Eukaryotic ribosomal assembly is a highly conserved, tightly regulated process, in response to environmental stress and to growth conditions. It accounts, in proliferationg cell cultures, for more than 50% of energetic expenses. This metabolic pathway begins in the nucleolus, goes on through the nucleoplasm and ends up in the cytoplasm (Figure 1).

Figure 1. An overview of ribosome biogenesis in Saccharomyces cerevisiae

My PhD thesis work focused on the dynamics of assembly, dissociation and recycling of proteins involved in the biogenesis of the large ribosomal subunit in Saccharomyces cerevisiae. It sheds some light on two control points in this metabolic pathway, localised in the nucleus and the cytoplasm respectively.

We have shown that the nuclear protein Nsa2, which is very conserved throughout the eukaryotic kingdom, is required for the correct maturation of the 27SB ribosomal RNA precursor. Nsa2 is an unstable factor, regulated in correlation with the activity of ribosome biogenesis; it thus constitutes a good candidate for the integration of various signals resulting in the regulation of this metabolic pathway. Besides, using the SILAC technique, we could define groups of early or late acting factors relative to the Nsa2 action time.

In the cytoplasm, we identified a protein network, which marks the end of ribosome biogenesis and triggers the entry of new ribosomal subunits into translation. The cytoplasmic protein Rei1 and the karyopherin Kap121 are both required for the recycling from the cytoplasm to the nucleus of a dimer of shuttling factors, Arx1-Alb1. This recycling enables the dissociation of the anti-association factor Tif6 from the large ribosomal subunit, which can consequently bind the small ribosomal subunit and enter translation.

The manuscript of this PhD thesis (in French) can be downloaded from the multidisciplinary thesis server of the Center for Direct Scientific Communication (CNRS).