The ribosomal proteins of Saccharomyces cerevisiae. Phosphorylated and exchangeable proteins

Sixty-seven ribosomal proteins of the yeast Saccharomyces cerevisiae have been distinguished by two-dimensional polyacrylamide gel electrophoresis. Five of the ribosomal proteins are phosphorylated in vivo. One of the phosphorylated proteins appears to correspond to the phosphorylated ribosomal protein, S6, of rat liver. Another of the yeast-phosphorylated proteins, which is highly acidic, may be related to the protein L7/L12 of Escherichia coli. The phosphorylation of four of the five ribosomal proteins depends on protein synthesis. Ribosome synthesis, however, is not necessary for the phosphorylation of any of them. Three proteins of the 60 S ribosomal subunit are "exchangeable" in vivo. Two of these are phosphoproteins, one of which is the very acidic protein possibly related to L7/L12 of E. coli. The possible significance of this phosphorylated, exchangeable protein in protein synthesis is discussed.