The eukaryotic ribosome contains nearly 80 intrinsic proteins. The high degree of similarity across species, from the primary amino acid sequences to their tertiary structures, suggests conserved functional roles beyond serving as mere scaffolding for the rRNAs. Ribosomal protein L11 of Saccharomyces cerevisiae is an essential, highly conserved component of the 60S subunit (in bacteria and archaea, the homologous protein is named L5; the yeast nomenclature is used throughout this text to minimize confusion). At the primary amino acid sequence level, L11 is well conserved among eukaryotes ($67-97% identity), while bacterial and archaeal L5 proteins are less well conserved (27-55% identical) (Supplementary Figure S1A ). L11 is uniquely positioned at the interface between the large subunit central protuberance ( Figure 1A and 1B) and the head of the small subunit ( Figure 1A ) (1) (2) (3) (4) (5) . In the small subunit, the head region undergoes significant rotational movement relative to the central protuberance between the pre-and post-translocational states (6) , and the protein-protein interactions between L11 and S18 (S13 in bacteria and archaea) on the small subunit (the B1b and B1c intersubunit bridges) undergo the largest intersubunit structural rearrangements between these two states (7) (8) (9) . These observations suggest that L11 may play a central role as an informational conduit between the two subunits. Detailed analysis of X-ray crystallographic and cryo-EM structures ( Figure 1B ) reveals that the concave surface of the b-sheet portion of L11 interacts with specific nucleotides in the minor groove of 23S rRNA helix 84 (10) . L11 also makes contacts with the helix III and loop C regions of 5S rRNA; these connections have been hypothesized to help stabilize 5S rRNA interactions and may participate in an information signal transmission network linking functional centers within the ribosome (1, 11) . Importantly, the B1b and B1c intersubunit bridges with S18 are the only protein-protein interactions between the two subunits (2, 3, 5, 8) . Analyses of these structures indicate that contacts involving L11 and S18 through the B1b and B1c bridges break and rearrange after eEF-2 binding and ribosome ratcheting, controlled in part by differentially charged amino acid side chains between the two proteins (2, 4, 5, 7, 9, 12) . An internal loop of L11 that we denote the 'L11 P-site loop', which is roughly formed by amino acid residues 48-68, also directly contacts the T-loop of the peptidyl-tRNA in the P-site through tRNA nucleotide 56 (3) (4) (5) . At the level of primary amino acid sequence, the P-site loop is highly conserved among eukaryotes (85-100% identity), while it is less well conserved among bacteria and archaea (42-57% identity) (Supplementary Figure S1B) . At the biochemical level, however, the P-site loop is significantly more homogeneous, containing a large number of well-aligned charged and aromatic amino acids. In particular, A50, F57, R60 and I65 (yeast numbering) are universally conserved. An alignment of the P-site loop structures from yeast, Haloarcula marismortui, Thermus thermophilus and Escherichia coli reveals that the P-site loop is extremely well conserved at the structural level (Supplementary Figure S1C) .