Aoki Keita, Niki Hironori
Microbial Genetics Laboratory, Genetic Strains Research Center, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan
Department of Genetics, SOKENDAI, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan.
Biol Open. 2017 Nov 15;6(11):1614-1628. doi: 10.1242/bio.027193.
After mitosis, nuclear reorganization occurs together with decondensation of mitotic chromosomes and reformation of the nuclear envelope, thereby restoring the Ran-GTP gradient between the nucleus and cytoplasm. The Ran-GTP gradient is dependent on Pim1/RCC1. Interestingly, a defect in Pim1/RCC1 in causes postmitotic condensation of chromatin, namely hypercondensation, suggesting a relationship between the Ran-GTP gradient and chromosome decondensation. However, how Ran-GTP interacts with chromosome decondensation is unresolved. To examine this interaction, we used , which is known to undergo partial breakdown of the nuclear membrane during mitosis. We found that Pim1/RCC1 was localized on nuclear pores, but this localization failed in a temperature-sensitive mutant of Pim1/RCC1. The mutant cells exhibited hypercondensed chromatin after mitosis due to prolonged association of condensin on the chromosomes. Conceivably, a condensin-dephosphorylation defect might cause hypercondensed chromatin, since chromosomal localization of condensin is dependent on phosphorylation by cyclin-dependent kinase (CDK). Indeed, CDK-phospho-mimic mutation of condensin alone caused untimely condensin localization, resulting in hypercondensed chromatin. Together, these results suggest that dephosphorylation of CDK sites of condensin might require the Ran-GTP gradient produced by nuclear pore-localized Pim1/RCC1.
有丝分裂后，核重组与有丝分裂染色体的解聚和核膜的重新形成同时发生，从而恢复细胞核与细胞质之间的Ran-GTP梯度。Ran-GTP梯度依赖于Pim1/RCC1。有趣的是，Pim1/RCC1缺陷会导致有丝分裂后染色质凝聚，即过度凝聚，这表明Ran-GTP梯度与染色体解聚之间存在关联。然而，Ran-GTP如何与染色体解聚相互作用仍未得到解决。为了研究这种相互作用，我们使用了……，已知其在有丝分裂期间核膜会发生部分破裂。我们发现Pim1/RCC1定位于核孔，但在Pim1/RCC1的温度敏感突变体中这种定位失败。由于凝聚素在染色体上的长时间结合，突变细胞在有丝分裂后表现出染色质过度凝聚。可以想象，凝聚素去磷酸化缺陷可能导致染色质过度凝聚，因为凝聚素的染色体定位依赖于细胞周期蛋白依赖性激酶（CDK）的磷酸化。事实上，仅凝聚素的CDK磷酸化模拟突变就导致凝聚素定位过早，从而导致染色质过度凝聚。总之，这些结果表明凝聚素CDK位点的去磷酸化可能需要由定位于核孔的Pim1/RCC1产生的Ran-GTP梯度。
