The 5S RNP and the regulation of the tumour suppressor

Abstract

Ribosomes are vital molecular machines involved in catalysing mRNA translation into proteins. Understandably, when ribosome production is disrupted, there are mechanisms available to prevent further cell growth. The main complex involved in this process is the 5S RNP, a ribosome assembly intermediate consisting of the ribosomal proteins RPL5 and RPL11, and the 5S rRNA. The 5S RNP induces the tumour suppressor p53 by interacting with, and inhibiting, MDM2 an ubiquitin ligase responsible for maintaining low levels of p53 in the cell. The 5S RNP-p53 pathway has been implicated in diseases known as ribosomopathies, however the regulation of these pathways have not been fully characterised and there is still much that is unknown. The aim of this project was to identify the components involved in regulating 5S RNP integration into the ribosome and to determine what further components may be required for 5S RNP-mediated p53 activation. There have been many recent studies elucidating the mechanism by which the 5S RNP is incorporated into the ribosome in yeast, however this mechanism appears to vary in humans. The data presented in this thesis determined that mimicking phosphorylation of RPL5 Y30 prevented 5S RNP incorporation into the ribosome. Furthermore, depletion of RPL7, RPL18, and RPL21 resulted in large subunit biogenesis defects and abrogated 5S RNP incorporation into the ribosome. Previous studies have suggested that SRSF1 and PRAS40 may be involved in the regulation of p53 via RPL5 and RPL11 respectively. In congruence with this, the data presented here showed SRSF1 interacts with the 5S RNP. Furthermore, depletion of SRSF1 results in reduction in 5S RNP-dependent p53 activation, mis-localisation of RPL11, and reduction in 5S RNP integration. Collectively, these data suggest that SRSF1 may be important for both ribosome production and p53 regulation. The role of PRAS40, however, still remains unclear. Despite not interacting with the 5S RNP, depletion of PRAS40 appeared to abrogate 5S-RNP mediated p53 induction. In summary, this work has provided a basis for studies into how the 5S RNP is recruited into the ribosome and provides evidence to suggest that SRSF1 is a component of the 5S RNP complex to activate p53.