Nucleoli are sites where both the large and small ribosomal subunits mature. Biochemical assays have suggested that a multivalent nucleolar protein, NPM1/nucleophosmin contributes to the formation of the outer layer of the nucleolus. Prior works show that NPM1 depletion disorganizes the nucleolar structure. However, the mechanism of how NPM1 regulates the nucleolar structure has been unknown. We demonstrated that NPM1 directly interacts with the large ribosomal subunits and maintains them in the nucleolus. Ectopically localized NPM1 efficiently recruits only the large ribosomal subunit precursors, while ectopically localized large ribosomal subunit by the ribosomal protein RPL4 efficiently recruits NPM1. These results suggest that the nucleolar localization of NPM1 and the large ribosomal subunit precursors are mutually dependent. Furthermore, proteomic and localization analyses suggest that NPM1 plays a crucial role in the accumulation of the late processing machinery of the large ribosomal subunits in the nucleolus. Our results suggest that NPM1 maintains the pre-ribosomes and assembly machinery in the nucleolus, which in turn determines the nucleolar volume.

核仁是大小核糖体亚基都成熟的位点。生化分析表明，多价核仁蛋白NPM1 / nucleophosmin有助于核仁外层的形成。先前的工作表明，NPM1耗竭会破坏核仁结构。但是，尚不清楚NPM1如何调节核仁结构的机制。我们证明NPM1直接与大核糖体亚基相互作用，并在核仁中维持它们。异位定位的NPM1仅有效地募集了大的核糖体亚基前体，而异位定位的较大的核糖体亚基由核糖体蛋白RPL4有效地募集了NPM1。这些结果表明，NPM1的核仁定位和较大的核糖体亚基前体是相互依赖的。此外，蛋白质组学和本地化分析表明，NPM1在核仁中大核糖体亚基的后期加工机制的积累中起着至关重要的作用。我们的结果表明，NPM1在核仁中保留了核糖体的前体和装配机制，这反过来又决定了核仁的体积。