Role of Accessory Proteins P50/P55 in SnoRNP Biogenesis

核仁小RNA（snoRNA）在真核生物核糖体的生物合成中起着重要作用。 关键snoRNA功能包括指导前核糖体RNA前体的切割和指导核糖体RNA中特定核苷酸的修饰。 在核仁中已经鉴定出超过100种snoRNA，从而确立了snoRNA作为真核细胞中发现的最大类别的小稳定RNA。 研究表明，盒C/D和H/ACA snoRNA表现出进化上保守的序列和每个家族特有的折叠结构。 这些RNA基序中的每一个结合家族特异性蛋白质以建立snoRNA：蛋白质（snoRNP）复合物。 盒C/D和H/ACA snoRNA基序及其相关的核心蛋白都是snoRNA加工、snoRNP转运和核糖体RNA的snoRNA引导的核苷酸修饰所必需的。 最近，已经确定了两个新的辅助蛋白，也是必不可少的snoRNP合成。 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具有重要的性质，但目前还不清楚p50/p55在合成新的snoRNP复合物中的作用。 因此，本研究的目标是双重的。 首先，p50/p55的snoRNP生物合成的作用将在体内爪蟾卵母细胞核中进行检查。 显微注射实验将确定这些辅助蛋白的snoRNP组装在核质中的可能作用，以及它们在运输成熟的snoRNP复合物进入核仁的潜在功能。 第二，结构研究将定义p50/p55与盒C/D核心snoRNP复合物的相互作用，提供调节snoRNP生物合成途径中辅助蛋白功能的机制细节。p50和p55是第一个鉴定的snoRNP生物合成所必需的snoRNP辅助蛋白。 定义它们的功能作用将为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合成表明，p50和p55是多功能蛋白质，并可能作为“分子桥梁”，以协调在真核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细胞核中RNA代谢的基本理解，并揭示新的基因表达调控模式。

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.