The Phosphorylation of Liver Ribosomal Proteins in Vivo: EVIDENCE THAT ONLY A SINGLE SMALL SUBUNIT PROTEIN (S6) IS PHOSPHORYLATED

Abstract

Abstract The phosphorylation of rat liver ribosomal proteins was increased by an order of magnitude during liver regeneration. A large amount of radioactivity was associated with preparations of liver ribosomes after administration of [32P]orthophosphoric acid to partially hepatectomized rats; however, only a small fraction was actually in ribosomal phosphoproteins. Radioactivity was removed from the ribosomal pellet by each of the following treatments: with high concentrations of potassium ions to form subunits, with cold trichloroacetic acid, with hot tichloroacetic acid, and with organic solvents; a small amount of radioactivity remained after treatment. The radioactivity associated with the 40 S ribosomal subunit was resistant to treatment with deoxyribonuclease and ribonuclease A but sensitive to trypsin and alkaline phosphatase; the radioactive phosphate was covalently bound to serine. It was shown by two-dimensional polyacrylamide gel electrophoresis that all of the radioactive phosphate incorporated into ribosomes was in a single small subunit protein, S6. There were, in addition, several (in some experiments as many as five) derivatives of S6 which contained increasing numbers of radioactive phosphoserine residues. No large subunit protein was phosphorylated.