Gene Center Munich and Center of Integrated Protein Science-Munich (CiPS-M), Department of Biochemistry, University of Munich, Munich, Germany.
Biochemie-Zentrum der Universität Heidelberg, Heidelberg, Germany.
Nat Struct Mol Biol. 2017 Nov;24(11):954-964. doi: 10.1038/nsmb.3476. Epub 2017 Oct 2.
The 40S small ribosomal subunit is cotranscriptionally assembled in the nucleolus as part of a large chaperone complex called the 90S preribosome or small-subunit processome. Here, we present the 3.2-Å-resolution structure of the Chaetomium thermophilum 90S preribosome, which allowed us to build atomic structures for 34 assembly factors, including the Mpp10 complex, Bms1, Utp14 and Utp18, and the complete U3 small nucleolar ribonucleoprotein. Moreover, we visualized the U3 RNA heteroduplexes with a 5' external transcribed spacer (5' ETS) and pre-18S RNA, and their stabilization by 90S factors. Overall, the structure explains how a highly intertwined network of assembly factors and pre-rRNA guide the sequential, independent folding of the individual pre-40S domains while the RNA regions forming the 40S active sites are kept immature. Finally, by identifying the unprocessed A1 cleavage site and the nearby Utp24 endonuclease, we suggest a proofreading model for regulated 5'-ETS separation and 90S-pre-40S transition.
40S 小核糖体亚基作为称为 90S 前核糖体或小亚基加工体的大型伴侣复合物的一部分，在核仁中进行共转录组装。在这里，我们呈现了嗜热毛壳菌 90S 前核糖体的 3.2Å 分辨率结构，这使我们能够构建 34 个组装因子的原子结构，包括 Mpp10 复合物、Bms1、Utp14 和 Utp18 以及完整的 U3 小核仁核糖核蛋白。此外，我们可视化了 U3 RNA 异源双链体与 5' 外部转录间隔区 (5'ETS) 和前 18S RNA 及其与 90S 因子的稳定作用。总的来说，该结构解释了组装因子和 pre-rRNA 的高度交织网络如何指导各个前 40S 结构域的顺序、独立折叠，同时保持形成 40S 活性部位的 RNA 区域不成熟。最后，通过鉴定未加工的 A1 切割位点和附近的 Utp24 内切酶，我们提出了一种用于调节 5'-ETS 分离和 90S-前 40S 过渡的校对模型。
