The puzzling lateral flexible stalk of the ribosome

The lateral flexible stalk of the large ribosomal subunit is made of several interacting proteins anchored to a conserved region of the 28S (26S) rRNA termed the GTPase-associated domain or thiostrepton loop. This structure is demonstrated to adopt puzzling changes of conformation following the different steps of the elongation cycle. Some of these proteins termed the P-proteins in eukaryotes and L10 and L7/L12 in bacteria, present little structural similarities between Eubacteria on one side and Archae and Eukaryotes on the other side. However, up to now, these proteins seem to present a similar macromolecular organisation and they have been involved in the same functions. Convincing evidence attests that these proteins participate in elongation factor binding to the ribosome, and it has been suggested that these proteins might be evolved in a GTP hydrolysis activating protein activity. Involvement of these proteins in the translational mechanism is discussed. Moreover, in eukaryotes, small P-proteins are also found as isolated proteins in a cytoplasmic pool that exchanges with the ribosome-associated P-proteins. Moreover, a part of the ribosomal proteins is phosphorylated (hence their P-protein names). The biological signification of these particularities is discussed.