The regulation of cellular signalling by the 5S RNP

Abstract

Ribosome biogenesis is a tightly regulated process that is essential for cell survival. Genetic mutations in genes encoding ribosomal proteins or ribosome biogenesis factors lead to the development of rare genetic disorders, known as ribosomopathies. The 5S RNP is a ribosome assembly intermediate composed of the 5S rRNA, ribosomal protein RPL5 and RPL11. During the 5S RNP assembly, the newly transcribed 5S rRNA is stabilized by RPL5 first, before RPL11 binds. Once it is assembled, the 5S RNP is incorporated into the large ribosomal subunit precursor. Ribosome biogenesis defects, which induce ribosomal stress, cause the accumulation of the 5S RNP. The accumulated non-ribosomal 5S RNP induces tumour suppressor p53, by binding and inhibiting its main negative regulator, MDM2. It has been suggested that any factor involved in the 5S RNP assembly and its integration into the ribosome will affect p53 signalling. I therefore set out to evaluate factors that have been proposed to be connected to the 5S RNP. Different proteins, including HEXIM1, Mybbp1a, B23, NML and SRSF1, have been suggested to be involved in the 5S RNP-p53 signalling, therefore I aimed to investigate their roles in this signalling pathway. Furthermore, I also aimed to investigate the role of RPL11 phosphorylation in ribosome biogenesis and the 5S RNP-p53 signalling. In this work, I present evidence supporting the proposed role of splicing factor SRSF1 in 5S RNP assembly. I also demonstrate that SRSF1 could be a part of the 5S RNP complex that is essential for p53 activation during ribosomal stress, while protein HEXIM1, Mybbp1a and NML are not needed for this pathway. I further provide evidence suggesting the threonine 73 phosphorylation in RPL11 could control its binding to the 5S RNP, whereas serine 51 phosphorylation in RPL11 could be important in the 5S RNP incorporation into the large ribosomal subunit. Last but not the least, my results also suggest that serine 140 in RPL11 could be transiently phosphorylated during the ribosomal maturation process. In conclusion, this work addresses the important role of the 5S RNP, in both ribosome biogenesis and the 5S RNP-p53 signalling.