Department of Cell and Chemical Biology, Oncode Institute, Leiden University Medical Center LUMC, Leiden, The Netherlands.
Department of Cell and Chemical Biology, Oncode Institute, Leiden University Medical Center LUMC, Leiden, The Netherlands
J Cell Sci. 2019 May 15;132(10):jcs225524. doi: 10.1242/jcs.225524.
Upon nuclear envelope (NE) fragmentation in the prometaphase, the nuclear and cytosolic proteomes mix and must be redefined to reinstate homeostasis. Here, by using a molecular GFP ladder, we show that in early mitosis, condensed chromatin excludes cytosolic proteins. When the NE reforms tightly around condensed chromatin in late mitosis, large GFP multimers are automatically excluded from the nucleus. This can be circumvented by limiting DNA condensation with Q15, a condensin II inhibitor. Soluble small and other nuclear localization sequence (NLS)-targeted proteins then swiftly enter the expanding nuclear space. We then examined proteasomes, which are located in the cytoplasm and nucleus. A significant fraction of 20S proteasomes is imported by the importin IPO5 within 20 min of reformation of the nucleus, after which import comes to an abrupt halt. This suggests that maintaining the nuclear-cytosol distribution after mitosis requires chromatin condensation to exclude cytosolic material from the nuclear space, and specialized machineries for nuclear import of large protein complexes, such as the proteasome.
在有丝分裂前期核膜（NE）碎片化时，核质和细胞质的蛋白质组混合，必须重新定义以恢复内稳态。在这里，我们通过使用分子 GFP 梯，表明在早期有丝分裂中，浓缩染色质排斥细胞质蛋白。当 NE 在后期有丝分裂中紧密围绕浓缩染色质重新形成时，大的 GFP 多聚体自动被排除在核外。通过用 Q15（一种凝聚素 II 抑制剂）限制 DNA 凝聚，可以避免这种情况。然后可溶性小分子和其他靶向核定位序列（NLS）的蛋白质迅速进入不断扩大的核空间。然后，我们检查了蛋白酶体，它们位于细胞质和细胞核中。在核重新形成后 20 分钟内，importin IPO5 将 20S 蛋白酶体的很大一部分导入核内，之后导入突然停止。这表明，维持有丝分裂后核质分布需要染色质凝聚将细胞质物质排除在核空间之外，并需要专门的机制来导入大型蛋白质复合物，如蛋白酶体。
