Ribosome Assembly
Structures of the mature ribosomal subunits in different functional states have been available for some years now, but our knowledge about their assembly at atomic resolution is only emerging.
As the composition of the mature ribosomes of eukaryotes, prokaryotes and organelles diverged through evolution, so did their assembly pathways, adapting to the presence of rRNA expansion segments, new or modified ribosomal proteins, and the need to travel through organelle membranes.
Eukaryotic Ribosome Biogenesis
In eukaryotes, ribosome assembly is a complex process involving more than 200 assembly factors and spreads from the nucleolus to the cytoplasm. The process starts with the synthesis by the RNA Pol-I of the 35S pre-rRNA, which is co-transcriptionally bound and folded by ribosomal proteins and assembly factors. Cleavage of this emerging rRNA releases the earliest small ribosomal subunit precursor, the 90S. The remaining rRNA will continue to be synthetized and become the large ribosomal subunit precursor.
To understand the pathway between those precursors and the mature ribosomes, our group, in collaboration with the group of external pageVikram Panse at the University of Zurich, visualized intermediates, revealing the interactions of assembly factors with the immature ribosomes. Those structures displayed how the assembly factors probe the integrity of important regions of the ribosome and lock the ribosomal proteins and the rRNA in an immature conformation, preventing premature progression through the maturation process. Additionally, the assembly factors were also shown to hamper the immature ribosomal subunits to take part in a possibly error-prone translation by shielding the functional regions of the ribosomal subunits.