Regulation of ribosome biogenesis in health and disease

Abstract

Ribosome biogenesis is one of the most energy consuming and regulated cellular processes. Due to the fundamental importance of ribosomes in cellular biology, it is unsurprising that defects in their biogenesis are linked to a range of human disorders, termed ribosomopathies. This project aimed to investigate different mechanisms through which ribosome production is regulated in relation to health and disease. When ribosome biogenesis is defective, the 5S RNP, a large subunit (LSU) assembly intermediate, accumulates, activating the tumour suppressor p53 through inhibition of its regulator. Unexpectedly, small subunit (SSU) production defects also activate p53 through the 5S RNP. In this study, SSU production defects were found to activate p53 early and in the absence of changes in mature SSU levels, contrary to previous research. SSU production defects result in impaired export of the pre-LSU and reduced levels of the late, cytoplasmic pre-5.8S ribosomal RNA. This implies that defective SSU production activates p53 through stalling the late stages of LSU maturation. Ribosomal proteins are produced in excess and are protected from aggregation by chaperones. RPL3 and its dedicated chaperone GRWD1 were both shown to be required to produce the LSU and SSU and were both shown to bind a novel inhibitor of ribosome biogenesis, C8ORF33. The GRWD1 binding site of RPL3 is required for this interaction and is required for ribosome production. These proteins had different but diverging roles during ribosome production and may be involved in regulatory mechanisms. The chaperone binding site of RPL3 is highly conserved between humans and yeast except for serine residues in humans that have been reported as phosphorylation sites. Phosphorylation offers a mechanism through which protein functions can be modified to diversify cellular responses. However, such modifications do not impact GRWD1 binding but are linked to minor changes in pre-ribosomal RNA processing.