Ribosomal Proteins Orchestrate the Biogenesis of Eukaryotic Large Ribosomal Subunits in a Sequential Fashion

Ribosome biogenesis in eukaryotes involves the transcription, folding, and processing of ribosomal RNA (rRNA), as well as the concomitant assembly of ribosomal proteins. Several hundred trans-acting assembly factors also play a role in the complex process of ribosome biogenesis. Investigations of the construction of ribosomes have focused primarily on the roles of these assembly factors. Little is understood about how ribosomal proteins (r-proteins) function in ribosomal subunit biogenesis in vivo, in either prokaryotes or eukaryotes. I began by focusing on a subset of r-proteins surrounding the polypeptide exit tunnel of the large ribosomal subunit in yeast. R-proteins in this neighborhood, namely L17, L26, L35, and L37, are of importance because they fail to assemble with preribosomes when early pre-rRNA processing steps are blocked. I showed that these rproteins are important for the next pre-rRNA processing, cleavage of the ITS2 spacer sequence in 27SB pre-rRNA. Interestingly, I showed that this biogenesis defect is not due to changes in structure of ITS2. Instead, these r-proteins are required for stable recruitment of key assembly factors that function in this event. I then carried out a global survey of the majority of r-proteins in the 60S subunit. I found that co-transcriptional binding of r-proteins influences post-transcriptional stabilization of 60S subunit structural neighborhoods. This led to a model wherein structural domains of eukaryotic large ribosomal subunits are constructed in a hierarchical fashion. Assembly begins at the convex solvent side, followed by the polypeptide exit tunnel, the intersubunit side, and finally the central protuberance. This hierarchy serves as an initial framework to further understand 60S assembly in vivo. I also showed that pre-ribosomes become more stable as assembly proceeds and that the final steps in 60S maturation occur around regions important for ribosome function. My results also support the hypothesis that the formation of the 3’ end of 27S pre-rRNA is important for early steps of 60S assembly occurring near the 5’ end of pre-rRNA. I also studied the functions of conserved and eukaryote-specific extensions of rproteins that are intrinsically disordered. This revealed distinct roles of extensions in 60S subunit biogenesis and supported a model for the sequential binding of globular and then extended domains of r-proteins during ribosome assembly. My surprising finding for a eukaryote-specific r-protein tail highlights the importance of understanding why several yeast r-proteins have evolved extra sequences that are conserved in higher eukaryotes. Together, these investigations revealed important principles governing ribosome assembly. Furthermore, striking similarities and differences between assembly of bacterial and eukaryotic large ribosomal subunits also emerged, providing insights into how these RNA–protein particles evolved.