Using a transport-reconstituted system, we found that the heat-shock-induced nuclear import of hsp70s is mediated by a novel receptor, which does not belong to well-characterized members of the importin β superfamily. In living cells, depletion of this molecule potently inhibits heat-shock-induced nuclear migration of hsp70s, reduces cell viability after heat-shock, and causes significant delay in attenuation of heat-shock response. Taken together, our data show a novel nuclear import receptor for hsp70s is required for protecting cell damages from the stress.

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