Eukaryotic ribosome
The ribosome selects aminoacylated transfer RNAs (tRNAs) based on the sequence of a protein-encoding messenger RNA (mRNA) and covalently links the amino acids into a polypeptide chain.
Compared to their prokaryotic homologs, many of the eukaryotic ribosomal proteins are enlarged by insertions or extensions to the conserved core.
Furthermore, several additional proteins are found in the small and large subunits of eukaryotic ribosomes, which do not have prokaryotic homologs.
Both 18S and 28S have multiple insertions to the core rRNA fold of their prokaryotic counterparts, which are called expansion segments.
For a detailed list of proteins, including archaeal and bacterial homologs please refer to the separate articles on the 40S and 60S subunits.
[8] More recently structures at sub-nanometer resolution were obtained for complexes of ribosomes and factors involved in translation.
Atomic coordinates (PDB files) and structure factors of the eukaryotic ribosome have been deposited in the Protein Data Bank (PDB) under the following accession codes: Some general architectural features of the ribosome are conserved across kingdoms:[20] The structure of the small subunit can be sub-divided into two large segments, the head and the body.
In the characteristic "crown view" of the large subunit, structural landmarks include the central protuberance, the L1-stalk and the P-stalk.
The ribosomal RNA core is represented as a grey tube, expansion segments are shown in red.
[23] Comparisons between bacterial, archaeal and eukaryotic ribosome structures reveal a very high degree of conservation in the active site—aka the peptidyl transferase center (PTC) -- region.
[16] It may link signal-transduction pathways directly to the ribosome though it also has a role in multiple translational processes that appear unrelated (reviewed in [25]).
Ribosomal protein eS6 is located at the right foot of the 40S subunit [16] and is phosphorylated in response to mammalian target of rapamycin (mTOR) signaling.
[27] The structures of the 40S:eIF1 [16] and 60S:eIF6 [17] complexes provide first detailed insights into the atomic interactions between the eukaryotic ribosome and regulatory factors.
However, structural information on the eukaryotic initiation factors and their interactions with the ribosome is limited and largely derived from homology models or low-resolution analyses.
Recent genetic evidence has been interpreted to suggest that individual proteins of the eukaryotic ribosome directly contribute to the regulation of translation.
Additional elements are restricted to the second tier of proteins around the tunnel exit, possibly by conserved interactions with components of the translocation machinery.
