Sen Gupta Ayantika, Sengupta Kundan
Biology, Indian Institute of Science Education and Research, Pune, India.
Biology, Indian Institute of Science Education and Research, Pune, India
Mol Cell Biol. 2017 Nov 28;37(24). doi: 10.1128/MCB.00274-17. Print 2017 Dec 15.
The nucleolus is required for ribosome biogenesis. Human cells have 2 or 3 nucleoli associated with nucleolar organizer region (NOR)-bearing chromosomes. An increase in number and altered nucleolar morphology define cancer cells. However, the mechanisms that modulate nucleolar morphology and function are unclear. Here we show that in addition to localizing at the nuclear envelope, lamin B2 localizes proximal to nucleolin at the granular component (GC) of the nucleolus and associates with the nucleolar proteins nucleolin and nucleophosmin. Lamin B2 knockdown severely disrupted the nucleolar morphology, which was rescued to intact and discrete nucleoli upon lamin B2 overexpression. Furthermore, two mutually exclusive lamin B2 deletion mutants, ΔHead and ΔSLS, rescued nuclear and nucleolar morphology defects, respectively, induced upon lamin B2 depletion, suggesting independent roles for lamin B2 at the nucleolus and nuclear envelope. Lamin B2 depletion increased nucleolin aggregation in the nucleoplasm, implicating lamin B2 in stabilizing nucleolin within the nucleolus. Lamin B2 knockdown upregulated nucleolus-specific 45S rRNA and upstream intergenic sequence (IGS) transcripts. The IGS transcripts colocalized with aggregates of nucleolin speckles, which were sustained in the nucleoplasm upon lamin B2 depletion. Taken together, these studies uncover a novel role for lamin B2 in modulating the morphology, dynamics, and function of the nucleolus.
核仁是核糖体生物合成所必需的。人类细胞有2个或3个与携带核仁组织区（NOR）的染色体相关的核仁。核仁数量增加和形态改变是癌细胞的特征。然而，调节核仁形态和功能的机制尚不清楚。在这里，我们表明，核纤层蛋白B2（lamin B2）除了定位于核膜外，还定位于核仁颗粒组分（GC）中靠近核仁素的位置，并与核仁蛋白核仁素和核磷蛋白相互作用。敲低核纤层蛋白B2会严重破坏核仁形态，而过表达核纤层蛋白B2后，核仁形态可恢复为完整且离散的状态。此外，两个相互排斥的核纤层蛋白B2缺失突变体ΔHead和ΔSLS分别挽救了核纤层蛋白B2缺失后诱导的核和核仁形态缺陷，这表明核纤层蛋白B2在核仁和核膜上发挥独立作用。核纤层蛋白B2的缺失增加了核仁素在核质中的聚集，这表明核纤层蛋白B2在稳定核仁内的核仁素方面发挥作用。敲低核纤层蛋白B2会上调核仁特异性的45S核糖体RNA和上游基因间隔序列（IGS）转录本。IGS转录本与核仁素斑点的聚集体共定位，在核纤层蛋白B2缺失后，这些聚集体在核质中持续存在。综上所述，这些研究揭示了核纤层蛋白B2在调节核仁形态、动态变化和功能方面的新作用。
