Ribosomal proteins L5 and L15 : Functional characterisation of important features, in vivo

Abstract: Protein synthesis is a highly regulated and energy consuming process, during which a large ribonucleoprotein particle called the ribosome, synthesizes new proteins. The eukaryotic ribosome consists of two unequal subunits called: small and large subunits. Both subunits are composed of ribosomal RNA (rRNA) and ribosomal proteins (r-proteins). Although rRNAs build the matrix of the ribosome and carries out catalysing of the peptide-bond formation between amino acids, r-proteins also appear to play important structural and functional roles. The primary role of r-proteins is to initiate the correct tertiary fold of rRNA and to organize the overall structure of the ribosome. In this thesis, I focus on two proteins from the large subunit of the eukaryotic ribosome: r-proteins L5 and L15 from bakers yeast S. cerevisiae. Both r-proteins are essential for ribosome function. Their life cycle is primarily associated with rRNA interactions. As a consequence, the proteins show high sequence homology across the species borders. Furthermore, both L5 and L15 are connected to human diseases, which makes the study their role in ribosome biogenesis and ribosome function important. By applying random- and site-directed mutagenesis, coupled with functional complementation tests, I aimed to elucidate functionally regions in both proteins, implicated in transport to the cell nucleus, protein-protein interactions and/or rRNA binding. The importance of individual and multiple amino acid exchanges in the primary sequence of rpL5 and rpL15 were studied in vivo. The obtained results show that S. cerevisiae rpL15 was tolerant to amino acid exchanges in the primary sequence, whereas rpL5 was not. Consequently, A. thaliana rpL15 could substitute for the function of wild type rpL15, whereas none of the tested orthologous proteins to rpL5 could substitute yeast rpL5 in vivo. These observations further emphasize the importance of studying r-proteins as separate entities in the ribosome context.