Acetylation of ribosomal proteins. I. Characterization and properties of rat liver ribosomal proteins.

Rat liver ribosomes have been acetylated both in vivo and in vitro. The in vivo acetylation of ribosomes occurred within 5 min of the administration of t3H]acetate, and 70 to 80% of the incorporated radioactivity remained in the ribosomal proteins after removal of RNA with urea-LiCl. Following treatment of the acetylated ribosomai proteins with ethanol-ether, RNase, and 0.5 M KCl, to remove possible contamination by lipids, nucleotides, and cytoplasmic proteins, respectively, the radioactivity still remained in the ribosomal proteins. However, hydrolysis of the acetylated ribosomal proteins with 6 N HCl at 110” for 18 hours released 50 % of the radioactivity. These results are regarded as evidence that labile acetyl groups were incorporated into the ribosomal proteins. By using exclusion chromatography and ion exchange chromatography, two major radioactive acetyl groups were identified as N-cY-acetyllysine and N-.$-acetyllysine. Analysis of ribosomal proteins by 10 % polyacrylamide gel electrophoresis also showed that 3H-acetyl groups were present in these proteins. Giving puromycin to the animals decreased acetylation of the ribosomal proteins. In vitro experiments showed that the acetylation of liver ribosomes could be accomplished by using cell sap or a partially purified acetyltransferase. This acetylation was not inhibited by the presence of puromycin. Acetylation of ribosomal proteins was shown also in heart, kidney, and thymus.