Role of Accessory Proteins P50/P55 in SnoRNP Biogenesis

小核仁rna （small nucleolar rna, snoRNAs）在真核核糖体生物合成中起着重要作用。关键的snoRNA功能包括指导核糖体前RNA前体的切割和指导核糖体RNA中特定核苷酸的修饰。在核仁中已鉴定出100多种snoRNAs，从而确定了snoRNAs是真核细胞中发现的最大一类稳定的小rna。研究表明，盒状C/D和H/ACA snoRNAs具有各自家族的进化保守序列和折叠结构特征。这些RNA基序中的每一个都与家族特异性蛋白结合，建立snoRNA:protein （snoRNP）复合物。盒C/D和H/ACA两个snoRNA基序及其相关的核心蛋白都是snoRNA加工、snoRNP运输和snoRNA引导的核糖体rna核苷酸修饰所必需的。最近，已经鉴定出两种新的辅助蛋白，它们也是snoRNP合成所必需的。辅助蛋白p50和p55与核质中的snoRNP核心复合物相互作用并调节snoRNP的产生。尽管p50/p55在合成新的snoRNP复合物中起着至关重要的作用，但目前还不清楚p50/p55是如何发挥作用的。因此，本研究的目的是双重的。首先，p50/p55在爪蟾卵母细胞核体内snoRNP生物发生中的作用将被研究。显微注射实验将确定这些辅助蛋白在核质中对snoRNP组装的可能作用，以及它们在将成熟的snoRNP复合物运送到核仁中的潜在功能。其次，结构研究将定义p50/p55与盒状C/D核心snoRNP复合物的相互作用，提供辅助蛋白在调节snoRNP生物合成途径中的功能机制细节。p50和p55是首次发现的snoRNP辅助蛋白，对snoRNP生物合成至关重要。确定它们的功能作用将为了解snoRNA/snoRNP生物合成途径提供基本的见解。p50/p55参与其他核质事件，如染色质重塑和RNA合成，这表明p50和p55是多功能蛋白，可能作为“分子桥梁”来协调真核生物细胞核中的各种过程。因此，这项研究有可能影响我们对真核生物细胞核中RNA代谢的基本认识，并揭示基因表达的新调控范式。

The small nucleolar RNAs (snoRNAs) play essential roles in eukaryotic ribosome biosynthesis. Critical snoRNA functions include directing cleavage of the pre-ribosomal RNA precursor and guiding the modification of specific nucleotides in the ribosomal RNAs. More than 100 snoRNAs have been identified in the nucleolus, thus establishing the snoRNAs as the largest class of small stable RNAs found in eukaryotic cells. Investigations have demonstrated that the box C/D and H/ACA snoRNAs exhibit evolutionarily conserved sequences and folded structures characteristic of each family. Each of these RNA motifs bind family-specific proteins to establish the snoRNA:protein (snoRNP) complex. Both the box C/D and H/ACA snoRNA motifs and their associated core proteins are required for snoRNA processing, snoRNP transport, and snoRNA-guided nucleotide modification of ribosomal RNAs. Most recently, two novel accessory proteins have been identified that are also essential for snoRNP synthesis. Accessory proteins p50 and p55 interact with the snoRNP core complex in the nucleoplasm and regulate snoRNP production. Despite their critical nature, nothing is yet known about how p50/p55 function in the synthesis of new snoRNP complexes. Therefore, the goals of this research are twofold. First, p50/p55's role in snoRNP biogenesis will be examined in vivo in the Xenopus oocyte nucleus. Microinjection experiments will determine the possible role of these accessory proteins for snoRNP assembly in the nucleoplasm as well as their potential function in transporting mature snoRNP complexes into the nucleolus. Second, structural studies will define the interactions of p50/p55 with the box C/D core snoRNP complex, providing mechanistic details of accessory protein function in regulating the snoRNP biosynthetic pathway. p50 and p55 are the first identified snoRNP accessory proteins essential for snoRNP biosynthesis. Defining their functional roles will provide fundamental insight into the snoRNA/snoRNP biosynthetic pathway. The involvement of p50/p55 in other nucleoplasmic events such as chromatin remodeling and RNA synthesis has suggested that p50 and p55 are multifunctional proteins and may serve as "molecular bridges" to coordinate various processes in the eukaryotic nucleus. Therefore, this research has the potential to impact our basic understanding of RNA metabolism in the eukaryotic nucleus and reveal new regulatory paradigms of gene expression.