AbstractsBiology & Animal Science

All ubiquitin molecules in the cell are initially produced as precursor proteins. In yeast, there are 4 ubiquitin coding genes, named UBI1, UBI2, UBI3 and UBI4 (Ozkaynak et al, 1987). The first three genes encode for a linear ubiquitin fusion with distinct r-proteins, thus, these genes are also known as RPL40A/UBI1, RPL40B/UBI2 and RPS31/UBI3 (Finley et al, 1989). These genes provide cells with ubiquitin under exponential growth conditions. UBI4 encodes a 5 head-to-tail repeats of ubiquitin and has an important role under stationary phase and stress conditions. UBI1 and UBI2 are duplicated genes that encode for the L40 r-protein. These genes produce ubiquitin fused to two identical LSU r-proteins of 52 amino acids, L40A and L40B. The ubiquitin moiety is interrupted at the same position by one intron that is not homologous in each gene. On the other hand UBI3 is a non-essential gene that encodes for a ubiquitin fused to the SSU r-protein S31 of 76 amino acids (Finley et al, 1989). L40 is a globular protein that assembles close to L12 at the base of the ribosomal stalk structure. S31 has a globular domain and an extension, but in contrast with other r-protein¿s extensions, this one stays at the surface rather than inside the structure. When both r-subunits join in an 80S particle, L40 and S31 are located opposed each other near the binding site of the elongation factors (Ben-Shem et al, 2011; Klinge et al, 2011). It has been proposed that the presence of the ubiquitin moieties within the r-proteins in the mature ribosome would prevent the proper association of the elongation factors. Thus, cleavage of ubiquitin will act as a mechanism of surveillance. However, this is only a speculation since the dynamics of ubiquitin processing still remain uncharacterised. The aim of my PhD project consists in understanding the role of the r-proteins that are naturally fused to ubiquitin in the synthesis and function of yeast ribosomes. In S. cerevisiae, these proteins are L40 and S31. To address this objective, we described the contribution of L40 in ribosome biogenesis. This study has been published as Fernandez-Pevida et al., 2012 (Fernández-Pevida et al, 2012). We are also interested in revealing the role of ubiquitin in the 60S r-protein assembly process. For this purpose, we analyse cis-mutations that affect the cleavage of the ubiquitin. Our group has previously studied the role of the ubiquitin moiety of the S31 r-protein in ribosome biogenesis (Lacombe et al, 2009). In the last part of my PhD project, we have studied the function of the eukaryotic-specific N-terminal extension of S31 in the assembly of 40S r-subunits. More specifically, we have analyzed how the deletion of this region, which contains a putative nuclear localization signal, affects ribosome biogenesis. In this study, we have undertaken the functional analysis of L40 in yeast ribosome synthesis. Homologues of L40 are found in archaea (archaeal L40e) but not in bacteria (Nakao et al, 2004; Wu et al, 2008). The role of the ubiquitin moiety of yeast Ubi3…