The acidic proteins of eukaryotic ribosomes A comparative study

The acidic proteins extracted by 0.4 M NH₄Cl and 50% ethanol from ribosomes from Saccharomyces cerevisiae, wheat germ, Artemia salina, Drosophila melanogaster, rat liver and rabbit reticulocytes have been studied comparatively in several structural and functional aspects. All the species studied have in the ribosome two strongly acidic proteins with $\text{p}\text{I}$ values not greater than pH 4.5, which appear to be monophosphorylated in the case of S. cerevisiae, A. salina, D. melanogaster and wheat germ. Rat liver proteins are multiphosphorylated, as possibly are those from reticulocytes. The molecular weight of these acidic proteins as determined by SDS electrophoresis ranges from around 13 500 to 17 000 and, except in the case of yeast, of which both proteins have the same molecular weight, the size of the two proteins in the other species differs by approx. 1000–2000. In general, the size of the proteins increases with the evolutionary position of the organism, as seems to be the case with the degree of phosphorylation. From an immunological point of view the ribosomal acidic proteins of eukaryotic cells are partially related, since antisera against yeast protein cross-react with proteins from wheat germ, rat liver and reticulocytes. Bacterial proteins L7 and L12 are very weakly recognized by the anti-yeast sera. Anti-bacterial acidic proteins do not cross-react with any of the protein from the species studied. The proteins from all the species studied are functional equivalents and can reconstitute the activity of particles of S. cerevisiae deprived of their acidic proteins.