Role of Accessory Proteins P50/P55 in SnoRNP Biogenesis

North Carolina State University, Raleigh NC

Investigators

Abstract

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.