Three binding sites for stalk protein dimers are generally present in ribosomes from archaeal organism.
Maki Y., Hashimoto T., Zhou M., Naganuma T., Ohta J., Nomura T., Robinson CV., Uchiumi T.
Ribosomes have a characteristic protuberance termed the stalk, which is indispensable for ribosomal function. The ribosomal stalk has long been believed to be a pentameric protein complex composed of two sets of protein dimers, L12-L12, bound to a single anchor protein, although ribosomes carrying three L12 dimers were recently discovered in a few thermophilic bacteria. Here we have characterized the stalk complex from Pyrococcus horikoshii, a thermophilic species of Archaea. This complex is known to be composed of proteins homologous to eukaryotic counterparts rather than bacterial ones. In truncation experiments of the C-terminal regions of the anchor protein Ph-P0, we surprisingly observed three Ph-L12 dimers bound to the C-terminal half of Ph-P0, and the binding site for the third dimer was unique to the archaeal homologs. The stoichiometry of the heptameric complex Ph-P0(Ph-L12)(2)(Ph-L12)(2)(Ph-L12)(2) was confirmed by mass spectrometry of the intact complex. In functional tests, ribosomes carrying a single Ph-L12 dimer had significant activity, but the addition of the second and third dimers increased the activity. A bioinformatics analysis revealed the evidence that ribosomes from all archaeal and also from many bacterial organisms may contain a heptameric complex at the stalk, whereas eukaryotic ribosomes seem to contain exclusively a pentameric stalk complex, thus modifying our view of the stalk structure significantly.