Science 334, 1524–1529 (2011)

Credit: SERGEY MELNIKOV & ADAM BEN-SHEM

Although the translation of mRNA to protein by ribosomes is generally conserved, there are ribosomal components specific to eukaryotes. Ben-Shem et al. now report the first atomic model of the 80S ribosome from Saccharomyces cerevisiae based on X-ray crystal data resolved to 3.0 Å. Their model includes two ribosomes in different conformations containing all 35 universally conserved proteins as well as the 44 eukaryote-specific ones. The latter components as well as eukaryote-specific rRNA are predominately located on the surface of the ribosome but away from the regions responsible for executing core ribosomal activities. The authors also noted that unlike in prokaryotic rRNAs, nonhelical elements of rRNA formed single-stranded stretches that serve as platforms for protein binding. The interface between the subunits was approximately doubled in eukaryotes, owing to the formation of more intersubunit bridges. Indeed, two bridges created by eukaryote-specific extensions were not buried between the subunits but were solvent exposed. Their location with respect to other known functional landmarks led the authors to hypothesize that they have a role in translational initiation. The structure also included a nonribosomal protein, the translational inhibitor Stm1, which appeared to block the mRNA access tunnel via the insertion of an α-helix that mimics the path of the mRNA to the active site of the ribosome. Although hypotheses resulting from this structure remain to be experimentally validated, this structure provides the first molecular-level model of the complete eukaryotic ribosome.