The variable location of phosphoproteins on eukaryotic ribosomal subunits

Accessibility of ribosomal proteins to lactoperoxidase-catalyzed iodination following phosphorylation and during subunit interaction

Lactoperoxidase-catalyzed iodination was employed as a probe to monitor conformational change in 40-S ribosomal subunits from rat liver. Using this probe, it was observed that phosphorylation of protein S6 resulted in no detectable change in the iodination pattern of 40-S subunit proteins. These results suggest that the conformation of the small subunit remains unaltered following phosphorylation. On the other hand, the differences noted in the iodination pattern between 40-S ribosomal proteins derived from isolated subunits and those from 80-S monosomes, suggest that the 40-S subunit undergoes a conformational change during association with the 60-S subunit. Following 40-S and 60-S subunit association, proteins S2, S3, S5, S6, S8, S10 and S14 became less accessible to iodination. It is suggested that these proteins may be located at the interface between the 40-S and 60-S subunits.

Characterization of the acidic phosphorprotein of eukaryotic ribosomes using a new system of two-dimensional gel-electrophoresis

1. A modified method of two-dimensional gel electrophoresis has been developed for detecting acidic eukaryotic ribosomal proteins. 2. Using this method it has been demonstrated that the major phosphoprotein (Lgamma) of mouse ascites and hamster fibroblast 60-S ribosomal subunits is an acidic protein, apparently analagous to L7/12 of Escherichia coli, and not a neutral protein as previously thought. Electrophoretic resolution of phosphorylated and non-phosphorylated forms of Lgamma has enabled the stoichiometric nature of the phosphorylation (previously deduced from measurement of the specific radioactivity of the ATP pool) to be confirmed. 3. When ascites cells were incubated for 3 h there appeared an altered form of Lgamma which is most likely produced by proteolytic cleavage of the original form. The extent of phosphorylation of Lgamma was decreased by one-half or more in these circumstances. 4. Phosphoprotein Lgamma was found to be almost completely phosphorylated in both polyribosomes and monoribosomes isolated from hamster fibroblasts. Thus the function of the phosphorylation of Lgamma appears not to be concerned with the inactivation of ribosomes.

The phosphorylation of an acidic protein of the large ribosomal subunit of Krebs II ascites cells

The ribosomes of Krebs II ascites cells contain an acidic protein, apparently analogous to proteins L7/12 of Escherichia coli. When ascites cells were incubated with [32P]Pi, this protein became labelled, indicating that it is a phosphoprotein.