Quality Control and Stress Response in Mammalian Ribosome Biogenesis

Author(s)

Minshi Wang

Date Approved

11-2014

Document Type

Dissertation

Degree Name

PhD in Cell and Molecular Biology

Department

Cell Biology

College

Graduate School of Biomedical Sciences

Funder

National Institutes of Health

First Advisor

Dimitri Pestov

Committee Member 1

Ronald Ellis, PhD

Committee Member 2

Randy Strich, PhD

Committee Member 3

Dmitry Temiakov, PhD

Committee Member 4

Venkataswar Venkataraman, PhD

Subject(s)

Ribosomes; Protein Biosynthesis; Ribosomal Peptide Biosynthesis; Eukaryota; Mammals; RNA Precursors

Abstract

Ribosomes are essential cellular components that translate mRNAs to proteins. The cytoplasmic ribosome in eukaryotes is a large molecular machine composed of 4 ribosomal RNAs (rRNAs) and ~80 ribosomal proteins (RPs). Ribosome biogenesis is a highly complicated process that involves transcription and processing of rRNA precursors (pre-rRNAs), their proper folding, assembly with RPs, and transport of pre-ribosomal particles from the nucleus to the cytoplasm. Besides rRNAs and RPs, numerous proteins and small RNAs work as trans-acting factors in this intricate process. Surveillance systems act along the entire ribosome synthesis pathway to eliminate defective processing intermediates and promote integrity of the final ribosomal particle. We have found that in mammalian cells, the S' exonuclease Xrn2 (homologous to yeast Rat1) plays a major role in both maturation of rRNA and degradation of a variety of discarded pre-rRNA species. We propose that probing of pre-rRNA maturation intermediates by exonucleases serves the dual function of generating mature rRNAs and suppressing suboptimal processing pathways during ribosome assembly. We have also developed a new approach, RAMP (ratio analysis of multiple precursors) to obtain a detailed picture of changes in pre-rRNA processing caused by deficiencies in RPs and ribosome synthesis factors. This part of our studies has revealed that the ability of each ribosomal subunit to progress through maturation determines how pre-rRNA transcript is cleaved to separate the two subunits during their assembly. Furthermore, we have discovered a previously unknown 5' truncated form of 5.8S rRNA, which we termed 5.8S_T, and shown that it can be assembled into mature ribosomes and participate in translation. Defects in assembly, such as deficiency of RPs, can lead to a dramatic increase in levels of the 5.8S_T rRNA in cells. These findings suggest that errors in ribosome assembly may promote synthesis of an altered form of the ribosome, which, in turn, may function differently in translation. This hypothesis could explain the diversity of developmental abnormalities observed in ribosomopathies, human disorders linked to defects in ribosome biogenesis. Finally, we have investigated the roles of an abundant nucleolar protein, NPM1, in ribosome biogenesis. Besides its known binding partners, such as Rps9 and Rp15, we have found that NPM1 also interacts with Rpl11. Contrary to a previously proposed model, our data indicate that NPM1 does not directly participate in ribosome synthesis, as an endonuclease or by facilitating nuclear export of pre-ribosomal particles.

Recommended Citation

Wang, Minshi, "Quality Control and Stress Response in Mammalian Ribosome Biogenesis" (2014). Graduate School of Biomedical Sciences Theses and Dissertations. 64.
https://rdw.rowan.edu/gsbs_etd/64